Thermochemical processing of biomass to obtain fuel and chemicals has received significant attention for a wide range of feedstocks. Early studies of thermochemical conversion of biomass focused mainly on fast pyrolysis, which involves rapid heating of biomass in the absence of air at approximately 500 °C. While fast pyrolysis is the simplest method of obtaining liquid from solid biomass, the resulting bio-oil contains a large number of oxygenated compounds (e.g., organic acids, aldehydes, ketones, etc.), which are chemically unstable and difficult to upgrade catalytically. Multi-stage thermal decomposition of biomass provides a promising alternative for overcoming these issues because biomass is fractionated into simpler streams (with a smaller number of species), thereby enabling their effective upgrading via tailored catalytic bed reactors. There are multiple options for the design of the catalytic upgrading system, including the number, types, and sequence of chemistries to perform (Figure 1). Therefore, identifying the optimal strategy, after accounting for performance metrics such as fuel-range carbon yield and hydrogen consumption, is a classic process-synthesis problem.
In this work, we use a multi-pronged approach to address the above problem. First, we use a superstructure-based optimization framework to identify the promising routes to convert biomass to liquid fuel. Second, based on reasoning, and insight obtained from experimental data (procured in part through our collaborations), we synthesize alternative catalytic upgrading strategies and rigorously evaluate them using Aspen Plus. For a realistic evaluation, our models in both the approaches are based on experimental data. Using a wide range of technoeconomic parameters, we identify (1) the relationship between process complexity and the resulting fuel-range carbon yields and economic feasibility, (2) the economic advantage of integrating different thermal decomposition fractions, and (3) the key cost drivers of the integrated processes.
