Patent Application
20250084990
The disclosure relates to a burner comprising a heat shield.
Burners are used in various industrial complexes including, but not limited to, ethylene and olefin plants, where feedstock is “cracked” using a process which involves heating the feedstock to “crack” the molecules into several smaller molecules. For example, an ethane cracker “cracks” a feedstock comprising ethane into a product stream comprising ethylene. High temperatures are required, so crackers typically employ a radiant furnace to raise the temperature of the feed to the required temperature of from 750° C. to 850° C.
Radiant furnaces typically use premix fuel gas burners (e.g. radiant wall burners) to provide high heat release adjacent to a wall. General premix fuel gas burner design includes a mixing chamber where a fuel gas is premixed with an oxidant (e.g., combustion air) before passing through a burner tip into the furnace where combustion occurs. A concern with premix burners is that combustion inside the burner tip or mixing chamber can lead to thermal damage, which may deleteriously impact efficiency of heat release or even render the burner inoperable. There are also potential safety concerns. Combustion inside the burner can occur when the premix of fuel and oxidant ignite before exiting the burner (autoignition), or when flame propagation velocity exceeds the discharge velocity of the fuel and air mixture exiting the burner outlet or tip (flashback).
Autoignition occurs when a mixture of fuel and oxygen are within the flammability limits and the temperature reaches a critical point, or autoignition temperature. The autoignition temperature is dependent on the fuel. Fuel composed of mainly natural gas has a higher autoignition temperature than fuel containing significant amounts of hydrogen. The temperature of the inside wall for a typical burner can approach ˜760° C., which can be high enough for autoignition.
Conventional strategies for dealing with flashback have focused mainly on using burner tip geometries that promote a fuel discharge velocity that exceeds flame propagation speed. For example, the outlet of a burner that is inside a furnace may include a geometry that comprises slots, holes, or a combination of slots and holes to promote such conditions. The geometry of the outlet (e.g., surface area and mass) can also be designed in such proportions to the flame properties that the outlet will quench the flame if it begins to travel backwards. This is commonly referred to as a flame arrester. The internal geometry leading up to the exit can also be tailored (e.g., by including turning vanes) to reduce the risk of flashback.
However, conventional burner designs having a reduced risk of flashback can be limited to certain fuel gas compositions because fuel gas composition can also affect flame propagation speed. For example, pure methane has a laminar flame speed of approximately 0.5 m/s, compared to pure hydrogen, which has a laminar flame speed of approximately 3 m/s. The higher discharge velocities necessary for higher flame speeds can result in a higher pressure drop, which can in turn limit the firing rate or amount of air in the mixture. Increased flame speeds (e.g., for fuel gases including hydrogen) can also make the burner less robust to disturbances to, for example, fuel supply rate and ambient conditions (e.g., wind, dust, or pollen), and can limit the manufacturing tolerances of the burner itself.
There accordingly remains a need for design solutions to reduce the risk of combustion inside the tip and mixing chamber of a premix burner to allow the use of fuels having lower autoignition temperatures.
Provided herein is a burner comprising a housing, the housing comprising an inlet for receiving gas, one or more outlets for expelling gas, and walls forming a gas cavity for directing the gas from the inlet to the outlet. The burner includes a heat shield comprising a shield layer configured to cover at least a portion of at least one wall, to thermally insulate the at least one wall from heat outside the gas cavity.
Also provided herein is a method of using a burner described herein, wherein the method comprises providing a mixture of combustible gas and oxygen to the gas cavity via the inlet and igniting the mixture of combustible gas and oxygen after the gas is expelled from the outlet.
Various objects, features and advantages of the disclosure will be apparent from the following description of particular embodiments of the disclosure, as illustrated in the accompanying drawings. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of various embodiments of the disclosure. Similar reference numerals indicate similar components.
Provided herein is a burner heat shield that can be affixed to premix burners to reduce the risk of combustion occurring inside the burner tip and mixing chamber, allowing use of fuel with lower autoignition temperatures. The burner heat shield can be included as a feature of pre-mix burners but can also be used with pre-mix burners already in operation. Retrofitting furnaces with burner heat shields for all pre-mix burners would be significantly cheaper than replacing all premix burners.
Reducing the metal surface temperature inside the burners described herein can help to prevent ignition from occurring within the burner. Previous solutions allowed ignition to occur in the burner but were intended to sweep out an igniting mixture before it flashed back. Preventing ignition in the burner may be achieved by placing a heat shield in front of the front surface (or around the surfaces forming the outlet) of the burner, thus decreasing the amount of radiation reaching the surface of the burner and lowering the temperature of the interior surface of the burner.
