MXPA03006141A - Nox reduction in combustion with concentrated coal streams and oxygen injection - Google Patents

Abstract

NOx formation in the combustion of solid hydrocarbonaceous fuel such as coal is reduced by obtaining, from the incoming feed stream of fuel solids and air, a stream having a ratio of fuel solids to air that is higher than that of the feed steam, and injecting the thus obtained stream and a small amount of oxygen to a burner where the fuel solids are combusted.

Description

COMBUSTION OF CONCENTRATED THREADED CURRENTS. WITH REDUCED NOx GENERATION FIELD OF THE INVENTION The present invention relates to the combustion of solid hydrocarbon fuel, such as coal, including the fuel containing bound nitrogen, and to the reduction of the generation of nitrogen oxides in the course of that combustion.

BACKGROUND OF THE INVENTION Coal combustion in thermoelectric furnaces continues to be an important means of generating energy. To the extent that said combustion continues to be believed to cause atmospheric emissions of NOx, which are still considered to contribute to air pollution, there is still substantial interest in identifying ways to reduce the amount of NOx emitted into the atmosphere in the course of that combustion. One way to reduce NOx emissions is to adopt gradual combustion techniques, either or both using aerodynamically graduated burners, with low NOx generation, and secondary air ports. In the aerodynamically graded burner, with low NOx generation, the mixing with fuel of a portion of the combustion air required for the complete combustion of the fuel is retarded, to produce a flame with a relatively large area of fuel rich flame, within the flame. In the globally graduated combustion, or gradual combustion, with secondary air, combustion air is fed, which provides only a portion of the total amount of oxygen necessary for the complete combustion of the coal, to the burner, in the primary combustion zone, with fuel, to create a fuel-rich flame area, followed by a fuel-poor area, to which the rest of the combustion air ("secondary air") is fed to complete fuel combustion. The entire primary combustion zone, with the exception of the area near the burner, where the combustion air is injected, and not yet mixed with the fuel, can become rich in fuel, under the overall graduated combustion, which provides a residence time prolonged, to reduce NOx emissions. In order to obtain fuel-rich conditions, the prior art suggests reducing the amount of combustion air fed with the fuel to the primary combustion zone; adding combustible gases to this region, or using recycled, oxygen-poor burner gases. Adding pure oxygen, at this point in the process, is discouraged by the teachings of the prior art, because it is inconsistent with the goal of removing excess oxygen from that area of the flame. In furnaces that burn pulverized coal, it is customary to transport the pulverized coal to the burner, and feed it through it as a flowing stream of pulverized coal solids, carried by, and intimately mixed with, the primary air (also known as "air conveyor"). The conveyor air also provides a portion of the combustion air requirement for coal. The conveyor air may contain recycled burner gas or products of combustion of the fuel used by the burners in the pipeline, to reduce the moisture content of the coal. Reducing the amount of air transporter is a recognized technique to reduce the amount of NOx formed by the combustion of coal. However, in most situations, reducing the amount of air conveyor is not feasible, as that adversely impacts the operation of the coal sprayer and the transport / distribution system. A second technique identified in the prior art, to reduce NOx emissions from pulverized coal burners, is the incorporation of coal concentrators or splitters. Coal concentrators or splitters coastingly separate the coal from its air conveyor, at or near the tip of the burner, in two or more separate streams of pulverized coal and air.; where one or more of the streams has a greater ratio of solids from coal to air conveyor, than one or more other streams. These devices inject the two (or more) streams in the furnace, separately, or highly stratified. In that way, the fuel-rich current is burned under more fuel-rich conditions, and the fuel-poor current would provide additional air and some heat to drive the devolatilization and denitrification of the coal. These devices also allow greater reduction of the burner because, as the coal load decreases with a decrease in the burning speed (less coal and the same amount of air conveyor), the concentrators allow denser currents of coal at the tip of the burner, which help maintain the ignition stability of the flame. Techniques of this type have become quite common in the industry, and there are now many coal concentrator devices designed for this purpose. However, the proposed NOx regulations from boilers that burn hard coal have become demanding, and the use of the combustion system with low NOx generation, according to the state of the art, with a coal concentrator only, does not it is enough to satisfy the new regulations regarding NOx. Additionally, the use of a coal concentrator can result in a lot of unburned carbon in the ash. Therefore, there is still a need for improved methods for combustion of coal, in order to reduce NOx emissions, without significant increases in the unburned coal in the ash, preferably while taking advantage of the aspects provided. by the coal concentrators.

