Patent Application
20200340667
This technology includes an apparatus and method for suppressing the production of NOx in a furnace combustion chamber, and particularly relates to the use of fuel-oxidant premix as a reactant to suppress the production of NOx.
The premixing of a fuel and an oxidant (typically combustion air) is common in many combustion processes. Thorough mixing has the advantage of an inlet stream combusting at a consistent fuel-to-air ratio. This premixing can be used for precise control of the combustion process, such as in the case of lean premix to limit combustion temperatures, resulting in significant reduction in the production and emission of NOx, a regulated pollutant.
Premix is limited with respect to thermal turndown, which is the ratio of the highest input to the lowest input. At a certain turndown ratio, the flow velocity of the premix is overcome by the flame speed of the premix, at which point flashback (the burning of the mixture back to the point of mixing) can occur, leading to deterioration in performance as well as damage to equipment. Complicated control systems are often programmed to facilitate operation near the point of flashback, but the systems are still limited by the physics of the flashback process.
An apparatus is provided for use with a source of fuel gas, a source of combustion air, and a furnace structure defining a combustion chamber. The apparatus includes first and second mixer tubes. The first mixer tube contains a first gas flow passage. The first gas flow passage has an inlet communicating with the source of fuel gas, and has an outlet to the combustion chamber. The second mixer tube contains a second gas flow passage, which has an inlet communicating with the source of combustion air and an outlet to the combustion chamber. The apparatus further includes premix control means for forming fuel gas-combustion air premix in the second passage by directing fuel gas from the first passage into the second passage. The premix control means alternatively forms fuel gas-combustion air premix in the first passage by directing combustion air from the second passage into the first passage.
In a given example, the premix control means directs fuel gas from the first passage into the second passage under the influence of a pressure drop from the first passage to the second passage. The premix control means alternatively directs combustion air from the second passage into the first passage under the influence of a pressure drop from the second passage to the first passage.
In another alternative, the premix control means directs only fuel gas into the first passage, while directing only combustion air into the second passage, with no substantial pressure differential between the first passage and the second passage.
A method of injecting reactants into a combustion chamber also is provided. The method injects the reactants from a burner having first and second gas flow passages. In a first mode, a premix of fuel gas and combustion air is injected from one of the passages. Fuel gas without combustion air is simultaneously injected from the other passage. Alternatively, in a second mode, a premix of fuel gas and combustion air is injected from one of the passages while combustion air without fuel gas is injected from the other passage. The method includes switching between the first and second modes in response to a pressure differential between the first and second passages.
The method can include a third mode that injects fuel gas without combustion air from one of the passages, while injecting combustion air without fuel gas from the other passage. The mode of operation can be switched to the third mode from either of the first and second modes in response to an equalization of pressures in the first and second passages.
In a given example, the method includes a step of directing fuel gas into a first gas flow passage having an outlet to the combustion chamber, and a step of directing combustion air into a second gas flow passage having an outlet to the combustion chamber. Premix is formed in alternative modes. In a first mode, fuel gas is directed from the first passage into the second passage to form premix in the second passage. In a second mode, combustion air is directed from the second passage into the first passage to form premix in the first passage.
The first mode can be a higher firing mode in which fuel gas is directed into the first passage at a pressure in a first range. The second mode can be a lower firing mode in which fuel gas is directed into the first passage at a pressure in a second range below the first range. In a third mode, only fuel gas is directed into the first passage, and only combustion air is directed into the second passage. The third mode is a premix-free mode in which the fuel gas is directed into the first passage at an intermediate pressure between the first range and the second range.
The apparatus shown schematically in the drawings has parts that are examples of the elements recited in the apparatus claims, and can be operated in steps that are examples of the steps recited in the method claims. These examples are described here to provide enablement and best mode without imposing limitations that are not recited in the claims.
As shown in
In this example the burner 10 is a premix burner with inner and outer mixer tubes 20 and 22. The inner mixer tube 20 is centered on a longitudinal axis 23, and contains a first gas flow passage 25 reaching from an inlet end 30 of the tube 20 to an outlet end 32. A pilot nozzle 34 is mounted on the outlet end 32 of the tube 20 to communicate the gas flow passage 25 with the process chamber 12.
The inner mixer tube 20 further includes a jet pump 40 defining an upstream portion of the first gas flow passage 25. As shown in enlarged detail in
Referring again to
As further shown schematically in
The fuel line 72 reaches from a fuel source 80, such as a plant supply of natural gas, to the inlet end 30 of the first gas flow passage 25. The air line 76 reaches from a source of combustion air, such as a blower 82, to the inlet end 60 of the second gas flow passage 65. A controller 84 operates the fuel and air valves 74 and 78 to initiate, regulate, and terminate flows of fuel gas and combustion air to the burner 10. The controller 84 may comprise any suitable programmable logic controller or other control device, or combination of control devices, that can be programmed or otherwise configured to perform as described and claimed herein.
