The Allison Engine Company has been developing a low emission, can-annular combustion system for the 501K industrial gas turbine engine to satisfy increasingly stringent environmental requirements. This paper describes the progress achieved, over that previously reported by Razdan et al. (1994), through subsequent design evolution, bench testing, and engine evaluation. Allison’s goal is to develop a retrofittable, can-annular combustion system that limits emission levels to less than 25 ppm nitrogen oxide (NOx), 50 ppm carbon monoxide (CO), and 20 ppm unburned hydrocarbon (UHC), while operating at full load conditions. The interim emissions goals for the combustion system are 37 ppm NOx, 80 ppm CO, and 20 ppm UHC (all dry 15% O2 corrected). The combustion system under development employs a dual mode combustion approach to meet engine operability requirements and high power emission targets without the use of combustor diluent injection or postcombustor exhaust treatment. A lean premixed combustion mode is used to minimize combustion zone temperature and limit NOx production during high power engine operation. The lean premix mode is augmented with a diffusion flame pilot mode for engine starting and low power operation. Initial engine testing showed a dry low NOx combustion system, designed to meet a 37 ppm NOx limit, produced less than 34 ppm NOx and less than 10 ppm CO and UHC in test stand verification test. Continued burner rig testing with modified primary combustion zone stoichiometry has demonstrated NOx less than 25 ppm, CO less than 50 ppm, and UHC less than 20 ppm with simulated engine conditions representing 20 to 100% power. Development activity continues on the combustion system as engine field evaluation trials proceed.

This paper describes the progress made in developing an external ultra-low oxides of nitrogen (NOx) ‘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 NOx, 50 ppm carbon monoxide (CO), and 10 ppm unburned hydrocarbon (UHC). Several lean premixed (LPM) module configurations were identified computationally for the best NOx-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 liner residence time on emissions was also examined. An LPM module featuring a radial inflow swirler demonstrated efficient fuel-air mixing and subsequent low NOx and CO production in extensive atmospheric bench and simulated engine testing. Measurements show the fuel concentration distribution at the module exit impacts the trade-off between NOx and CO emissions. The effect of varying the swirl angle of the module also has a similar effect with the gains in NOx emissions reduction being traded for increased CO emissions. A uniform fuel-air mixture (± 2.5% azimuthal variation) at the exit of the module yields low NOx (5–10 ppm) at inlet conditions of 1 MPa (∼10 atmospheres) and temperatures as high as 616 K (650°F). The combustion efficiency at the above conditions was also good (> 99.9%) 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 NOx 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 NOx 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.

This paper describes the development of an ultra-low emissions combustion system for Allison’s Advanced Turbine System (ATS) engine, which is being developed in cooperation with the U.S. Department of Energy. The simple cycle engine is designed to have a thermal efficiency that is 15% better than today’s best in class engine, and exhaust emissions of 9 ppm NOx, 20 ppm CO, and 20 ppm UHC. The approach taken to meet the low emissions goals is based on ultra-lean premixed fuel-air combustion supported by a catalyst. The progress toward development and integration of lean premix (LPM), catalytic and post-catalytic stages, and the combustor-to-turbine transition duct into an overall ATS combustion system is presented. A parametric computational fluid dynamics (CFD) study was conducted on the performance of lean premix modules at ATS conditions. Various lean premix modules were tested extensively under atmospheric conditions to determine airflow capacity, flashback propensity, lean blowout (LBO) fuel-air ratios, and fuel concentration profiles at the module exit. Kinetic modeling using the GRI mechanism has been used to estimate ignition delay times in the post-catalytic zone. Comparison between the modeling results and experimental data at high pressure shows good agreement. A detailed computational analysis was performed to design the combustion-to-turbine transition duct. The results indicate that the scroll duct configuration produces an acceptable mass flow uniformity and swirl angle exiting the duct into the turbine section. High pressure sector rig tests have been performed to evaluate staging interaction issues. The results indicate that the series staged approach can facilitate incorporation of the catalytic combustion system by expanding the operability range. NOx emissions levels of 9 ppm or less can be sustained over a wide range of equivalence ratios.

To meet the design requirements of next generation aircraft engines and the expected trends of more stringent emissions regulations, better calculation methods for aircraft mission emissions need to be established. In the present investigation, a number of steps were taken to define an appropriate calculation technique. The data obtained for a full-scale annular combustor rig were compared with engine emissions to illustrate that the level of rig-to-engine agreement was good enough to use the rig data to formulate a proposed mission NOx calculation technique. Conventional methods were then used to correlate the rig data in terms of various operating parameters. It was demonstrated that the level of agreement with data was improved by including both combustion and geometrical aspects in the correlations of NOx, CO, UHC, and smoke. A semianalytical approach, which was based on detailed chemical kinetic scheme and simulated the combustor by a number of reactors representing various combustion zones, was used to correlate the data of the annular combustor. The results illustrated that better estimates of emissions were obtained over conventional methods. Two mission profiles that represented the operation of turbofan-powered regional and business aircrafts were selected to evaluate their mission emissions using the semianalytical method. An approach that utilized only the four ICAO test points in the semianalytical method was formulated to provide the total aircraft mission emissions. Results obtained by this approach were comparable to those calculated using correlations based on extensive testing of the combustor; thus, by using such a method considerable savings in cost and effort could be achieved during combustor development. The results also demonstrated the possibility of correlating the emissions in terms of ambient pressure and temperature and fuel flow rate; thus, accurate estimates of altitude emissions could be obtained.

An operating cycle had been developed for a catalytic combustion system applied to the Allison 501-KB7 engine. This cycle used overboard bleed of diffuser air to maintain a high fuel/air ratio at the catalyst and thus achieve a high combustor outlet temperature with attendant low CO and UHC emissions. For the design point of this engine, the emissions measured at full pressure and temperature in a subscale catalyst test rig were <1 ppm NOx and <2 ppm CO and UHC. Tests over the full operating cycle showed that the catalytic combustor system would achieve low emissions from 20 to 100% load. The use of catalytic combustion on a high efficiency gas turbine engine design was also evaluated. Pressures up to 20 atm and combustor outlet temperatures up to 1500°C (2730°F) were demonstrated with NOx emissions <2.2 ppm and CO and UHC <2 ppm. These results show that catalytic combustion is a viable technology for application to a high pressure, high temperature industrial gas turbine engine design.