Experimental and Kinetic Investigation of the Influence of OH Groups on NOX Formation
AdvisorsRoberts, William L.
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AbstractThis work investigates the influence of one or more OH groups present on the fuel molecule and the resultant formation of NOX emissions. Combustion of oxygenated fuels has been increasing globally and such fuels offer significant potential in the reduction of pollutant emissions. One such emission class is the oxides of nitrogen, which typically form through a combination of two regimes: the thermal and non-thermal mechanisms. While thermal NO formation can be reduced by lowering the combustion temperature, non-thermal NO formation is coupled to the fuel chemistry. An experimental and computational investigation of NOX formation in three different burner configurations and under a range of equivalence ratios and temperature regimes explored the differences in NO formation. Measurements of temperature profiles and in-flame species concentrations, utilizing both probed and non-intrusive laser based techniques, allowed for the investigation of NO formation through non-thermal pathways and the differences that exist between fuels with varying numbers of OH groups. The first burner configuration was composed of a high swirl liquid spray burner with insulted combustion chamber walls designed specifically for the combustion of low energy density fuels. In this system the combustion of alcohols and glycerol (the largest by-product of biodiesel production), along with other fuels with multiple hydroxyl groups, was studied. Measurements of the mean flame temperature and exhaust gas measurements of NOX showed significant reductions in non-thermal NO concentrations with increasing numbers of OH groups. An accompanying modeling study and detailed reaction path analysis showed that fuel decomposition pathways through formaldehyde were shown a preference due to the presence of the OH groups which resulted in reduced contributions to the hydrocarbon radical pools subsequent reductions to the Prompt NO mechanism. Two burner configurations with reduced dimensionality facilitated measurements in premixed flames for temperature and species in high and low temperature flames. These measurements included probed thermocouple temperature measurements, extractive gas sampling for NO and intermediate hydrocarbon species, and planar Laser Induced Fluorescence (LIF) measurements for 2OH-LIF thermometry, semiquantitative CH2O LIF, and quantitative NO LIF. Additionally, the simplified nature of the burner geometries allowed for the modeling of the flames incorporating detailed reaction kinetics for fuel decomposition and NOX formation. Significant reductions in NO formation were observed in comparisons of alcohol and alkane flames, resulting in up to 50% reductions in the pollutant. Computational analyses and nitrogen flux accounting allowed for the identification of the reduction in NO formation through all the known NOX formation pathways. It was observed that all of the known pathways exhibited reductions in contributions to NO formation in the presence of OH functional groups, indicating a complex coupling of fuel and NOX chemistry.