Page 90 - Piston Engine-Based Power Plants

The Environmental Impact of Reciprocating Engine Power Plants 81

high temperatures to operate and as a consequence of this produces
relatively high levels of nitrogen oxides. Table 8.1 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
unburnt hydrocarbons. This can happen in any engine, particularly
when the load is changing and during start-up and shutdown. Carbon
monoxide is hazardous at low levels and its emissions are regulated
like those of NO x . Unburnt hydrocarbons are classified as VOCs and
their emissions are also controlled by legislation. A rich fuel air
mixture will produce more of both (as well as more NO x ) than a lean
mixture because with less oxygen available, there will be more fuel that
remains incompletely combusted. On the other hand, the combustion
temperature in an engine burning a rich fuel air mixture is higher
than with a lean mixture. This leads to greater efficiency because a
heat engine can extract more power from the engine if it has a higher
peak combustion temperature. In order to reduce emissions, efficiency
must therefore be compromised.

Particulates are another class of emissions. These small particles
result from incomplete combustion too; they are essentially a type of
soot. The smaller the particles, the more dangerous they become, with
the smallest able to enter the blood stream through the lungs if
breathed in with air. Diesel engines produce significantly higher levels
of particulates than natural gas engines or gasoline engines. Liquid
fuels, particularly heavy fuel, may also produce particles derived from
ash and metallic additives.

Natural gas contains negligible quantities of sulphur so gas engines
produce no sulphur dioxide. Gasoline contains no sulphur either.
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

Table 8.1 Emissions of Nitrogen Oxides From Internal Combustion Engines
Emission (ppmV) Emissions (g/kWh)
High-speed and medium-speed diesel engines 450 1800 7 20
Spark ignition natural gas engine 45 150 1 3
Source: US Environmental Protection Agency.