Patent Application
20170283713
This invention claims priority to and the benefit of U.S. Patent Application Ser. No. 62/316,246, filed Mar. 31, 2016, and European Patent Application No. 16170266.7 filed May 19, 2016, both of which are herein incorporated by reference.
This invention relates to burners, furnaces, fuel combustion processes using the same, and steam cracking processes using the same. In particular, it relates to burner sub-systems capable of burning fuel gas rich in hydrogen, furnaces comprising the same, hydrogen-rich fuel gas combustion processes using the same, and steam cracking processes using the same.
In gas fired industrial furnaces, NOx is formed by the oxidation of nitrogen drawn into the burner with the combustion air stream. The formation of NOx is widely believed to occur primarily in regions of the flame where there exist both high temperatures and an abundance of oxygen. Since ethylene furnaces are amongst the highest temperature furnaces used in the hydrocarbon processing industry, the natural tendency of burners in these furnaces is to produce high levels of NOx emissions.
The majority of recent low NOx burners for gas-fired industrial furnaces are based on the use of multiple fuel jets in a single burner. Such burners may employ fuel staging, flue-gas recirculation (“FGR”), or a combination of both. U.S. Pat. Nos. 5,098,282 and 6,007,325 disclose burners using a combination of fuel staging and flue-gas recirculation. Certain burners may have as many as 8-12 fuel nozzles in a single burner. The large number of fuel nozzles require the use of very small diameter nozzles. In addition, the fuel nozzles of such burners are generally exposed to the high temperature flue-gas in the firebox.
One technique for reducing NOx that has become widely accepted in industry is known as staging. With staging, the primary flame zone is deficient in either air (fuel-rich) or fuel (fuel-lean). The balance of the air or fuel is injected into the burner in a secondary flame zone or elsewhere in the combustion chamber. As is well known, a fuel-rich or fuel-lean combustion zone is less conducive to NOx formation than an air-fuel fuel ratio closer to stoichiometry. Combustion staging results in reducing peak temperatures in the primary flame zone and has been found to alter combustion speed in a way that reduces NOx. Since NOx formation is exponentially dependent on gas temperature, even small reductions in peak flame temperature dramatically reduce NOx emissions. However this is generally balanced with the fact that radiant heat transfer decreases with reduced flame temperature, while CO emissions, an indication of incomplete combustion, may actually increase.
In the context of premix burners, the term primary air refers to the air premixed with the fuel; secondary, and in some cases tertiary, air refers to the balance of the air required for proper combustion. In raw gas burners, primary air is the air that is more closely associated with the fuel; secondary and tertiary air is more remotely associated with the fuel. The upper limit of flammability refers to the mixture containing the maximum fuel concentration (fuel-rich) through which a flame can propagate.
U.S. Pat. No. 4,629,413 discloses a low NOx premix burner and discusses the advantages of premix burners and methods to reduce NOx emissions. The premix burner of U.S. Pat. No. 4,629,413 lowers NO emissions by delaying the mixing of secondary air with the flame and allowing some cooled flue gas to recirculate with the secondary air. The manner in which the burner disclosed achieves light off at start-up and its impact on NOx emissions is not addressed. The contents of U.S. Pat. No. 4,629,413 are incorporated by reference in their entirety.
U.S. Pat. No. 5,092,761 discloses a method and apparatus for reducing NO emissions from premix burners by recirculating flue gas. Flue gas is drawn from the furnace through recycle ducts by the inspirating effect of fuel gas and combustion air passing through a venturi portion of a burner tube. Airflow into the primary air chamber is controlled by dampers and, if the dampers are partially closed, the reduction in pressure in the chamber allows flue gas to be drawn from the furnace through the recycle ducts and into the primary air chamber. The flue gas then mixes with combustion air in the primary air chamber prior to combustion to dilute the concentration of oxygen in the combustion air, which lowers flame temperature and thereby reduces NO emissions. The flue-gas recirculating system may be retrofitted into existing burners or may be incorporated in new low NOx burners. The entire contents of U.S. Pat. No. 5,092,761 are incorporated herein by reference.
A drawback of the system of U.S. Pat. No. 5,092,761 is that the staged-air used to cool the FGR duct first enters the furnace firebox, traverse a short distance across the floor and then enter the FGR duct. During this passage, the staged air is exposed to radiation from the hot flue-gas in the firebox. Analyses of experimental data from burner tests suggest that the staged-air may be as hot as 700° F. when it enters the FGR duct.
