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IGCC is one more step in man's quest for high efficiency power generation with reduced emissions. The highly efficient and proven combined cycle, the main stay of power generation with Natural gas, is limited to firing gas or distillate oils. Aim of IGCC power plant is to close this gap by making it possible to fire coal in the combined cycle mode. The nett result is an increased efficiency coal based power plant with reduced emissions.
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Steps in an IGCC
There are three steps for the power generation in an IGCC.
- Produce clean gas from the coal also called syngas.
- Burn the gas in a gas turbine to produce power.
- Use the exhaust heat of the gas turbine to produce steam in an HRSG and use it in a steam turbine to produce power.
Step 1 is the new technology, which is in an advanced state of development with many plants already running in various parts of the world.
Step 2 is already proven for distillate oil and natural gas firing. However, for the coal gas firing the gas turbine technology require improvements and modifications to fire the synthetic gas.
Step 3 is the same as in the natural gas combined cycle except for some additional heat exchangers to recover heat from the hot syngas.
The gasification of coal produces heat. This heat recovery takes place by integrating the gasifier and the combined cycle. Hence the name 'Integrated Gasification Combined Cycle.'
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Gas from coal.
Gas from coal itself is not a new process. This has been in use for firing in large furnaces as producer gas and as town gas for domestic purposes and public lighting from the beginning of the twentieth century.
The basic gasifier consists of steam passing through a hot bed of coal. The air or oxygen provided is less than required for full combustion. In this reducing environment the following chemical reactions takes place.
Carbon + Oxygen giving Carbon monoxide and Carbon Dioxide
Carbon+ Water giving Hydrogen and Carbon monoxide.
Carbon monoxide + Water giving Carbon dioxide and Hydrogen also called the water shift reaction.
Carbon Monoxide + Hydrogen giving Methane.
The result is gas with a composition of Carbon monoxide and Hydrogen and Methane.
The problems of this earlier gasifier were the formation of tar and other impurities, which made it unsuitable for firing in gas turbines.
The modern gasifier tries to eliminate these drawbacks to give clean pure gas.
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A brief working of a an entrained flow gasifier for an IGCC plant is described in these pages.
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Irrespective of the type of gasifier the main the main ingredients are
- Water or Steam
- Air or Oxygen– less than the stoichiometric quantity.
There are three different types of gasifier.The difference is only in how these react.
- The fixed bed gasifier with large coal particle size where the bed temperature is around 400 °C.
- The fluidized bed gasifier with coal particle size in the range of 2-3 mm at temperatures around 600 °C.
- The entrained flow gasifier where pulverized coal is fed into the gasifier at temperatures of 1600 °C and pressures in the range of 25 to 60 bar.
Because of the impurities present in the gas and lower gas yields the fixed bed and fluidized bed gasifier are not economical for large capacity power plants.
The calorific value of the coal, ash content and fuel composition play an important part in deciding the type of gasifier. Also is of importance is the required composition of the product gas. Higher methane content is acceptable for power generation but not acceptable for other purposes.
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Entrained flow gasifier.
The most promising gasifier type for the power plant use is the entrained flow gasifier. Different commercial models are available and in development from various process technology companies.
The coal is first pulverised to a fine powder. This is fed into the gasifier and burned as a dry powder with steam , or as slurry with water. The oxygen required for combustion - less than the stoichiometric , is supplied as air or as oxygen from an air sepeartion unit.
The temperatures in the gasifier could go as high as 1600 °C. The pressures could be as high as 60 bar.
This temperature is higher than the coal ash fusion temperature resulting in the ash becoming molten slag. The hot gases and molten ash passes through a steam or water-cooled screen where the slag and ash particles separates form the gas stream. The slag cools and flows to the bottom of the gasifier for removal.
A typical molar gas concentration will be 30% Hydrogen, 50% Carbon monoxide, 10% Methane and balance CO2 percentage.
The hot gases are cooled in different heat exchangers and scrubbed for the removal of CO2, Sulphur Dioxide and other impurities.
The output is almost pure gas. This gas is fired in the gas turbine.