PROCESS AND APPARATUS FOR EMISSION REDUCTION FROM WASTE INCINERATION

Abstract: A furnace for combustion wherein a combustion chamber is configured such that waste can be advanced from a drying zone, to a combustion zone, to a burnout zone, and then into an ash pit. An air source provides air for drying, combustion and burnout in a primary combustion zone (PCZ). Fuel or a fuel/recirculated flue gas mixture is injected above the PCZ to create a mostly reducing substoichiometric secondary combustion zone (SCZ), to reduce NO x and decompose other nitrogen bearing compounds entering the SCZ. Vitiated air is injected into the combustion chamber above the mostly reducing SCZ. A process for combustion of the waste includes introducing the waste into the combustion chamber, advancing the waste through the combustion chamber, supplying combustion air to the combustion chamber for drying and combusting the waste and final ash burnout, and removing ash products from the combustion chamber. The fuel or fuel/recirculated gas mixture is supplied into the combustion chamber to create substoichiometric conditions for NO x reduction and nitrogen bearing compounds decomposition. Overfire air is supplied into the combustion chamber above the substoichiometric zone for thorough mixing and at least partial burnout of combustibles contained within the waste/fuel combustion products.

Description: BACKGROUND OF THE INVENTION Field of the Invention A process and apparatus for combustion of waste such as municipal solid waste (MSW), refuse derived fuel (RDF) or other comparable solid waste; the process results in simultaneous reduction in nitrogen oxides (NO x ), carbon monoxide (CO), hydrocarbons (THC), dioxins (PCDD), furans (PCDF), and other organic emissions. Description of the Prior Art Most of the existing processes and apparatuses for combustion of waste, such as municipal solid waste (MSW) or refuse derived fuel (RDF), include a combustion chamber equipped with a sloped or horizontal stoker that reciprocates or travels to move the waste from the waste inlet side of the combustor to the ash removal side of the combustor. A portion of the combustion air, generally equivalent to 1.0 to 1.3 of the waste stoichiometric requirement, is supplied under the stoker. Such combustion air is typically called undergrate air, or UGA, and is distributed through the stoker to dry and burn the waste present on the stoker. The waste is first dried on the drying portion or drying grate of the stoker, then combusted on the combustion portion or combustion grate of the stoker. The residual waste that primarily includes ash and carbon is then decarbonized or burned on the burnout portion or burnout grate of the stoker. The bottom ash is then removed through an ash pit. To assure carbon burnout, a high level of excess air, compared to the amount required for carbon burnout, is maintained at the burnout grate. In addition to other species, the products of waste drying, combustion and burnout contain products of incomplete combustion (PIC's) such as carbon monoxide (CO) and total hydrocarbons (THC), nitrogen bearing compounds (NBC's) such as NH₃, HCN and the like. The majority of NO x evolved from the stoker is believed to form from the oxidation of nitrogen bearing compounds and a smaller portion forms from the oxidation of molecular nitrogen. Additional air or overfire air (OFA) is usually introduced above the stoker and mixed with the products evolved from the stoker to burnout the combustibles and destroy NBC's. The excess air level downstream of the OFA injection is generally in the range of 60% to 100%. The NBC's that evolve from the waste react with oxygen in and downstream of the OFA injection zone forming significant additional NO x . Because of the low combustion temperatures in and downstream of the OFA injection most of the NO x formed in this zone is by the oxidation of NBC's (less than about 10%, are formed in this zone by the oxidation of molecular nitrogen). Based on measurements by the inventors, typical mass burn operations would result in about 30% of the total NO x formed on the stoker and about 70% in and downstream of the OFA injection. In most cases, a boiler is an integral part of the combustor to recover the heat generated by MSW combustion. In some cases, a portion of the cooled flue gases from downstream of the boiler are recirculated back into the combustion zone to reduce oxygen concentration and to lower combustion temperatures and thus are believed to decrease oxides of nitrogen formation. A disadvantage of flue gas recirculation is generally a higher concentration of PIC's within the flue gases and within the stack gases