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Show The Radially Stratified Flame Core Burner A Low-NOx Burner of Novel Type * J.M. Beer, M.A. Toqan, L. Berg, A. Testa and J.D. Teare Massachusetts Institute of Technology Cambridge, MA USA ABSTRACT PAPER NO. 41 To1tt-J1 I1h'( vi c,f/M.. k ReS. fau.'//Ce; Results of an investigation to establish the design principles of a 10w-NOx burner of novel design are discussed. The admixing of the combustion air to the fuel is staged by radial stratification in the flame due to the combination of swirling burner air flow and a strong positive radial density gradient. Fuel/air mixing is suppressed close to the burner but is promoted further downstream where vortex breakdown results in the development of a toroidal recirculation zone in the central region of the flame. A flexible experimental burner capable of varying the radial distributions of the combustion air flow and the swirl velocity has been designed and constructed. Computer modeling of the flow, temperature and stable species concentration distributions in natural gas-air turbulent flames guided parametric and detailed experimental studies which were carried out in the 1.2mx1.2mx4.Sm test section of the MIT Combustion Research Facility. Results of experiments showed that by the optimization of the flow in the radially stratified flame core (RSFC) burner, emission levels of 70ppm NOx and 56ppm CO can be obtained with natural gas fuel flames. Further reductions to below 15ppm NOx were obtained when flue gas recirculation and steam injection were added to the aerodynamically optimized natural gas flames. INTRODUCTION The most widely used method of NOx emission reduction from combustion is the creation of a sequence of fuel rich and fuel lean flame zones -by the staged introduction of air or fuel. The degree of NOx reduction achieved, however, varies widely and depen~s on the combustion system (Sarofim et al., 1978; Beer et al., 1981; Thompson et al., 1972). The practical realization of the well established principles of staged combustion for NOx control is hampered by the lack of information on the overlapping processes of the nitrogen/hydrocarbon chemistry and the concentration temperature history of the fuel in the flame. The problem is especially difficult in the case of internal staging, a process in which the fuel rich and fuel lean flame zones are produced by aerodynamic means using a single burner, rather than by physically separated regions of the combustion chamber using "overfire" air. The challenge is to produce a fuel rich high temperature zone near the burner which will provide sufficient residence time for fuel pyrolysis, followed by vigorous mixing of the rest of the combustion air and the remainder of the fuel, to ensure complete combustion. To achieve this objective we have adopted the prinCiple of radial 1 |
