Primary particulate matter is generated by a variety of physical and chemical processes. It is emitted to the atmosphere through combustion, industrial processes, fugitive emissions and natural sources. Secondary particulate matter is formed in the atmosphere from condensation of gases and is predominantly found in the fine range. During coal combustion, the mineral matter (inorganic impurities) is converted to ash. Part of the ash is discharged from the bottom of the furnace as bottom ash. The particles suspended in the flue gas are known as fly ash. Fly ash constitutes the primary particulate matter, which enters the particulate control device. Particulate matter is in general referred to as “PM”, “PM10”, “PM2.5” (particulate matter (PM) with an aerodynamic equivalent diameter of 10 microns or less and 2.5 microns or less, respectively).
Technologies used to control particulate emissions from coal combustion are:
- electrostatic precipitators (ESPs)
- fabric filters (baghouses)
- wet particulate scrubbers
- mechanical/inertial collectors (cyclones/multicyclones)
- high temperature, high pressure (HTHP) particulate control
Quantity and characteristics of the fly ash and particle size distribution depend on the coal mineral matter content, combustion system, and boiler operating conditions. Mineral composition of the coal and the amount of carbon in the fly ash determine the quantity, resistivity and cohesivity of the fly ash. Combustion technique mainly determines the particle size distribution in the fly ash and hence the final particulate emissions. Common combustion systems in pulverised coal firing include dry bottom, wall (front, opposed) and corner (tangential) burners and wet bottom cyclone furnaces. In dry bottom boilers, 10-20% of the ash is discharged as dry, bottom ash. In wet bottom boilers, 50-60% of the ash is discharged at the bottom of the boiler as slag. However, the higher temperatures in cyclone boilers result in higher emissions of NOx. The combustion temperature may also affect the cohesivity of the fly ash. Higher operating temperatures can result in greater particle cohesivity leading to improved fly ash cake removal by reducing re-entrainment. Boiler operating conditions can affect the amount of unburnt carbon in the fly ash.
The summarised description of the technologies is mainly drawn from the detailed IEA Coal Research reviews on particulate control by Soud (1995) and Klingspor and Vernon (1988).