CRADLE-TO-GRAVE LIFE CYCLE ASSESSMENT OF SYNGAS ELECTRICITY FROM WOODY BIOMASS RESIDUES

摘要

Forest restoration and fire suppression activities in the western United States have resulted in large volumes of low-to-no-value residues. An environmental assessment would enable greater use while maintaining environmental sustainability of these residues for energy products. One internationally accepted sustainable metric tool that can assess environmental impacts of new bioenergy conversion systems is the life cycle assessment (LCA). Using the LCA method, this study evaluated the synthesis gas (syngas) electricity produced via a distributed-scale biomass thermochemical conversion system called the Tucker renewable natural gas (RNG) system. This system converts woody biomass in a high-temperature and extremely low-oxygen environment to a medium-energy syngas that is burned to generate electricity. The system also produced biochar as a by-product and tar as a waste. Results from the life cycle impact assessment included an estimate of the global warming (GW) impact from the cradle-to-grave production of syngas for electricity. When the carbon sequestration effect from the biochar by-product was included, GW impact value (0.330 kg CO2-eq/kWh) was notably lower compared with electricity generated from bituminous coal (1.079 kg CO2-eq/kWh) and conventional natural gas (0.720 kg CO2-eq/kWh). Other environmental impacts showed that syngas electricity ranged between the direct-biomass-burned electricity and fossil-fuel-combusted electricity for different impact categories. This occurred because, although the woody biomass feedstock was from a renewable resource with less environmental impact, propane was consumed during the thermochemical conversion. Specifically, the evaluation showed that the highest greenhouse gas (GHG) emissions contribution came from burning propane that was used to maintain the endothermic reaction in the Tucker RNG unit. If the tar waste from the system were converted into a low-energy syngas and used to supplement propane consumption, a further decrease of 41% in GHG emissions (ie fossil CO2) could be achieved in this cradle-to-grave assessment.