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Page | 021 6.7. EMISSION CONTROL
As discussed in the earlier sections, natural gas engines are characterized by very
low emissions. They do not emit any sulfur oxides or particulate matter due to the
characteristics of the fuel. Pollutants that still need control are nitrogen oxides (NOX) and
carbon monoxide (CO). Nitrogen oxide may be controlled by optimizing the combustion
process. Adjusting the temperature of the air fed into the cylinders (effected by controlling
operation of charge air coolers) allows minimizing the NOX formation, which ensures
compatibility with many legal standards – notably current European Union regulations
– without a need to use any exhaust gas cleaning devices. If primary methods are not
sufficient (like in some areas of the United States), selective catalytic reduction (SCR)
process is used to reduce the NOX content in the exhaust gas. The SCR process
involves a reactor installed in the exhaust gas duct. The reactor needs to be supplied
with a reagent in the form of a water solution of urea or, in rare cases, ammonia. Layers
of catalytic material installed inside the reactor accelerate the reaction that breaks down
the NOX particles into oxygen and nitrogen. SCR reactors are always supplied as engine-
wise. To ensure concurrent maintainability, the reagent supply would be provided by a
looped system, just like the fuel gas.
The reduction of carbon monoxide, except for efficient combustion control, is
accomplished with an oxidation catalyst, which causes the further oxidation of CO into
CO2. This kind of catalytic converter is also installed in the exhaust gas duct and does
not require any reagent supply.
6.8. MAINTENANCE
Unlike small high-speed emergency diesel generators, medium-speed gas engines
used in commercial power industry are very robust and sturdy, which also makes them
relatively bulky and heavy. Therefore, once installed on-site, they are not moved for
maintenance. Instead all of the components that might need refurbishment, such as
cylinder heads, cylinder liners, pistons, connecting rods, etc., may be removed using
an on-site overhead crane and transported individually to refurbishment workshops,
while the heaviest components – the engine block, generator and crankshaft – stay at
the site for the entire lifetime of the plant, which may easily exceed 25 years of baseload
operation.
Individual engine-generator sets are maintained one at a time, along with engine-wise
auxiliaries, so the overhaul of one engine does not require outage of any other generating
set. While traditionally maintenance was performed according to predetermined schedule,
most modern plants use so-called condition-based maintenance (CBM). In this solution,
similar to what is used in modern commercial aviation, the condition of each engine
is continuously monitored by a service center, which then issues recommendations
concerning all scheduled and preventive maintenance procedures. Thanks to this
process, maintenance procedures are performed in certain “time windows” instead of
fixed moments of time, which improves operational flexibility.
Gas engine power plants with medium-speed engines are designed for minimum
operator intervention, can be remotely controlled and dispatched, if desired, and do
not require the presence of a permanent on-site crew. The most frequent maintenance
procedures, such as checks and spark plug replacements, are fast procedures carried
out in a matter of hours. Large, state-of-the-art designs require engine overhauls after
no less than 16,000 running hours, and the most extensive overhaul might be scheduled
only at 96,000 running hours, i.e. twelve years of continuous operation. This means that
for peaking power plants, the largest overhaul may not ever happen during the project’s
economic lifetime.
It needs to be noted that number of starts and stops of an engine has no impact on its
maintenance schedule, unlike in some other power generation technologies where a
single start may be counted as the equivalent to multiple running hours due to the extra
thermal stresses. This is inherent feature of the engine technology designed for cyclic
operation.
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