Life-Cycle Assessment of a Distributed-Scale Thermochemical Bioenergy Conversion System

Expanding bioenergy production from woody biomass has the potential to decrease net greenhouse gas (GHG) emissions and improve the energy security of the United States. Science-based and internationally accepted life-cycle assessment (LCA) is an effective tool for policy makers to make scientifically informed decisions on expanding renewable energy production from newly developed bioenergy technologies. A distributed-scale high-temperature thermochemical conversion system, referred to as the Tucker renewable natural gas (RNG) unit, was evaluated for producing medium-energy synthesis gas (syngas) and biochar along with its waste from harvested woody biomass. Mass and energy balances, cumulative energy demand, and life-cycle inventory (LCI) flows were calculated based on operational data from a 1-h continuous run. Emissions data summarized from the cradle-to-gate LCI showed biomass and fossil CO2 emissions of 0.159 and 0.534 kg, respectively, for each oven-dry (OD) kilogram of wood chips pyrolyzed. LCA, applied in accordance with International Organization for Standardization standards, was used to determine the potential environmental impacts. Total GHG was 0.595 kg CO2 eq per OD kilogram of wood chips processed. Contributions to total GHG were 20.7% from upstream forest resource extraction and chip processing at sawmills and 77.6% from the thermochemical conversion process with propane combustion. The remaining 1.62% was from parasitic electricity operating the Tucker RNG unit. Quantifying global warming 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. The use of low-energy syngas generated from what was originally a waste in the pyrolysis reaction to augment propane combustion would decrease GHG emissions (ie, fossil CO2) by about 30.4%.