Systematic Assessment of Straw as Potential Biogas Substrate in …756500/FULLTEXT01.pdf · The economic calculations show that it is proﬁtable to use straw with manure in a farm-based

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oping University

Department of Management and Engineering

Master’s thesis at Biogas Research Center and Biogas i Vadstena

Systematic Assessment of Straw asPotential Biogas Substrate inCo-digestion with Manure

Author:

Sutina Duong

Examiner:

Jonas Ammenberg

Supervisor:

Roozbeh Feiz

July 7, 2014

ISRN: LIU-IEI-TEK-A–14/01910—SE

Sammanfattning

Detta examensarbete har utförts i samarbete med Biogas Research Center (BRC) och företaget

Biogas i Vadstena. Målet med examensarbetet var att systematiskt utvärdera nya substrat för

biogasproduktion. Specifikt för det här fallet var att undersöka potentialen för halm i samrötning

med gödsel och flyt fr̊an svin, höns och nöt. Halm är intressant att utvärdera d̊a det tillhör andra

generationens biomassa och finns tillgängligt i stor mängd. Även rötning av gödsel är givande

d̊a den spontana metanemissionen uteblir och det ger en bättre gödselhantering. Det har satts

upp m̊al inom s̊aväl EU som i Sverige att mer förnybart bränsle bör produceras för att minska

växthusgasutsläppen fr̊an fossila bränslen.

Metodiken som använts har framarbetats av BRC. Det innebär att substrat granskas utifr̊an ett

flertal nyckelomr̊aden, s̊asom beskrivning och mängd biomassa, gasutbyte, synergie↵ekter, teknik,

ekonomi, miljöp̊averkan och energisystem, konkurrerande intressen och institutionella faktorer.

Dessa har utvärderats genom litteraturstudier och studie av fallet Biogas i Vadstena. Utifr̊an

resultatet görs en övergripande bedömning av substratet.

Resultatet visar att halm inte är lämpligt att röta enskilt p̊a grund av högt TS-värde, högt kolin-

neh̊all och att den är näringsfattig. Halm best̊ar även till stor del av lignocellosa-strukturer som

är sv̊ara att bryta ned, i synnerhet lignin. Mekaniska, termiska, kemiska och bioglogiska förbehan-

dlingar kan öka tillgängligheten och nedbrytbarheten av halm. Det kan även öka metanpotentialen

i vissa fall. Däremot fungerar halm bra som ett komplement i samrötning med gödsel som är ett

kväverikt substrat. Det finns teknik för rötning av halm för hela biogasprocessen, fr̊an transport,

förbehandling och rötning till uppgradering. Dock finns utrymme för tekniken att utvecklas ytterli-

gare. De ekonomiska beräkningarna visar att det är lönsamt att använda halm tillsammans med

gödsel i en jordbruksbaserad biogasanläggning för fordonsgasproduktion. Vidare visar beräkningar

för energisystemet att biogasproduktion är energie↵ektiv med energi input/output-kvot p̊a 18-23%.

Förutom fordonsgas produceras även biogödsel som är ett miljövänligt alternativ till konstgjord

gödsel.

Sammanfattningsvis, det är möjligt att producera biogas av halm tillsammans med gödsel och

det är fördelaktigt ur en s̊aväl miljömässigt som ekonomiskt perspektiv.

Abstract

This work was carried out at Biogas Research Center (BRC) and the company Biogas in Vad-

stena. The aim was to systematically evaluate new substrates for biogas production. In particular,

this case investigated the potential of straw in co-digestion with manure and slurry from pig, chicken

and dairy. Straw is interesting to evaluate since it is second generation biomass and available in

a large quantity. Also, anaerobic digestion (AD) of manure is beneficial because it deals with the

spontaneous methane emission and leads to a better manure handling. Goals within the EU as well

as in Sweden have been set up to reduce greenhouse gas emissions from fossil fuel and to produce

more renewable energy.

The methodology used is outlined by BRC in which a number of key areas, such as description of

biomass, amount biomass, gas yield, technology, economy, environmental performance and energy

system, competing interests and institutional factors, have been evaluated through literature studies

and case study Biogas in Vadstena. Based on the results an overall judgment is done to determine

the potential of straw.

The result shows that straw is not appropriate to digest solely because of high TS, high car-

bon content and lack of nutrients. Straw also has lignocellulosic structures, which are di�cult

to break down. Especially lignin limits the biodegradability. Mechanical, thermal, chemical and

biological pretreatments can increase the availability and biodegradability in the straw. In some

cases pretreatment can also increase the methane potential. However, straw works well as a carbon

complement in co-digestion with manure, which is a nitrogen-rich substrate. There are technolo-

gies available for AD of straw and manure for the whole biogas process, from transportation and

pretreatment to digestion and upgrading. Although, there is space for further development of pre-

treatment and upgrading technology. The economic calculations show that it is profitable to use

straw with manure in a farm-based biogas plant for vehicle gas production. Furthermore, the cal-

culations of the energy show that biogas production is energy e�cient with energy input/output

ratio of 18-23%. Besides production of biogas, the digestate could be used as an environmentally

friendly fertilizer.

In summary, it is possible to produce biogas from straw together with manure, and this is

beneficial from both an environmental and economic perspective.

Acknowledgement

I would like to give thanks to my supervisor Roozbeh Feiz and examiner Jonas Am-

menberg at the Department of Management and Engineering at Linköping University

for their guidance. Thanks to all members in the EP2 Biogas Research Center for

both the expert inputs and the not-so-rational-but-rather-amusing-comments; I value

them equally much. I also want to thank Thomas Malmström at Biogas i Vadstena for

helping me providing information.

Linköping, June 2014

Sutina Duong

Explanations

Abbreviations

AD - Anaerobic degradation

BRC - Biogas Research center

CH4 - Methane gas

C/N ratio - Carbon-nitrogen ratio

CO2 - Carbon dioxide

Digestate - The content that is left in the fermenter after biogas degradation

EP2 - project group within Biogas Research Center

GHG - Greenhouse gas

H2S - Hydrogen sulfide

HRT- Hydraulic retention time

iLUC - Indirect land use change

LBG - Liquid biogas

LUC - Land use change

MCA - Multi-criteria analysis

NOx

- Nitro oxides

NPV - Net present value

OLR - Organic loading rate

Straw- The stem of dried crops such as wheat

TS - Total solids, dry matter

VS - Volatile solids, organic matter

ww - Wet weight

Units

GJ - Gigajoule

ha - Hectare

kWh - Kilowatt hour

MJ - Mega joule

Nm3 - Normal cubic meter, pressure = 1 atm and temperature = 0 °C.

1 Nm3 CH4 = 9,8 kWh

1 MWh = 3,6 GJ

Contents

1 Introduction 1

1.1 Biogas Research Center . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Purpose and research questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Background to Biogas 4

2.1 The biogas process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2 Important factors and parameters for biogas production . . . . . . . . . . . . . . . 5

2.3 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3 Case Description 9

4 Methodology 11

4.1 Delimitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.2 Criticism to method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5 Description of the Feedstock 13

5.1 Straw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

5.2 Manure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

5.3 Co-digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

5.4 Key area synthesis: Description of the feedstock . . . . . . . . . . . . . . . . . . . . 17

6 Amount of Biomass 18

6.1 Amount of straw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

6.2 Amount of manure and slurry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

6.3 Key area synthesis: Amount of biomass . . . . . . . . . . . . . . . . . . . . . . . . 19

7 Gas yield and Potential Amount of Biogas 20

7.1 Methane potential and potential energy yield for Biogas i Vadstena . . . . . . . . . 20

7.2 Study: Algorithm to predict biodegradability and biochemical methane potential . 21

7.3 Key area synthesis: Gas yield and potential amount of energy . . . . . . . . . . . . 21

8 Other Products 23

8.1 Key area synthesis: Other products . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

9 Technology 24

9.1 Storage and pretreatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

9.2 Technology in the biogas process . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

9.2.1 Study: Solid-state plant in Trelleborg, Sweden . . . . . . . . . . . . . . . . . 27

9.2.2 Study: Co-digestion of swine manure with crop residues . . . . . . . . . . . 27

9.2.3 Study: The impact of pretreatment and process operating parameters . . . 28

9.2.4 Heating in Biogas i Vadstena . . . . . . . . . . . . . . . . . . . . . . . . . . 29

9.3 Upgrading to transportation fuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

9.4 Key area synthesis: Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

10 Economy For the Producer 33

10.1 Study: Lignocellulosic material for biogas production . . . . . . . . . . . . . . . . . 33

10.2 Study: Techno-economic assessment of agricultural based biogas production . . . . 33

10.3 Economic calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

10.4 Key area synthesis: Economy for the biogas producer . . . . . . . . . . . . . . . . . 39

11 Environmental Performance and Energy System 40

11.1 Environmental impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

11.2 Nutrients and soil quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

11.3 Environmental aspects of Biogas i Vadstena . . . . . . . . . . . . . . . . . . . . . . 41

11.4 The energy system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

11.5 Key area synthesis: Environmental performance and energy system . . . . . . . . . 45

12 Competing Interests and Suppliers 46

12.1 Land use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

12.2 The interest for straw . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

12.3 The suppliers’ role . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

12.4 Key area synthesis: Competing interests and suppliers . . . . . . . . . . . . . . . . 46

13 Institutional Factors and Other Societal Aspects 47

13.1 Institutional aims . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

13.2 Government financial support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

13.3 Certification of biofertilizer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

13.4 Swedish standardization of vehicle gas . . . . . . . . . . . . . . . . . . . . . . . . . 48

13.5 The biogas consumers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

13.6 Key area synthesis: Institutional factors and other societal aspects . . . . . . . . . 48

14 Semi-qualitative Assessment 49

15 Concluding Discussion 51

A Appendix: Calculations 58

B Appendix: The BRC-matrix For Straw 62

1 Introduction

There are several environmental challenges today such as eutrophication, pollution of air and wa-

ter and climate change. The Intergovernmental Panel on Climate Change (IPCC) of the United

Nations (IPCC, 2013) states:

”Warming of the climate system is equivocal, and since the 1950s, many of the

observed changes are unprecedented over decades to millennia. The atmosphere

and ocean have warmed, the amounts of snow and ice have diminished, sea level

has risen, and the concentrations of greenhouse gases have increased.”

