Supercritical Carbon Dioxide Pretreatment of Rye Straw for Biogas Generation by Lutz Christian KRAPF Master Thesis for the Degree of Master of Science (MSc) at the Institute of Chemical and Energy Engineering in Cooperation with the Division of Agricultural Engineering at the Department of Sustainable Agricultural Systems, University of Natural Resources and Applied Life Sciences, Vienna, Austria Supervisor: Ao.Univ.Prof. Dr.-Ing. Martin Wendland, Vienna, Austria Co-Supervisor: Assoc.Prof. Graeme D. Buchan, Lincoln, New Zealand Vienna, October 2008 MSc Natural Resources Management and Ecological Engineering NARMEE - International Joint Master Programme - Acknowledgements First of all, it’s a pleasure to thank these many people involved in this cooperative work between the Institute of Chemical and Energy Engineering under the direction of Prof. Fischer and the working group “Tierhaltungs-, Umwelttechnik” of the Division of Agricultural Engineering at the Department of Sustainable Agricultural Systems under the head of Prof. Amon. They made this thesis possible. In particular, I wish to thank Emmerich Haimer for his help with the experimental setup and general advice with all types of technical problems - at all times. His inspiration, and his great efforts to explain things clearly and simply, helped me to improve my technical understanding. His assistance was essential and greatly contributed to this work. I would like to gratefully acknowledge the supervision of Prof. Wendland. During our discussions, he provided me with many helpful considerations, important advice and constant encouragement. I thank Prof. Buchan whose editing suggestions and precise sense of language contributed to the final copy. I am much obliged to Alexander Bauer who gave me an understanding for the technical difficulties of fermentation experiments. Thank goes to Vitomir Bodiroza and the research group at the Versuchsanstalt in Groß Enzersdorf who supported me during this work. Throughout my thesis-writing period, many people provided encouragement, sound advice and motivation. Sincere thanks are given to all. II Zusammenfassung Über den anaeroben Abbauvorgang kann organisches Material in methanreiches Biogas umgewandelt werden. Die lignozelluläre Gerüststruktur verholzter Biomasse bildet hierbei ein Hindernis für die enzymatische Hydrolyse der Polysaccharide in Mono- und Oligosaccharide. Die Folge ist eine verringerte und zeitlich oftmals verzögerte Bildung von Biogas. Thermochemische Aufschlussverfahren stellen eine Möglichkeit dar, den enzymatischen Abbau von Lignozellulose während anaerober Vergärungsprozesse zu beschleunigen und somit eine verbesserte Biogasbildung zu ermöglichen. Mithilfe zweier unterschiedlicher Aufschlussverfahren wurde versucht, die Abbaubarkeit von Roggenstroh während anaerober Vergärung zu verbessern. Ziel hierbei war der Abbau des Kohlenhydrat-Ligninkomplexes infolge thermo- hydrolytischer Reaktionen. Die anschließende Herauslösung der verflüssigten Lignin- fragmente aus der Zellwand des Strohs, stellt eine weitere Anforderung an ein Voraufschlussverfahren dar. Beiden Verfahren gemein war die Verwendung überkritischen Kohlendioxides. Das eine Verfahren erfolgte in einem binären CO -H O 2 2 Gemisch. Das zweite Verfahren nutzte eine ternäre Mischung unter zusätzlicher Verwendung von Ethanol als einem organischen Lösungsmittel. Die Löslichkeitseigen- schaften eines überkritischen Fluides sowie die Bildung von Kohlensäure während des Aufschlussvorganges, mögen zu einer verbesserten Wirkung des Verfahrens beitragen. Die Ergebnisse dieser Arbeit ermöglichen keine eindeutige Bewertung der Wirkung des ersten untersuchten Verfahrens auf eine anschließende Fermentation des Roggenstrohs. So wurde mit der ternären Mischung, im Vergleich zu einer rein mechanischen Zerkleinerung des Strohs, eine Erhöhung der Methangaserträge erzielt. Die spezifische Wirkung des überkritischen CO konnte jedoch nicht befriedigend dargestellt werden. 