Hydrothermal gasification of waste biomass: process design and life cycle asessment

A process evaluation methodology is presented that incorporates flowsheet mass and energy balance modeling, heat and power integration, and life cycle assessment Environmental impacts are determined by characterizing and weighting (using CO2 equivalents, Eco-indicator 99, and Eco-scarcity) the flowsheet and inventory modeling results. The methodology is applied to a waste biomass to synthetic natural gas (SNG) conversion process involving a catalytic hydrothermal gasification step. Several scenarios are constructed for different Swiss biomass feedstocks and different scales depending on logistical choices: large-scale (155 MW(SNG)) and small-scale (5.2 MW(SNG)) scenarios for a manure feedstock and one scenario (35.6 MW(SNG))for a wood feedstock. Process modeling shows that 62% of the manure's lower heating value (LHV) is converted to SNG and 71% of wood's LHV is converted to SNG. Life cycle modeling shows that, for all processes, about 10% of fossil energy use is imbedded in the produced renewable SNG. Converting manure and replacing it, as a fertilizer, with the process mineral byproduct leads to reduced N20 emissions and an improved environmental performance such as global warming potential: -0.6 kg(CO2eq)/MJ(SNG) vs. -0.02 kg(CO2eq)/MJ(SNG) for wood scenarios.