- 1. ARTICLE IN PRESS Energy 32 (2007) 95107
www.elsevier.com/locate/energy Autothermal two-stage gasication of
low-density waste-derived fuelsStefan Hamel, Holger Hasselbach,
Steffen Weil, Wolfgang KrummUniversitat Siegen, Institut fur
Energietechnik, Paul-Bonatz-Str. 9-11, D-57068 Siegen, Germany
Received 2 December 2005AbstractIn order to increase the efciency
of waste utilization in thermal conversion processes, pre-treatment
is advantageous. With the HerhofStabilats process, residual
domestic waste is upgraded to waste-derived fuel by means of
biological drying and mechanical separation ofinerts and metals.
The dried and homogenized waste-derived Stabilats fuel has a
relatively high caloric value and contains high volatilematter
which makes it suitable for gasication. As a result of extensive
mechanical treatment, the Stabilats produced is of a uffyappearance
with a low density. A two-stage gasier, based on a
parallel-arranged bubbling uidized bed and a xed bed reactor, has
beendeveloped to convert Stabilats into hydrogen-rich product gas.
This paper focuses on the design and construction of the
conguredlaboratory-scale gasier and experience with its operation.
The processing of low-density uffy waste-derived fuel using
small-scaleequipment demands special technical solutions for the
core components as well as for the peripheral equipment. These are
discussed here.The operating results of Stabilats gasication are
also presented.r 2006 Elsevier Ltd. All rights reserved.Keywords:
Pyrolysis; Combustion; Fixed bed; Fluidized bed; Hydrogen;
Synthesis gas; Fuel feeding1. Introduction incineration plants [3].
The number of different uses forproduct gas clearly indicates the
exibility of gasication Gasication is by no means a new technology.
As earlyand therefore allows it to be integrated into severalas in
the mid 19th century, town gas obtained from theindustrial
processes as well as to be used with powergasication of coal was
used rst for illumination. This wasgeneration systems.followed by
heating, then as a raw material for theOne of the most difcult
aspects of waste as feedstock,chemical industry and more recently
for power generation. whether for gasication or for incineration,
is its hetero-With the decline of town gas, caused by the
widespreadgeneous nature. The broad variability in chemical
composi-introduction of natural gas and electricity,
gasicationtion, water and ash content, heating value and the
presence ofbecame a specialized niche technology [1,2]. The search
for a number of substances, such as sulphur, chlorides or metals,a
more efcient use of existing energy sources and rawaffects the
performance of thermal conversion processes.materials, initiated by
the exploitation of fossil fuels andWaste composition is inuenced
by numerous local condi-the release of greenhouse gases, has led to
a major tions, such, for example, as population structure and
localrenaissance of gasication technology. Nowadays, gasica- waste
separation and recycling regulations. In addition, thetion is
regarded as becoming the main technology for thecomposition of
local waste is subject to major seasonal orthermochemical
conversion of biomass to energy or even daily variations. For these
reasons, feedstock prepara-synthesis gas. Moreover, gasication
offers an attractive tion plays an important role in any thermal
waste conversionalternative to the thermal treatment of solid waste
inapproach. A number of different approaches for waste
pre-preparation are known, mostly involving mechanical
shred-Corresponding author. Tel.: +49 271 740 2634/2633; ding,
removal of metals and drying.fax: +49 271 740 2636. A promising
approach to improve homogeneity and toE-mail address:
[email protected] (W. Krumm). reduce the amount of
contaminants, which has already0360-5442/$ - see front matter r
2006 Elsevier Ltd. All rights
reserved.doi:10.1016/j.energy.2006.03.017
2. ARTICLE IN PRESS96 S. Hamel et al. / Energy 32 (2007)
95107been successfully demonstrated, is a mechanicalbiological
bottom. The fuel particles move slowly down the
reactorpre-treatment of waste. An accurate and efcient separa- and
react with the gases moving upwards. The fuel passestion of inert
materials such as glass, minerals, ferrous and through different
reaction zones where various processesnon-ferrous metals usually
requires several mechanical such as drying, pyrolysis and oxidation
take place.shredding and conditioning stages. The result is a
low-The maximum temperature at the bottom is normallydensity uffy
material which does not necessarily meet the 9301430 1C [8].
Another type of xed beds are co-currentrequirements of the
fuel-feeding system or the gasier itself. gasiers [9]. In a
co-current xed bed gasier, the fuel andIn order to avoid extensive
upgrading of the uff, either by the gasication agent move in the
same direction, wherebyfurther grinding to reduce its size or by
pellet forming tothe fuel must pass successively through the
drying,increase size and density, careful selection and design of
the pyrolysis, oxidation and reduction zones.gasication reactor and
its peripheral equipment must beThe advantages of counter-current
gasiers are fewerensured. restrictions on fuel moisture content and
fuel particle size. The purpose of this paper is to describe the
experience Thus no special fuel preparation is generally required
and agained in the design, construction and operation of a two-
wide range of biomass types can be used as feed material.stage
laboratory-scale gasier for processing uffy low- By comparison,
co-current gasiers produce a betterdensity Stabilats fuel derived
from waste which has been quality gas but place strict requirements
on fuel properties.mechanically and biologically pre-treated [4,5].
