Patent Application
20130260323
The use of high velocity gas burners is well known. In such burners, fuel gas and oxidant are mixed with one another and ignited in the interior of the burner. The resultant hot combustion gases then flow at high velocity through an outlet and into the furnace chamber for direct heating or into a radiant tube for indirect heating. The combustion of the fuel gas with an oxidant within the burner results in a greatly elevated temperature environment in the burner. To increase system efficiency, the oxidant can be pre-heated to result in higher temperatures. The preheating of the oxidant may be achieved by using a recuperative or regenerative system that uses the residual heat in the exhaust gas. This high temperature combustion environment provides two challenges. First, the internal components and surfaces of the burner and combustion chamber are exposed to the very high temperature environment. Second, when combustion is carried out at extremely high temperatures, nitrogen oxide (NOx) formation is promoted. As combustion temperatures increase, the levels of NOx production also increase. In order to deal with higher combustion temperatures, burners may be constructed from high temperature grade materials, for example, the combustion chambers can be made of ceramic materials which can withstand the high temperature environment. However, difficulties associated with high NOx emissions still remain.
A method and apparatus for a burner adapted to heat a furnace, radiant tube, or other environment of use is described herein. In particular, a burner for providing a fuel gas in combination with an oxidant to effect controlled combustion (or oxidation) of the fuel gas in a manner to reduce NOx emissions is described. Combustion of the fuel gas is shifted from within the burner combustor to a location outside the burner once the temperature within the furnace/radiant tube has reached a sufficient level to complete combustion of the fuel gas.
One embodiment provides a burner with a nozzle having a movable fuel tube to shift combustion from within the burner into the furnace/radiant tube. More particularly, this embodiment provides a burner in which fuel gas may be delivered through the fuel tube for discharge, such as axial and/or radial discharge, into a burner combustor for mixing with oxidant at a ratio that provides a combustible mixture to sustain the flame in the burner combustor. During a start-up stage, the fuel tube is in a retracted position, and the fuel gas/oxidant mixture is ignited with a spark igniter to combust within the burner combustor. During that period, the flame inside the burner combustor can be monitored with a flame sensor, such as a flame rod or UV scanner.
Once the temperature in the furnace/radiant tube reaches a pre-defined level above the auto-ignition temperature, the fuel tube can be moved toward the outlet of the burner to an extended position. During this time, the flame gradually moves with the fuel tube in the burner combustor towards the furnace chamber/radiant tube. As the fuel tube approaches a fully extended position near the outlet of the burner, the flame will be destabilized and extinguished such that all combustion will take place in the furnace chamber/radiant tube, and the flame sensor will detect a loss of flame inside the combustor. Due to the elevated temperature in the furnace/radiant tube, this movement of the flame to the furnace/radiant tube space leads to combustion in the furnace/radiant tube in the absence of a flame in the burner. The high velocity of oxidant and fuel exiting the burner combustor contributes to destabilization and extinguishment of flame in the furnace/radiant tube. While the temperature levels within the furnace/radiant tube are sufficient to cause combustion of the fuel gas, these temperature levels nonetheless are low enough to avoid substantial NOx generation. Moreover, the high exit velocity of the air and fuel provides substantial blending and recirculation of the furnace/radiant tube atmosphere with the air/fuel mix, resulting in reduced spikes of temperatures in the furnace/radiant tube, which are normally experienced during the standard operating mode of typical burners. After the flame ceases to exist in the burner combustor, the flow rate of the fuel gas and oxidant can be maintained, decreased, or increased, according to the needs of the furnace operator. The burner will begin to cool after the flame has ceased to exist in the burner combustor, and, thus, when the burner has cooled to a temperature below the auto-combustion temperature, the fuel tube may be retracted without the flame returning to the burner combustor. Alternatively, the fuel tube may stay in the extended position during the flameless mode operation. The latter creates a physical separation of the fuel and oxidant streams in the upstream portion of the burner combustor and may be advantageous when the auto-ignition temperature of the fuel/oxidant mixture is low, such as fuel gas mixture containing high percentage of hydrogen.
Before the embodiments of the burner and method are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and/or the arrangements of the components set forth in the following description or illustrated in the drawings. Rather, the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein are for purposes of description only and should not be regarded as limiting. The use herein of “including,” “comprising,” and variations thereof is meant to encompass the items listed thereafter and equivalents, as well as additional items and equivalents thereof.
Reference will now be made to the drawings wherein like elements are designated by like reference numbers in the various views.
As shown, an oxidant supply 30 provides combustion air for delivery from a blower or other supply source (not shown) to the annular air passageway 28 for transmittal to the nozzle assembly 24. An oxidant control valve 32 is used to control the flow of oxidant. In this regard, the oxidant control valve 32 may be operatively connected to a controller 34 such as a PLC, computer, or the like which opens or closes the oxidant control valve 32 in accordance with pre-established commands based on conditions in the furnace/radiant tube and/or the burner. Likewise, a fuel supply 40 provides natural gas or other gaseous fuel for delivery to the fuel tubes 21 and 22 for transmittal to the nozzle assembly 24. A fuel control valve 42 is used to control the flow of fuel gas. In this regard, the fuel control valve 42 may be operatively connected to the controller 34 which may adjust fuel feed in accordance with pre-established commands based on conditions in the furnace, radiant tube, and/or the burner.
