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Pyrolysis of Biomass Residues in a Screw Reactor
Ricardo Isidro Martins da Silva Maximino
Thesis to obtain the Master of Science Degree in
Mechanical Engineering
Examination Committee
Chairperson: Prof. Luis Rego da Cunha de Eça Supervisor: Prof. Doutor Mário Manuel Gonçalves da Costa
Co-supervisor: Doutor Rui Pedro da Costa Neto Members of the Committee: Doutor Abel Martins Rodrigues
October 2013
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Abstract
The present work aims to evaluate the potential of producing bio-oils and chars from biomass residues
through fast pyrolysis. Pinewood and agro-biomasses – olive bagasse, wheat straw and rice husk –
were pyrolysed in a bench scale screw reactor at 580 ºC using a carrier gas flow rate of 526 mL/min in
order to maximize the resultant bio-oil fraction. The yields of bio-oil, bio-char and gas obtained were
quantified. Bio-liquid yields varied between 51 wt.% for pinewood and 31 wt.% for olive bagasse and
the bio-char yields between 38 wt.% for wheat straw and 24 wt.% for pinewood. Subsequently, the
bio-oils and chars obtained were analyzed in terms of moisture content, elemental analysis, ash
content and heating value. The main conclusions are as follows. 1) pinewood showed the highest
potential to pyrolyse; the resultant bio-oil and char have potential to be used as fuels without further
treatments. 2) The substantial ash content in the feedstock of the agro-biomasses decreased
drastically their bio-oil quality and yield. Furthermore, the low conversion yields related to such
biomasses observed in the present work may not justify their valorisation through pyrolysis. 3) The
non-homogeneity of the bio-oils from agro-biomasses is a higher challenge to their use as fuel,
however, is an opportunity for recovering added-valued by-products. 4) The chars obtained from agro-
biomasses with higher ash content and lower energy densities may have potential to be used in the
preparation of active carbon when its pore structure and surface are appropriate.
Keywords: fast pyrolysis, screw reactor, pinewood, agro-biomasses, bio-oil, char
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Resumo
O presente trabalho tem como objetivo avaliar o potencial de produção de bio-óleos e carvões através
de pirólise rápida de resíduos de biomassa. pinho e agro-biomassas – bagaço de azeitona, palha de
trigo e casca de arroz - foram pirolisadas num reator de parafuso à escala laboratorial, a 580 ºC
usando um caudal volumétrico de gás de arrasto de
526 mL/min a fim de maximizar a fração de bio-óleo resultante. Os rendimentos de bio-óleo, carvão e
gás obtidos foram quantificados. A produção de bio-líquido variou entre 51 wt.% para madeira de
pinho e 31 wt.% para bagaço de azeitona e os rendimentos de carvão entre
38 wt.% para palha de trigo e 24 wt.% para pinho. Posteriormente, os bio-óleos e carvões obtidos
foram analisados em termos de humidade, análise elementar, teor de cinzas e poder calorífico. As
principais conclusões foram as seguintes. 1) O pinho mostrou o maior potencial na pirólise; o bio-óleo
e o carvão resultante têm potencial para serem usados directamente como combustível. 2) O teor de
cinzas substancial na matéria-prima das agro-biomassas diminuiu drasticamente a qualidade dos
seus bio-óleos e respectivos rendimentos. Os baixos rendimentos de conversão obtidos no presente
trabalho com estas biomassas podem não justificar a sua valorização através da pirólise. 3) A não-
homogeneidade dos bio-óleos das agro-biomassas é um desafio extra para a sua utilização como
combustível; no entanto, é uma oportunidade para obtenção de subprodutos de valor acrescentado.
4) Os carvões obtidos a partir das agro-biomassas com maiores teor de cinzas e poderes caloríficos
mais baixos podem ter potencial para ser utilizados na preparação de carvão activado quando a sua
estrutura de poros e superfície forem adequados.
Palavras-chave: pirólise rápida, reactor de parafuso, madeira de pinho, agro-biomassa, bio-óleo,
carvão
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Agradecimentos
Ao professor Mário Costa agradeço toda a disponibilidade e orientação prestada durante a
realização deste trabalho. O seu apoio foi sem dúvida uma forte ajuda e motivação.
Agradeço ao co-orientador, Doutor Rui Neto, pela orientação prestada ao longo deste
projecto, e pela sua receptividade de ideias e inegável capacidade de partilhar conhecimentos
laboratoriais. A sua disponibilidade e apoio técnico foram elementos fulcrais para a realização do
trabalho.
Ao Engenheiro Gonçalo Duarte agradeço os seus conhecimentos partilhados de LabVIEW e
à Engenheira Inês Monteiro agradeço todo o apoio moral na fase inicial deste trabalho de dissertação.
Quero deixar um agradecimento a todos os meus colegas do laboratório de combustão pelo
seu apoio e incentivo durante as várias fases deste trabalho, nomeadamente a António Duarte Silva,
Bruno Bernardes, Francisco Costa, Gongliang Wang, João Pina, Mafalda Henriques, Manuel Barbas,
Miguel Elias, René Zander, Tiago Carvalho e Ulisses Fernandes. Deixo um agradecimento especial à
minha colega e mais recente amiga Isabel Ferreiro pela sua ajuda e acompanhamento na última fase
do trabalho.
Aos amigos e colegas que conheci ao longo destes anos de vida académica deixo aqui o meu
profundo agradecimento pelo seu apoio, amizade e companheirismo. Várias cabeças sempre
pensaram mais que uma.
Aos meus amigos de longa data agradeço toda a amizade e encorajamento que de uma
maneira ou outra contribuíram para a realização do trabalho.
Agradeço também às oficinas do Departamento de Engenharia Mecânica do Instituto Superior
Técnico pela construção de parte do equipamento utilizado neste trabalho.
À minha Família deixo um agradecimento especial pelo apoio incansável e incondicional ao
longo deste trabalho e de toda a minha vida. Pai não me esqueço do que um dia me ensinaste:
“Esforço, Dedicação, Devoção e Glória”.
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Table of Contents
1. Introduction ........................................................................................................... 1
1.1 Background and objectives ........................................................................................... 1
1.2 Literature review ........................................................................................................... 5
1.2.1 Fast pyrolysis fundamentals ......................................................................................... 5
1.2.2 Lignocellulosic feedstock .............................................................................................. 9
1.2.3 Fast pyrolysis technologies ......................................................................................... 12
1.2.4 Related developed works ........................................................................................... 13
1.2.5 Bio-oil properties ......................................................................................................... 16
1.2.5.1 Chemical nature of bio-oil .............................................................................. 16
1.2.5.2 Physical properties ........................................................................................ 17
1.2.6 Bio-oil applications ...................................................................................................... 19
1.3 Present contribution .................................................................................................... 21
1.4 Thesis outline .............................................................................................................. 21
2. Facilities, Techniques and Procedures .............................................................. 22
2.1 Feedstock ................................................................................................................... 22
2.2 Experimental facilities ................................................................................................. 23
2.3 Measuring Techniques and uncertainties ................................................................... 31
2.3.1 Temperatures ............................................................................................................. 31
2.3.2 Biomass feed rate ....................................................................................................... 32
2.3.3 Nitrogen flow rate ........................................................................................................ 34
2.3.4 Reproducibility ............................................................................................................ 34
2.4 Experimental procedure .............................................................................................. 35
2.5 Analysis and characterization of products .................................................................. 36
2.5.1 Yields of the products ................................................................................................. 36
2.5.2 Bio-oil analysis ............................................................................................................ 37
2.5.3 Char analysis .............................................................................................................. 38
3. Results and Discussion ...................................................................................... 39
3.1 Experimental conditions .............................................................................................. 39
3.2 Analysis of the feedstock ............................................................................................ 39
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3.3 Yields of the pyrolysis products .................................................................................. 41
3.4 Analysis of bio-oils ...................................................................................................... 45
3.5 Analysis of chars ......................................................................................................... 49
4. Closure ............................................................................................................... 51
4.1 Conclusions ................................................................................................................ 51
4.2 Recommendations for future work .............................................................................. 52
5. References ......................................................................................................... 54
6. Appendices ........................................................................................................ 62
6.1 Appendix A – Mean value and standard deviation ..................................................... 62
6.2 Appendix B – Temperature control system ................................................................. 63
6.3 Appendix C – Thermal characterization of the reactor ............................................... 67
6.4 Appendix D – Biomass feed rate ................................................................................ 69
6.4 Appendix E – Estimation of the hot vapours residence time ...................................... 71
6.5 Appendix F – Trial tests on the reactor ....................................................................... 73
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List of Figures
Figure 1.1 – Modern renewables primary energy demand by region in the New Policies Scenario.
Source: Ref. [3] ....................................................................................................................................... 1
Figure 1.2 – Products from thermal biomass conversion. Source: Ref. [12] ....................................... 3
Figure 1.3 – Representation of the reaction paths for wood pyrolysis. Source: Ref. [20] ................... 7
Figure 1.4 – Pyrolysis mechanism proposed by Di Blasi. Source: Ref. [26] ....................................... 8
Figure 1.5 – Chemical structure of cellulose. Source: Ref. [16] ........................................................ 10 Figure 1.6 – Illustration of lignocellulosic structure. Source: Ref. [33] ............................................... 10
Figure 1.7 – Pyrolysis products from the main components of lignocellulosic biomass .................... 11
Figure 1.8 – Schematic diagram of the reactor configuration. Source: Ref. [62] ............................... 14
Figure 1.9 – Yields of pyrolysis products as a function of temperature. Source: Ref. [62] ................ 14
Figure 1.10 – Schematic diagram of fast pyrolysis apparatus. Source: Ref. [65] .............................. 15
Figure 1.11 – Applications for the products of fast pyrolysis. Source: Ref. [13] ................................ 19
Figure 1.12 – Fast pyrolysis-based biorefinery. Source: Ref. [13] .................................................... 20
Figure 2.1 – The different biomass samples: a) Pinewood, b) Olive bagasse, c) Wheat straw and d)
Rice husk .............................................................................................................................................. 22
Figure 2.2 – Schematic of the pyrolysis reactor ................................................................................ 23
Figure 2.3 – Facility of the screw reactor .......................................................................................... 24
Figure 2.4 – Detail view of the feeding system .................................................................................. 25
Figure 2.5 – Screw of the reactor ...................................................................................................... 26 Figure 2.6 – Heating resistance of the reactor .................................................................................. 26 Figure 2.7 – Temperature Control System diagram .......................................................................... 27
Figure 2.8 – Illustration of the temperature control system (not in scale) .......................................... 28 Figure 2.9 – Integrated heat exchanger of nitrogen .......................................................................... 30 Figure 2.10 – Illustrative detail view of the reaction zone .................................................................. 30 Figure 2.11 – Samples of char of pinewood pyrolysed under a screw velocity of a) 57 rpm
and b) 19 rpm (500 ºC, N2 flow rate of 526 mL/min) ......................................................................... 33
Figure 2.12 – Typical temperatures profile along an experimental run ............................................. 36
Figure 3.1 – Yields of the products obtained from the pyrolysis of the biomasses ........................... 41
Figure 3.2 – Correlation of the feedstock ash content to the bio-oil yields obtained from pyrolysis .. 42 Figure 3.3 – Contact of the pyrolytic hot vapours with char in the char’s flask .................................. 43
Figure 3.4 – Product distribution obtained from different modes of pyrolysis for woody biomass.
Source: Ref. [13] ................................................................................................................................... 44
Figure 3.5 – Viscous portion of bio-oil trapped in the first condenser ............................................... 47
Figure 3.6 – Bio-oil of a) pinewood and b) olive bagasse ................................................................. 48 Figure B-6.1 – Pyrolysis program main screen ................................................................................. 63 Figure B-6.2 – Auxiliary relay circuit .................................................................................................. 64
Figure B-6.3 – Heating pulsed action ................................................................................................ 65 Figure B-6.4 – Temperatures plot on Pyrolysis program ................................................................... 66
viii
Figure C-6.5 – Temperature distribution inside the pipe with a reactor temperature of
500º C ................................................................................................................................................ 67 Figure D-6.6 – Technique adopted to determine biomass feed rate ................................................. 69 Figure D-6.7 – Weight in the bowl as a function of time for the various biomasses with a screw
velocity of 19 rpm. ................................................................................................................................ 70
Figure F-6.8 – Product yields from the pyrolysis of the pinewood, olive bagasse, wheat straw and
rice husk in relation to temperature (N2 flow rate of 526 mL/min, 19 rpm) ........................................... 73
Figure F-6.9 – Product yields from the pyrolysis of the pinewood, olive bagasse, wheat straw and
rice husk in relation to N2 flow rate (reactor temperature of 580 ºC, 19 rpm) ....................................... 74
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List of Tables
Table 1.1 – Typical product weight yields (dry wood basis) obtained from different modes of
pyrolysis of wood. Source: Ref. [13] ....................................................................................................... 5
Table 1.2 – Fast pyrolysis chemical phenomena: consequence and prevention ................................ 8
Table 1.3 – Bio-oil and of heavy fuel oil typical properties. Source: Ref. [72] ................................... 17
Table 1.4 – Main properties of bio-oils derivate from various feedstocks. *on dry basis **only oil-
phase analysis ...................................................................................................................................... 17
Table 2.1 – Feed rates of the biomasses (19 rpm) ........................................................................... 33
Table 2.2 – Typical yield values for the pyrolysis of pinewood performed at 580 ºC with a nitrogen
flow rate of 526 mL/min ........................................................................................................................ 34
Table 3.1 – Main characteristics of pinewood, olive bagasse, wheat straw and rice husk. *as
received **on dry basis ......................................................................................................................... 39
Table 3.2 – Physical properties of the original feedstock and the bio-oils obtained from pyrolysis of
pinewood, olive bagasse, wheat straw and rice husk ........................................................................... 45
Table 3.3 – Physical properties of the char obtained from pyrolysis of pinewood, olive bagasse,
wheat straw and rice husk .................................................................................................................... 49 Table D-6.1 – Feed rates of the biomasses with a screw velocity of 19 rpm .................................... 70
Table E-6.2 – Estimated volumetric flow rates on the reaction zone to the various flow rates nitrogen
.............................................................................................................................................................. 72
Table E-6.3 – Estimated residence time of the hot vapours in the reaction zone ............................. 72
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Nomenclature
Symbols
mNTP – mass flow rate of nitrogen in the gas flow meter
ρNTP – density of nitrogen at NTP conditions
ρR – average density of nitrogen in the reaction zone
σ – standard deviation
TR – average temperature of nitrogen in the reaction zone
VU – useful volume of the reaction zone
VNTP – volumetric flow rate of nitrogen in the gas flow meter
VR – average volumetric flow rate of nitrogen in the reaction zone
wt.% – weight percentage
Abbreviations
as – as received
daf – dry and ash free basis
db – dry basis
FTIR – Fourier Transform Infrared Spectroscopy
GC – Gas Chromatography
GC-FID – Gas Chromatography - Flame Ionization Detector
GC-MS – Gas chromatography - Mass Spectrometry
GPC – Gel Permeation Spectroscopy
HHV – High Heating Value
HLPC – High-Pressure Liquid Chromatography
LHV – Low Heating Value
NMR – Nuclear Magnetic Resonance
NTP – Normal Temperature and Pressure
TCS – Temperature Control System
1
1. Introduction
1.1 Background and objectives
The continuous growth of energy demand seen in the last 40 years represents a panorama of
the world’s energy supply where fossil resources still prevail [1]. Nowadays the investment in
renewable resources became mandatory due to environmental considerations, particularly climate
change and sustainability reasons, restraining the heavy dependence on fossil fuels in the near future.
There are strong indications that the targeted introduction of renewable energies in
combination with a more rational use of energy will be able to achieve the required sustainability
criteria [2]. Renewable resources will play a central role in moving the world onto a more reliable
energy path, but their huge potential is highly hinged on the government support.
Fortunately, government intervention has grown making efforts to make renewables cost-
competitive and to stimulate technological advances in this field, for helping to diversify the energy
sources. In the “New Policies Scenario” the use of modern renewable energy – including hydro, wind
power, solar, geothermal, biomass and marine energy – triples till 2035 (see Fig. 1.1), with its share in
total primary energy demand increasing from 7% to 14%. European Union protrudes in this context,
where the increase is strongly encouraged by policies to raise the share of renewables to 20% in the
gross final consumption in 2020 [3].
From the renewables package, biomass is considered the renewable energy source with the
highest potential to contribute to the energy needs of modern society. It is considered a promising
non-nuclear form of energy, with environmental benefits. For both developed and developing
countries, it can provide the major part of the projected renewable provisions of the future [4]. In
countries heavily reliant on foreign sources with a considerable resource of biomass, its development
as a main source is an incentive to increase their own domestic energy generation, with all the
benefits that arise from there, as economical revitalization and job creation [5,6].
