NOx is a byproduct produced during the combustion of coal and other fossil fuels. Environmental concerns regarding the effects of NOx have prompted enactment of NOx emissions regulations requiring sharp NOx emission reductions from industrial and utility power plants in several countries including the United States. Current commercial methods and apparatuses for reducing NOx emissions have been successful in lowering NOx emissions from the levels emitted in previous years; however, further advances, beyond those of currently known methods and apparatuses, are needed to maintain compliance with current NOx emissions regulations.
A variety of low NOx burners are commercially available and widely used to fire pulverized coal (PC) and other fossil fuels in a NOx reducing manner as compared to conventional burners. Examples of such burners are The Babcock & Wilcox Company's DRB-XCL® and DRB-4Z® burners. Common to these and other low NOx burner designs is an axial coal nozzle surrounded by multiple air zones which supply secondary air (SA). During operation, PC suspended in a primary air (PA) stream, is injected into the furnace through an axial coal nozzle, as an axial jet, with little or no radial deflection. Ignition of the PC is accomplished by swirling SA, thereby causing recirculation of hot gases along the incoming fuel jet.
Typically a fraction of the SA is supplied to an air zone in close proximity to the coal nozzle and swirled to a relatively greater extent than the SA supplied to the other air zones to accomplish ignition. The remaining SA from the burner is introduced through air zones further outboard in the burner utilizing less swirl, so as to mix slowly into the burner flame, thereby providing fuel rich conditions in the root of the flame. Such conditions promote the generation of hydrocarbons which compete for available oxygen and serve to destroy NOx and/or inhibit the oxidation of fuel-bound and molecular nitrogen to NOx.
NOx emissions can further be reduced by staged combustion, wherein the burner is provided with less than stoichiometric oxygen for complete combustion. A fuel rich environment results at the burner flame. The fuel rich environment inhibits NOx formation by forcing NOx precursors to compete with uncombusted fuel in an oxygen lean environment. Combustion is then staged by providing excess oxygen to the boiler at a point above the burner wherein the excess fuel combusts at a lower temperature, thus precluding the production of thermal NOx as the combustion occurs at a lower temperature away from the burner flame. Staging also serves to lessen oxygen concentrations during the combustion process which inhibits oxidation of fuel bound nitrogen (fuel NOx).
Oxygen for staged combustion is normally provided in the form of air via air staging ports, commonly called Over Fire Air (OFA) ports, in a system utilizing low NOx burners. U.S. Pat. No. 5,697,306 to LaRue, and U.S. Pat. No. 5,199,355 to LaRue, herein incorporated by reference, disclose low NOx burners that may be combined with air staged combustion methods to further reduce NOx emissions.
Unlike conventional burners, low NOx burners tend to form long flames and produce higher levels of unburned combustibles. Long flames are not always desirable as they may be incompatible with furnace depth or height, and can impair boiler operation by causing flame impingement, slagging, and/or boiler tube corrosion.
Long flames result from an insufficient air supply to the fuel jet as it proceeds into the furnace. SA from the outer air zones of low NOx burners do not effectively penetrate the downstream fuel jet, such that unburned fuel persists due to a lack of air supply along the flame axis. High levels of unburned fuel are undesirable in both furnaces with OFA and those without. Unburned combustibles in the form of unburned carbon and CO reduce boiler efficiency and add operation expenses, whereas unburned pulverized coal, by nature of its abrasiveness, may cause undesirable erosive damage to the furnace itself.
Incomplete air/fuel mixing ahead of an OFA system can cause excessive amounts of unburned fuel to persist up to the OFA ports. When large amounts of unburned fuel try to burn with air at the OFA zone, NOx formation can increase, thereby minimizing or negating the benefit of staged combustion with OFA. In addition it becomes increasingly difficult to completely burn out these combustibles at and beyond the OFA ports, such that they add to inefficiency and operational difficulties.