Feasibility Study of the Thermal Performance Improvement of Natural Draft Dry Cooling Towers Due to Flue Gas Injection

In steam power plants, any improvement in the cooling towers thermal performance will cause the condenser's temperature to decrease; which in turn enhances the power plant's efficiency. Due to water shortage, many power plants all around the world rely on natural draft dry cooling towers (Heller type). In these cooling towers, condenser's exit hot water is pumped to vertical or horizontal finned radiators which are arranged all around of the tower. The tower should then be able to cool down the circulating water to the condenser’s temperature. Conversely ambient air is sucked naturally through these radiators due to heat gain from the hot water. The heated air is then discharged through the tower's top exit to the ambient. Environmental conditions strictly affect Heller cooling tower's performance. Temperature, density and the mass flow rate of sucked air into the cooling tower will completely influence on its operation. To improve the cooling tower's performance it is proposed to inject the steam generator’s flue gas into the tower for better air suction. In this study a natural draft dry cooling tower incorporating the flue gas duct is modeled to study the effect of flue gas injection on the sucked air flow rate and the radiator heat transfer. Considering the buoyancy effect term, for a 3-D incompressible flow, the navier stoke's equations as well as the energy equation are solved by computational fluid dynamic method (CFD) to obtain the air flow pattern in and around the tower as well as the heat transfer from the radiators. Results show that the flue gas injection will help to improve the performance of the cooling tower; however for this case (natural draft cooling) this efficiency enhancement is not significant. The maximum extra heat exchanged in the radiators due to flue gas injection is about 1.5 MW that can attain only a 0.07°C water temperature decrease. However, this amount is to be relied low to rely upon.