Figure 1.1 – Modern renewables primary energy demand by region in the New Policies Scenario. Source: Ref. [3].
2
By now, biomass already plays a vital role in meeting local energy needs from generating
electricity, heating homes and even industrial facilities, to fuelling vehicles with biogenic fuels, being
utilized with a varied portfolio of technologies [7]. Accounting for simple and still pollutant processes, it
represents 14% of the world total primary energy consumption, and for more than half of the final
energy produced from renewable energy sources [8].
In the last 30 years, the development of technologies capable of producing energy and
appreciated chemicals from biomass has been notable; however, these quite recent processes are still
not economically competitive with the large-scale production from the petrochemical industry. Besides,
biomass implies inherently a challenge to the production of energy from it – has to be harvested,
collected and transported. Most forms of biomass resource are widely dispersed, have poor energy
densities, high moisture contents and a wide range of sizes and shapes leading to problems of feed
handling. The low bulk densities of biomass explain the high transportation costs. Furthermore, the
use of biomass also has concerning points that must be tracked and if possible countered: possible
direct competition with the food chain, the likely negative savings in greenhouse gas emissions,
decrease of bio-diversity and soil exhaustion.
These issues, which justify the relative higher and non-competitive costs, have led researches
to seek solutions towards a more economical and efficient way to generate energy from biomass. As
response, energy production through what is called “biomass conversion routes” has became an area
of growing interest with substantial advantages. Via these conversion routes, biomass can be
converted to more useful forms of energy via a number of processes including mostly thermal and
biological processes [9]. Between both types of process, the thermal processing is often overlooked as
the most viable approach to biomass conversion in the short term [10].
Thermal (or thermochemical) processing of biomass uses heat and catalysts to transform
plant biomass into fuels, electric power and even chemicals [10]. There are three main thermal
processes for converting biomass into these more useful forms of energy and added value products –
direct combustion, gasification and pyrolysis [10].
Focusing our attention on pyrolysis, it is a well-known thermal process from which three
energetic products of distinct phase are always obtained: char, a pyrolytic oil, the so-called bio-oil, and
fuel gas. These products are of interest as they are possible alternate sources of energy. Figure 1.2
illustrates the fractionation of biomass pyrolysis products and their possible applications along with
other thermal processes.
3
Pyrolysis can be adjusted to favour char, bio-oil or gas production with high fuel-to-feed ratios
[11]. Depending on the conditions used, is possible to optimize the conversion of biomass into one of
the three products [12].
Fast pyrolysis is a singular category of pyrolysis that respect given conditions that tends to
maximize the liquid product – bio-oil. This advanced pyrolysis process is at early stage of development
compared to combustion, gasification or even conventional pyrolysis. In the last 30 years has become
of considerable interest since the process gives directly high yields of bio-oil that can be used directly
for subsequent processing to biofuels and chemicals, or as efficient energy carrier for heat and power
generation [13].
It is expected that fast pyrolysis play a major role in the near future since the resulting bio-oil
offers many advantages over the original raw biomass as an energy product, particularly: lower
transport costs, smaller storage requirements and ease of conveyance into current technologies. The
interest in bio-oil production from biomass has grown in recent years due to the possibilities of [10]:
§ Decoupling liquid fuel production (scale, time and location) from its utilization;
§ Producing a renewable fuel for engines, boilers, turbines, power stations and gasifiers;
§ Upgrading to motor fuels, additives or specific chemicals (biorefinery concept [13,15]).
Its ecological advantages over fossil resources became an important reason for its on-going
development and study: is CO2 neutral, and only generate insignificant amounts of other pollutants
(SOx, NOx etc.). On the other hand, since bio-oil is a carbonaceous liquid makes it a possible feed for
fossil based refineries or dedicated biorefineries for subsequent processing.
A particular, but very interesting aspect is that fast pyrolysis could establish a link between
petrochemical industry and conventional wood and agricultural industries. For instance, Portugal is a
Figure 1.2 – Products from thermal biomass conversion. Source: Ref. [13].
4
country where these industries have an important role on economy. The large amount of dispersed
residues obtained along several production stages signifies an abundant biomass resource. Dias [14]
has estimated about 5630 thousand dry tons/year of biomass residues generated in Portugal, a
number that justifies their evaluation as relevant feedstock for further processing. According to Butler
et al. [15], a possible solution is the “decentralised” densification of these biomass residues to bio-oil
through fast pyrolysis followed by “centralised” upgrading into several valued-added products as
biofuel, chemicals, power and heat. The centralised upgrading facilities might include existing crude oil
refineries or the so-called biorefineries, where the thermochemical processing is already dominated.
The development of the biorefinery concept along with the increase of applications for bio-oil
offers a very promising future for bio-oil. Expected technological improvements in the coming years
enable a cost reduction for the overall process.
In this context, the main purpose of the present study is to assess the potential of producing
bio-oils through fast pyrolysis. Specifically from abundant biomass residues in the Mediterranean
countries such as pinewood, olive bagasse, wheat straw and rice husk with few practical applications,
exploiting the possibility of their valorisation along this route. For this purpose, such biomass residues
were pyrolysed in a screw reactor under fast pyrolysis conditions. The yields of the products, together
with the foremost physiochemical characteristics of the bio-oils and chars were examined.
5
1.2 Literature Review
Section 1.2.1. reviews fast pyrolysis fundamentals and section 1.2.2 is dedicated to
lignocellulosic feedstock and their implications on pyrolysis. Section 1.2.3 summarizes the existing
technologies of fast pyrolysis and section 1.2.4 reviews related previous works, which helped to
establish the starting point of the present work. Section 1.2.5 is dedicated to bio-oil properties. At last,
section 1.2.4. is devoted to the bio-oil applications.
1.2.1 Fast pyrolysis fundamentals
In the broader concept, pyrolysis is the thermal decomposition of organic material in the
absence of oxygen, and at temperatures ranging from 300 ºC to 600 ºC. Three products of distinct
phase are produced – char (bio-char), bio-oil and fuel gas – but their yields can vary over a wide range
by adjustment of the process conditions, mainly temperature, vapours residence time and heating rate
[13]. While carbonisation, or slow/conventional pyrolysis, seeks regularly charcoal production,
gasification appeals to higher temperatures maximizing the gas phase. Fast pyrolysis aims to bio-oil
production, involving generally higher heating rates. All these processes can be seen as variants of
the pyrolysis process, although gasification is seen as an independent process in technical terms.
Table 1.1 indicates the product distribution obtained from the different processes, showing the
considerable flexibility achievable by changing process conditions.
For fast pyrolysis, bio-oil’s yields of 60-75 wt.% are usually obtained, with 10-25 wt.%
remaining for solid char and 10-20 wt.% for non-condensable gases. Bio-oil, char and remaining
gases typically contain about 70%, 25% and 5% of the energy in the feed material, respectively. The
process itself only needs 15% of the energy in the feed, and from the by-products, only the produced
char is sufficient to provide this heat, making pyrolysis an energy self-sufficient process. For this
reason, recent well-designed processes should not produce any emissions other than clean flue gas
i.e. CO2 and water [16].
Fast pyrolysis involves higher heating rates where biomass decomposes in a few seconds or
less, resulting in char, non-condensable gases and condensable volatiles. After a rapid cooling and
condensation of these last ones, results in a dark brown liquid – pyrolysis oil or bio-oil [13,16-18]. The
condensable volatiles consist of hot vapours and aerosols. For simplicity of reading, from now on
ModeLiquid Solid (char) Gas
Fast 75% 12% 13%Intermediate 50% in 2 phases 25% 25%Carbonisation (slow) 30% 35% 35%Gasification 5% 10% 85%
~ 400 °C, long vapour residence time (days)~ 750-900 °C, long vapour residence time
Product yield (%)Conditions
~ 500 °C, short hot vapour residence time ~ 1 s~ 500 °C, hot vapour residence time ~ 10 - 30 s
Table 1.1 – Typical product weight yields (dry wood basis) obtained from different modes of pyrolysis of wood. Source: Ref. [13].
6
“vapours” denote any combination of vapours and aerosols. The fuel gas product is a mixture of
gases, some of them flammable, among which stand out CO, H2, and CH4.
The yields and compositions of the products depends on biomass type and process main
parameters: size of the biomass particles, temperature, pressure, volatile and biomass particles
residence time (feed rate), char separation, and biomass minerals content that ends up in the
pyrolysis as ash [16,17]. It was found on literature the essential features of fast pyrolysis process that
favours bio-oil production [13-18]:
§ Very high heating rates and heat transfer rates at the reaction interface, which usually
requires a finely ground biomass feed as biomass generally has a low thermal conductivity;
§ A carefully controlled reaction temperature of around 500 ºC (range of 425-575 ºC usually);
§ Short vapour residence times of typically less than ~ 2 s to minimise secondary reactions;
§ Rapid removal of product char to minimise cracking of vapours;
§ Rapid cooling of the pyrolysis vapours to give the bio-oil, avoiding possible secondary
reactions.
From what has been reported in literature, pyrolysis is a complex reaction where mechanisms
are not quite clarifying yet. The products are the result of multiple parallel, consecutive, competitive
and multi-phase reactions [19]. Sinha et al. [19] enumerate the general changes that occur during
pyrolysis:
(1) Heat transfer from an heat source, to increase the temperature inside biomass particles;
(2) The initiation of primary pyrolysis reactions at this higher temperatures releases volatiles
and forms char;
(3) The flow of hot volatiles toward cooler solids results in heat transfer between hot volatiles
and cooler unpyrolysed particles;
(4) Condensation of some of the volatiles in cooler parts of the biomass, followed by
secondary reactions, can produce tar;
(5) Autocatalytic secondary pyrolysis reactions proceed while primary reactions (item 2,
above) simultaneously occur in competition; and
(6) Further thermal decomposition, reforming, water gas shift reactions, recombination of
radicals, and dehydrations can also occur, which are a function of the process’s
residence time/temperature profile.
Neves et al. [20] also reported a complete description of the thermal degradation (pyrolysis) of
biomass in an inert atmosphere where it refers the main possible routes for the formation of the
products. Figure 1.3 illustrates the thermal degradation process of pyrolysis.
7
Heat and mass transfer processes, and phase transition phenomena, as well as chemical
reaction kinetics, play important roles. Particularly, under fast pyrolysis conditions the temperature
development inside the feedstock particles and the corresponding reaction kinetics dominate the
decomposition rate and products distribution [13]. Along with other main features of fast pyrolysis,
temperature is considered the most important factor for the product distribution [21].
Emmons and Atreya [22] estimated that more than 200 intermediate products are formed
during pyrolysis of biomass. Since cellulose is the major constituent of wood and pyrolyses over
almost the entire range of temperature, several researches have studied cellulose pyrolysis in detail
(see [23,24]). An excellent review by Di Blasi [25] has described the classes of mechanisms that had
been previously proposed for wood pyrolysis and that of other cellulosic materials. Figure 1.4
represents a reaction mechanism proposed by Di Blasi [26].
These, and other similar studies, led to understand the main goal of fast pyrolysis: prevent the
primary decomposition products (i) being cracked thermally or catalytically (over char formed already)
to non-condensable gas and/or (ii) being recombined/polymerized to char on the other hand, while
primary and secondary reactions occur [10].
Figure 1.3 – Representation of the reaction paths for wood pyrolysis. *Bio-oil refers like tar. Source: Ref. [20].
8
In the case of wood, and other lignocellulosic materials, extensive decomposition and
fragmentation of the three key biomass building blocks – cellulose, hemicellulose, and lignin – occurs
when undergoing fast pyrolysis. The primary products may continue to decompose through cracking
processes to yield monomeric derivatives and other low molecular weight products (non-condensable
gas), or to cross-link (repolymerize) through dehydration reactions, resulting in char formation [16].
Hence, is possible to relate the main and required features of fast pyrolysis to the chemical reaction
phenomena – Table 1.2 [27-29].
Figure 1.4 – Pyrolysis mechanism proposed by Di Blasi. Source: Ref. [26].
Table 1.2 – Fast pyrolysis chemical phenomena: consequence and prevention.
Phenomena Consequence Prevention
Cracking - thermal and catalytic
Continuous desfragmentation of volatiles to yield low
molecular weight products (non-
condensable gas)
Repolymerization Char formation
! Short volatiles residence time in order to qualitatively and quantitatively optimise the yield of oil ! Rapid condensation to minimise the extent of vapour phase cracking and possible secondary reaction ! Rapid removal of char to minimise its catalytic cracking role!
! High heat transfer that minimises the rate of second-order repolymerisation reactions
9
Short vapour residence times, rapid condensation and rapid removal of char prevent cracking
of vapours into non-condensable gases, and a high heat transfer minimizes char formation. Thus
using small particles is appropriate, because under ideal conditions there is nearly no temperature
gradient within the heated sample, being decomposed instantaneously, giving time to primary volatile
products to escape from the solid residue without delay [27]. The bigger the size of biomass particles,
the bigger the temperature gradients leading to lower temperatures that favour formation of char.
Bridgwater [18] reviewed the principles and practice of biomass fast pyrolysis processes and
the variables influencing process design considerations. Design variables required for fast pyrolysis
include the following: feed drying, particle size, pre-treatment, reactor configuration, heat supply, heat
transfer, heating rates, reaction temperature, vapour residence time, secondary cracking, char
separation, ash separation, and liquid collection. Each one of the aspects was reviewed and
discussed. Because of the complexity of the process, the effect of operating conditions on the bio-oil
properties is very process-specific.
For more detailed information about fast pyrolysis fundamentals, the reader should read
extensive recent reviews written by Bridgwater [13,18], Mohan et al. [16], Venderbosch et al. [30], and
Meier et al. [27,31].
1.2.2 Lignocellulosic feedstock
Virtually any form of biomass can be considered for fast pyrolysis. Biomass resources include
wood, agricultural residues and their waste by-products, animal wastes, municipal solid waste, waste
from food processing, aquatic plants and algae.
Wood and other plant biomass (rice husk, wheat straw, and olive bagasse) are basically a
composite material constructed from oxygen-containing organic polymers. Besides specific sugar and
vegetable oils-based biomass, the major part of these plant resources consists of lignocellulosic
material, mainly composed of [16]:
- Cellulose – is the main component of lignocellulosic material and the most abundant
biopolymer. It is a glucose polymer consisting of linear chains of β-(1,4)-D-glucopyranose (chains of
glucose) with a relative high molecular weight (106 or more. The basic repeating unit of the cellulose
polymer consists of two glucose anhydride units, called a cellobiose unit. Figure 1.5 shows the
chemical structure of cellulose. Aggregation of these linear chains with micro fibrils provides a strong
crystalline structure (cellulose fibers) that is inert to chemical reagents, comprising ~40-50 wt.% of dry
wood. Shafizadeh [23] investigated the pyrolysis of cellulose along temperature rise. Cellulose
appears to be stable up to ~310 ºC, after which almost all cellulose is converted to char, non-
condensable gases and condensable organic vapours at 320-420 ºC. Its degradation provides mostly
anhydrocellulose and levoglucosan.
10
- Hemicellulose – or polyose, is a mixture of various polysaccharides such as glucose,
mannose, galactose, xylose, arabinose, 4-0 methylglucuronic acid and galacturonic acid residues, with
a molecular weight lower than cellulose. Unlike cellulose, is amorphous in structure and usually
accounts for ~25-35% wt.% of dry wood. According to Soltes and Elder [32], hemicellulose
decomposes at temperatures of ~200-260 ºC, giving more volatiles, less tars and less chars than
cellulose. The onset of hemicellulose thermal decomposition occurs at lower temperatures than
cellulose. Much of acetic acid liberated from wood pyrolysis is attributed to deacetylation of the
hemicellulose.
- Lignin – is an amorphous cross-linked resin with no exact structure, highly branched on
substituted mononuclear aromatic polymers, and often bound to adjacent cellulose and hemicellulose
fibbers to form a lignocellulosic complex (Fig. 1.6). It is considered as the main binder for
agglomeration of fibrous components, being the third major component accounting for ~15-30 wt.% of
dry wood. Its decomposition occurs at ~280-500 ºC, producing a high yield of char [32]. Lignin is
responsible for the presence of phenol and other aromatic composts in bio-oil.
-
Figure 1.5 – Chemical structure of cellulose. Source: Ref. [16].
Figure 1.6 – Illustration of wood composition and lignocellulosic structure. Source: Ref. [33].
11
Inorganic minerals & organic extractives – a small mineral content ends up in the pyrolysis
ash, affecting the yield and composition of bio-oil. Organics extractives are present like fats,
waxes, resins, essential oils, etc.
