Title:
BURNER AND METHOD OF OPERATING IT TO CONTROL THE PRODUCTION OF NITROGEN OXIDES
United States Patent 3729285
Abstract:
In the operation of a burner using gaseous fuel the production of nitrogen oxides is limited and controlled by providing successive stages of combustion and by holding down the temperature of combustion. This is accomplished by introducing the fuel in two streams of unequal volume and comprising respectively more and less than the stoichiometric percentage of air, igniting the streams in sequence, thus effecting first and second stages of combustion, and mixing and consuming the excess fuel and air of said streams respectively, thus effecting a third stage of combustion.
Application Number:
05/255532
Publication Date:
04/24/1973
Other Classes:
431/284, 431/351
International Classes:
C21C5/52; F23C6/04; F23D14/22; C21C5/00; F23C6/00; F23D14/00; (IPC1-7): F23C7/00
Field of Search:
431/2,10,187,174,284,285,351 239
Primary Examiner:
O'dea, William F.
Assistant Examiner:
Anderson, William C.
Claims:
What I claim is
1. In the operation of a burner the steps of,
2. The method claimed in claim 1 in which one of said streams is of greater volume than the other, and the sum of the percentages of air in the two streams approximates the optimum or stoichiometric air percentage.
3. A burner adapted to provide combustion of gaseous fuel and air in successive stages which comprises:
4. The burner claimed in claim 3 in which the said conduits are concentric and the choke collar is axially aligned with the center conduit means.
5. In the operation of a burner using a gaseous fuel/air mixture which when burned forms nitrogen oxides, the concentration of the nitrogen oxides being greatest when the volume of air employed is substantially the optimum for combustion and therefor when burned has a high temperature, the step of minimizing the temperature of combustion and the formation of the nitrogen oxides by providing multistage combustion at successive distances from the burner face said multistage combustion being achieved by dividing the gaseous fuel/air supply into two streams, one of the streams comprising excess air and the other stream comprising excess gaseous fuel, igniting the said two streams in successive stages and thereafter mixing and burning the remaining excess air and excess fuel from said streams respectively providing a third stage of combustion.
Description:
FIELD OF THE INVENTION
This invention relates to a burner of the kind in which propane gas or other gaseous fuel is combined with air and burned to provide heat, as for example to provide steam in an electric power generating plant. The invention is particularly directed to the method of operating such a burner to limit and control the amount of nitrogen oxides which are produced while obtaining substantially complete and therefor efficient combustion.
SUMMARY OF THE DISCLOSURE
In accordance with this invention the production of nitrogen oxides is kept at a low level by providing successive stages of combustion, each occurring at a temperature substantially less than would be obtained in a single stage of combustion using the same volume and proportion of gaseous fuel/air. Instead of injecting gaseous fuel and air into the burner to mix and provide a single stream in which for efficient heating the percentage of air would be the optimum for obtaining complete combustion of the gaseous fuel, the gaseous fuel and air are introduced so as to form two separate streams of different volume and each comprising different ratios of gaseous fuel to air. One of the streams comprises an excess of air and the other stream comprises an excess of gaseous fuel. One of the streams is first ignited effecting a first stage of combustion. It then ignites the second stream effecting a second stage of combustion, and a third stage of combustion is provided by the mixing and burning of the excess gaseous fuel in one of the streams with the excess air in the other of the streams. The temperature created in each stage is less than would have been created if the same volumes of gas and air had been mixed and consumed in a single stage, but the total of heat produced is substantially the same as would have been produced in a single combustion stage.
The invention will be best understood if the following description is read in connection with the drawing in which:
FIG. 1 is a side elevation showing a burner having two sets of gas and air inlets disposed to provide two separate gaseous fuel/air mixture streams and means for igniting them, and the excess air from one stream and the excess gaseous fuel from the other stream, in sequence providing a cascading three stage combustion, and
FIG. 2 is a vertical section taken on the line 2--2 of FIG. 1.
