Editing Combustor (section)

==Emissions==
One of the driving factors in modern gas turbine design is reducing emissions, and the combustor is the primary contributor to a gas turbine's emissions. Generally speaking, there are five major types of emissions from gas turbine engines: smoke, [[carbon dioxide]] (CO<sub>2</sub>), [[carbon monoxide]] (CO), [[unburned hydrocarbon]]s (UHC), and [[nitrogen oxides]] (NO<sub>x</sub>).<ref name="reg">Verkamp, F. J., Verdouw, A. J., Tomlinson, J. G. (1974). Impact of Emission Regulations on Future Gas Turbine Engine Combustors. ''Journal of Aircraft''. June 1974. Vol. 11, No. 6. pp. 340–344.</ref><ref name="reduct">{{cite journal |author1=Sturgess, G.J. |author2=Zelina, J. |author3=Shouse D. T. |author4=Roquemore, W.M. |title=Emissions Reduction Technologies for Military Gas Turbine Engines |journal=Journal of Propulsion and Power |date=March–April 2005 |volume=21 |number=2 |pages=193–217 |doi=10.2514/1.6528 |url=https://arc.aiaa.org/doi/10.2514/1.6528|url-access=subscription }}</ref>

Smoke is primarily mitigated by more evenly mixing the fuel with air. As discussed in the fuel injector section above, modern fuel injectors (such as airblast fuel injectors) evenly atomize the fuel and eliminate local pockets of high fuel concentration. Most modern engines use these types of fuel injectors and are essentially smokeless.<ref name="reg"/>

Carbon dioxide is a [[product (chemistry)|product]] of the [[combustion]] process, and it is primarily mitigated by reducing fuel usage. On average, 1&nbsp;kg of jet fuel burned produces 3.2&nbsp;kg of CO<sub>2</sub>. Carbon dioxide emissions will continue to drop as manufacturers improve the overall efficiency of gas turbine engines.<ref name="reduct"/>

Unburned-hydrocarbon (UHC) and carbon-monoxide (CO) emissions are highly related. UHCs are essentially fuel that was not completely combusted. They are mostly produced at low power levels (where the engine is not burning all the fuel).<ref name="reduct"/> Much of the UHC content reacts and forms CO within the combustor, which is why the two types of emissions are heavily related. As a result of this close relation, a combustor that is well optimized for CO emissions is inherently well optimized for UHC emissions, so most design work focuses on CO emissions.<ref name="reg"/>

Carbon monoxide is an intermediate product of combustion, and it is eliminated by [[oxidation]]. CO and [[hydroxyl radical|OH]] react to form CO<sub>2</sub> and [[hydrogen|H]]. This process, which consumes the CO, requires a relatively long time ("relatively" is used because the combustion process happens incredibly quickly), high temperatures, and high pressures. This fact means that a low-CO combustor has a long ''residence time'' (essentially the amount of time the gases are in the combustion chamber).<ref name="reg"/>

Like CO, Nitrogen oxides (NO<sub>x</sub>) are produced in the combustion zone. However, unlike CO, it is most produced during the conditions that CO is most consumed (high temperature, high pressure, long residence time). This means that, in general, reducing CO emissions results in an increase in NO<sub>x</sub>, and vice versa. This fact means that most successful emission reductions require the combination of several methods.<ref name="reg"/>
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