The main Fluid bed systems allow a more efcient gasication
duedesign parameters of the laboratory-scale plant were to the
elimination of hot spots inside the reactor. They aredened in
co-operation with the manufacturer of Stabi- suitable for various
types of feedstock and can be scaled uplats. The most important
requirement, which inuencedto relatively large plant sizes. Whilst
xed bed gasiers arethe design and the scale of the entire plant,
was to process used in the low-capacity range of up to a few
MWth,Stabilats as produced by the manufacturer, i.e. withoutuidized
bed reactors are typically applied in the rangeany further
treatment. over 5 MWth [10]. However, they are more expensive to In
the following section, the joint development of thebuild and need
better gas cleaning due to the highlaboratory-scale gasier and the
specic challenges pre- particulate content in the product gas.
Unlike xed bedsented by the small scale of the reactor and their
solutionreactors, uidized beds have no dened reaction zones. Theare
discussed in detail. The results from different gasica- conversion
of fuel particles and the secondary reactions oftion runs using
Stabilats are presented. pyrolysis products take place in the same
reaction volume. Tar conversion can be supported by the
introduction of2. Gasication conceptscatalytically active bed
materials. The large amount of solid material in the uidized bed
serves as heat storage and Gasication technology permits the
production of gasits permanent mixing induced by bubble movement
resultsfor a variety of applications such as fuels and chemicals.
in a uniform temperature distribution. The maximum bedThe type of
application for product gas is in practice temperature is
determined by the ash softening point and islimited by the gas
quality of the gasication process [6]. Tousually in the range of
8151040 1C [8].meet high gas quality requirements for product gas
use, Entrained ow gasiers require pulverized fuels. The fuelmost
research activities are focused on gas cleaning byparticles are
introduced into the steam/oxygen feed andprimary measures within
the gasier, such as reactor gasied at a residence time of a few
seconds. Gasiers candesign, optimization of operating parameters
and additionbe operated at lower temperatures to maintain ash as a
dryof catalytically active materials. Secondary gas cleaningsolid,
or at temperatures well above the ash fusion point inmeasures
include improvement of downstream clean-upthe slagging mode so that
ash is removed as molten liquidsuch as hot gas clean-up and
catalytic tar conversion. [11]. Operation at these high
temperatures makes itBasically, gasiers can be categorized in
several reactorpossible to produce a product gas which is virtually
freedesign groups such as single-stage and multi-stage of tar and
oil. Entrained ow gasiers have the ability toarrangements.handle
practically any fuel as feedstock, provided they are ground to the
right, i.e. small, size. Although entrained2.1. Single-stage
gasiers ow gasiers may seem attractive for the production of a
clean, tar-free synthesis gas for chemical applications, no The aim
of a single-stage gasier is to convert organic method of size
reduction has yet been found to mill thesubstances entirely in one
reactor. Depending on the typebrous biomass materials to a
satisfactory size [1].of operation, different gasication agents
such as oxygen,However, most of the single-stage gasiers for
solidair and/or steam are supplied. The most commonly usedbiomass
and waste can be used if the requirements placedgasication
technologies for single-stage processes areon product gas quality
are low, as is the case for directxed bed, uidized bed and
entrained ow reactors. thermal gas use such as co-combustion of hot
raw gas in Fixed bed or moving bed gasiers are often counter- coal
boilers [12] or use as fuel gas in a cement processcurrent ow
systems [7]. The fuel is fed in at the top of the[13]. To improve
the gasication process, modern andgasier and the gasication agent
is injected near the advanced gasication technologies separate the
drying, 3. ARTICLE IN PRESSS. Hamel et al. / Energy 32 (2007)
9510797devolatilization, gasication and combustion
reactiongasication reactors are kept separate and are onlyzones.
These multi-stage processes enhance process interconnected by heat
transfer. In principle, a pyrolysisefciency and product gas quality
by combining several stage is necessary to split the fuel into gas
and char. Toreaction zones under consideration of various fuel
char- provide the heat necessary for autothermal operation,
theacteristics such as high reactivity, low ash and sulphur char or
part of the pyrolysis gas must be oxidized outsidecontent and high
volatile matter.the pyrolysis reactor [18].A double-line plant
concept with heat production by2.2. Multi-stage gasierschar
combustion is the Fast Internal Circulating Fluidized Bed (FICFB)
process developed at the Technical Uni- Various multi-stage
processes are currently under versity of Vienna, Austria [19]. The
endothermic gasica-development or already in operation. The high
volatile tion of the fuel takes place in a stationary uidized
bedamount of biomass, which is released rapidly as gaseousconnected
via a chute to the combustion chamber which issubstances during
pyrolysis, is taken into account inoperated as a circulating
uidized bed. The char particlenumerous reactor concepts by spatial
subdivision of thecombustion heats up the bed material. The hot
bedfuel conversion steps. This makes it possible to inuencematerial
is separated by a cyclone and fed back as a heatand optimize the
operating parameters in each conversion carrier to the stationary
uidized bed. A plant with astep. These concepts can be categorized
as single-line orthermal capacity of 8 MWth is in operation in
Gussing, double-line processes. Single-line processes use only
oneAustria [20].main stream of mass through a number of reactors
whichA similar principle has been realized with FERCOsare arranged
in series. Double-line processes divide the SILVAGASs process which
consists of two circulatingmass stream into at least two partial
streams which pass uidized beds (CFB) [21]. The rst CFB is used
forthrough parallel-arranged reactors.pyrolysis and partial
gasication with steam. The second In Fig. 1(a), a two-stage
single-line gasication process CFB is used to combust the remaining
char from theaccording to Ref. [14] is shown. The fuel is dried and
gasication CFB.pyrolysed in the rst stage in an indirectly heated
pyrolyser.Another multi-stage double-line process is the so-The
pyrolysis products are subjected to partial oxidationcalled Staged
Reforming [22] which is also based on theby air in a narrow zone
between pyrolyser and char spatial separation of pyrolysis and heat
production. Thegasier. The product gas has to pass the hot char bed
whichinput material is fed into the pyrolysis unit, heated up
andleads to substantial tar cracking and results in low tar
pyrolysed in contact with a hot metallic or mineral heatcontent in
the product gas.carrier. The coke and heat carrier thus produced
are Other two-stage single-line concepts are, for example,
theseparated and the cold heat carrier is fed into a
heatcombination of a counter-current and a co-current xed exchanger
and heated up again using ue gas frombed gasier [15,16]. A
three-stage single-line congurationpyrolysis coke combustion. Raw
gas hydrocarbons areentitled Carbo-Vs has been successfully
demonstrated,cracked with steam in a reforming unit to form
carbondetails can be found in Ref. [17]. monoxide and hydrogen.
