The present invention generally relates to fuel-fired heating appliances, such as furnaces, water heaters and boilers and, in a preferred embodiment thereof, more particularly relates to apparatus and methods for reducing NOx emissions generated by the combustion systems in such appliances.
Nitrogen oxide (NOx) emissions in fuel-fired heating appliances, such as furnaces, water heaters and boilers, are a product of the combustion process, and are formed when the combustion reaction takes place at high temperature conditions typically encountered in such heating appliances. NOx emissions became an environmental issue in the late 1960's and early 1970's due to their detrimental role in atmospheric visibility, photochemical smog and acid deposition. Regulations in the subsequent decade led to significantly reduced amounts of NOx emissions.
Current SCAQMD (South Coast Air Quality Management District) regulations for residential furnaces and water heaters limit NOx emissions to 40 ng/j of useful heat generated by these types of fuel-fired appliances. Growing environmental concern is leading to even more stringent regulation of NOx emissions. For example, regulations currently being proposed by SCAQMD for water heaters and boilers limit NOx emission levels to 30 ppm at 3% oxygen, which is approximately 20.5 ng/j for middle efficiency water heaters and boilers. Conventional fuel-fired appliance combustion systems are not currently capable of meeting these more stringent limitations. For example, a typical in-shot burner system typically employed in these types of fuel-fired appliances produces NOx emission levels in the range of from about 50 ng/j to about 70 ng/j.
One technique currently used to lower NOx emissions in fuel-fired heating appliances is to position a heat absorbing flame insert within the burner flame path for "quenching" purposes. The resulting lowered combustion flame temperature results in lowered NOx emission rates. For example, as shown in U.S. Pat. No. 5,146,910, flame cooling can be achieved by placing an insert within the burner flame zone. The insert receives heat from the flame and radiates heat away to thereby cool the flame. Using this quenching technique, gas furnaces with flame inserts are now in commercial production and have NOx emission rates of somewhat less than about 40 ng/j.
Flame insert methods are relatively easy and inexpensive to implement. However, NOx reduction achieved by existing flame inserts is rather limited because conventional flame insert designs are operative solely through a flame cooling mechanism and, for a given combustion system, only limited flame cooling can be realized without jeopardizing the combustion process itself. Due to this practical limitation, existing flame inserts are able to reduce NOx emissions to about 30 ng/j--considerably short of the proposed emission limitation set forth above.
Some advanced combustion systems such as infrared/porous matrix surface burners, catalytic combustion and fuel/air staging could reach a very low NOx emission level in compliance with these proposed emission standards, but these methods tend to be quite expensive and usually require extensive system modification. Accordingly, they are not suited for retrofitting existing combustion systems to achieve the desired substantial reduction in system NOx emissions.
A particularly effective, retrofittable NOx reducing apparatus for this general application is illustrated and described in U.S. Pat. No. 5,370,529 to Lu et al and comprises a tubular metal mesh insert which is coaxially held in place within an inlet end portion of a fuel-fired furnace combustor tube by an elongated solid metal support member longitudinally extending through the interior of the tube. A first transverse end of the support member is removably secured to a rod transversely extending across the inlet end of the combustor tube, while an opposite transverse end of the support member slidably engages the interior side surface of the combustor tube. The mesh insert is laterally spaced inwardly from the inner side surface of the combustor tube and functions to receive the incoming burner flame in a manner substantially reducing the NOx emissions of the furnace during operation thereof.
While this NOx reduction insert structure operates to advantageously reduce the NOx emissions to below 20 ng/j, it carries with it two limitations. First, the insert structure is defined by two separate structures--the metal mesh tube, and its associated solid metal support member. This, of course, increases the production cost of the overall NOx reducing structure. Second, this NOx reducing structure must be installed in its associated combustor tube in a manner such that the inner end of its support member portion faces downwardly and rests on a bottom side portion of the interior surface of the combustion tube. Otherwise, the necessary centering of the metal mesh tube within the combustor tube cannot be achieved. Accordingly, the NOx reducing structure is not a multi-positional structure, and must be modified (by changing the orientation of the rod to which the support member attaches) to accommodate different orientations of its associated combustor tube.
From the foregoing it can readily be seen that it would be desirable to provide an improved NOx reducing insert structure that functioned in a manner generally similarly to the structure shown in U.S. Pat. 5,370,529 while at the same eliminating the two previously mentioned limitations associated therewith. It is accordingly an object of the present invention to provide such an improved NOx reducing structure.