What is General Electric LM6000 Combustor?
Another important aeroderivative gas turbine is General Electric’s LM6000. This premix combustor employs about twice the volume of the conventional annular combustor it replaces in order to maintain low levels of CO and UHC while greatly reducing the emissions of NOx. Part of the air used in combustion, which at maximum power is around 80% of the total combustor airflow, flows into the combustion zone through three annular rings of premixers.
The two outer rings each have 30 fuel–air premixers, whereas the inner ring has 15. This arrangement of premixers facilitates fuel staging at part-load operation. The total of 75 fuel nozzles is formed by having 15 stems with three premixers on each stem, plus 15 stems with two premixers on each stem.
Each stem incorporates two or three separate fuel circuits for independently fueling the premixers. A short annular liner was selected to minimize the amount of air needed for wall cooling. Only backside cooling is used, so a thermal barrier coating is applied to both the liner and in the dome area to keep the metal temperatures within acceptable limits.
The use of a multipass diffuser also permits further reduction in overall combustor length. Of special importance to the attainment of low emissions is the design of the premixers. The double annular counter rotating swirler (DACRS) was conceived to satisfy the restraints of autoignition and size. The duct diameter is reduced toward the exit in order to create an accelerating flow, thereby preventing flashback.
The conical centerbody located along the centerline of the premixer can be used to supply liquid fuel to an atomizer at its tip, and gas passages for diffusion burning at low-power conditions. The objective with this type of mixing device is to produce a completely homogeneous mixture of fuel and air at the premixer exit.
As the total area of the fuel-injection holes is fixed by the flow rate and the available fuel njection pressure, the design procedure is essentially one of finding the best compromise between the desire for small injection holes to give a large number of fuel-injection points, and the equally important requirement of large injection holes to allow the fuel jets to penetrate across the airstream.
A big advantage of the premixer module concept is that, once developed, it has broad applications to a wide range of combustor sizes and configurations, as discussed above in connection with the ABB-EV burner.
The basic module remains the same regardless of combustor size; only the number and
arrangement varies. Thus, according to Joshi et al,, the DACRS II and DACRS III mixers could be applied to a range of GE engines, including the LM1600, LM2500, and LM6000, because single digit NOx emissions have been attained with both these mixers at test conditions encompassing the operating ranges of these engines.
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