From the standpoint of NOx production, another drawback associated with the burner of U.S. Pat. No. 5,092,761 relates to the configuration of the lighting chamber, necessary for achieving burner light off. The design of this lighting chamber, while effective for achieving light off, has been found to be a localized source of high NOx production during operation. Other burner designs possess a similar potential for localized high NOx production, since similar configurations are known to exist for other burner designs, some of which have been described hereinabove.
Additionally, commercial experience and modeling have shown when flue-gas recirculation rates are raised, there is a tendency of the flame to be drawn into the FGR duct. Often, it is this phenomenon that constrains the amount of flue-gas recirculation. When the flame enters directly into the flue-gas recirculation duct, the temperature of the burner venturi tends to rise, which raises flame speed and causes the recirculated flue gas to be less effective in reducing NOx. From an operability perspective, the flue-gas recirculation rate needs to be lowered to keep the flame out of the FGR duct to preserve the life of the metallic FGR duct.
U.S. Pat. No. 6,877,980 discloses a burner for use in furnaces such as those used in steam cracking with increased FGR recirculation rate and low NOx formation. The burner includes a primary air chamber; a burner tube having an upstream end, a downstream end and a venturi intermediate said upstream and downstream ends, said venturi including a throat portion having substantially constant internal cross-sectional dimensions such that the ratio of the length to maximum internal cross-sectional dimension of said throat portion is at least 3; a burner tip mounted on the downstream end of said burner tube adjacent a first floor burner opening in the furnace, so that combustion of the fuel takes place downstream of said burner tip; and a fuel orifice located adjacent the upstream end of said burner tube, for introducing fuel into said burner tube. In the burner disclosed therein, a circular barrier wall is erected surrounding the floor burner opening, blocking the base of the floor burner flame from the flue-gas recirculation duct ports on the floor. The barrier wall serves the purpose of stabilizing the flame and reducing NOx formation.
It has been recently found that, however, the annular barrier wall in the burner of U.S. Pat. No. 6,877,980 also reflects the heat produced by the flame to the burner tip, thereby increasing the burner tip temperature. Where the fuel gas comprises primarily hydrocarbons such as methane, the burner tip temperature is generally reasonably low to provide a satisfactory life, even with the reflected heat from the barrier wall. However, where the fuel gas comprises primarily hydrogen (i.e., comprising at least 50 mol % of hydrogen), the flame speed and flame temperature are significantly higher, and so is the amount of heat reflected by the barrier wall to the burner tip. As a result, the burner tip is frequently overheated to an exceedingly high temperature, leading to premature failure, especially during burner turn-down process or flame flash-back.
Therefore, there is a need for an improved burner sub-system design with reduced overheating potential, especially when hydrogen-rich fuel gas is used. The present invention satisfies this and other needs.
It has been found that, by employing a barrier wall segment between the floor burner opening and the FGR duct opening capable of blocking direct gas flow between these two, in whole or in part, instead of a annular barrier wall surrounding the entirety of the floor burner opening, one can effectively reduce the amount of heat reflected to the burner tip, resulting in a lower burner tip temperature enabling satisfactory life thereof, even when hydrogen-rich fuel gas is used. A burner sub-system including such barrier segment can achieve a high level of FGR rate, a relatively low temperature inside the FGR, a low level of NOx emissions, without decreasing flame stability. Such a burner sub-system can be advantageously used in hydrocarbon steam cracking furnaces.
Thus, a first aspect of the present invention relates to burner sub-system comprising: (a1) a furnace floor segment having a floor burner opening and a flue-gas recirculation duct opening; (a2) a tile enclosure lining the periphery of the floor burner opening; (a3) a burner comprising a burner tip adjacent and surrounded by the floor burner opening, the burner tip configured to provide a floor burner flame through the floor burner opening and having a vertical centerline; (a4) a flue-gas recirculation duct opening adjacent the tile enclosure; and (a5) a barrier wall segment extending upwards from the upper surface of the furnace floor segment between the flue-gas recirculation duct opening and the burner tip, the barrier wall segment having an angle of view no greater than 180° when viewed from the point where the vertical centerline of the burner tip intercepts a plane of the furnace floor segment.
A second aspect of the present invention relates to a furnace comprising: (b1) at least one burner sub-system according to the first aspect of the present invention; (b2) a furnace floor comprising each of the furnace floor segment of the at least one burner sub-system; and (b3) one or more furnace side walls; wherein the furnace floor and the one or more furnace side walls form a furnace fire box.
A third aspect of the present invention relates to a fuel combustion process carried out in a furnace according to the second aspect of the present invention, the process comprising: (c1) supplying a fuel gas comprising at least 50 mol % of hydrogen into the at least one burner sub-system; and (c2) combusting the fuel gas to form a floor burner flame above the burner tip inside the furnace fire box.
