Lime and limestone play a significant role in the removal of pollutants from flue gas streams of coal-fired power plants, incinerators and industrial facilities. Flue gas desulfurization (FGD) primarily refers to the removal of sulfur dioxide (SO2). However, lime and limestone are also used in the removal of other pollutants such as hydrogen chloride (HCl), sulfur trioxide (SO3), fine particulates and mercury. In the US, air pollution control applications were the second largest use of lime in 2003, consuming over 3.4 million tons of lime. Lime and limestone products are used in both wet and dry FGD processes.
In the wet FGD processes these products are slurried with water and sprayed into a flue gas scrubber vessel. The acidic gases, normally SO2 and HCl, are absorbed into the water where they chemically react with the lime and limestone. The reaction products, primarily calcium sulfite, can then be oxidized to produce calcium sulfate, a salable gypsum byproduct.
There are three basic types of dry FGD processes.
- Dry injection processes inject dry hydrated lime directly into the flue gas stream.
- Spray dryer processes inject finely atomized lime slurry into a separate vessel. Water from the slurry is evaporated before the solids contact the vessel walls.
- Circulating fluidized bed processes inject dry hydrated lime in a separate fluidizing vessel.
With all three processes, the acidic gases combine with lime to form a dry product which is removed from the flue gas stream in particulate control devices such as bag houses or electrostatic precipitators (ESP's).
Graymont services all of these applications and is prepared to provide technical support to our customers.
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