1. Field of the Invention
This invention relates to a combustion process and apparatus for emissions reduction, in particular NO.sub.x emissions reduction, from the combustion of a combustible material in a combustion chamber of a heating device such as a boiler or water heater. More particularly, this invention relates to a process and apparatus for combustion of a combustible material in a combustion chamber wherein a primary combustion zone is formed and a fluid is introduced into the combustion chamber downstream of the primary combustion zone producing an oxygen deficient zone which reduces any undesirable NO.sub.x and/or NO.sub.x precursors to harmless molecular nitrogen.
2. Description of Prior Art
Conventional combustion of combustible materials with air in industrial heating devices such as water heaters and boilers typically produces elevated temperatures which promote complex chemical reactions between oxygen and nitrogen in the air or in the fuel, forming various oxides of nitrogen as by-products of the combustion process. These oxides, containing nitrogen in different oxidation states, generally are grouped together under the single designation of NO.sub.x. Concern over the role of NO.sub.X and other combustion by-products, such as sulphur dioxide and carbon monoxide, in "acid rain" and other environmental problems is generating considerable interest in reducing the formation of these environmentally harmful by-products of combustion.
Toward this end, there exist numerous methods and apparatuses which are designed to reduce undesirable emissions, such as NO.sub.x, from the combustion process. One such general methodology is the use of staged combustion in which elements for combustion are introduced in stages, such as into multiple combustion chambers, for combustion of a fuel. For example, U.S. Pat. No. 5,462,430 to Khinkis teaches a process and apparatus for cyclonic combustion with ultra-low pollutant emissions and high efficiency in which a fuel and primary combustion air mixture is tangentially injected into a reducing primary combustion zone of a cyclonic combustor. Secondary combustion air is then tangentially injected into an oxidizing secondary combustion zone of the cyclonic combustor where it mixes with primary combustion products from the reducing primary combustion zone. See also U.S. Pat. No. 5,209,187 to Khinkis which teaches a low pollutant-emission, high efficiency cyclonic burner for fire tube boilers and heaters in which the combustion air required for complete combustion is introduced into the burner in stages, and U.S. Pat. 5,441,403 to Tanaka et al. which teaches a method of low NO.sub.x combustion in which a primary fuel is injected from a periphery of a stream of combustion air toward the combustion air, thereby effecting a first combustion and creating a generally cylindrical primary flame covering the combustion air, and a secondary fuel is injected towards the combustion air, shielded by the primary flame from the combustion air while causing NO.sub.x in the primary flame to be reduced by the secondary fuel, after which the secondary fuel contacts a portion of the combustion air by penetrating through the primary flame at a downstream side thereof so as to effect a second combustion.
Staged combustion is also taught by U.S. Pat. No. 4,989,549 to Korenberg which teaches a combustion apparatus for staged combustion inside a Morison tube of a fire tube boiler in which the first combustion stage is sub-stoichiometric and the second stage is above stoichiometric; U.S. Pat. No. 4,505,666 to Martin et al. which teaches a low NO.sub.x burner for a furnace and a method for operating the burner involving a primary and secondary combustion zone in which staged fuel and air is provided to both combustion zones; U.S. Pat. No. 4,007,001 to Schirmer et al. which teaches a method of combustion for lowering emissions of nitrogen oxides and carbon monoxide in which a first stream of air is introduced into a first combustion zone of a combustor, a second stream of air is introduced tangentially into the first combustion zone, and a third stream of air is introduced tangentially into a second combustion zone of the combustor; and U.S. Pat. No. 4,021,188 to Yamagishi et al. which teaches a burner configuration for staged combustion in which a fuel-rich mixture of hydrocarbon fuel and air are introduced into a combustion chamber having a path for secondary air around the combustion chamber, a flame holding means in one end of the combustion chamber stabilizes the sub-stoichiometric combustion so that a partially burned gas containing mainly hydrogen and carbon monoxide as combustible components is obtained, and as the partially burned gas is discharged from the combustion chamber, secondary air is discharged in a pattern surrounding the partially burned gases exiting the combustion chamber so as to complete combustion.
U.S. Pat. No. 4,879,959 to Korenberg teaches a swirl combustion apparatus in which a peripheral swirl of air is supplied into a combustion chamber adjacent to the inner surface of a cylindrical wall, and partially pre-burned fuel is supplied from a pre-combustion chamber to the combustion chamber to mix with the swirl of air, burn in the combustion chamber, and form hot combustion gases.