In general, pyrolysis is the combined and complex degradation of biomass building blocks
discussed above. Figure 1.7 resumes the products from the pyrolysis of the main components of
lignocellulosic biomass.
The ash content is one of the most influential parameters in the pyrolysis process. Agricultural
residues generally have higher ash contents than woody-biomass. High ash contents in biomass are
not desirable because ash catalyses reactions that compete with biomass pyrolysis, leading to
increased formation of water and gas at the expense of liquid organics [34-37]. It also reduces the
temperature at which maximum organic liquids are yielded. Abdullah et al. [38] recommend a
maximum of 3 wt.% ashes in feedstock to avoid phase-separation of bio-oil. Nowakowski et al. [39]
refers potassium as the most problematic metal with a strong catalytic effect. Phosphorous also has
an undesired impact on the yield structure and product quality [40]. One possible way to overcome the
problem of high ash content is by pre-treatment of the feedstock – by water or acid washing [36,38].
This process prior to pyrolysis decreases ash content and results in better quality of bio-oil.
The relative portions of cellulose, hemicellulose and lignin in biomass feedstock also play an
important role on the quality of the bio-oil product. According to Oasmaa et al. [35], agricultural
residues generally contain less lignin and more hemicellulose and ash/alkali metals than wood
biomass, resulting in a higher O/C molar ratio than for woody biomass. Recent studies proved that
Figure 1.7 – Pyrolysis products from the main components of lignocellulosic biomass. Source: Ref. [94].
12
cellulose pyrolysis contributes mainly to bio-oil production (72 wt.%) by decomposing into sugars and
water, while hemicellulose-derived bio-oil (mostly acids) yields are much lower (42% wt.%) and
produces significant quantities of char (25 wt.%) and gases [41,42]. Yang et al. [43] confirms that most
of the cellulose is converted to bio-oil, while hemicellulose and lignin also yield substantial quantities of
tar, char and gas. Indirect evidence is given by the composition of the pyrolysis-derived-char, which
has an elemental composition close to that of lignin. Since agricultural residues contain higher
hemicellulose portions than wood, gas and char formation is more likely.
Nowakowski et al. [44] concluded that lignin-derived bio-oils have lower oxygen content and
therefore a higher energy density than conventional bio-oil. Since agricultural residues generally
contain less lignin, their resultant bio-oil has lower heating values than those from woody biomass
(with a comparatively higher lignin content).
1.2.3 Fast pyrolysis technologies
Fast pyrolysis is the least understood of the thermal degradation processes yet, but research
and technologic improvement have advanced significantly over the last years. Clear evidence is the
substantial number of laboratories and academic institutes around the world trying to commercialize
“fast wood pyrolysis” to a liquid [30,31,35].
Most research and development has focused on testing the different reactor configurations
with a variety of feedstock. In the last 20 years it has been done numerous researches on laboratory
scale reactors based on technological concepts of fast pyrolysis. High yields of bio-oil and operational
efficiency are the main goals. Advanced fast pyrolysis processes have been developed to convert
higher inputs of feedstock with high liquid yields. The reactor is considered from far the core and the
most distinguishing piece of equipment. Latest published reviews usually classify the reactors within
the following classification [10,13,15-18,27,31]:
§ Bubbling fluidized-bed reactor [45,46];
§ Circulating fluidized-bed reactor [47];
§ Rotating cone [48];
§ Ablative pyrolysis [49,50];
§ Entrained flow reactor [51];
§ Vacuum pyrolysis reactor [52];
§ Screw/auger reactor [53,54];
§ Fixed bed fast pyrolysis reactor [55];
§ Microwave pyrolysis and hydropyrolysis reactor [56].
Bridgwater [13,17,18] discussed all the reactor configurations investigated so far among
plentiful studies, explaining their main highlights, pro and con features, and detailed technical aspects
(e.g. heating, char removal and liquid collection methods). Meier and Faix [27] discussed the state-of-
the-art of reactors applied to fast pyrolysis of lignocellulosic materials and the emerging areas of
13
utilization. Isahak et al. [57] made a complete description of the fast pyrolysis technology focusing on
the characterization of feedstock, reactor design, products formation and upgrading. Meier et al. [58]
reported an overview of fast pyrolysis current activities in various countries both on research, pilot and
demonstration level. The commercial process must include necessarily the stages: feed reception,
storage, preparation and pre-treatment (drying and grinding), conversion of solid biomass by fast
pyrolysis into bio-oil, and conversion (upgrading) of this crude liquid product by processing, refining
and clean-up to a marketable end-product such as power, heat, bio-fuels and/or chemicals.
While all these reactors have been investigated on a laboratory scale, no single reactor has
emerged as being vastly superior to the other; however, certain reactors are more suitable for
commercial application than others. Scott et al. [59] and Brown et al. [60] concluded that only five main
technologies are suitable for commercialization, those which generate acceptable yields of bio-oil: a)
bubbling fluidized-bed reactors; b) circulating fluidized-bed reactors; c) vacuum reactors; d) ablative
reactors and e) screw reactors. The good results and the solid technology basis of these reactors
justify their market attractiveness. The fluidized-beds are by far the most used type of reactor due to
its well-understood technology, good temperature control and very efficient heat transfer crucial for
fast pyrolysis. Screw reactors have been known for their potential to lead with heterogeneous feed
materials, ease of handling and operation with no carrier gas required.
1.2.4 Related developed work
Hundreds of biomass feedstock have been screened [16,31,61]: bark, wood, agricultural
residues, nuts and seeds, algae, grasses, forestry residues, cellulose and lignin, miscellaneous, etc.
All of these types of biomass have been already tested and submitted to different conditions of
pyrolysis in different reactors [16]. Pinewood (woody biomass), olive bagasse, wheat straw and rice
husk (agro-biomasses) studied on the present work represent a significant portion of these
researches, however, most work has been carried out on wood because of its consistency and
comparability between testes [13].
Thangalazhy et al. [62] investigated the influence of temperature on the physic-chemical
properties of bio-oil obtained from pyrolysis in an auger reactor. Figure 1.8 shows the schematic
diagram of the reactor configuration. The experiments were carried out with 500 g of sample (dried
and ground) at four different temperatures (425, 450, 475 and 500 ºC). Nitrogen was used as purging
gas before each experiment to maintain an inert atmosphere. Figure 1.9 shows the yields of pyrolysis
products as a function of temperature. The maximum yield of bio-oil (50 wt.%) was obtained at 450 ºC.
The physical analysis of bio-oil for the selected temperatures has indicated a slight decrease in
density of bio-oil for the increase in pyrolysis temperature
(ρ@450 ºC = 1156 kg/m3). The water content (water wt.%@450 ºC = 21 %) and HHV
(HHV@450 ºC = 19.1 MJ/kg) remained constant with the increase in pyrolysis temperature.
14
A GC-MS analysis quantified thirty-two compounds from pinewood bio-oil and those were
grouped as carbohydrates, aromatics, furans, phenols, and cresols and guaiacols. Based on this
chemical analysis at different temperatures, some of the carbohydrate compounds and most of the
furan compounds had a higher concentration either at 450 ºC or at 475 ºC. The concentration of
phenol and its derivatives was decreased whereas the concentration of guaiacol and its derivatives
was decreased with the increase in pyrolysis temperature. It was concluded that a compromise in the
yield (5 wt.%, from 450 ºC to 475 ºC) could provide bio-oil with better composition. A temperature of
475 ºC would be the appropriate temperature for the production of bio-oil from pinewood.
Şensöz et al. [63] investigated the effect of temperature, heating rate, particle size and sweep
gas flow rate on the pyrolysis of olive bagasse. A maximum bio-oil yield (37.7 wt.%) was obtained at a
final temperature of 500 ºC with a heating rate of 10 ºC/min, a particle size of 0.224 mm and a sweep
gas flow rate of 150 cm3/min. For these conditions, bio-oil has shown a heating value of 31.8 MJ/kg
with an empirical formula of C1.65O0.25N0.03, a H/C molar ratio 1.65 and a O/C molar ratio 0.25. The H/C
ratio indicates that oil lies between light and heavy petroleum products. Further chemical
characterization through FTIR and GC-FID analysis showed that the bio-oil composition was
Figure 1.8 – Schematic diagram of the reactor configuration. Source: Ref. [62].
Figure 1.9 – Yields of pyrolysis products as a function of temperature. Source: Ref. [62].
15
dominated by oxygenated species, although may be potentially valuable as a fuel and chemical
feedstock.
Yanik et al. [64] studied the fast pyrolysis of wheat straw in a bench scale fluidized-bed reactor
at 500 ºC. Nitrogen was used as fluidizing gas. The pyrolysis liquid produced was in two separate
phases: oil and aqueous phase. For wheat straw, the yield of oil was 35 wt.% and the aqueous phase
was 6 wt.%. For characterization, the oil was fractionated by water extraction into two fractions: water-
soluble and water insoluble fractions. The water-soluble fraction and the aqueous phase of the
pyrolysis liquid were analysed by GC-MS and HPLC. The analysis showed that carboxylic acids,
mainly acetic acid, nonaromatic ketones, mainly acetone, methanol and phenols are the main organic
compounds. The moisture content of the oil was 4.68 wt.% and the elemental composition showed a
highly oxygenated oil. Based on the results, the pyrolysis liquid tends to be a source of speciality
chemicals and needs to be upgraded for use as a fuel source.
Heo et al. [65] assessed the fast pyrolysis of rice husk under different reaction conditions
(temperature, flow rate, feed rate and fluidizing medium) in a fluidized-bed reactor. Figure 1.10 shows
a schematic diagram of the pyrolysis apparatus. The reactor was made from SUS 306 stainless-steel
pipe with an internal diameter and height of 80 and 300 mm, respectively. Nitrogen was used to purge
out the reactor system and as fluidizing medium. In order to decrease the heat loss during the
experiments, nitrogen was preheated to 350 ºC before and then introduced to the reactor. Emery
(Al2O3 abrasives), 1000 g, with mean particle size of 40 was used as the bed material. The optimal
pyrolysis temperature were found to be between 400 ºC and 450 ºC. Higher gas flows and higher
biomass feed rates were more favourable for the production of bio-oil, but did not significantly affect
bio-oil yield. The use of the product gas as the fluidizing medium was most effective for bio-oil
production, leading to the highest bio-oil yield of approximately 60 wt.%. The major compounds found
in bio-oil by GC-MS analysis were phenolics, including phenol, cresols, guaiacols and benzendiols, as
well as acetic acid and ketones.
Figure 1.10 – Schematic diagram of fast pyrolysis apparatus. Source: Ref. [65].
16
The fast pyrolysis studies reported above helped to establish the starting point of the present
work focused on biomass residues. Other related works include those of DeSisto et al. [66],
Nokkosmaki et al. [67], Sipila et al. [68], Di Blasi et al. [69], Bakar et al. [70] and Ji-lu [71]. The
literature reveals a wide range of reactors and biomass feedstock used in several fast pyrolysis
studies. Thus, the conversion yields of the pyrolysis products and the respective properties vary
significantly in the existing literature.
1.2.5 Bio-oil properties
1.2.5.1 Chemical nature of bio-oil
Pyrolysis implies the rupture of carbon-carbon linkages and the later formation of carbon-
oxygen linkages. Chemically, it is considered an oxidation-reduction process in which a fraction of
biomass is reduced to carbon and the other is oxidised and hydrolysed. As expected, bio-oil from fast
pyrolysis is composed of a very complex mixture of oxygenated hydrocarbons with an appreciable
proportion of water. It approximates to biomass in elemental composition [72].
While water is the most abundant single component, other major groups of compounds are
found: hydroxyketones, hudroxyaldehydes, sugars, carboxylic acids (e.g. acetic and formic acids), and
phenolic compounds (mostly present as oligomers, derived mainly from lignin [16]). A more detailed
classification organizes compounds under the following categories: acids, alcohols, aldehydes, esters,
ketones, phenols, guaiacols, syringols, sugars, furans, alkenes, aromatics, nitrogen compounds, and
miscellaneous oxygenates [73]. In pyrolysis reaction: water is formed by dehydration; acetic acid come
from the elimination of acetyl groups originally linked to the xylose unit; furfural is formed by
dehydration of the xylose unit; and methanol arises from methoxyl groups of uronic acid [74].
Biomass pyrolysis liquids are either a homogeneous one-phase oil containing large amounts
of water or a heterogeneous fluid separated into an aqueous phase and an oily-phase. Piskorz et al.
[73] consider bio-oil as a micro-emulsion in which the continuous phase is an aqueous solution of
hollocellulose (cellulose and hemicellulose) decomposition products (pyroligneous acid, aqueous
phase) that stabilises a discontinuous phase of pyrolytic lignin macromolecules (“oligomeric lignin-
derived components” or tar). The agricultural residues contain extractive matter that yields a bigger
aqueous phase and the higher amount of ashes causes phase instability. Roy et al. [75] points out the
complex multiphase structure of biomass pyrolysis oils that “can be attributed to the presence of char
particles, waxy materials, aqueous droplets, droplets of different nature and micelles formed of heavy
compounds in a matrix of hollocellulose-derived compounds and water”.
The broad chemical characterization of bio-oils is difficult. Bio-oils contain higher-molecular-
weight species (e.g. complex phenolic species) that difficult the analysis. Only a portion of the bio-oil
can be detected via GC, even using robust columns and high-temperature programs. Furthermore, the
bio-oils contain polar, non-volatile components that are only accessible by HPLC or GPC analysis [76].
A complete analysis of bio-oil involves at least GC-MS (volatile compounds), HPLC and
17
HPLC/electrospray MS (non-volatile compounds), FTIR (functional groups), GPC (molecular weight
distributions), and NCR (types of hydrogen or carbon in specific structural groups, bonds, area
integrations).
1.2.5.2 Physical properties
Oxygen is present in most of the more than 300 compounds already identified, and its
substantial amount is the primary reason for the difference between fossil fuels and biomass pyrolysis
oil. Owing to the presence of large amounts of oxygenated compounds, bio-oil is immiscible with liquid
hydrocarbons, due to its high polarity and hydrophilic nature. The physical properties of bio-oil are well
described in literature (see, e.g., [72]). Table 1.3 compares the properties of a common wood derived
bio-oil with a conventional petroleum fuel. Table 1.4 resumes the main properties of bio-oils derivate
from various feedstocks in related studies.
Table 1.3 – Bio-oil and of heavy fuel oil typical properties. Source: Ref. [72]
Table 1.4 – Main properties of bio-oils derivate from various feedstocks. * on dry basis ** only oil-phase analysis
Physical propertiy Bio-oil Heavy fuel oil
moisture content, wt.% 15 ! 30 0.1pH 2.5 !specific gravity 1.2 0.94elemental composition, wt.% C 54 ! 58 85 H 5.5 ! 7.0 11 O 35 ! 40 1.0 N 0 ! 0.2 0.3 ash 0 ! 0.2 0.1HHV, MJ/kg 16 ! 19 40viscosity (at 50°C), cP 40 ! 100 180solids, wt.% 0.2 ! 1 1distillation residue, wt.% up to 50 1
Oil
Density (kg/dm3) Moist. (wt.%) C H O HHV (MJ/kg) * Phases1.142 20.3 - - - 23.2 1 [62]1.266 11.1 56.4 6.3 37.2 18.4 1 [66]
Olive bagasse ** 1.07 None 66.9 9.2 21.9 31.8 2 [63]Straw 1.186 19.9 55.3 6.6 37.7 19.6 1 [68]
- 25.2 55.1 7.2 37 24.8 2 [65]1.155 25.2 41.7 7.7 50.3 25.5 1 [71]
Pinewood
Rice husk
Feedstock Ref.
18
Water content appears inherently in bio-oil from both the original moisture and as reaction
product (dehydration reactions), ranging between 15 wt.% and 30 wt.%. Unless biomass is dried
below about 10 wt.% before pyrolysis, water content can range as high as 30-45 wt.% that can cause
spontaneous phase separation of bio-oil. Water in bio-oil cannot be removed by conventional
methods, since the heated oil tends do polymerize. The presence of water has contradictory effects on
oil properties: lowers oil-heating value, especially the LHV, but on the other hand reduces viscosity
and improve stability. Therefore, “careful control of moisture content of the bio-oil prior to pyrolysis is
important for assuring high-quality bio-oil” [10].
The density of the liquid is significantly higher than of the fuel oil, which can bring implications
for the design and specification of equipment (e.g. pumps and atomisers in boilers and engines).
Viscosity of the bio-oil can varies from as low as 35 cP to as high as 1000 cP (measured at 40 ºC) or
more, depending on the feedstock, process conditions, and the amount of light components in the oil.