BRIEF DESCRIPTION OF THE INVENTION The present invention comprises a combustion method comprising: providing a feed stream of solid hydrocarbon fuel, powder, in a gaseous carrier; obtaining from the feed stream at least one obtained stream comprising the fuel and the carrier, and having a ratio of fuel to carrier that is greater than the ratio of fuel to carrier of the feed stream; feed the current obtained and air from a burner to a combustion chamber, inject oxygen into the stream obtained at or near the burner, such as by injecting it directly into the fuel, when the fuel leaves the burner, or adding oxygen to the air that is fed through the burner, and combustion of the coal present in said obtained stream, in the combustion chamber, with air and oxygen, in a flame having a fuel-rich flame zone; where the amount of oxygen is less than 25 percent of the stoichiometric amount necessary for the complete combustion of the fuel, and maintain the area rich in fuel, while reducing the amount of air fed through the burner, in an amount that contains enough oxygen so that the stoichiometric ratio in the total combustion zone varies by no more than 10 percent, compared to the stoichiometric ratio, without said addition of oxygen.

In the preferred embodiments, the stoichiometric ratio of the fuel rich flame zone is between 0.6 and 1.0 and, more preferably, between 0. and 0.85. In another preferred embodiment, air is added from a source other than the burner, to a region within the combustion chamber, which is outside the fuel-rich flame zone, to establish a primary combustion zone, rich in fuel, in a an amount that contains at least enough oxygen so that the total amount of oxygen fed into the combustion chamber is at least the stoichiometric amount necessary for the complete combustion of said fuel. Preferably, in this embodiment, the stoichiometric ratio of the primary combustion zone is between 0.6 and 1.0 and, more preferably, between 0.7 and 0.85. It should be understood that the current or currents obtained from coal and air, which have a higher ratio of combustible solids to air than the feed stream, but which are not necessarily obtained, can be obtained, as can one or more streams that are physically separated from the stream or currents of combustible solids and air, of lower fuel-to-air ratio, which are also necessarily produced. That is, the current or currents obtained, with a high ratio of fuel-to-air solids, can constitute one or more regions that are part of a stream that also has one or more regions with a lower ratio of combustible solids to air.

