Gas Turbine Handbook : Principles and Practices

Gas Turbine Exhaust Treatment 137
obtain effective cooling a liquid with a high heat of vaporization is
preferred and water has a high heat of vaporization. Both water and
steam injection increase the power output of the gas turbine by adding
to the mass flow through the turbine.
WATER INJECTION:
During the early development of gas turbines, water was injected
into the compressor, diffuser, or combustor to increase power
output. This technique was applied to both the aircraft “jet” gas
turbine engine to increase takeoff thrust, and the stationary heavy
industrial gas turbines for peaking power. Today, in order to control
the formation of organic NO, demineralized/deionized water is injected
directly into the combustion zones of the gas turbine thereby
influencing the chemical reaction of the combustion process. In addition
to lowering the flame and gas temperatures, vaporized water
also increases the mass flow through the engine. As a result, at a
constant power output, combustion and turbine temperatures are
reduced. The combination of the reduced combustion temperatures
and changes in the chemical reaction can reduce NO x
formation up
to 80%. The amount of water necessary to accomplish this reduction
in NO x
is a function of the diffuser, combustor, and fuel nozzle
design. Water injection rates are generally quoted as a water-tofuel
ratio or as a percentage of compressor inlet air flow. The water
injection rate for a typical 80 MW heavy frame gas turbine would
be 0.6 water-to-fuel ratio or 1.15 % of total air flow. The amount of
water injected into the diffuser or the combustor is limited by several
factors. Water injection moves the operating line closer to the
compressor surge line (Figure 9-4). Also, too much water will quench
the combustion flame, resulting in flame-out.
The water used for NO x
control is demineralized and deionized
to eliminate deposits from forming on the hot metal surfaces of
the combustor, turbine nozzles, and turbine blades. When handling
demineralized/deionized water, care must be taken to select materials
that are resistant to its highly reactive attack. Therefore, piping
should be AISI 304L and valves and pumps should be 316L stainless
steel.