Global warming is said to be caused by greenhouse gases. The GHGs are, for example, methane

(CH4), nitrous oxides (NOx) and carbon dioxide (CO2) (Chandra et al., 2012), which are emitted

from combustion of fossil fuels and agricultural practices (UNEP, 2012). There is a significant risk

that the climate changes impact the nature and human systems on all continents and oceans on

earth (IPCC, 2014).

The GHG emissions add to the greenhouse e↵ect by excess GHG in the atmosphere, which

are not being taken up by plants. The GHG emissions also cause eutrophication, acidification and

toxicity (van der Voet et al., 2010). The biggest sources of GHG emissions in the world are the

energy sector (29%), industry (18%), transport (13%) and agriculture (11%) (UNEP, 2012). In

addition, the deforestation leads to reduced carbon dioxide uptake of trees and plants.

In order to reduce the GHG and work for a more sustainable environment in the European Union

(EU) has an adaption policy where environmental protection, water management and land planning

are some of the issues (IPCC, 2014). On a national level, Sweden aims to lower the emissions of GHG

with 40% by 2020 compared to 1990 (Näringsdepartementet, 2012). This corresponds to a decrease

to 20 million ton carbon dioxide equivalents per year. About one fourth of all energy use in Sweden

is for transportation (Energimyndigheten, 2013a). The main sources for road transportation are

fossil fuels such as petrol and diesel. Since fossil fuels are consumed faster than they are generated

more renewable energy and environmentally friendly fuels are needed to meet the energy demand

and reduce the GHG emissions (Weiland, 2010). There are several alternative fuels from organic

material, which are also known as biofuels. Some examples of biofuels are biodiesel, bioethanol and

biogas. This work will look more into biogas.

Biogas is methane gas and is also known as biomethane (Angelidaki et al., 2011). Biometha-

niation occurs naturally as a result of anaerobic degradation of organic materials, for example,

in landfills, sediments and in the intestines of animals. Artificial biogas is produced from sewage

sludge, slaughterhouse waste, household waste and biomass from the agriculture (Carlsson and

Uldal, 2009) and other types of feedstock.

In Sweden, the biogas use has increased steadily during the last ten years, especially in the public

1

transportation sector (Energimyndigheten, 2013b). Despite this increase, the share of renewable

fuels was only 8,1% of the total fuel in 2012, and only 12% from that portion is from biogas. There

is space to enhance the biogas production, both by optimization of the production on existing

biogas plants and examine new feedstock that can be used.

Research has been done to find more potential feedstock for biogas production. Particularly

second generation biomass is of interest, i.e. renewable, inedible biomass (Chandra et al., 2012).

Biogas production of second generation could reduce the dependence of fossil fuel combustion. It

also recycles the CO2, since biomass absorb CO2 during growth and emits it when combusted. In

contrast, fossil fuels have a much longer regeneration time and therefore contribute to a higher

netto CO2 emission when combusted. Examples of second generation biomass are manure and

lignocellulosic material from agriculture residues, such as ley, corn stalks, sugar beet tops and straw.

Lignocellulosic materials are available in big amounts (Isroi et al., 2011). Straw is of particular

interest, as wheat straw is the second most abundant crop residue in the world (Wu et al., 2010).

Straw has a certain energy potential, but it is known that there are some di�culties to convert the

lignocellulosic biomass to biogas due to the chemical structure (Isroi et al., 2011).

There are studies about straw (mostly wheat straw) respectively manure as biogas substrate,

but few about straw in co-digestion with farm animal manure (Wang et al., 2012). It is of interest to

examine this combination, because it can be applied on farm-based biogas plants. Co-digestion of

manure and straw can enhance the digestion e�ciency and there are possibly technical, economical

and ecological benefits to mix di↵erent feedstock.

1.1 Biogas Research Center

This work is a part of the EP2-project within Biogas Research Center (BRC). BRC is a competence

center and a cooperation between Linköping University, Energimyndigheten and several organiza-

tions and companies in Sweden (BRC, 2014). The ”E” in EP2 indicates that it is an exploratory

project. EP2 works with systematic assessment of feedstock for expanded biogas production. In

general, most feedstock assessments have a quite narrow and specific focus in, for instance technical

and biochemical performances. The delimited approach provides information about only a small

part in the biggest process and no overview. However, EP2 recognized this problem and aims to

give a broader picture about a certain feedstock by gathering information about key areas such as

technology, economy, environmental impact and institutional factors in a systematic matrix. The

pros and cons are assessed to provide an overall judgment about the feedstock. The matrix may

be useful for biogas producers as well as a basis or decision making in politics. This work is a part

of the EP2-project.

2

1.2 Purpose and research questions

This work has two major purposes. One is to assess new potential biogas feedstock; this specific

work aims to assess straw. The other purpose is to apply and evaluate the methodology developed

by EP2.

The main research questions are:

• What possibilities and obstacles are there to use straw as feedstock in co-digestion with

manure and slurry from pig, chicken and dairy?

• Would straw be worthwhile as biogas feedstock?

• How well does the EP2-methodology work to assess potential biogas feedstock?

3

2 Background to Biogas

Biogas is produced from organic material, and it mainly consists of methane (CH4) and carbon

dioxide (CO2) (Angelidaki et al., 2011). Other products are ammonia (NH3), hydrogen sulfide

(H2S), hydrogen (H2), water and nitrogen (N2). This section describes the biogas process, what

factors impact on the biogas production and the applications of biogas.

2.1 The biogas process

The biogas process involves anaerobic degradation (AD) of feedstock performed by microbes in a

tank fermenter. In the sludge, also called inoculum, there are a wide variety of microorganisms

that metabolize di↵erent substances. Examples of these are acetogens and methanogens (Weiland,

2010). In Figure 1 an overview of the biogas process is shown.

The methane formation can be divided into four biochemical reactions:

1. Hydrolysis

The carbohydrates, proteins and lipids are depolymerized. Enzymes produced by microor-

ganisms break down the substrate. Carbohydrates give monosaccharides, proteins give amino

acids, and lipids give long fatty chain and glycerol. Hydrolysis is often the rate determining

step.

2. Fermentation acidogenesis:

The monomers from the previous step are fermented by microbes and form carbon dioxide,

volatile fatty acids (VFA), alcohols and ammonia.

3. Acetogenesis

In acetogenesis, the alcohols, long-chain organic acids and fatty acids are converted to acetate

and H2. This step also gives CO2.

4. Methanogenesis

In this step, the acetate or CO2 plus H2 are converted to methane by methanogens. There

are two di↵erent chemical reactions:

a) Hydrogenotrophic methanogenesis with carbon dioxide: CO2+ H2 => CH4 + H2O

b) Acetoclastic methanogenesis with acetic acid: CH3COOH => CH4 + CO2

4

Figure 1: The figure shows the main steps in anaerobic degradation (de Mes et al., 2010).

2.2 Important factors and parameters for biogas production

The biogas process is complex and there are many factors that can a↵ect the choice of technology

and production. The gas yield depends on, for instance, the retention time, degradability of sub-

strates, organic loading rate, carbon-nitrogen ratio and temperature. Di↵erent parameters, such as

total solids (TS), volatile solids (VS), pH, volatile fatty acids (VFA), and the methane produced

can be measured to monitor and control the process (de Mes et al., 2010). The parameters are

described in more detail below.

Hydraulic retention time

The hydraulic retention time (HRT) is the time the sludge is in the tank fermenter (Pind et al.,

2003). HRT is used to give the volumetric loading of a tank. In general, a longer HRT results in

higher total volatile solid mass reduction, which in turn yields more biogas per unit of feedstock

(Chandra et al., 2012). Methanogens often have a long retention time and the HRT should be at

least 10-15 days to avoid washing out of them.

Feedstock and degradability

Feedstock can be divided into first, second and even third generation (Murphy et al., 2011).