A fourth aspect of the present invention relates to a steam cracking process comprising a fuel combustion process of the third aspect of the present invention, wherein a reactant stream comprising a hydrocarbon is heated inside a cracking tube which is heated inside the furnace by the flame.
These and other features of the present invention will be apparent from the detailed description taken with reference to accompanying drawings.
The invention is further explained in the description that follows with reference to the drawings illustrating, by way of non-limiting examples, various embodiments of the invention wherein:
Although the present invention is described in terms of a burner sub-system for use in connection with a furnace or an industrial furnace, it will be apparent to one of skill in the art that the teachings of the present invention also have applicability to other process components such as, for example, boilers. Thus, the term furnace herein shall be understood to mean furnaces, boilers and other applicable process components.
As used herein, a “hydrogen-rich” gas is a gas comprising at least 50 mol % of molecular hydrogen. Hydrogen-rich fuel gas comprising at least 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 98, or even 99 mol % of molecular hydrogen has become more readily available than before due to, inter alia, steam cracking of saturated hydrocarbon (ethane, propane, butanes, and the like) to make olefins. Such hydrogen-rich fuel gas may comprise, in addition to molecular hydrogen, hydrocarbons such as methane, ethane, propane, butanes, and the like. Flames produced from burning hydrogen-rich fuel gas tend to have higher flame speed than those produced from natural gas. Higher flame speed tends to cause the flame to attach more closely to the burner tip resulting in higher burner tip temperature. As a result, thermal management of burners burning hydrogen-rich fuel gas is more important than those burning natural gas.
Referring to the examples of burner sub-systems illustrated in
Multiple air ports 30 (
In order to recirculate flue gas from the furnace to the primary air chamber, FGR duct 76 extends from FGR duct opening 40, in the floor of the furnace into the primary air chamber 26. Alternatively, multiple passageways (not shown) may be used instead of a single passageway. Flue gas is drawn through FGR duct 76 by the inspirating effect of fuel passing through venturi 19 of burner tube 12. In this manner, the primary air and flue gas are mixed in primary air chamber 26, which is prior to the zone of combustion. Therefore, the amount of inert material mixed with the fuel is raised, thereby reducing the flame temperature, and as a result, reducing NOx emissions. Closing or partially closing damper 37b restricts the amount of fresh air that can be drawn into the primary air chamber 26 and thereby provides the vacuum necessary to draw flue gas from the furnace floor.
Mixing is promoted by providing two or more primary air channels 37 and 38 protruding into the FGR duct 76. The channels 37 and 38 are conic-section, cylindrical, or squared and a gap between each channel 37 and 38 produces a turbulence zone in the FGR duct 76 where good flue gas/air mixing occurs.
The geometry of channels 37 and 38 is designed to promote mixing by increasing air momentum into the FGR duct 76. The velocity of the air is optimized by reducing the total flow area of the primary air channels 37 and 38 to a level that still permits sufficient primary air to be available for combustion, as those skilled in the art are capable of determining through routine trials.
Mixing is further enhanced by a plate member 83 at the lower end of the inner wall of the FGR duct 76. The plate member 83 extends into the primary air chamber 26. Flow eddies are created by flow around the plate of the mixture of flue gas and air. The flow eddies provide further mixing of the flue gas and air. The plate member 83 also makes the FGR duct 76 effectively longer, and a longer FGR duct also promotes better mixing.
The improvement in the amount of mixing between the recirculated flue gas and the primary air caused by the channels 37 and 38 and the plate member 83 results in a higher capacity of the burner to inspirate flue-gas recirculation and a more homogeneous mixture inside the venturi portion 19. Higher flue-gas recirculation reduces overall flame temperature by providing a heat sink for the energy released from combustion. Better mixing in the venturi portion 19 tends to reduce the hot-spots that occur as a result of localized high oxygen regions.
Unmixed low temperature ambient air (primary air), is introduced through angled channels 37 and 38, each having a first end comprising an orifice 37a and 38a, controlled by damper 37b, and a second end comprising an orifice which communicates with FGR duct 76. The ambient air so introduced is mixed directly with the recirculated flue gas in FGR duct 76. The primary air is drawn through channels 37 and 38, by the inspirating effect of the fuel passing through the fuel orifice, which may be contained within gas spud 24. The ambient air may be fresh air as discussed above.
