Patent Grant
9989246
The present invention relates to furnaces.
Water evaporation, process gas heating, steam cracking, and pyrolysis of hydrocarbons are examples of processes often carried out in tubular coils, the process coils, inside furnaces. These processes are often considered heart of the industrial plant and have significant influence on the economics of the overall industrial plant. The duration that heater or tubular coils can operate without failure depends on two primary factors: fouling and crack initiation. Fouling and cracking are forms of coil degradation. Fouling occurs when deposits, such as coke and scale, build up on the inside surfaces of heating coil. These deposits in process fluid stream act as a resistance to heat flux and the outside metal temperature of the tube increases in response to allow for the equivalent flux through a higher resistance. The second factor is crack initiation, which depends strongly upon the makeup of the radiant heating coil, thermal stresses, and fatigues. Typically, the coil is made up of metal or metal alloy and has a nominal operating temperature range of from 400 K to 1400 K. Metals and metal alloys are sensitive to extreme temperatures. The coil will begin to deteriorate and become damaged or at least prone to damage when the coil is exposed to a temperature that exceeds the upper end of its nominal operating temperature range. As a result, a typical heater must be monitored carefully at substantial cost to maintain specific temperature ranges. This becomes problematic as deposits build up on the coil because more heat must be added to maintain the efficiency of the system.
For example, in a typical process gas heater used in the refineries or steel industries, the reformed gas, e.g. CO, H2, CO2, etc., mixture is preheated to a temperature of from about 400° C. to 600° C. This preheat occurs in the convection section of the heater. The mix then passes to the radiant section where a constant outlet temperature on the order of about 700° C. to 950° C. is maintained. The flue gas temperature exiting the radiant section of the fired heater is typically above 1,000° C. The heat transfer to the coils is primarily by radiation. In some conventional designs, such as boilers in power plants, approximately from 30% to 40% of the heat fired as fuel into the furnace is transferred into the coils in the radiant section. The balance of the heat is recovered in the convection section either as feed preheat or to superheat steam. Given the limitation of small tube volume to achieve short residence times and the high temperatures of the process, heat transfer into the reaction tube is difficult. As a result, high heat fluxes are used and the operating tube metal temperatures are close to the mechanical limits for even exotic metallurgies.
In most cases, tube metal temperatures limit the extent to which residence time can be reduced. A combination of higher process temperatures required at the coil outlet and the reduced tube length, i.e. the reduced tube surface area, results in higher flux and higher tube metal temperatures. Tube metal temperatures are also a limiting factor in determining the capacity of these radiant coils since more flux is required for a given tube when operated at higher capacity. The exotic metal reaction tubes located in the radiant section of the cracking heater represent a substantial portion of the cost of the heater. Therefore, it is important that they are operated at as high and as uniform a heat flux as possible consistent with the design objectives of the heater. This will minimize the number and length of the tubes and the resulting total metal surface area required for a given design capacity. Furthermore, having uniform heat flux across tube bundles will cause uniform thermal elongation of coils resulting in greater life of spring hangers on which coils are suspended and thus minimizes maintenance requirement.
In a typical furnace, the heat is supplied by burners, which can be mounted at the furnace floor, the furnace roof, the furnace sidewalls, or some combination thereof. The coils are typically suspended from the top of the radiant section and hang between the radiant walls. A small portion of the heat transferred is done convectively by the flue gases within the firebox transferring the heat directly to the coils. However in a typical furnace, greater than 85% of the heat is transferred by radiation.
In any flame from a burner, the flame has an inherent characteristic combustion profile, inherently generates heat, and inherently generates soot. As the fuel and air mixture leaves the burner, combustion begins. As the combustion reaction continues, the temperature of the combustion mixture increases and heat is released. At some distance from the burner, there is a point of inherent maximum combustion by the flame and hence an inherent maximum or peak heat release. During this process, heat is absorbed by the process coils. The characteristics of the flame, and its inherent maximum or peak heat, depend upon the total firing from that burner and the specifics of the burner design. Different flame shapes and heat release profiles are possible, depending upon how the fuel and air are mixed. Because of the characteristic heat release profile from these burners, an uneven heat flux profile, i.e. heat absorbed profile, is sometimes created. The typical flux profile for the radiant coil shows a peak flux near the center elevation of the firebox, i.e. at the point of maximum combustion or heat release for the hearth burners, with the top and bottom portions of the coil receiving less flux. In some heaters, radiant wall burners are installed in the top part of the sidewalls to equalize the heat flux profile in the top portion of the coil.
