Process and apparatus for ultra-low pollutant emission combustion

Abstract: A process and apparatus for ultra-low pollutant emission combustion of fossil fuel wherein an elongated cyclonic primary combustion chamber (10) has a cross-sectional area about 4% to about 30% that of an elongated cyclonic secondary combustion chamber (30) and a volume about 1% to about 20% of the combined primary and secondary combustion chamber volume. A first fuel portion of about 1% to about 20% of the total fuel and primary combustion air in an amount selected from about 40% to about 90% and about 140% to about 230% of the stoichiometric requirement for complete combustion of the first fuel portion is introduced into the primary combustion chamber (10). A second fuel portion of about 80% to about 99% of the total fuel ist introduced into the secondary combustion chamber (30) with secondary combustion air in an amount of about 150% to about 260% of the stoichiometric requirement for complete combustion of the fuel. In preferred embodiments cyclonic flow is maintained through the combustor.

Description: This invention relates to an apparatus and process for ultra-low pollutant emission combustion of fossil fuel using a primary combustion chamber with a relatively small amount of fuel and relatively low or high precentage of stoichiometric air requirement and a secondary combustion chamber with a large amount of fuel with excess air, both combustion chambers having cyclonic flow. The secondary combustion chamber is larger than the primary combustion chamber in a specified relation. A dilution chamber is used. Combustion under these conditions results in ultra-low nitrogen oxides (No x ), carbon monoxide (CO) and total hydrocarbon emissions (THC). Existing multi-stage combustors use nozzles to mix fuel and air within a combustion chamber and other exisiting designs use partially premixed fuel and air prior to introducing such fuel/air mixture into a combustion chamber. Other existing combustor designs which use fully premixed fuel and air prior to introducing the fuel/air mixture into a combustion chamber use a one-stage combustion process which does not provide high flame stability at very high excess air. U.K. Patent Application GB 2 082 756 A teaches a combustor for a gas turbine using staged combustion in which a first portion of fuel consisting of 1/4 to about 1/3 of the total amount of fuel consumed in the combuster and primary combustion air are premixed with an excess air ratio of about 1.2 to about 1.4 by weight and introduced into a primary combustion chamber of a combustor having two combustion chambers. A second portion of fuel consisting of about 2/3 to about 3/4 of the total amount of fuel consumed in the combuster and secondary combustion air, as well as dilution air, are introduced into the secondary combustion chamber. The proportions of fuel and air introduced into both the primary and secondary combustion chambers, as well as the velocities of the fuel and air, are indicated to be necessary in order to provide stable combustion without the risk of flashback in the combustor. It is the object of this invention to provide a process and apparatus for combustion of fossil fuel having high flame stability which produces ultra-low pollutant emissions of nitrogen oxides (No x ), carbon monoxide (CO), and total hydrocarbons (THC). Suitable fossil fuels include natural gas, atomized oils, and pulverized coals, natural gas being preferred. This object is achieved by a process with the characterizing steps as set forth in claim 1 and by means of an apparatus having the characterizing elements as set forth in claim 8. Special derivations of this inventive process are claimed in the depending process-claims and special embodiments of the inventive apparatus are claimed in the respective depending apparatus-claims. An advantageous variant of the inventive process for combustion of fossil fuel works as follows. A first stage of combustion burns a first fuel portion from about 1% to about 20% of a total fuel mixed with primary combustion air in an amount of about 140% to about 230% of the stoichiometric requirement for complete combustion of the first fuel portion. The second stage of combustion burns any unburned fuel from the primary combustion chamber and added second fuel portion of about 80% to about 99% of the total fuel mixed with secondary combustion air in an amount of about 150% to about 260% of the stoichiometic requirement for complete combustion of the second fuel in the secondary combustion chamber. In another embodiment, primary combustion air in an amount of about 40% to about 90% of the stoichiometric combustion of the first fuel portion is introduced to the primary combustion chamber. The reducing gases from the primary combustion chamber are passed to the secondary combustion chamber. The preferred apparatus for low pollutant emission combustion of fossil fuel has at least one first wall defining an elongated cyclonic primary combustion chamber having a first upstream and a first downstream end. At least one second wall defines an elongated cyclonic secondary combustion chamber having a second upstream end and a second downstream end. At least one dilution chamber wall defines an elongated dilution chamber having a dilution chamber upstream end and a dilution chamber downstream end. The primary combustion chamber is in communication with the secondary combustion chamber