The first generation is biomass from energy crops that also can be used in food production

5

like sugar beets, corn and rape seed oil. Second generation biomass is biomass with lignocellulosic

content such as crop residues, manure, and does not compete with food competition. An example

of third generation feedstock is algae.

The theoretical methanogenic potential di↵er among di↵erent biomass feedstock depending on

the degradability and carbon-oxidation state (Angelidaki et al., 2011). A high degradability and low

oxidation state yields more methane. The prediction of gas potential is central in AD to conclude

if it is worthwhile and can be calculated if the substrate composition is known. The practical

potential is always lower than the theoretical potential because of insu�cient nutrients available,

inoculate (microbial) activity, toxicants or heterogeneous substrate. The degradation pace can be

faster by mechanical, thermal, chemical or enzymatic pretreatment of the feedstock (Bruni et al.,

2010).

Organic loading rate

The organic loading rate (OLR) is the amount of VS or chemical oxygen compound (COD) com-

ponents that are fed per day per unit digester tank volume (Chandra et al., 2012). A higher OLR

may reduce the tank fermenter volume and thus investment cost.

The carbon-nitrogen ratio

The carbon-to-nitrogen (C/N) ratio is important for the fermentation process. A C/N=30 is benefi-

cial for the microorganism metabolism (Carlsson and Uldal, 2009). However, an exceeded nitrogen

level with C/N at 10-15 results in ammonium formation and high pH, which might be toxic for the

microbes. At C/R ratio higher than 30 the degradation process decreases. To regulate the ratio

a certain feedstock can be complemented by co-digestion of with other substrates with di↵erent

C/R.

Temperature

AD is often done in mesophilic or thermophilic conditions (Weiland, 2010). Mesophilic is at 35-

42°C and thermophilic is at 45-60°C. Changes in temperature may impact on the AD negatively.

The retention time is about 15-30 days in mesophilic conditions (de Mes et al., 2010). Anaerobic

bacteria are most active in mesophilic or thermophilic conditions (Chandra et al., 2012). However,

methanogens are sensitive to temperature changes and are inhibited at temperatures between 40-50

°C.

TS and VS

TS, also called dry matter (DM), is measured by drying the sludge 1 hour at 103-105°C and

indicates the measure of organic and inorganic matter content (Pind et al., 2003). TS could be

6

measured before and after the process to monitor the process e�ciency . Feedstock with high TS,

about 10-15% needs to be diluted before it could be treated in pumps and be stirred (Carlsson

and Uldal, 2009). However, there may be exceptions among some substances, e.g. pure glycerol has

100% TS but is pumpable.

VS, also called organic matter, is measured by drying at 550 C° for 1 h and is a measure of

organic matter only. (Pind et al., 2003). Generally a high VS value indicates a high gas yield as

it is only the organic matter that contribute to the biogas production (Carlsson and Uldal, 2009).

Therefore, a low VS results in an ine�cient use of the tank fermenter. On the other hand, a high

VS does not always give high methane yield as some of the VS cannot be degraded like in the case

of lignin and plastic.

Co-digestion of substrates with di↵erent TS and VS levels can balance the organic matter

content as well as the fluidity. It is of importance to determine the TS and VS separately in

di↵erent substrate and also to continue the measurements of the mix during the biogas process.

pH and VFA

In the biogas process pH is an important parameter for the performance (Chandra et al., 2012).

An optimal organic loading rate can help keep the pH within an acceptable range. The pH is an

indicator of the degradation e�ciency (Pind et al., 2003).

The pH is related to volatile fatty acid (VFA) level as fatty acids lower the pH (de Mes et al.,

2010). Many bacteria are sensitive to extreme pH levels, and especially the methanogens require

a neutral pH 6,5-7,5. The pH level can be regulated by adding base or acid (Pind et al., 2003). A

pH outside 6,0-8,5 starts a toxic e↵ect on methanogens (Chandra et al., 2012).

Wet digestion and dry digestion

There are two sorts of digestion; wet digestion and solid-state digestion. Wet digestion is often

applied when the organic material i.e. TS is less than 10% (Weiland, 2010). The slurry can be

stirred in the tank reactors. Mesophilic conditions are most common for wet digestion. Mainly wet

digestion in is considered in this work because it is most common in the agricultural sector.

Dry digestion, or solid-state digestion, is anaerobic degradation with biomass containing 10-40%

TS, which has a too high viscosity to be pumped, mixed or homogenize (ibid.). Batch fermentation

is applied for dry digestion and the process water is recycled and poured over the biogas substrate.

Other parameters

Other parameters that could be of interest to keep track of are the level of ammonia, heavy metals,

sulfide and xenobiotics because these substances are toxic in high concentrations (due Mes et al.

2010)

7

2.3 Applications

Biomethane can replace natural gas and be used as vehicle fuel, heat, electricity and in the pro-

duction of chemicals (Weiland, 2010). The biogas produced contains only 60-70% methane and has

to be upgraded to at least 95% before it can be used as a transport fuel and/or be injected in the

natural gas grid (Energimyndigheten, 2013a).

Below, Figure 2 illustrates the biogas production cycle for lignocellulosic material.

Biosynthesis*

Lignocellulosic*materials*

Pretreatments:*Mechanical*Thermal*Chemical*Biological*

Anaerobic*fermenta;on:*1.  Hydrolysis*2.  Acidogenesis*3.  Acetogenesis*4.  Methanogenesis*

Biofer;lizer*

•  Solubilize*or*remove*lignin*•  Reduce*the*crystallinity*of*celluloses*•  Increase*accessible*surface*area*•  Reduce*degree*of*polymeriza;on*of*

hemicelluloses*

CO2****Biofer;lizer*

CH4*

Figure 2: The principles of biogas production from lignocellulosic biomass. Information from (Monlau

et al., 2013).

8

3 Case Description

Thomas Malmström, who is a farm owner in Vadstena and one of the owners of Biogas i Vadstena

has provided information about the start of the company. Vadstena is situated in the southern part

of Sweden with many agricultural farms in the surrounding area.

The farms are farming hens and pigs mostly, and cows to a smaller extent. The farms also

cultivate cereal crops like wheat, rye, barley, flax and rape. The crop rotation is quite similar in

the di↵erent farms. The ears of corn are used as food and fodder. One important incentive to start

a biogas plant was to better handle the big amount of manure and slurry. According to Thomas,

there were only he and two other farmers in the beginning that were interested to start a biogas

production. Initial calculations showed no economic viability with these three farms, because the

capacity was too small to cover heat and electricity deliveries and the investment cost of upgrading

the system.

Later in 2010-2011, a pre-study project was performed to investigate the conditions for a bigger

biogas plant where 28 animal farms close to Vadstena were involved. The pre-study was done by

energy consultants in Hush̊allningssällskapet and Lovanggruppen and the result showed economic

viability (Halldorf and Örup, 2011). However, not all the farms were ready for the investment, thus

11 of the farms formed the company Biogas i Vadstena (Malmström, 2014). All the animal farms

are located within a radius of 10 km, see Figure 3 (Halldorf and Örup, 2011). The biogas plant

would preferably be placed near a good road and be convenient for transportation of manure into

and biofertilizer out from the plant. Other factors that have to be considered are the transportation

of biogas and the closeness to the contingent heat source. A self-owned heating system would allow

more freedom in the choice of location.

An application for an investment grant was done and approved, but then reversed and with-

drawn almost instantly (Malmström, 2014). The interest cooled o↵ a bit, but the project was then

given 250 000 SEK for an in-depth study, which is conducted during the year 2014. This study

will evaluate the conditions of building a biogas plant based on manure and equipped with an

upgrading system.

9

Figure 3: An overview map of county of Vadstena with a circle of 10 km drawn. The needle only

shows the distance and not the location of the where the biogas plant possibly is to be built (Halldorf

and Örup, 2011).

There is a big supply of straw from the cereal cultivation, which is a potential biomass resource.

Therefore, it is of interest to investigate if it is possible to co-digest with manure to produce biogas.

Currently, the straw is mainly ploughed down back to earth to recycle the nutrients. There is not

a big market for straw. A small part of the straw produced is pressed and sold for heating in farm

boilers. According to Thomas, about 25% of the straw could be devoted to biogas production.

Collection and storage of straw are important issues to address if straw is going to be used as

feedstock.

The plan for Biogas i Vadstena is to upgrade the biogas for use in vehicles and to involve the

public transportation company Östgötatrafiken as a stable consumer.

10

4 Methodology

This chapter describes the methodology, the scope and delimitations, and contains methodological

reflections. The methodical approach is largely determined by the BRC context. The EP2 method-

ology is best described as a Multi-Criteria Assessment (MCA) with semi-qualitative judgment in

di↵erent key areas. In contrast to other substrate evaluations, studies in lab-scale and pilot-scale,

life-cycle assessments or energy balances, this method is much broader. This approach is unique

for biogas feedstock assessment. However, a similar wide approach has been used in a life-cycle as-

sessment of cement by Feiz (2014). There is an MCA-matrix designed in Excel for this systematic

assessment with several key areas. The key areas in the systematic assessment are:

• Description of the feedstock

• Amount of biomass

• Gas yield and amount potential biogas

• Other products

• Technology

• Economy for the producer

• Environmental performance and energy system

• Competing interests and suppliers

• Institutional factors and other societal aspects

However, the MCA is complex and therefore each key area is synthesized to summarize the

most important information and make an overall synthesis. See the MCA-matrix for this case in

Appendix B. The synthesis contains the most important and relevant information in a specific

key area and is a base for the semi-qualitative assessment. There is also a matrix for the semi-

qualitative judgment, in which the reliability and relevance of data are ranged from very low-very

high. More about the semi-qualitative matrix can be read in Chapter 14. The whole working process

is iterative.