The invention maintains the goal of the process of a high temperature flame zone, rich in fuel, concentrating first the coal stream, and then applying oxygen to the concentrated coal stream, in a localized region, at the burner outlet. This allows a high concentration of oxygen to be brought into contact with the coal and to maintain the stoichiometric ratio at or below the original air values, depending on the degree of concentration of the coal reached. The combination of locally elevated oxygen concentrations, with low stoichiometric ratios, creates ideal conditions for suppressing NOx formation. The use of oxygen in concentrated coal streams can obtain these conditions, by excluding a portion of inert, that is, nitrogen contained in the air, from the normal coal burning process, which allows higher temperatures to be obtained, and with the result of lower global NOx emissions from the process, and less carbon not burned in the ashes. further, by applying oxygen in this way, less oxygen is needed to obtain the desired beneficial conditions, so that the economy of the use of oxygen is favored to a great extent. As used herein, "stoichiometric ratio" means the ratio of the oxygen fed, with respect to the total amount of oxygen that would have been necessary to fully convert all of the carbon, sulfur and hydrogen present in the substances comprising the feed , to carbon dioxide, sulfur dioxide and water. As used herein, "NOx" means nitrogen oxides, such as, but not limited to: NO, NO2, NO3, N2O, N2O3, N2O4, N3O4, and mixtures thereof. As used herein, "gradual combustion with burners with low NOx generation" means combustion in a furnace in which mixing with fuel, of a portion of the combustion air necessary for complete combustion of the fuel, is retarded for producing a flame with a relatively large fuel-rich flame zone As used herein, "globally graded combustion" or "gradual combustion with secondary air" means combustion in an oven in which only a portion of the combustion air necessary for the complete combustion of the fuel, it is fed to the furnace with the fuel, in the burners; and additional air (the "secondary air") is supplied which is at least sufficient air to complete the combustion of the fuel in the furnace, not through, or immediately adjacent to, any burner, but rather through one or more inlets located between the burner or burners, and the flue pipe device of the furnace, and is fed without an associated fuel feed. As used herein, "gaseous carrier" means a gaseous medium without oxygen or with an oxygen content suitable for transporting the fuel without risk of ignition occurring in the fuel system; the rest of the gaseous medium constituting inert species or the combustion products from a previous process. In practice, the volumetric flow of the gaseous carrier must be adequate to carry and displace the full scale of the sizes of the pulverized fuel particles. As used herein, "bound nitrogen" means the nitrogen that is part of a molecule that also contains carbon and hydrogen and, optionally, also oxygen.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a sectional representation of an embodiment of apparatus for carrying out the present invention. Figure 2 is a sectional representation of a burner useful for practicing the present invention. Figures 3 to 6 are sectional representations of devices useful in the present invention to obtain currents of edible solids and air that have a ratio of combustible solids to air greater than that of the feed. Figures 7 to 9 are sectional representations of devices useful in the practice of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Although the present invention is discussed below in terms of coal combustion and in terms of air which is the gaseous carrier, and both mentions represent preferred embodiments of the present invention, the techniques described would be applicable to any other pulverized fuel and any other gaseous carrier. The invention will be described with reference to the figures, although a description referring to the figures is also not intended to limit the scope of what is considered to be the present invention. In general terms, the invention first separates a coal / air conveyor stream into one or more streams (the stream or currents "obtained"), in which the first part of the stream has a solids ratio fuel to air greater than that of the feed stream Obtaining the second or second "obtained" streams will also produce one or more streams that have a ratio of combustible solids to air smaller than that of the inlet stream. then injects an amount of oxygen that is lower than the stoichiometric, relative to the fuel present in the stream or currents obtained, in or near the current obtained in the burner, to promote faster combustion, higher temperature, and the devolatilization of the fuel particles.Subject the fuel particles to these conditions, early in its combustion history, promotes the transformation of nitrogen not attached to the fuel to N2, instead of to NOx, and facilitates the complete burning of coal in a later period of time, within the combustion