Multistage torrefaction and upgrading for the coproduction of biofuels and specialty chemicals: Analysis of life cycle energy consumption and GHG emissions

Concurrent production of hydrocarbon biofuels and high-value bio-based chemicals via thermochemical conversion of lignocellulosic biomass and catalytic upgrading of bio-oil has gained attention as a promising option for... [ view full abstract ]

Concurrent production of hydrocarbon biofuels and high-value bio-based chemicals via thermochemical conversion of lignocellulosic biomass and catalytic upgrading of bio-oil has gained attention as a promising option for mitigating anthropogenic derived climate change and enhancing domestic energy independence and security. However, bio-oil produced via conventional thermochemical routes (i.e. fast pyrolysis) contains myriad chemical functionalities, making selective catalytic upgrading to fuels and/or chemicals challenging and a major technical bottleneck. This work explores an innovative design strategy, in which the primary constituents of lignocellulosic biomass (hemicellulose, cellulose, and lignin) are selectively decomposed via a series of torrefaction and pyrolysis reactors to produce several thermally fractionated bio-oil streams. The purposed multistage design permits tailored catalytic upgrading of the fractionated bio-oil streams to hydrocarbon fuels and/or specialty bio-based chemicals with minimal process hydrogen consumption, and thus holds promise for an improved environmental profile relative to traditional fast pyrolysis and hydroprocessing fuel platforms.

This work performs a prospective life cycle assessment to determine the environmental profile of concurrent hydrocarbon biofuel and specialty biochemical production via a series of novel multistage torrefaction and in-situ catalytic upgrading systems. Several multistage design cases, consisting of a combination of different catalytic upgrading strategies for targeted production of renewable fuels and/or bio-chemicals are considered. Detailed material and energy balances for the examined design cases are developed using a combination of experimental data and ASPEN process simulations. Several metrics including process hydrogen consumption, life cycle greenhouse gas (GHG) emissions, and Energy Return on Investment (EROI) are used to compare the performance of the multistage systems and benchmark against petroleum fuels. In addition, uncertainty in the process and life cycle inventory data is captured via Monte Carlo techniques, and thus provides a robust estimate of the anticipated environmental profiles for the examined design cases. This work seeks to answer several critical research questions including: (1) do renewable fuel(s) achieve minimum energy return on investment (EROI) criteria as well as meet compliance with life-cycle GHG emissions reductions thresholds set by U.S. federal regulatory programs, under different allocation schemes and coproduct scenarios? (2) Which unit processes are responsible for the highest environmental burdens in the supply chain? (3) Which parameters have the largest impact on the EROI and life cycle GHG emissions profile of renewable fuels?

Preliminary results reveal that the multistage design has the capacity of achieving over 85% GHG reductions relative to traditional petroleum diesel, with exogenous fossil-derived hydrogen constituting the principle GHG and primary energy burden across all examined cases. Further, partitioning a fraction of the produced bio-oil for biochemical production results in an improved environmental profile, but is sensitive to the choice of LCA coproduct handing method (allocation or displacement method). Additionally, while biochemical production is inherently constrained by the market size of specialty chemicals, the proposed modular design is flexible in its capacity for selective upgrading to fuels or chemicals, and can thus scale with growth in emerging markets. The implications of the results for decision-making, and efficacy of the bio-fuel/chemical industry for climate change mitigation will be discussed.