The methodology contains the following steps:

1. Selection of feedstock and case

Straw is assessed in this work. The specific case is the company Biogas i Vadstena. The case

is interesting because there are many farms within a small area with big amount of potential

biogas feedstock. There are similar farms in Sweden, which also need better manure handling

and have excess straw, which may consider to start-up a biogas production too. Data from

11

Thomas have been provided via email and a meeting. The data were used to find out if the

use of straw is economically viable.

2. Literature review for each key area and sub-area

Literature studies were done to find both quantitative and qualitative information to the

MCA-matrix. The literature search took place mainly in scientific databases (e.g. Web of

Science and Scopus). Firstly, a wide search of articles and other literature was done to find

relevant sources. The search results were stored in the reference program Zotero, which is used

in the whole BRC. Secondly, the literature was categorized, tagged and prioritized according

to subject and relevance.

3. The information is analyzed and documented in respectively chapter and part in the matrix.

4. A synthesis is done for each chapter

This information is used to make a semi-qualitative assessment of the gathered information.

There have been some co-operations during the work. The literature search was partially done

together with two other master students in the EP2-project to work more e�ciently. Discussions

have also taken place with these fellow students. Moreover, there have been two workshops with the

EP2-project group about the methodology, which has been under development. The first workshop

was about the MCA-matrix and the second one about the semi-qualitative matrix. During the

workshop feedback and inputs have been given from the biogas experts in EP2.

4.1 Delimitations

Only straw is evaluated in this work and its role in co-digestion with manure. The work is done

during 20 weeks in spring 2014. Further, this study considers a case in the southern part of Sweden

with certain geographical, agricultural and infrastructure conditions. The data is estimated to

provide an of the potential of straw.

4.2 Criticism to method

The method used consisted mainly of literature search, which can be a delimitation since studies

has been carried out in with di↵erent method designs. This makes it di�cult to provide a certain

potential for a certain feedstock. Assumptions about the biogas process conditions have been made

and used in the economic calculations. Further, the BRC method is designed to assess a single

feedstock and not cases with co-digestion of two or more feedstock. Thus, the matrix can in some

cases simplifying practical cases of reality. This case is restricted to the conditions of Biogas I

Vadstena, and can be hard to apply in other locations and cases. Systematic assessment is a very

wide approach, which could be both a strength and a weakness. It provides a whole picture, but it

could be discussed what role the small details has overall judgment.

12

5 Description of the Feedstock

Both straw and manure are considered as second generation substrates (Murphy et al., 2011).

The properties of straw and manure are described below with the main focus on straw and its

biodegradability. Thereafter, co-digestion is described and discussed.

5.1 Straw

Straw is the part of the cereal crop without the kernel and is an agricultural crop residue. In the

case of Biogas i Vadstena, the straw is from wheat, rye, barley, rape seed and flax. Straw is built

up of mostly cellulose, and to a smaller extent also hemicelluloses and lignin (Monlau et al., 2013).

In literature, the TS and C/N varies between di↵erent cereals and also for a certain crop the values

of these parameters di↵er. For instance, for wheat straw TS varies between 79,6-91,3 % (Chandra

et al., 2012). However, in this case the same TS is assumed for all types of straw in the calculations.

The TS and C/N are high in straw, about 78% TS and C/N ratio = 90 (Carlsson and Uldal, 2009).

The lignocellulosic compounds of wheat straw, barley straw and rye straw can be seen in Table 1.

Figure 4: The figure shows a straw bale (Strawbale, 2014).

13

Cellulose

Cellulose consists of glucose subunits, and has parts with crystalline structure and parts with not

well-organized structure (Hendriks and Zeeman, 2009). The glucose chains are bundled together in

cellulose fibrils. These cellulose fibrils are bound by weak hydrogen bonds. Cellulose is not soluble

in water and most organic solvents. However, it could be broken down by acids at high temperature

into sugars (Monlau et al., 2013).

Hemicellulose

Hemicelluloses are the heteropolymers of polysaccharides in plant cell walls (Monlau et al., 2013).

Hemicellulose is complex in structure and consists of pentoses (e.g. xylose and arabinose), hexoses

(e.g. mannose, glucose and galactose) and sugar acids (Hendriks and Zeeman, 2009). In contrast

to cellulose crystallinity, the alignment of hemicellulose is random with little strength (Monlau

et al., 2013). Xylose is the most abundant sugar monomer in hemicellulose. Hemicellulose is the

most thermo-chemical sensitive component of the lignocellulosic compounds (Hendriks and Zee-

man, 2009). The descending order of solubilization in hemicellulose compounds is mannose, xylose,

glucose, arabinose and galactose. Compounds of hemicellulose are solubilized in 150-180°C in water.

The solubilization also depends on pH and moisture content.

Xylan is the dominant hemicellulose component in agricultural plants like straw (Hendriks and

Zeeman, 2009). The xylan can be extracted in alkaline or acid conditions. Hemicellulose connects

cellulose and lignin fibers and provides the whole lignocellulosic network rigidity.

Lignin

Lignin consists of cross-linked network of hydrophobic polymers in plant cell walls (Monlau et al.,

2013). Lignin and lignin-carbohydrate are insoluble in all solvents and to some extent resistant to

anaerobic degradation and this limits the digestibility of lignocellulosic biomass. Lignin provides

structure and rigidity of plants as well as impermeability and resistance against microbial invasion.

In similar to hemicelluloses, lignin starts to dissolve in water at 180°C. The solubility in di↵erent

pH is determined by the precursor on the lignin.

Lignin is an important component to return to the soil for the humus content (Linné et al.,

1999). Therefore, some of the straw has to be plough down or some lignin must be intact from the

digestate.

Other factors in lignocellulosic mass

Other factors that a↵ect accessibility and biodegradability in lignocellulosic biomass are the degree

of polymerization, the crystallinity of the cellulose, the structure of hemicellulose, the structure of

the surface area and the pore volume (Monlau et al., 2013).

14

Table 1: The table shows the lignocellulosic content in wheat, barley and rye (Monlau et al., 2013).

Lignocellulosic compounds Wheat straw Barley straw Rye straw

Celluloses (%) 39,6 37,5 38,0

Hemicelluloses (%) 26,6 62,8 36,9

Lignin (%) 21,0 16,0 17,6

Degree of polymerization 1547 2085 1439

Crystallinity index 50,3 25,3 n/a

5.2 Manure

Manure works well as a base for biogas production, as it naturally has a good composition of

nutrients and contains important minerals that are suitable for biogas production (Carlsson and

Uldal, 2009). The main component in manure is carbohydrates followed by proteins and lastly fat.

Manure from chicken and pig are better than from ruminants because the manure is already partly

anaerobically degraded in the stomach of ruminants.

Manure naturally emits methane due to self-composition if it is left in a heap and thus the

methane is released to the atmosphere (Chandra et al., 2012). This can be avoided if manure

is used for biogas production instead. Both manure and slurry are considered, where the slurry

contains more water.

Pig manure and slurry

Pig manure is very rich in minerals. However, the minerals also tend to sediment and lay on the

bottom of the tank, which can be problematic in the biogas process. The TS is about 8% and VS

about 80% of TS (Carlsson and Uldal, 2009). Like chicken manure, manure from pig also has a

high nitrogen concentration and this may cause ammonia inhibition if digested solely. The C/N

ratio is 23 for manure and 5 for pig slurry (ibid.).

Chicken manure

Manure from chicken has a high level of phosphorus and nitrogen (Babaee et al., 2013). Due to

the latter the C/N ratio is about 3-10, which is very low and therefore not suitable to digest alone

(Carlsson and Uldal, 2009). Chicken manure also contains high levels of P. The TS is about 20-25

% and VS about 75% of TS. Moreover, feathers may cause the floating crust and sand may result

in sedimentation.

15

Dairy slurry and dairy litter straw

A small portion of the manure available is dairy slurry and dairy litter straw. Manure from dairy

di↵ers from chicken and pig as the content is already partly degraded be bacteria in the rumen

(Carlsson and Uldal, 2009). This leads to a lower gas yield. The C/N ratio in dairy slurry varies

from 6-20 and about 8% TS (ibid.). The litter may contain sand that sediments.