There have been a number of attempts to control the flux profile within a heater. It is known that staging the fuel to burners can be used to adjust the flame shape and thus impact the point of maximum heat release. Sometimes burners are designed with several differing fuel injection points. In some methods, side burners are used in combination with floor burners in a box chamber where combustion gases pass upwardly through the radiant chamber to a convention section. Methods of producing internal recirculation of combustion gases into the burner for producing a favorable influence on homogenizing combustible mixtures at the burners for reducing flame temperature and NOx emission have also been proposed. Still other methods have been proposed that depend on injection of steam in the furnace to reduce peak temperature and NOx emission. The results of these and other efforts, however, have not been entirely satisfactory, thereby necessitating further improvement in the art.
According to the principle of the invention, a method includes providing a furnace, the furnace includes a radiant heating zone having metal heating coils and burners, concurrently applying a combustion media, having a combustibility, and a diluent to the burners, the burners burning the combustion media producing flames heating the radiant heating zone, and the diluent reducing the combustibility of the combustion media for reducing peak heat generated by the flames for reducing heat degradation, such as fouling and cracking, of the metal heating coils. The combustion media includes fuel. In another embodiment, the combustion media includes fuel and air. The diluent is selected from a group consisting of flue gas, steam, hydrogen, carbon dioxide, and nitrogen. Flue gas and steam diluents each produce reduced soot formation by the flames, in accordance with the principle of the invention. Radiation flux directly correlates to flame emissivity. Accordingly, soot formation by the flames is reduced, which reduces flame emissivity of the flames, the ability of the flames to emit radiant energy, which, in turn, reduces local radiation flux, all of which contributes to reducing the temperature of the metal heating coils and reducing heat degradation of the metal heating coils, when the diluent is flue gas in one embodiment, and when the diluent is steam in another embodiment.
According to the principle of the invention, a method includes providing a furnace, the furnace includes a radiant heating zone having metal heating coils and burners, applying combustion media to the burners, the combustion media has a combustibility and includes, air, fuel, and a diluent in at least one of the air and the fuel, the burners burning the combustion media producing flames heating the radiant heating zone, and the diluent reducing the combustibility of the combustion media for reducing the peak heat generated by the flames for reducing heat degradation, such as fouling and cracking, of the metal heating coils. The combustion media includes fuel. In another embodiment, the combustion media includes fuel and air. The diluent is selected from a group consisting of flue gas, steam, hydrogen, carbon dioxide, and nitrogen. Flue gas and steam diluents each produce reduced soot formation by the flames, in accordance with the principle of the invention. Again, radiation flux directly correlates to flame emissivity. Accordingly, soot formation by the flames is reduced, which reduces flame emissivity of the flames, the ability of the flames to emit radiant energy, which, in turn, reduces local radiation flux, all of which contributes to reducing the temperature of the metal heating coils and reducing heat degradation of the metal heating coils, when the diluent is flue gas in one embodiment, and when the diluent is steam in another embodiment.
According to the principle of the invention, a method includes providing a furnace, the furnace includes a radiant heating zone having metal heating coils and burners, applying a combustion media to the burners, the combustion media having a combustibility, the burners burning the combustion media producing flames heating the radiant heating zone, applying a diluent to the radiant heating zone, and the diluent reducing the combustibility of the combustion media for reducing peak heat generated by the flames for reducing heat degradation, such as fouling and cracking, of the metal heating coils. The combustion media includes fuel. In another embodiment, the combustion media includes fuel and air. The diluent is selected from a group consisting of flue gas, steam, hydrogen, carbon dioxide, and nitrogen. Flue gas and steam diluents each produce reduced soot formation by the flames, in accordance with the principle of the invention. Again, radiation flux directly correlates to flame emissivity. Accordingly, soot formation by the flames is reduced, which reduces flame emissivity of the flames, the ability of the flames to emit radiant energy, which, in turn, reduces local radiation flux, all of which contributes to reducing the temperature of the metal heating coils and reducing heat degradation of the metal heating coils, when the diluent is flue gas in one embodiment, and when the diluent is steam in another embodiment.
Referring to the drawings:
Furnaces and methods of reducing heat degrading of metal heating coils of furnaces are disclosed.
