Dry processing of gaseous effluents involves injecting lime in the form of a dry powder to neutralize the acid gases. The fly ash, as well as the salts resulting from the reaction, are separated by an electrostatic precipitator or, preferably, by a fabric filter.

Dry processing is considered to be ideally suited to low-capacity incinerators. It can easily be incorporated into existing installations and has the following advantages:

• low investment cost
• simplicity of design and operation
• extremely space-efficient
• easy to automate
• low consumption of compressed air, electricity and water
• flexible operation as far as temperature and capacity are
  concerned

Moreover, the by-product from the purification process is a dry powder which is easy to handle.

In spray-dry flue gas treatment, lime is added in the form of a lime slurry (or lime milk) which is sprayed or atomised in the reactor. The latent heat in the treated gas evaporates the water in the lime slurry and creates a liquid-gas reaction. The by-product obtained is a dry powder.

In the wet process, acid gases are washed with water. Milk of lime or limestone slurry neutralizes the water that has been acidified by the dissolution of the acid gas. This type of process is typically used in large installations because of the higher investment cost and the need to treat the residual water.