The HHV of bio-oils is only 40-45 wt. % of that exhibited by hydrocarbon fuels, due to the superior
oxygen and water content. A typical higher heating value is ~17 MJ/kg [16]. Besides, bio-oil shows a
wide range of boiling temperature due to its complex composition. In addition, substantial amounts of
non-volatile components (e.g. sugars and oligomeric phenolics) difficult volatilization [72]. A pH of 2 –
3 is usual accounting for bio-oils substantial amount organic acids, mostly acetic and formic acids.
Higher water contents and increased temperatures reinforce this corrosiveness property.
Due to fast pyrolysis requirements (fast degradation followed by and “aggressive”
condensation) the oil contains many trapped reactive species that would further react if the residence
time was extended. For this reason, bio-oils contain many reactive species that contribute to their
unusual attributes and instability [13,16]. An important consequence is bio-oil “aging” after it is first
recovered: oil viscosity tends to increase and phase separation may also occur. This instability results
from a breakdown in the stabilized micro-emulsion and from slow chemical reactions that continue to
proceed in the oil. The detailed mechanism of this “aging”, the causes, and the consequences for
further use, are still unclear and will depend highly on the various oxygen functionalities in the oil (and,
thus, feedstock, initial quality, operating conditions, storage temperatures, etc.) [10].
Oasmma et al. [77] discussed various methods necessary for proper characterization of bio-oil
and its most common physical and chemical properties. Water content, poor volatility, high viscosity,
coking, corrosiveness, high solids content, incompatibility with conventional fuels and chemical
instability are probably the most challenging properties and have so far limited the range of bio-oil
applications. For this reason, and for specific applications, bio-oil from fast pyrolysis cannot be directly
used unless it is upgraded. Upgrading techniques control and improve oil quality through physical,
chemical or catalytic processes. The associated technologies are in the early stages of demonstration,
and it is likely to be some time before they are deployed on a commercial scale. Complete reviews
about bio-oil upgrade are found in literature (see [78,79]).
19
1.2.6 Bio-oil applications
Fast pyrolysis can be seen as pre-treatment method to convert decentralized voluminous
biomass into an added-value liquid, easier to handle, transport and store. It is a “densification” process
that presents both technical and economic advantages [80]. This leads to the concept of small-
decentralised fast pyrolysis plants for production of bio-oil to be transported to centralised upgrading
facilities [15]. These might include existing crude oil refineries or dedicated biorefineries that will be
explained more below.
After produced, pyrolysis bio-oil has pertinent applications. Czernik et al. [72] describes
extensively these applications and points out several related works being carried out. Figure 1.11
summarizes applications for the products of fast pyrolysis.
Bio-oil has been successfully used as a direct boiler/furnace fuel [81], and several works have
been carried out with positive results (see [82,83]). Furthermore, bio-oil also has shown promising
performance in diesel engines and gas turbine for heat and power generation. Bio-oil has been
successfully fired in a diesel test engine, where it behaves very similar to diesel in terms of engine
parameters and emissions [84,85]. Slight modifications of both the bio-oil and the diesel engine can
render bio-oils an acceptable direct substitute for diesel fuel in stationary engines [16]. Experience
with bio-oil combustion in gas turbines has also been reported [86,87]. Besides the main concerns for
operating due to undesirable properties (section 1.2.4), the potential advantages [86] of using bio-oil
as a direct applicable fuel led to important research activities. Oasmma et al. [88] discussed common
problems related to using bio-oils as a fuel in boilers, engines and turbines.
Figure 1.11 – Applications for the products of fast pyrolysis. Source: Ref. [13].
20
Co-firing is another feasible application for bio-oil, and large-scale tests where pyrolysis liquid
co-fired with coal for the commercial production of electricity already have been carried out [89].
It is widely accepted that the quality of bio-oil cannot be considered a realistic candidate for
large-scale liquid transport fuel production unless it is upgraded [15]. Through upgrading technologies,
bio-oil from fast pyrolysis can yield high quality biofuels. The most prominent upgrading routes include
[90]: 1) hydrodeoxygenation with typical hydrotreating catalysts (typically CoMo or NiMo supported on
alumina), 2) zeolite upgrading, or 3) blending with other fuels. Nguyen and Honnery [91] found that
fast pyrolysis bio-oil can be mixed up to 20 wt.% with ethanol and combusted at elevated pressures
(2.5 MPa at 827 ºC) without any significant drop in performance. Jiang and Ellis [92] studied the
emulsification of bio-oil with bio-diesel, which has shown promising results.
Besides the potential and advantages of biofuel production from fast pyrolysis bio-oil, several
difficulties still complicate the process. The delocalised distribution of biomass and its poor energy
density, reinforced by the large scale of production on which biomass-to-liquid will need to be
produced to produce an economically viable fuel, are examples of serious problems associated with
the use of biomass as a liquid transport fuel source [93]. One achievable solution goes through the
concept already referred: decentralised densification of biomass to bio-oil through fast pyrolysis
followed by centralised upgrading in biorefineries. Analogous to those in petroleum industry, a
biorefinery is a refinery particularly devoted to biomass and related processes, where the key feature
is the production of fuels, chemicals and energy from biomass. Figure 1.12 shows fast pyrolysis
process at the heart of a biorefinery. The resultant liquid, or bio-oil, is seen as the main feedstock
among the other by-products.
Apart from energetic purposes, bio-oil has the potential to supply a number of valuable
chemicals that offer a higher attraction as added value product than fuels. These chemicals come from
Figure 1.12 – Fast pyrolysis-based biorefinery. Source: Ref. [13].
21
the several compounds identified in bio-oil (section 1.2.4.1). Literature reports emerging areas of
utilization [16][94]: composts for food aromas and additives (syringol), solvents, adhesives, cosmetics,
plastics, varnishes, fuel enhancers, fertilizers, specific chemicals, etc. Effective
separation/fractionation methods have been developed to yield targeted chemicals, see [95,96,97].
The recognition of the strategic and economic potential of bioneferies is recent [13]. The fast
pyrolysis based biorefinery concept has gained attention since it is a optimised process that uses bio-
oil as a versatile feedstock and exploit its practical applications: chemicals and liquid transport fuels
production or power/heat generation.
Different feedstock, reactor configurations, and recovery systems results in different properties
of bio-oil, which turn large-scale applications a difficult task, however, more recent studies shown
substantial improvements in its consistency and stability, demonstrating the improvement in process
design and control as the technology develops. Minimum quality standards are being attained
enabling commercial applications. According to Meier et al. [58], the commercial implementation of
fast pyrolysis for bio-oil production is on the brink of the mark deployment, while other authors reports
that fast pyrolysis is already commercially successful for production (medium-scale) of chemicals and
is being actively developed for producing liquid fuel [98].
1.3 Present Contribution
The pyrolysis of pinewood, olive bagasse, wheat straw and rice husk was carried in a bench-
scale screw pyrolysis reactor heated with an electrical resistance. In order to maximize the bio-oil
yield, the pyrolysis tests covered specific conditions close to fast pyrolysis. The yields of the products
were determined. The physical foremost properties of the bio-oils and chars (moisture content,
elemental analysis, HHV) were evaluated.
1.4 Thesis Outline
This thesis is organized in four chapters, of which the present chapter constitutes the
introduction that includes a literature review and clarifies the main objectives of the work.
Chapter 2 describes the experimental facility, the measurement techniques and related
uncertainties, and the experimental procedure. It also includes all the details about the analysis and
characterization of the products of the pyrolysis.
Chapter 3 presents and discusses the results obtained during the present study.
Finally, chapter 4 summarizes the main conclusions of this work and provides
recommendations for future studies.
22
2. Facilities, Techniques and Procedures
2.1 Feedstock
The biomasses pyrolysed were pinewood, olive bagasse, wheat straw and rice husk.
Pinewood and straw was supplied in pellet form, while olive bagasse and rice husk came in pulverized
form. Before each run on the pyrolysis reactor, samples of each feedstock were properly prepared. In
order to enhance heat transfer and transport along the reactor, the biomass was properly milled into a
sample of smaller particles. Pinewood, straw and rice husk were milled in a stainless steel blender
(BECKEN, 500 W). Olive bagasse sample was not milled since it presented a fine granulometry by
itself. The samples obtained had similar particles sizes (< 2 mm). Figure 2.1 shows the different
biomass samples.
After milling, the biomass samples were completely dried in order to lose their initial moisture
(8 wt.% up to 12 wt.%) and, consequently, to reduce as much as possible the moisture in the resultant
bio-oil [20,72]. An oven (Memmert, Model 100 – 800) was used for this purpose. The samples were
dried at least for 24 hours at 110 ºC as in [65]. In these processes the samples were weight in a
Sartorius balance (model CP6201, ± 0.05 g).
a)
c)
b)
d)
Figure 2.1 – The different biomass samples: a) Pinewood, b) Olive bagasse, c) Wheat straw and d) Rice husk.
23
2.2 Experimental facilities
A bench-scale screw reactor was specifically designed for the present work to perform the
pyrolysis tests. The screw reactor runs discontinuously (batch operation) with biomass samples and
attains a temperature of 700 ºC. Figure 2.2 shows an overall scheme of the facility. This facility
configuration was based on previous works such as [53,62].
The main body of the reactor is a horizontal pipe 325 mm length with an inner diameter of 20
mm and 1 mm thick, made of AISI 316 steel (bold lines in Fig. 2.2). Inside the main pipe, an
assembled concentric screw goes along all pipe length. At left, an electric engine placed outside is
axially engaged to the screw imposing a rotation movement on it. The rotation of the screw carries the
biomass sample from the biomass inlet towards the heating/reaction zone till the products’ collection
point on the other end of the pipe. An electrical resistance coiled over 150 mm of the main pipe
provides heat for the reaction and defines the reaction zone. The temperature is monitored using
thermocouples located along the pipe. The reactor temperature control is done through a devised
LabVIEW interface and an auxiliary relay system, as shown in Figure 2.2. Nitrogen is used as inert
Figure 2.2 – Schematic of the pyrolysis reactor: 1 – Biomass feed; 2 – Electric engine; 3 – Screw and main pipe; 4 – Electrical resistance;
5 – Data acquisition & Control system; 6 – Nitrogen vessel; 7 – Nitrogen circuit; 8 - Char collector flask; 9 – Condensers; 10 – Refrigerator circuit.
24
and carrier gas. This is heated before its entry on the reactor by an integrated heat exchanger where
the heating resistance works as heat source.
After reaction, char is quickly collected in an enclosed flask and bio-oil is collected in flasks by
rapid condensation of the hot vapours in two condensation stages. The non-condensable gases
proceed their way to an exhaust system. Figure 2.3 shows a real image of the facility. The chimney of
the exhaust gases is not shown in the figure.
Two bases support the main pipe with clamps. Another base is used to sustain the engine with
a fixation. A more detailed explanation of each component of the facility is given below, as well as of
all the auxiliary equipment.
Biomass feed:
Figure 2.4 shows the biomass-feeding system placed at setup’s left side. It consists in a steel
tubular column with 150 mm height and 20 mm of inner diameter fixed at its bottom to a threaded steel
T-connection. The T-connection allows the “tee” assembly between the sample inlet, the main pipe
and the screw’s gearing. At the top of the column, a proper threated glass flask containing a biomass
sample is screwed upside down in a suitable fixed lid to guarantee sealing. Once the glass flask is
coupled, the biomass sample particles fall down by gravity along the column into the rotating screw
Figure 2.3 – Facility of the screw reactor.
Exhaust
Condensers
N2 flow meter
Bio-oil flasks
Feeding system
Engine
Char flask
Main body
25
placed below. Near the base, a small vibrator induces vibration in the column to avoid biomass
blockage due to its low “fluidity”. Nitrogen is directly introduced in the reactor on the feeding column.
Rotational engine:
In order to input rotation in the screw, a rotational electric engine was engaged axially to the
screw through a specifically designed gear. The gear allows movement transmission from the outer
engine to the inner screw maintaining sealing. Figure 2.4 above shows the designed gear and the
electric engine. The electric engine is a CTX peristaltic bomb adapted for the purpose with variable
rotation velocity. The engine is fitted in a support.
Screw:
Figure 2.5 shows the screw of the reactor made of stainless steel. The screw is 450 mm long
with 18 mm in diameter and a sharp nose at its tip. It is concentrically assembled on the main pipe
going along all its length. The rotation of the screw ensures the horizontal transport of biomass
particles from the feeding point towards the reaction zone where fast pyrolysis reaction occurs. It was
kept a gap of 1 mm between the screw and the inner diameter of the pipe to avoid situations of
biomass blockage or of thermal dilatation of steel that could lead to engine stoppage.
Figure 2.4 – Detail view of the feeding system.
Engine Transmission
gear
Vibrator
Lid
Biomass sample flask
N2 flow meter
N2 inlet
T connection
26
Heating resistance:
Fast pyrolysis is an endothermic reaction [16], and heat needs to be supplied to the
predefined reaction zone of the pipe. To ensure the suitable heating of this particular zone, an
appropriate electrical resistance designed for the work was attached to the pipe outer wall. Figure 2.6
shows this electrical resistance coiled over the tube with 150 mm wide. The pyrolysis reaction occurs
in this zone of the pipe.
The resistance is responsible for the heating. Its main characteristics are:
• Maximum acceptable temperature ~ 700 °C;
• Coil/helical shape to embrace firmly the main pipe and consequently provide a
uniform heating over the specified zone;
• Made of chromium-nickel alloy to resist thermal fatigue;
• 35 Ω (when plugged – 230 V – it reaches 1.1 kW of power);
Figure 2.5 – Screw of the reactor.
Figure 2.6 – Heating resistance of the reactor.
27
The electrical resistance is insulated to avoid heat losses. It is insulated with a glass wool
layer compacted with a stainless steel sheet. To reinforce insulation a rock wool layer covered in
aluminium foil is also used (see Fig. 2.8 below). This whole insulation is seen in Fig. 2.2 (the thicker
insulation in the main pipe).
It was observed in preliminary tests that the electrical resistance ensured an efficient and rapid
heating of biomass (fundamental feature of fast pyrolysis [13,20]). The heating resistance is also
responsible for heating nitrogen before its entrance into the reactor. A more detailed explanation is
given below in the nitrogen circuit topic.
Temperature control system - TCS:
A temperature control is needed to impose a set temperature in the reaction zone since
temperature is a key variable to be assessed. In order to fix and control temperature, a temperature
control system based on LabVIEW (VI. interface) was developed from scratch in the scope of the
present work. This system consists on a data acquisition board NI-9211 (National Instruments), a
Virtual Instrument (VI.) code/interface program for temperatures monitoring and control, and an
actuator system based on a multifunction board NI USB-6008 (National Instruments) settled in an
auxiliary relay circuit. The TCS is a feedback control system that establishes a fixed/mean
temperature (reactor temperature) on the reaction zone with the heating resistance. A desirable
temperature can be pre-defined in the VI. interface. Figure 2.7 illustrates the TCS diagram.
For each LabVIEW iteration, the acquisition board records the temperatures from four type-K
thermocouples placed along the pipe's outer wall in four relevant points. Figure 2.8 shows the location
of the thermocouples, their nomination, and a scheme of the overall control system as well. The
thermocouple placed in the reaction zone (between two coils of the resistance) monitors and helps to
control the temperature on the wall of this particular zone. Due to its importance and location, this
temperature is defined as the reactor temperature. Two other thermocouples monitor the temperature
Figure 2.7 – Temperature Control System diagram.
28
at the inlet and outlet of the reaction zone. Another one is placed along the pipe before the reaction.
Steel braces attach the thermocouples to the respective measuring points on the pipe outer wall.
The acquired temperatures are monitored on the computer through LabVIEW (VI.). Besides
monitoring temperatures, the VI. interface controls the reactor temperature: based on the immediate
measured reactor temperature and the desired pre-selected reactor temperature, the VI. turns
ON/OFF the heating resistance using an auxiliary relay circuit for the effect. The intermittent heating
establishes an average reactor temperature that is close to the pre-selected temperature defined on
the VI. interface. The VI. interface controls the reactor temperature throughout the experiments with
the help of the auxiliary relay circuit (actuator) and the temperatures acquisition board. In Appendix B
is present a more detailed explanation about the VI. interface operation and the overall control system.
Besides the insulation of the reaction zone, the remaining part of the main pipe is also
insulated with a fine rock wool layer for safety reasons. This rock wool layer is also covered in
aluminium foil (see Fig. 2.2).
Figure 2.8 – Illustration of the temperature control system.
29
Nitrogen circuit:
Nitrogen is used to maintain an inert atmosphere for the pyrolysis reaction. At the same time it
is used as a carrier gas for the volatile products (hot condensable vapours) as in [53]. For these
reasons, an appropriate sealing is required for the overall setup.
