WO2009061622A1

WO2009061622A1 - Burner control

Info

Publication number: WO2009061622A1
Application number: PCT/US2008/081063
Authority: WO
Inventors: Bruce E. Cain
Original assignee: NORTH AMERICAN MANUFACTURING Co Ltd
Current assignee: NORTH AMERICAN MANUFACTURING Co Ltd
Priority date: 2007-11-07
Filing date: 2008-10-24
Publication date: 2009-05-14
Anticipated expiration: 2010-05-07
Also published as: US20090117503A1

Abstract

A reactant supply and control system supplies a regenerative burner assembly with streams of pilot fuel and pilot air. The system can maintain a pilot flame throughout regenerative cycles in which a main flame is turned on and off, and supplies either or both of the pilot streams with an increased flow rate for shifting from a regenerative exhaust condition to a regenerative firing condition.

Description

BURNER CONTROL

TECHNICAL FIELD This technology relates to the operation of a burner for a furnace.

BACKGROUND

Regenerative burners may be used to heat a process chamber in a furnace. Each regenerative burner has a bed of heat-regenerative material, and is arranged in a pair with another regenerative burner. The two burners are cycled alternately such that one burner is actuated while the other is not. When a burner is actuated, it discharges fuel and combustion air into the process chamber for combustion to proceed in the process chamber. Much of the combustion air is pre-heated by driving it through the regenerative bed. Alternately, when a burner is not actuated, exhaust gases from the process chamber are drawn outward through the regenerative bed at that burner. The exhaust gases heat the regenerative bed to provide the thermal energy that pre-heats the combustion air when the burner is again actuated to fire into the process chamber.

SUMMARY A reactant supply and control system supplies a regenerative burner assembly with streams of pilot fuel and pilot air. The system can maintain a pilot flame continuously throughout consecutive regenerative cycles in which a main flame is turned on and off, and can supply either or both of the pilot streams with flow rates that differ between a regenerative exhaust condition and a regenerative firing condition. This can help to ensure that the pilot flame ignites a main flame for each regenerative firing condition. Lower flow rates of pilot reactants in the regenerative exhaust conditions can reduce fuel consumption and exhaust emissions. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic view showing parts of a furnace with regenerative burners.

Figure 2 is a perspective view of a regenerative burner assembly shown in Fig. 1. Figure 3 is a sectional view of the burner assembly of Fig. 2. Figure 4 is a sectional view of a part shown in Fig. 3.

Figure 5 is a side view of another part shown in Fig. 3. Figure 6 is a sectional view taken on line 6-6 of Fig. 5. Figure 7 is a rear view taken on line 7-7 of Fig. 5. Figure 8 is a front view taken on line 8-8 of Fig. 5. Figure 9 is a schematic view illustrating an operational feature of the furnace of Fig. 1.

Figure 10 also is a schematic view illustrating an operational feature of the furnace of Fig. 1.

DETAILED DESCRIPTION The furnace 10 shown in the drawings has parts that are examples of the elements recited in the claims. The following description thus includes examples of how a person of ordinary skill in the art can make and use the claimed invention. It is presented here to meet the statutory requirements of written description, enablement, and best mode without imposing limitations that are not recited in the claims.

As shown partially in the schematic view of Fig. 1, the furnace 10 has a wall structure 12 defining a process chamber 15. Burner assemblies 16, one of which is shown in Fig. 1, are arranged in pairs in which one burner assembly 16 fires into the process chamber 15 while the other exhausts from the process chamber 15. Each burner assembly 16 is mounted over a respective regenerative bed 18. When a burner assembly 16 fires into the process chamber 15, it receives preheated combustion air from the regenerative bed 18. Alternately, when a burner assembly 16 exhausts from the process chamber 15, it directs exhaust gases into the regenerative bed 18. This heats the regenerative bed 18 which, in turn, heats the combustion air when the burner assembly 16 once again fires into the process chamber 15.

As shown in Fig. 2, this example of a burner assembly 16 has a generally cylindrical body 20 with a central axis 21. A primary port 25 is centered on the axis 21 at the front of the body 20. A reactant delivery structure 26 extends along the axis 21 from the rear of the body 20 toward the primary port 25. Secondary ports 27 also are located at the front of the body 20. Air flow passages within the body 20 communicate the secondary ports 27 with a base 28 at the bottom of the body 20. The base 28 is configured to communicate with the regenerative bed 18 (Fig. 1).