Additional unmixed low temperature ambient air, having entered secondary air chamber 32 through dampers 34 is drawn through orifice 62, through bleed air duct 64, through orifice 97 into FGR duct 76 and into the primary air chamber 26 by the inspirating effect of the fuel passing through venturi portion 19. The ambient air may be fresh air as discussed above. The mixing of the cool ambient air with the flue gas lowers the temperature of the hot flue gas flowing through FGR duct 76 and thereby substantially increases the life of FGR duct 76 and allows use of this type of burner to reduce NOx emission in high temperature cracking furnaces having flue gas temperature above 1900° F. in the radiant section of the furnace. Bleed air duct 64 has a first end 66 and a second end 68, first end 66 connected to orifice 62 of secondary air chamber 32 and second end 68 connected to orifice 97 of FGR duct 76.
Additionally, a minor amount of unmixed low temperature ambient air, relative to that amount passing through bleed air duct 64, having passed through air ports 30 into the furnace, may also be drawn through FGR duct 76 into primary air chamber 26 by the inspirating effect of the fuel passing through venturi portion 19. To the extent that damper 37b is completely closed, bleed air duct 64 is desirably sized so as to permit the necessary flow of the full requirement of primary air needed by burner 10.
The flue-gas recirculated to the burner is mixed with a portion of the cool staged air in the FGR duct 76. This mixing reduces the temperature of the stream flowing in the FGR duct 76, and enables readily available materials to be used for the construction of the burner. This feature is useful for the burners of high temperature furnaces such as steam crackers or reformers, where the temperature of the flue-gas being recirculated can be as high as 1900° F.-2100° F. By combining approximately one pound of staged-air with each pound of flue-gas recirculated, the temperature within the FGR duct can be advantageously reduced.
One or more passageways connecting the secondary air chamber directly to the flue-gas recirculation duct induce a small quantity of low temperature secondary air into the FGR duct 76 to cool the air/flue-gas stream entering in the metallic section of the FGR duct 76. By having the majority of the secondary air supplied directly from the secondary air chamber, rather than having the bulk of the secondary air traverse across the furnace floor prior to entering the FGR duct, beneficial results are obtained, as demonstrated by the Examples below.
Advantageously, a mixture of from about 20% to about 80% flue gas and from about 20% to about 80% ambient air is drawn through FGR duct 76. It is particularly preferred that a mixture of about 50% flue gas and about 50% ambient air be employed. The desired proportions of flue gas and ambient air may be achieved by proper sizing, placement and/or design of FGR duct 76, bleed air ducts 64 and air ports 30, as those skilled in the art will readily recognize. That is, the geometry and location of the air ports and bleed air ducts may be varied to obtain the desired percentages of flue gas and ambient air.
A sight and lighting port 50 is provided in the primary chamber 26, both to allow inspection of the interior of the burner assembly, and to provide access for lighting of the burner 10 with lighting element (not shown). The burner plenum may be covered with mineral wool or ceramic fiber insulation 52 and wire mesh screening (not shown) to provide insulation therefor. The lighting chamber 99 is located at a distance from burner tip 20 effective for burner light off. A lighting torch or igniter (not shown) of the type disclosed in U.S. Pat. No. 5,092,761 has utility in the start-up of the burner. To operate the burner, the torch or igniter is inserted through light-off port 50 into the lighting chamber 99, which is adjacent burner tip 20, to light the burner 10.
In operation, fuel orifice 11, which may be located within gas spud 24, discharges fuel into burner tube 12, where it mixes with primary air, recirculated flue gas or mixtures thereof. The mixture of fuel, recirculated flue-gas and primary air then discharges from burner tip 20. The mixture in the venturi portion 19 of burner tube 12 is maintained below the fuel-rich flammability limit; i.e. there is insufficient air in the venturi to support combustion. Secondary air is added to provide the remainder of the air required for combustion.
In addition to the use of flue gas as a diluent, another technique to achieve lower flame temperature through dilution is through the use of steam injection. Steam can be injected in the primary air or the secondary air chamber. Steam may be injected through one or more steam injection tubes 15, as shown in
The cross-section of FGR duct 76 is substantially rectangular, typically with its minor dimension ranging from 30% to 100% of its major dimension. Conveniently, the cross sectional area of FGR duct 76 ranges from about 5 square inches to about 12 square inches/million (MM) Btu/hr burner capacity and, in a practical example, from 34 square inches to 60 square inches. In this way the FGR duct 76 can accommodate a mass flow rate of at least 100 pounds per hour per MM Btu/hr burner capacity, preferably at least 130 pounds per hour per MM Btu/hr burner capacity, and still more preferably at least 200 pounds per hour per MM Btu/hr burner capacity. Moreover, FGR ratios of greater than 10% and up to 15% or even up to 20% can be achieved.
