1. Field of the Invention
The present invention relates to a method and to a burner for burning of oxygen, and more particularly for use with heating furnaces.
2. Description of the Related Art
Normally, when hydrocarbons are burned in combination with oxidants having a high oxygen content, flame temperatures within the furnace exceed 2000° C. and furnace atmospheres with very high partial pressures of carbon dioxide and water vapor are present. That condition also gives rise to drawbacks, such as high NOx content and problems of local overheating.
There is a strong desire to provide burners that have emission-lowering properties.
In Swedish patent application number 0402223-2, a method is disclosed for burning a fuel with an oxidant in a heating furnace, wherein the fuel and the oxidant are supplied to a burner head. According to that application, in a first step fuel and oxidant are discharged from the burner head close to each other, so that the combustion essentially takes place close to and up to a certain distance from the burner head. Combustion proceeds until a temperature exceeding the auto ignition temperature of the fuel is reached inside the furnace, after which the fuel and the oxidant, in a second step, are instead discharged from the burner head at a distance from each other. Consequently, the combustion essentially takes place at a distance from the burner head at least as large as the diameter of the burner head, and outwardly from the burner.
According to that application, the intent is to lower the oxygen content in the combustion zone through separation, high pressure, and optimized positioning of the nozzles, even though the oxidant has an oxygen content of more than 80%. That is accomplished by the use of a nozzle configuration that gives rise to a large underpressure over those surfaces of the nozzle that do not have nozzles for the medium. Because of the underpressure, flue gases are sucked in from the furnace atmosphere and are rapidly and turbulently mixed with the out-flowing media. The mixture medium, i.e., the furnace atmosphere, typically has an oxygen content of 0.5-10%. The remaining gas is CO2, H2O, and N2 in various amounts.
Since the CO2, H2O, and N2 do not actively take part in the combustion, those compounds act as a “combustion brake.” The dilution of the oxygen and the fuel is very extensive, and oxygen rates during the combustion typically reach 7-15%, despite the use of pure oxygen gas.
The present invention is directed to overcoming those problems.
The present invention serves to further lower the NOx values, and also provides an even more uniform furnace interior temperature.
Thus, the present invention relates to a method and apparatus for the combustion of a fuel with an oxidant in the form of oxygen in a heating furnace, wherein fuel and oxidant are supplied to a burner head. Fuel and oxidant, respectively, are injected via the burner head through at least two pairs of nozzles, wherein one pair of nozzles is defined by a separate fuel nozzle and a separate oxidant nozzle. The nozzles of the nozzle pairs are uniformly distributed along and within the circumference of the burner head, and one of the fuel nozzles is provided with an oxidant nozzle at each side of the fuel nozzle.
The invention is described in greater detail below, partly in connection with embodiments of the invention shown in the appended drawings wherein:
The present invention concerns combustion in a heating furnace of a fuel with an oxidant, wherein the fuel and oxidant are supplied to a burner head. The burner head is secured in a furnace wall in a known fashion, so that the flame formed during combustion extends into the furnace interior.
Referring to the drawing, and particularly to
A burner head having three pairs of fuel and oxidant nozzles is shown in
According to the present invention, pairs of fuel nozzles and oxidant nozzles are provided, in contrast to the embodiment disclosed in the above-identified Swedish patent application, in which fuel is injected through one nozzle and the oxidant through a number of nozzles.
According to a preferred embodiment, the nozzles of the nozzle pairs shown and described herein are positioned along two mutually perpendicular diameters of the burner head, as shown in
Surprisingly, it was found that by injecting the same amount of fuel and oxidant through a number of pairs of fuel and oxidant nozzles, instead of using one fuel nozzle and a plurality of oxidant nozzles, the production of NOx is lowered even further. At the same time, local concentrations of heat and coolness within the furnace are reduced. A probable explanation is that the pairing of nozzles results in the formation of several zones with heavy turbulence, as compared to a burner head wherein fuel is injected through only one fuel nozzle.
According to a preferred embodiment, the burner head 1 includes an additional nozzle 7, a starting nozzle, wherein fuel and oxidant are discharged through respective individual and concentric channels 8, 9. Starting nozzle 7 is operated until the temperature within the furnace has reached the auto ignition temperature for the particular fuel and oxidant combination.
According to a preferred embodiment, the starting nozzle 7 is positioned at or near the center of the burner head 1.
A second combustion step, during which fuel and oxidant are injected through respective ones of the paired nozzles, can advantageously be initiated when the temperature within the furnace is above about 750° C.
According to a preferred embodiment, the discharge openings of nozzles 2, 3, and 4, 5, are outlets of laval or venturi nozzles.
The opening 22 is to allow supervision of the flame by means of the detection of ultraviolet light.
In
Thus, the burner disclosed permits two different modes of operation: on one hand as a normal oxyfuel burner, and on the other hand as a burner whose operation results in a flame with a substantially lower maximum temperature. The lower flame temperature is adapted to be below the temperature at which the production of NOx is limited by the reaction kinetics, which is about 1550° C.
The lower flame temperature is accomplished by the use of the mentioned positioning of the paired nozzles for fuel and for oxygen, whereby fuel and oxygen gas are combusted further away from the burner head as compared to what is the case for conventional oxyfuel combustion.
When carrying out the invention, a diffuse yet controlled combustion is achieved at process temperatures above the auto ignition temperature, substantially lowering the production of nitrous gases, mainly NO and NO2.
As a consequence, fuel and oxidant are mixed with flue gases of the furnace before the fuel and oxidant contact each other. In a way known per se, that results in a larger and cooler flame, in spite of the coefficient of utilization corresponding to combustion according to the prior art. Suitably, the nozzles can be directed straight ahead, that is, they do not need to be directed away from or toward each other. Instead, they can be angled toward or away from the longitudinal axis of the burner head.
According to a preferred embodiment, the oxidant is gaseous, and is an oxidant having an oxygen content of 85 vol % or above.
According to a principal feature of the invention, the oxidant is supplied to the burner at a pressure of at least 1 bar.
A normal pressure for normal applications is 4-5 bars.
The fuel is injected through normal nozzles at the available pressure.
A burner head according to the present invention is not larger than a known burner head for oxyfuel combustion. In a preferred embodiment, the burner head diameter is about 70 millimeters.
The compact structure provided by the present invention permits the invention to be applied to equipment already present at user premises. Also, the inventive structure can be positioned within a small, water-cooled protective jacket, for application at very high process temperatures.
According to the invention, the above-described advantages are achieved with any fuel, whether solid fuel, gaseous fuel, or liquid fuel. The apparatus according to the invention can replace existing combustion systems essentially without any reconstruction of the furnace equipment used in the process.
It is advantageous to choose as the fuel from among oil, propane, or natural gas.
Since the oxidant nozzles and the fuel nozzles can be directed straight ahead, a construction is achieved which is inexpensive, easy to maintain, and possible to apply to existing processes, and without any other measures other than exchanging the nozzle construction.
Several embodiments of the invention have been described herein. However, the design of the burner head can be varied, especially with respect to the number of pairs of fuel and oxidant nozzles.
Thus, the present invention should not be considered limited to the above-disclosed embodiments, but can be modified within the scope of the appended claims.
Number | Date | Country | Kind |
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0601374 | Jun 2006 | SE | national |
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Number | Date | Country |
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WO 2006031163 | Mar 2006 | WO |
Number | Date | Country | |
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20070298356 A1 | Dec 2007 | US |