Page 91 - Piston Engine-Based Power Plants

82 Piston Engine-Based Power Plants

Table 8.2 Range of Emissions From Diesel Engines
Emission Emission Range
Nitrogen oxides 50 2500 ppmV
Carbon monoxide 5 1500 ppmV
Particulate matter 0.1 0.25 g/m 3
VOC 20 400 ppmV
Sulphur dioxide 10 150 ppmV
Source: Nett Technologies.

cheap but which often contain significant levels of sulphur. Since
sulphur can damage the engine, it is normal to treat this type of fuel
first to remove most of the sulphur. The range of emissions from diesel
engines are shown in Table 8.2.

NITROGEN OXIDE EMISSIONS

The most significant exhaust emissions from a piston engine are nitro-
gen oxides. These can both cause and aggravate respiratory diseases
such as asthma. Continual exposure to high levels can lead to serious
respiratory problems. In addition, NO x can react with other airborne
pollutants to generate both particulates and ozone, both of which are
themselves harmful to health. These reactions are also responsible for
the haze above many cities. In the wider environment NO x can react
in the atmosphere to generate acid rain that can damage plants and
trees and acidify lakes and rivers, killing wildlife that relies on them.

There are a number of strategies for reducing the emissions of NO x .
Engine modifications that reduce the combustion temperature of the
fuel, such as the use of a lean fuel mixture and timing adjustments can
provide the first step in reducing these emissions. Natural gas engines
designed to burn a very lean fuel (excess air) provide the best perfor-
mance, 45 150 ppmV or 1 3 g/kWh. Diesel engines present a greater
problem because of the higher combustion temperature.

An additional technique that is being applied to internal combus-
tion engines to reduce NO x 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 concentra-
tion and thereby reduce the combustion temperature. This reduces
NO x production but will also reduce engine efficiency.