| OCR Text |
Show INTRODUCTION N O x formation in combustion processes is very complex. It is sensitive to temperature, local stoichiometry, and residence time. - The N O x produced is commonly attributed to three mechanisms: • thermal NOx resulting from fixation of molecular nitrogen by atomic oxygen at high temperatures in oxidizing atmospheres; • prompt NOx resulting from fixation of molecular nitrogen by hydrocarbon radicals in reducing atmospheres; and • oxidation of nitrogen compounds organically bound in the fuel. The general approach for controlling N O x emissions therefore requires an understanding of the formation and destruction mechanisms that affect the process. Ideally, the combustion process should be optimized recognizing the interrelationship of the burner and furnace whether it is a boiler, an air heater or a process fluid heater. For utility-scale fluidized bed coal-fired boilers, this is carefully considered since the integration of the combustion and heat transfer mechanisms are integrally linked. For liquid and gaseous fuels, however, the high volumetric heat release rates and combustion stability associated with these fuels have most often resulted in the design of the burner being engineered independent of the specific application. L o w N O x burners developed by most boiler and burner manufacturers generally are designed to achieve low emission levels by prolonging the combustion process through the way the air and fuel are introduced. The level of available oxygen is decreased in zones that are prone to high N O x formation. Flue gas recirculation is used to reduce the flame temperature and the partial pressure of the oxygen. While these techniques are very satisfactory for maintaining N O x emissions down to 40 ppmv levels, more exacting approaches are required to limit prompt and thermal N O x if single digit levels are to be attained. Thermo Power Corporation and John Zink Company with funding support from the United States Department of Energy, are working together to develop and bring to the market a natural gas burner capable of meeting ultra-low emissions for boilers and process heaters without costly post-processing of exhaust gases. The design approach is to combine the individual techniques necessary to achieve ultra-low N O x emissions into a single device, largely independent of the intended application, in sizes up to 120 MMBtu/hr. The specific objectives are to achieve N O x emission levels of 9 ppmv and C O levels of less than 50 ppmv, both at 3 % O2. Additional goals include high turndown ratio, and low levels of unburned hydrocarbons or air toxics. The implementation of these combustion goals into a single, cost effective burner represents a very challenging problem with a very high payoff upon its success. TECHNICAL APPROACH The VIStA burner is a two stage combustor. Figure 1 displays a schematic of the concept. In the first stage, natural gas and part of the combustion air are premixed and injected tangentially at high velocity into the combustion chamber through several ports located at the head end of the first stage combustion chamber. B y exploiting the radial pressure difference created by the resulting vortex, part of the combustion products are taken out of the first stage 'through tangential openings at the tail end of the first stage combustion volume. This gas is cooled as it passes through axially running recirculation tubes, and returned to the combustor axially through 3 |
