Degradation kinetics of steam exploded wheat straw during anaerobic digestion for biogas production

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1 Ref: C0472 Degradation kinetics of steam exploded wheat straw during anaerobic digestion for biogas production Javier Lizasoain, alps Centre for Climate Change Adaptation, Grabenweg 68, 6020 Innsbruck, Austria Franz Theuretzbacher, Cristoph Lefever, Ramon Enguidanos, Viktoria Werfring, Andreas Gronauer, Alexander Bauer*, University of Natural Resources and Life Sciences, Department of Sustainable Agricultural Systems, Division of Agricultural Engineering, Konrad-Lorenz- Strasse 24, A-3430 Tulln, Austria Nikolaus Weran, Biogas Systems GmbH, Am Futterplatz 3106, A-7111 Parndorf, Austria *Corresponding author. Phone: ; Fax: , alexander.bauer@boku.ac.at Abstract Cereal straw is a promising alternative for biogas production since the largest arable land areas in the EU are occupied by cereals and their residues are not fully utilized. Due to the recalcitrant nature of lignocelluloses, hydrolysis is the rate-limiting stage in the production of biogas from these materials and a pretreatment step e.g. steam explosion is required. The effects of steam explosion in the degradation kinetics of biomass during the anaerobic digestion process has not been entirely investigated. The main objective of this work was to examine the influence of different steam explosion conditions in the degradation kinetics of wheat straw. For this straw, pretreated at 140 ºC for 2 hours was kept in small fiber bags inside 25 L fermenters and the bags were progressively removed within the 45-day digestion test for the performance of detailed chemical analysis. Results showed the remarkable effects that steam explosion has on the degradation process of wheat straw. While the specific methane yields of untreated and pretreated samples were similar after the 45-day test, steam explosion greatly accelerated the biomass breakdown, increasing especially the degradation speed of cellulose. Moreover, the comparative analysis of native and pretreated wheat straw prior to the anaerobic digestion revealed that harsher pretreatment conditions resulted in an outstanding decrease in the hemicellulose content as well as a significant formation of pseudo-lignin, the latter being partially degraded along the biological process. Keywords: biogas, wheat straw, steam explosion, kinetics 1 Introduction The use of fossil fuels for energy production leads to the emission of greenhouse gasses. Its environmental consequences, together with the limitation of fossil fuels led to an increased interest in alternative energy sources. The European directive 2009/28/EC, which states that by 2020 a share of 20% of the overall energy consumption must come from renewable sources (Parliament, 2009), is encouraging the production of renewable energy. Within renewable sources, energy production from biomass is gaining interest and has a promising future. In the last decades, biodiesel, bioethanol and biogas production had an important increase. The production of these fuels is based on oil, starch and sugar and therefore rely to Proceedings International Conference of Agricultural Engineering, Zurich, /5

2 a huge part on the availability of traditional field crops such as maize, wheat or rapeseed (Borugadda & Goud, 2012). The production of biofuel from biomass, e.g. wheat grain and maize, can compete directly with food and feed production. The utilisation of lignocellulosic material is very promising, as lignocellulosic biomass is one of the most abundant and inexhaustible renewable natural resources and has an important potential as source for biogas production. Nearly 90% of the dry weight of most plant constituents is stored in the form of cellulose, hemicellulose, lignin, and pectin (Yat et al., 2008). However, the lignocellulose complex leads to a protective barrier that hinders cellulose and hemicellulose accessibility for microbial conversion. This is caused by many physicochemical, structural, and compositional factors. Therefore, efficient biochemical processing to biofuels requires a pretreatment step in order to prepare lignocellulosic materials for enzymatic attack. The goal of the pretreatment process is to break down the lignin structure. Hence, the acids or enzymes have access to cellulose at the same time that improves the complete solubilization of polymer to monomer sugars without the creation of degradation products. Moreover, the procedure should be economical, low energy demanding, and not cause any contamination (Kratky & Jirout, 2011). Currently, steam explosion is one of the most intensive investigated pretreatment technology of lignocellulosic material for both ethanol and biogas production. The aim of this research was to evaluate the influence of steam explosion in the degradation kinetics of wheat straw. Thus, steam explosion was performed and the resulting biomass tested. Since the main fraction of wheat straw is lignocellulose, a special focus was set on the degradation kinetics of the structural compounds (cellulose, hemicellulose and lignin), which have been examined using a modified method from animal feed analysis. A detailed knowledge of the digestibility and degradation speed of the single compounds can serve as a basis for further improvement of the steam explosion pretreatment technology. 2 Materials and methods 2.1 Raw material and steam explosion pretreatment The wheat straw used in the experiments was cultivated in eastern Austria in The steam explosion pretreatment was performed in a testing facility at a biogas plant placed in Parndorf, Austria. The wheat straw was first size reduced to particles smaller than 5 cm and then mixed with water to achieve a dry matter content of 30 %. After transferring the material into the reaction vessel, it was preheated in order to obtain a steam saturated atmosphere. Then, the temperature and pressure were increased by direct steam injection until 140 ºC were reached. After maintaining these conditions for 2 hours, the pressure was abruptly reduced. This causes rapid evaporation of the water inside the vessel, increasing the pore volume of the biomass. The pretreated material is then taken from the flash tank, vacuumed and stored at 4 C until further analysis were carried out. 2.2 Chemical analysis The untreated and all pretreated variants were analyzed regarding dry matter (DM) and volatile solids (VS) as well as cellulose (CEL), hemicellulose (H-CEL) and lignin (ADL) content. The DM was analyzed by drying the biomass in an oven at 105 C until constant weight was reached. The raw ash (XA) was analyzed by determining the residue left after dry oxidation of the oven dried material in a muffle furnace at 550 C (Sluiter et al., 2004). The VS were calculated by subtracting the raw ash content from the total solids (Naumann & Bassler, 1993). Proceedings International Conference of Agricultural Engineering, Zurich, /5