With reference to
U.S. Pat. No. 6,877,980 B2 discloses a substantially similar burner sub-system (shown in
In the burner-subsystem of the present invention, a non-annular barrier segment between the burner tip and the FGR duct opening is installed. It has been found that a partial barrier wall segment can be sufficient to block direct gas flow between the periphery of the tile enclosure and the FGR duct opening, preventing flame from entering the FGR duct, and achieving a sufficiently low NOx level in the exhaust. In addition, by employing only a segment of the barrier wall, the amount of heat reflected from the barrier wall to the burner tip can be reduced significantly, thereby reducing the burner tip temperature, preventing it from overheating especially during burner turn-down or fame flash back and premature failure. This design was found to be particularly advantageous in furnaces where hydrogen-rich flue gas is used, leading to prolonged burner tip life.
Thus, the barrier wall segment 60 in the burner sub-system of the present invention generally has a width resulting in an angle of view (alpha) no greater than 180° when viewed from the point where the vertical centerline of the burner tip intercepts the horizontal plane of the furnace floor segment. In general, alpha1≦alpha≦alpha2, where alpha1 and alpha2 can be, independently, 1, 3, 5, 10, 15, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, as long as alpha1<alpha2.
Exemplary barrier wall segment 60 in the burner sub-system of the present invention blocks at least 50% (or at least 60%, 70%, 80%, 90%, 95%, or even 100%) of the line of sight of the FGR duct opening, when viewed from the point where the vertical centerline of the burner tip intercepts the plane of the furnace floor segment. Preferably, the center lines of the angles of view of the barrier wall segment and the FGR duct opening, when viewed from the point where the vertical centerline of the burner tip intercepts the horizontal plane of the furnace floor segment, are substantially adjacent to each other. Thus, the angle formed between the center lines of the these two angles of views is desired to be no higher than 30° (or no higher than 25°, 20°, 15°, 10°, 5°, 3°, or even 1°).
Preferably, the angle of view of the barrier wall segment (alpha) is larger than the angle of view of the FGR duct opening (beta), when viewed from the point where the vertical centerline of the burner tip intercepts the plane of the furnace floor segment. Thus, r1≦alpha/beta≦r2, where r1 and r2 can be, independently, 1.0, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, as long as r1<r2.
Exemplary barrier wall segment has a height of from h1 centimeters to h2 centimeters extending above the furnace floor segment plane, where h1 and h2 can be, independently, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 35, 40, 45, or 50, as long as h1<h2. It is desired that the area of the furnace floor segment between the periphery of the floor burner opening and the air ports 30 is substantially flat.
In certain preferred examples, the barrier wall may comprise a center portion and one or two support structure portion(s) connected to the end of the center portion. The support structure portion(s) generally curve(s) away from the center of the floor burner opening. The support structure portion desirably has an average height lower than the center portion. The center portion may have a substantially uniform height, while the support structure portion may taper off from next to the center portion to the end thereof. The support structure portion can further deflect gas flow from the floor burner opening to the FGR duct opening, and provide mechanical support to the center portion. In a specific example, the barrier wall comprises a center portion blocking one side of a FGR duct opening, and two support structure portions blocking at least a portion of two other sides of the FGR duct opening. The barrier wall segment can be advantageously made from refractory materials such as ceramic, glass-ceramic, and the like.
The burner sub-system of the present invention may further include a centering plate as is now described with reference to
In one specific example, centering plate 160 also contains a pair of holes 168 to permit a corresponding pair of steam injection tubes 15 to pass through centering plate 160 to the extent such steam injection tubes 15 are present.
As noted above, the centering plate 160 is perforated to permit flow therethrough of air from the primary air chamber 26, which avoids flow losses that result from a normally tortuous flow pattern caused by a presently used solid centering plate. These flow losses are avoided because the perforated centering plate design smoothes out the flow vectors entering the venturi portion 19 of the burner tube to enable higher venturi capacity, higher flue-gas recirculation rate, lower flame temperature and lower NOx production.
Although centering plate 160 as shown in
The burner useful in the sub-system of the present invention may employ an advantaged fuel spud as is now described with specific reference to
While outer surface 21 may be helpful in the installation of fuel spud 24, as is illustrated by streamlines S of
An advantaged burner tip 20 useful in the burner sub-system of the present invention is now discussed with specific reference to
In connection with the advantaged burner spud 24 and burner tip 20 described above, the mixture of fuel, recirculated flue gas and primary air discharges from burner tip 20. The mixture in the venturi portion 19 of burner tube 12 is maintained below the fuel-rich flammability limit; i.e. there is insufficient air in the venturi to support combustion. Staged, secondary air is added to provide the remainder of the air required for combustion. The majority of the staged air is added a finite distance away from the burner tip 20 through staged air ports 30. However a portion of the staged, secondary air passes between the burner tip 20 and the annular tile enclosure 22 and is immediately available to the fuel exiting the side ports 568 of burner tip 20. As indicated, side-ports 568 direct a fraction of the fuel across the face of the annular tile enclosure 22, while main ports 564, direct the major portion of the fuel into the furnace.
