OCR Text |
Show INTRODUCTION The formation of N O x from combustion processes is a very complex mechanism sensitive to temperature, local stoichiometry, and residence time. The amount of N O x produced is commonly attributed to three mechanisms: thermal N O x resulting from fixation of molecular nitrogen by atomic oxygen at high temperatures in oxidizing atmospheres, prompt N O x 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 also decreased in zones that are prone to high N O x formation. Flue gas recirculation is also 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 p p m v levels, more exacting approaches are required to limit prompt and thermal N O x if single digit levels are to be attained. In this regard, 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 % 02. 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 is a very challenging problem with a very high payoff with its success. TECHNICAL APPROACH The VIStA burner concept, shown in Figure 1, consists of two combustion stages. In the first stage, natural gas and part of the combustion air are premixed and tangentially admitted at high velocity into the combustion chamber through multiple ports. B y exploiting the radial pressure difference created by the vortex, part of the combustion products are taken out through tangential openings, cooled, and returned into the combustor axially at a center opening via multiple recirculation tubes. Recirculation and cooling of the first stage combustion products is an important aspect of the VIStA technology. By controlling the stoichiometry, temperature, 2 |