Burner employing flue-gas recirculation system

A method and apparatus for reducing the temperature of the recirculated flue gas in a flue gas recirculation duct for burners in industrial furnaces such as those used in steam cracking. The apparatus includes a burner tube having a downstream end and an upstream end for receiving air, flue gas and fuel gas, a burner tip mounted on the downstream end of the burner tube adjacent a first opening in the furnace, so that combustion of the fuel takes place downstream of the burner tip; at least one passageway having a first end at a second opening in the furnace and a second end adjacent the upstream end of the burner tube, the passageway having an orifice in fluid communication with a source of air which is cooler than the flue gas; and a mechanism for drawing flue gas from the furnace through the passageway and air from the orifice of the passageway in response to an inspirating effect created by uncombusted fuel flowing through the burner tube from its upstream end towards its downstream end, whereby the flue gas is mixed with air from the orifice of the passageway prior to the zone of combustion of the fuel to thereby lower the temperature of the drawn flue gas.

FIELD OF THE INVENTION

This invention relates to an improvement in a burner such as those employed in high temperature furnaces in the steam cracking of hydrocarbons. More particularly, it relates to the use of a burner of novel configuration to reduce the temperature of recirculated flue gas.

BACKGROUND OF THE INVENTION

As a result of the interest in recent years to reduce the emission of pollutants from burners used in large industrial furnaces, burner design has undergone substantial change. In the past, improvements in burner design were aimed primarily at improving heat distribution. Increasingly stringent environmental regulations have shifted the focus of burner design to the minimization of regulated pollutants.

Oxides of nitrogen (NOx) are formed in air at high temperatures. These compounds include, but are not limited to, nitrogen oxide and nitrogen dioxide. Reduction of NOxemissions is a desired goal to decrease air pollution and meet government regulations. In recent years, a wide variety of mobile and stationary sources of NOxemissions have come under increased scrutiny and regulation.

A strategy for achieving lower NOxemission levels is to install a NOxreduction catalyst to treat the furnace exhaust stream. This strategy, known as Selective Catalytic Reduction (SCR), is very costly and, although it can be effective in meeting more stringent regulations, represents a less desirable alternative to improvements in burner design.

Burners used in large industrial furnaces may use either liquid fuel or gas. Liquid fuel burners mix the fuel with steam prior to combustion to atomize the fuel to enable more complete combustion, and combustion air is mixed with the fuel at the zone of combustion.

Gas fired burners can be classified as either premix or raw gas, depending on the method used to combine the air and fuel. They also differ in configuration and the type of burner tip used.

Raw gas burners inject fuel directly into the air stream, and the mixing of fuel and air occurs simultaneously with combustion. Since airflow does not change appreciably with fuel flow, the air register settings of natural draft burners must be changed after firing rate changes. Therefore, frequent adjustment may be necessary, as explained in detail in U.S. Pat. No. 4,257,763. In addition, many raw gas burners produce luminous flames.

Premix burners mix some or all of the fuel with some or all of the combustion air prior to combustion. Since premixing is accomplished by using the energy present in the fuel stream, airflow is largely proportional to fuel flow. As a result, therefore, less frequent adjustment is required. Premixing the fuel and air also facilitates the achievement of the desired flame characteristics. Due to these properties, premix burners are often compatible with various steam cracking furnace configurations.

Floor-fired premix burners are used in many steam crackers and steam reformers primarily because of their ability to produce a relatively uniform heat distribution profile in the tall radiant sections of these furnaces. Flames are non-luminous, permitting tube metal temperatures to be readily monitored. Therefore, a premix burner is the burner of choice for such furnaces. Premix burners can also be designed for special heat distribution profiles or flame shapes required in other types of furnaces.

One technique for reducing NOxthat has become widely accepted in industry is known as combustion staging. With combustion 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 NOxformation than an air-fuel ration 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 NOxformation is exponentially dependent on gas temperature, even small reductions in peak flame temperature dramatically reduce NOxemissions. However this must be balanced with the fact that radiant heat transfer decreases with reduced flame temperature, while CO emissions, an indication of incomplete combustion, may actually increase as well.