Specifically, the controller 84 is configured to operate the fuel and air valves 74 and 78 in a number of differing modes. In one such mode, the controller 84 directs the fuel valve 74 to provide the first gas flow passage 25 with a stream of fuel gas at a first pressure. The controller 84 simultaneously directs the air valve 78 to provide the second gas flow passage 65 with a stream of combustion air at a second pressure that is equal or substantially equal to the first pressure. With equal or substantially equal pressures at the fuel and air streams, there is no substantial pressure differential radially through the cross-jet mixing holes 49 at the jet pump 40. The fuel gas then flows through the jet pump 40 axially past the cross-jet mixing holes 49, and further through the first gas flow passage 25 to the pilot nozzle 34 from which it enters the combustion chamber 12. This is indicated schematically in
The premix-free mode of operation can be an intermediate mode in which the fuel gas pressure has a midpoint or other intermediate level. The intermediate pressure level can be, for example, 25% of available fuel gas pressure input. Accordingly, the burner 10 can be shifted from the intermediate mode of operation to an alternative mode by shifting the fuel gas pressure up or down from the intermediate level. If the intermediate level of fuel gas pressure is equal or substantially equal to the combustion air pressure as described above, shifting the fuel gas pressure away from the intermediate level will induce a pressure differential between the two reactant streams. A pressure drop will then act radially through the cross-jet mixing holes 49 between the first and second gas flow passages 25 and 65. A sufficient change in pressure will induce a pressure drop sufficient to drive either fuel gas or combustion air radially from the passage of higher pressure to the passage of lower pressure, thus forming premix in the passage of lower pressure.
For example, the controller 84 can operate the fuel valve 74 to increase the fuel gas pressure into a range above the intermediate level, including a level approaching or reaching 100% of available input. This will shift the burner 10 from the intermediate mode to a high-fire mode. In the high-fire range of fuel gas pressures at the first passage 25, the pressure drop to the second passage 65 will drive some of the fuel gas to flow radially outward through the cross-jet mixing holes 49. That fuel gas will mix with the combustion air in the second passage 65 to form premix as the two reactants flow axially toward the outlet 62. The premix will then emerge from the outlet 62 and mix further with the fuel gas emerging from the pilot nozzle 34. This is indicated schematically in
Alternatively, the controller 84 can operate the fuel valve 74 to decrease the fuel gas pressure into a range below the intermediate level, including a level of, for example, 10% of available input. This would shift the burner 10 to a low-fire mode. In the low-fire range of fuel gas pressures at the first passage 25, the pressure drop from the second passage 65 to the first passage 25 will drive some of the combustion air to flow radially inward through the cross-jet mixing holes 49. That combustion air will mix with the fuel gas in the first passage 25 to form premix as the two reactants flow axially toward the pilot nozzle 34. The premix will then emerge from the pilot nozzle 34 in streams that mix further with the combustion air emerging from the surrounding annular outlet 62 of the second gas flow passage 65, as indicated schematically in
The foregoing examples control a pressure differential between the two reactant streams by regulating the fuel gas pressure while maintaining a constant level of combustion air pressure. However, a pressure differential for forming premix can be induced or regulated by a change in either reactant pressure relative to the other. This can be accomplished, for example, by regulating the combustion air pressure while maintaining a constant level of fuel gas pressure, by increasing or decreasing both reactant pressures unequally, or by decreasing one reactant pressure while increasing the other. In each case, the mode of operation can be switched to the high-fire mode from either the intermediate mode or the low-fire mode by inducing a sufficient pressure drop from the first gas flow passage 25 to the second gas flow passage 65. The mode of operation can likewise be switched to the low-fire mode from either the intermediate mode or the high-fire mode by inducing a sufficient pressure drop from the second gas flow passage 65 to the first gas flow passage 25. The mode can also be switched to the intermediate mode from either the high-fire mode or the low-fire mode by equalizing the pressures in the first and second passages 25 and 65.
The invention can provide premix while simultaneously preventing flashback in the burner 10 throughout a wide operating range of firing levels. A fuel stream of relatively low velocity can be susceptible to flashback. In a high-fire mode, flashback is avoided by forming premix in the second gas flow passage 65 while the velocity of the fuel stream in the first gas flow passage 25 is high enough to prevent flashback. In a low-fire mode, flashback is avoided by forming premix in the first gas flow passage 25 where the relatively low velocity fuel would otherwise be susceptible to flashback. This avoids the risk of flashback at turndown ratios approaching or reaching lower stabilization limits, which provides a correspondingly greater range of firing levels without flashback.
This written description sets for the best mode of carrying out the invention, and describes the invention so as to enable a person skilled in the art to make and use the invention, by presenting examples of the elements recited in the claims. The patentable scope of the invention is defined by the claims, and may include other examples that do not differ from the literal language of the claims, as well as equivalent examples with insubstantial differences from the literal language of the claims.
