How Does a Combined-Cycle Plant Work?
A. Combustion Turbine
A combustion turbine unit operates much like a jet engine, drawing in air and compressing it. The compressed air is mixed with natural gas and ignited, creating rapidly expanding exhaust gases. These gases move through the gas turbine blades, making them spin. The blades are attached to a shaft that drives a generator on the cold end of the turbine converting a portion of the spinning energy into electricity.
B. Heat Recovery Steam Generators
The exhaust heat from the gas turbines is directed to boiler-like equipment known as Heat Recovery Steam Generators, or HRSGs. As their name implies, the HRSGs create steam by using heat recovered from the gas turbine flue gases. This steam is piped to the steam turbine.
The remaining exhaust gas is directed through the 195-foot-tall exhaust stack. Emissions are continuously monitored to ensure air quality regulations are met.
D. Steam Turbine
Steam generated by the HRSGs is used to drive a steam turbine that rotates another generator and makes more electricity.
E. Step-up Transformer
It steps up the generator voltage to transmission voltages and connects the generator to the grid, allowing the power to flow from the plant to thousands of homes and businesses. Three step-up transformers, one for each turbine, will be installed at St. Charles Power Station.
F. Cooling Tower
Cooling water, drawn from the Mississippi River, removes the small amount of remaining heat from the steam after it exits the steam turbine and is piped to the cooling tower where large fans move air to cool the water. The St. Charles Power Station will use a closed-loop system where water is reused and only make-up water for evaporation is required from the river.