Patent Grant
6887068
This invention relates to an improvement in a burner such as those employed in high temperature industrial furnaces in the steam cracking of hydrocarbons. More particularly, it relates to the use of a fuel riser centering plate designed for reducing flow losses in the region of the burner inlet.
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 NOx emissions is a desired goal to decrease air pollution and meet government regulations. In recent years, a wide variety of mobile and stationary sources of NOx emissions have come under increased scrutiny and regulation.
One strategy for achieving lower NOx emission levels is to install a NOx reduction 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, which patent is incorporated herein by reference. In addition, many raw gas burners produce luminous flames.
Premix burners mix 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.
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.
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 airfuel ratio closer to stoichiometry. 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 must be balanced with the fact that radiant heat transfer decreases with reduced flame temperature, while carbon monoxide (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 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.
Thus, one set of techniques achieves lower flame temperatures by using staged-air or staged-fuel burners to lower flame temperatures by carrying out the initial combustion at far from stoichiometric conditions (either fuel-rich or air-rich) and adding the remaining air or fuel only after the flame has radiated some heat away to the fluid being heated in the furnace.
Another set of techniques achieves lower flame temperatures by diluting the fuel-air mixture with diluent material. Flue-gas (the products of the combustion reaction) or steam are commonly used diluents. Such burners are classified as FGR (flue-gas-recirculation) or steam-injected, respectively.
Patents relating to improvements in burner design include U.S. Pat. No. 5,092,761, which discloses a method and apparatus for reducing NOx emissions 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 oxygen (O2) in the combustion air, which lowers flame temperature and thereby reduces NOx emissions. The contents of U.S. Pat. No. 5,092,761 are incorporated herein by reference.
In certain premix burners, a centering plate is utilized to assure that the fuel riser/burner spud assembly is aligned with the venturi to ensure maximum entrainment.
Analysis of burners of the type described in U.S. Pat. No. 5,092,761 has indicated the flue-gas-recirculation (FGR) ratio is generally in the range 5-10% where FGR ratio is defined as:
FGR ratio (%)=100[G/(F+A)]
The ability of these burners to generate higher FGR ratios is limited by the inspirating capacity of the gas spud/venturi combination. Further closing or partially closing the primary air dampers will produce lower pressures in the primary air chamber and thus enable increased FGR ratios. However, the flow of primary air may be reduced such that insufficient oxygen exists in the venturi for acceptable burner stability. Moreover, internal flow dynamics in the area of the fuel riser/burner spud assembly/venturi combination can affect the inspirating capacity of the combination, reducing the ability to achieve FGR rates in excess of 10%. In this regard, the fuel riser/burner spud centering plate, utilized to assure that the fuel riser/burner spud assembly is aligned with the venturi, can serve to negatively affect internal flow dynamics in the area of the fuel riser/burner spud assembly/venturi combination, reducing inspirating capacity.
Therefore, what is needed is a burner for the combustion of fuel gas and air that enables higher flue gas recirculation ratios (FGR) to be utilized, yielding further reductions in NOx emissions.
The present invention is directed to a burner capable of achieving lower levels of NOx emissions for use in furnaces such as those employed in steam cracking. The burner includes a burner tube having a downstream end and an upstream end. A burner tip is mounted on the burner tube adjacent a first opening in the furnace, so that combustion of the fuel takes place downstream of the burner tip. A fuel orifice is located adjacent the upstream end of the burner tube for introducing fuel into the burner tube. At least one passageway is provided for supplying flue gas from the furnace into a primary air chamber. The burner also has means responsive to an inspirating effect created by uncombusted fuel exiting the fuel orifice for drawing flue gas from the furnace through the passageway and the primary air chamber. The uncombusted fuel flows through the burner tube from its upstream end towards its downstream end, whereby the flue gas is mixed with air at the upstream end of the burner tube prior to the zone of combustion of the fuel and air to thereby lower the temperature of the drawn flue gas. The burner also has means including a plate for centering the fuel orifice in alignment with the burner tube, with the centering plate being perforated to permit flow therethrough from the primary air chamber. The perforated centering plate design reduces flow losses that result from a tortuous flow pattern caused by a solid centering plate.
Also provided is a method for reducing NOx emissions in a burner for the combustion of fuel, the burner being located adjacent a first opening in a furnace and including a primary air chamber, a burner tube including a downstream end and an upstream end, a burner tip adjacent the first opening in the furnace, so that combustion of the fuel takes place downstream of the burner tip and a fuel orifice located adjacent the upstream end of said burner tube, for introducing fuel into said burner tube, the fuel orifice being mounted on a riser; the method comprising installing a plate for centering the fuel orifice in alignment with said burner tube, the centering plate being perforated to permit flow therethrough from the primary air chamber.