which is in communication with the dilution chamber. The dilution chamber has a discharge outlet in communication with the outside atmosphere, a turbine, or the like. A first fuel portion inlet nozzle is in communication with the primary combustion chamber for introducing a first fuel portion of about 1% to about 20% of the total amount of fossil fuel to be combusted in the combustor. Primary combustion air is also introduced through the primary inlet nozzle into the primary combustion chamber in an amount of about 140% to about 230% of the stoichiometric requirement for complete combustion of the first fuel portion. The primary combustion air and the fuel portion are thoroughly mixed to form a primary fuel/air mixture which is then introduced into the primary combustion chamber. An ignitor is mounted within the primary combustion chamber for igniting the primary fuel/air mixture within the primary combustion chamber. The primary fuel/air mixture is combusted in the primary combustion chamber at about 1090° C to about 1485° C thereby producing initial combustion products having ultra-low pollutant emissions. The initial combustion temperature is controlled by the amount of primary combustion air introduced to the primary combustion chamber. In an alternative embodiment, primary combustion air is introduced into the primary combustion chamber in an amount of about 40% to about 90% of the stoichiometric requirement for complete combustion of the first fuel portion. Due to the incomplete combustion in the primary combustion chamber, the incomplete combustion products will include non-combusted fuel. The initial combustion products are introduced into the secondary combustion chamber. A second fuel portion, about 80% to about 99% of the total amount of fuel is introduced into the secondary combustion chamber through a secondary inlet nozzle. Secondary combustion air is also introduced through the secondary inlet nozzle into the secondary combustion chamber in an amount of about 150% to about 260% of the stoichiometric requirement for complete combustion of the fuel introduced to the secondary combustion chamber. The secondary combustion air and second fuel portion are mixed to form a secondary fuel/air mixture which is then introduced into the secondary combustion chamber. The secondary fuel/air mixture is combusted in the secondary combustion chamber at about 925° C to about 1430° C producing final combustion products having ultra-low pollutant emissions. The secondary combustion temperature is controlled by the amount of secondary combustion air introduced to the secondary combustion chamber. The final combustion products and the initial combustion products are mixed in the secondary combustion chamber to form mixed combustion products which are introduced into the dilution chamber. Dilution air is introduced into the dilution chamber thus producing ultra-low pollutant emission vitiated air at a temperature of about 35° C to about 1375° C. The ultra-low pollutant emission vitiated air is discharged from the dilution chamber. According to the invention, the primary combustion chamber, secondary combustion chamber and dilution chamber each have an approximately cylindrical shape and are longitudinally aligned. The downstream end of the primary combustion chamber is in communication with the upstream end of the secondary combustion chamber and the downstream end of the secondary combustion chamber is in communication with the upstream end of the dilution chamber. The cross-sectional area of the primary combustion chamber is about 4% to about 30% of the cross-sectional area of the secondary combustion chamber. The volume of the primary combustion chamber is about 1% to about 20% of the total combined volume of the primary and secondary combustion chamber. The volume of the dilution chamber is about 50% to about 250% of the volume of the secondary combustion chamber. At least one primary inlet nozzle is tangentially mounted through the first wall of the primary combustion chamber near the upstream end tangentially introducing the fuel and air with respect to the combustion chamber wall. At least one secondary inlet nozzle is tangentially mounted through the second wall near the upstream end of the secondary combustion chamber tangentially introducing the fuel and air with respect to the combustion wall. At least one dilution air inlet nozzle is tangentially mounted through the dilution chamber wall near the dilution chamber upstream end tangentially introducing air with respect to the dilution chamber wall. In a preferred embodiment of the invention, the primary combustion air and the first fuel portion fed to the primary combustion chamber are thoroughly premixed to form a primary fuel/air mixture prior to introduction into the at least one primary inlet nozzle. It is also preferred to premix the secondary combustion air and the second fuel portion fed to the secondary combustion chamber to form a secondary fuel/air mixture prior to introduction into the at least one secondrary inlet nozzle. In another preferred embodiment according to this invention, the downstream end of the primary combustion chamber may have a first orifice with a diameter less than that of the primary combustion chamber for exhausting initial combustion products from the primary combustion chamber into the secondary combustion chamber. The downstream end of the secondary combustion chamber may have a second