Pollutant formation and int ... combustion of heavy liquid fuels

3.2.3. The combustion stage in gas turbines: Combustors

Since the turbine power output is directly related to the thrust produced, the use of a large mass flow rate of air through the combustor is needed. The combustion chamber must be able to burn fuels efficiently with different operating conditions (depending on the application characteristics), must be stable against blow-out and with easy ignition (Harman (1981), Tuttle et al. (1976), Williams (1976)) . Moreover, gas turbine engines normally combust fuel-rich mixtures to prevent the exhaust temperature from exceeding that of the turbine materials limitations. The temperature of combustion gases of a stoichiometric mixture is 2,200 °C in a standard engine.

A typical gas turbine design may be as in the following Figure (Tuttle et al. (1976)) :

Figure 14: Typical design of a gas turbine combustor

Primary zone: The primary zone extends from the fuel nozzle face to the first row of air addition holes. Air flow enters at high velocity but it is decreased by means of diffusers. Ignition is caused by an applied source of energy, be it a high voltage spark, a torch supplied by an auxiliary burner... The flame size depends on the fuel burned and the flow pattern, and as a result it determines the combustor size. The flame intensity may be reduced by products of high emissivity that may radiate heat away.
Since even the reduced gas velocity is much higher than the actual velocity of the fuel spray, the flame must be stabilised by a high degree of recirculation, allowing ample time for reactions to take place.
In the primary zone equivalence ratios approach unity ( ~ 1), providing favourable conditions for fuel ignition and stability. Normally the air flow through the dome is swirled, creating a low pressure recirculation zone for flame holding. Between the higher velocity dome air flow and the lower velocity central recirculation zone flow, a region of high turbulence (the shear layer) is created due to the large velocity gradient.
The cone of fuel injected penetrates directly into the shear layer region where fuel vaporises, mixes with the oxidiser and burns. Typically 20 - 30 % of the air is injected in the primary zone, whereas the remainder is injected in the secondary zone.
Secondary zone: The secondary zone begins where the injection of air through the liner holes is produced. The equivalence ratio decreases ( ~ 0.5), providing an excess of air for all remaining fuel molecules, CO, H radicals to be fully oxidised. Air and fuel flow may recirculate back into the primary zone, but largely penetrate to the combustor centreline. In addition, the temperature is lower here than at stoichiometric mixture but still too high for the turbine blades.
Tertiary or dilution zone: The aim of this zone is to inject extra dilution air in order to reduce the combustion exit temperature to that acceptable for the turbine inlet.

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Pollutant formation and interaction in the combustion of heavy liquid fuels
Luis Javier Molero de Blas, PhD thesis, University of London, 1998
© Luis Javier Molero de Blas