Patent Grant
7025587
This invention relates to an improved burner of the type employed in high temperature furnaces. More particularly, the invention relates to a burner having a high capacity venturi so as to allow increased flue gas re-circulation and thereby reduce NOx emissions.
As a result of the interest in recent years to reduce the emission of pollutants from burners of the type used in large furnaces and boilers, significant improvements have been made in burner design. In the past, burner design improvements were aimed primarily at improving heat distribution to provide more effective heat transfer. However, 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.
The rate at which nitrogen oxide is formed is dependent upon the following variables: (1) flame temperature, (2) residence time of the combustion gases in the high temperature zone, and (3) excess oxygen supply. The rate of formation of nitrogen oxide increases as flame temperature increases. However, the reaction takes time, and a mixture of nitrogen and oxygen at a given temperature for a very short time may produce less nitric oxide than the same mixture at a lower temperature, over a longer period of time.
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, it represents a less desirable alternative to improvements in burner design.
Burners used in large industrial furnaces may use either liquid or gaseous fuel. Liquid fuel burners mix the fuel with steam prior to combustion to atomize the fuel to enable more complete combustion and mix combustion air with the fuel at the zone of combustion.
Gas fired burners can be classified as either pre-mix 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, such that 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.
Pre-mix burners mix some or all of the fuel with some or all of the combustion air prior to combustion. Since pre-mixing 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. Pre-mixing the fuel and air also facilitates the achievement of the desired flame characteristics. Due to these properties, pre-mix burners are often compatible with various steam cracking furnace configurations.
Floor-fired pre-mix 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 pre-mix burner is the burner of choice for such furnaces. Pre-mix burners can also be designed for special heat distribution profiles or flame shapes required in other types of furnaces.
One technique for reducing NOx that 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 NOx formation than an air-fuel ratio closer to stoichiometry. Combustion staging results in reducing peak temperatures in the primary flame zone and has been found to alter combustion speed in a way that reduces NOx. Since NOx formation is exponentially dependent on gas temperature, even small reductions in peak flame temperature can dramatically reduce NOx emissions. 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.
In the context of pre-mix burners, the term “primary air” refers to the air pre-mixed 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. 4,629,413 discloses a pre-mix burner that employs combustion staging to reduce NOx emissions. The pre-mix burner of U.S. Pat. No. 4,629,413 lowers NOx emissions by delaying the mixing of secondary air with the flame and allowing some cooled flue gas to recirculate with the secondary air. The entire contents of U.S. Pat. No. 4,629,413 are incorporated herein by reference.
U.S. Pat. No. 5,092,761 discloses a method and apparatus for reducing NOx emissions from pre-mix burners by recirculating flue gas. Flue gas is drawn from the furnace through recycle ducts by the inspirating effect of fuel gas and combustion air passing through a venturi portion of a burner tube. Air flow into the primary air chamber is controlled by dampers and, if the dampers are partially closed, the reduction in pressure in the chamber allows flue gas to be drawn from the furnace through the recycle ducts and into the primary air chamber. The flue gas then mixes with combustion air in the primary air chamber prior to combustion to dilute the concentration of oxygen in the combustion air, which lowers flame temperature and thereby reduces NOx emissions. The flue gas recirculating system may be retrofitted into existing pre-mix burners or may be incorporated in new low NOx burners. The entire contents of U.S. Pat. No. 5,092,761 are incorporated herein by reference.
Analysis of burners of the type disclosed in U.S. Pat. No. 5,092,761 has shown that the flue gas recirculation (FGR) ratio is generally in the range of 5 to 10%, where the FGR ratio is defined as:
The ability of existing burners of this type to generate higher FGR ratios is limited by the inspirating capacity of the fuel orifice/gas spud/venturi combination. Although further closing of the primary air dampers can further reduce the pressure in the primary air chamber and thereby enable increased FGR ratios, the resultant reduction of primary air flow is such that insufficient oxygen is present in the venturi for acceptable burner stability.
