Systematic Assessment of Straw as Potential Biogas Substrate in Co-digestion with Manure

Link¨

oping University

Department of Management and Engineering

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

Systematic Assessment of Straw as

Potential Biogas Substrate in

Co-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 utf¨orts i samarbete med Biogas Research Center (BRC) och f¨oretaget

Biogas i Vadstena. M˚alet med examensarbetet var att systematiskt utv¨ardera nya substrat f¨or

biogasproduktion. Specifikt f¨or det h¨ar fallet var att unders¨oka potentialen f¨or halm i samr¨otning

med g¨odsel och flyt fr˚an svin, h¨ons och n¨ot. Halm ¨ar intressant att utv¨ardera d˚a det tillh¨or andra

generationens biomassa och finns tillg¨angligt i stor m¨angd. ¨Aven r¨otning av g¨odsel ¨ar givande

d˚a den spontana metanemissionen uteblir och det ger en b¨attre g¨odselhantering. Det har satts

upp m˚al inom s˚av¨al EU som i Sverige att mer f¨ornybart br¨ansle b¨or produceras f¨or att minska

v¨axthusgasutsl¨appen fr˚an fossila br¨anslen.

Metodiken som anv¨ants har framarbetats av BRC. Det inneb¨ar att substrat granskas utifr˚an ett

flertal nyckelomr˚aden, s˚asom beskrivning och m¨angd biomassa, gasutbyte, synergie↵ekter, teknik,

ekonomi, milj¨op˚averkan och energisystem, konkurrerande intressen och institutionella faktorer.

Dessa har utv¨arderats genom litteraturstudier och studie av fallet Biogas i Vadstena. Utifr˚an

resultatet g¨ors en ¨overgripande bed¨omning av substratet.

Resultatet visar att halm inte ¨ar l¨ampligt att r¨ota enskilt p˚a grund av h¨ogt TS-v¨arde, h¨ogt

kolin-neh˚all och att den ¨ar n¨aringsfattig. Halm best˚ar ¨aven till stor del av lignocellosa-strukturer som

¨ar sv˚ara att bryta ned, i synnerhet lignin. Mekaniska, termiska, kemiska och bioglogiska

f¨orbehan-dlingar kan ¨oka tillg¨angligheten och nedbrytbarheten av halm. Det kan ¨aven ¨oka metanpotentialen i vissa fall. D¨aremot fungerar halm bra som ett komplement i samr¨otning med g¨odsel som ¨ar ett

kv¨averikt substrat. Det finns teknik f¨or r¨otning av halm f¨or hela biogasprocessen, fr˚an transport,

f¨orbehandling och r¨otning till uppgradering. Dock finns utrymme f¨or tekniken att utvecklas ytterli-gare. De ekonomiska ber¨akningarna visar att det ¨ar l¨onsamt att anv¨anda halm tillsammans med g¨odsel i en jordbruksbaserad biogasanl¨aggning f¨or fordonsgasproduktion. Vidare visar ber¨akningar

f¨or energisystemet att biogasproduktion ¨ar energie↵ektiv med energi input/output-kvot p˚a 18-23%.

F¨orutom fordonsgas produceras ¨aven biog¨odsel som ¨ar ett milj¨ov¨anligt alternativ till konstgjord g¨odsel.

Sammanfattningsvis, det ¨ar m¨ojligt att producera biogas av halm tillsammans med g¨odsel och

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 difficult 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 efficient 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 Link¨oping 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 Malmstr¨om at Biogas i Vadstena for helping me providing information.

Link¨oping, June 2014

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 _CH

4= 9,8 kWh

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

9.2.2 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

1

Introduction

There are several environmental challenges today such as eutrophication, pollution of air and wa-ter and climate change. The Inwa-tergovernmental 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 (N¨aringsdepartementet, 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.

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 difficulties 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 efficiency 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 Link¨oping 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.

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?

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

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

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 insufficient 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

measured before and after the process to monitor the process efficiency . 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 inefficient 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 efficiency (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.

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* CO_2* *** Biofer;lizer* CH₄*

Figure 2: The principles of biogas production from lignocellulosic biomass. Information from (Monlau et al., 2013).

3

Case Description

Thomas Malmstr¨om, 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 Hush˚allningss¨allskapet and Lovanggruppen and the result showed economic

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

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

are located within a radius of 10 km, see Figure 3 (Halldorf and ¨Orup, 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 (Malmstr¨om, 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.

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 ¨Orup, 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

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 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

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 efficiently. 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 difficult 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.

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.

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 (Linn´e 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).

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.

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 ratio

Straw 78 91 90

Pig slurry 6 80 5

Poultry manure 35 n/a n/a

Chicken manure 60 76 3-10

Chicken slurry 10 n/a n/a

Dairy slurry 9 80 6-20

Dairy litter straw 30 80 High

Total manure estimation 11 79 8

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 difficult 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.

6

Amount of Biomass

The estimated values of the agriculture area and straw per hectare for each crop were given from Thomas Malmstr¨om. 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 Malmstr¨om 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 (Malmstr¨om, 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

4 the amount of manure and slurry is presented.

Table 4: The table shows the amount of manure and slurry biomass (Halldorf and ¨Orup, 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.

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_CH

4/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 ¨Orup (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 _CH

4/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.

Table 5: The table shows the methane potential of straw, manure, and two cases of manure+straw. TS (ton) VS of TS (ton) CH 4/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^-6 n/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_CH

4/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

assumed that the methane potential for co-digestion is 300 m3_CH

4/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.

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 ¨Orup, 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

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 difficult 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 (Lehtom¨aki, 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 efficient method. The tube is placed outside near the biogas plant, and thus no investment cost is needed for building of straw stock storage.

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

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 efficiency 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 Cu}2+ _are

in-volved in the expression and production of certain enzymes. The degradation efficiency 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.

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 (Linn´e 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

CH4content 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_CH

4/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 difficult 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 difficult to achieve profitability in biogas production with available technology.

9.2.4 Heating in Biogas i Vadstena

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

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 efficient 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 efficient. 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 difficult 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 CO2and 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 CO2is 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,

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,

CO2is 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 CO2is done on materials like activated

carbon or molecular sieves (Persson et al., 2006) in an adsorption column, thus the CO2is 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 efficient 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