Dry Ultralow NO_x “Green Thumb” Combustor for Allison’s 501-K Series Industrial Engines

This paper describes the progress made in developing an external ultralow oxides of nitrogen (NO_x) “Green Thumb” combustor for the Allison Engine Company’s 501-K series engines. A lean premixed approach is being pursued to meet the emissions goals of 9 ppm NO_x, 50 ppm carbon monoxide (CO), and 10 ppm unburned hydrocarbon (UHC). Several lean premixed (LPM) module configurations were identified computationally for the best NO_x–CO trade-off by varying the location of fuel injection and the swirl angle of the module. These configurations were fabricated and screened under atmospheric conditions by direct visualization through a quartz liner; measurement of the stoichiometry at lean blow out (LBO); measurement of the fuel–air mixing efficiency at the module exit; and emissions measurements at the combustor exit, as well as velocity measurements. The influence of linear residence time on emissions was also examined. An LPM module featuring a radial inflow swirler demonstrated efficient fuel-air mixing and subsequent low NO_x and CO production in extensive atmospheric bench and simulated engine testing. Measurements show the fuel concentration distribution at the module exit impacts the tradeoff between NO_x and CO emissions. The effect of varying the swirl angle of the module also has a similar effect with the gains in NO_x emissions reduction being traded for increased CO emissions. A uniform fuel-air mixture (±2.5 percent azimuthal variation) at the exit of the module yields low NO_x (5–10 ppm) at inlet conditions of 1 MPa (~10 atm) and temperatures as high as 616 K (650°F). The combustion efficiency at these conditions was also good (>99.9 percent) with CO and UHC emissions below 76 ppm and 39 ppm, respectively. This LPM module was resistant to flashback, and stability was good as LBO was observed below φ = 0.50. Tests with multiple modules in a single liner indicate a strong intermodule interaction and show lower NO_x and CO emissions. The close proximity of adjacent modules and lower confinement in the liner most likely reduces the size of the recirculation zone associated with each module, thereby reducing the NO_x formed therein. The CO emissions are probably lowered due to the reduced cool liner surface area per module resulting when several modules feed into the same liner.