As shown in Fig. 3, the reactant delivery structure 26 includes an array of concentric conduits centered on the axis 21. These include a pilot fuel conduit 30 at the center of the array. The pilot fuel conduit 30 has an inlet 32 at its rear end and an outlet 34 at its front end. A flame stabilizer 36 projects from the outlet 34. Surrounding the pilot fuel conduit 30 is a pilot air conduit 40 with an inlet 42 at its rear end and an outlet 44 at its front end. A section 46 of the pilot air conduit 40 is located forward of the pilot fuel conduit 30. That section 46 is tapered radially inward to promote the mixing of fuel and air axially between the stabilizer 36 and the outlet 44. In this arrangement, these two conduits 30 and 40 together define a pilot burner that is configured to provide a pilot flame that projects axially forward from the outlet 44. A main fuel conduit 50 surrounds the pilot air conduit 40. A primary air conduit 52 surrounds the main fuel conduit 50. These conduits 50 and 52 have inlets 54 and 56 at their rear ends and outlets 58 and 60 at their front ends, respectively. This provides a main burner that is configured to provide a main flame that projects axially forward from the outlets 58 and 60. In the illustrated example, the concentric outlets 44, 58 and 60 are coplanar and radially adjacent. More specifically, the pilot burner outlet 44 is the circular space bounded by the surrounding edge of the pilot air conduit 40. It is spaced radially inward from the main fuel outlet 58 by only the thickness of the conduit 40 that is interposed radially between those two outlets 44 and 58. The main fuel outlet 58 is the annular space bounded by the concentric edges of the pilot air conduit 40 and the main fuel conduit 50. That outlet 58 is spaced radially inward from the surrounding outlet 60 by only the thickness of the main fuel conduit 50. The primary air outlet 60 likewise has an annular configuration defined by and between the concentric edges of the main fuel conduit 50 and the primary air conduit 52. The cylindrical body 20 in the illustrated example has three major portions. These include a rear portion 70, a central portion 72, and a front portion 74. The rear portion 70 includes a refractory structure 80 within a steel shell 82. Lower portions of those parts 80 and 82 define the base 28 at which the burner assembly 16 is mounted over a regenerative bed. The refractory structure 80 within the steel shell 82 defines a plenum 85 extending upward from a port 87 at the lower end of the base 28. The refractory structure 80 further defines a generally conical pocket 89 (Fig. 4) that is centered on the axis 21.

As shown separately in Figs. 5-8, the central portion 72 of the body 20 includes a refractory structure configured as a baffle 90. The baffle 90 in this particular example has a generally conical configuration centered on an axis 93. The primary port 25 is located on a circular front surface 98 of the baffle 90. A cylindrical bore 100 (Fig. 6) extends into the baffle 90 along the axis 93. A tapered bore 101 extends forward from the cylindrical bore 100, and has a front end at the primary port 25. The tapered bore 101 constricts radially inward from the cylindrical bore 100, and then flares radially back outward to the primary port 25. The secondary ports 27 also are located on the circular front surface 98 of the baffle 90. Two pairs 104 and 106 of air flow passages extend from the rear of the baffle 90 to the secondary ports 27 at the front surface 98. As shown in Fig. 8, the secondary ports 27 are arranged in an array that is asymmetrical with respect to a plane 109 containing the central axis 93. In this arrangement of the secondary ports 27, at least a major portion of their combined flow area is located at one side of the circular area of the front surface 98.

As shown in Fig. 3, the baffle 90 is fitted coaxially within the pocket 89 at the rear portion 70 of the body 20. The front portion 74 of the body 20 includes a ring-shaped refractory structure 122 that surrounds and projects axially forward from the baffle 90. The reactant delivery structure 26 extends fully into the cylindrical bore 100 in the baffle 90, with the coplanar outlets 44, 58 and 60 facing outward through the tapered bore 101 toward the primary port 25. The air flow passages 104 and 106 extending through the baffle 90 communicate the secondary ports 27 with the plenum 85 and the port 87 at the lower end of the base 28.

