SNCR distribution grid

An SNCR distribution grid for introducing a NOx reducing reactant into a flue gas flow. The grid is made of one or more elements which are formed by fluid-cooled tubes to which membrane pieces are attached, preferably by welding, to form conduits in between the tubes. The fluid-cooled tubes may be cooled by water and/or steam and the distribution grid is disposed in the flue gas flow. To admit the reactant into the flue gas, nozzles are provided in the membrane and the reactant is conveyed from a location external of the furnace or combustor enclosure, into the conduits so formed, and thence out into the Flue gas flow via the nozzles.

FIELD OF THE INVENTION

The present invention relates primarily to circulating fluidized bed (CFB) reactors, combustors and/or boilers having impact type particle separators used in the production of steam for industrial applications and/or utility power generation and, more particularly, to an apparatus for introducing ammonia or urea into the flue gas produced by such CFBs which, as part of a selective non-catalytic reduction (SNCR) system, is used to reduce NOx emissions from the CFB. The present invention may also be employed in connection with bubbling fluidized bed reactors, grate-type furnaces, etc.

BACKGROUND OF THE INVENTION

The typical operating temperature for the reactors or combustors of such CFBs, and thus the flue gases produced thereby, lies within a temperature range of approximately 1550-1650°F. This temperature range thus lies within an acceptable temperature “window” for the application of selective non-catalytic reduction (SNCR) techniques for reducing NOx emissions, since SNCR systems and their associated apparatus typically involve the introduction of a specific reactant into flue gases whose temperature lies within a temperature range of approximately 1400-2000° F. In SNCR, a reducing agent or reactant, typically ammonia or urea, is sprayed into the furnace flue gas for reducing NOx according to one of the following reactions, depending upon the reactant employed:
4NO+4NH3+O2→4N2+6H2O (ammonia-based)
2NO+(NH2)2CO+½O22N2+2H2O+CO2(urea-based).

SNCR is frequently used in CFB boilers which employ cyclone(s) for separating solids from the flue gas leaving the furnace to reduce NOx emissions. In such applications, the aforementioned reactant is sprayed at the inlet or outlet of the cyclone utilizing the high gas turbulence associated with the cyclone for mixing the flue gas with the reactant. These spray locations also take advantage of a relatively small cross-sectional flow area of the cyclone inlet or outlet, thereby allowing sufficient penetration of the jets of reactant into the flue gas flow to provide more uniform mixing of the reactant into the flue gas.

In contrast to the CFBs described above, another type of CFB reactor, combustor and/or boiler (hereinafter referred to as a CFB boiler for convenience) employs low velocity, impact-type particle separators, such as U-beams, for separating solids from the flue gas leaving the furnace and features a relatively large cross-sectional flow area for the flue gas flow. Utilizing nozzles to inject such reactants for SNCR which are installed only on the periphery of walls of the CFB which convey the flue gas flow might not achieve sufficient jet penetration of the reactant into the flue gas flow, resulting in poor mixing of the reactant with the flue gas.

SUMMARY OF THE INVENTION

One aspect of the present invention is drawn to an SNCR distribution grid for delivering a reactant for reducing NOx into a gas stream containing NOx. At least one element for conveying the reactant from a source outside of the gas stream is provided. The element has at least one nozzle for spraying the reactant from a conduit defined within the element into the gas stream. The conduit being formed by at least two fluid-cooled tubes and membranes located in-between the tubes, the at least one nozzle being located in at least one of the membranes.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The present invention overcomes the aforementioned difficulty by providing a particularly designed distribution grid for introducing the reactant into the flue gas flow. The grid comprises one or more elements which are formed by fluid-cooled tubes to which membrane pieces are attached, preferably by welding, to form conduits in between the tubes. The fluid-cooled tubes may be cooled by water and/or steam and the distribution grid is disposed into the flue gas flow. To admit the reactant into the flue gas, nozzles are provided in the membrane and the reactant is conveyed from a location external of the furnace or combustor enclosure, into the conduits so formed, and thence out into the flue gas flow via the nozzles. The spacing between the elements forming the distribution grid, as well as the spacing between the nozzles provided in the membrane is selected to achieve relatively uniform mixing of the dispersed reactant into the flue gas. As described above, the inlet to the conduits which convey the reactant into the flue gas is located outside of the furnace enclosure where it would be connected to a reactant feed line connected to a source of the reactant. Suitable valves and control devices would be provided in the reactant feed line to control the introduction of the reactant into the flue gas according to any particular control scheme desired by the operators of the CFB installation.

Preferably, the distribution grid can be placed at one or more of several locations: upstream of the impact type particle separators or U-beams, between the one or more rows of such U-beams, or downstream of the U-beams with respect to a direction of flue gas flow. An advantage of locating the distribution grid upstream of the impact type particle separator(s) is that the separator(s) can enhance the subsequent mixing of the reactant with the flue gas. A disadvantage of locating the distribution grid at this upstream location is that there is a higher solids loading in the Flue gas upstream of the separator(s) which could hamper penetration of the reactant jet into the flue gas. These factors would thus need to be considered when the desired location of the distribution grid is to be finalized.