The nitrogen circuit is illustrated in Fig. 2.2. Nitrogen is provided from a pressurized vessel (Air
Liquid TM) and presents a purity level of 99.9999% (H2O < 3 ppm, O2 < 2 ppm, CnHm < 0.5 ppm). A
pressure reducer valve regulates the pressure. The pressure is kept slightly above the atmospheric
pressure, only the minimum needed to maintain the nitrogen flow. Nitrogen is directly introduced in the
feeding column through a small stainless steel tube (blue colour line in Fig. 2.4). The inner diameter is
1 mm, while the outer diameter is 3 mm. The tube is introduced in a sealed orifice of the column and
directed upwards to the biomass flask. The outlet of the tube is pointed directly at the biomass sample
for the nitrogen flow prevent blocking of biomass in the flask.
The nitrogen flow rate is controlled upstream with a gas flow meter placed before its inlet into
the reactor (see Fig. 2.3). The gas flow meter is an AALBORG flow meter (model P single flow tube, ±
2 %) and operates in the range of 0 - 2 L/min. A clamp fixes the gas flow meter to a support (see Fig.
2.4)
Heo et al. [65], DeSisto et al. [68] and Ji-lu et al. [71] pre-heated nitrogen up to 350 ºC before
its inlet into the reactor in order to decrease heat losses in the pyrolysis reaction. A possible cooling
could represent a substantial impact in the reaction. For the same reason nitrogen is pre-heated
before its entrance in the reactor of the present work. After the gas flow meter, nitrogen flows through
an integrated heat exchanger where the heating resistance is used as heat source (see Fig. 2.2). The
heat exchanger consists in a portion of the nitrogen circuit made of a thin copper tube kept in direct
contact with the resistance (curled around its coils). Figure 2.9 shows this integrated heat exchanger
of nitrogen. The high temperature attained by the resistance while working heats up automatically the
nitrogen flow. The heated nitrogen follows to the nitrogen inlet referred in Fig. 2.3. The copper tube is
mutually insulated with the heating resistance (insulation already referred).
The temperature of the nitrogen before its entrance into the reactor depends on the
temperature of the resistance (reactor temperature) and on the nitrogen flow rate imposed in the
nitrogen circuit by the gas flow meter upstream.
30
Reaction zone:
The biomass particles are continuously driven along the pipe. Figure 2.10 illustrates a detail
view of the reaction zone, where the heating resistance is located as heat source. The TCS fixes a set
temperature on the wall of this zone. In contact with the wall, the biomass particles heat up and
undergo pyrolysis reaction – thermal decomposition in an inert nitrogen atmosphere – forming solid
particles of char that are collected afterwards, and a product gas stream. The product gas stream is
carried along with the nitrogen flow.
Figure 2.9 – Integrated heat exchanger of nitrogen.
Figure 2.10 – Illustrative detail view of the reaction zone.
31
A T-connection placed in the other tip of the tube next to the reaction zone allows a
reasonable separation of these two products in order to avoid secondary reactions, as refereed by
Bridgwater [13]. Char particles fall down and are collected in an enclosed glass flask; the product gas
stream proceeds rapidly upwards to the condensation stages through an elbow pipe connection (see
Fig. 2.2). The elbow pipe connection is isolated (not shown in Fig. 2.2) with a rock wool layer to
prevent cooling of the product gas stream and consequent pre-condensation before the respective
condensation step. Immediately after the elbow, a Teflon tube connects the outlet of the elbow
connection to the first condenser inlet.
The nose of the screw is placed on an intern cavity of the T-connection. The cavity is
concentric with the main pipe and allows for concentric alignment of the screw with the pipe (see Fig.
2.9).
Condensation stages:
The product gas stream proceeds to the condensation stages. Each condensation stage is a
glass coil condenser. In order to condensate efficiently the bio-oil from the hot condensable vapours
embedded in the product gas stream, two condensers/stages are used. Both condensers are cooled in
series with the help of a refrigerated circulator. The refrigerated circulator works at a variable set
temperature using ethylene glycol as refrigerant fluid. Bio-oil condensates and is collected into glass
flasks coupled to the condensers. The non-condensable gases continue their way until be expelled by
an exhaust system (see Fig. 2.2).
The refrigerated circulator is a model Haake C10-K15. The circulator had a maximum flow rate
capacity of 12.5 L/min and a cooling capacity of 200 W at 0 ºC. The equipment’s temperature
accuracy was of 0.04 ºC.
2.3 Measuring Techniques and Uncertainties
All measurements present uncertainties that must be quantified. The following sub-chapters
discuss the measuring techniques and uncertainties associated to the present experimental work.
2.3.1 Temperatures
The temperatures are acquired from four type-K thermocouples (Cr+, Al-) from Omega placed
along the main pipe of the reactor. Their signal is processed by the NI-9211 Data Logger into
temperature (º C) that is presented in the graphic interface of the program (VI) with a given associate
error. In order to estimate this error, the acquired temperature values were compared with a reference
temperature acquired by a high precision auxiliary thermocouple (Omega KMQSS-IM025U-300). Their
relative error was found to be ± 3 ºC (by increase).
32
Since the tube is made of stainless steel and has 1 mm thick, simple preliminary tests
concluded that any outer wall temperature of the pipe measured by the various thermocouples could
be assumed equal to the inner wall temperature, with no significant loss in accuracy.
The reactor temperature is by far the most important parameter of the facility. It is the
temperature controlled/measured in a strategic point of the outer wall of the reaction zone (see Fig.
2.8). The TCS is responsible to control this temperature. Preliminary tests concluded that the
temperature measured at this point is approximately kept constant throughout the wall of the reaction
zone, where the resistance is embraced. Therefore, the reactor temperature must be seen as the
temperature of the entire wall of the reaction zone. Once TCS controls the reactor temperature, it is
actually controlling the temperature of the entire wall of this particular zone that is kept uniform along
its length. The preliminary test results are present in Appendix C.
Temperature plays an important role in pyrolysis [20] and, therefore, the reactor temperature
has a crucial influence on the pyrolysis reaction. The biomass particles react when in contact with the
hot inner wall of the tube in the reaction zone, however, it is necessary to distinguish between
temperature of reaction and reactor temperature. The latter is much higher due to the inherent
temperature gradient associated to the heat transfer between the hot wall (reactor temperature) and
the biomass particles that pass through the reaction zone. The temperature attained by the particles
(temperature of reaction) is always lower and depends strongly on their position. The low thermal
conductivity of biomass reinforces this fact.
The reactor temperature controls the overall temperature distribution and is seen as the
characteristic temperature of the process. Any reference to temperature relates to the temperature of
the reactor, unless is refereed otherwise.
It is technically impossible to monitor the instantaneous temperature of reaction in the screw
reactor and its analytical estimation is a complex task, which must include the effect of the nitrogen
flow, the presence of the screw, movement of the particles, etc. For these reasons, the reaction
temperature was assumed approximately equal to the “Outlet temperature” of the reaction zone (see
Fig. 2.8), which translates the closest temperature attained by the product gas stream formed in the
reaction. In each experimental run, the reaction temperature is evaluated by the resultant “Outlet
temperature” attained.
2.3.2 Biomass feed rate
The biomass feed rate is an important parameter of pyrolysis. It influences the yields and
composition of the products (section 1.2.1). For a screw pyrolysis reactor the biomass feed rate is
directly proportional to the screw velocity. Trial tests have shown a slow rotation velocity as the most
indicated in order to ensure a more reliable thermal degradation (pyrolysis) of the biomass sample.
Figure 2.11 shows a microscopic view of two samples of char of pinewood pyrolysed under different
rotation velocities of the screw.
33
For higher rotation velocities the biomass particles are only partially pyrolysed as a
consequence of the minor contact time of the particles with the hot wall (see Fig. 2.10 a)). As the
rotation velocity of the screw is decreased, the contact time of the particles with the hot wall is
increased and the char sample shows clear evidences of a more successful thermal degradation
throughout the biomass particles (see Fig 2.10 b)). The minor residence time of the particles in the
reaction zone while the velocity is increased results in a partial pyrolysis of the particles and the
thermal degradation does not occur throughout the full particles.
In order to ensure as possible a reliable degradation of the biomass particles the screw
velocity was kept constant and low as possible for the present work: 19 rpm; consequently, the feed
rate is constant for each biomass type.
The biomass feed rate was determined. The technique carried out to calculate the biomass
flow rate and the results are present in Appendix D. Table 2.1 summarizes the feed rates of the
biomasses.
The screw velocity, imposed by the electric engine, is assumed as constant; however, it can
suffer some punctual variation due to biomass blockage between the screw and the inner wall of the
pipe along the experiments. It is a rare occurrence but still induces an error on the feed rate. For this
reason, it was kept a gap of 1 mm between the screw and the inner diameter of the pipe to avoid
blockage of particles that can lead to engine stoppage. The experiment runs were always carefully
followed to avoid this occurrence.
Pinewood Olive bagasse Wheat straw Rice husk
Biomass feed rate (g/min) 7.6 11 7.3 4.9
Table 2.1 – Feed rates of the biomasses (19 rpm).
a) b)
Figure 2.11 – Samples of char of pinewood pyrolysed under a screw velocity of a) 57 rpm and b) 19 rpm (500 ºC, N2 flow rate of 526 mL/min).
34
2.3.3 Nitrogen flow rate
The nitrogen flow rate in the nitrogen circuit is controlled upstream with a gas flow meter at
NTP conditions (20 ºC, 1 atm). Any reference to the nitrogen (carrier gas) flow rate should be related
to the flow in the gas flow meter at NTP conditions, unless is refereed otherwise.
The temperature of nitrogen after its heating with the electrical resistance, and before its
entrance into the reactor, is directly proportional to the temperature of the resistance (reactor
temperature) and to the nitrogen flow rate imposed upstream of the circuit with the gas flow meter. For
this reason, the nitrogen temperature at its inlet into the reactor was recorded for each experimental
run with an Omega KMQSS-IM025U-300 thermocouple. A multimeter True RMS Supermeter
(NEWPORT, ± 2 ºC) was used to process the thermocouple signal.
The hot vapours residence time is an important parameter of fast pyrolysis (section 1.2.1).
Nitrogen is used as carrier gas for these hot vapours of pyrolysis in the reactor of the present work. Its
flow rate affects directly their residence time in the reaction zone; thus, one can assume that their
residence time is somehow proportional to the nitrogen flow rate imposed. In order to obtain an
approximate residence time of the hot vapours, this was estimated based upon various nitrogen flow
rates and rough assumptions. The estimation of the hot vapours residence time is present in Appendix
E.
2.3.4 Reproducibility
All pyrolysis runs were performed at least in duplicate in order to obtain a more trustworthy
result. If the bio-oil, char and gas fractions were in agreement, one sample was chosen for detailed
chemical analysis. Table 2.2 shows typical yield values for the pyrolysis of pinewood performed at 580
ºC with a nitrogen flow rate of 526 mL/min. It shows the mean values and the standard deviation of
the various fractions. A maximum difference of ± 4 % in the yields was defined as the criteria to ensure
a reasonable reproducibility. Otherwise, the experimental run was repeated.
The blocking of biomass along the feeding column, and the locking of the screw due to
biomass particles entrained in the pipe/screw gap reduced in some tests the repeatability of
measurements, and in special cases sabotaged the whole test. Appendix A describes the formulas to
calculate mean values and standard deviations.
Yield (wt.%) First run Second run Mean value ± σ
Gas 25.9 23.2 24.5 ± 1.9
Bio-oil 51.0 51.7 51.3 ± 0.5
Char 23.1 25.3 24.2 ± 1.5
Table 2.2 – Typical yield values for the pyrolysis of pinewood performed at 580 ºC with a nitrogen flow rate of 526 mL/min.
35
2.4 Experimental Procedure
For each experimental run the experimental conditions such as weight of the initial biomass
sample, reactor temperature, nitrogen flow rate, velocity of the screw are defined at the start.
A typical run initiates with the preparation of the biomass sample to be pyrolysed. Biomass is
firstly milled and completely dried. An initial sample is weighed with the pre-defined weight and placed
in a proper enclosed glass flask apart from the reactor. The reactor temperature is set on the
computer program (VI. interface) and the reactor eventually heats up. The temperature from the four
thermocouples placed along the main pipe (including the reactor temperature) is monitored on the
computer. When the desirable reactor temperature is achieved, it is maintained constant till the end of
the experimental run through the Temperature Control System. The refrigerated circulator is turned
ON and the refrigerant (ethylene glycol) is pumped along both condensers connected in series. All the
equipment works continuously until all the monitored temperatures achieve steady-state conditions. At
this point the flask containing the initial biomass sample is screwed upside down in the fixed lid placed
at the top of the feeding column. Once the glass flask is coupled, the sample particles fall down by
gravity into the screw placed below, at witch point the nitrogen supply is opened and a slight flow is
kept for a few minutes in order to fully inertize the reactor. The correct pre-defined nitrogen flow rate is
then settled with the upstream gas flow meter and the rotational engine is turned ON. As the screw
rotates, the sample deposited on the feeding column is gradually carried on towards the reaction zone
where it undergoes pyrolysis. The experimental run is considered finished when no sample is seen on
the sample flask and no more visible gaseous product flows up to the exhaust exit. At this point the
heating (computer interface) and the screw engine are turned OFF. The nitrogen supply is completely
closed. The condensers are dissembled from the facility and the residual ethylene glycol is properly
cleaned. Each unit is weighed: char flask, both condensers containing residual bio-oil, bio-oil flasks,
the feeding column and the tubular set (main pipe plus elbow). The bio-oil (first flask) and the char
(char’s flask) obtained are collected for subsequent analysis and characterization. After the weighting
and collection all of these units are properly cleaned up with acetone. The pipes are also cleaned with
the help of a scraper to remove the char agglomerations and ashes deposits. The units are then
weighed and re-assembled before reutilization.
Figure 2.12 shows a typical temperature profile along an experimental run. It shows the
temperatures development for an experimental run of pinewood (50 g) with a reactor temperature, a
nitrogen flow rate of and a velocity of the screw of 580 ºC, 526 mL/min and 19 rpm, respectively. The
reactor temperature increases quickly till attain the set temperature and remains relatively constant
over time. As result the others temperatures increase progressively along time (“Transient”) till attain a
steady-state condition and remain constant. The typical timeout for steady-state condition is 40-45
min.
36
Biomass is pyrolysed once the steady state is attained (“Pyrolysis”). The reactor temperature
is held constant while the other temperatures vary as consequence of the biomass/products transport.
For this particular experimental run the reaction temperature was assumed as ≈ 500 ºC since that was
the temperature attained by the “Outlet temperature”.
2.5 Analysis and Characterization of Products
2.5.1 Yields of the products
In order to determine the yields of the products, the amounts of char and bio-oil obtained were
quantified by weighting the corresponding unit before and after each experiment. The weighting was
carried in a Sartorius balance (model CP6201, ± 0.05 g). The exhausted gases mass was calculated
by difference.
The mass of bio-oil is accounted for the bio-oil collected in the respective collection flasks plus
the residual bio-oil trapped in both condensers. Due to its high viscosity, a portion of bio-oil remains
always in the condensers, and therefore, this residual portion of bio-oil is also accounted. The weight
of the flasks and both condensers before and after each experiment dictates the bio-oil mass.
The mass of char obtained is accounted by the char collected in the respective collection flask
placed plus the residual char trapped in the tubular set (main pipe and elbow). After each experiment
the tubular set is cleaned and the residual char placed inside is weighed. The total weight of the flask
and the tubular set before and after each experiment dictates the char mass.
0
100
200
300
400
500
600
10 20 30 40 50 60
Logg
ed T
empe
ratu
res,
°C!!
Time, min
Reactor Temperature
Inlet temperature
Outlet Temperature
Duct temperature
Transient
Experimental run: - 50 g of Pine - Reactor temp. : 580 °C - Rotation: 19 rpm - N2 flow rate: 526 mL/min Pyrolysis
Figure 2.12 – Typical temperatures profile along an experimental run.
37
Between the initial sample and the whole pyrolysed portion, is necessary to account possible
biomass losses retained along the system that did not react. The biomass losses are accounted by the
initial sample retained in the biomass-feeding column and feeding flask. For this reason the feeding
column and the sample flask are weighed before and after each experiment.
The correct pyrolysed portion was obtained by the following equation:
m pyrolyzed (kg) = m initial - m losses (2.1)
The yield of the products is obtained with the masses, by the following equations:
Bio-oil yield wt. % (kg/kg) = m bio-oilm pyrolyzed
× 100 (2.2)
Char yield wt. % (kg/kg) = m charm pyrolyzed
× 100 (2.3)
Gases yield wt. % (kg/kg) = 100 – Bio-oil yield wt. % + Char yield wt. % (2.4)
2.5.2 Bio-oil analysis
The bio-oils obtained were analysed so as to determine their density, elemental composition,
moisture content, HHV and LHV. In order to determine density of bio-oils, a sample of 1 mL of each
bio-oil was collected into a calibrated flask using a micropipette (± 0.025 mL). The sample was
weighed and density was determined with the following equation:
Bio-oil's density (kg/m3) = m bio-oil
1 mL (2.5)
The sample was weighed in a Kern balance (model FKB, ± 0.1 g).