3 Cellulose and hemicellulose were determined by the Van Soest standard procedure, (Van Soest & Wine, 1967). The method determines the content of neutral detergent fiber (NDF), acid detergent fiber (ADF) Hemicellulose was calculated as the difference between NDF and ADF. The acid detergent lignin (ADL) was determined from the filtrates remaining in the glass filters after ADF determination. Cellulose was calculated by determining the difference between ADF and ADL. 2.3 Methane potential Anaerobic digestion batch trials were carried out in triplicate in accordance with VDI 4630 (VDI, 2006), employing eudiometer batch digesters of 0.25 l capacity. The substrates and the inoculum were weighed out in a ratio of 1:3 (based on volatile solids content). The inoculum utilized was taken from the biogas plant in Parndorf already mentioned. The digesters, incubated at 37.5 C, were continuously stirred and the biogas yields were monitored on a daily basis during the whole digestion process. Biogas and methane production were measured in norm liters (273 K and 1013 mbar) per kg of volatile solids (l N kg 1 VS). The portable gas analyzer Dräger X-AM 7000 was used to determine the biogas composition (CH 4 and CO 2 ). Standard cellulose was used to check the proper activity of the inoculum. 2.4 Degradation kinetics For investigating the degradation kinetics of the structural compounds in the biomass, a modification of the rumen simulation technique (RUSITEC) developed by Cerkawski and Breckenridge (1977) was used. This method has been already widely tested in the field of feedstuff analysis (e.g. for the investigation of feed additives on methane production (Machmüller et al., 1998)). There have been also first approaches of an utilisation of this method in the field of biogas production by Bayané and Guiot (2011), who explored the possibility of an adaptation of very effective animal digestion strategies for biogas production. In the original method, perforated nylon filter bags filled with feed are put into a digester filled with a solution that contains rumen microorganisms. For this study ash and nitrogen free nylon filter bags (Ankom Technology) of the type F57 with a 25 μm perforation were filled with sample material, sealed (Welding impulse sealer Type AIE 200) and fed into a 20 L digester filled with the same inoculum described in section 2.3. The inoculum to sample DM ratio was set to 2:1 by adding water to the inoculum, before putting the digesters into a water bath at 37.5 C. The digesters were equipped with a water lock to allow the produced biogas to exit the digester. At the start of the experiment and after every sample removal the headspace of the digesters were flushed with nitrogen. For each variant, one digester was used and filled with 8 nylon bags, which were removed from the digesters 1, 2, 5, 10, 15, 20, 36 and 45 days after the start of the experiment. Afterwards, the bags were washed with deionised water until the runoff was clear, and they were dried in an oven at 50 C. 3 Results and discussions Figure 1 shows the cumulative biogas production (l N kg 1 VS) for untreated and pretreated biomass at 140 ºC for 2 hours. During the first 20 days of the trial, the cumulative methane yield was higher in the steam exploded sample. This is due to the biomass degradation occurred in the pretreatment step. From day 20 to 45, despite the biogas yields seemed to be slightly higher for the untreated sample, the specific methane yields of both biomass was approximately the same (data not shown). Proceedings International Conference of Agricultural Engineering, Zurich, /5

