The Effect of Water on Soot Formation Chemistry

A combined, experimental and numerical program is presented. This work summarizes an internal research effort conducted at Southwest Research Institute. Meeting new, stringent emissions regulations for diesel engines requires a way to reduce NO x and soot emissions. Most emissions reduction strategies reduce one pollutant while increasing the other. Water injection is one of the few promising emissions reduction techniques with the potential to simultaneously reduce soot and NO x in diesel engines. While it is widely accepted that water reduces NO x via a thermal effect, the mechanisms behind the reduction of soot are not well understood. The water could reduce the soot via physical, thermal, or chemical effects. To aid in developing water injection strategies, this project's goal was to determine how water enters the soot formation chemistry. Linked burner experiments and modeling of a rich premixed flame were used to determine the magnitude of the thermal and chemical effect of water on soot formation and identify a possible kinetic mechanism to explain it. Following Dec's model for diesel combustion processes (Dec, 1997; Flynn, et al., 1999) [1,19] , soot inception results from rich premixed combustion; thus the rich premixed flame provides an appropriate venue in which to isolate the influence of water on the kinetics. Open flame, burner experiments have been performed to quantify the soot inception point and the relative amounts of soot formation in premixed flames with and without water addition. These results have been used to expand and compliment data available in the published literature. Subsequent modeling has been used to predict trends in soot inception using currently accepted kinetic soot mechanisms. Results from this effort led to a revised kinetic mechanism for the process. Comparison of the experimental and modeling data has been used to assess the accuracy of soot formation mechanisms and ultimately has yielded a new understanding of the soot formation chemistry and the role of added water.