orifice with a diameter less than that of the secondary combustion chamber for exhausting complete combustion products from the secondary combustion chamber into the dilution chamber. The dilution chamber downstream end may have a dilution chamber orifice with a diameter less than that of the dilution chamber for exhausting vitiated air to either the outside atmosphere, a turbine, or the like. The orifices are preferably concentrically aligend with the chambers. In one embodiment of this invention, at least one primary inlet nozzle may be positioned in the upstream end, axially with respect to the first wall, to introduce fuel and air into the primary combustion chamber. The above mentioned and other features of this invention and the manner of obtaining them will become more apparent, and the invention itself will be best understood by reference to the following description of specific embodiments taken in conjunction with the drawings, wherein; Figure 1 shows a cross-sectional side view of one embodiment of an apparatus according to this invention for ultra-low pollutant emission combustion of fossil fuel; Figure 2 shows a cross-sectional side view of another embodiment of an apparatus according to this invention for ultra-low pollutant emission combustion of fossil fuel; and Figur 3 shows a cross-sectional side view taken along line 3-3 as shown in figure 1. Figure 1 shows a cross-sectional side view of an apparatus for ultra-low pollutant emission combustion of fossil fuel according to one embodiment of this invention. Upstream end 11, downstream end 12 and at least one wall 13 define primary combustion chamber 10. Primary combustion chamber 10 has an approximately cylindrical shape. The first fuel portion of about 1% to about 20% of the total amount of fossil fuel to be burned in the combustor is introduced into primary combustion chamber 10 through primary inlet nozzle 15. At least one primary inlet nozzle 15 is one of tangentially mounted through wall 13, preferably near the upstream end of primary combustion chamber 10, and axially mounted through upstream end 11. The term "tangential" refers to a nozzle being attached to the side wall of a chamber in an non-radial position such that flow through the nozzle into the chamber creates cyclonic flow about the centerline of the combustion chamber. A cylindrical shaped combustion chamber best accommodates such cyclonic flow. Primary air is also introduced through primary inlet nozzle 15 into primary combustion chamber 10 in an amount of about 140% to about 230% or about 40% to about 90% of the stoichiometric requirement for complete combustion of a first fuel portion within primary combustion chamber 10 providing excess air or substoichiometric air, respectively. In a preferred embodiment of this invention, downstream end 12 is common with upstream end 31 of secondary combustion chamber 30. Downstream end 12 has orifice 19 with an opening smaller than the cross section of primary combustion chamber 10 which allows initial combustion products to be exhausted from primary combustion chamber 10 into secondary combustion chamber 30. It is apparent that orifice 10 can be positioned at any location in downstream end 12, preferably orifice 10 is concentrically aligend in downstream end 12. It is apparent that orifice 10 can be an orifice plate, a converging nozzle, or the like. Ignitor 21 is mounted whithin primary combustion chamber 10. Ignitor 21 provides ignition for the first fuel portion and primary air contained within primary combustion chamber 10. Ignitor 21 can be a spark plub, glow plug, continuous burner, or any other suitable ignition source familiar to the art. Upstream end 31, downstream end 32 and at least one wall 33 define secondary combustion chamber 30. Secondary combustion chamber 30 has an approximately cylindrical shape. The second fuel portion of about 80% to about 99% of the total fuel is introduced into secondary combustion chamber 30 through secondary inlet nozzle 35. At least one secondary inlet nozzle 35 is tangentially mounted through wall 33, preferably near the upstream end of secondary combustion chamber 30, to provide cyclonic flow. Secondary combustion air is also introduced through inlet nozzle 35 into secondary combustion chamber 30 in an amount of about 150% to about 260% of the stoichiometric requirement for complete combustion of the fuel in the secondary combustion chamber. Primary and secondary combustion air may flow through passage 46 into primary and secondary inlet nozzles 15 and 35, respectively. Downstream end 32 of secondary combustion chamber 30 is common with upstream end 51 of dilution chamber 50. Downstream end 32 has orifice 39 with an opening smaller than the cross section of secondary combustion chamber 30 through which combustion products can be exhausted to dilution chamber 50. Orifice 39 can be positioned at any location in downstream end 32, preferably orifice 39 is concentrically aligend in downstream end 32. Orifice 39 can be an orifice plate, a converging nozzle, or the like. Upstream end 51, downstream end 52 and at least one wall 53 define dilution chamber 50 in communication with secondary combustion chamber 30. Dilution chamber 50 is also in communication with either the outside atmoshpere, a turbine or other expanding device, or the like. Dilution chamber 50 has an approximately cylindrical shape. At least one dilution air inlet nozzle 56 is tangentially