Page 92 - Piston Engine-Based Power Plants

The Environmental Impact of Reciprocating Engine Power Plants 83

Such techniques improve emission performance and may be ade-
quate for automotive applications. However they are unlikely to be
sufficient for stationary engines for power generation. Depending on
the regulatory regime and the engine some additional form of post-
combustion NO x emission control will usually be required.

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 devel-
opment. Where a catalytic converter can be used, nitrogen oxide reduc-
tion is around 90% or more.
Automobile style catalytic converters are a relatively expensive
means of reducing NO x emissions. For large engines, the more eco-
nomical alternative is to use a selective catalytic reduction (SCR) sys-
tem and this system can be applied to both stationary engines and
large transportation engines. SCR also employs a catalyst, but in con-
junction 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 nitro-
gen oxides are reduced back to nitrogen. This type of system will cut
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.

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 burnt
within the engine. This is simplest in lean-burn engines but conditions
within these engines does compromise 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, and particularly for diesel engines, engine con-
trol systems will not maintain emissions sufficiently low to meet statu-
tory emission standards. In this case an oxidation catalyst will be
needed to treat the exhaust gases. When the hot gases pass over