Biological Production of Hydrogen and Methane: Process Evaluation and Design through Modeling

Abstract: A great deal of research is being carried out on the biological production of hydrogen and methane. However, little effort has been devoted to studying the technical and economic feasibility of these processes. The main objective of the work presented in this thesis was to perform techno-economic analyses to improve our understanding of the processes by identifying the bottlenecks, and to provide insight into the present status and potential of these processes from an economic perspective. The following processes were studied: biological hydrogen production (combining dark fermentation and photofermentation), biological hydrogen and methane production (combining dark fermentation and anaerobic digestion), and methane production (anaerobic digestion alone). Pretreatment and hydrolysis of the raw material (potato steam peels and barley straw in this work) and upgrading of the gaseous products were included in the investigations. An ethanol production process was included as a reference. The techno-economic aspects of the processes were evaluated by simulations using models implemented in the commercial flowsheeting software Aspen Plus™. In addition to the techno-economic evaluations, detailed studies of the dark fermentation step and the gas upgrading technique of water scrubbing were performed. The hydrogen production cost was found to be very high, primarily due to the high cost associated with the photofermentation step; the main bottlenecks being the low hydrogen productivity and the requirement of large amounts of buffers. The production cost in the biological hydrogen and methane process was shown to be significantly lower than that of the hydrogen process. However, the cost is still considerably higher than that of the methane and ethanol processes. Nonetheless, the process combining dark fermentation and anaerobic digestion has potential, for example, within a biorefinery, and deserves more attention.