Generating Equipment - Gas Turbines
types of emission controls in use for gas turbines are:
- Wet controls
(using steam or water injection) to reduce combustion temperatures for NOx control
- Dry controls
(using advanced combustor design) to suppress NOx formation and/or promote CO
catalytic control to selectively reduce NOx and/or oxidize CO emissions from the
developed technologies promise significantly lower levels of NOx and CO emissions
from diffusion combustion type gas turbines. These technologies are currently
being demonstrated in several installations. The uncontrolled, lean-premix, and
water injection emission factors for NOx and CO (Gas Turbine Emission Factors)
show the effect of combustion modification on emissions.
Water or steam injection is a technology that has been demonstrated to
effectively suppress NOx emissions from gas turbines. The effect of steam and
water injection is to increase the thermal mass by dilution and thereby reduce
peak temperatures in the flame zone. With water injection, there is an additional
benefit of absorbing the latent heat of vaporization from the flame zone. Water
or steam is typically injected at a water-to-fuel weight ratio of less than one.
Depending on the initial NOx levels, such rates of injection may reduce NOx by
60% or higher. Water or steam injection is usually accompanied by an efficiency
penalty (typically 2-3%), but an increase in power output (typically 5-6%). The
increased power output results from the increased mass-flow required to maintain
turbine inlet temperature at manufacturer's specifications. Both CO and VOC emissions
are increased by water injection, with the level of CO and VOC increases dependent
on the amount of water injected.
Since thermal NOx is a function of both temperature (exponentially) and
time (linearly), the basis of dry controls is to either lower the combustor temperature
using lean mixtures of air and/or fuel staging, or decrease the air/fuel mixture
residence time in the combustor. A combination of methods may be used to reduce
NOx emissions such as lean combustion and staged combustion (two-stage lean/lean
combustion or two-stage rich/lean combustion).
involves increasing the air-to-fuel ratio of the mixture so that the peak and
average temperatures within the combustor will be less than that of the stoichiometric
mixture, thus suppressing thermal NOx formation. Introducing excess air not only
creates a leaner mixture but it also can reduce residence time at peak temperatures.
combustors are essentially fuel-staged, premixed combustors in which each stage
burns lean. The two-stage lean/lean combustor allows the turbine to operate with
an extremely lean mixture while ensuring a stable flame. A small stoichiometric
pilot flame ignites the premixed gas and provides flame stability. The NOx emissions
associated with the high-temperature pilot flame are insignificant. Low-NOx emission
levels are achieved through cooler flame temperatures associated with lean combustion
and avoidance of localized "hot spots" by premixing the fuel and air.
combustors are essentially air-staged, premixed combustors in which the primary
zone is operated fuel rich and the secondary zone is operated fuel lean. The rich
mixture produces lower temperatures (compared to stoichiometric) and higher concentrations
of CO and H2, because of incomplete combustion. The rich mixture also decreases
the amount of oxygen available for NOx generation. Before entering the secondary
zone, the exhaust of the primary zone is quenched (to extinguish the flame) by
large amounts of air and a lean mixture is created. The lean mixture is pre-ignited
and the combustion completed in the secondary zone. NOx formation in the second
stage is minimized through combustion in a fuel-lean, lower-temperature environment.
Staged combustion is identified through a variety of names, including Dry-Low
NOx (DLN), Dry-Low Emissions (DLE), or SoLoNOx.