Humid Air Turbine Cycles With Water Recovery: How to Dispose Heat in an Efficient Way

Since the humid air turbine (HAT) cycle was first presented by Rao and Joiner (1990), several modifications were proposed to the original configuration to further improve its efficiency. In the last years, the attention was focused in the water recovery from flue gas and in determining the most suitable systems to separate water from gas and solving the problem of low temperature at the stack. In all the above studies it was shown that condensing water from flue gas requires a significant flow rate of a cooling medium (generally water) which is needed to remove condensation heat which must then be disposed in the environment. This worsens power plant performance because large cooling towers are needed. On the other hand, the reduced cost of water treatment may compensate the additional costs of the condensation equipment. In this paper, the introduction of an Organic Rankine Cycle (ORC), which transforms in mechanical power a fraction of the heat recovered from the HAT cycle, both in the water recovery system and in other heat exchangers, is presented. Results were obtained by using three different fluids and maximizing the ORC input exergy. The substances which were used are the conventional R502 refrigerant fluid, ammonia and the new HF134a, which is replacing phased-out CFCs in refrigeration systems.