Burner

Abstract: There is provided a combustion method, and apparatus (10), for gaseous fuels in which the emission of pollutants such as CO and NO₂ are reduced by promoting the completion of combustion reactions by at least temporarily separating ambient air from hot products (4) of combustion.

Description: BACKGROUND OF THE INVENTION AND RELATED ART The present invention relates to combustion methods and burners producing very low levels of pollutants such as CO and NO 2 . The invention is particularly applicable to gas-fired radiant burners having a combustion surface at or near which a gaseous fuel, such as natural gas, is combusted. The burners have been developed primarily for use in respect to gas burner space heating devices and will be described with reference to this particular use. However, it should be appreciated that the invention is not limited to this particular field of use and is equally applicable to other types of gas burners as well as other burners that use a variety of different gaseous fuels. Gas-fired burners are widely used commercially and in the domestic environment for heating including space heating for temperature conditioning interior space. Such burners are generally preferred over electricity due to their low cost and efficiency as well as their general flexibility. However, burners of all kinds are now known to be a source of indoor pollution especially in the amounts of oxides of nitrogen (NO x ) formed. NO x is a term used to describe the combined oxides of nitrogen and in particular NO, N 2 O and NO 2 . For example, NO and N 2 O are a concern in the outdoor environment, in particular with relation to acid rain, O-zone and photochemical smog. However, NO 2 , is of more concern to medical authorities due to its affect on lung function. Medical research during the 1980's has suggested that lung function will be affected by much lower levels of NO 2 that was previously thought. This has led to severe restrictions on the acceptable emission levels of NO x with particular emphasis on the emission of NO 2 . Radiant burners which include combustion surfaces, are generally preferred over blue flame burners which tend to produce NO 2 in the levels in the order of 15 to 30 ng/Joule and as such are not considered to have potential for the reduction of NO x . For this reason NO x gas burner research has centered primarily around surface combustion burners of different forms. In the last twenty years, research into the production of gas burners having lower NO x emission levels has concentrated on the use of excess air, alone or in combination with the incorporation of second stage burning. As a result, a number of these burners have become very complex in both design and operation procedures. For example, up until recently, the most successful burner design had been based on using pressurized pre-mixed air/fuel mixtures burned in a variety of metallic surface configurations, ceramic surfaces or after-burners. All such burners relied on high excess air and high combustion load. The associated requirements of pressurizing systems, after-burners and high combustion loads resulted in gas burners that were often bulky, complicated and inflexible in their operation as well as costly. Furthermore, while some of these burners succeeded in reducing NO x emission levels relative to the older types of burners, it still appeared impossible to approach the desired target levels. However, a recent development in surface combustion burners by the applicants has resulted in the production of a surface combustion gas burner having unexpectedly low emission levels of NO x including both NO and NO 2 . This burner will hereinafter be referred to as the "Bowin" burner. The present invention arises from further development work performed initially on the low NO x burner described above. This work has shown that further substantial reductions in the emission levels of NO 2 can be achieved and in some cases all measurable traces of NO 2 can be completely eliminated. Through a series of experiments it was shown that most burners can be modified to inhibit the production of NO 2 in a controlled manner previously unknown within the industry. The test work performed during development of the previously described Bowin burner indicated that the production of oxides of nitrogen can be controlled by, among other things, controlling the flame temperature within certain limits to inhibit the formation of NO. It is believed that NO is the precursor of NO 2 which forms by subsequent oxidation of the NO. Accordingly, by reducing the formation of NO and suppressing the combustion temperature, it was also possible to also limit the formation of NO 2 to very low levels. However, up until now it has not been possible to eliminate the production of NO 2 , completely, since at least a portion of the NO 2 is formed from NO outside the burner at a stage over which there is little or no control. The present invention provides a mechanism to prevent the suspected- conversion of NO to NO 2 by a simple technique that can be adapted to apply to most types of burners. The radiant burners of particular interest herein are non-powered burners in that they do not include a powered fan or blower assisted supply of combustion air. A pressurized source of fuel is used to aspirate the required combustion air at levels in excess of stoichiometric for delivery to the combustion surface. Heretofore, surrounding or ambient air was freely available to the combustion process occurring at or near the combustion surface. The ambient air is typically at a relatively cooler temperature as compared with the combustion