Page 89 - Piston Engine-Based Power Plants

80 Piston Engine-Based Power Plants

type of engine there may be regular fuel deliveries which will add to
local traffic movement. All these factors have to be taken into account
when considering a new engine installation.

THE ORIGIN OF RECIPROCATING ENGINE EMISSIONS

Piston engines in stationary power units virtually all utilise a fuel that
is burnt to release energy which is then exploited in the heat engine to
provide electrical power. The energy source is most commonly a fossil
fuel such as gasoline, diesel or natural gas, although some engines
burn sustainable organic fuels such as bio-diesel and ethanol. The com-
bustion of all these fuels, including the biofuels, produces a range of
chemical by-products, many of which are toxic or harmful. If these are
not removed or rendered harmless they will be released into the envi-
ronment along with the exhaust gases from the combustion process. In
the case of internal combustion engines the main emissions are nitro-
gen oxides, carbon monoxide, VOCs and particulates. Large diesel
engines burning heavy diesel fuel may also produce some sulphur
dioxide. The emissions of all these can be reduced by application of
appropriate technology.

In addition, the combustion process will produce carbon dioxide. It
is unlikely there will ever be a cost-effective method of removing the
carbon dioxide from the exhaust of a reciprocating engine. The only
solution is to burn a carbon neutral biofuel such as bio-diesel or
ethanol.

Nitrogen oxides, NO x , are formed during combustion, primarily by
a reaction between nitrogen and oxygen in the air mixed with the fuel.
NO x can also be produced from nitrogen contained in fuel but there is
little nitrogen in the liquid and gaseous fuels burnt by most reciprocat-
ing engines.

The main NO x product in an internal combustion engine is nitric
oxide, NO. The reactions that lead to NO take place more rapidly at
higher temperatures when oxygen and nitrogen in air are at their most
reactive. In lean-burn gas engines, spark ignition engines where the
fuel is burned with an excess of air, temperatures can be kept low
enough to that nitrogen oxide emissions are within local limits. The
engine timing can also be adjusted to reduce production of NO x but
this may effect engine efficiency. The diesel cycle depends on relatively