Referring again to Fig. 1, the furnace 10 has a reactant supply and control system 140. This system 140 connects the furnace 10 with a source of fuel 142, which is preferably the plant supply of natural gas, and a source of combustion air 144, which may include one or more blowers. The reactant supply and control system 140 includes a controller 146 and a valve assembly 148. It further includes fuel lines and air lines that connect each burner assembly 16 with the valve assembly 148 in the manner shown schematically in Fig. 1. The burner assemblies 16 and regenerative beds 18 are thus connected with the sources of fuel and air 142 and 144 for combustion to proceed in the process chamber 15, and are also connected with a flue 150 for discharging exhaust gases from the process chamber 15. Specifically, a pilot fuel line 160 delivers pilot fuel to the inlet 32 of the pilot fuel conduit 30. A pilot air line 162 delivers pilot air to the inlet 42 of the pilot air conduit 40. A main fuel line 164 delivers main fuel to the inlet 54 of the main fuel conduit 50. A primary air line 166 delivers primary combustion air to the inlet 56 of the primary air conduit 52. Moreover, a secondary air line 170 carries secondary combustion air to the regenerative bed 18, and an exhaust line 172 carries exhaust gases away from the regenerative bed 18 for transmission to the flue 150.

The controller 146 has hardware, software, or a combination of hardware and software that is configured to control the valve assembly 148. The controller 146 may thus comprise any suitable programmable logic controller or other control device, or combination of control devices, that is programmed or otherwise configured to perform as recited in the claims. As the controller 146 carries out those instructions, it actuates the valve assembly 148 to initiate, modulate, and terminate independent flows of reactant streams through the burner assembly 16. In one particular example of a start-up sequence, the controller 146 first directs the valve assembly 148 to supply the reactant delivery structure 26 with streams of pilot fuel, pilot air, and primary air, and also actuates an igniter (not shown). This causes a pilot flame to project axially forward toward the primary port 25 (Fig. 3). The controller 146 then monitors a pilot flame supervisory device 180 for a period of time, such a five seconds, to confirm the presence of the pilot flame. If the pilot flame is not confirmed, the controller 146 directs the valve assembly 148 to terminate the stream of pilot fuel. If the pilot flame is confirmed, the controller 146 next directs the valve assembly 148 to supply the reactant delivery structure 26 with a stream of main fuel. The main fuel stream flows through the main fuel conduit 50 to emerge from the annular outlet 58 over the pilot flame. The primary air stream flowing through the primary air conduit 52 emerges from the annular outlet 60 over the main fuel stream. The streams of main fuel and primary air begin to mix as they flow together through the tapered bore 101 toward the primary port 25, and continue to mix as they flow outward from the port 25 into the process chamber 15. The mixture surrounds, ignites and begins to combust over the pilot flame. As shown schematically in Fig. 9, this stage of combustion occurs in a primary reaction zone 185 defined by the main fuel and primary air streams as they form a main flame projecting axially and radially outward from the primary port 25.

Secondary combustion air flows through the secondary air line 170 to the regenerative bed 18. The plenum 85 (Fig. 3) receives the secondary combustion air as it flows from the regenerative bed 18 upward through the port 87 in the base 28. The air flow passages 104 and 106 in the baffle 90 (Fig. 5-7) convey the secondary combustion air from the plenum 85 to the secondary ports 27. The air streams emerging from the secondary ports 27 enable secondary combustion to occur in the process chamber 15 at locations spaced axially downstream from the baffle 90. This occurs as the secondary air streams project axially and radially outward from the secondary ports 27 to form secondary reaction zones 187 (one of which is shown schematically in Fig. 10) where they intersect the primary reaction zone 185. With the combined flow area of the secondary ports 27 located on one side of the plane 109 (Fig. 8), the secondary reaction zones 185 are likewise located predominantly on that side of the plane 109. This enables a recirculation zone 189 to form beside the front surface 98 of the baffle 90 on the opposite side of the plane 109. A main flame supervisory device 186 monitors combustion in the primary reaction zone

185. If the main flame supervisory device 186 fails to confirm combustion of the main fuel and primary air, the controller 146 directs the valve assembly 148 to terminate the main fuel stream. If combustion of the main fuel and primary air is confirmed, the controller 146 directs the valve assembly 148 to continue supplying those reactant streams to maintain a regenerative firing condition until the burner assembly 16 is switched to a regenerative exhaust condition.

The flame supervisory devices 180 and 186, which may be UV or other sensors for detecting a flame, are configured in a known manner for the pilot sensor 180 to sense the pilot flame, or optionally to sense both the pilot flame and the main flame, and for the main sensor 186 to sense the main flame but not the pilot flame. This prevents the pilot frame from being mistaken for a main flame, which permits the pilot flame to be maintained continuously throughout consecutive cycles in which the main flame is turned on and off for regenerative operation of the burner assembly 16. The reactant supply and control system 140 is configured accordingly. Specifically, when the burner assembly 16 is in the regenerative exhaust condition, the pilot sensor 180 senses the pilot flame but the main sensor 186 does not sense a main flame. The controller 146 directs the valve assembly 148 to supply the reactant delivery structure 26 with first streams of pilot fuel and pilot air in that condition. Since there is no stream of main fuel and no need for a main flame, either or both of the first streams of pilot fuel and pilot air can have a flow rate that is lower than the flow rate ordinarily provided for the pilot flame to ignite a main flame.