process. The stream or relatively dilute coal stream (s) also produced is or is used in other ways; for example, they can be injected in the furnace away from the oxygen stream, or even directed to a location separate from the burner, for example, as secondary air or as another graduated air stream). In general, the speed of the feed stream and the most concentrated coal-air stream is from 7.62 to 762 m / sec., And preferably from 15.24 to 60.96 m / sec. In general, the ratio of combustible solids to air, in the feed stream, is 0.25 to 1.5 parts by weight of fuel per part by weight of air and, preferably, from 0.35 to 0.7 parts by weight of fuel / part by weight. air weight. In general, the ratio of the combustible solids to air, in the most concentrated stream obtained is 0.4 to 10 parts by weight of fuel / part by weight of air, and preferably, 0.5 to 3 parts by weight of fuel / part in weight of air. Figure 1 shows the combustion device 1, which can be any apparatus in which combustion is carried out inside the device 2. Preferred combustion devices include furnaces and boilers which are used to generate electric power by conventional means, not shown. Each burner 3, in a side wall or an end wall of the combustion device 1, feeds fuel, air and oxygen, from sources thereof, located outside the combustion device 1, to the interior 2 of the combustion device 1. The suitable fuels include pulverulent hydrocarbon solids (ie, finely divided), a preferred example of which is pulverized coal or pulverized petroleum coke. As seen in Figure 1 and more in detail in Figure 2, the burner 3 preferably consists of several concentrically arranged passages, although other constructions can be used for the same effect. The fuel is fed to the combustion device 1 through the annular passage 4. Preferably the fuel and the conveying air are transported from a power source 20, to one or more burners 3, where it passes through the concentrator 18 of coal and the resulting streams are propelled into the interior 2 of the combustion device 1. An effective amount, typically about 1.5 to 2.0 parts by weight of primary air, is used to transport one part by weight of coal, which corresponds to approximately 20 percent of the stoichiometric combustion air necessary for the complete combustion of bituminous coal. The combustion air 22 is fed by a forced draft fan ("FD"), to one or more wind boxes 21, and is fed to the air passages. of one or more burners 3. The secondary combustion air 15 is fed through the burner 3, towards the combustion device 1, preferably through annular passages 11, arranged concentrically, which surround the annular space through which feeds the hydrocarbon fuel. Preferably, tertiary combustion air 16 is fed through the burner 3 to the combustion device 1, preferably through annular passages 12, arranged concentrically, surrounding the secondary air passage. It is preferable to also feed the combustion air through the secondary air port 7 (see FIG. 1) to the combustion device 1. The preferred low-NOx burners have passages for primary and secondary air (fuel). , for good aerodynamic adjustment capacity. However, other designs of burners with low NOx generation can be used, using only primary and secondary air feeds. Once the optimal settings with the three passages have been determined, the agitation blades and the passage for the secondary air can be designed, in order to create approximately the same aerodynamic mixing characteristics as with the design of three passages. Alternatively burners may be used with an additional (quaternary) passage (such as the RSFC ™ burner, described in U.S. Patent No. 5,960,724). The combustion is carried out between the hydrocarbon fuel, the oxygen present in the combustion air and the oxygen, which results in the formation of a flame 6. The region 8 of the flame, closest to the end of the burner 3, that is, where the hydrocarbon fuel leaves the burner, it is a zone rich in fuel. The area of the flame 6, around its periphery, is relatively poor, since the secondary and tertiary combustion air has not been fully reacted with the fuel. When a sufficient amount of air is fed from the secondary air port 7, for overall combustion grading, the entire lower zone of the furnace, or primary combustion zone (PCZ, acronym for its designation in English: Primary Combustion Zone) 10 , under the port 7 of secondary air becomes rich in fuel, except the areas near the burners 3, where the air is injected and still does not react completely with the fuel. Preferably, air is fed through the opening 7 of the secondary air port, which is richer in fuel, and provides additional oxygen, which helps to obtain complete combustion of the fuel in the burning zone 9. The oxygen present in the air of combustion fed through the burner 3, combined with the oxygen fed into the opening 7, are at least sufficient to allow complete combustion of the fuel, and typically contain from 10 to 15 volume percent of excess oxygen, with respect to the amount necessary for complete combustion of the fuel. Preferably the secondary and tertiary combustion air is fed into