because of reduced combustion efficiency. U.S. Patent 3,781,162 teaches an apparatus for mixing recirculated flue gases with combustion air before the gases reach an ignitor. The '162 patent discloses combustion without recirculating vitiated air from over a burnout grate for overfiring. The '162 patent teaches neither fluid swirling in the combustion chamber nor injecting fuel above a stoker. U.S. Patent 3,938,449 discloses a waste disposal facility which uses a rotary kiln that differs from a stoker. The rotary kiln includes a hollow, open-ended circular tube body mounted for rotation about its circular axis. Hot flue gases are recirculated to dehydrate the waste material and remove oxygen. The '449 patent does not disclose fluid swirling in the combustion chamber or fuel injection downstream of the primary waste combustion zone. U.S. Patent 4,336,469 teaches a method of operating a magnetohydrodynamic (MHD) power plant for generating electricity from fossil fuel. The MHD combustor has a first stage which operates substoichiometrically, second stage natural gas injection, and third stage air injection for complete combustion. The '469 patent does not disclose the use of vitiated air from the combustor for overfiring and does not disclose fluid swirling within the combustion chamber. The '469 patent discloses a dwell chamber downstream of the MHD generator for reducing nitrogen oxides, but does not disclose nbc's decomposition. U.S. Patent 4,672,900 teaches a tangentially-fired furnace having injection ports for injecting excess air above a fireball of the combustion chamber to eliminate the flue gas swirl as the flue gas flows into a convection section. The furnace uses pulverized coal as a fuel. Secondary air is tangentially injected into the furnace and swirls in the direction opposite of the flue gas swirl. The '900 patent does not suggest the use of recirculated vitiated air from the main combustor for overfiring, fluid swirling within the combustion chamber, or fuel injection downstream of the primary combustion zone. U.S. Patents 4,013,399, 4,050,877 and 3,955,909 teach reduction of gaseous pollutants in combustion flue gas. The '909 patent discloses tow-stage combustion within a combustion chamber. Heat removal occurs in the first, second or both combustion stages to reduce nitrogen oxides. Secondary combustion air is injected or diffused through tubes into the stream of gaseous combustion products flowing from a primary combustion chamber to promote mixing and complete combustion without an excessive amount of secondary air. D-B-1'019'788 discloses an apparatus for combustion of coal, with all features of the state of art as described above and with all procedural steps in accordance with the first portion of claim 1. Here vitiated air is recirculated from over a burnout grate and mixed with fresh air for overfiring, creating a secondary oxidizing combustion zone above the usual primary combustion zone. It does not suggest the use of recirculated flue gas to create a reducing combustion zone between the primary combustion zone and the oxidizing combustion zone. SUMMARY OF THE INVENTION It is an object of this invention to provide a process and apparatus for combustion of wastes such as MSW, RDF or other comparable solid waste where fuel, and recirculated flue gases from the boiler exit are injected above the burning waste providing a sufficient temperature, from about 1600°F to about 2000°F, and a sufficient length of time, from about 1.0 sec to about 4.0 sec, to create a mostly reducing zone which decomposes nitrogen bearing compounds (NBC's) and reduces nitrogen oxides (NO x ) entering the reducing zone to N₂ and uses secondary air or overfire air (OFA) to reduce other emissions such as carbon monoxide (CO), total hydrocarbons (THC), dioxins (PCDD), and dibenzofuranse (PCDF), without forming significant additional NO x . BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a diagrammatic cross-sectional front view of a furnace for combustion of MSW or other solid waste, according to one embodiment of this invention; Fig. 2 shows a cross-sectional side view of an upper wall having nozzles secured at an angle with respect to the horizontal, according to one embodiment of this invention; and Fig. 3 shows a cross-sectional top view of the upper walls of the combustion chamber having secured nozzles that can be used to tangentially inject a gas, according to one embodiment of this invention. DESCRIPTION OF PREFERRED EMBODIMENTS For purposes of this invention, the term "waste" or "solid waste" is synonymously used throughout this specification and in the claims as municipal solid waste (MSW), refuse