2 Bei der Verwendung der binären Mischung konnte bei der anschließenden anaeroben Vergärung keine erhöhte Biogasbildung festgestellt werden. Neben den Anforderungen hinsichtlich einer Erzielung erhöhter Biogaserträge ist der Energiebedarf eines solchen Verfahrens von Bedeutung bei der Bewertung bezüglich seiner Geeignetheit für moderne Bioenergiesysteme. Durch eine Reduktion des Lösungsmittelanteils, gemessen an der Menge aufgeschlossenen Strohs, erforderte der Aufschlussprozess mit dem binären Gemisch einen deutlich geringeren Energiebedarf als das Verfahren auf Basis eines ternären Gemisches. III Summary The process of anaerobic digestion describes the transformation of organic material into methane-rich biogas. The utilization of lignin-rich biomass is characterized by a retarded transformation of this process as the lignocellulosic matrix constitutes a barrier against enzymatic hydrolysis of the polysaccharides into oligo- and monosaccharides. Hydrothermal pretreatment processes are considered one possibility for improving enzymatic degradation of the lignocellulose during anaerobic digestion resulting in higher overall biogas yields. By application of two different pretreatment methods, this study aimed for a better transformation of rye straw into biogas via anaerobic digestion. The aim was to rupture the lignin-carbohydrate complexe by means of thermo-chemical hydrolysis. Subsequent solvolysis of the lignin and transportation out of the cell wall of the straw describes a further requirement on pretreatment processes. Both methods tested in this work included the utilization of supercritical carbon dioxide. The first pretreatment process applied a binary CO -H O system. The second pretreatment utilized a ternary mixture of 2 2 carbon dioxide, ethanol, and water. Ethanol is a useful organic solvent for non-polar substances such as lignin. The application of a supercritical fluid is believed to increase solvolysis of lignin and subsequent removal out of the cell wall. Formation of carbonic acid due to dissolution of CO in water at elevated pressure is believed to improve 2 pretreatment processes. Results from this work do not show a clear tendency regarding the impact of the application of supercritical carbon dioxide on subsequent fermentation of the rye straw. An increase in methane formation has been achieved for the pretreatment with the ternary mixture, compared to milled raw straw. However, the specific effect of supercritical carbon dioxide remained rather unclear. No increase in biogas formation could be achieved with regard to the pretreatment process applying the binary mixture. Besides physico-chemical requirements on pretreatment processes, energy demand for pretreatment is important for assessing its applicability for biobased energy systems. Compared to the pretreatment with the ternary mixture, energy demand for the binary mixture could be reduced by reducing the amount solvent required for a certain amount of straw. As a consequence, less energy was needed for co-heating of the liquid phase. IV Table of Contents 1 INTRODUCTION 1 1.1 Statement of the Problem 4 1.2 Objectives and Procedures 4 1.3 Composition and Structure of Lignocellulose 7 1.3.1 Cellulose 7 1.3.2 Hemicelluloses 8 1.3.3 Lignin 9 1.4 Factors Limiting Enzymatic Hydrolysis 10 1.5 Pretreatment of Lignocellulose 11 1.5.1 Requirements 11 1.5.2 Pretreatment Methods 15 1.5.3 Reaction Severity and Effect of CO on pH Value 20 2 1.6 Cubic Equations of State 23 1.6.1 Phase Equilibrium 25 1.6.2 Pure Substances 26 1.6.3 Binary System 27 1.6.4 Ternary System 31 1.6.5 Calculation of Enthalpy Changes 33 1.7 Material and Energy Balances 33 1.8 Anaerobic Digestion 36 1.8.1 Biochemistry and Microorganisms 36 1.8.2 Process Conditions 37 1.8.3 Biogas Yields 39 2 MATERIAL AND METHODOLOGY 42 2.1 Technical Equipment 42 2.2 Chemicals 42 V 2.3 Composition of the Raw Material 43 2.4 Inoculum for Anaerobic Fermentation 43 2.5 Pretreatment Process 44 2.5.1 Preparation and Composition of the Mixture 44 2.5.2 Experimental Design for Pretreatments 44 2.5.3 Operation of the Autoclave 45 2.6 Separation and Analysis of the Autoclaved Straw 48 2.6.1 Washing and Filtration of the Straw 48 2.6.2 Distillation of the Filtrate and Sedimentation of the Water-Insoluble