Piston engine power units generally burn fossil fuels, and the environmental considerations that need to be taken into account are exactly the same as those that affect all coal-, oil-, and gas-fired power plants—that is, all the emissions resulting from fuel combustion. In the case of internal combustion engines the main emissions are nitrogen oxide, carbon monoxide, and volatile organic com- pounds (VOCs). Larger diesel engines, particularly those burning heavy diesel fuel, will also produce particulate matter and some sulfur dioxide.
Nitrogen oxide is 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 oxide. Table 5.3 compares emissions from the two types of engine.
When the fuel in an internal combustion engine is not completely burned the exhaust will contain both carbon monoxide and some unburned hydrocarbons. Carbon monoxide is hazardous at low levels and its emissions are regulated like those of nitrogen oxide. Unburned hydrocarbons are classified as VOCs and their emissions are also controlled by legislation.
Natural gas contains negligible quantities of sulfur so gas engines produce no sulfur dioxide. Diesel fuels can contain sulfur. Small- and medium-size diesel engines generally burn lighter diesel fuels that contain little sulfur. Larger engines can burn heavy residual oils that are comparatively cheap but often contain significant levels of sulfur. Since sulfur can damage the engine, it is normal to treat this type of fuel first to remove most of the sulfur.
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 emissions of particulate matter.
The most serious exhaust emissions from a piston engine is nitrogen oxide. Engine modifications that reduce the combustion temperature of the fuel, such as the use of a lean-fuel mixture described earlier, provide the first step in reducing these emissions. Natural gas engines designed to burn a very lean fuel (excess air) provide the best performance: 45–150 ppmV or 1–3 g/kWh. Diesel engines present a greater problem because of the higher combustion temperature (Table 5.4).
An additional technique that is being applied to internal combustion engines to reduce nitrogen oxide is exhaust gas recirculation. This involves taking some of the exhaust gas and mixing it with the air used to feed the engine. The effect is to reduce the overall oxygen concentration and thereby reduce the combustion temperature. This reduces nitrogen oxide production but will also reduce engine efficiency. However, depending on the regulatory regime, all types of internal combustion engines may require some additional form of post-combustion nitrogen oxide emission control.
For small gasoline engines a simple catalytic converter of the type used in automobiles is often the most effective solution. However, this type of system cannot be used with diesel or with lean-burn engines. New catalysts for use with lean-burn engines are currently under development. Where a catalytic converter can be used, nitrogen oxide reduction is around 90% or more.
Automobile-style catalytic converters are a relatively expensive means of reducing nitrogen oxide emissions. For large engines, the more economical alter- native is to use a selective catalytic reduction (SCR) system that can be applied to both stationary engines and large truck engines. An SCR system 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 nitrogen oxide react on the catalyst and the nitrogen oxide is reduced to nitrogen. This type of system will reduce emissions by 80% to 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.
Carbon Monoxide, VOCs, and Particulates
The emission of carbon monoxide, VOCs, and some particulate matter can be partially controlled by ensuring that the fuel is completely burned within the engine. This is simplest in lean-burn engines but conditions within these engines compromises efficiency. With all engines, careful control of engine conditions and electronic monitoring systems can help maintain engine conditions at their optimum level. Old engines as they become worn can burn lubrication oil, causing further particulate emissions.
For larger engines, particularly diesel engines, engine control systems also will not maintain emissions sufficiently low to meet statutory emission standards. In this case an oxidation catalyst will be needed to treat the exhaust gases. When the hot gases pass over the oxidation catalyst, carbon monoxide, unburned hydrocarbons, and carbon particles are oxidized by reacting with oxy- gen remaining in the exhaust gases, completing the combustion process and converting all the materials into carbon dioxide.
Sulfur emissions can be found in diesel engines that burn fuel containing sulfur. Many engines now burn low-sulfur fuels with less than 0.05% sulfur content. However, some diesels and the heavy fuel oils that very large engines burn may contain significant amounts of sulfur. The latter may contain as much as 3.5% sulfur. The best way of controlling sulfur emissions from internal combustion engines is to remove the sulfur from the fuel before use. However, in the worst case a sulfur capture system can be fitted. This is likely to be similar to the scrubbing tower used in a coal-fired power plant but at a much smaller scale. The use of such a system adds to both capital and maintenance costs and affects plant economics. It is only likely to be cost effective in the very largest reciprocating engine-based power plants.
Internal combustion engines, in common with all heat engines that burn carbon- based fuel, generate carbon dioxide, which is released in the exhaust gases leaving the engine. The relative amount produced during electricity generation depends on the efficiency of the engine. A large, high-efficiency diesel engine operating at close to 50% efficiency will produce significantly less carbon dioxide for each unit of electricity it generates than a small gasoline engine operating at perhaps 20% efficiency.
Currently the only way of effectively eliminating carbon dioxide emissions from such engines is to run them on a biofuel such as ethanol or biodiesel that has been derived from plants. The principle here is that although the combustion of the fuel will still produce carbon dioxide, the regrowth of the plants that were used to produce the fuel will absorb the same amount of carbon dioxide from the atmosphere, so that for a full cycle of growth, fuel production and combustion, the net amount of carbon dioxide added to the atmosphere is zero.
Research is underway to develop systems to capture carbon dioxide from the exhaust of reciprocating engines and a variety of techniques are being explored based on some form of post-combustion capture. Whether such systems will ever be used extensively on reciprocating engines seems doubtful since the cost is likely to be prohibitive. However, for very large power generating systems it might eventually be both technically feasible and economical.