Water recovery from HAT cycle exhaust gas: A possible solution for reducing stack temperature problems

For several years the injection of steam into the combustion chamber has represented a common way to improve the performance of gas turbine power plants, increasing both the power output and the efficiency and reducing, at the same time, NOx emissions. Starting from the first STeam Injected Gas turbine (GE STIG) cycles, several types of gas turbine cycle with steam or water injection (dual fluid cycles) have been proposed. Among them, the most interesting results were obtained with the Cheng and the humid air turbine (HAT) cycles. In particular, the HAT cycle (which is a gas turbine cycle featuring intercooled compression, an air-water mixing evaporator before the combustion chamber, and a recovery system for the exhaust gases) has been identified as a promising way to generate electric power at high efficiency, low cost and with a system that is simple compared with the combined cycles (Stecco et al. and Gallo et al.). However the associated water consumption, about 1210-2420 m3 per day for a 100 MW unit, continues to represent a significant drawback to the spread of the HAT cycle, as well as of other steam injected cycles. In fact, such a high spread water consumption means high operational costs for water treatment, eventual legislative restrictions limiting the use of water, not to mention the environmental impact of the depletion of water resources. Some different solutions to those problems have been proposed, such as the introduction of a mixing exchanger (Bettagli and Facchini), or of a surface exchanger (Bombarda, Bidini et al.) on the exhaust of the cycle to recover water and heat from the flue gases. Further cooling of the exhaust gases, which is necessary to condense the stream fraction, lowers the stack temperature so much that the stack draft may become too low to prevent sufficient diffusion of the emission; this is one of the main drawbacks for large-scale water and heat recovery. One of the possible solutions is the introduction an induced draft fan (IDF), the other is recuperative heating of the exhaust gases after the condenser. The aim of this paper is to estimate the cycle performance with an IDF placed between the turbine and the stack