The Hydrogen-Fueled Allam Cycle: Thermodynamic Evaluation and Optimization After a Fuel Switch to Hydrogen

Abstract The Allam Cycle is one of the most promising concepts in thermal power generation, especially due to its high thermal efficiency (>60%) and near-zero emissions while being fossil-fueled. In view of a global shift toward renewable energies and the imminent scarcity of fossil energy sources, the transition of thermal power generation to hydrogen as fuel becomes increasingly relevant. This raises questions about the performance, design, and operational challenges associated with switching to hydrogen. Although various combinations of working fluids and fuels have been discussed before, a thorough design and a thermodynamic analysis of the H2-fired Allam Cycle have yet to be carried out. This work presents an Aspen HYSYS-based first thermodynamic analysis of the H2-fueled Allam Cycle and compares it with the fossil-fueled cycle. Preventing and compensating the inevitable CO2 release via the condensate becomes the key challenge, for the solution of which three methods are proposed and evaluated. The resulting, newly proposed LSM model allows a net zero emission H2-Allam Cycle. Based on the lower heating value (LHV), the thermal efficiency of the H2-Allam Cycle is 5% higher than for the NG-Allam Cycle. This is primarily due to the higher specific turbine work resulting from the increased specific isobaric heat capacity of the steam-enriched working fluid and secondarily due to the higher LHV of hydrogen. When accounting for the energy requirements of hydrogen processing and losses via turbine cooling, the thermal efficiency of the new cycle drops significantly but remains competitive with the NG-Allam Cycle.