IMPACT OF WATER ADDITION TO DIESEL FUEL ON NOx EMISSIONS FROM A MICROTURBINE GENERATOR

To combat urban pollutants such as ozone and particulate, it is becoming increasingly important to reduce the rates at which their precursors such as oxides of nitrogen (NOx) are produced. The goal of this project is to explore the use of water to reduce NOx emissions from a diesel fueled Microturbine Generator (MTG). While research has been conducted on NOx emissions from liquid fueled turbines, little has been done on evaluating the impact that water addition to liquid fuel (i.e., an emulsion) will have on these emissions. It is well established that NOx formation rates depend strongly on the temperatures of the combustion process. The hypothesis underlying this work is that the added water will reduce peak temperatures and therefore reduce NOx. In this project, water is added to Military Diesel (F-76) to create an emulsion mixture with varying water-mass fraction (Φ). To help explain possible differences in observed NOx emissions, experiments have been done to measure the spray droplet size (i.e., the Sauter mean diameter) using a Malvern laser-diffraction setup. It is well established that smaller droplets will evaporate faster leading to more rapid mixing with air and, in principle, r reduced NOx emissions. While adding water should reduce combustion temperatures, if, in the process, it interferes with the atomization process and results in large drop sizes, the benefits may be reduced. During these experiments, Φ and air-to-liquid ratio (ALR) were varied from 0-0.5 and 0.25-1.0, respectively. The results obtained show significantly better atomization (reduced drop sizes) at higher ALRs for all Φ tested. Interestingly, the addition of water to diesel fuel was found to have little effect on the SMD for the system studied. Preliminary emissions results from the MTG operated on the various emulsions indicate that increasing Φ does decrease NOx emissions as expected. However, increased atomizing air (which was shown to further reduce droplet size) does not significantly reduce the NOx emissions for any of the emulsions studied. Further, an increase in atomizing air flow beyond ALR of 0.5 increases NOx emissions. Thus, while the present results are promising, they demonstrate that an intrinsic coupling exists between the fuel preparation process (i.e., atomization and evaporation) and the resulting combustion behavior, which creates additional complexity warranting further study in order to attain optimal performance.