Another gasication approach is to divide the mass Another two-stage
parallel-arranged gasier presented instream into at least two
partial streams which are processed this paper is the
Herhof-IPV-Verfahrens. A brief descrip-in several parallel-arranged
reactors. The heat needed for tion of the process principle is
given in the followinggasication can be produced separately by
using air forsection, further details of the design, construction
andcombustion without affecting the gas quality of theinitial
operating results are discussed in the subsequentgasication
reactor, see Fig. 1(b). Combustion and chapters.Two-stage parallel
gasification Two-stage gasificationGasifier GasifierSteamPyrolysis
1. Stage2. StageRawgas FuelDrying & gasRaw FuelDrying &
Tar- & char pyrolysis gas pyrolysisgasification Char CharHeat
Heat Gasification Ash Air Flue agentChar combustion gas
(a)(b)AshFig. 1. Two-stage single-line and two-stage double-line
gasication concept. 4. ARTICLE IN PRESS98S. Hamel et al. / Energy
32 (2007) 951072.3. The Herhof-IPV-Verfahrens dilution of the
product gas by nitrogen from the ambient air required for
combustion can be avoided. Downstream The Herhof-IPV-Verfahrens is
characterized by the of the gasication reactor, the remaining tars
and otherparallel arrangement of a xed bed and a bubblingimpurities
are separated from the product gas by a gasuidized bed reactor (see
Fig. 2). The fuel is fed directlyscrubber and an electrostatic
precipitator. The water/tar/into the xed bed reactor lled with hot
bed material. dust mixture generated in the gas cleaning can be
burntThrough its contact with the hot bed material, rapid
dryingdirectly in the uidized bed combustor to ensure economicand
pyrolysis of the fuel occurs. The volatiles released,operation of
the plant.including tars, pass through the hot ash layer above
thefuel feed and leave the reactor at the top. Adding steam in3.
Fuel characteristicsthe upper part of the bed enhances tar
conversion bycatalytic and/or thermal cracking in the presence of
the hot 3.1. Waste treatment for Stabilats productionash particle
surface. The remaining char and the bedmaterial move towards the
bottom of the gasication The rst step after the delivery of the
residual domesticreactor and are transported by a screw conveyor
into the waste is its preconditioning with a shredder and a
rstuidized bed combustor. The char combustion leads to a separation
of ferrous metals. The shredded residual wasteheat-up of the
uidized bed material which is then is then kept in a composting box
for 7 days. During thisdischarged towards the gasication reactor by
a uidized time, a forced air ow through the waste creates
optimumloop seal. The energy required for pyrolysis and
gasica-conditions for microbial respiration and biological
break-tion in autothermal plant operation is generated by char down
of organic matter. Warm moist air removed fromcombustion. The
combustion and gasication reactors arethe boxes is passed over a
heat exchanger, enabling theconnected by circulating solid
material, ensuring separa-condensate to be captured, cleaned and
used within thetion of gasier raw gas and combustor ue gas. Thus
aprocess. The air passed through the boxes is
re-circulatedElectrostaticprecipitatorScrubberProductgasFly-ash,
Rawtar, water gas Flue gas Fixed bedgasificationBed
Bubblingreactormaterial fluidized bed (GR)combustor (BFB)
Condensate to analysisFuel Steam Fuel particle Raw Drying
Pyrolyzing Bed material + CokeCokeAshAir Fig. 2. The two-stage
parallel-arranged gasier designed for the processing of
waste-derived fuels. 5. ARTICLE IN PRESS S. Hamel et al. / Energy
32 (2007) 9510799which signicantly reduces the volume of exhaust
air required for autothermal operation. The external charemissions.
