Thermo-economic optimization of a new solar-driven system for efficient production of methanol and liquefied natural gas using the liquefaction process of coke oven gas and post-combustion carbon dioxide capture

Abstract

The suitable use of hydrogen-containing industrial by-products such as coke oven gas (COG) leads to a reduction of environmental pollution and energy waste. This paper develops a novel integrated structure for the simultaneous production of portable and relatively clean liquid fuels from COG and power plant exhaust gases. The subsystems include a CO2 capture unit to separate carbon dioxide from exhaust gases, natural gas purification and liquefaction process to produce liquefied natural gas (LNG), and a methanol production cycle. Photovoltaic panels with a geographical location in Iran are used to provide the required power. This process provides 1.287 kg/s of methanol and 1.678 kg/s of LNG as the main product and 46 kg/s of hot water as an industrial utility. The integrated structure's total energy and exergy efficiencies are 65.36% and 68.72%, respectively. The largest share of exergy degradation belongs to the photovoltaic panels (81.33%), heat exchangers (5.66%), and distillation towers (3.152%). The results of the economic assessment show that the payback period and the prime cost of product are 4.29 years and 0.3967 US$/kg methanol. The sensitivity analysis outcomes demonstrate that the efficiency of the whole process and the prime cost of the product increase to 69.05% and 0.4005 US$/kg methanol when the hydrogen content of the coke oven gas decreases from 65 to 54 mol%. Moreover, the flow rate decrease of COG from 991.6 to 901.6 kmol/h, reduces the prime cost of the product and the irreversibility of the whole system to 0.3855 US $/kg methanol and 55382 kW. Thermo-economic optimization of the hybrid system based on the non-dominated sorting genetic algorithm is implemented with TOPSIS and LINMAP being the decision-making methods. From the optimization results, the optimum values for the efficiency of the whole hybrid system and the prime cost of the methanol are calculated to be 65.83% and 0.3611 US$/kg methanol.