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Carbon Dioxide (CO2)
CO2 was thought of as a product of combustion and not as a pollutant. Kyoto protocol, effects of Green House gases and global warming issues have changed the way we look at CO2. CO2 has turned to be the major greenhouse gas. A fossil fuel power plant is the major contributor of CO2.
One MJ of heat input produces 0.1 kg of CO2. The only way to eliminate CO2 is to capture it before leaving to atmosphere. After capturing it has to be stored permanently or sequestered. Commercially viable capture and sequestration systems are yet to be in place. Till such time the only way is to
- Improve the power plant efficiency so that the reduced coal consumption reduces CO2 per kwhr.
- Switch over from Fossil based energy sources to renewable sources like wind, solar or hydro power.
- Reduce Deforestation and increase Afforestation to absorb the excess CO2 produced.
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Sulphur Dioxide (SO2)
This is a product of Combustion and depends on the amount of Sulfur in Coal.This is also referred to as SOx.
Sulphur in Coal ranges for 0.1 % to 3.5% depending on type and rank. During combustion Sulfur combines with Oxygen to form SO2.
Power plants are the largest emitters of SO2. In the presence of other gases SO2 forms Sulphuric acid and can precipitate down as acid rain leading to destruction of eco systems.
Use of low Sulfur coals is the best ways to reduce the SO2 emissions. Desulphurisation plants downstream of the boilers also reduce emissions. Fluidized bed combustion of coal is another effective method to reduce SO2 emissions..
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Ash is the residue after the combustion. A 500 MW coal fired power plant burning Coal with around 20 % Ash, collects ash to the tune of Two Million Tons in Five years. Cement plants may utilize a small portion of the ash. Disposing bulk of it on a long term basis can raise major environmental issues.
- Ash contains toxic elements that can percolate into the drinking water system.
- The wind, breach of dykes or ash spills can carry away the ash particles to surrounding areas causing harm to humans and vegetation.
Considering the life of a power plant is 20 years, great foresight , planning and commitment is required to dispose the ash in an eco friendly way.
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Power plants have elaborate arrangements to collect the ash. A small quantity still goes out through the stack and is categorized as Particulate Matter emission.
The very tall stacks in power plants disperse this ash over a very wide area reducing the concentration levels to human acceptable levels at ground levels.
The particles of size less than 2.5 microns called PM 2.5 is of great concern since these are responsible for respiratory illness in humans.
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Nitrogen Oxides (NOx)
Nitrogen in fuel and in the air reacts with Oxygen at high temperatures to form various oxides of Nitrogen collectively called NOX. Fossil fuel power plants are the second largest emitter of NOX.
This is a hazardous pollutant creating visual and respiratory problems. Also NOX combines with water to form acid rain, smog, and ground ozone.
Design changes in combustion technology have helped in reducing the NOX emissions. Methods like Selective Catalytic Reactors are used in power plants to meet the emission regulations.