Field Experience of a Dry Low Emissions Combustion System for Allison 501-K Series of Engines
Allison Engine Company has introduced a dry low emissions lean premixed combustion system, designated LE4, for the 501-K series industrial gas turbine engines. The design goals were 1) to develop a retrofittable combustion system which limits exhaust gas emission levels to less than 25 ppm NOx, 50 ppm CO and 20 ppm UHC while operating on natural gas fuel at full load conditions, and 2) to maintain system cost to less than that for alternate control methods. Extensive in-house engine tests were completed to ensure successful combustion system operation including acceptable engine transient operation during load dumps, and also to optimize the window of operation for emissions performance. These tests have demonstrated engine emissions levels which are below the goals, with NOx less than 15 ppm, CO less than 20 ppm, and UHC less than 10 ppm, all corrected to 15% O2. These emissions can be maintained at the target levels for engine operation from 85 to 100% power. For applications requiring wider power operation, a diffuser bleed system has been engine demonstrated which maintains less than 25 ppm NOx, 50 ppm CO and 25 ppm UHC from 50 to 100% power. The combustion system employs a dual mode combustion approach to meet engine operability requirements and emissions targets. The control algorithm developed for the LE4 combustion system allows easy tailoring of the pilot-to-main fuel ratio schedule setting to meet the customer needs on a site by site basis to account for different ranges of ambient conditions. Use of Streamwise Oriented Effusion Cooling (SOEC) design in the liner wall met the maximum wall temperature goals of less than 1650°F. The LE4 combustion system is operating currently in two applications: 501-KC5 ANR Pipeline application in Woodstock, IL, and 501-KB7 Cogeneration application in Scandiano, Italy. Measured emissions over time and a range of ambients in these engines show NOx, CO and UHC results which are better than the goals. The 501-KC5 engine has accumulated more than 3500 hours, and the 501-KB7 engine has accumulated more than 5500 hours. Both sites have been running with problem free operation, and borescope inspections have indicated excellent condition of the combustion systems.