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WORKING OF A COMBINED CYCLE POWER PLANT
Air Inlet
The amount of air needed for combustion is 800,000 cubic feet per minute. This air is drawn though the large air inlet section where it is cleaned, cooled and controlled, in order to reduce noise.
Two GAS Turbine-Generators:
The air then enters the gas turbine where it is compressed, mixed with natural gas and ignited, which causes it to expand. The pressure created from the expansion spins the turbine blades, which are attached to a shaft and a generator, creating electricity. Each gas turbine produces 185 megawatts (MW) (pragati III)of electricity .The blades are attached to a rotor, which spins the generator, and makes electricity. Think of a generator as a huge spinning magnet inside a coil of wire. As the magnet spins, electricity is created in the wire loops.
Heat Recovery Steam Generator (HRSG)
The hot exhaust gas exits the turbine at about 600 degrees Celsius and then passes through the Heat Recovery Steam Generator (HRSG).In the HRSG, there are 18 layers of 100-foot tall tube bundles, filled with high purity water. The hot exhaust gas coming from the turbines passes through these tube bundles, which act like a radiator, boiling the water inside the tubes, and turning that water into steam. The gas then exits the power plant through the exhaust stack at a much cooler 180 degrees, after having given up most of its heat to the steam process. About 1 million pounds of steam per hour (pragati III) is generated in this way and sent over to the steam turbine through overhead piping.
Steam Turbine
The steam turbine is a Turbine Generator, capable of producing up to 240 MW(pragati III). It is located on top of the condenser, across from the cooling tower. Steam enters the turbine with temperatures as high as 550 degrees Celsius and pressure as strong as 2,200 pounds per square inch(psi). The pressure of the steam is used to spin turbine blades that are attached to a rotor and a generator, producing additional electricity, about 100 megawatts per HRSG unit. After the steam is spent in the turbine process, the residual steam leaves the turbine at low pressure and low heat, about 100 degrees. This exhaust steam passes into a condenser, to be turned back into water. By using this “combined-cycle” process, two gas turbines and one steam turbine, we can produce a total of about 600 megawatts of electricity.
Condenser and Cooling Tower
The purpose of the condenser is to turn low energy steam back into pure water for use in the Heat Recovery Steam Generator. The purpose of the cooling tower is to cool the circulating water that passes through the condenser. It consists of ten cells with large fans on top, inside the cone-like stacks, and a basin of water underneath. The cool basin water absorbs all of the heat from the residual steam after being exhausted from the steam turbine and it is then piped back to the top of the cooling tower. As the cool water drops into the basin, hot wet air goes out of the stacks. Normally, hot moist air mixes with cooler dry air, and typically a water vapor plume can be formed, one that may travel hundreds of feet in the air and be seen from miles away. The cooling tower evaporates about three-fourth’s of the processed, recycled water, then we send about one-fourth of it back through the sewer lines for re-treatment by the City.
COMBINED CYCLES
Combined Cycle power plant integrates two power conversion cycles namely. Brayton Cycle Gas Turbines) and Rankin Cycle (Conventional steam power plant) with the principal objective of increasing overall plant efficiency.
Brayton Cycle
Gas Turbine plants-operate on Brayton Cycle in which air is compressed (process 1-2, in P-V diagram of Figure-1), this compressed air is heated in the combustor by burning fuel combustion produced is allowed to expand In the turbine (process 3-4) and the turbine is coupled with the generator. Without losses the theoretical cycle process is represented by 1’ 2’ 3’ 4’ In the actual process losses do occur. Deviation from the theoretical process, results from the fact that compression and expansion are not performed isentropically but polytropically which is conditioned by heat dissipation (expansion) and heat supply (Compression) caused by various flow and fraction by losses. In the combined cycle mode, the Brayton Cycle is chosen as the topping cycle due to the high temperature of the exhaust of the gas turbine (point 4 in the P.V diagram). In modern gas turbines the temperature of the exhaust gas is in the range of 500 to 550 o C.
1) Thermal efficiency 2) Process working capacity Thermal efficiency is obtained from chemical binding energy of the fuel and mechanical energy available at the shaft of the gas turbine. Thermal efficiency ( th) as follows:
Nth = Energy at GT shaft /Chemical Energy of fuel = Q Input. Q output 1 — Q Output Qinput Q Input Working capacity is also obtained from the difference between the amounts of heat supplied and removed. This is achieved by increasing P 2 that is increasing gas inlet temperature T 3.
Combining two Cycles to Improve Efficiency We have seen in the above two cycles that gas turbine exhaust is at a temperature of 500 –550 o C and in Rankine Cycle heat is required to generate steam at the temperature of 500-550 o C. so, why not use the gas-turbine exhaust to generate steam in the Rankine cycle and save the fuel required to heat the water ? Combined Cycle does just the same.
The efficiency of Gas Turbine cycle alone is 30% and the efficiency of Rankine Cycle is 35%. The overall efficiency of combined cycle comes to 48%.