In general, an exemplary method includes providing a furnace, the furnace includes a radiant heating zone having metal heating coils and burners, concurrently applying a combustion media, having a combustibility, and a diluent to the burners, the burners burning the combustion media producing flames heating the radiant heating zone, and the diluent reducing the combustibility of the combustion media for reducing heat generated by the flames for reducing heat degradation, such as fouling and cracking, of the metal heating coils. The combustion media includes fuel. In another embodiment, the combustion media includes fuel and air. The diluent is selected from a group consisting of flue gas, steam, hydrogen, carbon dioxide, and nitrogen. Flue gas and steam diluents each produce reduced soot formation by the flames, in accordance with the principle of the invention. Radiation flux directly correlates to flame emissivity. Accordingly, soot formation by the flames is reduced, which reduces flame emissivity of the flames, the ability of the flames to emit radiant energy, which, in turn, reduces local radiation flux, all of which contributes to reducing the temperature of the metal heating coils and reducing heat degradation, such as fouling and cracking, of the metal heating coils, when the diluent is flue gas in one embodiment, and when the diluent is steam in another embodiment.
Another method embodiment of the invention includes providing a furnace, the furnace includes a radiant heating zone having metal heating coils and burners, applying combustion media to the burners, the combustion media has a combustibility and includes, air, fuel, and a diluent in at least one of the air and the fuel, the burners burning the combustion media producing flames heating the radiant heating zone, and the diluent reducing the combustibility of the combustion media for reducing the heat generated by the flames for reducing heat degradation, such as fouling and cracking, of the metal heating coils. The combustion media includes fuel. In another embodiment, the combustion media includes fuel and air. The diluent is selected from a group consisting of flue gas, steam, hydrogen, carbon dioxide, and nitrogen. Flue gas and steam diluents each produce reduced soot formation by the flames, in accordance with the principle of the invention. Again, radiation flux directly correlates to flame emissivity. Accordingly, soot formation by the flames is reduced, which reduces flame emissivity of the flames, the ability of the flames to emit radiant energy, which, in turn, reduces local radiation flux, all of which contributes to reducing the temperature of the metal heating coils and reducing heat degradation, such as fouling and cracking, of the metal heating coils, when the diluent is flue gas in one embodiment, and when the diluent is steam in another embodiment.
Yet another method embodiment of the invention includes providing a furnace, the furnace includes a radiant heating zone having metal heating coils and burners, applying a combustion media to the burners, the combustion media having a combustibility, the burners burning the combustion media producing flames heating the radiant heating zone, applying a diluent to the radiant heating zone, and the diluent reducing the combustibility of the combustion media for reducing heat generated by the flames for reducing heat degradation, such as fouling and cracking, of the metal heating coils. The combustion media includes fuel. In another embodiment, the combustion media includes fuel and air. The diluent is selected from a group consisting of flue gas, steam, hydrogen, carbon dioxide, and nitrogen. Flue gas and steam diluents each produce reduced soot formation by the flames, in accordance with the principle of the invention. Again, radiation flux directly correlates to flame emissivity. Accordingly, soot formation by the flames is reduced, which reduces flame emissivity of the flames, the ability of the flames to emit radiant energy, which, in turn, reduces local radiation flux, all of which contributes to reducing the temperature of the metal heating coils and reducing heat degradation, such as fouling and cracking, of the metal heating coils, when the diluent is flue gas in one embodiment, and when the diluent is steam in another embodiment.
According to the principle of the invention, a furnace or heater has a radiant section or zone and a convection heating section or zone where the radiant section consists of a furnace floor and side walls including multiple burners and multiple heating coils of metal, wherein the term “metal” means a metal or a metal alloy. An illustrative embodiment of operating such a furnace includes introducing a diluent, such as a product of combustion from a flue gas stack of a furnace in an illustrative example, into an air stream and or a gas stream that is fed to a burner section of the furnace for combustion. The product of combustion is applied to the air via a prevailing difference in static pressure at a flue gas stack of the furnace and a suction line of a combustion fan that delivers air to the burners. In this embodiment, a flue gas recirculation (FGR) ratio in the range of 5-15% is achieved, wherein the FGR ratio is defined as:
FGR ratio (%)=100[G/(F+A)]
Where G=Flue gas flow drawn into air (lb/hr);
A=Air drawn into burner (lb/hr); and
F=Fuel flow drawn into burner (lb/hr).
In other embodiments, the ability to generate a high flue gas ratio can be achieved by using at least one flue gas fan. In other embodiments, CO2 generated by other processes in the industrial plant have sufficient static pressure. Therefore, another aspect of the invention includes diverting a diluent, such as a stream of CO2, generated by the plant processes in an example, to mix with air prior to feeding into the radiant heating zone of the furnace. Still other embodiments of the invention include diverting diluent process steam, diluent hydrogen, diluent carbon dioxide, diluent nitrogen, a combination of two or more of the foregoing diluents or other diluent or combination of diluents, such as from a combustion process, to be mixed with fuel prior to feeding into the radiant heating zone of the furnace, or to be injected directly into the radiant zone of the furnace as a separate stream.