The heat produced during this biologicalcombustion has to provide
sufcient energy to facilitateconversion is used in the plant for
waste drying [5].pyrolysis and gasication. In the case of low char
yields, The prerequisite for the subsequent separation of the part
of the Stabilats can be fed directly into the uidizedwaste mixture
into various fractions is moisture removal.bed to support heat
generation. In the case of high charDuring several downstream
process steps, different materi- yields, the gas yields are
affected which results in lower coldals are sorted in order nally
to obtain Stabilats, plastics gas efciency.(as an option), ferrous
and nonferrous metals, batteries, Table 1 presents the elemental
and proximate analysisglass (with high separation efciency
regarding colour) andof Stabilats compiled on the basis of
different references.other inert substances (mixture of ceramics,
stones andFig. 5 shows experimental results on char yields
fromporcelain). The dry consistency of Stabilats permits
easyStabilats pyrolysis at different temperatures and
carriedstoring and makes it suitable as a fuel for industrial out
with different equipment. The results published by Ref.processes. A
picture of Stabilats is shown in Fig. 3. The[23] were obtained in
an externally heated batch-fed xedbulk density of the sample
displayed is as produced, bed retort with an initial amount of
Stabilats of 250300 gusually in the range of 150250 kg/m3. A
compaction into for each experiment. The heating rates were
comparablypellets is possible if required. However, further
pelletizingslow at some 3 K/min with the aim of producing ashould
be avoided for economic reasons and the Stabilats maximum char
yield. Pyrolysis experiments in a batchpreferably used as produced.
The statistical average rotary kiln, which was also externally
heated, werecomposition is summarized in Fig. 4. performed by Ref.
[24] using an amount of 1000 g for each run. The heating rates of
individual particles introduced3.2. Pyrolysis of Stabilatsinto an
already formed bed inside the rotary kiln were some 550 K/min for
wall temperatures of 900 1C.For the intended processing of
Stabilats in a two-stage For our own pyrolysis experiments, small
amounts ofgasier, pyrolysis behaviour is critical for the process
some 15 g were used for each data point. The sample wasdesign. The
pyrolysis conditions and thus the resultingdivided into eight
portions of 12 g, each placed in a cokingpyrolysis products
determine the distribution of energy into crucible and exposed to
the designated temperature in athe gaseous phase and char. A
certain amount of char ismufe furnace for 7 min. The procedure
follows the determination of the volatile matter content of solid
fuels (900 1C, 7 min) according to DIN 51720. The range of data
points for the char yields obtained is displayed in Fig. 5 within
the grey area, the average is depicted as a dashed line. Firstly,
the expected decrease of char yield with increasing temperature is
obvious. Secondly, the inuence of sample size is indirectly
visible. Whilst the data points of Ref. [24], who used an amount of
1000 g, show less deviation, our own results obtained from small
sample sizes show a large range of remaining char yields (grey
area). This effect may be explained by heterogeneous fuel
composition. The composition of Stabilats can be considered
homogeneous compared to the original waste source and can, of
course, be called homogeneous when large quantities are processed.
However, with decreasing sample size, waste-derived material
appears to be of a Fig. 3. Stabilats sample. heterogeneous nature.
It is no longer possible with Inert materials Materials from
(stones, glass,metal)biomass sources < 1 wt.-%(paperboard/paper,
textiles, wood, organic compounds)about 65 wt.-% Material of fossil
sources (textiles, vulcanized rubber, composite materials,
etc.)Plastics about 25wt.-% about 9 wt.-% Fig. 4. Main constituents
of Stabilats, as determined in long-term statistical analysis [4].
6. ARTICLE IN PRESS100S. Hamel et al. / Energy 32 (2007) 95107Table
1Comparison of different published compositions of
StabilatsReference[23][24] [25]Own results No. 1No. 2No. 1 No.
2RangeAverageWater content (raw)10.5012.7 24.09.411.5 15Mass%Ash
content (dry)33.1026.3020.627.5 17 23.5Mass%Volatile matter
(dry)57.0064.4071.166.9 Mass%C (daf)59 59 57.659.247.6354.43
48.9958.34 Mass%H (daf)7.17.17.7 8.16.808.847.48 7.97Mass%O (daf)30
30 32.128.836.7451.71 40.8230.80 Mass%N (daf)1.9 1.90.952.721.50
1.80Mass%Lower heating value (dry)14,900 18,700 18,25016,770
13,00015,70014,300 kJ/kg 0.7Literature data Own results 0.6Retort
[23] AverageRotary kiln [24]RangeChar yield in kg/kg(daf) 0.5 0.4
0.3 0.2 0.1Feedstock: Stabilat 0.0300400500 600 700 800900
1000Pyrolysis temperature in C s Fig. 5. Char yields from
Stabilatpyrolysis, comparison of literature data and own
results.decreasing sample size to ensure a representative material
amount of fuel into the reactor at a rate as steady andmixture with
the original particle size. This is also reected uniform as
possible. At the same time, the feeding systemin the pyrolysis
results in Fig. 5.has to seal the reactors against environment and
serve as fuel storage in order to supply fuel for about 30 to 60
min.4. Design, construction and operation of the gasierDuring
design of the system, the advantages and disadvantages of fully
automated fuel-feeding apparatus The laboratory plant, with a