The system 140 is further configured to continue supplying pilot fuel and pilot air to the reactant delivery structure 26 to maintain the pilot flame, but to shift from the first streams to second streams that differ from the first streams when the burner assembly 16 is being shifted from a regenerative exhaust condition to a regenerative firing condition. The controller 146 then directs the valve assembly 148 to supply a main fuel stream, and also to provide either or both of the second streams of pilot fuel and pilot air with a flow rate that exceeds the corresponding first stream flow rate sufficiently to ensure that the pilot flame ignites a main flame. The increased pilot flow rate or rates can be shifted back to a lower level during the regenerative firing condition, when shifting to the next subsequent regenerative exhaust condition, or during the next subsequent regenerative exhaust condition. In each case lower pilot flow rates can reduce both fuel consumption and exhaust emissions as the pilot streams are continued without interruption but are shifted back and forth between the first and second flow rates throughout multiple cycles of shifting back and forth between the regenerative exhaust condition and the regenerative firing condition.

The patentable scope of the invention is defined by the claims, and may include other examples of how the invention can be made and used. Such other examples, which may be available either before or after the application filing date, are intended to be within the scope of the claims if they have elements that do not differ from the literal language of the claims, or if they have equivalent elements with insubstantial differences from the literal language of the claims.

Claims

1. An apparatus comprising: a regenerative bed; a burner assembly in air flow communication with the regenerative bed; and a reactant supply and control system configured a) to shift from a regenerative exhaust condition in which the system does not supply the burner assembly with a main fuel stream to a regenerative firing condition in which the system supplies the burner assembly with a main fuel stream, b) to supply the burner assembly with a flow rate of pilot fuel and a flow rate of pilot air in the regenerative exhaust condition, and c) to supply the burner assembly with a different flow rate of pilot fuel, or a different flow rate of pilot air, or different flow rates of both pilot fuel and pilot air, in the regenerative firing condition.

2. An apparatus as defined in claim 1 wherein each of the different flow rates in the regenerative firing condition is greater than the corresponding flow rate in the regenerative exhaust condition.

3. An apparatus as defined in claim 1 wherein the reactant supply and control system is configured to shift back and forth between the flow rates throughout consecutive cycles of shifting back and forth between the regenerative exhaust condition and the regenerative firing condition.

4. An apparatus as defined in claim 3 wherein the reactant supply and control system is configured to supply the pilot burner with pilot fuel and pilot air to form a pilot flame continuously throughout the consecutive cycles of shifting back and forth between the regenerative exhaust condition and the regenerative firing condition.

5. An apparatus comprising: a regenerative bed; a pilot burner configured receive streams of pilot fuel and pilot air to form a pilot flame; a main burner configured to receive streams of main fuel and primary air to be ignited by the pilot flame and thereby to form a main flame; a first supervisory device configured to sense the pilot flame; a second supervisory device configured to sense the main flame but not to sense the pilot flame; and a reactant supply and control system configured a) to alternate between a regenerative exhaust condition and a regenerative firing condition, b) to identify a first condition in which the first supervisory device senses the pilot flame and the second supervisory device does not sense a main flame, c) to identify a second condition in which the second supervisory device senses a main flame, d) to supply the pilot burner with a flow rate of pilot fuel and a flow rate of pilot air in the first condition, and e) to supply the pilot burner with a different flow rate of pilot fuel, or a different flow rate of pilot air, or different flow rates of both pilot fuel and pilot air, in the second condition.

6. An apparatus as defined in claim 5 wherein each of the different flow rates is an increased flow rate.

7. An apparatus as defined in claim 5 wherein the first condition is a regenerative exhaust condition and the second condition is a regenerative firing condition.

8. An apparatus as defined in claim 7 wherein the reactant supply and control system is configured to shift back and forth between the flow rates throughout consecutive cycles of shifting back and forth between the regenerative exhaust condition and the regenerative firing condition.

