Biomass to liquid transportation fuels (BTL) systems: process synthesis and global optimization framework†
An optimization-based process synthesis framework is proposed for the thermochemical conversion of biomass to liquid fuels (BTL). Gasification of biomass is used to generate synthesis gas which can be converted to raw hydrocarbons either directly using Fischer–Tropsch synthesis or indirectly using catalytic conversion of methanol over ZSM-5. Multiple technologies are considered for generation of the raw hydrocarbons including (i) six types of Fischer–Tropsch units with different temperatures, catalyst types, and hydrocarbon effluent compositions, (ii) methanol conversion using methanol-to-gasoline, and (iii) methanol conversion using methanol-to-olefins. The hydrocarbons are upgraded into the final liquid fuel products (i.e., gasoline, diesel, and kerosene) using one or more technologies including ZSM-5 catalytic conversion, oligomerization, hydrocracking, isomerization, alkylation, and hydrotreating. A simultaneous heat, power, and water integration is included within the process synthesis framework to directly examine the costs associated with utility production and wastewater treatment with a particular topological design. A rigorous global optimization branch-and-bound strategy is implemented to mathematically guarantee the development of a BTL refinery that is economically and environmentally superior to all competing designs. Twenty-four case studies are investigated to determine the effect of refinery capacity, liquid fuel composition, and biomass feedstock on the overall system cost, the BTL refinery topological design, the process material/energy balances, and the lifecycle greenhouse gas emissions.