(379f) A System-Level Analysis on Biomass Thermal Fractionation and Catalytic Upgrading Processes

Biomass presents a promising source for sustainable liquid fuels production. There are three principle routes for converting biomass into liquid fuels, classified by the initial treatment of the biomass: gasification, hydrolysis, or pyrolysis and liquefaction. The pyrolysis route involves thermal decomposition of the biomass at high temperature (500-800°C), and it has demonstrated high yields. Unfortunately, the wide range of components in the resulting pyrolysis vapor (oil) makes it unstable, corrosive, and challenging to catalytically upgrade. Furthermore, the high content of light (<C6) oxygenates in the pyrolysis are unsuitable for liquid fuels and are therefore of limited utility. Multi-stage torrefaction of biomass with catalytic upgrading presents a promising method of overcoming these challenges. Importantly, the decomposition of the biomass over several stages can reduce the number of product species within each fraction, allowing one to mold the catalytic upgrading strategy to target a subset of the chemical functionalities.

Although staged thermal decomposition of biomass coupled with targeted upgrading has a number of advantages over single stage pyrolysis with hydro-treating, there are many unknowns regarding the best way to design such a system. Importantly, the optimization of the number and conditions of each thermal decomposition stage is a complex problem that cannot be solved without knowing the influence on the final yields and process complexity resulting from the subsequent catalytic upgrading. Furthermore, there are a wide variety of chemistries available to perform upgrading. In addition, the reaction sequence must also be configured to optimize the overall process. In order to improve the impact of such an approach, one must also determine if general practices from this strategy may be applicable to different types of biomass feeds.

Accordingly, the ultimate goal of this work is to design a broad roadmap that can be used to identify the key parameters and trade-offs in a bio-refinery process based on multi-stage torrefaction of biomass with catalytic upgrading. In order to investigate the effect of the number of torrefaction stages and the specific chemistries on the yield of fuel-range grade product and the cost, we first derived a series of process alternatives based on experimental torrefaction yields obtained for a 3-stage torrefaction process (270 °C, 360 °C, and 500 °C). In designing the alternatives, we carefully selected the chemistries that are used to upgrade the thermal decomposition fractions, to maximize the yield of fuel-grade product and to minimize the hydrogen consumption, considering the abundance of various chemical functionalities within the biomass fractionation product. We suggested twelve cases where there is 1, 2, or 3 thermal decomposition stages and where each fraction is upgraded 1, 2, 3, or 4 times. Through the analysis, we found that there exists a trade-off between fuel-range carbon yields versus the number of upgrading steps. In general, increasing the complexity of the fraction upgrading systems increases the ultimate yield of C6+ product, thought there are diminishing returns on the increase in product yield versus the complexity of the catalytic upgrading sequences. In contrast, however, the choice of the number of thermal decomposition stages is found to be not simple, requiring careful consideration of the chemistries available to upgrade different components and the relative abundances of these different components. More thermal decomposition stages allow for more fine-tuning of the upgrading processes but lead to more complex and potentially more expensive processes. Thus, the design of the decomposition conditions and number of stages must be done simultaneously with the design of the fraction upgrading system. Finally, we also suggest some hybrid processes by eliminating several upgrading steps that have small marginal effects on the fuel-range carbon yield and discuss about separations that could enhance the process performance via prevention of unwanted side reactions or protection a functionality from conversion.