Thermochemical conversion of lignocellulosic biomass via single-stage fast pyrolysis and catalytic hydroprocessing of resultant bio-oil has gained attention as a commercial platform for producing transportation fuel. However, single stage pyrolysis suffers from several limitations including high hydrogen consumption, high production of light (C1-C4) hydrocarbons, and low liquid fuel yield. A multi-tiered torrefaction and pyrolysis system in which input biomass undergoes staged thermal decomposition is purposed to address these limitations, while concurrently increasing the overall performance and efficiency of the system. Thermal fractionation of lignocellulosic biomass produces intermediate product streams of varying chemical composition, which can be selectively upgraded via carbon-carbon coupling reactions to increase the carbon chain length of the resultant biofuel while simultaneously reducing overall hydrogen consumption.
We present a life cycle framework to proactively evaluate the potential environmental impacts of a novel multistage torrefaction/pyrolysis system for conversion of woody biomass to liquid transportation fuels. Several multistage configurations, consisting of a combination of stream specific bio-oil upgrading strategies and coproduct scenarios are modeled and compared with a single-stage direct hydrodeoxygenation (HDO) pyrolysis platform. The experimental results are coupled with detailed ASPEN process models to simulate several multistage process configurations, while life cycle assessment is performed to evaluate environmental impacts. Monte-Carlo simulation is used to quantify uncertainty for several key sustainability metrics including energy return on investment (EROI) and life cycle greenhouse gas (GHG) emissions.
Preliminary results at the process scale reveal that the multistage system(s) have several advantages over a single stage fast pyrolysis and direct hydrotreating platform including: (1) product distribution for multistage system(s) are skewed towards higher carbon compounds which is more amicable to gasoline and diesel range fuels, (2) multistage system(s) have less hydrogen consumption, and (3) the multistage system(s) are able to retain significantly more carbon in the final liquid transportation fuel. Preliminary LCA results show that the EROI and life cycle GHG emissions (gCO2 eq/MJ) for the multistage system(s) range from 1.20 to 4.70 and -17.9 to 27.8 respectively, depending on the choice of coproduct scenario. Tradeoffs between EROI, life cycle GHG emissions, product distribution, liquid carbon yield, and process complexity for the evaluated multistage configuration(s) will be discussed.