As may be envisioned, two combustion zones are established. A small combustion zone is established across the face of the peripheral tile enclosure 22, emanating from the fuel combusted in the region of the side ports 568, while a much larger combustion zone is established projecting into the furnace firebox, emanating from the fuel combusted from the main ports 564. In operation, the larger combustion zone represents an approximately cylindrical face of combustion extending up from the burner, where the staged air flowing primarily from air ports 30 meets the fuel-rich mixture exiting from the burner tip main ports 564.
The combustion zone adjacent to the side ports 568 and peripheral tile enclosure 22 contributes to flame stability. To provide adequate flame stability, the air/fuel mixture in this zone, which comprises the air/fuel mixture leaving the side ports 568 of burner tip 20, plus the air passing between the burner tip 20 and the peripheral tile enclosure 22, is desirably above the fuel-rich flammability limit.
While a mixture above the fuel-rich flammability limit in the combustion zone adjacent to the side ports 568 and peripheral tile enclosure 22 assures good burner stability, combustion in this zone tends to generate relatively high NOx levels compared to the larger combustion zone. Overall NOx emissions may be reduced by minimizing the proportion of fuel that is combusted in this smaller combustion zone. More particularly, in a staged-air, pre-mix burner employing integral flue-gas recirculation, when the quantity of fuel discharged into the combustion zone adjacent to side ports 568 and peripheral tile enclosure 22 does not exceed about 15% of the total fuel fired in the burner, lower overall NOx emissions are experienced. This is achieved by further assuring that the gas flow between burner tip 20 and the peripheral tile enclosure 22 is such that combustion takes place within this zone with a mixture sufficiently above the fuel-rich flammability limit to assure good burner stability, but without the high oxygen concentrations that lead to high NOx emissions.
The advantaged burner tip design described above limits the fuel discharged into the combustion zone adjacent to the side ports 568 and peripheral tile enclosure 22 to about eight percent of the total fuel. This design advantageously maintains the desired air/fuel ratio in this combustion zone, while maintaining a burner-tip-to-peripheral-tile enclosure gap of between about 0.15″ to about 0.40″. As shown, rather than have two rows of about thirty side ports, as is common in conventional designs, the advantaged burner tip 20 has two rows of 16 side ports 568, each side port having a diameter of about 6 mm. Advantageously, with this design, NOx emissions are reduced without the problems normally associated with reduced flame temperature and flame speed. The result is a very stable flame that is not prone to “lift-off” Reducing the diameter of the side ports 568 to about 5 mm also helps limit the fuel discharged into the combustion zone adjacent to the side ports 568 and peripheral tile enclosure 22 to between about 5 and 15 percent of the total fuel fired, while still producing a very stable flame.
In one example, burner tip 20 has an upper end 566 which, when installed, faces the burner box and a lower end adapted for mating with the downstream end 18 of burner tube 12. Mating of the lower end of burner tip 20 to the burner tube 12 can be achieved by swaging or, more preferably, by welding or threaded engagement.
Referring specifically to
Referring now to
The reduction in the number of side ports necessary to achieve a low NOx emissions level is dependent upon a number of factors including the properties of the fuel, itself, the dynamics of fluid flow and the kinetics of combustion. While the burner tips 20 described above having about a 53% reduction in the number of side ports, it would be expected that reductions in the number of side ports ranging from about 25% to about 75% could be effective as well, so long as each side port and the burner-tip-to-peripheral-tile enclosure gap is appropriately sized.
In the advantaged burner tip design described above, preferably the dimensions of the burner-tip-to-peripheral-tile enclosure gap are such that the total air available to the fuel gas exiting the side ports (i.e. the sum of air exiting the side ports with the fuel gas, plus the air supplied through gap), is between about 5 to about 15 percentage points above the Fuel Rich Flammability Limit for the fuel being used. For example, if the fuel being used has a Fuel Rich Flammability Limit of 55% of the air required for stoichiometric combustion, the air available to the fuel gas exiting the side ports desirably represents 60-65% of the air required for stoichiometric combustion.
Use of the advantaged burner tip described above serves to substantially minimize localized sources of high NOx emissions in the region near the burner tip.