A sensor 46, such as a flame sensor, or the like, may be present to continuously monitor the presence of a flame and to communicate such data to the controller 34. As will be described further herein, the controller 34 may utilize the data from the sensor 46 in combination with temperature data from the furnace/radiant tube to control the movement of the fuel tube 22. It will be appreciated that the sensor 46 can be any suitable sensor and can be disposed in any suitable location.
Referring now to
The radial disk portion 52 can include a pattern of interior air passages 58. As will be described further herein, during operation, oxidant delivered from the oxidant supply 30 may flow through an annular gap 56 and the interior air passages 58 towards the burner outlet as shown by the arrows in
As mentioned, the burner 10 may be operated in a flame mode with ignition within the burner combustor or in a flameless mode during which the oxidant and fuel gas combusts only downstream of the combustor outlet 80. The flameless mode may also be referred to as a volume combustion mode, i.e., when combustion is occurring in the volume of the furnace chamber or radiant tube in the absence of a flame in the burner. The flame mode can provide the initial start-up of the furnace/radiant tube 16 using combustion of fuel gas and oxidant in the burner combustion chamber 26 to heat up the furnace/radiant tube. The flame mode can be followed by the flameless mode during which the fuel gas and oxidant are ejected from the burner 10 and allowed to undergo combustion downstream of the combustor outlet. This dual mode operation results in substantially reduced NOx emissions.
Referring again to
Referring to
During the flameless mode, the fuel gas and oxidant are passed out of the burner 10 without undergoing combustion. Upon entering the high-temperature furnace/radiant tube environment, the fuel gas is raised to a temperature sufficient to activate combustion. Thus, the location of the onset of combustion is moved from the burner combustor 26 downstream to the furnace chamber/radiant tube 16. Due to the relatively disperse combustion zone outside of the burner 10 and the entrainment of the flue gas within the fuel/oxidant mixture, a substantial localized temperature spike does not occur. NOx production is thereby substantially reduced. As will be appreciated, once the flameless combustion mode has been initiated, the flows of fuel gas and oxidant may thereafter be cycled on and off, or otherwise maintained, decreased, or increased, to adjust the temperature within the furnace/radiant tube as desired.
The movement of the fuel tube 22 toward the outlet 80 can cause a physical separation of the fuel gas and oxidant streams passing through the nozzle assembly 24, which results in a decrease in the effective residence time for reaction within the burner combustor 26. Combustion occurs at finite rates and therefore requires a certain residence time to finish. The decrease of residence time can extinguish combustion within the burner combustor. Flow recirculation helps stabilize the combustion within the burner combustor 26. Due to the converging shape of the burner combustor, the extension of the fuel tube 22 results in a reduction of flow recirculation within the burner combustor 26. Thus, the extension of the fuel tube 22 de-stabilizes and extinguishes the flame in burner combustor 26. Accordingly, available fuel gas and oxidant can be delivered into the furnace/radiant tube 16 prior to combustion. Due to the elevated temperature in the furnace/radiant tube 16, the fuel gas undergoes combustion downstream from the burner combustor 26. While the temperature levels within the furnace/radiant tube 16 are sufficient to cause combustion of the fuel gas, these temperature levels nonetheless are low enough to avoid substantial NOx generation. Moreover, the high exit velocity of the oxidant and fuel provides substantial blending and recirculation of the flue gas with the oxidant/fuel mix, resulting in reduced combustion temperatures in the furnace/radiant tube 16. As noted above, after flame extinction and the initiation of flameless combustion, the flow rate of the mixture can be maintained, decreased, or increased according to the process needs.
As shown in
Accordingly, the burner 10 may be provided with a suitable structure to reduce the amount of fuel gas leakage between the movable fuel tube 22 and the fixed fuel tube assembly 21 and/or a nozzle extension tube 23 without interfering with the movement of the movable fuel tube 22. As shown in
As shown in
The movement rod 292 can be moved by the device 290 toward and way from the outlet 280 to push and pull the movable fuel tube 222 along the same direction. For example,
As noted, the fuel tube can be moved via any suitable mechanism. In addition, the fuel tube can be constructed and attached in any suitable way to permit the fuel gas exit to be moved from a position in the burner upstream and relatively far away from the burner outlet to a position at or near the burner outlet. It will also be appreciated that the nozzle assembly can be fixed, or can be moved with or moved independent from the fuel tube via any suitable mechanism.
All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.
The use of the terms “a” and “an” and “the” and “at least one” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The use of the term “at least one” followed by a list of one or more items (for example, “at least one of A and B”) is to be construed to mean one item selected from the listed items (A or B) or any combination of two or more of the listed items (A and B), unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.
This patent application claims the benefit of U.S. Provisional Patent Application No. 61/619,771, filed Apr. 3, 2012, which is incorporated by reference in its entirety herein.
| Number | Date | Country | |
|---|---|---|---|
| 61619771 | Apr 2012 | US |
