Highly efficient heat integration of a power-to-liquid process using MILP

Abstract

Synthetic fuels are needed to decarbonize the non-electrifiable parts of the transportation sector, such as shipping and aviation. The synthesis of fuels using power-to-liquid (PtL) processes has already been investigated and tested. However, economic implementation has failed to gain traction due to high plant and product costs and insufficient PtL-efficiencies. In this paper, we will use heat exchanger network synthesis (HENS) to optimize the heat integration of a novel 1 MW PtL-process. A unique feature of this case study is that the internal heat is supplied by oxidizing a Fischer-Tropsch tail gas within a combustion system (CS). The inlet and outlet temperatures of the exhaust gas flow of the three serially connected CS can be adjusted by the mass flow of excess air and the tail gas. Using HENS implies specifying inlet and outlet temperatures. We apply an adaptation of the HENS that allows for variable stream temperatures and flow capacities. This allows the CS, an internal hot utility, to be optimally tailored to the specific process. Linearization of the non-linear relations, such as stream- and stage-wise energy balance, is done with simplices. The linear approximation is transferred to MILP using highly efficient logarithmic coding. We show that implementing the CS in HENS significantly improves the total annual costs and PtL-efficiency. With the developed method, the design engineering of highly efficient PtL-processes can be successfully supported and the technology can be brought closer to an economically viable market maturity.