derived fuel (RDF) and/or other comparable solid waste. It is conceivable that waste may also have glass, metals, paper and/or plastic removed from the composition (RDF) and still be used as a fuel in the furnace of this invention. NO x is oxides of nitrogen or nitrogen oxides such NO, NO₂, N₂O. NBC's are compounds such as HCN and NH₃ that can oxidize to NO x , in the presence of oxygen. The secondary combustion zone (SCZ) is the volume of the combustion chamber that is downstream of the primary combustion chamber but below the location of overfire air (OFA) injection. The tertiary combustion zone (TCZ) is the volume of the combustion chamber downstream of the SCZ. The drying grate portion of the stoker also means the drying grate or drying zone and vice versa; and likewise for the combustion and burnout grate portions. The apparatus for waste combustion, furnace 10, is shown in a diagrammatic cross-sectional front view in Fig. 1. A plurality of walls 12 define combustion chamber 15. A stoker generally comprises at least one drying grate portion 20, at least one combustion grate portion 25 and at least one burnout grate portion 30 located within combustion chamber 15, preferably within a lower portion. At least one ash pit outlet 35 is located within combustion chamber 15, downstream of burnout grate portion 30. At least one fuel inlet 37 is positioned in wall 12 above the stoker such that the waste enters combustion chamber 15, then flows onto drying grate portion 20. The waste is advanced from drying grate portion 20, over combustion grate portion 25, over burnout grate portion 30, and then into ash pit outlet 35. At least one undergrate air conduit 40 is in communication with an undergrate air source and a space beneath at least one of drying grate portion 20, combustion grate portion 25 and burnout grate portion 30. Undergrate air conduit 40 is used to supply undergrate air beneath and then through the stoker. An undergrate air source and at least one space beneath the stoker are in communication with undergrate air conduit 40 and are also used to provide undergrate air beneath and then through the stoker. At least one fuel/flue gas nozzle 43 is secured to wall 12 and in communication with combustion chamber 15. Each fuel/flue gas nozzle 43 is positioned on wall 12 such that fuel/flue gases are injected into combustion products within combustion chamber 15. At least one overfire air nozzle 45 is sealably secured to wall 12 and in communication with combustion chamber 15. Each overfire air nozzle 45 is secured to wall 12 in such a position that a fluid, preferably vitiated air, is injected into combustion chamber 15, above the reducing SCZ. In a preferred embodiment according to this invention, each overfire air nozzle 45 and each fuel/flue gas nozzle 43 is either positioned or has internal mechanical components known in the art for tangentially or radially injecting each respective fluid into combustion chamber 15, above the reducing SCZ and the stoker, respectively. It is apparent that internal baffles, internal or external nozzles, or the like, can be used to tangentially or radially direct the fluid into combustion chamber 15. Thus, fluid swirl which enhances mixing can be accomplished in combustion chamber 15 having any type of cross section, even a rectangular cross section, as shown in Fig. 3. Referring to Fig. 3, overfire air nozzles 45 can be positioned at angles relative to wall 12 such that at least one swirl, preferably multiple swirls, are formed within combustion chamber 15. It is apparent that the fluid can be injected into combustion chamber 15 at an angle with respect to the horizontal by positioning secondary air nozzle 45 at an angle with respect to the horizontal, as shown in Fig. 2. In one embodiment according to this invention, at least one induced draft (ID) fan 33 is mounted within exhaust opening 32, which is preferably above burnout grate portion 30. ID fan 33 is used to exhaust vitiated air from above burnout grate portion 30, within cobustion chamber 15. In another embodiment according to this invention, ID fan 33 and a discharge nozzle are used to inject the vitiated air into combustion chamber 15, above a reducing SCZ. In a preferred embodiment, the vitiated air is mixed with fresh air and then injected through nozzle 34 as the OFA. Exhaust opening 32 can be positioned at any suitable location within wall 12, above burnout grate portion 30, preferably within the top section of wall 12, as shown in Fig. 1. Vitiated air duct 31 is sealably secured to wall 12 around exhaust opening 32. It is apparent that ID fan 33 can be a blower, a suction nozzle of a compressor, or any other