DESCRIPTION
Great difficulty has been experienced in attempting to reduce the amount of nitrogen oxide produced in air-gas burners. It is well established by experiments that the production of nitrogen oxides is highest when the mixture of gaseous fuel and air employed is close to the optimum mixture for combustion, or, in other words close to the point where the temperature of combustion is the highest. Reference to the Proceedings, Conference on Natural Gas Research and Technology, sponsored by American Gas Association, Inc. and Institute of Gas Technology, held at Chicago, Ill., Feb. 28-Mar. 3, 1971, and particularly to Session IV, Paper 2, and the curve which is shown on page 20 of that Paper and is identified as FIG. 7., Variation of Nitrogen Oxides Concentration with Stoichiometry for a Premixed Natural Gas/Air Flame consuming 200 Standard Cubic Feet Per Hour of Gas, indicates that the maximum production of nitrogen oxide occurs when a percent of air slightly in excess of the optimum amount of air for combustion, about 107 percent of stoichiometric air, is mixed with the gaseous fuel, and that the production of nitrogen oxides falls off rapidly when the percentage of stoichiometric air by volume is further increased, or is decreased. The curve indicates that when the gas/air mixture burned comprises approximately 107 percent of stoichiometric air by volume the accompanying production of nitrogen oxides is 120 parts per million by volume, dry basis, whereas when 140 percent of stoichiometric air is employed the nitrogen oxides concentration falls to approximately 80 parts per million by volume. Similarly the curve indicates that if only 90 percent of stoichiometric air by volume is present in the gas/air mixture production of nitrogen oxides falls to approximately 70 parts per million by volume.
In order to reduce the temperature at the time of ignition and thereby substantially reduce the production of nitrogen oxides I have employed the information indicated by the said curve to limit and control the production of nitrogen oxides and at the same time obtain an efficient and substantially complete combustion. In order to accomplish this, instead of introducing into the burner a single mixture of gas and air, in accordance with my method two mixtures of gaseous fuel and air are introduced separately into the burner and ignited in sequence. One of the gaseous fuel-air mixtures comprises substantially more than 100 percent of stoichiometric air and the other gaseous fuel-air mixture comprises substantially less than 100 percent of stoichiometric air. The first mentioned mixture therefor comprises a substantial excess of air and the second gaseous fuel-air mixture comprises a substantial excess of gaseous fuel. When the two streams are ignited the temperature produced by each will be substantially less than the temperature produced by igniting a single gaseous fuel-air stream having the volume of the combined two streams, and comprising 100 percent of stoichiometric air. The temperature close to the burner face is therefor greatly reduced, the exact amount of reduction varying to some extent depending upon the gaseous fuel employed.
With a single stream of gaseous fuel/air, if the ratio is the optimum for obtaining complete combustion, the high flame temperature realized at the burner will fall off gradually as the gases travel to the flue. In the case of the two streams which are ignited in succession and comprise the same total volume of gaseous fuel/air as a single stream, the flame temperature initially adjacent the burner face is lower because of the excess air and excess fuel in the two streams, but the temperature of the gases is sustained by the combustion in the second and third stages, and that temperature will be substantially the same as that of the single stream when the gases of the two streams reach the furnace flue and therefor the total transfer of heat is essentially the same.
By properly proportioning the volume of the two gaseous fuel/air mixtures and the ratio of gaseous fuel to air in each mixture the excess of air in one of the mixtures can be proportioned to the amount of excess fuel in the other of the mixtures so that during the three stages of combustion complete or substantially complete combustion will be realized and a total production of heat obtained equal to, or substantially equal to the amount of heat which can be obtained from a single stream of gaseous fuel equal in volume to the gaseous fuel in the two streams and mixed with 100 percent of stoichiometric air, the essential difference being that by the single flame a flash temperature approximating 2,800° F. - 3,200° F. would be obtained whereas by employing three stages of combustion as described above a much lower temperature near the burner face is obtained, a temperature which in the case of natural gas would be in the neighborhood of 2,100° F. - 2,400° F. Reference to FIG. 8 on page 21 of Paper 2, Session IV of the aforesaid Proceedings of the Conference on Natural Gas Research and Technology shows that close to the burner face a higher temperature results in a proportionately greater concentration of nitrogen oxides measured in parts per million by volume on a dry basis.