5.3 Co-digestion

Co-digestion is when two or more di↵erent substrates are in homogeneous mix and is very common

for wet digestion (Carlsson and Uldal, 2009). It is beneficial to mix carbon-rich substrate with

nitrogen-rich substrate to gain an appropriate C/N ratio that is suitable for AD (Chandra et al.,

2012). Also, co-digestion compensate macro and micronutrients, dry matter, pH level and inhibitors

(Wang et al., 2012). Since crop residues have a high C/N ratio the pH is low, has poor bu↵er

capacity and may possibly cause high VFA accumulation during AD. Co-digestion of crop residues

and manure improves the C/N ratio, stabilizes the pH and decreases the ammonia level. The

assumed TS, VS and C/N ratio are summarized in Table 2.

The synergistic e↵ects can enhance the methane potential. In a study with cow manure and

wheat straw, the highest methane yield was when 40% of total solids came from wheat straw (Wu

et al., 2010). It is not economically sustainable to add urea or glucose to adjust the C/N ratio in

large-scale methane production.

A Chinese study investigated the impact of feeding composition and carbon-nitrogen ratios on

the methane yield for co-digestion of dairy, chicken manure and wheat straw (Wang et al., 2012).

The study was done in lab scale. The study showed that co-digestion improved the methane po-

tential compared to individual feedstock. Moreover, the synergistic e↵ect was even better including

both diary and chicken manure with wheat straw than a single manure. A C/N ratio of 25:1 and

30:1 gave a stable pH and low concentration of ammonia.

Table 2: The table shows the TS, VS and C/N ratio in straw and manure.

TS (%) VS of TS (%) C/N ratioStraw 78 91 90

Pig slurry 6 80 5Poultry manure 35 n/a n/aChicken manure 60 76 3-10Chicken slurry 10 n/a n/aDairy slurry 9 80 6-20Dairy litter straw 30 80 HighTotal manure estimation 11 79 8

n/a = no answer

16

5.4 Key area synthesis: Description of the feedstock

The feedstock that has been assessed is straw from wheat, rye, barley, rapeseed and flax, but it is

assumed that all crops have the same TS and C/N. TS is assumed to be 78% TS and C/N ratio =

90. The TS is high for both wet and dry digestion. As mentioned above, the optimal C/N ratio is

around 30. The straw C/N ratio is therefore considered very high and therefore it is not suitable

to digest straw solely. For manure and slurry it is di�cult to be certain about the TS as the water

content varies much and therefore assumptions have been done for further work and calculations.

The means of TS% and C/N ratio for manure are used i.e. 11% respectively 8, see calculations in

Appendix A.

17

6 Amount of Biomass

The estimated values of the agriculture area and straw per hectare for each crop were given from

Thomas Malmström. The date of assessment was in February 2014. The geographical area for straw

and manure is municipal of Vadstena in southern Sweden.

6.1 Amount of straw

The amount of straw is calculated for an upper and a lower case. The lower case includes 8 farms in

the company that can supply straw, see Table 3. In the upper case, there are totally 28 farms that

cultivate cereal crops. The estimation shows that the 8 farms produce about 6100 ton straw/year

and the 28 farms produce 21 000 ton straw/year. Wheat is the most dominating crop with 45% of

agriculture area.

Though, according to Thomas Malmström some of the straw should be ploughed back down

into the soil to recycle the organic matter and nutrients. Therefore, about 25% of the straw could

be reserved as biogas substrate. The lower case i.e. 6100 ton corresponds 25% of the total amount

and is therefore considered to be available.

Table 3: The table shows the amount of straw biomass the a lower and upper case (Malmström,

2014).

8 farms 28 farms

Crop Agriculture

area (%)

Straw

(ton/ha)

Area

(ha)

Amount

(ton)

Area (ha) Amount

(ton)

Wheat 45 3 1 100 3 200 3 800 11 000

Barley 20 2 480 960 1 800 3 300

Rape seed 12 2 290 570 1 000 2 000

Rye 10 4 240 960 840 3 400

Flax 8 2 190 380 670 1 300

Total 95 2 300 6 100 8 000 21 000

6.2 Amount of manure and slurry

The manure and slurry are from 30 di↵erent farms in Vadstena. Pig slurry is the most abundant

animal waste in Vadstena with 69 000 ton/year, followed by chicken manure. The total amount of

slurry and manure is 90 600 ton/year and everything is assumed to be biogas feedstock. In Table

18

4 the amount of manure and slurry is presented.

Table 4: The table shows the amount of manure and slurry biomass (Halldorf and Örup, 2011).

Manure Amount ton/year

Pig slurry 69 000

Poultry manure 5 740

Chicken manure 2 450

Chicken slurry 3 000

Dairy slurry 5 100

Dairy litter straw 5 330

Total 90 600

6.3 Key area synthesis: Amount of biomass

The total amount of straw is 21 000/year ton in Vadstena, but only 6100 tons is considered available

for biogas production. The total amount of manure and slurry is 90 600 ton/year. The whole amount

of manure mix is assumed to be available for biogas production.

19

7 Gas yield and Potential Amount of Biogas

In this chapter the methane potential and potential amount of biogas are presented and discussed

for Biogas i Vadstena. A study about the impact of lignin in gas yield is also presented below.

7.1 Methane potential and potential energy yield for Biogas i Vadstena

The methane potential for straw only is 207 m3 CH4/ton VS (Carlsson and Uldal, 2009), see Table

5. For the lower case, straw from 8 farms, the energy potential is 8,8 GWh. In the upper case the

energy potential is 30,8 GWh/year.

In the case of manure only the methane potential is now known, but the energy potential is

18,1 GWh/year according to Halldorf and Örup (2011). The manure only case is a reference.

However, Biogas i Vadstena has plentiful of manure and straw in itself does not have a favorable

nutrient composition (Carlsson and Uldal, 2009). Therefore the focus is on co-digestion cases. By

using the TS from Table 2 and the amount of biomass in Table 3 and 4 the methane potentials

have been calculated. The methane potential calculations consider two co-digestion cases, one case

with manure+straw (8 farms) and the other case is manure+straw (28 farms), see Table 5. The

methane potential for co-digestion of straw and manure is assumed to be 300 m3 CH4/ton VS

(Berglund Odgner et al., 2012). This particular methane potential is from a lab-scale experiment

with swine manure+30% wheat straw, TS 15%, at 30 °C, 30 days digestion time in a continuously

fed and mixed reactor. Wet digestion is assumed to be applied. Biogas i Vadstena di↵er from

the literature methane potential by a manure mix and slurry mix from hen, swine and cow, as

well as a lower percentage of straw. This is the closest information found to somehow fit the

feedstock of Biogas i Vadstena. However, it is more complex to produce biogas in a full-scale biogas

plant compared to lab-scale, and therefore the literature values are only rough approximations. It

has been shown that co-digestion with manure and up to 10% of straw added is not a problem

(Oosterkamp, 2011). Straw from 8 farms is 6,3% in wet weight of the total available feedstock and

should then not be a problem to add to the manure mix.

The energy potential for manure+straw (8 farms) is 35,9 GWh/year and for manure+straw

(28 farms) is 67,8 GWh/year. The C/N ratio for manure+straw (8 farms) is 14, which is rather

low compared to manure+straw (28 farms), which has a ratio of 27. The low C/N ratio is not

optimal for biogas production. However, the TS is 15% for manure+straw (8 farms) and thus more

convenient for wet digestion in comparison to the upper case manure+straw (28 farms) that has a

TS of 24%. Because of this too high TS, the upper case has been excluded in further calculations,

since wet digestion is assumed to be applied. This is also in line with limited availability of straw

for biogas production. Even the lower case has a slightly too high TS for optimal wet digestion

that is 10%, but the sludge can be diluted to some extent.

20

Table 5: The table shows the methane potential of straw, manure, and two cases of manure+straw.

TS (ton) VS of TS (ton)

CH4/VS (m3/ton)

CH4 (m3 ) Energy (GWh)

Straw, 8 farms 4 800 4 300 2071 898 000 8,8 Straw, 28 farms 16 700 15 200 2071 3 141 000 30,8 Manure only 10 000 7 900 n/a 1 853 000 18,1

C/N ratio TS (%)Manure+straw (8 farms) 14 15 14 700 12 200 3002 3 664 000 35,9 Manure+straw (28 farms) 27 24 26 600 23 000 3002 6 914 000 67,8

1 = (Carlson and Uldal, 2009)2 = (Berglund Odgner et al., 2012)

Methane energy factor = 9,81*10^-6n/a = no answer

7.2 Study: Algorithm to predict biodegradability and biochemical methane

potential

It is known that hydrolysis of lignocellulosic biomass in the AD process are limited by lignin and

hemicelluloses as these biochemical structure acts as a protective coat to cellulose. Manure has a

high concentration of lignin since the easily degradable components has already been digested by

the animals. This is particularly apparent in ruminant manure.

In a study by Triolo et al. (2011), an algorithm was developed to characterize the biodegrad-

ability of biomass during AD. The study modeled the impact of fibrous content on the biochemical

methane potential (BMP) in energy crops (grass, maize and straw), manure, and a combination of

energy crops and manure.

The experiment was carried out in batch at 37 °C for 90 days. Biochemical and physiochemical

analyses were done before the BMP test to find the lignocellulosic composition of the biomasses.

