Energetic, economic and environmental optimization of a combined cycle power plant by recovering thermal energy from the exiting gases.

ERICA CORREA; CARLOS SICCATTO; PABLO ARENA

Buenos Aires , Argentina

Conferencia; Escuela de Estudios Energéticos.EdEE; 1999

The scope of the present work is the energetic and environmental optimization of a combined cycle power plant in Mendoza (Argentina). To reach this aim, operative data corresponding to working conditions and emissions released have been collected over a whole year. These data have been checked against field measurement, and the final information has been used for the development of a simulator of the real behavior of the cycle. The simulator allows for the evaluation of the significance of suggested technological measures for the optimization of the plant, giving the energetic and economic saving potential for each one of them. The data has been collected and analyzed through campaign carried out by professionals and students from the ‘Clíope’ group (Universidad Tecnológica Nacional, Facultad Regional Mendoza). The combined cycle power plant is made up by two 20 MW turbogas groups. The exiting gases pass through a single-pressure Heat Recovery Steam Generator (HRSG), with a mass flow rate of 138 kg/s, from where they exit at a temperature of about 185ºC. The produced steam feeds a 29 MW condensing steam turbine with two steam extractions. The cycle close up with a condenser, two steam ejectors, a steam seal condenser, a low pressure heater and a degasifier. The first extraction (high pressure, high steam quality) is used for feeding the degasifier, while the second one preheats the water in a low pressure heater. Once the collected data has been statistically treated, the simulation of the cycle was performed, and in a first instance the impact of substituting the low pressure heater by a heat exchanger which recovers the heat content of the exiting gases was evaluated. In this way, instead of using the enthalpy of the steam extracted from the turbine as a heat source the heat recovered from the flue gases from the HRSG is used, which otherwise would be lost. In this way, a reduction in the temperature of the exiting gases and an increase of about 400 kW in the steam turbine is reached. If 7000 working hrs/year are assumed, it results that the low required investment has a quick return, even without considering any improvement in the efficiency of the steam turbine. The economic benefit of the improvement of the steam turbine performance has been also analyzed. Besides the economic return, the more efficient energy system consume less fuel, which represents a non-renewable resource saving, an a better specific emission factor. The proposed solution is very simple, and it requires only a few modifications in the plant lay-out. Since only a small fraction of the recoverable heat is used, further modifications are been analyzed in order to complement this proposal and to increase the use of the heat content of the exiting gases, but at a higher cost. The use of simulation models is very useful for the evaluation of the technological modifications proposed for the optimization of a plant, which permits for a quick calculation of the economic, energetic and environmental consequences of different solutions. In a future work the different technological improvements will be analyzed from the thermoeconomic point of view.   Key words: Combined cycle power plant, rational use of energy, simulation, models, environmental impact, costs, optimization.