Coal contains variable amounts of halogens, with chlorine generally present in the highest concentrations, followed by fluorine, bromine and iodine. When coal is burned, most of the halogen content is emitted as acid gases such as hydrogen chloride and hydrogen fluoride.
The largest emissions of man-made hydrogen chloride are from coal burning and waste incineration. In 1984, the UK released 240,000 tonnes from coal burning, out of a total estimated western European emission of 483,000 tonnes. The UK's contribution has remained high despite the subsequent decline in its coal consumption, largely because of the high chlorine content of British coals.
Waste incineration contributes another 138,000 tonnes of hydrogen chloride per year, and is the major source of the gas in Sweden, the Netherlands, Denmark, Switzerland and France. France has the largest emission from this source, estimated at 64,000 tonnes per year.
Fluorine and fluoride emissions have been quantified for few countries, but the report concludes that the major sources are coal combustion and the steel, ceramics and aluminium industries. In the UK, coal contributes 5,400 tonnes to the total western European emission of 14,000 tonnes per year.
The report summarises what is known of the environmental fates and effects of the gases. Hydrogen chloride and hydrogen fluoride are highly reactive and soluble and tend to be rained out or dry-deposited close to sources. Both can be locally important sources of acidity near combustion plants.
Control of halogen emissions from combustion plants is not generally required, the report notes, although Germany has set emission limits for inorganic chlorine and fluorine. Limits on hydrogen chloride have already been set at EC level for refuse incinerators, and tight emission standards for both chloride and fluoride are contained in the recent EC proposal on hazardous waste incineration (ENDS Report 208, pp 36-7 ). Both gases are readily removed by flue gas desulphurisation plants.