Industrial Emission Controls
Acidic emissions of sulphur dioxide and nitrogen oxides arise from many industrial sources as a result of combustion processes. In the UK power stations contributed 65% of all sulphur dioxide emitted in the UK in 1999. Other industries were responsible for 22%. Industries also emit nitrogen oxides which can also cause rainfall to become more acidic. While road transport is the major source of nitrogen oxides in the UK (44% in 1999), power stations accounted for 21% and other industries 13% in 1999. There are many technologies which can be used in industry to reduce the emissions of pollutants to the atmosphere and these can be applied before, during or after combustion.
Examples of pre-combustion sulphur control technology (removing sulphur before burning) include coal scrubbing and oil desulphurisation. Another removal process is to change the design of the boiler and to install pressurised fluidised bed combusters (FBC) which removes sulphur from coal during the burning process. Another process which removes sulphur dioxide from coal during combustion is the Integrated Gasification Combined Cycle. Coal is gasified under pressure with a mixture of air and steam which results in the formation of gas which can then be burned to produce electricity.
One of the post-combustion sulphur controls (removing sulphur after burning) is Flue Gas Desulphurisation (FGD). In FGD processes, waste gases are scrubbed with a chemical absorbent such as limestone to remove sulphur dioxide. There are many different FGD processes, the main ones being the limestone-gypsum process and the Wellman-Lord regenerative process. The limestone-gypsum FGD involves mixing limestone and water with the flue gases to produce a slurry which absorbs the sulphur dioxide. The slurry is then oxidised to calcium sulphate (gypsum) which can then be used in the building trade.
Unlike sulphur, it is not possible to reduce the nitrogen content of the fuel before combustion by physical cleaning as it is combined within the organic matter of the fuel, and at present there are no commercially available methods to reduce organic nitrogen. Instead, nitrogen oxides can be removed during combustion. Low nitrogen oxides burners ensure that the fuel is burnt in low oxygen concentrations, such that any nitrogen oxides produced are reduced to nitrogen gas. Once initial combustion has taken place, further air is added to the combustion chamber to ensure that the fuel is completely burnt. Advanced low nitrogen oxides burners can reduce emissions by up to 30%. Such burners can be installed on either new or existing combustion plants.
Emissions of nitrogen oxides, like for sulphur dioxide, can also be reduced by treating the flue gases. One method involves mixing the flue gases with ammonia, converting the nitrogen oxides to nitrogen and water. This process is suitable for fitting to existing plant and new build applications, and can achieve an emissions reduction of up to 80 to 90%.
Some fuels, for example natural gas, are naturally less polluting in terms of acidic emissions, whilst traditional coal power generation is more polluting, depending on the amount of sulphur there is in the coal being burnt. To help reduce atmospheric emissions of sulphur dioxide and nitrogen oxides in the UK, many of the more recent power stations have been built to operate on gas rather than coal.