Low-temperature corrosion of preheater tubes in Fars combined cycle power plant

In this paper, the effects of HRSG (Heat Recovery Steam Generator) preheater tubes on the performance of Fars (Iran) combined cycle power plant are evaluated. Since different fuels are used in combined cycle power plants, for high sulfur fuels (normally distillate oil as a liquid fuel), the preheater is totally bypassed and the water enters directly into the low-pressure drum. In fact, the combustion products contain sulfur and also the gas-side temperature of the preheater tubes is lower than the dew point temperature of sulfur. Therefore, in order to prevent fouling and corrosion of the preheater tubes, this part of boiler (HRSG) is used only when gas fuel (natural gas) is consumed. However, observations in Fars power plant show that combustion products still contain sulfur, which over time causes corrosive acids on preheater tubes surface. In this regard the effects of the preheater on energy generation and other cycle parameters are investigated in order to adopt a suitable method to reduce corrosion. The combined cycle power plant is modeled and the mathematical results obtained from the model are validated by manufacturer data and performance results from Fars power plant. Due to the existing conditions in Fars combined cycle power plant, the results show that controlled bypass in the preheater by metal temperature prevents corrosion. Finally, the proposed method reduces maintenance costs even when natural gas fuel is used.