N2 Brayton Cycle in Oxy-combustion Power Plants

The appropriate placement of compressors and expanders was recently shown to be at the pinch temperature, meaning that the inlet temperature to these units should be at the pinch. However, the application of the rule is not obvious when multiple values of minimum approach temperature for heat transfer exist, such as in oxy-combustion power plants where both subambient (cryogenic air separation) and above ambient (the steam cycle) processes are involved. For cryogenic low purity O-2 production processes, high pressure N-2 is available from the distillation column and the pressure based exergy is normally recovered in the cold box. When the O-2 is supplied to an oxy-combustion power plant to concentrate the CO2, considerable low temperature heat in the flue gas is lost after heat recovery in the preheater. A N-2 Brayton cycle is proposed in this paper. The high pressure N-2 is preheated to the pinch temperature of the preheater in the boiler area before expansion. The entire plant thermal efficiency increases by 0.69-0.85 % points.