Multistage torrefaction and in situ catalytic upgrading to hydrocarbon biofuels: analysis of life cycle energy use and greenhouse gas emissions†
A well-to-wheel life cycle assessment (LCA) model is developed to characterize the life cycle energy consumption and greenhouse gas emissions profiles of a series of novel multistage torrefaction and pyrolysis systems for targeted thermochemical conversion of short rotation woody crops to bio-oil and in situ catalytic upgrading to hydrocarbon transportation fuels, and to benchmark the results against a base-case fast pyrolysis and hydrodeoxygenation (HDO) platform. Multistage systems utilize a staged thermal gradient to fractionate bio-oil into product streams consisting of distinct functional groups, and multi-step chemical synthesis for downstream processing of bio-oil fractions to hydrocarbon fuels. Results at the process scale reveal that multistage systems have several advantages over the base-case including: (1) ∼40% reduction in process hydrogen consumption and (2) the product distribution for multistage systems are skewed towards longer carbon chain compounds that are fungible with diesel-range fuels. LCA reveals that the median Energy Return On Investment (EROI) and life cycle greenhouse gas (GHG) emissions for multistage systems range from 1.32 to 3.76 MJ-fuel/MJ-primary fossil energy and 17.1 to 52.8 gCO2e/MJ-fuel respectively, over the host of co-product scenarios and allocation schemes analyzed, with fossil-derived hydrogen constituting the principle GHG and primary energy burden across all systems. These results are compelling and indicate that multistage systems exhibit comparatively higher gasoline/diesel-range fuel yield relative to current technology, and produce a high quality infrastructure-compatible hydrocarbon transportation fuel capable of achieving over 80% reduction in life cycle GHG emissions relative to baseline petroleum diesel.