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 are 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. 4,004,875, the contents of which are incorporated by reference in their entirety, discloses a low NOxburner, in which combusted fuel and air is cooled and recirculated back into the combustion zone. The recirculated combusted fuel and air is formed in a zone with a deficiency of air.

U.S. Pat. No. 4,629,413 discloses a low NOxpremix burner and discusses the advantages of premix burners and methods to reduce NOxemissions. The premix burner of U.S. Pat. No. 4,629,413 lowers NOxemissions by delaying the mixing of secondary air with the flame and allowing some cooled flue gas to recirculate with the secondary air. 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 NOxemissions from premix burners by recirculating flue gas. Flue gas is drawn from the furnace through a pipe or pipes by the inspirating effect of fuel gas and combustion air passing through a venturi portion of a burner tube. The flue gas mixes with combustion air in a primary air chamber prior to combustion to dilute the concentration of O2in the combustion air, which lowers flame temperature and thereby reduces NOxemissions. The flue gas recirculating system may be retrofitted into existing premix burners or may be incorporated in new low NOxburners. The contents of U.S. Pat. No. 5,092,761 are incorporated by reference in their entirety.

A drawback of the system of U.S. Pat. No. 5,092,761 is that the staged-air used to cool the FGR duct must first enter 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.

Despite these advances in the art, a need exists for a burner having a desirable heat distribution profile that meets increasingly stringent NOxemission regulations and results in acceptable FGR duct temperatures.

Therefore, what is needed is a burner for the combustion of fuel gas and air wherein the temperature of the fuel/air/flue-gas mixture is advantageously reduced and which also enables higher flue gas recirculation ratios (FGR) to be utilized in order to meet stringent emissions regulations. The required burner will provide extended FGR duct life as a result of the lower temperature of the recirculated gas.

SUMMARY OF THE INVENTION

The present invention is directed to a method and apparatus for reducing the temperature of recirculated flue gas in a flue gas recirculation duct for use in burners of furnaces such as those used in steam cracking. The apparatus includes a burner tube having a downstream end, and having an upstream end for receiving air, flue gas and fuel gas, a burner tip mounted on the downstream end of said burner tube adjacent to a first opening in the furnace, so that combustion of the fuel takes place downstream of the burner tip, at least one passageway having a first end at a second opening in the furnace and a second end adjacent the upstream end of the burner tube, the passageway having an orifice; at least one bleed air duct having a first end and a second end, the first end in fluid communication with the orifice of the at least one passageway and the second end in fluid communication with a source of air which is cooler than the flue gas, and means for drawing flue gas from the furnace through the at least one passageway and air from the at least one bleed air duct through said at least one passageway in response to an inspirating effect created by uncombusted fuel flowing through the burner tube from its upstream end towards its downstream end, whereby the flue gas is mixed with air from the air bleed duct prior to the zone of combustion of the fuel to thereby lower the temperature of the drawn flue gas.

The method of the present invention includes the steps of combining fuel, air and flue gas at a predetermined location, combusting the fuel at a combustion zone downstream of said predetermined location, drawing a stream of flue gas from the furnace in response to the inspirating effect of uncombusted fuel flowing towards the combustion zone; and mixing air drawn from a duct, the air having a temperature lower than the temperature of the flue gas, with the stream of flue gas so drawn and drawing the mixture of the lower temperature air and flue gas to the predetermined location to thereby lower the temperature of the drawn flue gas.

An object of the present invention is to provide a burner arrangement that permits the temperature of the air and flue gas mixture in the FGR duct to be reduced, thus prolonging the life of the FGR duct. Alternatively, the arrangement permits the use of higher FGR ratios at constant venturi temperature.

These and other objects and features of the present invention will be apparent from the detailed description taken with reference to accompanying drawings.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Reference is now made to the embodiments illustrated inFIGS. 1-10wherein like numerals are used to designate like parts throughout.