type of suitable air compressing device or blower means. A process for combustion of the waste begins with introducing the waste through waste inlets 37 into combustion chamber 15 and onto drying grate portion 20 of the stoker. The waste is further advanced, preferably by reciprocating motion and gravity over combustion grate portion 25 and burnout grate portion 30. Undergrate air is supplied beneath and then through drying grate portion 20, combustion grate portion 25 and burnout grate portion 30 for drying and combusting the waste. Ash products are removed from combustion chamber 15 through ash pit outlet 35 which is located downstream of burnout grate portion 30, within combustion chamber 15. Fuel is injected into combustion chamber 15 above the stoker to form a mostly reducing SCZ (60% to 100% of SCZ volume) for decomposing NBC's as well as reducing NO x entering the SCZ. The fuel can be in either a solid, liquid or gaseous form, any of which contain insignificant amounts of fuel-bound nitrogen. In a preferred embodiment, the fuel is natural gas. The fuel represents about 5 percent to about 25 percent of the waste heating value. The fuel, which is contained in a stream of recirculated flue gases, is injected through at least one fuel/flue gas nozzle 43, as shown in Fig. 1, to provide an average stoichiometric ratio of about 0.6 to about 1.05 within combustion chamber 15, above the stoker. Flue gases representing about 5% to about 30% of the flue gases at the boiler exhaust are recirculated and injected into the SCZ to enhance mixing and improve temperature and gas composition uniformity. In one embodiment of this invention, vitiated air is ejected from above burnout grate portion 30, mixed with fresh air at fresh air nozzle 34, and injected as OFA into combustion chamber 15 above the reducing SCZ. The OFA is preferably injected through at least one overfire air nozzle secured to wall 12 and in communication with combustion chamber 15, above the SCZ. OFA is supplied into combustion chamber 15 through at least one overfire air nozzle 45 for thorough mixing and at least partial burnout of combustibles contained within the waste combustion products. In a preferred embodiment of this invention, OFA is tangentially or radially injected, with respect to wall 12, into combustion chamber 15, above the reducing SCZ. In one embodiment of this invention, OFA representing about 5 percent to about 50 percent of a total air supply is injected above the reducing SCZ. OFA is injected above the reducing zone only after allowing a sufficient residence time, preferably about 1 sec. to about 4 secs., in the mostly reducing SCZ for significant decomposition of NBC's and NO x reduction. The preferred residence time of about 1-4 sec. is due to the relatively low temperatures in waste combustors. It is apparent that the residence time may vary according to the specific waste, amount of fuel injected and the combustor operating temperature. In another preferred embodiment according to this invention, the ejected vitiated air is mixed with fresh air prior to injecting into combustion chamber 15, above the SCZ. An air deficiency level achieved in the SCZ is about 0 percent to about 40 percent and the overall excess air level achieved downstream of OFA nozzle 45 is about 40 percent to about 100 percent. In another embodiment according to this invention, flue gas is recirculated for drying and preheating waste on the drying grate portion 20. In still another preferred embodiment according to this invention, natural gas, flue gases, a natural gas/flue gas mixture, and/or OFA, all generally referred to as a fluid, can be tangentially or radially injected, with respect to wall 12, into combustion chamber 15, above the stoker. In another embodiment according to this invention, the fluid can be injected into combustion chamber 15 above the stoker, at an angle with respect to the horizontal, as shown in Fig. 2. This invention uses a combination of low excess air or substoichiometric combustion of the waste on the stoker. Natural gas or any other solid, liquid, or gaseous fuel that does not contain significant fuel-bound nitrogen is injected into combustion chamber 15 above the stoker to provide a mostly reducing zone, having a 0.6 to 1.05 average stoichiometric ratio above the stoker, but with 60% to 100% of the SCZ volume at a stoichiometric ratio of less than 1.0, which decomposes NBC's and reduces NO x . OFA is injected above the reducing zone to provide a relatively strong mixing zone which assures high efficiency/low pollutant emission combustion within combustion chamber 15, providing low air emissions such as CO, THC, PCDD and PCDF.

