LIFE-CYCLE ASSESSMENT OF A DISTRIBUTED-SCALE THERMOCHEMICAL BIOENERGY CONVERSION SYSTEM

AbstractExpanding bioenergy production from woody biomass has the potential to reduce net greenhouse gas (GHG) emissions and improve nation’s energy security. Science-based and internationally accepted life cycle assessment (LCA) is a tool essential for policy makers to decide on expanding renewable energy production from newly developed technologies. A distributed-scale high-temperature thermochemical conversion system, referred to as the Tucker renewable natural gas (RNG) unit was evaluated on producing medium-energy synthesis gas (syngas) and biochar along with its waste from harvested woody biomass. Mass and energy balances, cumulative energy demand (CED), and life cycle inventory (LCI) flows were found based on operational data from a 1-hr continuous run. Emission data summarized from the cradle-to-gate LCI showed biomass and fossil CO2 emissions of 159 g and 534 g, respectively, for each oven-dry kg of wood chips pyrolyzed. LCA, applied in accordance with ISO 14040:2006, was used to determine the potential environmental impacts. Total GHG is 0.595 kg CO2 eq/ OD kg of wood chips processed. Contributions to the total GHG contribution were 20.7% from upstream forest resource extraction and chip processing at sawmill and 77.6% from thermochemical conversion process with propane combustion. The remaining1.62% was from parasitic electricity operating the Tucker RNG unit. Quantifying Global Warming Potential (GWP) showed the carbon benefits (eg low GHG emissions) along with the carbon “hotspots” from burning propane to maintain the endothermic reaction in the Tucker RNG unit. Using low-energy syngas generated from what was originally a waste in the pyrolysis reaction to augment propane combustion would reduce GHG emissions (ie fossil CO2) by about 30.4%.