EP0076036B1

EP0076036B1 - Method and apparatus for burning fuel in stages

Info

Publication number: EP0076036B1
Application number: EP82304508A
Authority: EP
Inventors: Richard R. Martin; Kurt S. Jaeger
Original assignee: John Zink Co
Current assignee: Zinklahoma Inc
Priority date: 1981-09-28
Filing date: 1982-08-26
Publication date: 1987-04-29
Also published as: DE3276191D1; EP0076036A1; CA1212617A; JPS5875606A

Description

This invention relates to a method and apparatus for burning fuel resulting in low NO,, formation. More specifically, this invention relates to a staged fuel and air injection burner.
With the advent of contemporary environmental emission standards being imposed by various governmental authorities and agencies involving ever stricter regulations, methods and apparatus to suppress the formation of oxides of nitrogen during combustion with air are becoming increasingly numerous. Various techniques have been suggested and employed in the design and operation of burners and furnaces to meet these regulations. Thus it is known that to burn a hydrocarbon fuel in less than a stoichiometric concentration of oxygen intentionally produces a reducing environment of CO and H₂. This concept is utilized for example in EP-A-0006358 and US-A-4004875 in a staged air type low NOx burner wherein the fuel is first burned in a deficiency of air in one zone producing a reducing environment that suppresses NO_X formation and then the remaining portion of the air is added in a subsequent zone. Staged fuel also has been suggested by Hitachi Zosen of Japan in brochure No. D-148 of July 1978 wherein all of the air and some of the fuel is burned in the first zone and then the remaining fuel is added in the second zone. The presence of an over abundance of air in the first reaction zone acts as a diluent thus lowering the temperature and suppressing formation of NO_X. It has also been proposed to recirculate flue gas to accomplish the lowering of the flame temperature.
However, each of the prior art processes have certain inherent deficiencies and associated problems which have led to limited commercial acceptance. For example, when burning fuel in'a sub-stoichiometric oxygen environment the tendency for soot formation is increased. The presence of even small amounts of soot will alter the heat transfer properties of the furnace and heat exchanger surfaces downstream from the burner. Also flame stability can become a critical factor when operating a burner at significantly sub-stoichiometric conditions.
It is an object of the present invention to provide a method and apparatus for burning a hydrocarbon fuel resulting in reduced emission of nitrogen oxides generated by the combustion.
According to the present invention a method for burning fuel-air mixture for a furnace chamber that will result in low NO_x products of combustion comprising the steps of supplying, at a given instant of burning, a given total amount of pressurized fuel and a given total amount of air, the total amount of air being at least substantially stoichiometrically sufficient to burn the total amount of fuel supplied to a burner, is characterized in that a primary reaction zone is created in the burner that begins outside of the furnace chamber but extends into the furnace chamber by supplying a first portion of the total fuel and a portion of the total air which exceeds the stoichiometric requirements for burning the first portion of the total fuel, separately injecting the remaining portion of the total fuel and the remaining portion of the total air into the furnace chamber so as to create a substantially unconfined secondary reaction zone about and reacting with a substantial portion of the primary reaction zone, the remaining portion of the total air being less than the stoichiometric requirement to burn the remaining portion of the total fuel. The fuel may be proportioned from about 40% to 60% to the primary reaction zone and then from about 60% to 40% to the second reaction zone while the air is proportioned from about 80% to 95% to the primary zone (preferably 90%) and from about 20% to 5% to the secondary zone (preferably 10%).
Also according to the present invention a fuel-air burner for a furnace chamber, comprising an air-fuel mixing and injection burner housing attached to the furnace wall such that th.e downstream face of the burner housing terminates substantially adjacent an inner wall of the furnace chamber, means being provided to supply to the housing at a given instant of burning, a given total amount of fuel under pressure and a given total amount of air, the total amount of air being at least substantially stoichiometrically sufficient to burn the total amount of fuel supplied to the burner housing, means being provided to create a primary reaction burning zone that begins in an enclosed space upstream of the inner wall and extends downstream of the inner wall into the furnace chamber and at least one secondary fuel supply and injection nozzle disposed adjacent the enclosed space, is characterized by means for supplying, to the primary burning zone, a first portion of the total fuel and a portion of the total air which exceeds the stoichiometric requirements for burning the first portion of fuel thereto, at least one conduit through the burner housing located adjacent the enclosed space, the or each conduit, providing communication between the total air supply and the furnace chamber, a secondary fuel supply and an injection nozzle being positioned within the or each conduit such that there is passage of air thereabout, the nozzle terminating adjacent the inner wall of the burner housing, means for supplying the remaining portion of the total fuel to the secondary fuel supply and injection nozzle and means for supplying the remaining portion of the total air through the conduit surrounding the nozzle, such that the remaining portion of the total air is less than the stoichiometric requirements to burn the remaining portion of the total fuel, the nozzle directing the remaining portion of the total fuel and the remaining portion of the total air so as to contribute to the formation of a substantially unconfined secondary reaction burning zone about and reacting with a substantial portion of the effluent of the primary-reaction zone within the furnace chamber.
The invention will now be described further, by way of example, with reference to the accompanying drawings, in which:-