gases. Accordingly, it is an object of the present invention to provide non-powered radiant burner apparatus and methods of combustion so as to overcome or substantially ameliorate the foregoing disadvantages of the existing prior art burners and combustion techniques. It is a further object of the present invention to enable production and operation of pertinent burner types with reduced pollutants so that emissions of CO and NO 2 are substantially reduced and/or eliminated. U.S. Patent 3199572 discloses a gas burner which includes a screen or grid arranged to cause the combustion mixture to exit at low velocity. Above the screen or grid there is provided one or more chimneys of a height of the same order of magnitude as the transversal dimension thereof. The invention is directed to a method of combusting a gaseous fuel in a gas burner apparatus having a naturally aspirated, non-powered radiant burner including a combustion surface having a periphery and an imperforate elongate wall extending along a major portion of the periphery of combustion surface, comprising the steps of mixing the fuel with a preselected amount of combustion air greater than the amount required for theoretically complete combustion of the fuel to form an air/fuel mixture, delivering the air/fuel mixture to the combustion surface, and combusting the air/fuel mixture in a combustion zone at or near the combustion surface to form hot products of combustion including reactive molecular species in the zone, characterised by restricting the addition of relatively cooler ambient air to the reactive species in the combustion zone by blocking laterally inward ambient air flow across the combustion surface using the imperforate elongate wall which projects in the direction of flame buoyancy to a height above the adjacent edge of the combustion surface substantially less than the adjacent lateral width of the combustion surface prior to the substantial completion of reactions of combustion to thereby suppress the formation of CO and NO 2 in the hot products of combustion exiting the combustion zone. The invention is also directed at a gas-fired burner apparatus including a naturally aspirated, non-powered radiant burner comprising a combustion surface for combusting an air/fuel mixture to form reactive molecular species which further react in a combustion zone to form hot products of combustion, and gas flow means for removing the products of combustion from the apparatus, the gas flow means including imperforate wall means extending along a major portion of the periphery of the combustion surface, characterised in that the wall means project from the combustion surface in the direction of flame buoyancy to a height above the adjacent edge of the combustion surface substantially less than the adjacent lateral width of the combustion surface to obstruct the flow of ambient air directly to the combustion zone from the lateral extremities thereof, and to substantially prevent contact of the reactive species with relatively cooler ambient air prior to the substantial completion of reactions of combustion whereby the formation of increased amounts of CO and NO 2 is suppressed as compared with an otherwise similar burner apparatus which does not include said imperforate wall means. In accordance with the present invention, the formation of pollutants is suppressed by promoting completion of the combustion reactions. Ambient air is kept separated from the hot products of combustion in order to reduce the formation of pollutants such as CO and NO 2 . The combustion process is controlled or regulated in a manner which at least initially separates and delays the contact of the hot products of combustion with the relatively cooler ambient air. The mechanism by which the separation or relatively cool ambient air from the hot products of combustion suppresses the formation of increased pollutants such as CO and NO 2 is not entirely understood. The cooler ambient air may inhibit combustion reactions which normally consume such pollutants or their precursors and effectively freeze the gas composition in an undesirable condition. Alternatively, the cold ambient air may yield or promote undesired reactions resulting in such pollutants. Further, a combination of both inhibiting desired reactions and promoting undesired reactions may occur. In any case, it has been found that the separation and/or delayed addition of ambient air to the products of combustion results in reduced pollutants, particularly, CO and NO 2 species. In part, the present invention is based on the insight that the combustion reactions in respect to pollutants may be significantly altered by regulation of the combustion process adjacent the combustion surface. That is, it has been determined that the levels of various pollutants may be substantially reduced by restricting, regulating or controlling the ambient air flow to the combustion at or near the surface itself. In other words, it has been discovered that the combustion phenomenon may be substantially affected by process controls and apparatus substantially operating or disposed only in close proximity to the combustion surface. For example, processing and apparatus within less than about one inch spacing from the surface have been found to effectively suppress the formation of CO and NO 2 in the resulting flue gas. Consistent with these process and apparatus developments, it has been found that the majority of combustion reactions tend to occur and to be completed in relatively close proximity to the combustion surface in a radiant burner. Laser fluorescence techniques indicate that the maximum concentrations of various