Although the present invention is described in terms of a burner 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.

Referring now toFIGS. 1-4, a premix burner10includes a freestanding burner tube12located in a well in a furnace floor14. Burner tube12includes an upstream end16, a downstream end18and a venturi portion19. Burner tip20is located at downstream end18and is surrounded by an annular tile22. A fuel orifice11, which may be located within a gas spud24, is located at upstream end16and introduces fuel gas into burner tube12. Fresh or ambient air is introduced into primary air chamber26through adjustable damper28to mix with the fuel gas at upstream end16of burner tube12. Combustion of the fuel gas and fresh air occurs downstream of burner tip20.

A plurality of air ports30originates in secondary air chamber32and pass through furnace floor14into the furnace. Fresh air enters secondary air chamber32through adjustable dampers34and passes through staged air ports30into the furnace to provide secondary or staged combustion, as described in U.S. Pat. No. 4,629,413.

In order to recirculate flue gas from the furnace to the primary air chamber, ducts or pipes36,38extend from openings40,42, respectively, in the floor of the furnace to openings44,46, respectively, in burner10. Pipes36and38are preferably formed from metal and are inserted in openings40and42so as to extend only partially therethrough and not directly meet with the interior surface of the furnace as shown in FIG.2. This configuration avoids direct contact with and radiation from the very high gas temperatures at openings40and42.

Flue gas containing, for example, 0 to about 15% O2is drawn through pipes36,38, with about 5 to about 15% O2preferred, about 2 to about 10% O2more preferred and about 2 to about 5% O2particularly preferred, by the inspirating effect of fuel gas passing through venturi portion19of burner tube12. In this manner, air and flue gas are mixed in primary air chamber26, which is prior to the zone of combustion. Therefore, the inert material mixed with the fuel reduces the flame temperature and, as a result, reduces NOxemissions.

Closing or partially closing damper28restricts the amount of fresh air that can be drawn into the primary air chamber26and thereby provides the vacuum necessary to draw flue gas from the furnace floor.

Unmixed low temperature ambient air, having entered secondary air chamber32through dampers34is drawn from air port30through orifice62, through bleed air duct64, through orifice60into pipes36,38into the primary air chamber by the inspirating effect of the fuel gas passing through venturi portion19. 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 pipes36,38and thereby substantially increases the life of the pipes36and38and allows use of this type of burner to reduce NOxemission in high temperature cracking furnaces having flue gas temperature above 1900° F. in the radiant section of the furnace. Bleed air duct64has a first end66and a second end68, first end66connected to orifice60of pipe36or38and second end68connected to orifice62of air port30.

Additionally, a minor amount of unmixed low temperature ambient air, relative to that amount passing through duct64, having passed through air ports30into the furnace, may also be drawn through pipes36,38into the primary air chamber by the inspirating effect of the fuel gas passing through venturi portion19. To the extent that damper28is completely closed, bleed air duct64should be sized so as to permit the necessary flow of the full requirement of primary air needed by burner10.

Advantageously, a mixture of from about 20% to about 80% flue gas and from about 20% to about 80% ambient air should be drawn through pipes36,38. 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 pipes36,38, bleed air ducts64and air ports30, 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 port50is provided in the burner10, both to allow inspection of the interior of the burner assembly, and to provide access for lighting of the burner. The burner plenum may be covered with mineral wool and wire mesh screening54to serve as insulation.

An alternate embodiment to the premix burner ofFIGS. 1-4is shown inFIG. 5, wherein like reference numbers indicate like parts. As may be seen, the main difference between the embodiment ofFIGS. 1-4, and that ofFIG. 5, is that the latter employs only a single recirculation pipe56. In this embodiment, for example, a single 6-inch diameter pipe is used to replace two 4-inch diameter pipes. Once again, the desired proportions of flue gas and ambient air may be achieved by the proper sizing, placement and/or design of pipe56, bleed air duct64and air ports30. In this embodiment, furnace floor14, comprised of a high temperature, low thermal conductivity material, which may, for example, be selected from ceramics, ceramic fibers or castable refractory materials, includes a wall portion65having an air bleed duct64formed through the wall portion65of the furnace floor14. In this configuration, the temperature of the metallic recirculation pipe56is minimized.