As disclosed in “The Design of Jet Pumps” by A. E. Knoll, appearing in Vol. 43 of Chemical Engineering Progress, published by the American Institute of Chemical Engineers (1947), it is known to optimize the operation of venturis used in air and steam operated air movers at relatively mild (roughly ambient) temperatures. In contrast, in the burner of the invention, combustible gaseous fuel (including, but not limited to, methane, H2, ethane, and propane) is used to move a combination of very hot (above 1000° F., 540° C.) flue gases, hot air, hot uncombusted fuel, CO, and ambient air.
In one aspect, the present invention is directed to an improved burner for the combustion of fuel in a furnace, said burner comprising:
(a) a burner tube having an upstream end, a downstream end, and a venturi intermediate said upstream and downstream ends, said venturi including a throat portion having substantially constant internal cross-sectional dimensions such that the ratio of the length to maximum internal cross-sectional dimension of said throat portion is at least 3; and
(b) a burner tip mounted on the downstream end of said burner tube adjacent a first opening in the furnace, so that combustion of the fuel takes place downstream of said burner tip.
Preferably, the ratio of the length to maximum internal cross-sectional dimension of said throat portion is from about 4 to about 10, more preferably from about 4.5 to about 8, more preferably from about 6.5 to 7.5, and most preferably from about 6.5 to 7.0.
In a further aspect, the invention resides in a method for combusting fuel in a burner of a furnace, comprising the steps of combining fuel gas and air at a pre-determined location, drawing the fuel gas and air so combined through a venturi, and combusting said fuel gas at a combustion zone downstream of said pre-determined location and said venturi, wherein said venturi includes a throat portion having substantially constant internal cross-sectional dimensions such that the ratio of the length to maximum internal cross-sectional dimension of said throat portion is at least 3.
The invention is further explained in the description that follows with reference to the drawings.
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 originate in secondary air chamber 32 and pass through furnace floor 14 into the furnace. Fresh air enters secondary air chamber 32 through adjustable dampers 34 and passes through staged air ports 30 into the furnace to provide secondary or staged combustion.
In order to re-circulate flue gas from the furnace to the primary air chamber, ducts or pipes 36, 38 extend from openings 40, 42, respectively, in the floor of the furnace to openings 44, 46, respectively, in burner plenum 48. Flue gas is drawn through pipes 36, 38 by the inspirating effect of fuel gas passing through venturi 19 of burner tube 12. In this manner, the primary air and flue gas are mixed in primary air chamber 26, which is prior to the zone of combustion. The amount of inert material mixed with the fuel is raised, thereby reducing the flame temperature, and as a result, reducing NOx emissions. Closing or partial 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 floor.
Unmixed low temperature ambient air, having entered secondary air chamber 32 through dampers 34 and having passed through air ports 30 into the furnace, is also drawn through pipes 36, 38 into the primary air chamber by the inspirating effect of the fuel gas passing through venturi 19. The ambient air may be fresh air as discussed above. The mixing of the ambient air with the flue gas lowers the temperature of the hot flue gas flowing through pipes 36, 38 and thereby substantially increases the life of the pipes and permits use of this type of burner to reduce NOx emissions in high temperature cracking furnaces having flue gas temperature above 1900° F. (1040° C.) in the radiant section of the furnace.
It is preferred 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 pipes 36, 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 pipes 36, 38 in relation to air ports 30, as those skilled in the art will readily recognize. That is, the geometry of the air ports, including but not limited to their distance from the burner tube, the number of air ports, and the size of the air ports, may be varied to obtain the desired percentages of flue gas and ambient air.
A sight and lighting port 50 is provided in the primary air chamber 26, extending into secondary air chamber 32, both to allow inspection of the interior of the burner assembly, and to provide access for lighting of the burner.
