Impact of changes in the syngas-biochar mix and plant size on the economics and environmental performance of distributed biomass gasification systems

Abstract

Agriculture and forestry residues are potential sources of sustainable energy that do not compete with food or demand land use changes. Small-scale biomass gasification could be used to generate decentralized renewable electricity where these biomass stocks are locally available, while co-producing biochar to sequester carbon. This study evaluated how the scale and the syngas-biochar trade-offs impact the economics and decarbonization potential of a gasification system. A small-scale downdraft gasifier fed with logging residues in Michigan was used as case study. A Life Cycle Assessment (LCA) approach was used to formulate Economic Benefit (EB) and Carbon Abatement (CA) objective functions that formed a Multi Criteria Decision Analysis (MCDA) problem. Feasible product mix and scale configurations were mapped, and a pareto frontier was identified. EB is maximized when the electricity generation and the scale are maximized, at expense of emitting 1.683 kg CO2eq/kWh. Conversely, CA is maximized to 0.348 kg CO2eq abated per kWh for the highest biochar production and the smallest scale. Results were found to be sensitive to external factors: EB optimum shifted to maximize biochar when the carbon price was increased from 5 $/ton CO2eq to match the social cost of carbon (50 $/ton CO2eq) and 2030 projections (100 $/ton CO2eq), CA increased 112.0% when grid electricity emissions were increased from 0.48 kg CO2eq/kWh (Michigan’s) to 0.87 kg CO2eq/kWh (West Virignia’s), and EB reached 0.147 $/kWh when a high electricity price of 33 ¢/kWh (Hawaii’s) is considered instead of Michigan’s 13 ¢/kWh. For different stakeholders and contexts, the maximization of positive impacts can require different technology configurations. The developed LCA-MCDA combined methodology provides an example of a framework that could inform decision-making in the deployment of biomass gasification to reconcile economic and climate change mitigation objectives.