This invention relates generally to a burner assembly and, more particularly, to an inproved burner assembly and method which operate in a manner to reduce the formation of nitrogen oxides as a result of fuel combustion.
Considerable attention and efforts have recently been directed to the reduction of nitrogen oxides resulting from the combustion of fuel, and especially in connection with the use of coal in the furnace sections of relatively large installations such as vapor generators and the like. In the burning of coal, nitrogen oxides are formed by the fixation of atmospheric nitrogen available in the combustion-supporting air, and is a function of the flame temperature. When the flame temperature exceeds 2800.degree., the amount of fixed nitrogen removed from the combustion-supporting air rises exponentially with increases in the temperature. Nitrogen oxides are also formed from the fuel-bound nitrogen available in the fuel itself, which is not a direct function of the flame temperature, but is related to the quantity of available oxygen during the combustion process.
In a typical arrangement for burning coal in a vapor generator, for example, one or more burners are usually disposed in communication with the interior of the furnace, and operate to burn a mixture of air and pulverized coal. The burners used in these arrangements are generally of the type in which a swirling fuel and air mixture is continuously injected through a single nozzle so as to form a single, relatively large flame. As a result, the surface area of the flame is relatively small in comparison to its volume, and therefore the average flame temperature is relatively high. This condition leads to the production of high levels of nitrogen oxides in the final combustion products, which cause severe air pollution problems.
Since the formation of nitrogen oxides increases with increases in the burner temperatures, attempts have been made to supress the latter temperatures and thus reduce the formation of nitrogen oxides. Attempted solutions have included techniques involving two stage combustion, flue gas recirculation, the introduction of an oxygen-deficient fuel-air mixture to the burner and the subsequent introduction of additional combustion-supporting air exteriorally of the burner itself, and the breakup of a single, large flame into a plurality of smaller flames. However, these attempts have often resulted in added expense in terms of increased construction costs, and the like, and have lead to other related problems, such as the production of soot and complex mechanisms to achieve the solutions.
Heretofore, registers positioned within a windbox disposed adjacent to a lower portion of the furnace have been used to jointly control the volume flow and the turbulence of the secondary combustion-supporting air from the windbox to support the burning of coal. These registers, which generally comprise mechanically-complex assemblies of rotatable vanes and associated control mechanisms, are designed primarily to induce turbulence, or swirl, in the flow of the mixture of fuel and combustion-supporting air. Secondarily, these registers were designed as damper or flow volume control devices. However, depending upon the operating condition, existing registers only function with one degree of control. That is, if they are operating in the closed-down, or slightly open condition, they function primarily as a damper to control the quantity flow of combustion-supporting air through the register and through the burner assembly. On the other hand, if they are operating at larger openings, the dampening effect achieved by further opening of the register is considerably reduced. At the more fully-open conditions, however, relatively more swirl is induced in the flow of the combustion-supporting air by only slight changes in the opening of the register. Thus, the prior art registers either function effectively as dampers or as turbulence creating devices, but do not function with equal effectiveness in both modes. The large numbers of components associated with the prior art registers also present problems of reliability.