mounted through wall 53, preferably near the upstream end of dilution chamber 50. Downstream end 52 of dilution chamber 50 has orifice 59 with an opening smaller than the cross section of dilution chamber 50 for exhausting vitiated air to the outside atmosphere, a turbine or other expanding device, or the like. Orifice 59 can be positioned at any location in downstream end 52, preferably orifice 59 is concentrically aligned with downstream end 52. Orifice 59 can be an orifice plate, converging nozzle, or the like. Primary combustion chamber 10, secondary combustion chamber 30 and dilution chamber 50 are longitudinally aligned. It is preferred that the cross-sectional area of primary combustion chamber 10 be about 4% to about 30% of the cross-sectional area of secondary combustion chamber 30. The volume of primary combustion chamber 10 is preferred to be about 1% to about 20% of the total combined volume of primary combustion chamber 10 and secondary combustion chamber 30. The volume of dilution chamber 50 is preferred to be about 50% to about 250% of the volume of secondary combustion chamber 30. In one embodiment according to this invention, primary inlet nozzle 15 is passed through upstream end 11 to provide axial introduction into primary combustion chamber 10. In the embodiment shown in figure 1, primary combustion air and the first fuel portion are thoroughly mixed within primary inlet nozzle 15 to form a primary fuel/air mixture. Likewise, secondary combustion air and the second fuel portion are thoroughly mixed within secondary inlet nozzle 35 to form a secondary fuel/air mixture. Figure 2 shows a cross-sectional side view of a combustor wherein the primary combustion air and the first fuel portion are thoroughly premixed and the secondary combustion air and the second fuel portion are thoroughly premixed prior to being introduced into primary fuel/air mixture nozzle 18 and fuel/air mixture nozzle 38, respectively. At least one primary fuel/air inlet nozzle 18 is tangentially mounted through wall 13, preferably near the upstream end which provides cyclonic flow through primary combustion chamber 10. At least one secondary fuel/air inlet nozzle 38 is tangentially mounted through wall 13 preferably near the upstream end which provides cyclonic flow through secondary combustion chamber 30. Figure 3 shows a cross-sectional view along line 3-3, as shown in figure 1 showing secondary inlet nozzle 35 in the outermost tangential location with respect to wall 33. It is apparent that the term "tangential " applies to any nozzle whose centerline does not intersect with the centerline of the chamber.

Claims: A process for ultra-low pollutant emission combustion of fossil fuel in which combustion is carried out in stages in a combuster, where premixed fuel and air are introduced into an upstream primary combustion chamber of the combuster and fuel and air are introduced into a downstream secondary combustion chamber of said combuster, and where dilution air is introduced into a dilution region of said combuster, the process comprising the steps of: a) introducing a first fuel portion of about 1% to about 20% of a total fuel to be combusted and primary combustion air in an amount selected from about 40% to about 90% or about 140% to about 230% of the stoichiometric requirement for complete combustion of said first fuel portion into the primary combustion chamber (10); b) combusting said first fuel portion with said primary combustion air in said primary combustion chamber (10) at a temperature about 1090°C to about 1485°C producing initial products of combustion; c) passing said initial combustion products into the secondary combustion chamber (30); d) introducing a second fuel portion of about 80% to about 99% of the total fuel and secondary combustion air in an amount of about 150% to about 260% of the stoichiometric requirement for complete combustion of said second fuel portion into the secondary combustion chamber (30); e) combusting said second fuel portion and any remaining fuel in said initial combustion products in said secondary combustion chamber (30) at a temperature of about 925°C to about 1430°C producing final products of combustion; f) passing said final combustion products into the dilution region within a dilution chamber (50); g) introducing dilution air into said dilution chamber (50), producing ultra-low pollutant emission vitiated air at a temperature between about 38°C to about 1375°C; and h) discharging said ultra-low pollutant emission vitiated air from said dilution chamber (50). Process according to claim one, wherein the first fuel portion and the primary air are introduced separately and mixed within primary inlet means (15;18) and wherein the second fuel portion and the secondary air are introduced separately and mixed within secondary inlet means (35;38). Process according to claim one, wherein said first fuel portion and said primary combustion air are thoroughly pre-mixed forming a primary fuel/air mixture prior to introducing said primary fuel/air mixture into primary inlet means (15;18) and wherein said second fuel portion and said secondary combustion air are thoroughly pre-mixed forming a secondary fuel/air mixture prior to introducing said secondary fuel/air mixture into secondary inlet means (35;38). Process according to one of the foregoing claims, wherein at least one of said first fuel portion and said primary combustion air is introduced