Combined Cycle Technology

Burning gas in a gas turbine (GT) produces not only power – which can be converted to electric power by a coupled generator – but also fairly hot exhaust gases.

Routing these gases through a water-cooled heat exchanger produces steam, which can be turned into electric power with a coupled steam turbine and generator.

High power outputs at high efficiencies (up to 55%) and with low emissions.

Abstract

The Combined Cycle Power Plant or combined cycle gas turbine, is a gas turbine generator generates electricity, and waste heat is used to make steam to generate additional electricity via a steam turbine. The gas turbine is one of the most efficient for the conversion of gas fuels to mechanical power or electricity. The use of distillate liquid fuels (usually diesel) is also a common as alternate fuel.

More recently, as simple cycle efficiencies have improved and as natural gas prices have fallen, gas turbines have been more widely adopted for base load power generation, especially in combined cycle mode, where waste heat is recovered in waste heat boilers, and the steam is used to produce additional electricity.

This system is known as a Combined Cycle.combined-cycle-technology-illustration

The basic principle of the Combined Cycle is simple: burning gas in a gas turbine (GT) produces not only power – which can be converted to electric power by a coupled generator – but also fairly hot exhaust gases.

Routing these gases through a water-cooled heat exchanger produces steam, which can be turned into electric power with a coupled steam turbine and generator.

This type of power plant is being installed in increasing numbers around the world where there is access to substantial quantities of natural gas.

A Combined Cycle Power Plant produces high power outputs at high efficiencies (up to 55%) and with low emissions. Conventional power plants we are producing only 33% electricity and the remaining 67% is waste.

By using combined cycle technology we are producing 68% electricity.

It is also possible to use the steam from the boiler for heating purposes so such power plants can operate to deliver electricity alone or in combined heat and power (CHP) mode.

Mechanism

A combined cycle power plant as the name suggests, combines existing gas and steam technologies into one unit, yielding significant improvements in thermal efficiency over a conventional steam plant. In a CCGT plant, the thermal efficiency is extended to approximately 50-60 percent, by piping the exhaust gas from the gas turbine into a heat recovery steam generator.

However the heat recovered in this process is sufficient to drive a steam turbine with an electrical output of approximately 50 percent of the gas turbine generator.

The gas turbine and steam turbine are coupled to a single generator. For startup, or "open cycle", operation of the gas turbine alone, the steam turbine can be disconnected using a hydraulic clutch. In terms of overall investment, a single-shaft system is typically about 5 percent lower in cost, with its operating simplicity typically leading to higher reliability.

Working Principle of a CCTG Plant

The first step is the same as the simple cycle gas turbine plant. An open circuit gas turbine has a compressor, a combustor, and a turbine. For this type of cycle the input temperature to the turbine is very high. The output temperature of flue gases is also very high.

This high enough to provide heat for a second cycle, which uses steam as the working medium, i.e. thermal power station.

Air Inlet

This air is drawn though the large air inlet section where it is cleaned, cooled, and controlled. Heavy-duty gas turbines are able to operate successfully in a wide variety of climates and environments due to inlet air filtration systems that are specifically designed to suit the plant location.

Under normal conditions the inlet system has the capability to process the air by removing contaminants to levels below those that are harmful to the compressor and turbine.

In general, the incoming air has various contaminants. They are:

Corrosive Agents

Chlorides, nitrates, and sulfates can deposit on the compressor blades and may result in stress corrosion attack and/or cause corrosion pitting. Sodium and potassium are alkali metals that can combine with sulfur to form a highly corrosive agent and that will attack portions of the hot gas path. The contaminants are removed by passing through various types of filters which are present on the way.

Gas phase contaminants such as ammonia or sulfur cannot be removed by filtration. Special methods are involved for this purpose.

Turbine Cycle

The purified air is then compressed and 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.

In the second step the heat of the gas turbine’s exhaust is used to generate steam by passing it through a heat recovery steam generator (HRSG), with a live steam temperature between 420 and 580 °C.