maximum thermal power of were discussed in detail. Since the focus
of the project was150 kW, consists of many parts other than the
core reactor the development and testing of the gasication reactor,
thecomponents, such as extensive peripheral equipment for decision
was ultimately taken to build rst a partly manualthe supply of
auxiliary energy, gasication agent, pressur-feeding system with the
option of full automation in theized air for pneumatic devices,
handling of product gas andfuture. The main reason for this was
that in the case of fullcontrol and measurement systems. Since the
peripheralautomation, a scale-down of the entire conveying
andequipment is usually regarded as state-of-the-art, the storage
devices would be necessary.following description focuses on the
newly developed main Although the manufacturer has extensive
experience incomponents which had to be carefully designed for the
task the design and operation of large-scale plants for thein
question. production, handling, storage and conveying of Stabilats,
the prevailing opinion was that problems were to be4.1. Stabilats
feeding systemexpected from a scale-down to feeding rates of some
1030 kg/h.The current feeding system performs several tasks which
The aim of the solution identied is to feed Stabilatsare slightly
different to those of a full-scale plant. In themanually in a cycle
of about 30 min from barrels or bagscase of the laboratory-scale
equipment, the general task into the lock hopper which simply
consists of a chargesimilar to that of a full-scale plant is to
feed a denedhopper (a in Fig. 6) and a lock hopper (c in Fig. 6)
between 7. ARTICLE IN PRESSS. Hamel et al. / Energy 32 (2007)
95107101Fuel a Lock hopper Seal of the b slide valvesFlangeI c N2+
air dN2openclose Slider e N2 II Cooling waterf gWaterWaterFuelWater
Expansion WaterjointFig. 6. The Stabilats feeding system.two
pneumatic access slide valves (b+d in Fig. 6). This is a fuels or
biomass are caused by a mismatch of the internalwell-known
principle which has proven to be reliable for transport system to
the fuel properties. A particularfeeding solids into pressurized
reactors [26]. After placing problem, called bridging, which causes
interruptions inthe fuel in the lock hopper, it was ushed with
nitrogen the material ow, is described as the phenomenon where
abefore being released into a vertical bin (e in Fig. 6) below.
particulate solid fuel forms a stable structure across anThis bin
serves as fuel storage for the fuel screw feeder (g inopening.
Several parameters are known to increaseFig. 6) which is mounted at
the bottom. To ensure theparticulate fuels tendency to bridge over
openings. Thiscontinuous transportation of Stabilats, a paddle
mixer (fincludes particle shape and size distribution, depth of
thein Fig. 6) was additionally installed above the screwfuel bed
over an opening and moisture content. Investiga-conveyer to prevent
blockage or bridging. The completely tions for different biomasses
indicate that especially thecooled screw, comprising separate
cooling of the conveying proportion of long and hook-shaped
particles inuencestube jacket, the double central tube and the
screw wings signicantly the tendency to form bridges [27,28].
Increas-(see detail II in Fig. 6), forces the Stabilats into the
xeding moisture content usually accentuates this inuence. Inbed
reactor. A considerable part of the Stabilats is madethe current
task of handling refuse-derived fuel, not onlyup of different types
of plastics with low softening the presence of long particles is
seen as a cause oftemperatures and also of substances with
comparably lowconveying difculties but also their concurrence with
largestarting temperatures for pyrolysis (see Fig. 4).
Thereforeamounts of soft and low-density uffy matter.
Further-extensive cooling of the screw is necessary to prevent
more, the small diameters of hopper and auger makemelting and
pyrolysis in the feeding system, especiallyhandling difcult.inside
the conveying tube.In order to avoid problems with the hopper
system, the It is known that many of the operating problems
diameter from the charge hopper down to the screwexperienced in
plants for processing waste, refuse-derived conveyor is enlarged.
Before selecting a pneumatic slide 8. ARTICLE IN PRESS102 S. Hamel
et al. / Energy 32 (2007) 95107valve, the clear diameter of the
opening has to be carefullygasier, from fuel feed down to the
bottom, is designed tochecked. The best results for the ow through
the slide be 2030 min, depending on the desired solid
circulationvalves were achieved when choosing not the nominal size
rate. The amount of circulating solid heat carrier
materialaccording to the pipe diameter but the next standard
size.is about 10 kg/kgfuel for the experiments presented here.In
turn, the clear diameter is large enough to be able toThe residence
time of the released volatiles through theextend the pipe length of
the hopper beyond the connectionhot bed material layer is in the
range of 24 s. Due toange so that the pipe ends as closely as
possible above thedrying and pyrolysis reactions inside the gasier,
the hotslider (see detail I in Fig. 6). This avoids rough edges on
the bed material cools down and internal temperature zonesone hand
and on the other hand prevents, as far asare formed. The
temperature level at the top of the gasierpossible, contact of the
