Patent Application
20070295250
Stream 3 of ash passes into the combustor, preferably near one end of the combustor. Ash treated in accordance with the present invention is typically comprised of the residue of a carbonaceous ash-containing fuel that has been exposed to a high temperature process, such as a combustor or gasifier.
Stream 5 of oxidant is fed into combustor 1, preferably at or near the same end of combustor 1 into which ash stream 3 is fed. Oxidant stream 5 contains more than 21 vol. % oxygen, but may contain higher oxygen concentrations such as at least 25 vol. %, at least 50 vol. %, or even at least 90 vol. %. One preferred embodiment of feeding ash stream 3 and oxidant stream 5 is illustrated in
Optionally, supplemental oxidant 7 can also be fed into the interior of combustor 1. As described in more detail below, the supplemental oxidant is fed so that it comes into contact with the ash after unburned carbon in the ash has ignited and begun to combust within combustor 1. Supplemental oxidant 7 can comprise air, oxygen-enriched air, or oxygen in commercially available purity. The supplemental oxidant thus will generally contain 21 vol. % to 99.9 vol. % oxygen.
The desired oxygen composition of the oxidant stream and of any supplemental oxidant streams can be attained by employing oxygen in a commercially available purity, or by combining oxygen in a commercially available purity with air until the desired overall oxygen content is attained.
Stream 9 contains ash having a reduced unburned carbon content compared to the unburned carbon content of the ash fed into combustor 1. Stream 9 also contains gaseous combustion products such as carbon dioxide and water vapor, and may contain carbon monoxide. Stream 9 passes from a suitable outlet in combustor 1, preferably at the end opposite from the end into which ash stream 3 is fed, to a separatory apparatus 11 in which solids, comprising the ash, are separated from gaseous components of stream 9. Stream 13 represents solids recovered from separatory apparatus 11. Stream 15 comprises substantially all of the gaseous component of stream 9. A typical preferred separatory apparatus 11 is a cyclone, of conventional design provided that it is capable of sustained operation at the elevated temperatures to which it is exposed from the material conveyed in stream 9 into apparatus 11. If desired, the stream 9 can be cooled, for instance by heat transfer via indirect heat exchange with another stream, or by quenching.
Referring to
Stream 5 of oxidant is fed into the interior of combustor 1 at a sufficient velocity and momentum that it entrains the particles of ash in ash stream 3 into the oxidant stream, thereby forming a flowing mixed stream of oxidant and ash. The minimum velocity of oxidant stream 5 effective to achieve entrainment of the ash into the oxidant stream can readily be determined based on the feed rate and the particle size distribution of the ash stream.
The stream of ash 3 and oxidant stream 5 are preferably fed separately into combustor 1, as shown in
The relative mass flow rates of the stream of ash particles, the oxidant stream, and the stream (or streams) of supplemental oxidant (if used), fed to combustor 1, should be established so that the total amount of oxygen fed in the oxidant stream and in any supplemental oxidant streams contains enough oxygen relative to the amount of unburned carbon in the ash stream to combust the desired amount of unburned carbon that is in the ash stream fed into combustor 1 (which may be all, or less than all, of the unburned carbon). The amount of oxygen in the oxidant stream(s) and the mass flow rates of the oxidant stream(s) are adjusted accordingly. While combustion of less than 100% of the imburned carbon is sometimes sufficient, it is preferred to provide enough oxygen (as the aggregate of oxygen contained in all oxygen-containing streams fed) to combust all of the unburned carbon in the ash. Thus, it is preferred to feed sufficient oxygen (as the aggregate of all oxygen-containing streams fed) to comprise 100% to 105 or 110% of the amount of oxygen required to combust all of the unburned carbon in the ash.