Processes and apparatuses for emissions reduction from waste incineration in which a reducing/oxygen deficient secondary combustion zone is formed downstream of a primary combustion zone are taught by U.S. Pat. No. 5,020,456, U.S. Pat. No. 5,105,747, U.S. Pat. No. 5,205,227, and U.S. Pat. No. 5,307,746, all to Khinkis et al. Each of the disclosed processes and apparatuses utilizes the introduction of one or more of a fuel, a fuel/carrier fluid, fuel/recirculated flue gases, or the output of a calciner comprising combustion products and a calcined sorbent downstream of the primary combustion zone to form the desired reducing oxygen deficient secondary combustion zone.
Flue gas recirculation in boilers and furnaces is a technique generally known to those skilled in the art for reducing emissions from combustion processes. The use of flue gas recirculation typically involves the introduction of flue gases into a combustion chamber within a boiler or furnace above a burning fuel bed, that is, downstream of a primary combustion zone. This continual recycling of the flue gas results in a further burning of smoke and other particulate matter contained therein. In addition, the formation of various nitric and nitrous oxides and carbon monoxide gases in the flue gas is reduced, thereby minimizing the release of these undesirable gases into the atmosphere. In some instances, flue gas recirculation is also used under the grate which supports the burning fuel bed for coal-fired stoker boilers. Flue gas recirculation under the grate has been applied to coal-fired stoker boilers for a number of years as evidenced by Maloney, K. L., "Recycle Flue Gas to Cut Emissions, Improve Boiler Performance," Power, pages 97-99, June 1983, U.S. Pat. No. 4,335,660, and U.S. Pat. No. 5,020,456, U.S. Pat. Nos. 5,105,747, 5,205,227, and 5,307,746 to Khinkis et al. as discussed hereinabove.
One of the drawbacks of flue gas recirculation is the requirement of a large quantity of completely combusted gases for injection into the furnace to promote mixing and help make the temperature profile more uniform, which large volume of gases then must be removed from the exhaust. Depending on the removal location, this combusted gas may contain large quantities of particulate which may cause erosion and corrosion in the duct work of conventional systems. An additional problem is the presence of oxygen in the exhaust gases. This oxygen, when present in the furnace, diminishes the net effect of the reducing atmosphere.
For applications in which fuel is introduced into a combustion chamber downstream of a primary combustion zone to form a reducing secondary combustion zone, a technique known as "reburn," the volume of fuel introduced in this manner is generally so low compared to the total volume of combustion products from the primary combustion zone that mixing of the fuel uniformly therein is difficult, thereby requiring special injection techniques for enhancing mixing, or the use of a carrier fluid, such as flue gases, steam, water, air, and/or nitrogen, the resulting increase of volume enhancing the mixing and improving the temperature and composition uniformity within the combustion chamber. At least one disadvantage of this technique is the requirement of additional hardware, such as blowers, duct work, piping, etc. which increase the capital cost to retrofit a furnace for NO.sub.x reduction. In addition, there is considerably more maintenance cost associated with such a reburn system, and operating expenses are also higher, particularly where flue gas recirculation is employed, due to the large blower and blower motor requirements which consume considerable quantities of electricity. See also U.S. Pat. No. 5,176,513 which teaches a pulse combustor apparatus in which air or a gaseous, low nitrogen fuel for air staging or reburning is introduced into a combustion zone downstream of a primary combustion zone; U.S. Pat. No. 5,161,471 which teaches an apparatus for reburning ash material of a previously burned primary fuel in which combustion air is introduced at a level above a primary combustion zone; U.S. Pat. No. 5,139,755 which teaches a combustion process for reducing NO.sub.x emissions in which a reburned fuel is mixed with combustion emissions in a gaseous reburn zone so as to produce a substantially oxygen deficient reburn zone; U.S. Pat. No. 4,759,340 which teaches a fire grate having air delivery passages for supplying fresh air directly to the top of the grate for assistance in reburning volatile products; and U.S. Pat. No. 4,516,510 and related U.S. Pat. No. 4,438,705, both of which teach a method of incineration in a main combustion chamber having two consecutive reburn stages.