Claims: In a process for waste combustion in which waste is introduced (37) into a drying zone (20) within a combustion chamber (15), air for preheating, drying and partially combusting the waste is supplied (40) to the drying zone (20), the waste is advanced within the combustion chamber (15) to a combustion zone (25), air for further combustion of the waste is supplied (40) to the combustion zone (25), the waste is advanced within the combustion chamber (15) to a burnout zone (30), air for final burnout of organics in the waste is supplied (40) to the burnout zone (30), ash is removed (35) from the combustion chamber (15), and fuel and recirculated flue gases are injected (43) into the combustion chamber (15) above the waste to create a secondary combustion zone; the improvement comprising the steps of: injecting (43) the fuel and the recirculated gases maintaining an air deficiency level to form a reducing combustion zone; ejecting (32) vitiated air from the burnout zone (30); and injecting (45) vitiated air into the combustion chamber (15) above the secondary combustion zone for thorough mixing and final burnout of combustibles in combustion products of the waste in a tertiary combustion zone. In a process for waste combustion according to Claim 1 further comprising mixing of the exhausted vitiated air (32) with fresh air (34) prior to injecting (45) the exhausted vitiated air into the combustion chamber (15). In a process for waste combustion according to Claim 1 further comprising maintaining an air deficiency level in the secondary combustion zone of about 0 percent to about 40 percent. In a process for waste combustion according to Claim 1 further comprising maintaining an overall excess air level downstream of overfire air inlet means (45) at about 40 percent to about 100 percent. In a process for waste combustion according to Claim 1 wherein the fuel is at least one of a solid fuel, a liquid fuel and a gaseous fuel containing relatively insignificant fuel-bound nitrogen. In a process for waste combustion according to Claim 1 wherein the fuel is natural gas. In a process for waste combustion according to Claim 1 wherein the fuel represents about 5 percent to about 40 percent of the waste heating value and the fuel is injected (43) into the combustion chamber (15) to maintain an average stoichiometric ratio of about 0.6 to about 1.05 within the secondary combustion zone. In a process for waste combustion according to Claim 2 wherein overfire air consisting of vitiated air and fresh air is about 5 percent to about 50 percent of a total air supply. In a process for waste combustion according to Claim 1 wherein the fuel and the vitiated air are injected above the waste at an angle with respect to the horizontal. In a furnace for performing the process for waste combustion according to Claim 1 comprising: a plurality of walls (12) defining a combustion chamber (15); a stoker comprising at least one drying grate portion (20), at least one combustion grate portion (25), and at least one burnout grate portion (30) located in a lower portion of said combustion chamber (15); ash pit means (35) within said combustion chamber (15) located downstream of said burnout grate (30) for discharging ash from said combustion chamber (15); waste inlet means (37) located in at least one of said walls (12) in a position such that the waste is introduced into said combustion chamber (15) on said drying grate portion (20); undergrate air supply means (40) for supplying air to said stoker; recirculated flue gases and/or fuel injection means (43) for injecting recirculated flue gases and/or fuel above the waste to produce a secondary combustion zone; and exhaust means (32) for ejecting vitiated air from above said burnout grate portion (30) and vitiated air injection means (45) for injecting said vitiated air into said combustion chamber (15) within said combustion chamber (15), characterized in that said air injection means (43,45) are positioned at an angle with respect to at least one combustion chamber wall (12) of the furnace, so that swirl zones are created, and that air vitiated air injection means (45) are positioned above the recirculated flue gases and/or fuel injection means (43).

Inventors: ABBASI, HAMID A., ITSE, DANIEL C., KHINKIS, MARK J., LISAUSKAS, ROBERT A.
Applicants: INSTITUTE OF GAS TECHNOLOGY
Application year: 1991
Main classification: F23G5
IPC codes: F23G5, F23L9