Heat Recovery Steam Generator

In the Heat Recovery Steam Generator, highly purified water flows in tubes and the hot exhaust gases heat it to produce steam. The steam then rotates the steam turbine and coupled generator to produce electricity. The hot gases leave the HRSG at around 140 degrees centigrade and are then discharged into the atmosphere.

The steam condensing and water system is the same as in the steam power plant.

Typical Size and Configuration of CCGT Plants

The combined-cycle system includes single-shaft and multi-shaft configurations. The single-shaft system consists of one gas turbine, one steam turbine, one generator, and one Heat Recovery Steam Generator (HRSG), with the gas turbine and steam turbine coupled to the single generator on a single shaft.

Multi-shaft systems have one or more gas turbine generators and HRSGs that supply steam through a common header to a separate single steam turbine generator. In terms of overall investment, a multi-shaft system is about 5% higher in costs.

The primary disadvantage of multiple stage combined cycle power plants is that the number of steam turbines, condensers and condensate systems—and perhaps the cooling towers and circulating water systems—increases to match the number of gas turbines.

Efficiency of CCGT Plant

The steam turbine cycle produces roughly one-third of the power and the gas turbine cycle produces two-thirds of the power output of the CCPP. By combining both gas and steam cycles, high input temperatures and low output temperatures can be achieved. This adds efficiency to the cycles, because they are powered by the same fuel source.

To increase the power system efficiency it is necessary to optimize the HRSG, which serves as the critical link between the gas turbine cycle and the steam turbine cycle, with the objective of increasing the steam turbine output. HRSG performance has a large impact on the overall performance of the combined cycle power plant.

The electrical efficiency of a combined cycle power station may be as high as 58 percent when operating at continuous output with ideal conditions. As with single-cycle thermal units, combined-cycle units may also deliver low temperature heat energy for industrial processes, district heating, and other uses. This is called cogeneration and such power plants are often referred to as a Combined Heat and Power (CHP) plant.

The efficiency of CCPT is increased by Supplementary Firing and Blade Cooling. Supplementary firing is arranged at the HRSG. In the gas turbine a part of the compressed air flow bypasses and is used to cool the turbine blades. It is necessary to use part of the exhaust energy through gas to gas recuperation. Recuperation can further increase the plant efficiency, especially the when gas turbine is operated under partial load.

Fuels for CCPT Plants

The turbines used in Combined Cycle Plants are commonly fueled with natural gas, and is more versatile than coal or oil and can be used in 90% of energy applications. Combined cycle plants are usually powered by natural gas, although, fuel oil, synthesis gas, or other fuels can be used.

Emissions Control

Selective Catalytic Reduction (SCR):

  • To control the emissions in the exhaust gas so that it remains within permitted levels as it enters the atmosphere, the exhaust gas passes though two catalysts located in the HRSG.
  • One catalyst controls Carbon Monoxide (CO) emissions and the other catalyst controls Oxides of Nitrogen, (NOx) emissions. In addition to the SCR, Aqueous Ammonia (a mixture of 22% ammonia and 78% water) is injected into the system to even further reduce levels of NOx.

Merits

Demerits

  1. The gas turbine can only use natural gas or high-grade oils like diesel fuel.
  2. Being dependant on the above fuels, combined-cycle can be operated only in locations where these fuels are available and cost effective.

Conclusions

Combined cycle power plants continue to meet the growing energy demand, and hence special attention must be paid to the optimization of the whole system. Developments for the gasification of coal to use in gas turbines are in advanced stages.

Once this is proven effective, coal can be used in combined cycle power plants as the main fuel source to meet the growing energy demand.

Advances in cogeneration—the process of simultaneously producing useful heat and electricity from the same fuel source—increases the efficiency of fuel burning from 30% to 90%, thereby reducing damage to the environment while increasing economic output through more efficient use of resources.

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