reactive molecular species occur within about one inch of the combustion surface at temperatures in the range of about 830 to 870° c. The present invention is also founded on the realization that the application of the foregoing process and apparatus controls to a radiant combustion surface may be effected at the combustion surface periphery or extremities. Accordingly, the separation of the ambient air from the hot products of combustion at the periphery of the combustion surface and in close proximity therewith effectively reduces the levels of pollutants. It is believed that the interior regions of the combustion surface are effectively shielded from deleterious ambient air contact or effects by the hot combustion products themselves as they rise upwardly due to their natural buoyancy. In the illustrated embodiment, radiant burners having controlled ambient air flow in accordance with the invention have been found to produce reduced levels of pollutants as compared with otherwise identical burners not having controlled ambient air. In accordance with the invention, primary combustion air is provided at levels in excess of stoichiometric or that required for theoretically complete combustion so that no secondary combustion air is required. As described above the control is achieved by use of a radiant burner which includes an air baffle or barrier extending along the burner periphery adjacent the burner surface to restrictively control the egress of ambient air into the burner flames and/or hot products of combustion adjacent the combustion surface. The barrier extends along a majority of the periphery of the combustion surface so as to restrict the direct lateral flow of ambient air onto the combustion surface. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an elevational end view, partly in section, showing a burner apparatus with parts broken away or omitted for clarity of illustration; Fig. 2 is a perspective view showing a burner in accordance with the invention with parts broken away or omitted for clarity of illustration; and Fig. 3 is an elevational view of the burner of Fig. 2. DETAILED DESCRIPTION OF THE DRAWINGS A burner apparatus comprising a space heater 10 suitable for indoor heating is shown in Fig. 1. The heater 10 includes a gas-fired burner 1 of the type referred to herein as a "Bowin" burner. The burner 1 according to Fig. 1 is not in accordance with the invention but is included for the purpose of description of parts of the burner of the invention. The burner 1 includes an air inlet and fuel mixing device generally indicated at 2 arranged to deliver a combustible air/fuel mixture to a combustion zone generally indicated at 3. More particularly, the mixing device 2 comprises a fuel gas injector nozzle 12 arranged to deliver a flow of fuel gas into a venturi 14 with aspiration and mixing of a primary air component to form the combustible air/fuel mixture. The air/fuel mixture is delivered by the venturi into a tubular plenum chamber 16 for distribution to a porous combustion surface 18 formed of wire mesh, sintered metal or ceramic material in a known manner. The air/fuel mixture passes through the combustion surface 18 with combustion occurring at or near the combustion surface 18 within the combustion zone 3. The combustion along the surface 18 includes a plurality of flames extending from associated openings in the surface 18 to a height generally less than the height of the combustion zone. The hot products of combustion or flue gases indicated by the arrows 4 rise due to their natural buoyancy. Ambient air indicated by the arrows 5 is prevented from contacting and/or mixing with the reactive species of the flue gas 4 by the housing 6. The housing 6 includes a base portion 7 which extends from the plenum chamber 16 to housing sidewalls 8. The sidewalls 8 terminate at an upper opening 9 provided for venting the flue gas 4 to the atmosphere. The lower interior region of the housing 6 forms a combustion chamber 20 which encloses the combustion surface 18. More particularly, the housing base portion 7 cooperates with a lower portion of a corrugated interior housing wall 22 to enclose the combustion surface 18 and the combustion zone 3 to prevent contact of the combustion products or flue gas with ambient air. The combustion products move upwardly from the combustion surface 18 and away from the combustion zone 3 due to their natural buoyancy for venting to the atmosphere through the upper portion of the housing 6 for discharge via opening 9. It should be appreciated that combustion reactions are completed and the flue gas 4 is cooled by time it reaches the opening 9. The flue gas 4 may be cooled by recovery of heat energy therefrom via indirect heat transfer and radiant heat transfer. Through a series of tests it was shown that by preventing the ambient air 5 from coming into contact with the hot reactive combustion species until they reach the opening 9, the production of CO and NO 2 can be inhibited. The housing 6 is designed to facilitate cooling of the flue gas 4. The burner 1 is provided with at least 100% premixed primary air, and it is capable of running in a totally sealed enclosure where the hot reactive combustion products have no interaction with the ambient air as described above. Because of this arrangement, the heater 10 enjoys an added benefit of being capable of running at much higher efficiencies than contemporary burner configurations. This is due to the relatively lower volumes of air flow through the combustion chamber 20 which reduces the energy losses from the system. In the heater 10, it is possible to use a fairly large housing 6 and to