Introduction
There has been a steady increase in demand for steaming coal over the last 30 years. The demand rising mostly from increase in number of coal fired power stations.
Coal is the preferred fossil fuel for power generation. The main factors in choice of coal are the cost of heat content and suitability for boiler. This depends on mining, processing and transport costs and the characteristics of the coal.
Newer types of boilers can fire a wider range of coal qualities. When designing the boiler and plant the coal types that will be used are subjected to pilot combustion tests to obtain performance data.
Outline of Combustion Reactions
Coal produces energy in the form of heat when combusted in an atmosphere of air.
Coal is typically composed of :
- Carbon 60 to 80% depending on rank
- Hydrogen about 5%
- Nitrogen about 1 to 2%
- Sulphur about 0.1 to 2%
- Oxygen amount depends on other ultimate parameters
- Non-coal material, inherent and extraneous ash
- Water as total moisture and chemically bound moisture
Carbon, the major component of coal, is the principal source of energy. About 14 500 Btu/lb or 8055 kcal/kg of energy liberated for carbon.
Hydrogen generates about 62 000 Btu/lb or 34 444 kcal/kg but not all of this energy is available for heat as part of the hydrogen combines with oxygen to form water vapor.
Sulphur generates about 4 000 Btu/lb or 2 222 kcal/kg.
The higher the oxygen content of coal the lower the heating value. This is because the oxygen is chemically bound to the carbon in the coal structure. Thus the carbon is partially oxidised lowering the heating value.
This follows the coal rank – higher rank higher carbon, lower hydrogen and oxygen and higher heating values.
The by-products of coal combustion are:
• Bottom Ash
• Fly Ash
• Sulphur Dioxide SO2
• Nitrous Oxides NOx
• Water H2O
• Carbon Dioxide CO2