the burner 3, so as to swirl around a longitudinal axis, thereby creating a recirculation zone near each burner, and improving the intermixing of air and fuel. The whirlwind can be obtained by known techniques, such as by providing baffles 13 and 14, in the annular passages for the secondary and tertiary air flow of the burner, which direct the air flow of the currents, in the desired directions of rotation. It is preferred to provide a high vortex grade, preferably a vortex number of 0.6 to 2.0, as defined by Combustion Aerodynamics, JM Beer and NA Chigier, Robert E Krieger Publishing Company, Inc., 1983. Preferably, the total amount of air fed through the burner 3, that is, the sum of the primary air, secondary air and tertiary air, is between 60 and 100 percent of the requirement of stoichiometric air, for complete combustion. It is highly preferable that the total amount of air fed through the burner 3 be about 70 to 85 percent of the stoichiometric air requirement, for complete combustion. When the oxygen is premixed or mixed rapidly in the coal transport stream, using 20 percent stoichiometric air and the stoichiometric ratio of total combustion is 1.15, the following average oxygen concentrations in the transport air stream are calculated, and in total combustion air:% SR air replacementO2 concentrationOverage concentration of O2 (*) in the air transport- O2 in the total combustion air (% by volume) (% by volume). 0 21.0 21.0 5 24.9 21.7 10 28.5 22.5 15 31.7 23.4 20 34.7 24.3 25 37.4 25.4 (*) for example, 141.55 dm3 of air replaced with 29.72 dm3 of pure O2, to give the same amount of O2 The amount of oxygen fed to the burners should be sufficient to establish a stoichiometric ratio, in the fuel rich zone of flame 6, that is less than about 0.85. The amount of oxygen fed through line 5 must be less than 25 percent of the stoichiometric amount needed for complete combustion of the fuel. It is more preferred that the amount correspond to less than 15 percent of the stoichiometric amount necessary for complete combustion of the fuel. At the same time it is necessary to decrease the amount of the secondary and tertiary combustion air, fed through the burner 3, to the combustion device 1, in an amount corresponding to the quantity of oxygen fed through the line 5. More specifically, the amount of secondary and tertiary combustion air, and quaternary if used, fed through burner 3, must be reduced by an amount containing within 10 percent of the amount of oxygen fed via line 5, to the fuel. The NOx emission depends strongly on the total stoichiometric conditions. When oxygen injection makes the local stoichiometric condition poorer, the change in local stoichiometric conditions must be considered after oxygen injection. For example, the injection of oxygen, equivalent to 10 percent of the stoichiometric air, into a locally rich area, at a stoichiometric ratio of 0.4 (SR = 0.4), without changing the amount of combustion air that is fed, would alter the conditions local stoichiometric to SR (stoichiometric ratio) = 0.5, and it would be expected that NOx emissions would decrease substantially. This effect is much greater than that of "replacing 10 percent of the air with oxygen", at the same time that the stoichiometric condition is kept constant, at SR = 0.4. If the same amount of oxygen is injected into the flame zone, without changing the combustion air, when the local stoichiometric condition is SR = 0.95, it is expected that the NOx emission will increase strongly if the local stoichiometric condition is increased to the stoichiometric ratio = 1.05 The invention maintains the goal of the process of a high-fuel, high-temperature flame zone, concentrating the coal stream first, lowering the stoichiometric ratio, and then applying oxygen to the concentrated coal stream, in a localized region, at the burner outlet. This allows a fairly high concentration of oxygen to come into contact with the coal, and can maintain the stoichiometric ratio at or below the original air values, depending on the degree of coal concentration achieved. The combination of locally high oxygen concentrations, with low stoichiometric ratios, creates ideal conditions for suppressing NOx formation. The use of oxygen in concentrated coal streams can obtain these conditions, excluding the inerts of the coal combustion process, which allows reaching higher temperatures, and results in lower total NOx emissions, from the process, and less carbon without burn in the ash. In addition, by applying oxygen in this way, less oxygen is required to obtain the desired beneficial conditions, so that the economy of oxygen use is greatly favored. For example, when a coal feed stream containing 20 percent of the stoichiometric air is concentrated as transport air, and the concentrated coal stream then contains 10 percent stoichiometric air, the following average oxygen concentrations are calculated in the transport air stream and in the total combustion air, assuming that the oxygen in the concentrated coal is premixed or mixed rapidly, and that the stoichiometric ratio of total combustion is 1.15.