The burners described herein can be particularly advantageous in an ethane cracking process, where byproduct hydrogen can be captured and then used as a source of fuel for the burners. Because the burner heat shield described herein can reduce the risk of ignition within the burner, the burner fuel gas can include relatively more captured hydrogen, decreasing the natural gas demand of the burner, and decreasing the combustion emissions of the burner.
Various aspects of the disclosure will now be described with reference to the figures. For the purposes of illustration, components depicted in the figures are not necessarily drawn to scale. Instead, emphasis is placed on highlighting the various contributions of the components to the functionality of various aspects of the disclosure. A number of possible alternative features are introduced during the course of this description. It is to be understood that, according to the knowledge and judgment of persons skilled in the art, such alternative features may be substituted in various combinations to arrive at different embodiments of the present disclosure.
In this case, the housing 101 is substantially rotationally symmetric about an axis. The housing comprises cylindrical side walls 121 configured to direct gas axially from the inlet 128, and a blocking wall 123 configured to redirect the gas to one or more laterally oriented outlets 125a-c. In this case, the shield layer 126 of the heat shield 102 is configured to cover the blocking wall 123.
In this case, combustible gas and oxygen or air is introduced into the inlet 128 at one end of the cylindrical housing. It will be appreciated that the components may be pre-mixed or introduced into the cavity via separate inlets and mixed within the cavity. The gas mixture moves along the axis of the housing until it is redirected laterally out through the outlets 125a-c.
In this case, the outlets are formed between a fluted or flared section 122 of the side wall 121, two fluted or flared guide vanes 124a-b and the blocking wall 123. These form three outlets 125a-c. As shown in
Regarding the temperature of the housing 101, under normal operation, the interior surfaces of the housing next to the cavity 127 will be cooled by the passage of gas within the chamber. In contrast, the exterior of the housing would be subject to the heat of the burning gas outside the housing and within the furnace. If the housing walls 121,123 transmit the heat into the interior of the housing to the extent that the temperature of the gas mixture exceeds the ignition temperature, the mixture may ignite within the cavity and damage or destroy the burner 100.
In this case, the volume within the cavity, which is most susceptible to heating, is next to the blocking wall 123. This may be because the blocking wall 123 is typically oriented to face the interior of the furnace and so receives more heat than the side walls 121. This is shown in
In a furnace, there may be dozens, or even hundreds, of burners mounted in the wall of the heater. The entire wall of the furnace would be heated and radiate heat to the process tubes in the center of the furnace. The radiated heat is accomplished by the number of burners and the shape of the flame. These burners have a radial flame shape that is directed along the wall and transfers heat from the combustion reaction to the hot surface of the heater wall.
Because the blocking wall is particularly susceptible to heating, as shown in
In this case, the blocking wall 123 and the shield layer 126 are separated by a gap. This prevents heat being transmitted directly to the blocking wall 123 through radiation and conduction. In this case, the heat shield 102 is attached to the blocking wall 123 only adjacent to the outlet 125c by clips 127a-b. The clips 127a-b of this embodiment are folded sections of metal which grip the edge of the blocking wall 123 at multiple discrete positions around the periphery of the blocking wall (e.g., 3-4 positions). This reduces the area through which heat can be transmitted to the covered wall through conduction. In addition, to the extent that heat can be transmitted to the blocking wall, the heat is transmitted to an area where the blocking wall experiences greatest cooling from the gas (as the velocity of the gas is highest adjacent to the outlet), and where, even if the gas were to be heated to a dangerous level, spontaneous ignition would have the lowest risk, as it would not occur within the body of the cavity.
To further reduce the heating of the shield layer 126, the shield may be concave in shape. This means that the center of the shield layer is recessed towards the cavity and away from the furnace interior. This reduces exposure of the heat shield to radiative heating and to convection currents. It may also help trap a layer of gas which is cooler than the body of the furnace.
In addition, in this case, the housing 101 is configured such that the outlets 125a-c are configured to eject the gas at an angle of more than 90° with respect to the axis of the burner. That is, gas travelling along the axis of the burner is turned more than 90° such that it is ejected slightly backwards (e.g. by the blocking wall, the vanes and the fluted section of the side wall) towards the surface 130 on which the burner is mounted. This allows the surface of the furnace to be heated so as to provide a more uniform heating within the furnace. It also means that when the gas is ignited, the direct exposure of the heat shield to the burning gas is reduced.
The gap is in this case isolated from (i.e. not in direct fluid communication with) the gas cavity. That is, gas from the cavity 127 can only enter the gap after being expelled via the outlets 125a-c. Combustion products can also enter the gap from the furnace chamber. The gap is configured to trap gas between the heat shield and the wall, to act as an insulating layer and to prevent direct exposure of the outside surface of the blocking wall 123 to radiation.