Phase 49 2.6.3 Separation of Lignin and Water-Soluble Organics 49 2.6.4 Determination of Klason-Lignin Content 50 2.6.5 Elementary Analysis 50 2.7 Anaerobic Batch Fermentation Tests 51 3 RESULTS AND DISCUSSION 53 3.1 Supercritical CO -Organosolv Pretreatment 53 2 3.1.1 Results 53 3.1.1.1 Pretreatment Results 56 3.1.1.2 Results for Methane Production 57 3.1.1.3 Pressure Effects on Pretreatment and Methane Generation 59 3.1.2 Discussion 59 3.1.2.1 Impact of Reaction Severity on Solid Residue Yield 59 3.1.2.2 Relation between Solid Residue Yield and Klason Lignin Content 60 3.1.2.3 Impact of Klason Lignin Content and Co-Fermentation of the Water-Soluble Phase on Methane Production 61 3.1.2.4 Pressure Effect on Solid Residue Yield and Klason Lignin 62 3.1.2.5 Pressure Effects on Methane Production 63 3.2 Supercritical CO -Explosion 65 2 3.2.1 Results 65 3.2.1.1 Pretreatment Results 67 3.2.1.2 Results for Methane Production 68 3.2.2 Discussion 69 3.2.2.1 Evaluation of Pretreatment Results 69 3.2.2.2 Evaluation of Methane Production 71 VI 4 MATERIAL AND ENERGY REQUIREMENTS 74 4.1 Quantitative Assessment of the Feedstock Material 75 4.2 Enthalpy Changes and Assessment of Pretreatment Processes 78 5 CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK 80 5.1 Conclusions 80 5.2 Suggestions 82 6 LITERATURE 84 7 APPENDIX 93 VII List of Tables Table 1: Energy requirement of mechanical comminution of lignocellulosic materials with different size reduction 16 Table 2: Results of thermodynamic determination of carbonic acid pH in a binary carbon dioxide-water system at elevated temperature and pressure 23 Table 3: Phase equilibrium for temperature, pressure and fugacity 26 Table 4: Optimal growth temperatures for selected methanogenic bacteria 38 Table 5: Theoretical biogas yields and composition of the biogas from different substrates 40 Table 6: Equipment used for experiments and analysis conducted in this work 42 Table 7: Chemicals required for experiments and analysis carried out in this work 42 Table 8: Elementary composition of rye straw (Secale cereale) and theoretical methane yield 43 Table 9: Caloric value of rye straw (Secale cereale) 43 Table 10: Pretreatment results for the scCO -organosolv method 54 2 Table 11: Biogas and methane yields for rye straw pretreated with the scCO -organosolv 2 method 54 Table 12: Elementary composition of the solid residue after pretreatment and the raw straw 54 Table 13: Experimental design and results for the scCO -explosion pretreatment 66 2 Table 14: Experimental design and biogas and methane yields for fermentation tests from straw pretreated with the scCO -explosion method 66 2 Table 15: Elementary composition of the pretreated straw 66 Table 16: Supercritical CO -organosolv process: Overview about the amount of feedstock 2 material required for fermentation samples and the converted methane values for the raw straw 76 Table 17: Supercritical CO -explosion pretreatment: Overview about the amount of feedstock 2 material required for fermentation samples and the converted methane values for the raw straw 76 Table 18: Supercritical CO -organosolv pretreatment: Influence of temperature and pressure on 2 enthalpy changes during pretreatment and calculation of the difference between to energy output and the input 78 Table 19: Supercritical CO -explosion: Influence of temperature and pressure on enthalpy 2 changes during pretreatment and calculation of the difference between the energy output and the input 78 VIII List of Figures Figure 1: Process scheme of the hydrothermal pretreatment of rye straw with separation of the product streams and subsequent anaerobic digestion 6 Figure 2: Schematic structure of lignocellulose 7 Figure 3: Scheme of the arrangement of cellulose microfibrils and cellulose 8 Figure 4: Lignin/phenolics–carbohydrate complex in wheat straw 10 Figure 5: Schematic of goals of pretreatment on lignocellulosic material 12 Figure 6: Reactions occurring during hydrolysis of lignocellulosic materials 13 Figure 7: Schematic of a (physico-) chemical pretreatment process 16 Figure 8: Left: Literature values for Henry’s