The methane produced was measured. The results showed that the lignin concentration was the

best predictor of BMP in all substrates out of the investigated variables lignin, cellulose, acidic

determined fibers, neutral determined fibers and hemicelluloses. The method needs to be further

developed to increase the enhance BMP prediction.

7.3 Key area synthesis: Gas yield and potential amount of energy

The methane potential for straw only is 207 m3 CH4/ton VS and the energy potentials 8,8 GWh for

the lower case and 30,8 GWh for the upper case. However, digestion of straw only and the upper case

manure+straw (28 farms) are excluded because they are not convenient for wet digestion. In further

work the co-digestion case manure+straw (8 farms) and the case manure only are considered. It is

21

assumed that the methane potential for co-digestion is 300 m3 CH4/ton VS. The energy potentials

for manure only and co-digestion of manure are 18,1 GWh and 35,6 GWh respectively.

The study by (Triolo et al., 2011) suggests the level of lignin content can be used to predict the

biodegradability and biomethane potential in lignocellulosic material.

22

8 Other Products

The digestate is the mass left after fermentation and has a high nutrient content. It could be used

as biofertilizer and is a another product of the biogas process. By using the digestate as biofertilizer

the nutrient is recycled to the soil and closing the global energy (Arthurson, 2009). However, the

feedstock composition should have C/N ratio that is appropriate as biofertilizer (Weiland, 2010).

The quality of biogas residues is evaluated based on the chemical, physical and biological properties

(Arthurson, 2009). These properties depends on the type of biomass. Pathogens are killed o↵ in

the AD. The digestate is enriched in potassium, (de Mes et al., 2010) nitrogen and phosphorus

and can be used as environmentally friendly biofertilizer (Monlau et al., 2013), since conventional

biofertilizer require much energy to produce. Further, the digestate has better flow properties and is

easier and faster sunken into the soil which decreases ammonia emissions and that in turn reduces

nitrogen losses. Digestate from biogas production can therefore replace mineral fertilizers.

No studies have been found with straw only as biomass when producing biofertilizer from AD,

but there is information available about biofertilizer from other biomasses such as household wastes

and sewage sludge. However, straw has a low nutrient content and a low nitrogen content, which

makes it not so suitable as biofertilizer when digested solely. The nutrient content can be improved

when straw and manure are co-digested. In a study where di↵erent fertilizers (biogas residues, pig

slurry and mineral fertilizer) were compared to the e↵ect of wheat growth, biogas residue performed

well (Abubaker et al., 2012). Although biogas residues yielded the lowest overall biomass, it did

compensate with increased ear mass along with increasing fertilizer rate.

The digestate has high water content and separation is a good way to reallocate nutrients

like N2H, P and K (Halldorf and Örup, 2011). Separation of digestate gives a solid phase and a

liquid phase, which makes it easier to handle and the nutrients can be spread out where it is most

needed. Separation would give phosphorus in the solid fraction. In Denmark separation of digestate

is common and decanter centrifugation is a recommended method.

For Biogas i Vadstena the idea is to exchange the manure for biofertilizer. The amount of

biofertilizer assumed to be produced is about 90 000 ton with 6% TS (Swedish Biogas International,

2012). It is also an option to distribute biofertilizer to farms outside of the company for instance

to an organic farm close by that do not have animal farming.

More information about biofertilizer and its environmental impact is discussed in Chapter 11.

8.1 Key area synthesis: Other products

Digestate can be used as biofertilizer. It has been shown that digestate from straw and manure

co-digestion perform fairly well in cultivation compared biogas residues, pig slurry and mineral

fertilizer. It is preferable to separate the biofertilizer into a solid and a liquid fraction when spreading

in order to easier make a proper dosage.

23

9 Technology

The main issues with straw as biogas substrate are the big volume storage and low biodegradability

of the biomass. The technology should suit both the biogas production and be economically feasible.

This chapter discuss storage and di↵erent pretreatment methods followed by technology in the

biogas process and lastly upgrading technologies.

9.1 Storage and pretreatment

For storage and pretreatment ensilaging is an alternative. Pretreatment of biomass may increase

the degradation rate, but this is not necessarily equal with higher methane production (Weiland,

2010). It is important to hygenize the feedstock to remove any pathogens, either by pasteurization

at 70°C or sterilization at 130°C.

As mentioned before, straw is a lignocellulosic biomass that is di�cult to degrade. In particular,

lignocellulosic biomass is problematic in the hydrolysis step as the glucose units are inaccessible

(Monlau et al., 2013). However, reducing the crystallinity can enhance the digestibility and biogas

yield and degree of polymerization, increase the surface area accessibility and weaken the strong

structure of lignin. The biogas yield and production rate can be increased by various pretreatments

to make the biomass more accessible. The di↵erent storage and pretreatment methods that will be

described below are ensilage, physical, chemical, thermal and biological.

Ensilage

Crop residues can be stored and pretreated by ensilaging, which is a biochemical conversion of the

carbohydrate and a lowered pH to values of pH 3-4 (Weiland, 2010). Ensilaging can be regarded

as a type of pretreatment as the degradation of polysaccharides have started. The process can be

sped up by adding formic acid, starter cultures or enzymes (Lehtomäki, 2006). Pretreatment with

ensilage and formic acid as additive gives a higher methane yield in comparison with no additive,

enzymes and lab inoculate. Ensilaging is optimal when the biomass is cut into pieces of 10-20 mm

and have a TS of 25-35% (Weiland, 2010). However, ensilaging results in energy losses between

8-20% due to unwanted AD. There are di↵erent ensilage methods such as bunker silos, bales and

tubes (Wennerberg, 2012). With bunker silos there is risk for press water leech, which can cause

water pollution. Bales are storage in smaller round plastics, which is suitable for lower volumes.

However, for larger volumes tube ensilaging is a good alternative, in which the straw is packed in

a long tube instead. By tube ensilaging less plastic is needed, the environment inside is kept free

from oxygen and it does not let press water out. Ensilaging is a cost e�cient method. The tube is

placed outside near the biogas plant, and thus no investment cost is needed for building of straw

stock storage.

24

Physical pretreatment

Physical treatments involve milling (Hendriks and Zeeman, 2009) cutting, grinding and chipping

(Monlau et al., 2013). This results in smaller particles, less degree of polymerization and increased

accessible surface area. The bigger surface area is beneficial in the hydrolysis step of AD and reduces

the digestion time by 23-59%. It has been shown that milling increased the biomethane yield with

5-25%. However, milling is very energy consuming and is therefore not a good alternative from an

economical perspective.

Thermal pretreatment

Examples of thermal pretreatments are steam treatment, steam explosion, liquid hot water and

ammonia fiber explosion (Hendriks and Zeeman, 2009). The biochemical bonds start to break

at 150-180°C and the lignocellulosic mass become more solubilized. Heat pretreatment can form

compounds such as vanillin, furfural and HMF that can have an inhibitory e↵ect, but this is more

common in acid conditions.

The steam treatment is carried out in a tank with the biomass and steamed with temperatures

up to 240°C. In addition to the steam, the steam explosion treatment also involves a step with

rapid depressurization and cooling, which results in explosion of the water in the biomass. The

enzymatic digestibility may increase six times after a steam treatment. However, there is a risk for

condensation and precipitation of soluble lignin content which makes the biomass less degradable

and reduce the biomethane production.

In a study and steam exploded pretreated straw were co-digested with cattle manure (Ris-

berg et al., 2013a). The result showed similar gas yields for both untreated and steam exploded

pretreated straw that is the pretreatment did not seem to have a significant impact on the gas

yield.

In liquid hot water is used to solubilize most of the hemicellulose in order to make the cellulose

more exposed (Hendriks and Zeeman, 2009). It is important to keep a pH level between 4-7 to avoid

formation of inhibitors. More solubilized products are gained with this treatment in comparison

to steam treatment, but the product concentration is lower, probably because of higher loads of

water. Liquid hot water can result in increase 2-5 fold of the enzymatic hydrolysis.

Chemical pretreatment

Chemical pretreatment is addition of either alkaline, acid, oxidizing agents or organic solvents.

Alkaline treatment makes the biomass swell and enhanced surface gives a better accessibility for

bacteria and enzymes. The hemicellulose and partly the lignin become solubilize which is positive

for the degradability. Noteworthy is that the microbes consume some of the alkali. Acids make the

cellulose more accessible by solubilizing the hemicellulose. Like steam treatment, there is risk of

25

condensation and precipitation of soluble lignin.

In oxidative pretreatment an oxidation compound is added e.g. hydrogen peroxide or peracetic

acid to solubilize hemicellulose and lignin. There is a high risk of inhibitor formation and also loss

of sugars as the oxidation is non-selective.

It is also possible to increase the e�ciency by combining thermal pretreatments with chemical

treatments like adding alkaline, oxidative agent or ammonia (Hendriks and Zeeman, 2009). Exam-

ples of additives are lime pretreatment, peracetic acid, and ammonia and carbon oxide pretreatment

(AFEX).

Biological pretreatment

Biological treatments can be done with white-rot fungi or with enzymes. Biological pretreatment is

an environmentally friendly alternative as it lowers the activation energy and reaction temperature.