Fig. 1 is a vertical cross-section of one embodiment of the invention illustrating a T-bar primary nozzle and a pair of secondary nozzles;
Fig. 2 is a side-view cross-section of the T-bar primary nozzle of Fig. 1;
Fig. 3 is a lower end view of the burner of Fig. 1;
Fig. 4 illustrates the orifice configuration of the secondary nozzles for the burner illustrated in Fig. 1;
Fig. 5 is a cross-sectional view of an alternative embodiment of this invention illustrating a domed nozzle;
Figs. 6, 7 and 8 illustrate alternative secondary nozzle and orifice configurations for the burner of Fig. 5;
Fig. 9 is another embodiment ilustrating a pair of flat flame design primary nozzles; and
Fig. 10 is a graph showing the NOx levels achieved for each set of tips at various fuel split ratios.

In the embodiment shown in Figs. 1, and 3 the burner is indicated generally by the numeral 10. This particular embodiment involves a primary burnertube 12 leading to a T-bar primary nozzle 14 along with a pair of secondary burner tubes 16 and secondary burner nozzles 18 all being supplied hydrocarbon fuel from a common source through tube 20. The fuel exiting primary nozzle 14 enters the primary combustion zone 22 wherein it is burned in the presence of a significant stoichiometric excess of air flowing through the interior 24 of the burner and entering the primary reaction zone 22 through an annular space 26 surrounding the primary nozzle 14, as indicated by the presence of arrows.
The effluent from the primary reaction zone 22 enters a larger secondary reaction zone 28. Simultaneously, the fuel exiting the secondary nozzle 18 is mixed with air from the interior 24 of the burner 10 passing through annular conduits 30 surrounding burner tubes 16 and is then burned in the secondary reaction zone 28 in the presence of the effluent from the first reaction zone 22.
The orifices of the respective T-bar nozzle 14and secondary nozzles 18 are sized such that the fuel is proportioned between the primary reaction zone and the secondary reaction zone. Preferably from about 40 to about 60% of the fuel is directed through the primary nozzle 14 and the remaining fuel is directed to the secondary nozzles 18. Similarly, the cross-sectional area of the annular space 26 and the annular conduits 30 for conducting air to primary and secondary reaction zones are selectedsuch as to deliver about 80% to 95% of the total airto the primary reaction zone 22 and the remaining 20% to 5% of the total air to the secondary reaction zone 28.
Fig. 4 illustrates the directional characteristics of the orifices of each secondary nozzle 18. As illustrated, the five fuel ports 32 will issue a fan like sheet of fuel directed towards the effluent of the primary combustion zone.
In Fig. 5 an alternate forced draft burner 10 is illustrated involving a single gas nozzle 34 that directs the fuel delivered through conduit 36 into the primary combustion zone 3.8 defined by the refractory walls 40 of the burner. Riser pipes 42 fitted with orifice tips 44 extend through this refractory wall 40 such as to deliver the secondary fuel to the secondary combustion zone 46. Similar to Fig. 1, combustion air flows through the interior 48 of burner 10 into the primary zone 38 by way of annular conduit 49 and into secondary combustion zone 46 through annular openings 50. Figs. 6, 7 and 8 illustrate the basic orifice or port configuration 52 of the secondary nozzles 44 including alternate angles of inclination (Fig. 7) towards the axial direction of the flow in the primary reaction zone 38.
Fig. 9 illustrates another alternate embodiment of a staged fuel and air burner 10 of the present invention wherein the particular burner is a flat flame design involving a pair of primary nozzles 54 and 56 each essentially adjacent to the refractory walls forming the primary reaction zone 58. Similar to the previous embodiments, secondary fuel conduits 60 and 62 pass through the refractory material such as to deliver fuel to the secondary reaction zone 64.
In order to evaluate the principle of separating the gaseous fuel into two essentially equal but sequential burning stages wherein a significant stoichiometric excess or major portion of the air is employed in the first stage with the remaining minor portion of the air in the second stage, a series of tests were conducted using a burner configuration as illustrated in Fig. 5. The burnerwas of a forced draft design using natural gas. A center mounted gas gun was mounted to fire inside a refractory chamber. Four riser pipes fitted with orifice tips were installed through the refractory wall of the combustion chamber parallel to the center line of the burner. Three sets of tips were tested, each having orifices discharging at different angles to the tip centerline. The burner was tested by firing vertically upward into a furnace.
Three series of tests were conducted; one series for each set of secondary riser tip drillings. The tip drillings included three orifices and were oriented in the first series discharging vertically upward (parallel to the centerline of the burner), in the second series discharging at a small angle, e.g. 15° off vertical (towards the burner centerline) and in the third series discharging 30° off vertical (towards the burner centerline). Each test series of each set of tips included variations of primary/ secondary fuel ratio and turned down tests.
Fig. 10 shows the graphs plotted as a result of the tests. The burner was also fired on center gas only to establish the base point for non-staged operation of 80 ppm NO_x. The lowest NO,, levels were obtained with secondary orifices discharging parallel to the burner axis, but this set of tips also produces the highest level of combustibles. Turn down on 30° tips was about 3:1 on a fifty/fifty fuel split and turn down on 15° tips was about 2:1 on a forty/sixty split. Flame appearance was generally good on all arrangements. From the data and test results it is readily apparent that the basic concept of staged air and fuel combustion is capable of producing NOX levels significantly lower than conventional combustion. The test results have also established that these low NO_X levels are achieved in the absence of significant soot formation or flame instability. Additional advantages of the present invention include the fact that the NO_X levels achieved are lower than those associated with staged air combustion and the fact that the basic concept of staged air and fuel is compatible with a wide variety of types of burners.