In this case, the blocking wall is also connected to a thermally conducting projection 129 positioned within the housing. In this case, the projection is a spindle supported by the blocking wall and/or ribs 131. But the projection 129 also acts a cooling mechanism as the exterior surfaces of the projection are exposed to the cooling effect of the gas moving within the cavity 127. This will help cool the blocking wall and distribute the heat more evenly. In some embodiments, thermally conducting projection 129 is a pure fuel line, i.e., free of oxidizer, with an outlet at the tip to expel pure fuel into the burner without a risk of combustion in the pure fuel line.
Using the burner 100 described above typically involves placing the burner within a furnace, such as a cracking furnace. A mixture of combustible gas and air or oxygen is provided to the gas cavity via one or more inlets. As noted above, the mixture may be premixed, or provided to the cavity separately and allowed to mix within the cavity.
The combustible gas may comprise any gaseous fuel, such as natural gas. The combustible gas may comprise hydrogen and/or methane. The mixture is ignited after the gas is expelled from the outlet.
The gas would be provided at a rate such that the speed of the gas exiting the outlet is greater than a predetermined speed. The predetermined speed would typically be greater than the flame speed of the mixture for the conditions of the furnace. The predetermined speed may be dependent on the combustible gas and the proportion of combustible gas to oxygen. The predetermined speed may be greater than 0.5 m/s. The predetermined speed may be greater than 3 m/s.
As noted above, the heat shield is configured to help ensure that the temperature of the interior walls of the housing do not reach the spontaneous ignition temperature of the gas mixture within the cavity. This may allow higher hydrogen ratios to be used than would otherwise be possible for a burner lacking the heat shield described herein. For example, this may allow for a hydrogen content of the fuel up to or even above 85%.
Using clips, threaded nuts or other releasable connectors allow the heat shield to be retrofitted to existing burners. In other embodiments, the heat shield may be permanently connected to the housing and/or the furnace wall.
The heat shield may be formed from a single sheet of metal, cut and folded and/or stamped into shape. The clip connectors may be formed by bending tabs around the periphery of the shield layer. The inherent resilience of the metal would allow the clips to be bent out to position the heat shield over the blocking wall and grip the blocking wall when released. In some embodiments, the heat shield may be cast to the required shape.
The heat shield may be made into a concave shape from a planar sheet of metal by stamping or any other typical methods of manufacture. The heat shield may be disc shaped. The heat shield may be integrally incorporated into the burner (i.e., forming a unit with the housing), or as a retrofit to an existing burner. The heat shield may be of unitary construction.
The heat shield may include a refractory material (e.g., having a low thermal conductivity). The refractory material may make up at least a portion of surface of the heat shield (e.g., including the surfaces of shield layer 126 facing towards and/or away from the blocking wall 123). Or the heat shield may be formed from the refractory material. The refractory material may be a ceramic material.
The heat shield may cover any portion (e.g., at least about 25%, or at least about 50%, or at least about 75%) of at least one of the walls (e.g., the blocking wall 123) that is sufficient to lower the temperature of the covered wall relative to a corresponding, uncovered wall. The heat shield may substantially or even entirely cover at least one of the walls (e.g., the blocking wall 123).
The blocking wall 123 and the shield layer may be separated by a gap of up to 0.5 inches. Or the blocking wall 123 and the shield layer 126 may be separated by a solid layer comprising a refractory material having a low thermal conductivity (i.e., in contact with the blocking wall and the shield layer). The refractory material may be a ceramic material. In this case, the low thermal conductivity of the refractory material (e.g., a ceramic material) limits heat transmission to the blocking wall 123.
The projection 129 may also be a fuel pipe introducing secondary (also known as staged) fuel to the furnace (and not into the cavity 127). In this case, the projection 129 provides additional cooling. The heat shield 102 may also be affixed mechanically (e.g., by a nut) to the projection 129 as an alternative or additional fixture to clips 127a and 127b. Or, as the thermally conducting projection is not necessary to the burner or the heat shield, the burner may not include a thermally conducting projection at all.
The burner may be a radiant wall burner. The burner may be a premix gas burner. The burner may be a radial burner (e.g. configured to eject gas radially around a circumference).
The burners may form part of a radiant furnace. The radiant furnace may comprise a chamber with burners arranged along the walls. The radiant furnace may be configured to heat a feedstock to between 750-850° C. (1380-1560° F.). The radiant furnace may be configured to heat a feedstock to facilitate cracking of the feedstock. The radiant furnace may form part of an ethylene or olefin plant.