In general, biological pretreatment leads to loss of polysaccharides and requires long process time

(Isroi et al., 2011). A combination of chemical or physical pretreatment prior to biological can

enhance lignin degradation and the accessibility of substrate.

Biological pretreatments with white-rot fungi is most common in solid-state fermentation.

White-rot fungi is Basidiomycetes and grow on hardwood and softwood. Various species are suit-

able for degradation of wheat straw and biogas production, for instance, P. chrysosporium. The

fungi degrade lignin and transform the lignocellulosic biomass into a white, fibrous mass. There are

selective and non-selective decays. The selective depends on type of lignocellulosic material, culti-

vation time and other factors. Selective degradation degrades lignin and hemicellulose and almost

no cellulose, whereas non-selective fungi degrade all lignocellulosic components almost equally.

White-rot fungi produces enzymes such as manganese peroxidase, laccase and lignin peroxidase,

which promote lignin degradation. The white-rot fungi depolymerize by cleaving the carbon-carbon

linkages and mineralizes lignin with the ligninolytic enzymes. In a study the lignin loss was 39,7%

in wheat straw after pretreatment with the white-rot fungi P. ostreatus.

In enzymatic pretreatment, the nitrogen concentration is important in the culture medium for

the production and activity of ligninolytic enzymes. Other additives like Mn2+ and Cu2+ are in-

volved in the expression and production of certain enzymes. The degradation e�ciency also depends

on aeration, moisture contents (in solid-state fermentation), pH and temperature. Pretreatment

by white-rot fungi can be applied in the production of biopulp, biogas, bioethanol and chemicals,

while the enzymes can be used in for example biobleaching.

In a study, the degradation rate increased after addition of enzymes, but did not significantly

a↵ect the methane yield (Weiland, 2010). Enzymes also reduce the viscosity of substrate and de-

crease the formation of floating layers. Nevertheless, the enzyme e↵ect may be reduced by proteases

produced by the microorganisms.

26

Discussion: Pretreatments

In the choice of pretreatment method a cost e↵ective method is wishful. It is also important to

avoid methods that form inhibitors and toxification (Hendriks and Zeeman, 2009). The biomass and

its composition are important in the choice of a pretreatment method. In comparison, biological

pretreatment leads to loss of polysaccharides and require a longer time than chemical and physical

pretreatment (Isroi et al., 2011). More research is needed in the field of pretreatment to enhance

the biodegradability and the gas yield. E↵ective methods, but too expensive in relation to the

sugar are concentrated acids, wet oxidation, solvents and metal complexes. Steam treatment, lime

pretreatment, liquid hot water and ammonia based systems are economically feasible and e↵ective

(ibid.).

9.2 Technology in the biogas process

The biogas process is complex and depends on the technology used as well as many other factors

to function well. The following subchapters will describe three studies that have been done with

straw and the technologies could possibly be applied on Biogas i Vadstena.

9.2.1 Study: Solid-state plant in Trelleborg, Sweden

In a pilot-scale study in Trelleborg in Sweden, straw was co-digested in solid-state with manure

and other wastes (Linné et al., 1999). It had been shown in an earlier study that it is economically

feasible using crop residues and waste from the city. The fermenter tank had the volume of 600

m3 and had a hygenization and heating part. The AD requires long retention time and each batch

was loaded with dry matter in September and unloaded in April. During this period wet matter,

mainly pig slurry, was added. Since straw is very dry and have a high TS a lot of process liquid was

needed. The energy yield was 50% of the theoretical potential. The methane potential increased

after optimizing the feedstock composition and bacteria culture. However, the methane potential

was lower than expected due to inability to maintain appropriate temperature and problem with

the recirculation of process liquids. In this case, no pretreatment was done to break down the

lignin, which possibly have a↵ected the methane potential. Solid-state digestion excludes the costs

of storage and pretreatment, but on the other hand a longer retention time is necessary and less

biogas conversion. It was found that the key economic parameters were investment cost, reception

cost, gas price and straw price.

9.2.2 Study: Co-digestion of swine manure with crop residues

In an American study by Wu et al. (2010), swine manure was co-digested with three crop residues;

corn stalks, oat straw and wheat straw. The experiment design investigated these three crop residues

at three di↵erent C/N levels (16:1, 20:1, 25:1). The digestion was in batch and the CH4 volume,

27

CH4 content in the biogas and the net CH4 volume were measured. The crops were pretreated by

cutting and grinding into particles of 0,42 mm. Digestion was conducted at 37 °C during 25 days.

The results showed that the biogas production increased with crop residues added at all C/N

ratios. However, the corn stalk showed the biggest biogas increase in daily maximum volume

(11,4-fold), in comparison to the control. Next was oat straw (8,45-fold) and lastly wheat straw

(6,12-fold). A 20:1 in C/N ratio show better performance. Moreover, the highest methane content

was with corn stalk (68%), followed by oat straw (57%), control with manure only and lastly straw

(47%). Wheat straw had the lowest biogas productivity of the crop residues in the study, although

wheat has higher carbon content than both corn stalks and oat straw.

To achieve the determined C/N ratios, wheat straw was added in less amount in terms of

weight. Thus, the surface availability of degradable material is reduced for straw, which could have

an impact on the performance. In addition, it was suggested that the higher lignin content in wheat

limited the degradation. The lignin content in wheat straw, oat straw and corn stalks was found

to be 18%, 13% and 8,4% respectively. This could also explain the better biogas and methane

performance of corn stalks and oat straw.

9.2.3 Study: The impact of pretreatment and process operating parameters

A study in Sweden by Risberg et al. (2013b) investigated the biogas production of non-treated

or steam-exploded wheat straw in co-digestion with cattle manure. This was compared to sole

manure respectively wheat straw as biogas substrate. The experiment was done in laboratory-scale

continuously stirred tank reactor batch reactors with a volume of 5 litre. The HRT was 25 days

and temperature at 37 °C, 44 °C or 52 °C. The dry (not steam-exploded) straw was milled to 10

mm particles. Thermal pretreatment consisted of steam at 210 °C for 10 min.

The results showed stable but low methane yield (0,13-0,21 Nm3 CH4/kg VS) in the biogas pro-

cess with sole manure, sole dry wheat straw and co-digestion of manure and steam-exploded straw.

The small di↵erence in methane yield between non-pretreated straw and pretreated straw could

be due to conversion of pentoses to furans and polymerization of pseudo lignin for lignocellulosic

biomass in high temperatures. The accumulation of lignin and pseudolignin possibly made the pre-

treated straw too di�cult to degrade. Furthermore, the microorganisms for anaerobic degradation

use pentoses for methane production, which is probably why the methane yield was low. In con-

trast, in ethanol production S. cerevisiae only ferment on hexoses and thus loss of pentoses do not

a↵ect the ethanol production. The study showed it is possible to get a stable biogas process with

wheat straw, both non-pretreated and steam-exploded pretreated, together with cattle manure at

mesophilic and thermophilic temperatures. However, the low methane yield makes it di�cult to

achieve profitability in biogas production with available technology.

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9.2.4 Heating in Biogas i Vadstena

It has been discussed whether or not to use distance heating as energy source (Halldorf and Örup,

2011). In this case it would result in unnecessary long transportation of the heat. Furthermore,

another heat source would be needed for hygenization. It is suggested to use a chipping pan, which

would provide a more steady temperature and a complementary big bale pan may be useful. The

chipping pan would be e�cient due to the hygenization requirement.

Discussion: Technical studies

The studies show that straw can be used as biogas feedstock in co-digestion with manure in both

solid-state digestion and wet digestion. Solid-state digestion has a much longer digestion time

compared to wet digestion, eight months compared to about 25 days. Solid-state digestion is not

so e�cient. Thus, solid-state digestion is not suitable for the case of Biogas i Vadstena, which has

a big amount of feedstock. Furthermore, straw seems to enhance the methane potential due to

better nutrient composition and C/N ratio. In some cases, pretreatment of straw seems to have

limited impact on the methane yield due to the high lignin content. Thus, it is not fully clear

if pretreatment of straw is worthwhile. It is of great importance to have appropriate operation

conditions such as right temperature, as could be seen in the solid-state pilot study. The biogas

process is stable in mesophilic temperature. However, the studies had used di↵erent method designs

and conditions, which makes it di�cult to compare the results.

9.3 Upgrading to transportation fuel

Raw biogas contains primarily methane and carbon oxide and smaller portions of (H2S) and (NH3)

(Persson et al., 2006). The raw biogas is also saturated with water vapor (Weiland, 2010). Biogas

from co-digestion of manure and harvesting residues contains 100-3000 ppm H2S.

The biogas has to be purified from CO2 and H2S, and dried before it can be used in transporta-

tion (Persson et al., 2006). CO2 is removed in order to increase the heating value and the driving

distance. The removal of CO2 is also for standardizing the composition of biogas, if it is injected in

gas grid. Some methane loss occurs when removing CO2 (Weiland, 2010). It is important to keep

the methane losses as low as possible for economic and environmental reasons, because methane

is 23 times stronger GHG compared to CO2. Sulfur gases have a corrosive e↵ect on compressors,

gas tanks and engines. Desulfurization is most commonly done by biological treatment through

oxidation of H2S in which a small amount of air is injected and presence of the bacteria Sulfobacter

oxydans.