Claims

1. A fuel-air burner for a furnace chamber, comprising an air-fuel mixing and injection burner housing attached to the furnace wall such that the downstream face of the burner housing terminates substantially adjacent an inner wall of the furnace chamber, means being provided to supply to the housing at a given instant of burning, a given total amount of fuel under pressure and a given total amount of air, the total amount of air being at least substantially stoichiometrically sufficient to burn the total amount of fuel supplied to the burner housing, means being provided to create a primary reaction burning zone that begins in an enclosed space upstream of the inner wall and extends downstream of the inner wall into the furnace chamber and at least one secondary fuel supply and injection nozzle disposed adjacent the enclosed space, characterized by means for supplying, to the primary burning zone, a first portion of the total fuel (14, 34, 54, 56) and a portion of the total air (26, 49) which exceeds the stoichiometric requirements for burning the first portion of fuel thereto, at least one conduit (30, 50) through the burner housing located adjacent the enclosed space (22, 38, 58), the or each conduit (30, 50), providing communication between the total air supply and the furnace chamber (28, 46, 64), a secondary fuel supply (16, 42, 60, 62) and an injection nozzle (18, 44) being positioned within the or each conduit (30, 50) such that there is passage of air thereabout, the nozzle terminating adjacent the inner wall of the burner housing (40), means for supplying the remaining portion of the total fuel to the secondary fuel supply (16, 42, 60, 62) and injection nozzle (18, 44) and means for supplying the remaining portion of the total air through the conduit (30, 50) surrounding the nozzle (18, 44), such that the remaining portion of the total air is less than the stoichiometric requirements to burn the remaining portion of the total fuel, the nozzle (18, 44) directing the remaining portion of the total fuel and the remaining portion of the total air so as to contribute to the formation of a substantially unconfined secondary reaction burning zone about and reacting with a substantial portion of the effluent of the primary reaction zone within the furnace chamber (28, 46, 64).

2. A burner according to claim 1, characterized in that it includes means for supplying within the range of about 40% to about 60% of the total fuel to the primary reaction zone and about 60% to about 40% of the fuel being supplied to the secondary reaction zone.

3. A burner according to claim 1 or 2, characterized in that it includes means for supplying in the range from about 80% to about 95% of the total air to the primary reaction zone.

4. A method for burning fuel-air mixture for a furnace chamber that will result in low NO_X products of combustion comprising the steps of supplying, at a given instant of burning, a given total amount of pressurized fuel and a given total amount of air, the total amount of air being at least substantially stoichiometrically sufficient to burn the total amount of fuel supplied to a burner, characterized in that a primary reaction zone is created in the burner that begins outside of the furnace chamber but extends into the furnace chamber by supplying a first portion of the total fuel and a portion of the total air which exceeds the stoichiometric requirements for burning the first portion of the total fuel, separately injecting the remaining portion of the total fuel and the remaining portion of the total air into the furnace chamber so as to create a substantially unconfined secondary reaction zone about and reacting with a substantial portion of the primary reaction zone, the remaining portion of the total air being less than the stoichiometric requirement to burn the remaining portion of the total fuel.

5. A method according to claim 4, characterized in that the fuel is proportioned from about 40% to about 60% to the primary reaction zone and from about 60% to about 40% to the second reaction zone.

6. A method according to claim 4 or 5 characterized in that the portion of air in the primary reaction zone is from about 80% to 95% of the total air.