(647b) Optimization Framework for the Process Synthesis and Simultaneous Heat and Power Integration of a Thermochemical Coal, Biomass, and Natural Gas to Liquids Facility

Thermochemical conversion of biomass or coal proceeds via either direct liquefaction which produces a synthetic crude oil or indirect liquefaction which produces synthesis gas (syngas) that can be converted to liquid hydrocarbons directly via the Fischer-Tropsch (FT) process [1]. Thermochemical conversion of natural gas generally occurs through indirect liquefaction using an autothermal reactor. A coal, biomass, and natural gas to liquids (CBGTL) process was recently developed [2,3] that utilizes input hydrogen to have an almost-100% conversion of the carbon in the feedstock to liquid fuels. While the use of hydrogen in the CBGTL process can produce near-zero CO₂ quantities without the need for carbon sequestration, it remains unclear if that is the most cost-effective option to produce liquid fuels. To facilitate the transition from petroleum-based transportation fuels to new carbon-based systems, it is essential to determine the process topology that can deliver the fuels at the lowest levelized cost while using the limited supply of coal, biomass, and natural gas available throughout the country.

To address the above problem, a process synthesis model is proposed to find the CBGTL refinery system topology that provides the lowest levelized cost of transportation fuel production [4]. The model utilizes a combination of continuous variables to model stream flow rates, process operating conditions, and thermodynamic properties and binary variables to model the logical use of a process unit or a process stream. Several constraints dictating the conservation of mass and energy, equilibrium of reactor species, split fractions, and extents of reaction are imposed to ensure that a feasible solution is found.  The model is formulated as a mixed-integer non-linear non-convex optimization problem (MINLP).  The MINLP model includes simultaneous heat and power integration utilizing heat engines to recover electricity from the process waste heat.  Four case studies are presented to investigate the effect of CO₂ sequestration (CCS) and GHG reduction targets on the process topology along with detailed parametric analysis on the role of biomass and electricity prices. Optimal process topologies, the complete heat and power integration, and the overall costs of each case study are compared.

 [1] National Academy of Sciences, National Academy of Engineering, and National Research Council. Liquid Transportation Fuels from Coal and Biomass: Technological Status, Costs, and Environmental Issues. Prepublication. Washington, D. C., EPA, 2009. 

[2] R. C. Baliban, J. A. Elia, and C. A. Floudas. Toward Novel Biomass, Coal, and Natural Gas Processes for Satisfying Current Transportation Fuel Demands, 1: Process Alternatives, Gasification Modeling, Process Simulation, and Economic Analysis. Ind. Eng. Chem. Res., 49:7343-7370, 2010.

[3] J. A. Elia, R. C. Baliban, and C. A. Floudas. Toward Novel Biomass, Coal, and Natural Gas Processes for Satisfying Current Transportation Fuel Demands, 2: Simultaneous Heat and Power Integration. Ind. Eng. Chem. Res., 49:7371-7388, 2010.

[4] R. C. Baliban, J. A. Elia, and C. A. Floudas. Optimization Framework for the Simultaneous Process Synthesis, Heat and Power Integration of a Thermochemical Hybrid Biomass, Coal, and Natural Gas Facility. Comp. Chem. Eng., doi:10.1016/j.compchemeng.2011.01.041,2011.