Flue Gas
Flue gas (sometimes called exhaust gas or stack gas) is the gas that emanates from combustion plants. It contains the reaction products of fuel and combustion, air and residual substances such as particulate matter, sulphur oxides, nitrogen oxides and carbon monoxide [1]. When burning waste materials hydrogen chloride and hydrogen fluoride may be present in the flue gas as well as hydrogen and heavy metal derivatives.
Exhaust gases must comply with the strict environmental limits as set out within the Industrial Emissions Directive (2010/75/EU) and transposed within England and Wales as part of the Environmental Permitting regime to limit the amount of pollutants entering the environment and local atmosphere. In order to minimise these pollutants produced from the combustion of waste in energy from waste plants lime is used to ‘scrub’ acid pollutants such as hydrochloric acid and sulphur dioxide from the exhaust gases. The introduction of limestone, quick lime or hydrated lime as a powder or slurry promotes a reaction to form insoluble calcium sulphate or gypsum which can then be collected. If carried out under the correct conditions this process can produce a saleable gypsum co-product [2].