4 Relative weight to starting material (g/g) Cumulative biogas production (l N kg 1 VS) C 2h Untreated Fermentation time (days) Figure 1. Cumulative biogas production for untreated and pretreated biomass. Figure 2 show the degradation kinetics of the pretreated biomass. When comparing the initial values, prior to the anaerobic digestion experiments, the hemicellulose content in the steam exploded sample is lower than in the untreated one. In addition, the degradation of hemicellulose along the process is faster in the pretreated biomass than in the untreated. Regarding lignin (ADL), there is an important increase of this fraction in the pretreated sample in comparison with the untreated one. Moreover, part of the ADL content in the steam exploded material was degraded during the process. Some researchers have reported lignin increments and hemicellulose reductions for other steam-exploded biomass (Ballesteros et al., 2004; Vivekanand et al., 2013). These changes are caused by different reactions occurred during the pretreatment. The lignin increase could be related to the degradation of hemicellulose sugars since their monosaccharides lead to the formation of different degradation products (Nelson et al., 1988). These products can be transformed to acidic insoluble pseudo-lignin (Sannigrahi et al., 2011), which can be partially degraded in the anaerobic digestion process. 100% 80% 60% 40% Organic rest Ash Protein Cellulose Hemicellulose AIL 20% 0% Fermentation time (days) Figure 2. Degradation kinetics of the different fractions of the pretreated biomass. Proceedings International Conference of Agricultural Engineering, Zurich, /5

5 4 Conclusions The present study based on the Rumen Simulation Technique concluded that steam explosion is a good pretreatment method for biogas production from wheat straw, enhancing the microbial availability of the lignocellulosic fraction. Steam explosion led to higher degradation of hemicellulose and an increment of the ADL content, probably caused by the formation of pseudo-lignin. Along the anaerobic digestion process, the hemicellulose degradation of the pretreated biomass was faster in comparison with the native material. In addition, the ADL fraction of the pretreted biomass was partially degraded during the anaerobic digestion. The comparison of cumulative biogas production of untreated and pretreated samples showed that steam exploded biomass had higher biogas yields in the first 20 days of the process. 5 References Ballesteros, M., Oliva, J.M., Negro, M.J., Manzanares, P., Ballesteros, I Ethanol from lignocellulosic materials by a simultaneous saccharification and fermentation process (SFS) with Kluyveromyces marxianus CECT Process Biochemistry, 39(12), Bayané, A., Guiot, S.R Animal digestive strategies versus anaerobic digestion bioprocesses for biogas production from lignocellulosic biomass. Reviews in Environmental Science and Biotechnology, 10(1), Borugadda, V.B., Goud, V.V Biodiesel production from renewable feedstocks: Status and opportunities. Renewable and Sustainable Energy Reviews, 16(7), Czerkawski, J.W., Breckenridge, G Design and development of a long-term rumen simulation technique (Rusitec). British Journal of Nutrition, 38(3), Kratky, L., Jirout, T Biomass Size Reduction Machines for Enhancing Biogas Production. Chemical Engineering and Technology, 34(3), Machmüller, A., Ossowski, D.A., Wanner, M., Kreuzer, M Potential of various fatty feeds to reduce methane release from rumen fermentation in vitro (Rusitec). Animal Feed Science and Technology, 71(1-2), Naumann, C., Bassler, R The chemical analysis of animal feed (Original title: Die chemische Untersuchung von Futtermittel), Vol. 3th ed., VDLUFA-Verlag. Darmstadt. Nelson, D.A., Hallen, R.T., Theander, O Formation of aromatic compounds from carbohydrates: Reaction of xylose, glucose, and glucuronic acid in acidic solution at 300 C. ACS Symposium Series, Denver, CO, USA. Publ by ACS. pp Parliament, E Directive 2009/28/EC on the promotion of the use of energy from renewable sources and amending and subsequently repealing Directives 2001/77/EC and 2003/30/EC, (Ed.) E. Parliament. Sannigrahi, P., Kim, D.H., Jung, S., Ragauskas, A Pseudo-lignin and pretreatment chemistry. Energy and Environmental Science, 4(4), Sluiter, A., Hames, B., Ruiz, R.O., Scarlata, C., Sluiter, J., Templeton, D., Energy, D.o Determination of Ash in Biomass. National Renewable Energy Laboratory; National Bioenergy Center; Department of Energy. Van Soest, P.J., Wine, R.H Use of detergents in the analysis of fibrous feeds. IV. Determination of plant cell-wall constituents. Journal of the Association of Official Analytical Chemists, 50, VDI VDI Fermentation of organic materials. Characterisation of substrate, sampling, collection of material data, fermentation tests. Vivekanand, V., Olsen, E.F., Eijsink, V.G.H., Horn, S.J Effect of different steam explosion conditions on methane potential and enzymatic saccharification of birch. Bioresource Technology, 127, Proceedings International Conference of Agricultural Engineering, Zurich, /5

The objective of this work was the production of ethanol

1.12 Bioethanol from sugar cane bagasse Urribarrí, Lauris 1 * Ferrer, Alexis 2 Aiello, Cateryna 3 Rivera, Jhoandry 4 Abstract The objective of this work was the production of ethanol by simultaneous saccharification