The inventors have performed experiments and CFD (Computational fluid dynamics) simulations, which have shown that the inside surface of the burner outlet can be above the auto-ignition temperature of the air fuel mixture.
These simulations are consistent with photographs taken from the inside of a unshielded burner during operation, where the temperature can be measured from the radiation spectrum emitted by the surfaces of the housing.
These findings are consistent with an upper limit of hydrogen content (e.g. as stipulated for commercially available design) and indicates why the conventional designs cannot ensure that ignition does not occur inside the burner for higher hydrogen content. The flow inside the outlet or tip must then be fast enough and orderly enough that the mixture that is beginning to ignite is swept out of the outlet before it achieves self-sustaining combustion.
The present disclosure addresses this problem by lowering the temperature of the inside surface of the blocking wall by shielding this region from heat within the furnace.
Testing of a commercially available lean premix low NOX burner demonstrated an increase in hydrogen content to 92% from an 82% limit that existed across a range of firing rates and numerous outlet or tip designs.
Although the present disclosure has been described and illustrated with respect to preferred embodiments and preferred uses thereof, it is not to be so limited since modifications and changes can be made therein which are within the full, intended scope of the disclosure as understood by those skilled in the art.
One embodiment of the disclosure is a burner comprising a housing, the housing comprising an inlet for receiving gas, one or more outlets for expelling gas, and walls forming a gas cavity for directing the gas from the inlet to the outlet. The burner includes a heat shield comprising a shield layer configured to cover at least a portion of at least one of the walls, to thermally insulate the at least one of the walls from heat outside the gas cavity.
In an aspect, the burner comprises side walls configured to direct gas axially from the inlet, and a blocking wall configured to redirect the gas to one or more laterally oriented outlets, wherein the heat shield is configured to cover at least a portion of the blocking wall.
In an aspect, the covered at least one wall and the shield layer are separated by a gap.
In an aspect, the heat shield is attached to the covered at least one wall only adjacent to the outlet.
In an aspect, the shield layer is separated from the covered at least one wall by a gap, the gap not being in direct fluid communication with the gas cavity.
In an aspect, the shield layer is separated from the covered at least one wall by a refractory material having a low thermal conductivity, the refractory material being in contact with the shield layer and the covered at least one wall.
In an aspect, the burner is configured to be connected to a surface, the shield layer being configured to cover at least a portion of the at least one wall which has the same orientation as the surface.
In an aspect, the walls are metallic.
In an aspect, the heat shield comprises a metallic material.
In an aspect, the heat shield comprises a refractory material.
In an aspect, the refractory material is a ceramic material.
In an aspect, the shield layer consists of a single layer of material.
In an aspect, the shield layer comprises a first layer of a metallic material and a second layer of a refractory material.
In an aspect, the second layer makes up at least a portion of a surface of the shield layer.
In an aspect, the housing comprises one or more turning vanes for redirecting the gas towards the one or more outlets.
In an aspect, the covered at least one wall is connected to a thermally conducting projection positioned within the housing.
In an aspect, the heat shield is releasably connected to the housing.
In an aspect, the heat shield is integral to the housing.
In an aspect, the shield layer has a concave shape.
In an aspect, the shield layer is disc shaped.
In an aspect, the shield layer is ring shaped.
Another embodiment of the disclosure is a method of using a burner described herein, wherein the method comprises providing a mixture comprising combustible gas and oxygen to the gas cavity via the inlet, and igniting the mixture of combustible gas and oxygen after the gas is expelled from the outlet.
In an aspect, the combustible gas comprises hydrogen.
In an aspect, the combustible gas comprises methane.
In an aspect, the combustible gas is provided at a rate such that the speed of the gas exiting the outlet is greater than a predetermined speed.
In an aspect, the combustible gas comprises more than 85% hydrogen by volume.
Other than in the operating examples or where otherwise indicated, all numbers or expressions referring to quantities of ingredients, reaction conditions, etc. used in the specification and claims are to be understood as modified in all instances by the term “about”. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the present specification and attached claims are approximations that can vary depending upon the desired properties, which the present disclosure desires to obtain. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.
Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical values, however, inherently contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.
Also, it should be understood that any numerical range recited herein is intended to include all sub-ranges subsumed therein. For example, a range of “1 to 10” is intended to include all sub-ranges between and including the recited minimum value of 1 and the recited maximum value of 10; that is, having a minimum value equal to or greater than 1 and a maximum value of equal to or less than 10. Because the disclosed numerical ranges are continuous, they include every value between the minimum and maximum values. Unless expressly indicated otherwise, the various numerical ranges specified in this application are approximations.
The present disclosure relates to a heat shield for pre-mix burners.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2023/050372 | 1/16/2023 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63301637 | Jan 2022 | US |