The elemental analysis (wt.%) – C, H and N – was determined according to M.M. 8.6 (A.E)
(2009-05-06). The oxygen content (wt.%) was obtained by direct calculation. The moisture content
(wt.%) was determined according to EN 12880:2000. The LHV and HHV (MJ/kg) were measured
according to CEN/TS 15400. The HHV was determined with a Parr oxygen bomb calorimeter while
LHV was determined according to the following formula:
LHV (kJ/kg) = HHV – 218.3 x H (wt.%) (2.6)
All the analyses were carried out at Laboratório de Análises from Instituto Superior Técnico.
The standard methods refereed above are internal standards of the laboratory.
38
2.5.3 Char analysis
The chars obtained were analysed so as to determine their density, elemental composition,
ash and moisture content, HHV and LHV. The elemental analysis (wt.%) – C, H, and N – was
determined according to M.M. 8.6 (A.E) (2009-05-06). The oxygen content (wt.%) was obtained by
direct calculation. The moisture content (wt.%) was determined according to M.M. (GRAV). The ash
content was determined according to CEN/TS 15403. The LHV and HHV (MJ/kg) were measured
according to CEN/TS 15400. The HHV was determined with a Parr oxygen bomb calorimeter while
LHV was determined according to the equation 2.6, as for bio-oils.
39
3. Results and Discussion
3.1. Experimental Conditions
Pinewood, olive bagasse, wheat straw and rice husk were pyrolysed at 580 ºC (reactor
temperature) with a nitrogen flow rate of 526 mL/min at atmospheric pressure. Upon these conditions
the nitrogen temperature at its inlet into the reactor was found to be 65 ± 4 ºC. The rotational velocity
of the screw was held at 19 rpm (see section 2.3.2). The tests were conducted with biomass samples
of 50 g properly prepared as described in section 2.1. The temperature of the condensation/refrigerant
fluid (ethylene glycol) was kept at 15 ºC.
The experimental conditions were based on trial studies carried out prior to conducting the
experimental runs. The trial studies pointed out the above conditions as the most promising and close
to a fast pyrolysis process with the highest yields of bio-oil, without compromising its properties. The
trial tests are described and discussed in Appendix F.
3.2. Analysis of the Feedstock
The physical properties of the feedstock play a major role in bio-oil yield and its properties.
Table 3.1 shows the main characteristics of pinewood, olive bagasse, wheat straw and rice husk, used
as feedstock for the present work.
Table 3.1 – Main characteristics of pinewood, olive bagasse, wheat straw and rice husk. *As received, **on dry basis
Parameter Pinewood Olive bagasse
Wheat straw Rice husk Method
Proximate analysis (wt.%, ar*) Volatiles 72.7 57.8 64.9 65.5 ASTM E872, ASTM E897 Fixed Carbon 13.5 19.7 11.5 14.6 By difference Moisture 13.6 9.4 8.9 9.4 ASTM E871 Ash 0.2 13.1 14.7 10.5 ASTM 1101, ASTM E830
Ash Analysis (wt.%, db**) SiO2 10.8 30.4 34.3 88.2 Al2O3 4.2 10.6 7.7 0.3 P2O5 4.8 7.4 3.4 1.6 K2O 5.2 18.7 15.1 3.7Ultimate analysis (wt.%, ar*) Carbon 46.48 43.2 58.7 40.7 Hydrogen 6.85 5.6 0.5 6.0 Nitrogen 0.0 1.9 1.1 0.5 ASTM E778 Sulphur < 0.02 < 0.02 < 0.02 < 0.02 ASTM E775 Oxygen 32.87 26.8 16.1 32.9 By difference
High Heating Value (MJ/kg, ar*) 18.1 17.5 19.0 15.7
Low Heating Value (MJ/kg, ar*) 16.7 16.4 18.8 14.4
ASTM D3682, ASTM D279,
ASTM D4278, AOAC 14.7
ASTM E777
ASTM D2015, E711
40
Pinewood shows the lower ash content (0.2 wt.%) when compared to those of olive bagasse,
wheat straw and rice husk (up to 14.7 wt.% for straw). Its lower ash content and high volatile matter
content (72.7 wt.%) implies a bigger portion available for energy conversion [53]. As described in
section 1.2.2, biomass materials such as olive bagasse, wheat straw, and rice husk with such high ash
contents may not make a good source of pyrolysis bio-oil, since ash plays a catalytic role on the
pyrolysis reactions affecting bio-oil yield and quality. Moreover, a high amount of phosphorous or alkali
metals as potassium, as for the case of olive bagasse and straw, potentiates this catalytic effect
[39,40].
The agro-biomasses present higher nitrogen amount in their elemental composition (up to 1.9
wt.% for olive bagasse), which will naturally result in bio-oils and chars with reasonable nitrogen
amount. Wheat straw presents the highest HHV (19.0 MJ/kg) as result of its high carbon content, and
rice husk presents the lower (15.7 MJ/kg).
The moisture content, which varies from 13.6 wt.% for pinewood to 9.4 wt.% for olive bagasse
or rice husk, is initially removed from the biomasses prior to conducting the tests in order reduce as
possible the moisture in the resultant bio-oil [20,72].
Once the biomasses were pyrolysed under the same conditions, the properties of the bio-oils
and chars are just related to the biomass initial composition. The characteristics described above
affect directly the yields of the products and their quality.
41
3.3. Yields of the Pyrolysis Products
Pinewood, olive bagasse, wheat straw and rice husk were pyrolysed at the conditions
described in section 3.1. Figure 3.1 shows the yields of the products obtained from their pyrolysis. The
reaction temperature was considered to be the “outlet temperature” achieved when the pyrolysis
reaction was taking place, which gave approximately ≈ 500 ºC for all the experiments.
The highest bio-oil yield was obtained with pinewood (51 ± 0.5 wt.%) and the lowest with olive
bagasse (31 ± 1.8 wt.%). Pinewood has shown the highest bio-oil yield and the lowest char yield with
numbers comparable to similar screw reactor studies, see Thangalazhy et al. [63] and Ingram et al.
[54]. Its relative good performance is related to its low ash content (0.2 wt.%) [35-37], and to its usual
major cellulose content, as refereed in Oasmaa et al. [35], from which most part is converted in bio-oil
[43]. The agro-biomasses (olive bagasse, wheat straw and rice husk) presented lower yields of bio-oil
than that of pinewood as a consequence of their higher ash content (see Table 3.1). Figure 3.2 shows
a correlation of the feedstock ash content to the bio-oil yields obtained from pyrolysis. As the ash
content of the feedstock increases the bio-oil yield decreases with a linear behaviour comparable with
the results of Fahmi et al. [36] and Oasmaa et al. [35], which also pyrolysed agro-biomasses with
substantial ash content in a fluidized-bed reactor.
After to pinewood, rice husk have shown the second best bio-oil yield with 40 ± 1.5 wt.%,
comparable to that obtained by Di Blasi et al. [69] in a packed bed reactor. The considerable ash
content (10.5 wt.%) of rice husk promoted the char and gas formation, which affected the bio-oil yield,
see Fahmi et al. [36].
24 34 38 34
51 31 33 40
25 34 29 26
0
20
40
60
80
100
Pinewood Olive bagasse Wheat straw Rice husk
wt. %
Feedstock
Gas
Bio-oil
Char
Figure 3.1 – Yields of the products obtained from the pyrolysis of the biomasses.
42
Olive bagasse showed the lowest bio-oil yield due to its relative high ash content (13.1 wt.%),
and to its typical high lignin content, as referred in Panopoulos et al. [99], which yield substantial
amounts of char and gas. Şensöz et al. [64] obtained comparable yield values for a fixed-bed reactor
at 500 ºC. According to Coulson [100], the alkali metals present in the olive bagasse ashes, as
potassium (K2O) in high percentage (18.7 wt.%), catalysed pyrolysis reactions to yield extra water and
gas and decrease the bio-oil yield. As result, olive bagasse produced the greatest gas yield (34 ± 0.04
wt.%). The yields of wheat straw are comparable with those of olive bagasse, although, it has
generated more char than gas. The reason may also be linked to a lower heating rate [64].
The yields obtained for the products are comparable with those obtained in other studies
involving screw reactors and fixed-bed reactors [54,63,64], however, previous fast pyrolysis works that
pyrolysed pinewood, wheat straw and rice husk in fluidized-bed reactors at 500 ºC have shown larger
yields of bio-oil (60 %), see [65-71]. Such discrepancy is due to the own nature of the screw reactor
where a very short residence time and high heating rates comparable to those of fluidized-beds are
difficult to achieve. According to Bridgwater [13], the “hot vapour residence time can range from 5 to
30 s depending on the design and size of the screw reactor”. The lower bio-oil yields obtained are an
indicative of an inherent larger residence time in the screw reactor that is obviously superior to the
estimated residence time (2.8 s) with the nitrogen flow rate of 526 mL/min. The larger residence time
in the reaction zone tended to increase thermal and catalytic cracking of pyrolytic vapours into
gaseous products, which increased gas yield. Moreover, the significant and higher yields of char
obtained are an evidence of lower heating rates accomplished within the screw reactor, a common
occurrence in these reactors already refereed by Bridgwater [13,18]. The own geometry of the reactor
and screw may have compromised an efficient heat transfer between the hot wall and the biomass
particles in the reaction zone, which resulted in a deficient heating rate not high enough. Such low
heating rate led to an appreciable repolymerisation of char, increasing its production (see Table 1.2 in
section 1.2.1). As overall consequence of cracking and repolymerisation, the bio-oil yields obtained
Pinewood
Olive bagasse
Wheat straw Rice husk
0
10
20
30
40
50
60
0 2 4 6 8 10 12 14 16 18
Bio
-oil
yiel
d, w
t.%
Ash, wt.%
Figure 3.2 – Correlation of the feedstock ash content to the bio-oil yields obtained from pyrolysis.
43
with the reactor of the present work tended to be somewhat lower than those of fluidized-beds
reactors found in literature.
The possibility of such low heating rates are related with a possible over feeding of biomass in
the reactor [101] is discarded, since the biomass feed rates (Table 2.1) are lower than those used by
Ingram et al. [54] (1kg/h) in a screw reactor with comparable dimensions and slight higher yields.
Therefore, the own geometry of the screw/reactor is pointed out as the main reason for the low
heating rates developed in the reactor.
The weaker pre-heating of nitrogen up to 65 ºC before its entrance into the reactor when
compared to other works that pre-heated nitrogen up to 350 ºC in fluidized-beds [65,68,71] is also a
possible reason to obtain distinguished lower bio-oil yields. A lower pre-heating of the fluidizing gas
represents bigger heat losses that could affect the own pyrolysis reaction.
After the pyrolysis reaction it was observed a deficient separation of products instead of a
rapid removal of char, which decrease bio-oil formation [13]. For all the experiments a visible portion of
hot vapour was trapped in the char flask and kept in direct contact with char before its follow to the
condensers. Figure 3.3 shows the contact of the pyrolytic hot vapours with char in the char’s flask. The
significant time in contact generated substantial extra-repolymerisation of char and catalytic cracking.
In technical terms, a not achievable residence time of ~ 2 s with a nitrogen flow rate of 526
mL/min, an evident low heating rate in the reaction, and an unable separation of the products while
using the screw reactor configuration were the main experimental justifications for the low bio-oil
yields. The weak pre-heating of nitrogen may also have had significant influence.
Attending the woody biomass reference yields for fast pyrolysis (see Table 1.1 in section
1.2.1) and to other related works in fluidized-beds with superior bio-oil conversion yields (> 55%), is
Figure 3.3 – Contact of the pyrolytic hot vapours with char in the char’s flask.
44
not possible or either correct to associate such covered experimental conditions to those of fast
pyrolysis. When comparing the yield values of pinewood with reference yields for woody biomass, one
concludes that the experimental conditions reached a regime of fast pyrolysis so-called intermediate
pyrolysis [13,16]. Figure 3.4 indicates the product distribution obtained from different modes of
pyrolysis.
Intermediate pyrolysis is a middle way regime of fast pyrolysis with longer hot vapour
residence times (5 – 30 s) and consequent lower bio-oil conversions yields (for woody biomass: bio-
oil: 50 wt.%, gas: 25 wt.%, char: 25 wt.%). The clear signs of a longer residence time obtained in the
screw reactor when compared to other fluidized-bed studies and the consequent yields obtained for
pinewood, which are strictly close to the referred values, let one conclude that the covered
experimental conditions led to an intermediate pyrolysis.
The bio-oil yields obtained with the other agro-biomasses are even lower than these woody
references due to the own nature of feedstock (ash content and chemical structure). It is interesting to
note that their difference (high as 20 wt.%) from the yield of pinewood bio-oil equals the difference
obtained with other agro-biomasses from the yield of pinewood in other comparative studies [99]
Nevertheless the oil yields seem to be reasonable while considering other fast pyrolysis
studies that reported bio-oil yields low as 17 wt.% [102] for rapeseed or 14 wt.% for rice straw,
sugarcane bagasse and coconut shell [103].
Figure 3.4 – Product distribution obtained from different modes of pyrolysis for woody biomass. Source: Ref. [13].
45
3.4. Analysis of the Bio-oils
The bio-oil obtained in the first condensation flask for each biomass was collected and
subsequently analysed as described in section 2.5.2. Table 3.2 shows the physical properties of the
original feedstock and the bio-oils obtained from the pyrolysis of pinewood, olive bagasse, wheat straw
and rice husk.
The pinewood oil analysis have shown a single-phase oil with a specific gravity, water content
and heating value of 1.2, 30 wt.% and 16.6 MJ/kg, respectively. Such properties matches the
conventional values refereed by Czernik et al. [72] for a typical woody pyrolysis bio-oil. The analysis of
the oils obtained from olive bagasse, wheat straw and rice husk have shown non-homogeneous oils
(two visible phases) with higher water contents (49-67%) and lower densities, which correlate to their
water content.
The heating values ranged from 9.5 MJ/kg to 19.6 MJ/kg for rice husk and for olive bagasse,
respectively. Olive bagasse bio-oil has shown the highest heating value even with the worst bio-oil
conversion yield (31 ± 1.8 wt.%). According to Butler et al. [15], this result is a clear sign of a large
portion of lignin in the olive bagasse, as in accordance with Panopoulos et al. [99]. The large lignin
portion was cracked better due to the catalysing effect of alkali metals present in the initial feedstock in
significant quantities, such as potassium (K2O - 18.7 wt.%). The better degradation of the lignin portion
led to a bio-oil with lower oxygen content and therefore with an energy density higher than the own
raw material (17.5 MJ/kg) [44], even with substantial water content (48 wt.%). Its heating value is also
superior to that of conventional woody bio-oil as that of pinewood [72].
Table 3.2 – Physical properties of the original feedstock and the bio-oils obtained from pyrolysis of pinewood, olive bagasse, wheat straw and rice husk.
Oil Raw Oil Raw Oil Raw Oil Raw
Density, kg/m3 1249 - 1212 - 1101 - 1050 -
Moisture, wt.% 30 13.6 49 9.4 58 8.9 67 9.4
C (wt.%) 45 46.48 56 43.2 57 58.7 62 40.7
H (wt.%) 7 6.85 8 5.6 7 0.5 7 6
N (wt.%) < 0.5 0.0 3.1 1.9 1.5 1.1 0.9 0.5
O (wt.%) 47.5 32.87 32.9 26.8 34.5 16.1 30.1 32.9
O/C 1.06 0.71 0.59 0.62 0.61 0.27 0.49 0.81
H/C 0.16 0.15 0.14 0.13 0.12 0.01 0.11 0.15
HHV, MJ/kg 16.6 18.1 19.6 17.5 11.7 19 9.5 15.7
LHV, MJ/kg 15.1 16.7 17.9 16.4 10.2 18.8 8.0 14.4
Phases 1 - 2 - 2 - 2 -
AnalysisPinewood Olive bagasse Wheat straw Rice husk
46
Pinewood has shown the second higher heating value with 16.6 MJ/kg, a value that is below
of that obtained by Thangalazhy et al. [63], 19.1 MJ/kg, due to its higher water content of 30 wt.%.
Although, the heating value obtained for pinewood still matches the conventional heating value for
woody biomass, according to Czernik et al. [72]. Besides the water content, its higher oxygenated
composition than that of the raw material with an O/C ratio of 1.06 is another reason for not reaching a
higher heating value, maybe similar to that of the raw material (18.1 MJ/kg) [72].