The improved flue gas recirculating system of the present invention may also be used in a low NOxburner design of the type illustrated inFIGS. 6,6A,7and8, wherein like reference numbers indicate like parts. As with the embodiment ofFIGS. 1-4, a premix burner10includes a freestanding burner tube12located in a well in a furnace floor14. Burner tube12includes an upstream end16, a downstream end18and a venturi portion19. Burner tip20is located at downstream end18and is surrounded by an annular tile22. A fuel orifice11, which may be located within gas spud24, is located at upstream end16and introduces fuel gas into burner tube12. Fresh or ambient air is introduced into primary air chamber26through adjustable damper28to mix with the fuel gas at upstream end16of burner tube12. Combustion of the fuel gas and fresh air occurs downstream of burner tip20.

A plurality of air ports30originate in secondary air chamber32and pass through furnace floor14into the furnace. Fresh air enters secondary air chamber32through adjustable dampers34and passes through staged air ports30into the furnace to provide secondary or staged combustion.

In order to recirculate flue gas from the furnace to the primary air chamber, a flue gas recirculation passageway76is formed in furnace floor14and extends to primary air chamber26, so that flue gas is mixed with fresh air drawn into the primary air chamber from opening80. Flue gas containing, for example, 0 to about 15% O2is drawn through passageway76, with about 5 to about 15% O2preferred, about 2 to about 10% O2more preferred and about 2 to about 5% O2particularly preferred, by the inspirating effect of fuel gas passing through venturi portion19of burner tube12. As with the embodiment ofFIGS. 1-4, the primary air and flue gas are mixed in primary air chamber26, which is prior to the zone of combustion. Closing or partially closing damper28restricts the amount of fresh air that can be drawn into the primary air chamber26and thereby provides the vacuum necessary to draw flue gas from the furnace floor.

Unmixed low temperature ambient air, having entered secondary air chamber32through dampers34is drawn from secondary chamber32through orifice62, through bleed air duct64, through orifice60into flue gas recirculation passageway76into the primary air chamber26by the inspirating effect of the fuel gas passing through venturi portion19. Again, the ambient air may be fresh air, as discussed above. Bleed air duct64has a first end66and a second end68, first end66connected to orifice60of flue gas recirculation passageway76and second end68connected to orifice62and in fluid communication with secondary chamber32. As with the embodiment ofFIG. 5, furnace floor14comprises a high temperature, low thermal conductivity material, and includes a wall portion65having an air bleed duct64formed through the wall portion65of the furnace floor14to minimize the temperature of the metallic flue gas recirculation passageway76.

Additionally, a minor amount of unmixed low temperature ambient air, relative to that amount passing through duct64, having passed through air ports30into the furnace, may also be drawn through flue gas recirculation passageway76into the primary air chamber26by the inspirating effect of the fuel gas passing through venturi portion19.

As with the embodiments ofFIGS. 1-4and5, a mixture of from about 20% to about 80% flue gas and from about 20% to about 80% ambient air should be drawn through passageway76. 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 flue gas recirculation passageway76, bleed air ducts64and air ports30; 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.

Sight and lighting port50provides access to the interior of burner10for lighting element (not shown).

A similar benefit can be achieved simply by providing a hole or holes in the FGR duct as it passes through the staged-air plenum or chamber. Such a feature can be employed in flat-flame burners, as will now be described by reference toFIGS. 9 and 10. A burner110includes a freestanding burner tube112located in a well in a furnace floor114. Burner tube112includes an upstream end116, a downstream end118and a venturi portion119. Burner tip120is located at downstream end118and is surrounded by an annular tile122. A fuel orifice111, which may be located within gas spud124, is located at upstream end116and introduces fuel gas into burner tube112. Fresh or ambient air is introduced into primary air chamber126to mix with the fuel gas at upstream end116of burner tube112. Combustion of the fuel gas and fresh air occurs downstream of burner tip120. Fresh secondary air enters secondary chamber132through dampers134.

