Development of a tool for simulating performance of sub systems of a combined cycle power plant

Abstract: Abstract In Sri Lanka, around 50% of the electrical energy generation is done using thermal energy, and hence maintaining generation efficiencies of thermal power plants at an acceptable level is very important from a socio-economic perspective for the economic development of the country. Efficiency monitoring also plays a vital role as it lays the foundation for maintaining and improving of generation efficiency. Heat rate, which is the reciprocal of the efficiency, is used to measure the performance of thermal power plants. In combined cycle power plants, heat rate depends on ambient conditions and efficiencies of subsystems such as the gas turbine, Heat Recovery Steam Generator (HRSG), steam turbine, condenser, cooling tower etc. The heat rate provides only a macroscopic picture of the power plant, and hence it is required to analyse the efficiency of each subsystem in order to get a microscopic picture. Computer modelling is an efficient method which can be used to analyse the each subsystem of a combined cycle power plant. Objective of this research is to develop a computer based tool which simulates the performance of subsystems of a combined cycle power plant in Sri Lanka. At the inception of the research, only heat rate was measured, and performances of subsystem were unknown.                  During the analysis, plant is divided into main systems, in order to study them macroscopically. Then, these main systems are divided into subsystems in order to have a microscopic view. Engineering equation solver (EES) was used to develop the tool, and the final computer model was linked with Microsoft excel package for data handling. Final computer model is executed using both present and past operating data in order to compare present and past performance of the power plant.             In combined cycle power plants steam is injected into the gas turbine to reduce the NOx generation and this steam flow is known as NOx flow. According to the result it was evident that turbine efficiency drops by 0.1% and power output increase by 1MW when NOx flow increases from 4.8 to 6.2kg/s. Further it was possible to conclude that gas turbine efficiency drop by 0.1% when ambient temperature increased by 3 C; and gas turbine power output decrease by 2MW when ambient temperature increases from 27 to 31 degrees.   Regarding the steam cycle efficiency it was found that steam turbine power output drops by  0.5MW when ambient temperature increases from 27 to 31 degrees; and steam cycle efficiency increases by 1% when NOx flow increases from 4.8 to 6.2kg/s. Further, steam turbine power output decreases by 0.25MW When NOx flow increases from 4.8 to 6.2kg/s                 Heat rate, which is the most important performance index of the power plant, increases by 10units (kJ/kWh) when ambient temperature increases by 3 C. Heat rate also increases with raising NOx flow which was 6.2kg/s in 2007 and 4.2kg/s in 2011. Hence, heat rate of the power plant has improved (decreased) by 10units (kJ/kWh) from 2007 to 2011.                Other than above, following conclusions were also revealed during the study.                         1)       HRSG efficiency has not change during past 4 years 2)     Significant waste heat recovery potential exists in the gas turbine ventilation system in the form of thermal energy