At last, wheat straw and rice husk bio-oils have shown poor heating values of 11.7 and 9.5
MJ/kg, respectively, as a direct result of their higher water contents (58 and 67 wt.%, respectively).
Such conclusion are possible in view of other studies that obtained bio-oils from wheat straw and rice
husk with low heating values of 16.9 MJ/kg and 17.42 MJ/kg and moisture contents of 19.9 wt.% and
25.2 wt.%, respectively [64,65].
Regarding the O/C ratios, pinewood bio-oil is the most oxygenated. Such result may be a
consequence of its lower ash content in the initial feedstock. The oxygen present in the mineral matrix
of the ashes in the initial agro-biomasses followed to char rather than for bio-oil, which may have
resulted in less oxygenated bio-oils even with higher water contents.
The agro-biomasses have shown a residual amount of nitrogen as a direct result of their initial
feedstock. It varied from 3.1 wt.% to olive bagasse to 0.9 wt.% to rice husk. It is clear that the amount
of nitrogen in the bio-oil correlates to the amount of nitrogen in the initial feedstock.
The higher water contents in the agro-biomass bio-oils are a consequence of their higher ash
content in the feedstock that catalysed pyrolysis reactions to yield extra water, as concluded by
Coulson et al. [100]. Oasmaa et al. [35] have proven such influent effect of the ashes when pyrolysed
agricultural residues in a fluidized-bed reactor at ~ 500 ºC and obtained moisture contents as high as
51.1 wt.%. Tsai et al. [103] also obtained large amounts of water in bio-oils (>65 wt.%) when
pyrolysing rice straw, sugarcane bagasse and coconut shell, with poor yields of bio-oil (down to 17
wt.%). The evident larger resident times of the hot vapours and low heating rates in the screw reactor
may also increased even more this catalyst effect [18]. Besides the catalyst effect of the ashes, an
insufficient drying prior to the tests is also a possible reason for such moisture contents.
It also seems reasonable to question how good was the performance of the condensation
stages once in the trial tests prior to conducting the present tests they have shown a compromising
behaviour (Appendix E). The analysis on Table 3.3, including the moisture contents, relate to the
representative bio-oil samples retained in the first condensation flask at the end of each test (Fig. 2.2
in section 2.2). It was observed throughout the tests that major part of the water steam has condensed
into this first flask, as in accordance with Chen et al. [104], and that a substantial portion of bio-oil,
heavier and more viscous, was trapped in the channels of the condensers without flowing into the
flasks (e.g. for wheat straw 36 wt.% of the whole bio-oil was trapped in condensers). Both combined
effects may have resulted in a representative bio-oil sample in the first flask of condensation with
47
superior water content, in relative terms, when compared to the gross bio-oil obtained. Figure 3.5
shows a viscous portion of bio-oil trapped in a condenser.
For these reasons, would be reasonable to assume that the covered methodology and the
own inability of the condensers to deal with viscous bio-oil tented to increase the water content in the
representative sample of the bio-oils, which influenced the other properties. For the same reasons,
authors such as DeSisto et al [66] analysed the bio-oil collected in the electrostatic precipitator rather
than the bio-oil fraction condensed in the condenser.
Figure 3.6 shows the bio-oils of pinewood and olive bagasse. The bio-oil of pinewood
presented a homogeneous aspect while the bio-oil of olive bagasse, as the bio-oils from the other
agro-biomasses, presented a heterogeneous biphasic aspect as consequence of their superior water
content, which caused phase instability [16]. According to Czernik et al. [72], the lower water content
of pinewood bio-oil (30 wt.%) enabled the miscibility of water in the whole emulsion that resulted in a
single-phase oil. Adversely, for the agro-biomass bio-oils with higher water contents the solubilizing
effect of the hydrophilic compounds was not enough to prevent phase separation into two phases [73],
a water-soluble (aqueous phase) and a water-insoluble phase (tar).
Figure 3.5 – Viscous portion of bio-oil trapped in the first condenser.
48
The phases are visually distinguishable in the bio-oils (Fig. 3.4 b)): the hydrophilic aqueous
phase (top phase) and the heavier non-soluble phase (tar) that settled at the bottom. Besides the own
water content, the usual higher amount of extractive matter (neutral substances) contained in the
feedstock of the agro-biomasses may have helped to yield a bigger aqueous phase [99].
These phase-separated oils may be desirable in some applications where fractionation is
required [13]. Şensöz et al. [63] and Yanik et al. [64] are examples of studies where these phases
were fractionated/separated and the only fraction taken into analysis was the heavier phase (tar),
which has the higher carbon content and consequent energy density.
The yields of the products already discussed together with such water content values (“half
water, half organics”) are important evidences from which one can conclude that the pyrolysis process
carried with the described experimental conditions was in fact an intermediate pyrolysis process,
according to Bridgwater [13].
a) b)
Figure 3.6 – Bio-oil of a) pinewood and b) olive bagasse
49
3.5. Analysis of Chars
The char obtained in the char’s flask for each biomass was collected and subsequently
analysed as described in section 2.5.6. The characteristics of chars obtained from pyrolysis are
depended on the pyrolysis conditions such as temperature and heating rate as well as the composition
of the biomass [64,105]. Once the biomasses were pyrolysed under the same conditions, the
properties of the chars are just related to the biomass initial composition.
Table 3.3 shows the physical properties of the char obtained from the pyrolysis of pinewood,
olive bagasse, wheat straw and rice husk.
The ash analysis has shown that the ash content in the char correlates to that in the initial
biomass. Pinewood with the lowest ash content in its initial composition (0.2 wt.%) had de lowest ash
content in its char (11 wt.%) while the agro-biomasses with ash contents as high as (14.7 wt.% for
straw) in their initial composition had ash contents as high as 43 wt.% for straw.
The ash contents in the chars agro-biomasses are a prove that the oxygen settled in the
mineral matrix of initial feedstock followed to char rather than for bio-oil, which may have resulted in
less oxygenated bio-oils but higher oxygenated chars for the agro-biomasses. The high O/C ratios are
evidences of such oxygenation. As result, pinewood with the lowest ash content had the highest
carbon content and the lowest O/C ratio.
Due to the higher content of carbon (75 wt.%), the char obtained from pinewood had the
higher heating value of 27.2 MJ/kg. This value is in agreement with Thangalazhy et al. [63] and
Table 3.3 – Physical properties of the char obtained from pyrolysis of pinewood, olive bagasse, wheat straw and rice husk.
Ash, wt.% 11 36 43 37
Moisture, wt.% 3.1 1.3 2.3 3.9
C (wt.%) 75 60 47 53
H (wt.%) 3.5 2.9 2 2.6
N (wt.%) < 0.5 2.2 < 0.5 < 0.5
S (wt.%) < 2 < 2 < 2 < 2
O (wt.%) 19 32.9 48.5 41.9
O/C 0.25 0.55 1.03 0.79
H/C 0.09 0.05 0.04 0.05
HHV, MJ/kg 27.2 21.6 17.7 21.3
LHV, MJ/kg 26.4 21.0 17.3 20.7
Wheat straw char Rice husk charAnalysis Olive bagasse
charPinewood char
50
DeSisto et al [66], which obtained a heating value for the char from pinewood of 28.1 MJ/kg and 28.5
MJ/kg at 500 ºC, respectively.
The char from wheat straw had the worst heating value of 17.7 MJ/kg with the worst carbon
content (47 wt.) and, consequently, the higher oxygenation (O/C ratio of 1.03). Such fact is in
agreement with its highest ash content in the initial feedstock that turns the char into a more
oxygenated product. Yanik et al. [64] also obtained a char from the pyrolysis of wheat straw at 500 ºC
with a heating value of 19 MJ/kg and an ash content of 38.3 wt.%.
The olive bagasse pyrolysis resulted in a char with a heating value of 21.6 MJ.kg, a slightly
lower than that reported by Şensöz et al. [63] of 24.8 MJ/kg. Such fact may be related to the lower
heating rate imposed in the pyrolysis of olive bagasse, which result in a char with higher carbon
content (73.1 wt.%).
The char from rice husk presented a heating value of 21.3 MJ/kg, higher than that obtained by
Di Blasi et al. [69], which also pyrolysed rice husk at 580 ºC in a packed bed and obtained a char with
a heating value of 18.7 MJ/kg with less carbon content (51.5 wt.%)
51
4. Closure
4.1 Conclusions
The main conclusions of the present study are as follows:
1) The ash content in the feedstock plays a major role in pyrolysis. It has been shown by the
relative behaviour of pinewood and of agro-biomasses that ashes, mainly alkali metals,
catalyse pyrolysis reactions to decrease the bio-oil production and yield extra water, which
decreased drastically the bio-oil quality:
2) The bio-oils of the agro-biomasses present higher amounts of nitrogen as a result of their
initial composition;
3) High water contents (> 30 wt.%) causes phase separation of bio-oils;
4) The O/C ratio analysis for bio-oils and chars suggested that oxygen settled in the mineral
matrix of initial feedstock followed to char rather than for bio-oil, which resulted in a char
more oxygenated and a bio-oil less oxygenated for the agro-biomasses when comparing
to those of pinewood;
5) The char from the agro-biomasses present higher ash contents and lower energy
densities than that of pinewood;
As result,
6) Pinewood has shown the more consistent results with the highest yield of bio-oil (51 ± 0.5
wt.%) and the lowest yield of char (24 ± 1.5 wt.%). The bio-oil produced from pinewood
met the specifications of the ASTM standard (D 7544-12) for the measured properties,
and has potential to be used as a direct liquid biofuel in industrial burners equipped to
handle these types of fuels. Though, its utilization as a transport liquid fuel would just to
be possible with an upgraded to reduce its considerable oxygen content [78,79,90].
7) Agro-biomasses have pyrolysed into bio-oils with low energetic content (apart from olive
bagasse), due to the higher water content, and with significant nitrogen amounts (3.1 wt.%
for olive bagasse). The combustion implications with so high water contents discard these
agro-biomasses as a potential feedstock to pyrolyse into a direct fuel liquid unless they
are upgraded [15,78]. Their significant nitrogen content would require an appropriate
emission control. Furthermore, the low conversion yields related to such bio-oils in the
present work (low as 31 ± 1.8 wt.%) may not justify their production with pyrolysis.
8) The non-homogeneity (biphasic) of the bio-oils from agro-biomass is a high challenge to
their use as fuel, however, is an opportunity for recovering added-value by-products with
particular properties [13,35]. Particular studies [63,64] have concluded promising potential
for the non-aqueous phase of bio-oil.
52
9) The resultant char from pinewood has shown the highest heating value with 27.2 MJ/kg, a
considerable value comparable with those of solid fuels ranging from lignite to anthracite
[105], suggesting its potential to be used as solid fuel (e.g. in the form of briquettes or in
char-oil water slurry [106]). The chars obtained from the agro-biomasses with higher ash
contents and lower energy densities may be used in the preparation of active carbon
when its pore structure and surface are appropriate [107].
4.2 Recommendations for future work
The feeding system has shown some limitations namely dealing with the low “fluidity” of
biomass in the feeding column, which led to blockage of biomass in many cases and the sabotage of
the test. Furthermore, bigger amounts of biomass sample to pyrolyse led to bigger cases of blockage.
A new configuration for the feeding system (e.g. hopper and screw feeder) should be useful to
pyrolyse bigger quantities of biomass and to permit a continuously operation for the reactor that
simplifies the experiments in many aspects.
A preliminary project of the screw, attending fast pyrolysis features, would be an extra aspect
to optimize results. The material and geometry of the new screw would have to permit a consistent
transport of the particles, an efficient and rapid heating rate with a good escape of the hot vapours. A
more rapid heating rate would be achievable if the screw was heated internally through an
incorporated resistance. This would be an interesting point to keep in mind for a new design.
The pre-heating of nitrogen to considerable temperatures was an experimental fact shown in
other works and has been pointed out as a possible reason to decrease the performance of the
reactor, as far as it may increase thermal losses. So, it would be interesting to pre-heat nitrogen to
higher temperatures and analyse its effect on the results. A new design of the main pipe with a more
efficient separation of products at its end would also be an improvement to achieve better results. The
char must be removed as soon as possible from the reaction to avoid secondary reactions. This would
be achievable through a possible cyclone connected directly to the main pipe that would separate the
solid phase from the gas/vapour phase. A thermal blanket at 400 ºC applied around the cyclone till the
condensation stage would avoid the pre-condensation of vapours.
In regard to the condensation, it would be interesting to analyse the influence of other type of
condensers on the results since condensation is crucial in any pyrolysis process. As said before, the
condensers used in the present work showed a compromising behaviour since the trial tests and may
have induced bigger water content, in relative terms, in the bio-oil sample.
After the optimization process, it would be interesting to achieve oils with better quality and
higher conversion yield, and test the influence of important process parameters such as temperature,
residence time, pre-heating of nitrogen and velocity of the screw. The study could be also extended to
other biomasses. As the ash content in the agro-biomass had a drastic role on the quality and yields of
their bio-oils, it would be interesting to compare the present results with the possible results of pre-
53
treated olive bagasse, wheat straw and rice husk with no ash content in order to better quantify the
effects of ash in pyrolysis.
A chemical characterization of the bio-oils through a GC-MS analysis would further identify
their major chemical compounds and assess their potential as chemical feedstock. A porosimetry
analysis of the chars would also assess their potential as industrial adsorbents.
54
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6. Appendices
6.1 Appendix A - Mean value and standard deviation
Mean values were obtained from:
x =1N xi
N
i=1
where N is the number of runs and xi is the yield of char/bio-oil/gas.
The standard deviation was calculated with:
σx = 1
N - 1(xi - x)
2N
I=1
63
6.2 Appendix B - Temperature Control System
A VI. interface program was developed for this work in order to monitor all the temperatures
measured by the acquisition board NI-9211, and specially to control the reactor temperature. The
program was nicknamed “Pyrolysis program” and was developed in LabVIEW 8.5 language (Virtual
Instruments – VI. code). This is a visual programming language that processes several signals by
cyclic iteration. In the case of “Pyrolysis program”, it processes the signal coming from the various
thermocouples with a sample rate of 1 Hz.
After each program iteration the information is presented to the user through a graphic
interface. Figure B-6.1 shows the main screen of the graphic interface. It presents the instantaneous
temperature values and experimental information that is directly introduced in the program (screw
rotation velocity; nitrogen flow rate; type and quantity of biomass sample). Through the main screen is
possible to monitor the temperatures and to define/control the desirable reactor temperature. A LED
called “ALARM” ensures that the reactor temperature does not increase abruptly threatening the own
system integrity.
A reactor temperature is defined introducing the wanted temperature on the respective field
(“Set Temperature” field in Fig. B-6.1). Based on the instantaneous measured reactor temperature, the
VI. acts on the auxiliary relay circuit. Figure B-6.2 shows the auxiliary relay circuit where a relay works
as direct actuator on the heating resistance of the reactor. This circuit is compost by a transistor
CTBC548ALE, a 6 AC V relay with 10 A, a NI USB-6008 multifunction board with analogue outputs,
Figure B-6.1 – Pyrolysis program main screen.
64
and a small rheostat with 1 kΩ of capacity. A 5 AC V transformer with 0.5 A is needed to feed this
circuit.
The NI-USB-6008 multifunction board links the control action of the VI. and the relay action
(actuator) on the heating resistance (system). The board inputs a minimum voltage in the circuit when
the VI. transmits that order from the computer (digital signal). When is the case, the transistor is
actuated, the relay acts and closes the heating resistance circuit (AC), and the reaction zone heats up
by Joule effect. Otherwise, the heating resistance circuit is open and there is no heating. The LED
named “Power” on Fig. B-6.2 indicates to the user if the reaction zone is heating up or not. Eventually,
heating up the reaction zone ends up by heat the all system.
The TCS is a feedback control system that is constantly comparing the instantaneous/on-line
reactor temperature with the pre-selected one. Its control/action logic is the following:
(1) If the selected temperature is below the on-line measured temperature, the VI. activates the
relay (by the process explained above), the resistance turns ON and the reaction zone heats
up till the measured temperature attains the selected one.
(2) If the set temperature is above the on-line temperature, the VI. does not activate the relay, the
resistance is switched OFF and there is no heating. If the system is already heated up above
the on-line temperature, the reaction zone turns out to cool down.
The VI. interface applies the control logic explained above (temperatures comparison), for
each program iteration along time. The perfect feedback control is impossible and the response of the
controlled variable – reactor temperature – is not perfect. Eventually, to maintain a given reactor
temperature process (1) is preceded by process (2) and vice-versa. The control maintains a reactor
Figure B-6.2 – Auxiliary relay circuit.