fully enclose the burner 1. This results in the substantially complete elimination of all measurable NO 2 from the exhaust products. The temperature of the exhaust/ambient air mixture at the point of measuring was approximately 550°C. Referring to Fig. 2, an embodiment of the invention is shown and described hereinafter with the use of similar reference numerals for like parts and the addition of a prime designation to indicate modified parts or arrangements. A burner 1 has a modified housing 6' comprising a pair of baffle or barrier walls 24 which extend along a major portion of the periphery of the combustion surface 18. In this embodiment, relatively cooler ambient air is prevented from direct lateral flow into the combustion zone 3 adjacent the combustion surface 18. The combustion zone 3 extends a small distance, e.g. equal to at least the flame height and usually about 13 to 25mm (0.5 to 1 inch) or more, above the combustion surface 18 depending upon the combustion loading, the size of the openings in the combustion surface and other burner operating characteristics. The hot products of combustion within the zone 3 contain the maximum concentrations of reactive molecular species and they are maintained separate from significant contact with relatively cooler ambient air in accordance with the invention. It was found therefore that the simple expedient of a shroud or baffle in the path from which the ambient air was to be admitted, so as to provide a labyrinthine type ambient air by-pass, is effective to substantially prevent the production of NO 2 . The barrier walls 24 comprise elongate strips of metal about 25mm (one inch)wide mounted to the burner 1 in a suitable manner such as by threaded fasteners. The walls 24 are imperforate and extend along a major portion of the periphery of the combustion surface 18. As shown by dotted outline, a wall portion 24' may be omitted to provide clearance for other burner apparatus such as a pilot lighting device. Each of the walls 24 is disposed in a plane parallel to a plane passing through a diameter of the tubular plenum chamber 16 of the burner and radially spaced from the outer periphery of the burner by a distance of about 6.4mm (0.25 inch). Each of the walls 24 extends away from the combustion surface 18 in a direction generally corresponding with the direction of flow of the flue gases due to their natural buoyancy. As shown in Fig. 3, the combustion surface 18 has a semi-cylindrical configuration including major longitudinal edges 18a connected by arcuate end edges 18b. The periphery of the combustion surface 18 extends along the opposed pairs of longitudinal edges 18a and end edges 18b. The combustion surface 18 is secured to the burner 1 by a crimp arm 26 extending along each of the major edges of the combustion surface 18. Each of the walls 24 extends beyond the associated crimp arm 26 and combustion surface edge 18a a distance equal to about 0.3 inch or 8 mm. The walls 24 may extend a greater distance beyond the combustion surface edge 18a in accordance with the size of the combustion zone 3 and other operating characteristics of the particular burner as well as the desired reduction in pollutant emission levels. This distance may be easily determined by trial and error. The barrier walls 24 have been found effective to reduce the level of NO 2 . Further, the rapid influx of relatively cool ambient air is believed to yield increased amounts of CO, at least in part, due to excessive reduction of the flue gas temperature. The barrier walls 24 are therefore effective in reducing pollutant levels. In accordance with the embodiment of Figs. 2 and 3, variously sized burners were operated with and without barrier walls 24 to demonstrate the effectiveness of the present invention to reduce levels of pollutants. Each barrier wall 24 extended above the associated lowermost edge 18a of the combustion surface 18 a distance of about 8 mm (0.3 inch) in the direction of flue gas flow as described above in respect to the burner 1. Each burner was operated with free access to ambient air so that the effects of the barrier walls 24 could be compared. The results of these tests are reported below in Table I. WITH BARRIER WITHOUT BARRIER BURNER INPUT SCREEN SIZE LEN. X WID. PERIPHERAL PERCENT CO/CO 2 NO 2 CO/CO 2 NO 2 9 MJ 178 X 86 54.8 0.007 3.5 0.006 3.9 17 MJ 360 X 86 73.5 0.005 2.3 0.009 3.6 22 MJ 490 X 86 79.6 0.005 1.5 0.010 2.1 28 MJ 490 X 102 77.4 0.004 2.3 0.007 3.1 The tested burners were of various sizes having burner input ratings ranging from 9 to 28 MJ/m 2 hr as indicated in Table I. The combustion surfaces were formed of three tightly secured layers of 30x32, 0.36 mm (0.014 inch) diameter, nickel-based steel wire having an overall porosity of about 32%. For each burner size, CO and NO 2 pollutants were measured in the vented flue gases at steady state operating conditions. The CO/CO 2 ratio was calculated based on CO and CO 2 measured values using standard techniques. The NO 2 values were measured using a Bendix 8101-B oxides of nitrogen analyzer or a Neotronics Exotox 75 analyzer. As shown by the results of Table I, the barriers 24 are surprisingly effective in reducing NO 2 levels; the improvements ranging from about 11% to 40% reduction. The percent reduction is indicated to be proportional to the peripheral percent of the combustion surface which is shielded by a barrier wall. The shielding of a major portion of the periphery of the combustion surface results in about a 10% reduction in NO 2 level. The CO/CO 2 ratio also tends to be reduced as increased proportions of the combustion surface periphery are shielded by the barrier wall.