Air% SR Concentration of O2 in Average concentration of replacement with the air carrier (% O2 in the total air of com- QoO in volume) bastion (% by volume). 0 21.0 21.0 5 28.5 21.7 10 34.7 22.5 15 39.9 23.4 20 44.4 24.3 25 48.2 25.4 (*) for example, 141.55 dm3 of air replaced with 29.72 dm3 of pure O2, to give the same amount of O2 The previous example shows that the requirement of oxygen to obtain the same concentration of oxygen in the air carrier is reduced by half, by using the concentrator. The injection or mixing of oxygen in the tertiary and quaternary air, if the latter is used, should be avoided in an aerodynamically graduated burner, without OFA. In theory, optimization of the local stoichiometric condition can be obtained with any oxidants, including air. However, oxygen is more effective, because only a small volume is required and the local stoichiometric condition can be changed without great impact on the total aerodynamic mixing conditions of the flame. Burner 3 includes structure 18, through which the coal and air feed stream passes. From the structure you get one or more streams that have a ratio of combustible solids to air greater than the ratio that exists in the feed stream, and one or more streams that have a fuel to air solids ratio lower than the ratio in the feeding current. The known coal concentrators, useful to achieve this objective, are primarily of an inertial design, whereby the mass difference between the coal particle and the carrier gas is used to separate the two media. The coal particles will tend to move in a straight line, and are influenced primarily by the high velocity of the gas in a passage. The gas can be made to change direction and speed much more easily than coal and, therefore, the two are allowed to separate quite easily. Coal injectors for burners generally take one of three basic forms: a central axial passage (ie, a tube), an annular passage (eg, the space between two concentric tubes) and a square or rectangular section (eg. , boiler injectors tangentially ignited), which could be classified in one of the two previous categories, but may have other geometric considerations, which make it different from the cylindrical coal injectors. The coal that exits from each of the burner shapes can be concentrated in a radial direction (ie edge against center) or in a circumferential direction (ie, high and low density alternate areas, scattered around the cross section). of the opening), or a combination of both. The primary devices that concentrate the hard coal are venturi that accelerate the mixing and then decelerate the gas portion; tangential entries that produce vortex flows; pallets that produce swirling flows, and dividing plates, alternating convergent and divergent ducts, that collect the particles and push them together. Figures 3 to 6 show some typical examples of these devices, along with the concentrated coal streams they produce. Figure 3 shows a coal concentrator 30 which employs a venturi 31 to obtain coal-air streams having higher and lower ratios of coal to air, than the ratio of the incoming feed stream 32. The passage of the feed stream through the restricted annular throat of the venturi 31, forms a more concentrated coal stream in region 33, closer to the inner surface of unit 30, and a less concentrated coal stream, which flows through the central region 34. Figure 4 shows a coal concentrator 40 having blades 41 imparting eddy motion to the incoming feed stream 42. The swirling movement causes more hard solids to move to the region radially out of the stream, obtaining in that region a stream of concentrated fuel. The paddle 44 straightens the flow when it leaves the unit. Figure 5 shows a coal concentrator 50, in which the coal and air feed stream 51 is fed tangentially to the axis of the unit 50. Then the flow is channeled through the unit, making the circumferential movement of the stream of food that move more combustible solids closer to the internal surface of the unit. This obtains the most concentrated, desired current of combustible solids, such as a current 52 surrounding current 53, less concentrated. Figure 6 shows a coal concentrator 60, provided with vanes defining alternating converging and divergent passages for the flow of incoming feed stream. For example, pallets 61 and 62 converge towards each other; while the vanes 62 and 63 diverge from each other. The most concentrated stream of coal in air is obtained in the passage between each convergent vane, and a less concentrated stream is produced in the passages between each divergent pair of vanes. This type of concentrator produces currents in which the concentration of combustible solids in the air