65
temperature response with an inherent perturbation/variation around the selected/targeted
temperature:
− For case (1) the reaction zone heats up and when the on-line reactor temperature exceeds
the selected one, the heating resistance is turned OFF. However, the reactor temperature response
presents an overshoot. This is mainly due to: a) delay in the thermocouple response, b) significant
heat power (1.1 kW) and c) heating inertia. In order to reduce this response perturbation, the heating
is pulsed (like a common oven). The relay turns ON/OFF the heating at a rate of 2 Hz. The VI. is
programmed to send pulsed orders to the relay circuit, which result in a pulsed heating. Figure B-6.3
illustrates this heating pulsed action.
This way, the heat transfer is reduced to half per iteration and the temperature increase is
fainter, which “gives time” to the thermocouple delay. It has been seen that the pulse heating
decreases the overshooting magnitude.
− Eventually for case (2) when the reaction zone cools down below the targeted reactor
temperature, the delay in the thermocouple response generates an undershooting in the reactor
temperature response.
The temperatures are plotted along time (30 min range) in other tab of the program Figure B-
6.4 presents a concrete example of the plot. Firstly, at the start up of any experiment, the reactor
temperature increases till attain a set temperature and the other temperatures end up increasing.
Eventually, the steady state is reached and all the temperatures keep constant along time.
Figure B-6.3 – Heating pulsed action.
66
A mean value of the reactor temperature based upon the last 3 min of data is also presented
on the plot. The constant noise on the reactor temperature is a result of the cyclic process of heating
(1) and cooling (2) of the reaction zone to maintain a constant temperature (already explained above).
All the data recorded along the experiments is recorded into a .txt file for posterior analysis.
Figure B-6.4 – Temperatures plot on Pyrolysis program.
67
6.3 Appendix C – Thermal characterization of the reactor
In order to characterize the reactor thermally, the temperature distribution inside the main pipe
was obtained. The screw was removed from the pipe and the T-connection placed at right of the
reactor (orientation of Fig. 2.2) was disassembled. A reactor temperature of 500 ºC was set as
calibration temperature and the system heated up. A type-K thermocouple KMQSS-IM025U-300
(Omega) was properly introduced inside the main pipe and recorded temperature in several points.
Two temperature distributions were obtained based upon the measured temperature from points
located on the upper and lower position inside the pipe along its length. Both distributions start at
begin of the pipe (point 1, placed on left side) and continue for more 20 points all spaced 1.5 cm ones
from the others. A multimeter True RMS Supermeter (NEWPORT, ± 2 ºC) was used to process the
thermocouple signal. The process was carried without any nitrogen flow and biomass feed. Figure C-
6.5 shows the temperature distribution (upper and lower distribution) inside the pipe with a reactor
temperature of 500 ºC.
The heating resistance (150 mm wide) is placed between points 9 (13.5 cm from the
beginning of the pipe) and 20 (30 cm), which define the reaction zone. Figure C-6.5 reveals that the
temperature on the inner wall among these points is nearly constant and close to the defined reactor
temperature (500 ºC). The temperature in points 9 and 20 (reaction zone limits) present a maximum
percentage difference of 8 % and 18 %, respectively, from the reactor temperature. The disassembly
of the T-connection from the right side of the reactor justifies the bigger temperature difference on
point 20. Figure C-6.5 also shows that the temperature tends to increase sharply while one
approximates the reaction zone. The similarity of the two temperature distributions obtained from the
0
50
100
150
200
250
300
350
400
450
500
550
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
Tem
pera
ture
[ºC
]
Points of measurement
Upper distribution Lower distribution
Figure C-6.5 – Temperature distribution inside the pipe with a reactor temperature of 500º C.
68
upper and lower points confirms the equality of temperature for points of the same cross section of the
pipe.
From the results, it is reasonable to assume that the temperature distribution should be
qualitatively the same regardless of the pre-defined reactor temperature: 1) the imposed reactor
temperature is nearly kept constant between the reaction zone and 2) temperature tends to increase
while one approximates the reaction zone. This is an important conclusion since one can interpret the
reactor temperature as the temperature of the entire reaction. Once TCS is controlling the reactor
temperature, it is actually controlling the temperature of the entire wall of this particular zone that is
nearly kept uniform along its length.
The slight and gradual relative increase of temperature towards the reaction zone focuses the
pyrolysis reaction on this particular zone, as it is supposed to be. It is assumed that the reaction
occurs totally on the reaction zone with no major loss in accuracy, however, the biomass samples can
start reacting before, attending the wall temperature increase along their route towards the reaction
zone.
Important factors that influence the wall temperature along the pipe such as nitrogen flow and
biomass/products transport are not being considered on this characterization, although, these factors
only affect the temperature on the wall outside the reaction zone. The “Inlet temperature” decreases
as a consequence of biomass transport into the reaction zone, and the “Outlet temperature” increases
as a result of the transport of hot products outside the reaction zone. The TCS controls and fixes
constantly the reactor temperature along the defined reaction zone.
69
6.4 Appendix D – Biomass feed rate
Figure D-6.6 illustrates the technique adopted to determine the biomass feed rate in the
reactor. For each biomass type, a prepared sample (milled and completely dried) was introduced in
the reactor feeding point. The rotation of the screw (19 rpm) carried the sample towards the other end
of the pipe where it fell into a bowl placed upon a digital balance. The weight variation indicated by the
balance while biomass sample falls into the bowl was timed. According to the mass balance, the
weight variation along time obtained in the bowl quantifies the biomass feed rate along the reactor
during its operation.
For each biomass type, a prepared sample of 50 g was used. The screw transported the
samples with a constant velocity of 19 rpm while the reactor was kept “cold” (heating resistance turned
OFF) and there was no nitrogen flow. The digital balance was a Sartorius balance (model CP6201, ±
0.05 g). The weight variation was recorded with a simple stopwatch: 5 g marked the beginning of the
timing and 45 g marked the end (50 g is a rare or even impossible occurrence due to mass losses
along the screw). The procedure was repeated three times for each biomass type in order to obtain a
more trustworthy result. Figure D-6.7 shows the weight in the bowl (mean value) as a function of time
obtained for the various biomasses with a velocity of 19 rpm.
In order to obtain a weight variation value in time, which actually represents the biomass feed
rate, a linear regression was estimated for each scatter. Each biomass feed rate is the own slope of
each respective linear regression. Table D-6.1 summarizes the linear regressions and the estimated
feed rates of the biomasses with a velocity of 19 rpm. The estimated feed rates are consistent with the
different density of the four biomasses.
It has been estimated an error of ± 0.3 for all the biomass feed rates based upon the
uncertainty of the balance and the timing of reaction time (stopwatch).
Figure D-6.6 – Technique adopted to determine biomass feed rate.
70
Feedstock Liner regression (t-time) Biomass feed rate (g/s)
Biomass feed rate (g/min)
Pinewood 0.127 t + 4.47 ; R2 = 0.998 0.127 7.6
Olive bagasse 0.181 t + 5.57 ; R2 = 0.996 0.181 11 Wheat straw 0.122 t + 6.01 ; R2 = 0.996 0.122 7.3
Rice husk 0.083 t + 5.90 ; R2 = 0.997 0.083 4.9
Table D-6.1 – Feed rates of the biomasses with a screw velocity of 19 rpm.
0
5
10
15
20
25
30
35
40
45
50
0 60 120 180 240 300 360 420 480 540
Wei
ght (
g)
Time (s)
Pinewood
Olive bagasse
Straw
Rice husk
Figure D-6.7 – Weight in the bowl as a function of time for the various biomasses with a screw velocity of 19 rpm.
71
6.5 Appendix E – Estimation of the hot vapours residence time
By the mass conservation law, the mass flow rate of nitrogen (mNTP) at the flow meter at NTP
conditions (ρNTP = 1.25 x 10-3 g cm3) is equal to the mass flow rate of nitrogen in the reaction zone
(the reactor is sealed). Estimating an average density (ρR) of the nitrogen flow based on its average
temperature (TR) in the reaction zone allow the estimation of a volumetric flow rate on the reaction
zone (VR). With that volumetric flow rate one can estimate the residence time of nitrogen in the
reaction based on the useful volume of the reaction zone (VU). Assumptions: 1) steady-state condition,
2) constant properties, 3) ideal gas behaviour, 4) negligible pressure variations (atmospheric
pressure), 5) nitrogen carries perfectly the hot vapours (same spatial velocity) 6) the reaction occurs
totally in the reaction zone and 7) despising the presence of biomass. The T-connection placed at right
of the reactor (orientation of Fig. 2.2) was disassembled and the screw was kept on its position inside
the reactor. A reactor temperature of 550 ºC was set as calibration temperature since it is the average
temperature of the covered spectrum of temperatures (480 - 620 ºC, in the trial tests), and the system
heated up. Varying the flow rate (VNTP) with the gas flow meter upstream of the nitrogen circuit, a type-
K thermocouple KMQSS-IM025U-300 (Omega) was carefully introduced within the reaction zone
(between the wall and the screw) and the temperature of nitrogen was recorded for each flow rate, at
steady state. The density of nitrogen in the reaction zone was determined based on that temperature
(using a common table of thermophysical properties for nitrogen at atmospheric pressure). The
volumetric flow rate of nitrogen in the reaction zone was calculated with the following mass balance
equation:
VR = mNTPρR
(E-6.1)
The useful volume is the volume between the inner diameter of the main pipe (2 cm) and the
screw volume along the length of the reaction zone (15 cm), which was found to be VU = 25.1 cm3.
The estimated residence time of the hot vapours was then obtained with the equation:
Hot vapours residence time = VUVR (E-6.2)
Table E-6.2 summarizes the estimated volumetric flow rates on the reaction zone to the
various flow rates of nitrogen controlled upstream. Table E-6.3 summarizes the consequent estimated
residence time of the hot vapours in the reaction zone.
72
In order to impose a residence time on the order of fast pyrolysis (~ 2 s), a minimum nitrogen
flow rate of 526 mL/min is needed.
VNTP (mL/min)
mNTP ∙ !"! (g/s) TR (ºC) ρR ∙ !"! (g/cm3) VR (cm3/s)
267
44
5.57 329 1.03 5.40
526
10.9
15.7
337 1.03
10
10.6
755 15.7 339 1.02 15.4
938 19.5 343 1.00
19.5
1200 25 349 0.96 25.8
VNTP (mL/min)
VR (cm3/s) H. v. residence time
267
44
5.40 4.65
526
10.6 2.37
755 15.4 1.63
938 19.5 1.29
1200 25.8 0.97
Table E-6.2 – Estimated volumetric flow rates on the reaction zone to the various flow rates of nitrogen.
Table E-6.3 – Estimated residence time of the hot vapours in the reaction zone.
73
6.6 Appendix F – Trial tests on the reactor
Trial tests were carried out in the screw reactor with the different biomasses in order to attain
a reasonable operation condition that yields significant bio-oil conversions and ensures a fast pyrolysis
process. The tests were performed with biomass samples of 50 g properly prepared as explained in
section 2.1. The experiments were carried out in two series and followed the experimental procedure
stated in section 2.3. For both series the condensation temperature was held at - 5 ºC in order to
ensure an efficient condensation, and the velocity of the screw was kept at 19 rpm (feed rates
refereed in section 2.3.2). The first series was carried out to determine the effect of the reactor
temperature on the pyrolysis yields. The nitrogen flow rate was held constant at 526 mL/min (2.4 s),
once it is the largest flow rate value that does not carry any ashes and char into the condensers, and
by the estimation made (Appendix F) is on the order of fast pyrolysis residence time (~ 2 s). Four
reactor temperatures were examined: 480, 530, 580 and 630 ºC. Figure F-6.8 shows the product
yields from the pyrolysis of the pinewood, olive bagasse, wheat straw and rice husk in relation to the
reactor temperature (N2 flow rate of 526 mL/min, 19 rpm). The second group of experiments was
performed in order to establish the effect of carrier gas (nitrogen) flow rate on the pyrolysis yields. The
reactor temperature was kept at 580 ºC, based on the results of the first group of experiments. Four
nitrogen flow rates were experienced: 1200, 755, 526 and 276 mL/min.
!"
#!"
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%!"
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480 530 580 630
Mas
s yi
eld
[%]
Temperature [ºC]
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Temperature [ºC]
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480 530 580 630
Mas
s yi
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Temperature [ºC]
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+!"
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480 530 580 630
Mas
s yi
eld
[%]
Temperature [ºC]
!"#$%&'%($)&
Bio-oil
Char
Gases
Figure F-6.8 – Product yields from the pyrolysis of the pinewood, olive bagasse, wheat straw and rice husk in relation to temperature (N2 flow rate of 526 mL/min, 19 rpm).
74
For both group of experiments the yields of the by-products were obtained as described in
section 2.4.
Figure F-6.9 shows the product yields from the pyrolysis of the pinewood, olive bagasse,
wheat straw and rice husk in relation to the nitrogen flow rate (reactor temperature of 580 ºC, 19 rpm).
For the wheat straw special case, for a nitrogen flow rate of 1200 mL/min and 756 mL/min the ashes
and some char penetrated into the first condenser and its cleaning was difficult. The bio-oil weighting
with these flow rates had to account this portion of char/ashes.
Temperature was expected to have the largest effect on the pyrolysis yield and chemical
composition, and for this reason a range from 480 ºC to 630 ºC was used to cover the typical
temperature range of pyrolysis. As the temperature was increased, the yields on bio-oil and char were
reduced as a result of the increased gasification regime. The condensable vapours are further cracked
into low molecular weight organic compounds and gaseous products. The increased amount of char at
lower temperatures could result from either incompletely or unpyrolysed biomass. The yields obtained
were consistent with previous works.
The effect of residence time is clear evident for pinewood and rice husk. The char maintained
a constant average yield, while bio-oil reached a maximum at 526 mL/min. The gas yield increased at
lower nitrogen flow rates as a result of the higher residence time of vapours on the reaction time
!"
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&!"
'!"
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+!"
#!!"
1200 755 526 267
Mas
s yi
eld
[%]
N2 flow rate (mL/min)
!"#$%&&'(
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Char
Gases
!"
#!"
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&!"
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+!"
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1200 755 526 267
Mas
s yi
eld
[%]
N2 flow rate (mL/min)
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Char
Gases
!"
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&!"
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+!"
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1200 755 526 267
Mas
s yi
eld
[%]
N2 flow rate (mL/min)
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1200 755 526 267
Mas
s yi
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[%]
N2 flow rate (mL/min)
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Bio-oil
Char
Gases
Figure F-6.9 – Product yields from the pyrolysis of the pinewood, olive bagasse, wheat straw and rice husk in relation to N2 flow rate (reactor temperature of 580 ºC, 19 rpm).
75
leading to more secondary reactions (cracking); and increases at higher flow rates, which could result
from the increased cooling effect on the pyrolysis reaction. Similar trends have already been seen in
previous works. Olive bagasse and wheat straw have not shown significant effects of residence time
for this range, only a small increase in bio-oil over the reduction in gas yield. For both series of tests,
olive bagasse and wheat straw pyrolysis indicated a higher tendency to char formation.
In general, and taking into account all the results of the various biomasses, the reactor
temperature and the N2 flow rate that seemed most appropriate to achieve a bigger yield of bio-oil
were 580 ºC and 526 mL/min, respectively. Therefore, the corresponding bio-oils were analysed in
order to quantify their foremost properties (elemental analysis, moisture content and HHV). The results
have shown bio-oils with “bad quality” possessing high weight percentages of Oxygen, high moisture
contents and low HHV’s. The cause for such “bad quality” properties was found to be the actual
operation of the condensation setup: the condensation temperature was too low (- 5 ºC) and the high
viscosity fractions in the bio-oil (heavy-molecular-weight compounds and tars) rich in Carbon got stuck
in both condensers, not reaching the bio-oil sample flask. The low condensation temperature ended
up reinforcing condensation of low-molecular-weight compounds and water into the bio-oil flask. The
resultant bio-oil samples taken to analyse were then, in percentage terms, rich in low-molecular-weight
compounds and water, which explains the “bad quality” of the bio-oils (high moisture contents and low
HHV’s).
As solution, the condensation temperature was increased up to 15 ºC in order to avoid the
blockage of heavier fractions in the condensers. The resultant bio-oils have shown a more
homogeneous look and better quality (lower moisture contents and higher HHV’s), however, higher
yields of bio-oil were compromised in view of a better quality of bio-oil.
In order to not compromise the quality of the bio-oils rather than their conversion yields,
pinewood, olive bagasse, wheat straw and rice husk were pyrolysed in the present work with a reactor
temperature and a nitrogen flow rate of 580 ºC and 526 mL/min, respectively, and with a condensation
temperature of 15 ºC. The velocity of the screw was always held constant at 19 rpm.