In a specific aspect of the present invention, the centering plate has an outer ring member, an inner centering member and a plurality of radially extending spokes connecting the outer ring member and the inner centering member. Further, the outer ring member fixedly secures the centering plate within the primary air chamber; and the riser extends through the inner centering member and is supported therein. In addition, the outer ring member is connected to a surface of the primary air chamber by a plurality of support members. The perforated centering plate design smoothes out the flow vectors entering a venturi portion of the burner tube to enable higher venturi capacity. The reduced flow losses at the venturi inlet results in higher venturi capacity, higher flue gas recirculation rate, lower flame temperature and lower NOx production.
An object of the present invention is to provide a burner arrangement that permits higher flue gas recirculation rates to be employed, thus reducing NOx emissions.
These and other objects and 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 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 to
A plurality of air ports 30 (
Unmixed low temperature fresh or ambient air, having entered the secondary air chamber 32 through the dampers 34, and having passed through the air ports 30 into the furnace, is also drawn through a passageway 76 into a primary air chamber 26 by the inspirating effect of the fuel gas passing through the venturi portion 19. The passageway 76 is preferably metallic and is employed as a FGR duct. The mixing of the staged or secondary air with the flue gas lowers the temperature of the hot flue gas flowing through the passageway 76 and thereby substantially increases the life of the passageway 76 and allows the use of this type burner to reduce NOx emissions in high temperature cracking furnaces having flue gas temperature above 1900° F. in the radiant section of the furnace. As an alternative embodiment, shown in
The recirculated flue gas drawn into the primary chamber 26 is mixed with fresh or ambient air drawn into the primary air chamber 26 from an opening 80 through adjustable dampers 28 before the mixture enters the venturi portion 19.
With reference to
In one embodiment of the present invention, centering plate 60 also contains a pair of holes 68 to permit a corresponding pair of steam injection tubes 15 to pass through centering plate 60 to the extent such steam injection tubes 15 are present.
As noted above, the centering plate 60 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 60 as shown in
Sight and lighting port 50 provides access to the interior of the burner for the lighting element (not shown).
Flue gas containing, for example, from about 6 to about 10% O2 is drawn through passageway 76, by the inspirating effect of fuel gas passing through venturi portion 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. This is in contrast to a liquid fuel burner, such as that of U.S. Pat. No. 2,813,526, in which the combustion air is mixed with the fuel at the zone of combustion, rather than prior to the zone of combustion.
Closing or partially closing damper 28 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. A mixture of from about 20% to about 80% flue gas and from about 20% to about 80% ambient air should be drawn through 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 flue gas recirculation passageway 76 and air ports 30. That is, the geometry and location of the air ports may be varied to obtain the desired percentages of flue gas and ambient air.
A similar benefit can be achieved in flat-flame burners, as will now be described by reference to
In order to recirculate flue gas from the furnace to the primary air chamber, a flue gas recirculation passageway 176 is formed in furnace floor 114 and extends to primary air chamber 126, so that flue gas is mixed with fresh air drawn into the primary air chamber from opening 180 through dampers 128. Flue gas containing, for example, 0 to about 15% O2 is drawn through passageway 176 by the inspirating effect of fuel gas passing through venturi portion 119 of burner tube 112. Primary air and flue gas are mixed in primary air chamber 126, which is prior to the zone of combustion.
In operation, the fuel orifice 111, which may be located within gas spud 124, discharges fuel into burner tube 112, where it mixes with primary air, recirculated flue gas or mixtures thereof. The mixture of fuel gas, recirculated flue-gas, and primary air then discharges from burner tip 120. The mixture in the venturi portion 119 of burner tube 112 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.
Referring now to
As with the previous embodiment and again referencing
Although the burners 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. In fact, it has been found that the pre-mix, staged-air burners of the type described in detail herein can be operated with the primary air damper doors closed, with very satisfactory results.
Thus, it can be seen that, by use of this invention, NOx emissions may be reduced in a burner without the use of fans or otherwise special burners. The centering plate of the burner of the present invention can also easily be retrofitted to existing burners.
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.
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.
This patent application claims priority from Provisional Application Ser. No. 60/365,151, filed on Mar. 16, 2002, the contents of which are hereby incorporated by reference.
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