Claims: A method of combusting a gaseous fuel in a gas burner apparatus having a naturally aspirated, non-powered radiant burner (1) including a combustion surface (18) having a periphery and an imperforate elongate wall (6', 24) extending along a major portion of the periphery of combustion surface, comprising the steps of mixing the fuel with a preselected amount of combustion air greater than the amount required for theoretically complete combustion of the fuel to form an air/fuel mixture, delivering the air/fuel mixture to the combustion surface (18), and combusting the air/fuel mixture in a combustion zone (3) at or near the combustion surface (18) to form hot products (4) of combustion including reactive molecular species in the zone, characterised by restricting the addition of relatively cooler ambient air to the reactive species in the combustion zone (3) by blocking laterally inward ambient air flow across the combustion surface (18) using the imperforate elongate wall (6', 24) which projects in the direction of flame buoyancy to a height above the adjacent edge (18a) of the combustion surface (18) substantially less than the adjacent lateral width of the combustion surface (18) prior to the substantial completion of reactions of combustion to thereby suppress the formation of CO and NO 2 in the hot products of combustion exiting the combustion zone (3). A method as in Claim 1, including combusting the air/fuel mixture to continuously form a plurality of flames and wherein the elongate wall (6', 24) is located substantially only along the combustion surface periphery. A method as in Claim 1 or Claim 2, wherein the elongate wall (6',24) has a height of from about 1.27cm (0.5 inch) to about 2.54cm (1 inch) in the direction of flame buoyancy. A method as in any preceding Claim, including cooling the products of combustion as they move away from the combustion surface (18) and combustion zone (3) prior to venting them to the atmosphere. A method as in any preceding Claim, wherein the major portion of the periphery is equal to at least about 50% of the periphery of the combustion surface (18). A method as in Claim 5, wherein the major portion of the periphery is equal to at least about 70% of the periphery of the combustion surface (18). A method as claimed in any preceding claim, wherein the elongate wall (6',24) includes at least two portions and each wall portion is mounted to the burner to form the elongate wall and prevent the flow of ambient air. A gas-fired burner apparatus (10) including a naturally aspirated, non-powered radiant burner (1) comprising a combustion surface (18) for combusting an air/fuel mixture to form reactive molecular species which further react in a combustion zone (3) to form hot products of combustion, and gas flow means for removing the products of combustion from the apparatus, the gas flow means including imperforate wall means (6',24) extending along a major portion of the periphery of the combustion surface (18), characterised in that the wall means (6',24) project from the combustion surface (18) in the direction of flame buoyancy to a height above the adjacent edge (18a) of the combustion surface (18) substantially less than the adjacent lateral width of the combustion surface (18) to obstruct the flow of ambient air directly to the combustion zone (3) from the lateral extremities thereof, and to substantially prevent contact of the reactive species with relatively cooler ambient air prior to the substantial completion of reactions of combustion whereby the formation of increased amounts of CO and NO 2 is suppressed as compared with an otherwise similar burner apparatus which does not include said imperforate wall means (6', 24). An apparatus as in Claim 8, wherein the combustion surface (18) is porous such that combusting the air/fuel mixture continuously forms a plurality of flames, and the wall means (6', 24) project from the combustion surface (18) substantially only along the periphery of the combustion surface (18). An apparatus as in Claim 8 or Claim 9, wherein the burners (1) includes air/fuel supply means for delivering the air/fuel mixture to the combustion surface (18), the gas flow means include a combustion chamber (20) enclosing the combustion surface for combustion of the air/fuel mixture substantially free of contact with relatively cooler ambient air and thereafter venting the products of combustion. An apparatus as in any one of Claims 8 to 10, wherein the wall means (6',24) extend at least 70% of the periphery of the combustion surface (18). An apparatus as in any one of Claims 8 to 10, wherein the wall means comprises a first wall member (6', 24) which has a wall height extending from a lower boundary on the burner below the combustion surface to an upper boundary positioned above the periphery of the combustion surface a distance of at least 0.762 cm (0.3 inch), the first wall member (6',24) also having a wall length extending along at least about 50% of the periphery of the combustion surface (18). An apparatus as in Claim 12, wherein the wall means includes a second wall member (6',24) which extends from the burner, and the wall members (6',24) together extend along at least about 70% of the periphery of the combustion surface (18). An apparatus as in any of Claims 8 to 13, wherein the wall means (6',24) comprises an elongate imperforate strip of metal having opposed first and second longitudinal edges, the first edge being secured to the burner (1) and the second edge being positioned above the periphery of the combustion surface (18).

Inventors: JOYCE JOHN VINCENT, JOYCE, JOHN VINCENT
Applicants: BOWIN DESIGNS PTY LTD, BOWIN DESIGNS PTY. LTD.
Application year: 1992
Main classification: F23D14
IPC codes: F23D14