(558a) Thermochemical and Catalytic-Upgrading System Design to Convert Biomass to Liquid-Fuel Using a Superstructure-Based Approach
The general multistage thermal decomposition process comprises of either one, or two, or three thermal decomposition stages for biomass pretreatment, each at different temperatures. While the streams leaving each of these stages, in practice, contain a wide variety of species, here, for the purpose of tractability, we represent all species using a fixed set of model-compounds, namely, acetic acid, acetol, furan, furfural, levoglucosan, toluene, guaiacol, char, carbon-di-oxide and water. The composition of these model-compounds in each stream leaving the thermal decomposition stages is dependent on the number of thermal decomposition stages used during pretreatment, which accordingly influences the sequence of downstream catalytic-upgrading chemistries to be used. The catalytic-upgrading steps are needed to increase the carbon-chain-length of the molecules, and reduce/eliminate oxygen-content so as to finally obtain fuel-like molecules. In this work, we consider the following kinds of chemistries for catalytic-upgrading as they are applicable to a variety of organic-molecules: acylation, aldol-condensation, alkylation, hydroxy-alkylation, ketonization, hydrogenation, oxidation and hydrodeoxygenation. With so many options for both thermal decomposition and catalytic-upgrading, it is non-trivial to identify cost-effective pathways to convert biomass to liquid fuel. To make the process-layout decisions and design the biorefinery in a systematic manner, we use a superstructure-based approach where we develop a mixed-integer nonlinear programming (MINLP) model.
We first develop a novel superstructure for a torrefaction-based biorefinery. The superstructure consists of reactors that are linked by a network of connecting streams associated with heat exchangers for temperature-conditioning. Further, we devise connectivity rules between the reactors. Some of these rules eliminate cyclical-paths and symmetrical solutions. Thereafter, we present strategies to model the individual elements of the superstructure.
If a reactor is present in the final solution, it is assigned one of many possible tasks. Each task in the thermal decomposition section is associated with a specific thermal decomposition temperature, while a task in the catalytic upgrading section is associated with a specific catalyst carrying out a specific kind of chemistry (listed earlier). For example, if acylation is the task assigned to a catalytic upgrading reactor, then, all molecules in the inlet stream to this reactor amenable for acylation react. So, multiple reactions can take place inside a reactor. To model the reactivities of the various molecules for a specific task, we use conversion data from literature and experiments. We also develop new methods to handle parallel reactions that any molecule can participate inside a reactor, alongside identifying the limitting reactant species of a reaction, which is unknown because stream flows are unknown. Furthermore, for each reactor, we determine the heating/cooling requirement based on the extents of reactions that occur in them. A new approximate method is proposed for the calculation of heating and cooling duties for phase- and temperature-conditioning the streams that connect reactors. While the overall heating/cooling duties along with the biomass and hydrogen usage contribute to the overall operating costs, capital costs are governed by the reactors used and the tasks that are assigned to them.
Using this general framework, we identify cost-optimal pathways to covert biomass to liquid fuels based on multistage thermal decomposition, and study the sensitivity of the solutions to different parameters like biomass costs, hydrogen costs, and fuel-range carbon-yield.
 M.J. Prins, K.J. Ptasinski, F.J.J.G. Janssen, More efficient biomass gasification via torrefaction, Energy. 31 (2006) 3458â3470.
 J.A. Herron, T.Vann, N.Duong, D.E.Resasco, S. Crossley, L.L. Lobban, C.T. Maravelias, A Systems-Level Roadmap for Biomass Thermal Fractionation and Catalytic Upgrading Strategies, Energy Technology. 5 (2017) 130â150.