In the embodiment illustrated in the drawings a burner 10 comprises a burner face 12 disposed between air and gaseous fuel supply conduits 14, 16 and 18, 20 and a burner tile chamber 22 which at its far end opens into the combustion chamber 24 of a furnace or the like indicated generally by the numeral 26. As shown the burner tile chamber 22 projects into the furnace combustion chamber 24 and a wall 28 of the furnace extends around the burner tile chamber.
Gaseous fuel conduit 14 and the surrounding air conduit 16 communicate with the interior of the burner tile chamber 22 through orifices 30 and 32 respectively in the burner face 12. Gas and air is supplied under pressure through said conduits to mix adjacent the burner face and provide a first gaseous fuel/air stream. This stream is ignited by a suitable ignition means 36, preferably disposed at the burner face, and it may have the ignition wire 36a extending to it through conduit 14, and the flame is directed into the tubular and perforate choke collar 34.
Gaseous fuel conduit 18 and the air conduit 20 communicate with the interior of the burner tile chamber through the orifices 40 and 42. Gas and air is supplied through conduits 18 and 20 respectively to mix and form a second gaseous fuel/air stream which is concentric with, and spaced radially outwardly of, the first stream. The second stream is not directed into the choke collar 34 but flows around it, and is ignited after the first stream by the flame of the first stream issuing through the perforations 35 in the choke collar 34. The excess of fuel in one stream mixes with the excess air in the other stream and is ignited within the merged flame 44 resulting from the sequential ignition of the two streams thus providing a third stage of ignition.
It will be noted that wall 46 separates the two sets of conduits which supply the two streams, and that orifices 40 are inclined to direct the gaseous fuel from conduit 18 to mix with air from conduit 20 and not to be injected into the choke collar 34. The burner tile is shaped so as to bring the excess fuel into the center stream thus mixing it with the excess air causing them to burn and thus release all of the heat from the fuel supplied in the two streams.
It will be understood that if desired the two streams of gaseous fuel/air may be premixed before entering the burner.
The conduits 14 and 16 may be employed to provide a constant minimum supply of gaseous fuel and the conduits 18 and 20 may be controlled to increase the fuel supply as may be desired.
To illustrate my method more specifically I prefer to use two mixed streams of gaseous fuel/air which differ in volume by the ratio of one to three, with a 1 to 1.75 proportion of gas to stoichiometric air in the smaller stream, and a 1 to 0.75 proportion of gas to stoichiometric air in the larger stream. This means that in the smaller stream there will be 75 percent excess air, and in the larger stream there will be a 25 percent deficiency of air, or stated oppositely a 25 percent excess of gas. When these two streams are ignited in sequence all of the gas in the smaller stream will be ignited and burned close to the burner leaving approximately 75 percent unconsumed air which will flow outwardly within the burner tile chamber and the combustion chamber. All of the air in the second stream will combine with gas in the second stream and will be ignited and burned a little further from the burner face, leaving an excess of 25 percent of gas which will flow outwardly within the burner tile chamber and the combustion chamber and become mixed with the excess air from the first stream. Since the volume of the larger stream is three times as great as that of the first stream there will be relative volumes of gaseous fuel and air which are the optimum for complete combustion and this mixture will burn in a third stage of combustion which is a continuation of the first and second stages, the three stages occurring at progressively greater distances from the burner face. Thus in successive stages complete or substantially complete combustion of the total amount of gaseous fuel and air will be achieved, but the temperature of each stage will be very substantially less than in the case of a single stage of combustion employing 100 percent stoichiometric air and the same total volume of gaseous fuel, and the total amount of heat produced will be the same or approximately the same as would have been obtained from a single stage of combustion at a much higher initial temperature.
LIST OF PARTS
10 burner
12 burner face
14 gaseous fuel conduit
16 air conduit
18 gaseous fuel conduit
20 air conduit
22 burner tile chamber
24 combustion chamber
26 furnace
28 wall of 26
30 orifice for conduit 14
32 orifices for conduit 16
34 choke collar
35 perforations in 34
36 ignition means
36a ignition wire
40 orifices for conduit 18
42 orifices for conduit 20
44 merged flame
46 wall of burner