tangentially near an upstream end (11) of said primary combustion chamber (10) and wherein at least a portion of one of said second fuel portion and said secondary combustion air is introduced tangentially near an upstream end (31) of said secondary combustion chamber (30) and further wherein dilution air is introduced tangentially into said dilution chamber (50). Process according to one of claim 1 to 3, wherein at least one of said first fuel portion and said primary combustion air is introduced axially into said primary combustion chamber (10). Process according to one of claim 1 to 3, wherein at least one of said first fuel portion and said primary combustion air is introduced at the same time both axially and tangentially into said primary combustion chamber (10). Process according to one of the foregoing claims, wherein the initial combustion products are passed through an orifice (19) having an opening with a cross-sectional area smaller than the cross-sectional area of said primary combustion chamber (10) in passing to said secondary combustion chamber (30) and wherein said final combustion products are passed through an orifice (39) having an opening with a cross-sectional area smaller than the cross-sectional area of said secondary combustion chamber (30) in passing to said dilution chamber (50). An apparatus for carrying out the process for ultra-low pollutant emission combustion of fossil fuel comprising: a) at least one first wall (13) defining an elongated cyclonic primary combustion chamber (10), having a first upstream end (11) and a first downstream end (12), said primary combustion chamber (10) having a cross-sectional area about 4% to about 30% of the cross-sectional area of a secondary combustion chamber (30) and a volume about 1% to about 20% of the combined volume of said primary (10) and secondary combustion chamber (30); b) at least one second wall (33) defining an elongated cyclonic secondary combustion chamber (30) having a second upstream end (31) and a second downstream end (32), said primary combustion chamber (10) in communication with said secondary combustion chamber (30); c) at least one dilution chamber wall (53) defining an elongated cyclonic dilution chamber (50) having a dilution chamber upstream end (51), a dilution chamber downstream end (52), and dilution chamber discharge means in communication with said dilution chamber (50), said secondary combustion chamber (30) in communication with said dilution chamber (50); d) primary inlet means (15;18) in communication with said primary combustion chamber (10) for introducing a first fuel portion and primary combustion air into said primary combustion chamber (10); e) said primary inlet means (15;18) at least one of tangentially and axially mounted with respect to said first wall (13), ignition means (21) for igniting said primary fuel/air mixture within said primary combustion chamber (10); f) secondary inlet means (35;38) in communication with said secondary combustion chamber (30) for introducing a second fuel portion and secondary combustion air into said secondary combustion chamber (30); g) said secondary inlet means (35;38) tangentially mounted with respect to said second wall (33); h) dilution air inlet means (56) in communication with said dilution chamber (50) for introducing dilution air into said dilution chamber (50); i) all of said chambers being cylindrical and longitudinally aligned; j) said primary inlet means (15;18) mounted proximate said first upstream end (11); and k) said secondary inlet means (35;38) mounted proximate said second upstream end (31). Apparatus according to claim 8, herein said dilution chamber (50) has a volume equal to about 50% to about 250% of the volume of said secondary combustion chamber (30). Apparatus according to one of the claims 8 or 9, wherein said first downstream end (12) has a first orifice (19) with an opening cross-sectional area smaller than a cross-sectional area of said primary combustion chamber (10) through which initial combustion products are exhausted into said secondary combustion chamber (30) and wherein said second downstream end (32) has a second orifice (39) with an opening cross-sectional area smaller than a cross-sectional area of said secondary combustion chamber (30) through which complete combustion products are exhausted into said dilution chamber (50) and further wherein said dilution chamber downstream end (52) has a dilution chamber orifice (59) with an opening cross-sectional area smaller than a cross-sectional area of said dilution chamber (50). Apparatus according to one of the claims 8 to 10, wherein said first orifice (19) is concentrically aligned with said first downstream end (12) wherein said dilution chamber orifice (59) is concentrically aligned with said dilution chamber (50) and wherein said second orifice (39) is concentrically aligned with said second downstream end (32). Apparatus according to one of the claims 8 to 11, further comprising mixing means (46) for mixing said first fuel portion and said primary air prior to introduction to said primary inlet means (15;18) and mixing means (46) for mixing said second fuel portion and said secondary air prior to introduction to said secondary inlet means (35;38).

Inventors: KHINKIS MARK J, KHINKIS, MARK J.
Applicants: INST GAS TECHNOLOGY, INSTITUTE OF GAS TECHNOLOGY
Application year: 1990
Main classification: F23C6
IPC codes: F23R3, F23C6, F23C3, F23C7