In order to recirculate flue gas from the furnace to the primary air chamber, a flue gas recirculation passageway176is formed in furnace floor114and extends to primary air chamber126, so that flue gas is mixed with fresh air drawn into the primary air chamber from opening180through dampers128. Flue gas containing, for example, 0 to about 15% O2is drawn through passageway176by the inspirating effect of fuel gas passing through venturi portion119of burner tube112. Primary air and flue gas are mixed in primary air chamber126, which is prior to the zone of combustion.

Unmixed low temperature ambient air, having entered secondary air chamber132through dampers134is drawn from secondary air chamber132through orifice162, through at least one bleed air duct164, through orifice160into flue gas recirculation passageway176into the primary air chamber126by the inspirating effect of the fuel passing through venturi portion119. The ambient air may be fresh air as discussed above. Each bleed air duct164has a first end166and a second end168, first end166connected to orifice160of flue gas recirculation passageway176and second end168connected to orifice162and in fluid communication with secondary air chamber132. As is preferred, furnace floor114comprises a high temperature, low thermal conductivity material and includes at least a portion of air bleed duct164formed within furnace floor114to minimize the temperature of the flue gas recirculation passageway176.

Once again, it is desirable that a mixture of from about 20% to about 80% flue gas and from about 20% to about 80% ambient air should be drawn through passageway176. 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 and placement of passageway176and bleed air ducts164. Additionally, a plurality of bleed ducts164may be employed to obtain the desired percentages of flue gas and ambient air.

In operation, the mixture in the venturi portion119of burner tube112is 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. The majority of the secondary air is added a finite distance away from the burner tip120.

As may be appreciated, a feature of the burner of the present invention is that the flue-gas recirculated to the burner is mixed with a portion of the cool staged air in the FGR duct. This mixing reduces the temperature of the stream flowing in the FGR duct, and enables readily available materials to be used for the construction of the burner. This feature is particularly important 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.

It may be recognized that prior flat flame burner designs have employed the use of one or more holes placed in the metal portion of an FGR duct, within the secondary air chamber, in an attempt to reduce the overall temperature of the flue gas. While of some benefit, such a design has only a minimal effect on duct life and temperature reduction, since the cooler secondary air enters the FGR duct after the metal portion has been exposed to hot flue gas before any significant mixing with secondary air can take place. As may be appreciated by those skilled in the art, the flat flame burner design of the present invention overcomes these shortcomings.

Unlike prior designs, 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 to cool the air/flue-gas stream entering in the metallic section of the FGR duct. 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.

EXAMPLES

To assess the benefits of the present invention, an energy and material balance was performed for each of the configurations described below.

In order to demonstrate the benefits of the present invention, the operation of a pre-mix burner employing flue gas recirculation of the type described in U.S. Pat. No. 5,092,761 (as depicted in FIG. 5 of U.S. Pat. No. 5,092,761), was calculated using data from existing burner designs to set the energy and material balance. Results of the detailed material and energy balance are illustrated in Table 1 for the baseline burner of Example 1.

In Example 2, the same material balance is maintained as in the existing burner. As indicated in Table 1, the detailed material and energy balance calculated was calculated to be reduced by over 100° F. Note that the momentum ratio of the venturi (momentum of fuel jet in:momentum of air/fuel/flue-gas stream after mixing) is reduced, indicating that the load on the venturi mixer has been reduced.

As may be appreciated by those skilled in the art, the present invention can be incorporated in new burners or can be retrofitted into existing burners by alterations to the burner surround.

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 injection may occur through, for example, steam injection tube15, as shown inFIG. 2, or steam injection tube184, as shown in FIG.9. Preferably, steam may be injected upstream of the venturi.