constant describing CO solubility at elevated 2 temperatures. Right: Literature values for the first dissociation constant of H CO in water 2 3 at elevated temperatures 22 Figure 9: Vapour pressure curve of water 27 Figure 10: P, T, x-diagram of a binary system with one gaseous and one liquid phase 28 Figure 11: Schematic representation of phase equilibrium of a binary system with one supercritical component in a P, x-diagram 29 Figure 12: Pressure dependent phase behaviour of a ternary system 31 Figure 13: Flowsheet-simulation with Chemcad 35 Figure 14: Typical shapes of gas formation curves from batch fermentation tests 41 Figure 15: Pretreatment scheme for the autoclavation of rye straw 46 Figure 16: Separex Autoclave SFP1 47 Figure 17: Description of the autoclave used for pretreatment experiments 47 Figure 18: Product streams obtained from the separation process 49 Figure 19: Schematic description of anaerobic batch fermentation apparatus 52 Figure 20: Methane production of fermentation run F1-F7 and the raw straw 55 Figure 21: A: Relationship between reaction severity during autoclavation and solid residue yield. B: Dependency between solid residue yield and pH of the hydrolysate. C: Relation of the solid residue yield to the Klason lignin content. D: Effect of Klason lignin content on methane yield at different pressure 56 Figure 22: Cumulative methane formation curves for the fermentation runs F8-F13 67 Figure 23: Product streams of Ligno-Cellulosic Feedstock Biorefinery 75 Figure 24: Supercritical CO -organosolv: Relationship between methane yield calculated for 2 the feedstock material to its percentage share taken for fermentation 77 IX 1 Introduction Increasing environmental awareness of the negative impacts of the consumption of fossil fuel, particularly the emission of greenhouse gases (GHG), is a key incentive to look for renewable alternatives to petroleum, coal and natural gas. Concerns about the security of oil supplies, due to an accelerating depletion rate of crude oil further motivates developing biobased energy systems and technologies [1, 2]. Based on the commitment to reduce emissions of GHG by 8% in 2010 compared to 1990, the European Union (EU) promotes the use of renewable sources for energy supply [3]. Several political documents underline the importance of investigating into alternative energy-strategies and their implementation [4-7]. Biomass from agricultural production and forestry is supposed to play a major role in developing sustainable energy-supply systems in the mid-term perspective. Direct thermal combustion is the most common way of using chemical energy stored in plant biomass. Thermal gasification and subsequent liquefaction (BtL-Process) as well as transformation of organic matter into bioethanol or biogas are alternative technologies for the generation of renewable secondary energy sources. In comparison to direct thermal combustion of the feedstock, the generation of a secondary energy-carrier allows a more flexible use of the stored energy e.g. in mobile systems including the transport sector [8, 9]. Alternative propellants such as bioethanol and biogas are substitutes for fossil fuels. The EU’s goal is to increase the share of biobased fuels for vehicles to 5.75% by the year 2010 [10, 11]. For stationary applications, methane-rich biogas can be converted into electricity and heat in combined heat and power units. Purified biogas can be introduced into the gas distribution-system resulting in a high temporal and spatial flexibility [12]. Due to the potentially high energy output-to-input ratio of biogas systems, conversion of agrarian biomass into methane-rich biogas via anaerobic digestion is seen a key technology for implementing sustainable energy supply systems [9, 13]. Following Amon et al. [13], in Europe up to 320 million tons of crude oil equivalent (COE) could be substituted by biogas production from energy crops every year. This figure accounts for about 96% of the total energy demand from road traffic within the EU. On a European level about 1500 million tons of biomass could be anaerobically digested each 1