The flowing mixture of oxidant into which the ash has been entrained should contain sufficient heat so that the unburned carbon in the flowing mixed stream of ash and oxidant ignites, i.e. begins to combust (whether in the combustion chamber, or upstream from the combustion chamber but after the ash has been entrained into the oxidant), and so that the combustion is maintained in the combustion chamber. This heat content can be provided by heating the oxidant before the oxidant is mixed with the ash, or by combining the oxidant and the ash with supplemental combustible matter that ignites and combusts in the combustion chamber and thereby provides additional heat to the ash. However, the total heat content of the mixture of the oxidant and the ash should be below the point at which particles of ash entrained in the oxidant stream begin to melt, such they become adherent to each other.
The temperatures at which the particles of ash risk beginning to melt and becoming adherent to each other may vary among different ash compositions, and can readily be determined for a given ash. Avoiding the situation in which the particles begin to melt requires taking into consideration several factors, including the heat content of the oxidant stream as it first contacts the ash particles, and the heat content within the combustion chamber which in turn is affected by the evolution of heat of combustion as unburned carbon in the ash combusts, the evolution of heat of combustion of any other matter that is also combusting in the combustion chamber, and heat losses to the combustion chamber walls, especially if heat is transferring through the walls such as to circulating water or steam as in an electric power generation boiler. Thus, the heat content of the oxidant stream should be sufficiently low that, even as heat of combustion of the unburned carbon and of any other combusting matter present raises the heat content of the mixture of oxidant and combusting ash, the heat content of the mixture still remains below a point at which the particles of ash begin to melt.
In embodiments such as those that promote ignition and combustion of the ash by entraining the ash into a stream of oxidant that is at a temperature above ambient, the temperature and heat content of the oxidant stream can be raised in any of several ways. The oxidant stream can be heated by passing it through a chamber that is already heated, so that the oxidant is heated by direct heat transfer from the interior surfaces of the chamber, or by passing the oxidant stream through a heat exchanger in which the stream is heated by indirect heat exchange from a hotter stream such as flue gas from another combustion operation, or such as the hot gas recovered from separator 11 as stream 15.
A preferred technique for providing a heated oxidant stream is illustrated in
Referring to
While the oxidant in chamber 34 can be heated by any suitable means such as by combustion, by an electric arc, by electrical resistance heating, or by heat exchange as described above,.
The fuel provided into chamber 34 combusts with oxidant in chamber 34 to produce heat and combustion reaction products such as carbon dioxide and water vapor. Within chamber 34 the combustion reaction products mix with the remaining oxygen thus providing heat to the remaining oxidant that was fed as stream 33 and raising its temperature. Preferably, fuel stream 36 is provided into chamber 34 at a high velocity, typically greater than 200 fps and generally within the range of from 500 to 1500 fps. The high velocity serves to entrain oxidant into the combustion reaction 38 thus establishing a stable flame. The high velocity enables further entraining of combustion reaction products and oxidant into the combustion reaction, thus improving the mixing of the hot combustion reaction products with the remaining oxygen within chamber 34 and thus more efficiently heating the remaining oxygen.
The heated oxidant formed in chamber 34 passes out through opening 35 as hot oxidant stream 5. The oxidant stream 5 formed in this way preferably contains at least 50 vol. % oxygen. The stream formed in this manner can be expected to have a velocity sufficient to entrain the stream of particles of ash. The velocity can be adjusted, as desired, by controlling the temperature of the stream of hot oxidant (with higher temperatures correlated with higher velocities) and by suitably dimensioning the orifice through which the stream emerges. This mode of forming a hot oxidant stream and of providing the desired velocity are described further in U.S. Pat. No. 5,266,024.
Another mode of providing sufficient heat so that the flowing stream of oxidant and ash to be combusted ignites in the combustion chamber and combusts so that the combustion is maintained, is to mix with the oxidant and ash supplemental combustible matter that ignites in the combustion chamber and liberates heat of combustion which promotes ignition and combustion of the unburned carbon in the ash.
In
Stream 41 of supplemental combustible matter can be preblended with the ash so that a combined stream of ash and the combustible matter is entrained into the flowing stream of oxidant. Alternatively, stream 41 can be fed to combustor 1 as a stream separate from the stream of ash, in which case the stream of ash particles and the stream of supplemental combustible matter are each entrained into the flowing stream of oxidant (either within combustor 1, or upstream from combustor 1). The supplemental combustible matter, if fed separately, should be fed near the entrance of the oxidant into combustor 1 so that the supplemental combustible matter is entrained into the stream of oxidant.