The firebox temperature profile, i.e. the temperature of the radiant heating zone of the firebox, and reduction in peak metal skin temperatures is improved by injection of the diluent in the combustion media (air and/or fuel stream) without having to raise the fuel firing rate. In other words, injecting the chosen diluent, which can be one diluent or a combination of diluents, in the combustion media, the air and/or the fuel feed to the burners at the radiant heating zone of the furnace without raising the fuel firing rate has a favorable influence on the firebox temperature profile, i.e. the temperature of the radiant heating zone, and the reduction in peak metal skin temperatures. Injection of the diluent not only lowers the burner flame temperature of the burners but also raises the temperature at the end portion of firebox thereby leveling out the temperature profile across the longitudinal dimension of the firebox and distribution of the more heat into downstream sections such as convection box or air preheater. Flue gas and steam diluents each produce reduced soot formation by the flames of the burners, in accordance with the principle of the invention. Accordingly, soot formation by the flames is reduced, which reduces flame emissivity of the flames, the ability of the flames to emit radiant energy, which, in turn, reduces local radiation flux, all of which contributes to reducing the temperature of the metal heating coils and reducing heat degradation, such as fouling and cracking, of the metal heating coils, when the diluent is flue gas in one embodiment, and when the diluent is steam in another embodiment, such as fouling and cracking,
The invention relates to improvements in furnaces and furnace operations with an objective of improving the operational life of metal heating coils, namely, reducing heat degradation, such as fouling and cracking, of the metal tubes, and reducing burner combustion for providing a uniform heat flux across the metal heating coils. In one aspect, the invention relates to the use of a flue gas as the diluent that is recirculated with combustion air for producing homogeneous temperature profiles in the furnace and across the metal heating coils. The flue gas is mixed with air upstream or downstream of a fan used to deliver combustion air to the burners at the radiant heating zone of the furnace. In other aspects, a stream of flue gas or other gas or diluent, such CO2 or steam generated by other processes in the industrial plant, are mixed with the combustion media to the burners, i.e. the fuel, the air, or both the fuel and the air, for reducing combustion of the flammable gas by the burners for reducing heat degradation of the metal heating coils by favorably influencing heat flux across the heating coils in the radiant heating zone. Other benefits of the invention are reduction in NOx emissions, reduction in soot formation by the flames when the diluent is flue gas or steam, improved heat distribution across radiant and convection zones for providing improving efficiency, and uniform thermal elongation of metal tubes. In other words, other benefits of the invention include favorably influencing reduction in NOx emissions, reduction in soot formation by the flames, heat distribution across radiant and convection zones for favorably influencing efficiency, and uniform thermal elongation of metal tubes. The invention has application in heaters for steam generation, and fired heaters in chemical and metal manufacturing industries as well as petroleum refining such as high-temperature cracking of hydrocarbon gases, thermal polymerization of light hydrocarbons, or hydrogenation of oils.
The present invention is described in terms of a burner for use in connection with a furnace, which can be an industrial furnace. It will readily occur to those skilled in the art that the teachings of the present invention also have applicability to other process components, such as boilers in a particular example. Thus, the term “furnace” used herein shall be understood to mean a furnace, a boiler, and other applicable process heaters.
Operation of furnace 1 generates flue gas 23, a product of combustion of the combustion media by burners 15, which is ejected through stack 22 at the top of convection box 3A. Flue gas 23 is exhausted through stack 22 either by an induced draft fan or with no further assistance when heater 1 is operated at a positive pressure, such as from 5 mbar to 100 mbar in an illustrative embodiment. In a preferred embodiment, conduit 40 couples stack 22 to air header 16 in gaseous communication. Flue gas 23 is a diluent. Part of flue gas 23 is harvested from stack 22 via conduit 40, which transfers the harvested flue gas 23 from stack 22 to air header 16 where it is mixed with air 18 at header 16. Control valve 25 incorporated in conduit 40 is used to control and set the amount of flue gas 23 applied to air 18 in header 16 from stack 22.
