Thermodynamic assessment of zero-emission power, hydrogen and methanol production using captured CO₂ from S-Graz oxy-fuel cycle and renewable hydrogen

Thermodynamic analysis of a novel combined system, including geothermal driven dual fluid organic Rankine cycle (ORC), S-Graz cycle, proton exchange membrane electrolyzer (PEME) and methanol synthesis unit (MSU) are carried out using energy and exergy principles. The presented system produces zero emission power and hydrogen using oxy-fuel combustion technology and splitting water into hydrogen and oxygen in the PEME, which consumes the dual fluid ORC produced power. Then, methanol production from captured CO2 and produced H2 is suggested to eliminate the captured CO2 from S-Graz cycle. The energy and exergy efficiencies and sustainability index of 14.7%, 42.43% and 1.737 are obtained for the presented cogeneration system, respectively. Also, results associated with the exergy destruction reveal that most of the destroyed exergy in the system is due to the dual fluid ORC in compare with the other units. Furthermore, it is observed that, within the dual fluid ORC, low pressure evaporator (LPE) is responsible for 33% of exergy destruction. Moreover, the results indicate that increasing higher pressure of the dual fluid ORC improves the system energetic and exergetic performance while an increment in geothermal hot water temperature leads to a reduction in system energy and exergy efficiencies.