The USA Clean Air Act in effect since 1970 and the Kyoto agreement on reduction of fossil fuel emissions has placed stringent air pollution controls in many parts of the world including Indonesia.
Pollution control equipment can account for up to 40% of the cost of a new power plant and 35% of operating costs.
Environmental concerns about coal use center on emission of sulphur, nitrogen and carbon dioxide. Coal is not the only contributor of these pollutants but is a significant one. Advanced coal technologies are being investigated to reduce air pollution and make more efficient use of coal reserves in power generation.
Clean coal technologies range from use of ultra clean coal to improved combustion processes.
• Ultra clean coal
Ultra clean coals with lower ash, sulphur and nitrogen contents and good combustion performance.
Either naturally occurring such as Indonesia’s sub-bituminous coals that has low pollutant properties. Or coals cleaned to achieve these properties.
A majority of the ultra clean coals are lower rank. The market for these coals has grown substantially as more power stations are designed to handle these types of coal.
• Fluidized Bed Combustion
Crushed coal mixed with limestone is supported on a strong current of air. The “fluidized” mixture acts as a boiling liquid, mixing turbulently, thus assuring efficient combustion. The limestone reacts with and removes over 90% of the sulphur. Scrubbers (FGD) are not required for sulphur removal. This improves the thermal efficiency.
The operating temperature is lower than conventional boilers so less nitrogen oxides are formed.
This technology is suited to smaller boilers fabricated as modular units. The capital cost is less than conventional boilers of same capacity.
• Coal Gasification
Instead of directly firing the coal it is gasified (hydrocarbon gas). The product gas is burned to power gas turbines and the remaining heat is used to produce steam to turn turbines.
This is called combined cycle gasification, which has been reported as extremely efficient and clean.
It can have transport cost savings as if gasification is done close to coal source there is no need to transport bulky coal long distances.
Summary of Coal Quality Effect on Combustion
Below Table details the effect of coal quality on combustion.
