Parametric and economic analysis of condensing heat exchanger combined with organic rankine cycle for heat and water recovery from boiler flue gas

This study concerns with heat and water recovery from the flue gas of a natural gas-fired thermal power plant. A combined system of condensing heat exchanger (CHE) and Organic Rankine Cycle (ORC) is proposed. The CHE acts as the super-heater of the ORC. The flue gas enters to the CHE with a temperature of 160 celcius and is cooled to under the water vapor dew point temperature and leads to water vapor condensation, therefore latent and sensible heat are recovered. The condensed water is used as cooling tower make-up water. The ORC refrigerant enters to the CHE as a saturated vapor and is superheated by the recovered heat. In order to achieve the best ORC performance and highest water recovery simultaneously, parametric analysis was done in terms of evaporator and superheating temperature. It was found that by increasing evaporator and superheating temperature, the efficiency and generated power of the ORC increase, although water recovery decreases. Also an increase in evaporator and superheating temperature raises the heat transfer area of CHE, particularly in the non-condensing zone. Therefore to find the optimum evaporation temperature an economic analysis based on NPV method was done and 43 & DEG;C was determined as the optimum evaporation temperature. Considering this condition, flue gas enters the CHE with a temperature of 160 & DEG;C and leaves it with a temperature of 57.85 & DEG;C, and on the other side, the refrigerant enters it in a saturated state and leaves it with a temperature of 75 & DEG;C. The total area of the CHE is about 39,490 m(2). The amount of recovered water is 36.78 kg/s, which saves 34.2 % of the make-up water consumption. Also, the production power of the ORC-CHE was calculated as 24.12 MW and the additional power produced by ORC-CHE compared with bare ORC is 3.97 MW.