Exhaust Gas Recirculation to Improve Part Load Performance on Combined Cycle Power Plants

The common approach for part load operation of a combined cycle power plant is to maintain the turbine inlet temperature as high as possible without exceeding the temperature limits of the gas turbine exhaust. High part load firing temperature will give high cycle efficiency and low HC and CO emissions. The common approach is to reduce the flow by decreasing the compressor inlet flow-angle by turning the compressor variable guide vanes. This is done to control the turbine inlet temperature while the load is reduced by decreasing the fuel flow. However, using the variable guide vanes to reduce the flow renders in an offset of the compressor stage loading which has a negative impact on the efficiency. Compressors are basically volumetric flow machines and if operated on a fixed speed, a change in inlet gas density will alter the mass-flow. This means that if the inlet air is heated, the mass-flow and hence load will be reduced if turbine inlet temperature is kept constant. Thanks to the more or less maintained volume flow the compressor is operated closer to its design point and efficiency remains high. A heat exchanger, preferably with water or steam from the bottoming cycle on the hot side, would be a simple solution to heat the inlet gas. A better use of the available energy would be to semi-close the cycle by recirculating a part of the exhaust gas flow. Semi-closing the cycle means that less oxygen will be available in the combustion process and this will be one of the limiting factors for the recirculation rate. However, the fuel to air ratio decreases at part load and hence the oxygen surplus increases. Therefore, higher recirculation rates may be acceptable at part load compared to full load. The results from this thermodynamic study are very promising and show that a 40% recirculation rate can improve part load efficiency by as much as 4.1%.