solids with the sealing of the slider. is up to 920 1C, depending
on uidized bed operation The bin below the lock hopper is also
designed so that temperature, and reaches about 500600 1C at the
bottomthe diameter is enlarged in ow direction towards the of the
gasier.paddle mixer. The paddle mixer prevents the formation of The
addition of steam into the conical part above thebridges and
ensures the continuous lling of the screwfuel feed is carried out
by a ring line and several nozzlesfeeder with fuel. Experience
clearly showed that the paddle mounted along the perimeter. Steam
serves on the onemixer is the most important component for
ensuringhand as a reactant to support tar conversion and gas
phasereliable material ow. The positioning of the hopper
binreactions and on the other hand effects a continuousdirectly
either on a small or a large diameter screw feeder loosening of the
bulk solid.without a paddle mixer was not successful. After a
coupleIn the upper zone of the gasier, the hot bed materialof
minutes, a bridge formed over the auger. This simple layer, in
combination with the added steam, supports targravity-driven
movement of the material downwards ontocracking before the raw gas
produced is piped at the top ofthe screw was obviously not
effective enough to ll the the gasier into a gas-cleaning unit. The
gas is analysed in ascrew. Visual inspections indicate that the
uffy bulk is gas analysis device and combusted in a
post-combustionhighly elastic and tends to spring back from the
auger so unit without any further utilization.that the screw wings
were unable to force the material intothe conveying tube. Feeding
was only possible once the4.3. Fluidized bed combustion
reactorpaddle mixer was installed, but the conveying
character-istics were ultimately improved by designing the auger
withThe task of the uidized bed is to burn the char from thea
conical shape at the end. This affects the way in which the gasier
and simultaneously to ensure the transportation ofpaddle mixer
pushes the material onto the conical part of bed material in an
upward direction. The coke/bedthe auger which then forces the
material in the conveying material mixture is fed with a
temperature of somedirection as it rotates.500600 1C from the xed
bed gasier into the uidizedbed. The feeding point is in the lower
part of the uidized4.2. Gasication reactorbed above the air
distributor. The preheated air enteringthe uidized bed oxidizes the
char, which mainly consists of The gasier is designed as a
cylindrical reactor with acarbon, and the bed material is heated up
as a result ofnarrow cone above the fuel feeding point. The
subsequentexothermic combustion reactions. The operation is
carriedenlargement of the diameter downwards aims to achieve out in
bubbling uidization mode, thus a simple overowbetter mixing and
heat transfer between the hot heat carrierwas chosen to adjust the
bed height and to discharge theand the Stabilats introduced and
thus lead to better fuel bed material towards the gasication
reactor.conversion. A cooled screw conveyor forces the Stabilats In
order to be able to investigate experimentally thedirectly into the
bulk of hot bed material, see Figs. 6 and 2.inuence of relevant
parameters and their interaction, theThrough its contact with the
hot bed material, drying and plant design must be as exible as
possible in order to takepyrolysis take place, the gas moves in
counter-current owinto account different operating parameters. In
the currentto the bed material and leaves the reactor at the top.
Thecase, the uidized bed is inuenced directly or indirectly bybed
material, together with the remaining coke, isnumerous factors. The
properties of the char remainingdischarged at the bottom by another
cooled screw feeder from pyrolysis, such as heating value, yield
and reactivity,directly into the uidized bed combustion reactor.
Theare expected to be in a wide range due to the
heterogeneousmixture of coke and bed material is cooled down on its
waynature of the feedstock. The char yield, which dependsdownwards
and has a temperature of about 500600 1C atmainly on fuel
properties and feed rate but also onthe bottom. Therefore in order
to minimize heat loss, the pyrolysis conditions, determines the
amount of requiredcooling of the screw conveyor is dimensioned so
that thecombustion air. The uidized bed temperature is a result
ofsteel of the screw is just protected against high tempera-heat
release from char combustion, temperature and owtures and no
further unnecessary cooling of the solid rate of the primary air
itself, temperature and ow rate ofoccurs. the bed material fed from
the gasier and of the reactor The solid material ow inside the
gasier is driven by heat loss. Air ow rate and resulting uidized
bed temp-gravity. The mean solid fuel residence time inside
theerature in combination with geometric design determine 9.
ARTICLE IN PRESS S. Hamel et al. / Energy 32 (2007) 95107103the
subsequent uidization patterns. The desired andThe solids slide
over the inclined plane into the dischargepossible hydrodynamic
uidized bed operation mode pipe which is emptied manually by a
slide valve. The hotdecides nally about the allowed and realizable
range ofsolids fall into a sealed box and are collected until the
endthe above-mentioned parameters. Since some of the of the
experiment.parameters are not known a priori and others
uctuateconsiderably, it is not possible to dene the optimal 4.4.
