A combined cycle power plant is a system of power generation that integrates a Brayton cycle and a Rankine cycle to produce the shaft work more efficiently. The combined cycle power plant utilizes the hot exhaust gases from the gas turbine’s exhaust to generate steam and drive a steam turbine, producing additional work.
The combined cycle power plant consists of an air compressor, a gas turbine, a heat recovery steam generator and a steam turbine. In the first stage, fuel is burned with compressed air and it expands in the gas turbine, producing work. The exhaust gases from the gas turbine, which is still hot at around 500-600 °C, enters the heat recovery steam generator, where the heat is recovered and high pressure steam is produced. This steam then expands in the steam turbine, producing additional work without consuming additional fuel.
The main objective of the combined cycle power plant is to maximize the energy extraction from the fuel and minimize the waste heat loss. Using the exhaust gas heat from the gas turbine in steam turbine, the plant converts a larger fraction of the fuel’s energy into useful work.
Table of Contents
Integration of Brayton and Rankine cycle in Combined Cycle Power Plant
Brayton cycle operation
The gas turbine operates on Brayton cycle and is the topping cycle. In this cycle, the air compressor (1-2), compresses the air increasing its temperature and pressure. In the combustor, the fuel is injected and is burned at nearly constant pressure (2-3), producing high temperature gas, which is then expanded in the gas turbine (3-4) to generate the power.
The work done by the Gas turbine, WGT = h3 – h4
Work done in the compressor, WC = h2 – h1
The Heat supplied, Q1 = h3– h2
Waste Heat Recovery
The exhaust gases leaving the gas turbine still has a high temperature of around 500-600 °C. The exhaust gas is thus made to pass through heat recovery steam generator, which is basically a heat exchanger. The Heat Recovery Steam Generator takes up the heat from the exhaust gases and generates steam at a high pressure for the steam turbine cycle.
Rankine Cycle operation
The steam turbine operates on the Rankine Cycle and in combined cycle power plant, is the bottoming cycle. The water is converted to steam in the Heat Recovery Steam Generator, utilizing the heat from the exhaust gases of the topping cycle (5-6). The steam expands in the steam turbine (6-7) in isentropic process and generates work. This work generated by the steam turbine is additional work in context to the fuel and Brayton cycle. The exhaust steam condenses in the condenser to form saturated liquid (7-8) and is pumped back to the steam generator (8-5).
The work output from steam turbine, WST = h6 – h7
The work consumed in the pump, WP = h5 – h8
The heat absorbed in the steam cycle, Q2 = h6 – h5
Combined Cycle Power Plant Work output
The net work output from the combined cycle power plant is
WCC = WGT -WC + WST – WP
= (h3 – h4)– (h2 – h1) + (h6 – h7) – (h5 – h8)
Combined Cycle Power Plant Efficiency
Since the Rankine cycle utilizes the waste heat from the Brayton cycle, no additional fuel is required. Therefore, the efficiency is
ηCC = WCC / Q1 = [(h3 – h4)– (h2 – h1) + (h6 – h7) – (h5 – h8)] / [h3– h2]
Why is Combined Cycle Power Plant more efficient
The combined cycle power plant achieves significantly higher thermal efficiency than conventional gas turbine plant because of its ability to extract more useful energy from the same quantity of fuel.
Better Utilization of Fuel Energy
A simple gas turbine power plant has a thermal efficiency of about 35-40 %, a simple steam power plant has a thermal efficiency of about 35-42% but because of better utilization of fuel from heat recovery steam generator, the efficiency of combined cycle power plant reaches 55-65%.
Reduced heat rejection losses
The exhaust gas temperature of gas turbine is typically between 500-600°C. Recovering this heat reduces the heat rejected to the environment, which lowers the cycle’s irreversibility and improves the thermal efficiency.
Two stage power generation
Form the same amount of fuel, the combined cycle power plant generates power or work in two stages:
First Stage: The work output is generated by the Gas turbine working on Brayton Cycle.
Second Stage: The work output is generated by the steam turbine working on Rankine cycle from steam produced from waste heat of the first stage.
Lower Specific Fuel Consumption
Since more work is generated per unit of fuel consumed, the plant consumes less fuel for each unit of electricity produced. Therefore, it lowers the operating cost and reduces the greenhouse gas emission while achieving higher overall plant efficiency.
Sources
This article is a part of thermal system, where other related articles are discussed.
