Bocandé, David Marcel Pa

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 towards 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 has 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.

To face the challenge regarding reconversion of stored green hydrogen into electricity, zero emission oxyfuel hydrogen cycles with steam as a working fluid are very promising in both thermal efficiency and large-scale applicability. Previous studies suggest that with Turbine Inlet Temperatures (TIT) of 1700 °C thermal efficiencies in excess of 70% based on Lower Heating Value (LHV) can be reached. This work starts with a historical review, from the early 80s until today, of the processes proposed in the literature which can be categorized in partly condensing and fully condensing cycles. Also, since the calculations in the literature are based on different assumptions of cycle parameters, component efficiencies and material limitations, a selection of state-of-the-art process and component parameters in the turbomachinery and power plant industry will be established to help identify the technically realizable and promising cycles. Those parameters will also be used as a common base for thermodynamic simulations of the selected cycles and can serve as a reliable reference for further calculations. The simulations show that thermal efficiencies up to 73% are achievable under conditions reflecting the present state-of-the-art and 75% in the near future, considering current development. Mitsubishi’s intercooled topping recuperation cycle shows the highest thermal efficiency with 75% based on LHV. Moreover, a higher TIT goes along with increased cooling demand and thus higher losses in the turbine, counteracting the efficiency increase due to the elevated temperature. A parametric analysis will identify the optimum operating point of each cycle regarding TIT with consideration of the cooling efficiency.