Fuel 21 is fed to each burner 15 via fuel header 20. Again, the combustion media applied to burners 15 is a mixture of fuel 21 fed to burners 15 via fuel header 20, and air 18 mixed with flue gas 23 applied to the burners 15 from header 17 that is delivered to header 17 from header 16 in this example. And so the air 18 component of the combustion media, which incorporates the diluent flue gas 23, is continuously applied to burners 15 from air header 17, and the fuel 21 of the combustion media is continuously applied to burners 15 from fuel header 20. Flue gas 23 is continuously recirculated via conduit 40 and headers 16 and 17 from stack 22 to burners 15. Control valve 19 in fuel header 20 regulates the amount of fuel 21 flowing to burners 15. Air 18 incorporating the flue gas 23 is delivered into air header 17 by a forced draft fan 24 from air header 16, and then to burners 15 from air header 17. Application of the diluent flue gas 23 mixed with air 18 in this example provides reduced burning of the combustion gas by burners 15 for producing lowered temperature of flames 15a maintained by burners 15 for achieving the various objectives of the invention, namely, favorably influencing NOx emissions, i.e. reducing NOx emissions, favorably influencing soot formation by flames 51a, i.e. reducing soot formation by flames 15a, favorably influencing heat distribution across radiant and convection zones for favorably influencing efficiency, i.e. reducing heat distribution across radiant and convection zones, and favorably influencing uniform thermal elongation of metal tubes that form heating coils 7, 8, and 9, all for favorably influencing metal heating coil life, i.e., reducing heat degradation of the metal heating coils 7, 8, and 9, according to the principle of the invention. Flue gas 23 produces reduced soot formation by flames 15a, in accordance with the principle of the invention. Radiation flux directly correlates to flame emissivity. Accordingly, soot formation by flames 15a is reduced, which reduces flame emissivity of the flames 15a, the ability of the flames 15a to emit radiant energy, which, in turn, reduces local radiation flux, all of which contributes to reducing the temperature of the metal heating coils 7, 8, and 9 and reducing heat degradation of the metal heating coils 7, 9, and 9, when the diluent is flue gas 23.
According to the principle of the invention with reference to
In the embodiment discussed in
According to the principle of the invention, an alternate method includes providing furnace 1, the furnace includes radiant heating zone 2 having metal heating coils 7,8,9 and burners 15, applying the combustion media to the burners, the combustion media has an inherent combustibility and includes, air 18, fuel 21, and a diluent, diluent flue gas 23 in this example, in at least one of air 18 and fuel 21, the burners 15 burning the combustion media producing flames 15a heating radiant heating zone 2, and the diluent reducing the combustibility of the combustion media for reducing heat generated by the flames 15a for reducing heat degradation of the metal heating coils 7,8,9. In this example, the diluent is flue gas 23. As in the previous embodiment, soot formation by flames 15a is reduced, which reduces flame emissivity of the flames 15a, the ability of the flames 15a to emit radiant energy, which, in turn, reduces local radiation flux, all of which contributes to reducing the temperature of the metal heating coils 7, 8, and 9 and reducing heat degradation of the metal heating coils 7, 9, and 9, when the diluent is flue gas 23. As in the prior embodiment, the diluent can be selected from a group consisting of flue gas, steam, hydrogen, carbon dioxide, and nitrogen, and can be composed of two or more diluents, such as from the group consisting of flue gas, steam, hydrogen, carbon dioxide, and nitrogen. Again, steam produces reduced soot formation by flames 15a, in accordance with the principle of the invention. Accordingly, in this alternate embodiment soot formation by flames 15a is reduced, which reduces flame emissivity of the flames 15a, the ability of the flames 15a to emit radiant energy, which, in turn, reduces local radiation flux, all of which contributes to reducing the temperature of the metal heating coils 7, 8, and 9 and reducing heat degradation of the metal heating coils 7, 9, and 9, when the diluent is steam.
Referring to
Referring to
Referring to
The present invention is described above with reference to illustrative embodiments. However, those skilled in the art will recognize that changes and modifications may be made in the described embodiments without departing from the nature and scope of the present invention. Various further changes and modifications to the embodiment herein chosen for purposes of illustration will readily occur to those skilled in the art. To the extent that such modifications and variations do not depart from the spirit of the invention, they are intended to be included within the scope thereof.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/051,822, filed Sep. 17, 2014, and U.S. Provisional Patent Application Ser. No. 62/165,718, filed May 22, 2015, the entire contents of which are incorporated herein by reference.
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| Number | Date | Country | |
|---|---|---|---|
| 20160076761 A1 | Mar 2016 | US |
| Number | Date | Country | |
|---|---|---|---|
| 62051822 | Sep 2014 | US | |
| 62165718 | May 2015 | US |
