Page 72 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Power Equipment and Systems 51

difference between a winter night dry bulb temperature and that of a summer day dry
bulb temperature. Combustion turbine inlet cooling (CTIC) systems lower or maintain
the low intake temperature to ensure the stable output of power at all times. While
indirect/direct evaporative cooling is the most common CTIC system type, chilled
water coils and direct expansion (DX) refrigerant coils can be used to provide even
greater benefit in situations with high outside air temperatures especially those with high
humidity. The use of CTIC systems has several benefits including increased power output
capacity, lower heat rate, extended turbine life, and system efficiency improvements.
Emissions Control Types
The amount of thermal NO generated is directly related (linear function) to the amount
x
of time the hot gases are at flame temperature in the combustor, and related exponen-
tially to the temperature of the flame. The flame temperature is the variable that is more
easily controlled and can be adjusted in order to achieve reduced NO emissions
x
levels. The flame temperature is a unique function of the equivalence ratio, and, there-
fore, the rate of NO production is likewise a unique function of the equivalence ratio as
x
previously defined.
NO production is highest when λ = 1, and is lowered as the fuel mixture is either
x
richened or leaned (λ < 1 or λ > 1, respectively). As the equivalence ratio increases above
λ = 1, so do smoke emissions. As the equivalence ratio decreases below λ = 1, the carbon
monoxide emissions also increase.
The most common emissions control strategy for combustion turbine exhaust is to
install selective catalytic reduction (SCR) in the discharges gases. This process requires
injecting ammonia into the exhaust air stream. The ammonia reacts with NO on the
x
catalyst surface which lowers the NO in the exhaust gases. As noted, emissions and
x
emission control methods are discussed further in Chap. 7.
Noise/Vibration
Combustion turbines exhibit noise differently than reciprocating engines, and generate
high-frequency noise and vibrations. An operating combustion turbine generator set
often sounds similar to a stationary jet airplane and its sound can be loud and uncom-
fortable to the casual observer. Manufacturers often attenuate this noise by enclosing
their turbine generators in sound-insulated enclosures. Locating the CTG in an enclo-
sure reduces sound levels considerably, but will not attenuate the sound completely.
Additional sound attenuation equipment is typically installed on turbine generators to
further lower noise to acceptable levels. Sound attenuation equipment includes inlet air
silencers and exhaust silencers, although, if the CTG exhaust flows through a HRSG
and exhaust stack, the HRSG may attenuate the exhaust noise sufficiently without an
exhaust silencer.
Vibration levels in combustion turbines are generally low as the rotational nature of
the assembly does not include reciprocating parts; however, structural engineers
must account for high-frequency vibration in their foundation and housekeeping
pad calculations.

Controls
Similarly to internal combustion reciprocating engine generators, CTGs normally have
their own stand-alone control system to start and stop safely the CTG based on equip-
ment permissives, and to help ensure the proper operation of the CTG(s) including
controlling rotational speed by controlling fuel flow. Of course, as with reciprocating