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Electrical Generating Equipment - Gas Turbines

Three generic 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 burnout
  • Post-combustion catalytic control to selectively reduce NOx and/or oxidize CO emissions from the turbine

Other recently 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.

Wet Controls
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.

Dry Controls
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).

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.

Two-stage lean/lean 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.

Two-stage rich/lean 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.