The burner 10 useful in the burner sub-system of the present invention may also comprise a venturi 19 as now discussed. Referring now to
Increasing the ratio of length to internal cross-sectional dimensions in the throat portion of the venturi can reduce the degree of flow separation that occurs in the throat and cone portions of the venturi which increases the capacity of the venturi to entrain flue gas thereby allowing higher flue-gas recirculation rates and hence reduced flame temperature and NOx production. A longer venturi throat also promotes better flow development and hence improved mixing of the fuel gas/air stream prior to the mixture exiting the burner tip 20. Better mixing of the fuel gas/air stream also contributes to NOx reduction by producing a more evenly developed flame and hence reducing peak temperature regions.
The non-limiting burner 10 particularly useful in the burner sub-system of the present invention may include a lighting chamber arrangement as will now be discussed with particular reference to
To operate the burner 10 useful in the burner-system of the present invention, a torch or igniter is inserted through light-off tube 50 into the lighting chamber 99, which is adjacent to the primary combustion area and burner tip 20, to light the burner. Following light-off, the lighting chamber 99 is plugged-off by inserting removable lighting chamber plug 362 through light-off tube 50 into the lighting chamber 99, for normal operation, eliminating the zone of high oxygen concentration adjacent to the primary combustion zone, and thus reducing the NOx emissions from the burner. For ease of installation, the lighting chamber plug 362 may be affixed to an installation rod, to form lighting chamber plug assembly 368, which is inserted through light-off tube 50 into lighting chamber 99. The use of the removable lighting chamber plug assembly 368 allows convenient attachment to the burner plenum through mechanical attachment of installation rod to burner plenum.
The removable lighting chamber plug 362 and assembly is advantageously constructed of materials adequate for the high temperature environment inside the furnace. The face 364 of the removable lighting chamber plug 362, which is the surface exposed to the furnace and which fits into burner tile enclosure 22, may be profiled to form an extension of the axi-symetric geometry of the burner tile enclosure 22, thus creating a flush mounting with the burner tile enclosure 22, as shown in
The burner 10 useful in the burner sub-system of the present invention may also include a tip seal arrangement as will now be discussed in connection with
To establish a uniform dimension between the burner tip 20 and the peripheral burner tile enclosure 22 for the air gaps 70, a burner tip band 85, which may be formed of steel or other metal or metal composite capable of withstanding the harsh environment of an industrial burner, is attached to the outer periphery of burner tip 20, by tack welding or other suitable means. Advantageously, a compressible high temperature material 87 is optionally employed in the unwanted gap between the burner tip band 85 and the peripheral tile enclosure 22 to further reduce or eliminate the gap. Burner tip band 85 may further include a peripheral indentation 81 (see
Compressible material 87 is desirably rated for high temperature service since it is very close to the burner side port flames. A material that expands when heated is very useful as compressible material 87 because it makes the initial installation much easier. Examples of suitable materials include, but are not limited to; Triple T™ by Thermal Ceramics and Organically Bound Maftec™ (OBM Maftec™) distributed by Thermal Ceramics of Atlanta, Ga., a division of Morgan Crucible. OBM Maftec™ is preferable since it held together better after being exposed to high temperatures. OBM Maftec™ is produced from high quality mullite fiber. This material is known to possess low thermal conductivity and heat storage and is resistant to thermal shock and chemical attack. It additionally is highly flexible, has a maximum temperature rating of 2900° F. and a continuous use limit of up to 2700° F., making it ideal for this application. While the Triple T™ material expands more than the Maftec™, it was found to flake apart more easily after heating.
Referring now to
The burner sub-system of the present invention also comprises a FGR duct, which may be angled, as next discussed in connection with
With reference to the non-limiting example shown in
Referring again to
In addition to the use of flue gas as a diluent, another technique to achieve lower flame temperature through dilution is through the use of steam injection. Steam can be injected in the primary air or the secondary air chamber. Preferably, steam may be injected upstream of the venturi.
Referring now to
A first plane of radiant coils 1420 is arranged parallel to a plane P passing through the centerline L of the furnace floor 1414 and perpendicular to the furnace floor 1414. First plane of radiant coils 1420 is spaced at a distance greater than the distance that the first line of burners 1416 is spaced from the centerline L of the furnace floor 1414 and on the same side of the centerline L as the first row of burners 1416. A second plane of radiant coils 1422 is arranged parallel to plane P passing through the centerline L of furnace floor 1414 and perpendicular to furnace floor 1414. Second plane of radiant coils 1422 is spaced at a distance greater than the distance that the second line of burners 1418 is spaced from the centerline L of furnace floor 1414 and on the same side of the centerline L as the second row of burners 1418.
In one form, furnace 1410 may also include a second plurality of burners 1411 arranged along at least two parallel lines D3 and D4 to form a third line of burners 1426 and a fourth line of burners 1428, each line of burners spaced a substantially equal distance from the centerline L of the furnace floor 1414 at a distance greater than the distance that the first plane of radiant coils 1420 and the second plane of radiant coils 1422 are spaced from the centerline L of the furnace floor 1414, respectively.