Loop sealoperation point. The design of the air distributor is
important for exibleMany chemical or energy-related processes such
asoperation. During operation, the gas distributor must combustion
or gasication recirculate solids and useensure that the gas is
distributed on entry as evenly aspreferably non-mechanical solid ow
devices. In the valvepossible across the uidized bed. When the gas
ow is mode of operation, the solid ow through the non-stopped, the
solids must be prevented from raining throughmechanical device can
be controlled by the amount ofthe distributor into the plenum. In
most small-scale studies, uidization gas, whilst in automatic mode,
solid owporous plate or perforated plate distributors are used. The
corresponds to the natural circulation rate of the
process.advantage is that they are inexpensive and easy toA
comprehensive overview of different standpipes andmanufacture and
normally ensure good gas distribution non-mechanical valves and
their operation is given by Ref.for a wide range of operating
parameters [29]. For large- [32].scale applications, especially
with difcult operating con-In the current laboratory-scale plant,
hot bed material isditions such as high temperature or a highly
reactive transported between the two reactors and the mixing of
ueenvironment, tuyere designs are used. A detailed compar-gas and
product gas is avoided by using a loop seal. A loopison concerning
design, construction, advantages andseal is essentially a variation
of the well-known seal potdisadvantages of all types of gas
distributors can be found [32]. Like the seal pot, it comprises a
standpipe and ain Refs. [2931]. uidized bed section. Hot bed
material leaves the uidized It must furthermore be taken into
account in the current bed combustor via an overow into the
standpipe of thecase that inert material particles of larger size
and particles loop seal. The bed material from the standpipe enters
theunsuitable for uidization (e.g. metal) will most probablysteam
uidized bed section of the loop seal from the sidebe fed in with
the fuel. The possibility for a discontinuousbefore it is
discharged towards the gasication reactor.discharge of solids at
the bottom of the uidized bed must To ensure the separation of the
ue gas from thetherefore be taken into account in the design. The
aircombustion and the product gas from the gasier, rst
ofdistributor realized is displayed in Fig. 7. A nozzle-typeall the
pressure in the freeboard of both reactors has to bedesign with a
comparably high specic pressure drop was kept at the same level. In
fact, there are slight pressurebuilt in order to ensure that air
distribution is as uniform as uctuations in both reactors. In the
bubbling uidized bedpossible over a wide air ow and to prevent a
backow ofcombustor, these are mainly caused by bubble activity
andsolids into the plenum. At the same time, large inert in the
gasication reactor by gas generation which is notparticles have the
opportunity to settle between the nozzles. always uniform. To
enhance the pressure sealing of theloop seal, the height of the
solid packing in the standpipeFluidized can be increased.bedA
pressure measurement below the air distributor of the
Nozzlesuidized bed combustion reactor provides a signal which
isproportional to the bed height. A steady-state pressurelevel is
achieved during operation with constant bed height.Stopping the
loop seal uidization interrupts the transportof solids towards the
gasication reactor and, as a Inclined consequence, the height of
the uidized bed in the planecombustor rises. The increase of bed
height in the uidizedbed combustor becomes apparent through the
increasingSolid pressure across the bed (see Fig. 8). To increase
the dischargeefciency of the gas separation, the steam injection
for loopAir seal uidization was carried out discontinuously.
Whenstopping loop seal uidization, rst the height of the
solidpacking increases until the standpipe is completely lledand
then the uidized bed height in the combustorincreases. After the
addition of steam into the loop sealrecommences, the transport of
the solids continues and thePlenumprevious pressure level is
reached again. The discontinuousBed materialoperation of the loop
seal is shown in Fig. 9. Although Fig. 7. Design of air
distributor, solid discharge and plenum. solid temperatures of 900
1C are reached in the uidized 10. ARTICLE IN PRESS104S. Hamel et
al. / Energy 32 (2007) 95107bed combustor, the recorded pressure
indicates a reliable 5. Gasication resultsoperation of solid ow.
The gasier at the University of Siegen (see Fig. 2) was4.5. Bed
material operated using Stabilats and wood as feedstock. A plot
ofproduct gas concentrations during operation with Stabi- The task
of the bed material is to transfer heat producedlats fuel is shown
in Fig. 11. In the laboratory plant,in the combustion reactor to
the gasication reactor. The nitrogen was used to inert the lock
hopper fuel feedinghot bed material is transported, as described
earlier, by a system. Nitrogen content in the dry product gas was
in theuidized loop seal into the gasier, providing the heat range
of 37 vol% as a result of the inerting procedure. Toneeded for
drying, pyrolysis and gas phase and tarbe able to compare the
experimental results from differentconversion reactions. For the
start-up, silica sand with aruns the gas concentrations are related
to dry and N2-freeparticle size between 0.2 and 0.6 mm was used as
initial bedbasis.material. As the experimental run time progresses,
the Both reactors were heated up simultaneously for theoriginal
silica sand is slowly replaced by fuel ash. Fig. 10start-up using
preheated hot air to reach operationpresents the size distribution
of initial silica sand and bed temperature. Solid circulation was
adjusted to ensurematerial after some 40 h of operation. It appears
that theuniform heat-up of both reactors and bed material. Oncesize
distribution shifts towards larger particle sizes due to the
operating temperature had been reached, air wasthe accumulation of
fuel ash. The diameter of the average replaced by nitrogen in the
pyrolysis reactor until themass of bed material shifts from 0.277
to 0.385 mm. So far, measured oxygen concentration was almost zero.
Fuelthe change in particle size has not noticeably affected the
feeding then commenced, followed by replacement ofoperating
characteristics of the uidized bed, loop seal andnitrogen with
steam after a certain period of time, seemoving bed.1000.5 800
Cumulative amount in kg/kg10-1 600pBelow distributor BFB Pressure
in hPa10-2 400Initial silica sand10-3Bed material after 200 Steam
into 40 h experiment loop seal ON ON BFB OFF10-4 0 Steam05:45
05:5506:05 06:15 0.010.11.0 10Run time in hh:mmDiameter in mmFig.
