Thermodynamic analysis of the Allam cycle and its pressure sensitivity

The Allam cycle is a novel system which operates the oxy-combustor at a high pressure under supercritical conditions, uses a single gas turbine, circulates carbon dioxide as the working fluid in a semiclosed-loop and utilizes low-pressure-ratio recuperated Brayton cycle. These properties of the Allam Cycle enable to reach high efficiencies. This system was simulated using Chemcad software for a methane feed flow rate of 1 kmol/s. The pressure at the inlet of turbine was taken as 285 bar. The net power generation rate was estimated considering the generation in the turbine and the usage in the compressors for circulating carbon dioxide and for oxygen and natural gas feeds. The calculations using energy balance for the process gave 388 MW net power output, whilst Chemcad software gave 392 MW. The thermal energy recovered by lowering the temperature of the flue gas before condensing its water content was also considered in the overall efficiency of the system. The net power cycle efficiency was determined as 48.89%. The second law analysis of the cycle was also made. Entropy generation rate, Sgen, was determined as 965.79 kW/K and exergy destroyed, ψdestroyed, was found as 287.81 MW from entropy balance for the process. The exergy balance for the process was also made and the results were compared with the findings from the entropy balance. The second law efficiency of the process was found as 62.54%. Working at high pressure, naturally, affects the material of construction and consequently the fixed capital investment as well as operating and maintenance costs. Therefore, in this work, a sensitivity analysis is also made to see the effect of pressure on power generation and efficiency. The sensitivity analysis was made using Chemcad software simulation. It was found that the optimum pressure range for operation of the system was between 250 and 350 bar.