In most of the industries, steam is used as a medium for transferring energy. Steam is generated by the combustion of various types of fuels. All fossil fuels and biomass contain some quantity of non-combustibles (except natural gas) which form the majority of the particulate. At the exhaust the unburned carbon also appears as particulate.
When coal and biomass is burned in conventional boilers, a part of the ash settles at the bottom of the furnace and the remaining part is carried out of the furnace with the flue gases (fly ash). The amount of ash in the flue gas depends upon the method of firing. Stoker and grate fired boilers have 30% of fly ash and 70% of grate (bottom ash), for fluidized bed boiler it is the reverse.
However the extent of fly ash also depends on the type of fuel for example, in case of Pulverized coal or saw dust where there is high content of fines has higher quantity of fly ash.
The Ash components are basically oxides of various elements such as iron oxide, aluminum dioxide, calcium oxide, silicon dioxide, titanium dioxide, magnesium oxide, diphosphorous pentoxide, sodium oxide, sulfur trioxide and potassium oxide. Other than these some Trace quantities of various elements are also found in ash.
Other significant properties of coal ash are particle size distribution and shape, both of which depends on the type of firing method, and influences the selection of the type of pollution control equipment.
There are wide number of air pollution control devices. The selection of the appropriate control technology depends on the characteristics of pollutants collected, operating conditions, and the emission control efficiency required. In some cases, pollutant emissions can be reduced significantly by modifying the processes and efficiently controlling the combustion. However, to achieve allowable emission limits as per regulations some form of air pollution equipments are to be installed.