8. Effect of discontinuous loop seal operation on bubbling
uidizedFig. 10. Particle size distribution of initial silica sand
and bed materialbed combustor.after 40 h of operation. 1.0005800 4
Temperature in C TBFB pBelow distributor BFBSteam in kg/h Pressure
in hPa600TGR crackzoneSteam into loop seal3400 2200 1 0009:1509:20
09:25 09:3009:3509:4009:45 Run time in hh:mm Fig. 9. Discontinuous
operation mode of the loop seal. 11. ARTICLE IN PRESS S. Hamel et
al. / Energy 32 (2007) 95107 105 Run no.: TS10 Fuel:Stabilat60
mfuel: 8.31 kg/h Concentration in vol.-% [dry + N2 free]Steam
addition (replacing N2 with N2 inerting of steamin gasification
reactor) Steam to fuel ratio:0.44 kg/kg gasification50 reactor H240
TBFB= 920 C30Start of TGR crackzone = 860 Cfuel feed CO220CH4O210
CO N is used for inerting the lock hopper fuel feeding system
2001:00 02:00 03:0004:00 05:0006:00 07:0008:00Run time (hh:mm)Fig.
11. Product gas concentrations during operation of the
Herhof-IPV-Verfahrens with Stabilats.Fig. 11. During the following
5 h (displayed in Fig. 11), theConcentration in vol.-% Lower
Heating Value (LHV) gas in MJ/moperating parameters were kept
constant. Gas sampling(dry + N2 free) (std., dry + N2 free)16was
carried out continuously after the scrubber unit. The scrubber unit
consists of a quench cooler whichcools the raw gas by injecting
water. The cooled productgas subsequently moves through a column
lled with Pall14rings in counter-current ow to the washing liquid.
Exp. Data TrendEntrained nes and condensed tars are collected in
the LHV gasliquid which was analysed after each experiment.50 12
The gas sample was dried for analytical purposes, i.e. to
Meas.Trenddetect the main gas components. A time-averaged hydro-
H2gen concentration of about 45 vol% (dry and N2-free gas) CO40
CH4was achieved in this particular run. The cumulated gas
CO2production was measured during the experiment for severaltime
intervals of some 10 min. The dry gas yields obtained30were in the
range of 0.560.7 m3 (std. dry) per kg Stabilatsfor run No. 10 (Fig.
11). The composition corresponds toan average lower heating value
(LHV) of the producer gasof some 13:3 MJ=m3 std:;dryN2 free ; one
third of natural gas20LHV. In Fig. 12, the time-averaged
concentrations fromseveral runs with Stabilats are plotted over the
tempera- 10Fuel: Stabilatture in the pyrolysis zone near the fuel
feeding port. AfterSteam/Fuel: 0.15 - 0.5 kg/kgrst start-up of the
equipment, the gasier was operated N2 is used for inerting the lock
hopper fuel feeding systemsystematically from low to higher
temperatures. The0preferred operating point is in the range of
high600700800900temperatures where a maximum gas yield and
highTemperature pyrolysis zone in Chydrogen content can be
obtained. The effect of tempera-Fig. 12. Concentrations of gas
produced in the processing of Stabilats.ture in the pyrolysis zone
of the gasier on gas compositionand heating value is shown in Fig.
12. The evaluation of all the experimental data obtained tofuel
feeding was only involved during the fuel operationdate
substantiates the repeatability of the measured values.period of
each run. However, the results showed that theOne run usually takes
about 24 h from start-up of the coldequipment components worked as
planned for an overallplant to shutdown, during which the period of
fueltime of approximately 450 h including some 100 h of
fueloperation was usually in the range of 68 h. Part of
theoperation. The next targets are:equipment, e.g. that needed for
the solid circulation such asloop seal, bed material screw and
uidized bed, worked forto increase fuel operation time which is
mainly athe whole run time, whilst the equipment concerned
withquestion of the availability of scientic staff; 12. ARTICLE IN
PRESS106 S. Hamel et al. / Energy 32 (2007) 95107 to vary further
operating parameters in order tostoffen/ Trockenstabilats in der
Mono-/Mitverbrennung). Presenta-determine their inuence on gas
quality;tion at VDI-Seminar (46-56-02) Abfall- und Kostenmanagement
furVerbrennungsanlagen, Bamberg, 2122 June 2001 [in German]. to use
different bed materials in order to improve gas[5] Wengenroth K.
New developments in the dry stabilate
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in the optimization of the process is toProceedings of an expert
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installation of a tar measurementbiomass and waste. Strasbourg,
France: 2003. ISBN:1-872691-77-3.[7] Teislev B. Harboore-Woodchips
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directly after the gasicationengines operating at 32% power
efciency in CHP conguration. In:reactor. To date, tar yields and
compositions are detectedProceedings of 12th European conference on
biomass for energy,only by analysing the quench water and washing
liquid.industry and climate protection. Amsterdam, Netherlands:
2002.Tar values so far obtained give only a lump-sum yield.p.
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experiment, [8] Klass DL. Biomass for renewable energy, fuels, and
chemicals. NewYork: Academic Press; 1998.whether from unsteady
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Holzvergaser-Technik, Landesgewer-parallel-arranged approaches
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[in German]. advantages. In general, parallel-arranged generation
of heat[13]Scur P, Rott A. Environmental compatibility and plant
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[14]Henriksen U, Ahrenfeldt J, Kvist Jensen T, Gobel B, Bentzen
JD,Hindsgaul C, et al. The design, construction and operation of a
75 kW In the Herhof-IPV-Verfahrens presented here, the two-two
stage gasier. Energy 2005, in press,
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according to the Carbo-required specic solutions for several plant
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