Visualization of Spatially Resolved Thermal Nitric Oxide Production Rates as a Tool for Emissions Reduction and Combustion Optimization

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Visualization of Spatially-Resolved Thermal Nitric Oxide Production Rates as a Tool for Emissions Reduction and Combustion Optimization Kurt D. Annen, David B. Stickler, and Robert C. Brown Aerodyne Research, Inc., Billerica, M A Abstract A new diagnostic technique has been developed that directly visualizes regions of high NOx formation in combustion flowfields. The technique is based on the familiar "green" chemiluminescent emission from boron combustion that is produced by the reaction BO + O + M -> B02* + M. The strongest emission bands are at 518, 548, and 580 nm. This reaction has similarity with the rate-limiting step in the Zel'dovich mechanism, N2 + O -> NO + N, in that both reactions have a first order dependence on the O atom concentration. Initial experimental studies of turbulent jet diffusion flames have shown that the boron chemiluminescence intensity closely tracks the N O production rate. Photographic and digital images of chemiluminescent emission from the flames, acquired with high spatial and temporal resolution, clearly visualize the three-dimensional flame sheet where N O x formation rates are highest. Modeling studies were also performed using detailed kinetic mechanisms for the N O x chemistry and boron seed decomposition and reaction. The results indicated that the boron chemiluminescence intensity closely follows the N O production rate over a range of pressures and equivalence ratios. This N O x production rate visualization technique promises to be a useful tool for reducing N O x emissions and for diagnosing and improving mixing characteristics in practical burners and combustors. Introduction Reduction of NOx emissions is a major priority for power production and process heating systems. For most of these systems, N 0 X emissions are primarily produced by the Zel'dovich or thermal mechanism. A reduction in N 0 X emissions is usually obtained by designing the combustion equipment to operate under either fuel lean conditions, or a combination of fuel rich and fuel lean conditions, minimizing or eliminating regions in the combustor that have near unity stoichiometry and correspondingly high N O production rates. In both of these approaches, uniform and rapid mixing of the fuel and oxidizer streams is crucial to the achievement of low N O x emissions. 1