There are several upgrading methods to remove CO2; water scrubber, pressure swing adsorption,

membrane separation and organic physical scrubber and cryogenic separation will be presented

below (Persson et al., 2006; Bauer et al., 2013).

29

Water scrubber

Water scrubbing is one example of absorption and is the most common upgrading method in Sweden

(Persson et al., 2006). In absorption methods biogas is separated through di↵erences in polarity.

Since CO2 and H2S are more polar than CH4, CO2 is solubilized in water. In water scrubbing,

CO2 is dissolved in water under high pressure. Newer water scrubbers have a recirculation system

for water, which is more stable.

Pressure swing adsorption

Pressure swing adsorption (PSA) is a method where gases are separated according to di↵erent

physical properties (Bauer et al., 2013). The adsorption of CO2 is done on materials like activated

carbon or molecular sieves (Persson et al., 2006) in an adsorption column, thus the CO2 is retained

but not the CH4 (Bauer et al., 2013). PSA is carried out under an elevated pressure and when the

pressure is decreased the material can be reused.

Membrane

Separation through membrane, so called dry membranes, can be done in gas phase (Persson et al.,

2006). It can also be a gas-liquid absorption in which a liquid absorbs the CO2 on one side of

the membrane. In both cases, methane is retained on one side of the membrane. Besides CO2,

membrane separation also separates water vapor, hydrogen and partly oxygen (Bauer et al., 2013).

The membrane permeation rate depends on molecule size and hydrophilicity.

Amine scrubbing

Amine scrubbing is a chemical scrubber that has become more established in the recent years.

In amine scrubbing, carbon dioxide is removed from the biogas by a water solution with amines

(compounds with carbon and nitrogen) that chemically bond to the CO2 molecule. The most

common amine used today is activated methyldiethanolamine , which is a mixture of piperazine,

and methyldiethanolamine. The raw gas is injected into upgrader where the CO2 and H2S react

with the amines and turn from gas phase to liquid phase. The methane gas exits on the top of the

tank. The amine scrubber is the most e�cient separation method of CO2, where 99,8% is removed.

(Bauer et al., 2013)

Organic physical scrubber

Among organic physical scrubber Genosorb 1753 is the most common solvent used today. The

principles for organic physical scrubber is the same as for water scrubber, although the solubility

of CO2 is much higher for Genosorb solvent than for water. Thanks to the high solubility less

solvent needs to be recirculated in comparison with water scrubber. (Bauer et al., 2013)

30

Cryogenic upgrading

Liquid biogas (LBG) is a result of cryogenic technology where the methane is purified and upgraded

by cooling down to -161 °C. In the raw gas, CO2, CH4 and N2 are present with condensation

temperatures at -78,5°C, -161°C and -196°C respectively. However, the di↵erent compounds have

di↵erent condensation temperatures and can separated when chilled down. The cryogenic technique

requires much energy, about 0,8-1,8 kWh electricity/Nm3 clean biogas, but may be worthwhile in

some aspects. For instance, LGB is five times more space e�cient than biogas and better for

long transportations. LBG also possibly allows heavy vehicles to run longer distances as it can be

stored on the vehicle. Cryogenic upgrading is used to a smaller extent, but is still having operational

problems. Moreover, cryogenic upgrading is more appropriate for landfill gases, since landfill biogas

contains more N2 that is more easily separated. LBG can be produced in large scale with cryogenic

liquification technology, but as a subsequent step after traditional upgrading treatment. It would

be interesting to develop LBG to be used in heavy vehicles in the future. (Johansson, 2008)

Discussion: Upgrading of biogas

The technology for upgrading biogas has developed and become more mature (Bauer et al., 2013).

Today the most dominating technologies are water scrubbing, PSA and amine scrubbing. The cost

for upgrading is a significant part of the investment (Persson et al., 2006). Upgrading costs are

specific investment cost, electricity, maintenance, sta�ng and in some cases water and chemicals.

In the upgrading process, the removal of CO2 is the most expensive part of the process. For mid-

scale biogas production the most common upgrading technologies mentioned above are viable. In

common for the scrubbing technologies, PSA and membrane separations is that they have about

the same investment costs. Amine scrubbing has a higher purity and lower methane loss. However,

water scrubbing is simple, reliable and perform well and this is preferable in the choice of technology.

In addition, water scrubber does not involve chemicals, which is both a cost and unpleasant to work

with. The membrane technology is not an established method, but has potential and may be an

option due to low methane slip.

Cryogenic technology to make liquid biogas is interesting, but it is more suitable for landfill gas

and is a costly technology that has not really matured yet. Thus, for the case of Biogas i Vadstena

LBG will not be considered in further work.

9.4 Key area synthesis: Technology

Pretreatment of straw can increase the biodegradability and sometimes the biogas yield by partly

destructing the lignocellulosic structure. There are a number of di↵erent methods. However, steam

treatment, lime pretreatment, liquid hot water and ammonia based systems are known to be

e�cient and cost e↵ective. Ensilaging works both as pretreatment and storage, and has also shown

31

positive e↵ects on the methane yield.

The biogas technology studies show that solid-state digestion of straw is ine�cient and therefore

not preferable for Biogas i Vadstena. Furthermore, it is important to maintain proper process

conditions, for instance, keep the temperature to get a stable process. Co-digestion of manure and

straw can increase the biogas yield due to better C/N ratio. It is not clear if pretreatment of straw

gives a higher methane yield.

The most common upgrading technologies today are water scrubber, PSA and amine scrub-

bing. Water scrubber does not involve chemicals and new method allows recirculation of water,

which is more environmentally friendly. Water scrubber is used in further work and calculations.

More technologies are available with various maturity. Upgrading cost is a significant part of the

investment in a biogas plant. Cryogenic technology to produce liquid biogas is not recommended

since the technique is not mature and costly.

32

10 Economy For the Producer

In this chapter two reports that have similarities with Biogas i Vadstena are described. Thereafter

the production conditions are described for the cases. Economical calculations are presented and

discussed for the cases manure only and manure+straw (8 farms).

10.1 Study: Lignocellulosic material for biogas production

In this extensive report by Berglund Odgner et al. (2012) the available knowledge of lignocellulosic

material was summarized. The economy and benefit was analyzed on a commercial scale. Wheat

straw, paper and forest residues were the investigated biomasses.

With extruded (non-pretreated) straw the methane potential is 281 m3 CH4/ton VS which is

a significantly higher yield compared to untreated straw of 165 m3 CH4/ton VS.

From the literature study in pretreatment methods, steam explosion pretreatment is recom-

mended for wheat straw. The conditions were 180 °C for 15 min, organic load of substrate/inoculum

DM ratio 1:3. The methane yield was 331 ml/g VS or 283 ml/g raw material at 37,5 °C. The in-

vestment cost for steam explosion pretreatment was estimated to 15 million SEK and 2,6 million

SEK in operational cost/year with organic load of 5000 ton/yr. The results of pretreatment was a

positive net result and can be cost e�cient.

There are few options of suppliers for pretreatment equipment. It is of importance look at the

flow of energy and material in the process. Excess heat and steam and recirculation of water could

be applied. This needs to be planned when constructing the plant. Straw is not used commonly

for combustion due to formation of corrosive flue gases.

The report showed that biogas production of straw is overall economical feasible. The price of

sold methane is important according to the sensitivity analysis. In this calculation it is assumed

that straw had to be bought from a number of farmers and transported to the biogas plant, which

was a big cost. This cost would be reduced in the case of Vadstena where the straw is free of charge

and short distances to transport.

10.2 Study: Techno-economic assessment of agricultural based biogas

production

Berglund et al. (2012) investigated biogas production for vehicle fuel from agricultural feedstock

from a technical and economical perspective for two cases: 1) a centralized biogas plant with

upgradation on site and 2) decentralized biogas plants that are connected via a raw gas grid and

with central upgrading. The total amount of manure as substrate was assumed to be 100 000 ton

manure, which corresponded to about 18 GWh. In the decentralized system it was assumed to be 20

smaller plants each with a capacity of 5000 ton manure. It was shown that there are few technical

33

di↵erences between centralized and decentralized plants, but much more transportation costs for

the centralized plant. The results showed high production costs for production of vehicle biogas, and

thus di�cult to reach economic viability for both cases decentralized production and centralized

production. However, centralized system showed lower costs with 0,8 SEK/kWh compared to 1,00

SEK/kWh for decentralized system. For centralized plant the viability would improve significantly

with financial support of 0,2 SEK/kWh, see 13.2 .

10.3 Economic calculation

Below economic calculations are done for two di↵erent cases, for manure only and manure+straw

(8 farms). A budget is presented for each case. The net present value (NPV) and annuity are

calculated to. NPV is when all the future costs are recalculated to the present currency value.

The annuity allocates the costs and income over the

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Systematic Assessment of Straw as Potential Biogas Substrate in …756500/FULLTEXT01.pdf · The economic calculations show that it is proﬁtable to use straw with manure in a farm-based