Development of a Biomass-to-Methanol Process Integrating Solid State Anaerobic Digestion and Biological Conversion of Biogas to Methanol DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Johnathon Patrick Sheets, M.S. Graduate Program in Food, Agricultural and Biological Engineering The Ohio State University 2017 Dissertation Committee: Dr. Jay Martin, Advisor Dr. Gönül Kaletunç Dr. Ajay Shah Dr. Zhongtang Yu Copyright by Johnathon P. Sheets 2017 Abstract Solid-state anaerobic digestion (SS-AD) can be used to convert abundant, low moisture feedstocks, such as switchgrass, to methane (CH4)-rich biogas. However, SS-AD of energy crops is a relatively nascent technology that requires further optimization to increase biogas yields. Additionally, biogas from anaerobic digestion is a gas under ambient conditions, and impurities such as carbon dioxide and hydrogen sulfide need to be removed before it can be upgraded to purified biogas for pipeline injection or to transportation fuels, such as compressed natural gas. These issues have sparked interest in technologies that can convert biogas to methanol, a liquid chemical that can be used directly as a fuel or can be upgraded to a variety of other products. However, the thermochemical process for methanol production has high capital costs, operates at high temperatures and pressures, and requires a CH4 feedstock with few impurities. In contrast, the biological process for conversion of biogas to methanol may not require biogas purification and can operate under low temperatures and pressures, reducing capital costs and energy demands. Integration of SS- AD with biological conversion of biogas to methanol has great potential as an environmentally friendly technology to produce methanol from renewable feedstocks. For this research, there were five inter-related projects: 1) investigation of impacts of environmental conditions on SS-AD of switchgrass for biogas production; 2) isolation of methanotrophs (CH4 oxidizing bacteria) from SS-AD that can directly convert biogas to methanol; 3) development of a trickle-bed bioreactor (TBR) to improve gas-to-liquid mass ii transport and produce methanol from biogas; 4) mathematical modeling of TBRs to identify the impacts of key operating parameters on biological conversion of biogas to methanol; and 5) techno-economic analysis to compare the economic feasibility of biological conversion of biogas to methanol to other biogas upgrading technologies. The first project showed that limited air exposure had a minimal effect on SS-AD performance, indicating that this popular method to reduce hydrogen sulfide levels in biogas could be used in scaled-up reactors. Thermophilic temperature (55°C) enhanced biodegradation of cellulosic materials and improved biogas yields (102–145 L CH4 kg -1 -1 VSadded ) compared to mesophilic (37°C) temperature (88–113 L CH4 kg VSadded ). Net energy analysis of a theoretical scaled up “garage-type” SS-AD reactor (operating in Ohio) suggested that positive net energy could be obtained using either thermophilic or mesophilic conditions at elevated total solids contents (≥ 20% TS). A new methanotroph strain, called Methylocaldum sp. 14B, was isolated from the digestate of a mesophilic SS-AD reactor in the second project. Methylocaldum sp. 14B had comparable physiological characteristics and had similar 16S rRNA gene sequence identity to strains of the genus Methylocaldum. This methanotroph had comparable growth rates (0.06 h-1) in nitrate mineral salts (NMS) medium using either biogas from a commercial anaerobic digestion facility (70% CH4, 30% CO2, <50 ppm H2S) or purified CH4 (99% CH4) as the primary carbon sources and air as the oxygen source (O2). Strain 14B successfully converted biogas to methanol using phosphate as a methanol dehydrogenase (MDH) inhibitor and formate as an electron donor. The maximum methanol concentration (0.43±0.00 g L-1) and CH4 to methanol conversion ratio (25.5±1.8%) were obtained using strain 14B suspended in iii NMS medium (0.4 g dry cells/L) containing 50 mM phosphate and 80 mM formate and headspace gas composed of biogas and air at a 1:2.5 ratio (v/v). Biological conversion of biogas to methanol is likely limited by the low solubility and mass transport of gaseous substrates (O2, CH4) in NMS medium. From the third project, it was determined that abiotic mass transport of O2 to DI water in a TBR packed with ceramic balls was almost two times higher than an unpacked TBR. The TBR was inoculated with Methylocaldum sp. 14B and then operated non-sterilely under different dilution rates and biogas:air ratios. Maximum CH4 uptake rates were observed at high biogas:air ratios (1:2.5), and the results suggested that the TBR enhanced gas oxidation compared to shake flasks. The non-sterile TBR also was used to convert biogas to methanol using formate as an electron donor and phosphate as MDH inhibitor. Maximum methanol productivity (0.9 g/L/d) was obtained at 12 mmol formate