OCR Text |
Show and other reagents, have been developed and are emerging as the primary method of control for certain applications. In general, NOx formation can be retarded by reducing the concentration of nitrogen and oxygen atoms at the peak combustion temperature or by reducing the peak flame temperature and residence time in the combustion zone. This can be accomplished by using combustion modification techniques such as changing the operating conditions, modifying the burner design, or modifying the combustion system. Of these alternatives, burner design modifications are the most widely used. Low NOx burners are generally of the diffusion burning type and are designed to reduce flame turbulence, delay mixing of fuel and air, and establish fuel-rich zones where combustion is initiated. Manufacturers of such burners have claimed nominal reductions of 40 to 50% compared to conventional burners, but significant differences in the predicted NOx emissions and those actually achieved in practice have been noted. The underlying cause for these discrepancies is due to the complexity in trying to control simultaneous heat and mass transfer along with the complex reaction kinetics occurring during diffusion burning. Investigations of new burner designs have recently intensified, motivated in part by the need for more efficient heat transfer but more so by the current emphasis arising from the adoption of more stringent emissions standards for the oxides of nitrogen (NOx ) for stationary combustion systems in California's South Coast Air Quality Management District. One new burner concept that appears to have promise is the porous media (PM) burner. PM burners imbed the combustion process within a solid matrix which has a high porosity. The premixed gases are constrained to flow through the open structure of the solid matrix. Because of the high emissivity of the solid relative to a gas, radiation from the high temperature postflame zone not only yields a high radiant output but also serves to heat the preflame zone of the porous material which, in turn, convectively heats the incoming reactants. This internal heat feedback mechanism results in several interesting characteristics relative to a free-burning flame (e.g., 1,2). The potential advantages include increased burning rates, increased flame stability, and the ability to achieve a high radiant output from a compact combus-tor. Additionally, since the pore size and material properties of the porous matrix can be varied, superior control of the tempera- -2- |