The combustion of organic material such as gas, heavy oil, and coal in power plants, of domestic and industrial waste in waste incineration plants, but also various process control procedures in the chemical industry result in polluted exhaust gases which must not be released into the environment without secondary treatment.
■ Legal basis
The emission limits for the corrosive gases SO2, SO3, NOX, (HCl, HF) are governed by standard rules which apply throughout the EU. In the USA the Clean Air Act applies. Almost all newbuilt plants, particularly in China and South-East Asia, use this state-of-theart technology so that there are almost no new plants without modern flue gas cleaning.
From impurities in the source materials to be combusted mainly sulfur dioxide results. There are several procedures to
extract it. In the most cases the most efficiently working so-called “wet procedure” is applied. The sulfur dioxide, which is acidic in solutions, is bound by an alkaline suspension in an absorbtion tower. Mostly a limestone suspension or hydrated lime (milk of lime) are used:
Calcium sulfite accumulates in the lower part of the tower, the so-called sludge. By injecting air (forced oxidation) it is converted into gypsum.
The resulting gypsum is dewatered and can then be used as building material. Gypsum from waste incineration plants may be more contaminated and must possibly be disposed of. In that way the SO2 emissions have been reduced from 1.6 mio t to 120.000 t within 20 years….