addition and 3.6 mmol phosphate addition and a biogas:air ratio of 1:2.5 (v/v). Operation under non-sterile conditions caused differences in the microbial community of the TBR. A mathematical model that considered gas-to-liquid mass transport and methanotroph gas consumption kinetics was developed to analyze the use of TBRs for biological conversion of biogas to methanol. The model was used to generate results that were fairly comparable to selected semi-batch data from the lab-scale TBR project. The model was then used to identify the impacts of key operating parameters on biogas to methanol conversion in a theoretical large scale TBR (H=20 m, D=2 m) that had methanotrophs with good methanol tolerance (30 g/L). The results suggested that maximum methanol yields (18 g/L) could be obtained at high gas velocities (>500 m/h), high methanotroph cell density (40 kg cells/m3), and elevated pressure (up to 3 atm), because those conditions improved mass transport and iv gas uptake kinetics. Sensitivity analysis indicated that TBR packings with high specific surface area should be used to enhance gas-to-liquid transport, and that methanotrophs with higher CH4 oxidation rates and higher methanol tolerance will improve methanol production rates. Finally, techno-economic analysis was used to compare the economic feasibility of biological conversion of biogas to methanol to conventional and emerging methods for biogas upgrading at a large-scale biogas production facility (5900 Nm3/h, 5,000,000 sft3/d). Biogas cleaning via pressurized water scrubbing (PWS) for compressed natural gas production (Bio-CNG) had the highest net present value, followed by PWS for purified biogas production, biological conversion of biogas to methanol, and thermochemical conversion of biogas to methanol. Biological conversion had slightly higher methanol production costs than thermochemical conversion because of low gas conversion rates by methanotrophs. Sensitivity analysis indicated that the costs of biological conversion could be reduced if methanotrophs are modified to have higher CH4 oxidation rate, higher CH4 to methanol conversion ratio, and higher methanol tolerance. Furthermore, the cost of formate needs to be significantly reduced or alternative electron donors are needed for biological conversion of biogas to methanol to be economically feasible. These results from this research indicate that an integrated process consisting of SS- AD of switchgrass and biological conversion of biogas to methanol is technically feasible. The knowledge obtained from these studies could be used to assist in the optimization and scale-up of SS-AD and biotechnologies for biogas valorization. v Acknowledgments I am incredibly grateful to my original advisor, Dr. Yebo Li, for his guidance throughout the past five years. I did not come into graduate school with the same accolades or test scores as other students, and to an outsider, Dr. Li’s commitment of research funding to me could have been perceived as a risk. However, Dr. Li took a chance on my work ethic and aspirations to improve as an engineer and researcher. He knew how to encourage my strengths and helped me improve on my weaknesses. He introduced me to some of the most genuine and intelligent people I now call friends. There is no doubt that the time working with Dr. Li will go down as one of the most influential experiences in my life. I encourage all those involved in research to tackle their projects with the tenacity and positive attitude that Dr. Li brought to the Bioproducts and Bioenergy Research Laboratory every single day. Many thanks go to my dissertation committee members including Dr. Jay Martin, Dr. Gönül Kaletunç, Dr. Zhongtang Yu, and Dr. Ajay Shah for their advice and collaboration throughout the development of my dissertation. I am especially thankful to Dr. Martin and Dr. Kaletunç for their advisement during my TA appointment and throughout the last year of my PhD work. Several members of the Department of Food, Agricultural and Biological Engineering were crucial, including Mrs. Mary Wicks for her time reviewing publications and proposals, Ms. Candy McBride for her administrative assistance and encouragement, Mrs. Peggy Christman for her administrative assistance, and Mr. Michael Klingman and Mr. Scott Wolfe for their expertise in the engineering and fabrication of experimental equipment. vi One of the great joys of graduate school was getting to know the diverse and incredibly talented members of the Bioproducts and Bioenergy Research Laboratory. My sincere thanks go to Dr. Xumeng Ge, Dr. Xiaolan Luo, Dr. Fuqing Xu, Dr. Stephen Park, Dr. Shengjun Hu, Dr. Liangcheng Yang, Dr. Zhiwu Wang, Ms. Juliana Vasco, Ms. Long Lin, Ms. Kathryn Lawson, Mr. Adam Khalaf, Mr. Josh Borgemenke, Ms. Lo Niee Liew, Mr. Jia Zhao, Ms. Zhe Liu, and several others for their willingness to provide advice and mentorship throughout this journey. I especially want to thank Juliana Vasco, Dr.