varies around the circumference of the unit, while the units illustrated in Figures 3 to 5 produce currents in which the concentration varies along the radio of the unit. For flat tubes, through which coal and air are flowing, coal usually concentrates on the inner edge of the tube wall or, less commonly, in the center of the tube. This is done because the more concentrated coal stream is usually positioned to interact with the enlarged body flame stabilizers. Under conditions in which the most concentrated coal stream is obtained, on the inner surface of the tube, an annular flow of oxygen, which would surround the coal tube, would be better. This flow must be injected in a direction along the axis of the coal stream, or converging to the coal stream. This configuration is illustrated in Figure 7, where the feed stream 71 is fed tangentially to the unit 70 and comes out as a stream 72, shown with black arrows, having a coal concentration greater than that of the feed stream 71.; and the stream 73, shown as lighter arrows, having a coal concentration lower than that of the feed stream 71. The oxygen stream 74 is fed to the annular passage 75 from which the oxygen exits and is brought into contact with stream 72 of concentrated coal-air. Baffles can be used at the tip of the burner to slow the whirlpool flow; but usually the coal comes out in a whirling direction out. The whirlwind movement, due to the tangential injection of the coal, also causes the air to move radially outwards, although less than coal, and therefore, the ratio of coal / air can not be as high as with other techniques. . Figure 8 illustrates the practice of the present invention, which uses a unit that is based on a coal concentrator that uses a throttle vnturi 81, installed in the tube 80. The flow of the feed stream 82 through the venturi 81 makes that the coal solids are concentrated towards the center of the tube 80, and leaves the current flowing closer to the inner surface of the tube, relatively poor in coal. In that situation, an oxygen lance 84, installed up to the center of the tube, with axial or slightly diverging injection ports 86, will mix oxygen with the more concentrated coal stream. Depending on the position of the throttle vinturi 81, it may be necessary to provide a concentrically located baffle, to prevent the concentrated coal stream from re-mixing with the diluted stream, before leaving the burner tip. In both cases discussed above, deflectors or vortex provocative vanes can be installed in the tube; to take the radially stratified flow and break it to separate segments of dense and dilute coal streams, around the circumference of the tube. In these cases, the same two oxygen injectors would be used; except that the end of the injectors would be plugged in and holes would be drilled in different circumferential positions, so that the injection point of the oxygen would coincide with the location of the dense coal stream. As shown in Figure 9, the coal concentrator of the type shown in Figure 6 can be adapted to the present invention by using an axial oxygen lance 91, having a closed end and having holes 92 punched through the end of the shaft. Spear, in number equal to the number of streams in which the ratio of combustible solids to air is greater than that of the feed stream. Each orifice may be radial or may converge or diverge with respect to the flow of the concentrated streams, at an appropriate angle, and be located so as to intersect the concentrated coal streams emanating from the narrow gap between the vanes. In cases where the dense coal stream is circumferentially segmented and directed to the outermost radial position, then an oxygen ring with openings or grooves located adjacent the openings of the dense coal stream is preferred. Due to its design, tangentially lit combustion chambers do not have normal burners. At each corner of the square combustion chamber there is an air bank and fuel injection nozzles that inject coal or air into the combustion chamber to mix the reagents for combustion. Due to their non-cylindrical geometry, most of these types of injectors use deflectors arranged to convergent and divergent ducts. By having the same minimum cross section at the end and at the beginning of the passages, the air flow to them is divided fairly evenly. However, the converging passage acts as a funnel and picks up more coal particles on the inlet side and concentrates them at the narrow outlet end, to produce a concentrated coal stream. In this configuration oxygen could be applied through the ring surrounding the coal nozzle (usually a secondary air source) or through multiple nozzles installed in the dense coal passage of the burner.