As is shown in
Referring now to
Increasing the ratio of length to internal cross-sectional dimensions in the throat portion of the venturi is found to reduce the degree of flow separation that occurs in the throat and cone portions of the venturi which increases the capacity of the venturi to entrain flue gas thereby allowing higher flue gas recirculation rates and hence reduced flame temperature and NOx production. A longer venturi throat also promotes better flow development and hence improved mixing of the fuel gas/air stream prior to the mixture exiting the burner tip 20. Better mixing of the fuel gas/air stream also contributes to NOx reduction by producing a more evenly developed flame and hence reducing peak temperature regions.
In addition to the use of flue gas as a diluent, another technique to achieve lower flame temperature through dilution is the use of steam injection. Steam can be injected in the primary air chamber 26 or the secondary air chamber 32. Preferably, steam is injected through steam injection tube 15, upstream of the venturi, for mixing with the primary air and recirculated flue gas to further reduce flame temperature and hence NOx emissions. The steam is conveniently provided through tube(s) terminating adjacent the gas spud 24, as shown.
The increased capacity venturi shown in
A plurality of air ports 30 originate in secondary air chamber 32 and pass through furnace floor 14 into the furnace. Fresh air enters secondary air chamber 32 through adjustable dampers 34 and passes through the air ports 30 into 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 passageway 76 is formed in furnace floor 14 and extends to primary air chamber 26, so that flue gas is mixed with fresh air drawn into the primary air chamber from opening 80. Flue gas containing, for example, about 6–10% O2 is drawn through passageway 76 by the inspirating effect of fuel gas passing through venturi portion 19 of burner tube 12. As with the embodiment of
Referring now to
Referring now to
The high capacity venturi disclosed herein can also be applied in flat-flame burners, as will now be described by reference to
In the embodiment shown in
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, a 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
Again, in addition to the use of flue gas as a diluent, another technique to achieve lower flame temperature through dilution is the use of steam injection. Steam can be injected in the primary air chamber 126 or the secondary air chamber 132. Preferably, steam is injected through steam injection tube 184, upstream of the venturi, for mixing with the primary air and re-circulated flue gas to further reduce flame temperature and hence NOx emissions. The steam is conveniently provided through tube(s) terminating adjacent the gas spud 124, as shown.
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.
The invention will now be more particularly described with reference to the following Examples.
Table 1 below summarizes the geometry of a conventional pre-mix burner with FGR (Example 1) and five pre-mix burners (Examples 2–6) having modified venturi throat portions.
To assess the results of modifying the venturi throat portion, computational fluid dynamics (CFD) were used to evaluate the configurations summarized in Table 1. FLUENT™ software from Fluent, Inc. was used to perform the analysis. (Fluent, Inc., USA, 10 Cavendish Court, Centerra Resource Park, Labanon, N.H., 03766-1442). The fluid flows calculated for the various venturi designs are summarized in Table 2 below.
As will be seen from Table 2, except for the burner of Example 2, increasing the length/diameter ratio of the venturi throat portion increased the total mass flow through the burner tube. For a given flow rate, in addition to an optimum L/D ratio, there is also an optimum diameter for the venturi. If the diameter is too small, it causes excessive frictional losses that limit the venturi capacity. If the diameter is too big (as in Example 2), flow separation occurs in the throat, which also reduces capacity.
Although increasing the length and hence the length/diameter ratio of the venturi throat portion increases the total mass flow through the burner tube, frictional losses overtake the advantage of increased flow if the throat portion becomes too long. Thus the length/diameter ratio of the venturi throat portion should preferably not exceed 10, more preferably is between about 6.5 and about 7.5, and most preferably is between about 6.5 and about 7.0.
Although illustrative embodiments have been shown and described, a wide range of modification change and substitution is contemplated in the foregoing disclosure and in some instances, some features of the embodiment may be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the embodiments disclosed herein.
This patent application is a Continuation of application Ser. No. 10/388,910, filed Mar. 14, 2003, now U.S. Pat. No. 6,881,053 claims priority from Provisional Application Ser. No. 60/365,218, filed on Mar. 16, 2002, the contents of which are hereby incorporated by reference.
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| Number | Date | Country | |
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| Parent | 10388910 | Mar 2003 | US |
| Child | 11072843 | US |
