Environmental considerations

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Piston engine power units generally burn fossil fuels and the environmental considerations that need to be taken into account are exactly the same considerations that affect all coal-, oil- and gas-fired power plants; the emissions resulting from fuel combustion. In the case of internal combustion engines the main emissions are nitrogen oxides, carbon monoxide and volatile organic compounds (VOCs). Diesel engines, particularly those burning heavy diesel fuel will also produce particulate matter and some sulphur dioxide.

Nitrogen oxides are formed primarily during combustion by a reaction between nitrogen and oxygen in the air mixed with the fuel. This reaction takes place more rapidly at higher temperatures. In lean-burn gas engines where the fuel is burned with an excess of air, temperatures can be kept low enough to maintain low nitrogen oxide emissions. The diesel cycle depends on relatively high temperatures and as a consequence of this produces relatively high levels of nitrogen oxides. Table 6.2 compares emissions from the two types of engines.

Table 6.2 Emissions of nitrogen oxides from internal combustion engines





High- and medium-speed diesel



Natural gas burning spark-ignition engine



Source: US Environmental Protection Agency.

Source: US Environmental Protection Agency.

When the fuel in an internal combustion engine is not completely burned the exhaust will contain both carbon monoxide and some unburnt hydrocarbons. Carbon monoxide is poisonous and its levels should be minimised. The unburnt hydrocarbons are classified as VOCs, and both their emissions and those of carbon monoxide are controlled by legislation.

Natural gas contains negligible quantities of sulphur so gas engines produce no sulphur dioxide. Diesel fuels can contain sulphur. Small- and medium-sized diesel engines generally burn lighter diesel fuels which contain little sulphur. Larger engines can burn heavy residual oils which are comparatively cheap but which often contain significant levels of sulphur. As sulphur can damage the engine, it is normal to treat this type of fuel first to remove most of the sulphur.

Liquid fuels may produce particulate matter in an engine exhaust, the particles derived from ash and metallic additives. Incomplete combustion of heavy fuel can also lead to the emission of particulate matter.

Emission control

The most serious exhaust emissions from a piston engine are nitrogen oxides. Engine modifications that reduce the combustion temperature of the fuel, such as the use of a pre-combustion chamber and lean fuel mixture described above, offer the best means of reducing these emissions. Natural gas engines designed to burn a very lean fuel (excess air) provide the best performance. Diesel engines present a greater problem but water injection can reduce emission levels by 30-60%.

Where these measures are insufficient to keep emissions below regulation levels, exhaust gas treatment will be necessary. For small gasoline engines a simple catalytic converter of the type used in automobiles is often the most effective solution. This type of system cannot be used with diesel or with lean-burn engines although new catalysts for use with lean-burn engines are currently under development. Where it can be used, the catalytic converter will reduce nitrogen oxide emissions by 90% or more.

The alternative is a selective catalytic reduction system. This also employs a catalyst, but in conjunction with a chemical reagent, normally ammonia or urea, which is added to the exhaust gas stream before the emission-control system. The reagent and the nitrogen oxides react on the catalyst, and the nitrogen oxides are reduced to nitrogen. This type of system will reduce emissions by 80-90%. However care has to be taken to balance the quantity of reagent added so that none emerges from the final exhaust to create a secondary emission problem.

The emission of carbon monoxide, VOCs and some particulate matter can be controlled by ensuring that the fuel is completely burnt within the engine. Careful control of engine conditions and electronic monitoring systems can help maintain engine conditions at their optimum level. Exhaust gas catalytic oxidation systems can also be used to keep levels below prescribed limits. Old engines as they become worn can burn lubrication oil, causing further particulate emissions.

Sulphur emissions are only likely to be met with large diesel engine power plants burning heavy fuel oil. Some of these oils can contain as much as 3.5% sulphur. Normally this sulphur can be removed by pre-treating the fuel. However in the worst case, a sulphur capture system can be fitted to the exhaust system. This adds to both capital and maintenance costs, and affects plant economics.

Carbon dioxide

The combustion of all carbon-base fuels results in carbon dioxide. This is as true of natural gas, oil or biodiesel as it is of coal. Coal is predominantly composed of carbon and it produces the greatest amount of carbon dioxide for each unit of heat energy. Liquid and gaseous fuels normally produce less. However in all cases significant emissions are inevitable.

Emission levels can be minimised by operating the engine at the highest efficiency possible. Use of waste heat increases efficiency and so helps minimise emission. Bio-derived fuels are generally considered carbon dioxide neutral since although their combustion generates carbon dioxide, production of more fuel results in the capture of the carbon dioxide again.

The only method of physically reducing carbon dioxide emission from fossil fuel combustion is to capture it and store it. Technology to achieve this is being developed but the cost is likely to be extremely high. It seems unlikely that it will ever be an economic option for small piston engines.

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