Thermodynamic modeling and optimization of a combined biogas steam reforming system and organic Rankine cycle for coproduction of power and hydrogen

Abstract

Abstract This paper aims to use waste heat of the biogas steam reforming (BSR) system for organic Rankine cycle (ORC) for simultaneous power and hydrogen production. A comprehensive thermodynamic modeling of the proposed combined system is carried out. In addition, optimization of the proposed system is conducted, using genetic algorithm (GA). Four different working fluids of R600, R245fa, R123, and R113 are used in ORC, where among all of them R600 is recommended due to its high performance and environment benefits. Using R600 as working fluid in ORC, results of the optimization demonstrated that the proposed system performs in an optimum state based on the selected objective functions when steam to carbon molar ratio, carbon dioxide to methane molar ratio, reaction pressure, reactor temperature, pump pressure ratio and pinch point temperature difference of the internal heat exchanger (IHE) are set in 2.99, 0.502, 1.004 bar, 998.85 K, 7.21 K, and 5.44 K, respectively. In this case, the optimum net output power, hydrogen production rate, energy efficiency and exergy efficiency are obtained 15.9 kW, 0.02529 kg s−1, 45.63%, and 74.89%, respectively. Moreover, the results of exergy analysis indicated that among all components, recuperator and reactor are accountable for the highest exergy destruction through the system. To better understand the effect of various parameters on performance of the system, a comprehensive parametric study of some key thermodynamic parameters on the main performance criteria is performed. It is concluded that the energy and exergy efficiencies of the combined BSR-ORC system can be increased by increasing steam to carbon molar ratio and pump pressure ratio or decreasing reaction pressure, carbon dioxide to methane molar ratio and pinch point temperature difference of the IHE.