When the oxygen is injected into the dense coal stream, it must make contact with the stream and mix with it; but it is not convenient to make the oxygen jet create or alter the total flow pattern, at the outlet of the nozzle. With this in mind, oxygen must be injected at a speed similar to the speed of coal. A typical scale is 50 percent of the speed of the coal stream obtained, up to 150 percent of the speed of the coal stream obtained, with the design goal being 100 percent. With dividing air currents, and injectors that occupy some of the volume in the coal chute, it can be difficult to accurately determine the velocity of the coal in the dense coal stream. In addition to the inertial devices described above, other techniques can be used to concentrate the coal, before mixing it with oxygen. You could mount external devices, such as a cyclone, adjacent to a burner, and then you could introduce two streams stratified to the burner, through separate passages. The oxygen would then be injected at some point into, along, or around the dense coal passage. External separation devices, such as those just mentioned, can be extremely useful if the diluted coal stream is to be injected into the furnace, at some point distant from the dense coal stream.

Claims (12)

1. CLAIMS 1.- A combustion method, characterized in that it comprises: providing a stream of solid, pulverulent hydrocarbon fuel feed in a gaseous carrier; obtaining from the feed stream at least one current obtained, comprising the fuel and the carrier, and having a ratio of fuel to carrier that is greater than the ratio of fuel to carrier of the feed stream; feeding the obtained current and air from a burner (3) to a combustion chamber (1); injecting oxygen into the current obtained, in or near the burner (3), and combustion of the coal in the current obtained, in the combustion chamber (1), with air and oxygen, in a flame that has a zone (8) of flame rich in fuel; where the amount of oxygen is less than 25 percent of the stoichiometric amount required for the complete combustion of the fuel, and keeps the area rich in fuel, while reducing the amount of air fed through the burner (3) by an amount which contains sufficient oxygen so that the stoichiometric ratio of the total combustion zone varies by no more than 10 percent, compared to the stoichiometric ratio, without the addition of oxygen.
2. 2. A method according to claim 1, further characterized in that the stoichiometric ratio of the fuel-rich flame zone is between 0.6 and 1.0.
3. 3. A method according to claim 1, further comprising additionally adding air from a source (7) different from the burner, to a region (9) within the combustion chamber (1) external to the area (8). ) of fuel-rich flame, to establish a primary fuel-rich combustion zone, in an amount containing at least sufficient oxygen so that the total amount of oxygen fed into the combustion chamber (1) is at least the amount stoichiometric necessary to complete fuel combustion.
4. 4. A method according to claim 3, further characterized in that the stoichiometric ratio of the primary combustion zone is between 0.6 and 1.0.
5. 5. A method according to claim 1, further characterized in that the fuel is coal.
6. 6. A method according to claim 5, further characterized in that the stoichiometric ratio of the fuel-rich flame zone is between 0.6 and 1.0
7. 7. A method according to claim 5, further characterized by additionally comprising: adding air from a source (7) different from the burner (3), to a region (9) inside the combustion chamber (1), outside the fuel-rich flame zone (8), to establish a primary combustion zone , rich in fuel, in an amount containing at least sufficient oxygen so that the total amount of oxygen fed into the combustion chamber is at least the stoichiometric amount necessary for the complete combustion of the fuel.
8. 8. A method according to claim 7, further characterized in that the stoichiometric ratio of the primary combustion zone is between 0.6 and 1.0.
9. 9. A method according to claim 5, further characterized in that the gaseous carrier is air.
10. 10. A method according to claim 9, further characterized in that the stoichiometric ratio of the fuel-rich flame zone is between 0.6 and 1.0.
11. 11. A method according to claim 9, further characterized in that it additionally comprises: adding air from a source (7) different from the burner (3), to a region (9) inside the combustion chamber (1) outside of the the zone (8) of fuel-rich flame, to establish a primary combustion zone, rich in fuel, in an amount containing at least sufficient oxygen so that the total amount of oxygen fed into the combustion chamber is at least the stoichiometric amount necessary for the complete combustion of the fuel.
12. 12. A method according to claim 11, further characterized in that the stoichiometric ratio of the primary combustion zone is between 0.6 and 1.0.