In operation of furnace 1410, hydrocarbon feed is first preheated and, in the case of liquid feeds commonly at least partially vaporized, and mixed with dilution steam in the convection section 1432 of furnace 1410. The temperature exiting convection section 1432 is generally designed to be at or near the point where significant thermal cracking commences.
Typically, for example, this temperature is about 1050° F. (565° C.) to about 1150° F. (620° C.) for gas-oil feeds, about 1150° F. (620° C.) to about 1250° F. (675° C.) for naphtha feeds, and about 1250° F. (675° C.) to about 1350° F. (730° C.) for ethane feed. After preheating in convection section 1432, a vapor feed/dilution steam mixture is typically rapidly heated in the radiant section 1434 to achieve the desired level of thermal cracking. The coil outlet temperature (COT) of radiant section 1434 commonly can be in the range of from 1450° F. (790° C.) to about 1500° F. (815° C.) for gas oil feeds, about 1500° F. (815° C.) to about 1600° F. (870° C.) for naphtha feeds, and about 1550° F. (845° C.) to about 1650° F. (900° C.) for ethane feeds. After the desired degree of thermal cracking has been achieved in radiant section 1434, the furnace effluent is rapidly quenched in either an indirect heat exchanger 1436 and/or by the direct injection of a quench fluid stream (not illustrated).
In various examples, the plurality of burners 1411 of furnace 1410 may include raw gas burners, staged-fuel burners, staged air burners, premix staged air burners or combinations thereof. In another form the plurality of burners 1411 of furnace 1410 may include premix staged air burners and optionally with combinations including the preceding listed burners. Examples of premix staged air burners may be found in U.S. Pat. Nos. 4,629,413; 5,092,716, and 6,877,980, the contents of which are hereby incorporated by reference in their entirety. With burners of these types, tall flames are produced and commercial experience has confirmed there is no need for supplementary wall mounted burners. While the third line of burners 1426 and the fourth line of burners 1428 may of the same type as the first line of burners 1416 and the second line of burners 1418, flat-flame burners may be employed the third line of burners 1426 and the fourth line of burners 1428. As those skilled in the art will readily understand, a flat-flame burner is one that is typically stabilized, at least in part, by the furnace wall.
Highly stable flames with a tall height can be achieved by using the burner sub-system of the present invention. Thus it is highly desirable that the furnace firebox has a height of at least 8.0, 8.5, 9.0, 9.5, 10.0, 10.5, 11.0, 11.5, 12.0, 12.5, 13.0, 13.5, 14.0, 14.5, 15.0, 15.5, or even 16.0 meters. The tall walls of the furnace enable tall, stable flames having a height H(f) with a height in the range from Hf(1) to Hf(2),where Hf(1) and Hf(2) can be, independently, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, or even 10.0, as long as Hf(1)<Hf(2).
It is desired that the distance of the vertical centerline of any burner tip to any side wall is at least 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100 centimeters. Such relatively large distance between the flame and the side wall reduces the erosion of the side wall.
Because the stability of the flames achievable by the burner sub-system of the present invention, it is desirable that in certain examples, there is no intermediate partitioning wall between adjacent burner sub-systems. The burner sub-system enables large furnaces housing multiple rows of burners providing multiple flames capable of heating cracking tubes installed in between to the desired temperature ranges with the desired level of temperature variation. Side wall burner flames produced by side wall burners installed on the side walls of the furnace firebox may be eliminated, substantially reducing the overall cost of the furnace.
Although the burner sub-system, the furnace, and the processes of this invention have been described in connection with floor-fired hydrocarbon cracking furnaces, they may also be used in furnaces for carrying out other reactions or functions.
It will also be understood that the teachings described herein also have utility in traditional raw gas burners and raw gas burners having a pre-mix burner configuration wherein flue gas alone is mixed with fuel gas at the entrance to the burner tube.
Thus, it can be seen that, by use of this invention, burner tip can avoid premature failure due to overheating caused by reflection from the barrier wall, especially where hydrogen-rich fuel gas is used. In addition, NOx emissions may be reduced without the use of fans or otherwise special burners.
Although the invention has been described with reference to particular means, materials and embodiments, it is to be understood that the invention is not limited to the particulars disclosed and extends to all equivalents within the scope of the claims.
Thus, non-limiting aspects and embodiments of the present invention include:
| Number | Date | Country | Kind |
|---|---|---|---|
| 16170266.7 | May 2016 | EP | regional |
| Number | Date | Country | |
|---|---|---|---|
| 62316246 | Mar 2016 | US |