The supplemental combustible matter can be added in any amount that maintains or increases the ability of the unburned carbon in the ash to ignite and combust in the combustor 1. Preferred amounts of the supplemental combustible matter are up to 15 wt. % based on the mass of the ash stream.
In the embodiments which employ supplemental combustible matter to promote ignition and combustion of the unburned carbon in the ash, separate heating of the oxidant stream before or after it is mixed with the ash stream may not be necessary.
The present invention is also useful when practiced in conjunction with a coal-fired combustion apparatus, such as a utility boiler that combusts coal to generate steam and/or electric power. Ash formed in the combustion of the coal, if it contains unburned carbon in amounts that one would like to reduce, can be treated by the method described herein to reduce the unburned carbon content of the ash. If desired, a suitable amount of the coal that is already available for use in the coal-fired boiler can be pulverized and used as supplemental combustible matter in the embodiment described above with respect to
Burner 61 is provided through a surface of combustion device 51. In actual practice, anywhere from 1 to 20 or more burners may be provided, depending on the size of the installation. Furthermore, the burners can be wall-mounted, roof-mounted, or corner-mounted. Fuel-air stream 62 comprising a mixture of fuel and air, and primary air stream 63, are fed through burner 61 and combusted in combustion chamber 53. Preferred fuel is pulverized coal, which is mixed with transport air to form the fuel-air mixture that is carried to and through the burner.
The burner can be configured, if desired, to feed secondary and even tertiary streams of air into combustion chamber 53. The combustion of the fuel and air forms flame 65 whose base is at the burner. Optional overfire air stream 64 of air is fed into combustion chamber 53 downstream from flame 65, between flame 65 and flue 55. When more than one burner is employed, the air streams 63 (and overfire air streams 64, when used) can be fed from a common windbox or plenum (not shown) which is conventional in current industrial practice. In addition, small amounts of oxygen can also be fed into the flame, from suitable feed lance in or near the burner.
Injector 71 receives ash stream 73 and oxidant stream 75. Ash stream 73 can comprise all or a portion of the ash produced by combustion in combustion chamber 53 of the fuel that is fed through burner 51. The ash from stream 73 is entrained into oxidant from stream 75 within injector 71. Accordingly, oxidant stream 75 is fed with sufficient velocity to entrain the ash as desired. As with the embodiments described above, the oxidant stream in which the ash is entrained preferably contains at least sufficient oxygen to combust 100% of the unburned carbon in the ash.
Injector 71 feeds the resulting flowing mixed stream of ash and oxidant into combustion chamber 53. In this embodiment of the invention, the ongoing combustion in combustion chamber 53 of the fuel fed through burner 51 ignites and maintains combustion of the unburned carbon in the ash. The oxygen in the oxidant stream in which the ash is entrained also promotes ignition and combustion of the unburned carbon.
The present invention presents several advantages.
One advantage is that by entraining the particles of ash into the oxidant stream, the unburned carbon content of the ash can then be reduced to levels of below 2 wt. % or even below 1 wt. % in a remarkably short residence time, typically of two minutes or less and even of one minute or less.
Another advantage is that there is no need to retain inert solids in the combustion chamber, such as are provided in some prior art processes purportedly for purposes of heat transfer or solids dispersion. Thus, the only solids that would be present within the combustion chamber are the ash that is fed into the combustion chamber and passes through it, combusting therein to be withdrawn as a product stream.
Also, combustion chambers such as the combustor 1 are advantageous in that ash can be treated in accordance with the present invention in a method which completes the reduction of the unburned content to the desired low final levels in only one passage of the ash through the combustor. That is, if desired, recycle back into the combustor of all or of a portion of the ash that is obtained from the combustor is not necessary and can be avoided.
