Biomass conversion through syngas-based biorefineries : thermochemical process integration opportunities

Abstract: The replacement of fossil resources through renewable alternatives is one way to mitigate global climate change. Biomass is the only renewable source of carbon available for replacing oil as a refining feedstock. Therefore, it needs to be utilized not just as a fuel but for both biochemical and thermochemical conversion through biorefining. Optimizing and combining various conversion processes using a system perspective to maximize the valorization, biomass usage, and environmental benefits is of importance. This thesis work has evaluated the integration opportunities for various thermochemical conversion processes within a biorefinery system.The aim for all evaluated concepts were syngas production through gasification or reforming. Two potential residue streams from an existing biorefinery were evaluated as gasification feedstocks, thereby combining biochemical and thermochemical conversion. Torrefaction as a biomass pretreatment for gasification end-use was evaluated based on improved feedstock characteristics, process benefits, and integration aspects. A system concept, “Bio2Fuels”, was suggested and evaluated for low-temperature slow pyrolysis as a way to achieve simultaneous biomass refinement and transport driven CO2 negativity.Syngas was identified as a very suitable intermediate product for residue streams from biochemical conversion. Resulting syngas composition and quality showed hydrolysis residue as suitable gasification feedstock, providing some adjustments in the feedstock preparation. Gasification combined with torrefaction pretreatment demonstrated reduced syngas tar content. The co-gasification of biogas and wood in a FBG was successfully demonstrated with increased syngas H2/CO ratio compared to wood gasification, however high temperatures (≥1000°C) were required for efficient CH4 conversion. The demonstrated improved feedstock characteristics for torrefied biomass may facilitate gasification of biomass residue feedstocks in a biorefinery. Also, integration of a torrefaction unit on-site at the biorefinery or off-site with other industries could make use of excess low-value heat for the drying step with improved overall thermal efficiency. The Bio2Fuels concept provides a new application for slow pyrolysis. The experimental evaluation demonstrated significant hydrogen and carbon separation, and no significant volatilization of ash-forming elements (S and Cl excluded)  in low-temperature (<400°C) pyrolysis. The initial reforming test showed high syngas CH4 content, indicating the need for catalytic reforming.The collective results from the present work indicate that the application of thermochemical conversion processes into a biorefinery system, making use of by-products from biochemical conversion and biomass residues as feedstocks, has significant potential for energy integration, increased product output, and climate change mitigation.