Page 93 - Piston Engine-Based Power Plants

84 Piston Engine-Based Power Plants

oxidation catalyst, carbon monoxide, unburnt hydrocarbons and car-
bon particles are oxidised by reacting with oxygen remaining in the
exhaust gases, completing the combustion process and converting all
the materials into carbon dioxide.

SULPHUR DIOXIDE

Sulphur emissions can be found in diesel engines that burn fuel con-
taining sulphur. Many engines now burn low-sulphur fuels with less
than 0.05% sulphur content. However some diesels and the heavy fuel
oils that very large engines burn may contain significant amounts of
sulphur. Heavy fuel, or residual oil, may contain as much as 3.5%
sulphur. The best way of controlling sulphur emissions from internal
combustion engines is to remove the sulphur from the fuel before use.
However in the worst case a sulphur capture system can be fitted. This
is likely to be similar to the scrubbing tower used in a coal-fired power
plant but at much smaller scale. The use of such a system will add 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.

CARBON DIOXIDE

Internal combustion engines, in common with all heat engines that
burn carbon-based fuel, generate carbon dioxide which is released into
the atmosphere with the exhaust gases leaving the engine. The relative
amount produced during electricity generation depends both on the
fuel and 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 etha-
nol or bio-diesel that has been derived from plants. The principle here
is that although the combustion of the fuel will still produce carbon
dioxide, the re-growth 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.