9. An apparatus as defined in claim 8 wherein the reactant supply and control system is configured to supply the pilot burner with pilot fuel and pilot air to form a pilot flame continuously throughout the consecutive cycles of shifting back and forth between the regenerative exhaust condition and the regenerative firing condition.

10. An apparatus comprising: a pilot burner configured to receive fuel and air flows to form a pilot flame; a reactant supply and control system configured to supply the pilot burner with a first combination of fuel and air flows, and a second combination of fuel and air flows distinct from the first combination by virtue of differing amounts of either or both of fuel flow and air flow, and capable of switching between the two combinations; a main burner configured to receive streams of main fuel and primary air to be ignited by the pilot flame and thereby to form a main flame; a first supervisory device configured to sense the pilot flame under either combination of fuel and air flows, regardless of whether or not the main fuel stream is being delivered to the main burner; and a secondary supervisory device configured to sense the main flame when the main fuel stream is present, but which will not sense the pilot flame when the main fuel stream is absent and the pilot burner is being supplied with the first combination of fuel and air flows; wherein the reactant supply and control system is further configured a) to identify a first condition in which the first supervisory device senses the pilot flame, the second supervisory device does not sense the main flame, and there is no main fuel stream supplied to the main burner, b) to identify a second condition in which the second supervisory device senses the main flame, c) to supply the pilot burner with the first combination of fuel and air flows in the first condition, and d) to supply the pilot burner with the second combination of fuel and air flows in the second condition.

11. An apparatus as defined in claim 10 wherein the reactant supply and control system is configured to shift back and forth between the first and second combinations throughout consecutive cycles of shifting back and forth between the first and second conditions.

12. An apparatus as defined in claim 11 wherein the reactant supply and control system is configured to supply the pilot burner with fuel and air flows to form a pilot flame continuously throughout the consecutive cycles of shifting back and forth between the first and second conditions.

13. An apparatus as defined in claim 10 further comprising a regenerative bed in air flow communication with the reactant supply and control system, and wherein the first condition is a regenerative exhaust condition and the second condition is a regenerative firing condition.

14. A method comprising: shifting a reactant supply and control system from a regenerative exhaust condition in which the system does not supply a burner assembly with a main fuel stream to a regenerative firing condition in which the system supplies the burner assembly with a main fuel stream and with preheated air from a regenerative bed; operating the reactant supply and control system to supply the burner assembly with a flow rate of pilot fuel and a flow rate of pilot air in the regenerative exhaust condition; and operating the reactant supply and control system to supply the burner assembly with a different flow rate of pilot fuel, or a different flow rate of pilot air, or different flow rates of both pilot fuel and pilot air, in the regenerative firing condition.

15. A method as defined in claim 14 wherein each of the different flow rates in the regenerative firing condition is greater than the corresponding flow rate in the regenerative exhaust condition.

16. A method as defined in claim 14 wherein the reactant supply and control system is shifted back and forth between the flow rates throughout consecutive cycles of shifting back and forth between the regenerative exhaust condition and the regenerative firing condition.

17. A method as defined in claim 16 wherein the reactant supply and control system is operated to supply the burner assembly with pilot fuel and pilot air continuously throughout the consecutive cycles of shifting back and forth between the regenerative exhaust condition and the regenerative firing condition.

18. A method of operating an apparatus comprising a pilot burner configured to receive streams of pilot fuel and pilot air to form a pilot flame, and a main burner configured to receive streams of main fuel and primary air to be ignited by the pilot flame and thereby to form a main flame, the method comprising: operating a first flame sensor that is configured to sense the pilot flame; operating a second flame sensor that is configured to sense the main flame but not to sense the pilot flame; identifying a first condition in which the first flame sensor senses the pilot flame but the second flame sensor does not sense the main flame; identifying a second condition in which the second flame sensor senses the main flame; supplying the pilot burner with a flow rate of pilot fuel and a flow rate of pilot air in the first condition; and supplying the pilot burner with a different flow rate of pilot fuel, or a different flow rate of pilot air, or different flow rates of both pilot fuel and pilot air, in the second condition.

19. A method as defined in claim 18 wherein each of the different flow rates is an increased flow rate.

20. A method as defined in claim 18 wherein the flow rates are shifted back and forth throughout consecutive cycles of shifting back and forth between the first condition and the second condition.

21. A method as defined in claim 18 wherein the pilot flame is formed at the pilot burner continuously throughout the consecutive cycles of shifting back and forth between the first condition and the second condition.

22. A method as defined in claim 18 wherein the first condition is a regenerative exhaust condition and the second condition is a regenerative firing condition.