Low-Emissions Technology Development for Auxiliary Power Unit Combustion Systems

The aircraft Auxiliary Power Unit (APU) is required to provide power to start the main engines, conditioned air and power when there are no facilities available and, most importantly, emergency power during flight operation. Given the primary purpose of providing backup power, APUs have historically been designed to be extremely reliable while minimizing weight and fabrication cost. Since APUs are operated at airports especially during taxi operations, the emissions from the APUs contribute to local air quality. There is clearly significant regulatory and public interest in reducing emissions from all sources at airports, including from APUs. As such, there is a need to develop technologies that reduce criteria pollutants, namely oxides of nitrogen (NOx), unburned hydrocarbons (UHC), carbon monoxide (CO) and smoke (SN) from aircraft APUs. Honeywell has developed a Low-Emissions (Low-E) combustion system technology for the 131-9 and HGT750 family of APUs to provide significant reduction in pollutants for narrow-body aircraft application. This article focuses on the combustor technology and processes that have been successfully utilized in this endeavor, with an emphasis on abating NOx. This paper describes the 131-9/HGT750 APU, the requirements and challenges for small gas turbine engines, and the selected strategy of Rich-Quench-Lean (RQL) combustion. Analytical and experimental results are presented for the current generation of APU combustion systems as well as the Low-E system. The implementation of RQL aerodynamics is well understood within the aero-gas turbine engine industry, but the application of RQL technology in a configuration with tangential liquid fuel injection which is also required to meet altitude ignition at 41,000 ft is the novelty of this development. The Low-E combustion system has demonstrated more than 25% reduction in NOx (dependent on the cycle of operation) vs. the conventional 131-9 combustion system while meeting significant margins in other criteria pollutants. In addition, the Low-E combustion system achieved these successes as a “drop-in” configuration within the existing envelope, and without significantly impacting combustor/turbine durability, combustor pressure drop, or lean stability.