Thermoeconomic Analysis of an Innovative Integrated System for Cogeneration of Liquid Hydrogen and Biomethane by a Cryogenic-Based Biogas Upgrading Cycle and Polymer Electrolyte Membrane Electrolysis

Abstract

Hydrogen (H2) as an ecofriendly alternative to fossil fuels faces challenges in storing large capacities for extended transportation due to its low volumetric energy density. Also, common large-scale storage techniques for H2 are associated with high energy consumption, which is potentially at odds with the environmental benefits. The multiproduct H2 storage systems are associated with enhancing efficiency and environmental sustainability, leading to significant cost savings and minimizing waste production. In this paper, an integrated system that cogenerates liquids H2 and biomethane is designed using low-temperature cascade refrigeration processes, cryogenic-based biogas purification cycle, proton exchange membrane (PEM) electrolysis, and organic Rankine plant. A two-stage mixed refrigerant refrigeration process is employed for purifying untreated biogas, liquefying biomethane, and precooling H2 generated by a PEM electrolyzer. Four H2 Joule-Brayton refrigeration systems are used to liquefy H2 in the hybrid system. This hybrid configuration purifies 2917 kg/h unrefined biogas and uses 27.71 MW power to produce 416.6 kg/h liquid H2 and 674 kg/h liquid biomethane. The energy and exergy efficiencies for the designed integrated process are calculated at 54.65% and 56.67%, respectively. The system exergy analysis reveals that the electrolyzer (77.94%), heat exchangers (10.61%), and compressors (5.69%) are the principal equipment in exergy destruction. Pinch analysis is employed to design heat exchanger networks and reduce energy consumption effectively. According to the economic analysis, the investment return period and the prime cost of H2 are 5.589 years and 3.539 USD/kgLH2, respectively. The parametric sensitivity analysis demonstrates that lowering the electricity cost from 0.1 to 0.03 USD/kWh leads to a decrease in the prime cost of liquid H2 and the investment return period to 2.205 USD/kgLH2 and 4.251 years, respectively. Moreover, increasing the outlet pressure of the turbo-expander in the cryogenic refrigeration cycle from 100 to 220 kPa results in an increase in exergy efficiency and net annual benefit to 56.67% and 13.89 MMUSD/year, respectively, and a reduction in the prime cost of liquid H2 to 3.536 USD/kgLH2.