Referring to the drawings annexed to and forming a part of this disclosure, wherein like reference numerals designate the same or functionally similar elements throughout the several drawings, and toFIG. 1in particular, there is shown a sectional side view of a typical CFB boiler10having a furnace or reactor enclosure12, typically rectangular in cross-section, defined by fluid-cooled enclosure walls14. The enclosure walls14are typically tubes separated from one another by a steel membrane to achieve a gas-tight enclosure12. The reactor enclosure12has a lower portion16, an upper portion18, and an exit opening20located at an outlet of the upper portion18. Fuel, such as coal, and sorbent, such as limestone, schematically indicated at22, are provided to the lower portion16in a regulated and metered fashion by any conventional means known to those skilled in the art. By way of example and not limitation, typical equipment that would be used includes gravimetric feeders, rotary valves and injection screws. Primary air, indicated at24, is provided to the lower portion16via windbox26and distribution plate28connected thereto. Bed drain schematically indicated at30removes ash and other debris from the lower portion16as required, and overfire air supply ports32,34supply the balance of the air needed for combustion.

A flue gas/solids mixture36produced by the CFB combustion process flows upwardly through the reactor enclosure12from the lower portion16to the upper portion18, transferring a portion of the heat contained therein to the fluid cooled enclosure walls14. A primary, impact type particle separator38is located within the upper portion18of the reactor enclosure12. In a preferred embodiment, the primary, impact type particle separator38comprises several rows of U-beams40which may be arranged in two groups; an upstream group42and a downstream group44U-beams40may be supported from roof46of the reactor enclosure12, as disclosed in U.S. Pat. Nos. 4,992,085 and 5,343,830, or they may be supported by cooled tubes as disclosed in U.S. Pat. No. 6,454,824, the entire texts of which are hereby incorporated by reference as though fully set forth herein.

The furnace enclosure12of the CFB reactor10may be provided with division wall heating surface48, wing wall heating surface50, or both types of heating surface, depending upon the steam generation requirements of the given CFB installation. In some installations, neither type of surface may be required for steam generation requirements. In addition, there will be provided downstream superheater heating surface52, as shown.

Referring toFIG. 2, which is a sectional view through the upper portion18, there are illustrated several locations where individual elements60can be located and used to inject a reactant62supplied by an SNCR system64(schematically illustrated inFIG. 1), and which collectively form a distribution grid80. As shown, the elements60may be located on the division wall heating surface48, the wing wall heating surfaces50, and/or the superheater heating surface52. WhileFIG. 2shows the preferred location as being on what can be referred to as the “trailing edges” of any of these heating surfaces, this is not essential and the elements60can be located anywhere, including being on the surfaces48,50and/or52, and in single or multiple locations on the surfaces48,50and/or52. Further, while we have described the present invention as a distribution grid, it will be appreciated that certain applications may require only a single element60with a single nozzle72. Conversely, a plurality of elements60may be employed on one or several of the surfaces48,50and/or52across a width W of the CFB boiler10, and at various locations spaced along any such surface48,50and/or52, so that the reactant62is injected into the flue gas at many locations across a cross-section of the flue passage conveying the flue gas.

FIGS. 3 and 4are close-up views of two preferred embodiments, designated I inFIG. 2, of an individual element60containing a conduit70. Solely for ease of illustration, and not in any way limiting the application of the elements60according to the present invention, assume the elements60are formed as part of a wing wall heating surface50, comprised of fluid-cooled tubes66, some or all of which may be connected to one another by membrane68. InFIG. 3, the elements60are formed by two pieces of membrane68extending in between two adjacent fluid-cooled tubes66, thereby creating a conduit70therein which is used to convey the reactant62from a source thereof to one or more apertures or nozzles72for injecting the reactant62into the flue gas. The apertures or nozzles72may be comprised of small pieces of tube or pipe or a more particularly designed shape as dictated by jet penetration and/or pressure drop requirements. If required for erosion resistance and/or heat absorption reduction, the elements60may be provided with a coating of refractory74, as shown. InFIG. 4, a larger conduit70may be employed, if required by the quantity of reactant62which must be conveyed along any individual conduit70, by increasing the number of fluid-cooled tubes66used to form the conduit70, with an associated increase in the number of membrane pieces68as shown.

Alternatively, and as shown inFIG. 5, protective tiles82may be employed instead of refractory74to protect the membrane68as well as the tubes66adjacent thereto. The protective tiles82may be made of any suitable high-temperature and erosion-resistant material such as ceramics or metals such as stainless steel. The protective tiles82may be attached to the membrane68by any suitable means, such as by fastening the tiles82to the nozzle72with a washer84welded, as at W, to the nozzle72. A spacer or washer86may be employed to position the tiles82relative to the nozzle72and to provide a gap88between the tile82and the membrane68for reducing heat absorption by the membrane68. The protective tile82may thus be provided with an aperture90for this purpose, the aperture90being such that it will accept the nozzle72. If there is an appreciable difference in the diameter of the aperture90and the outside diameter of the nozzle72which would be inserted into the aperture90, the spacer or washer86may also be provided with a portion which would also extend around the outside diameter of the nozzle72and within the aperture90to prevent excessive movement of the protective tile82during operation. The protective tiles82between locations on the elements60where the nozzles72are provided may be similarly attached to the membrane68; of course, at these locations the nozzles72would be replaced by simple pins since no reactant62is provided or supplied into the flue gas36at these intermediate locations.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, those skilled in the art will appreciate that changes may be made in the form of the invention covered by the following claims without departing from such principles. For example, the present invention may be applied to new construction involving circulating fluidized bed reactors or combustors, or to the replacement, repair or modification of existing circulating fluidized bed reactors or combustors. It may be applied in non-CFB applications, as well, such as in bubbling fluidized bed boilers or furnaces.

In addition, while the distribution grid has been shown as being located in the vicinity of the exit opening, and/or just upstream or downstream thereof, it may be desirable to locate the distribution grid at other locations within the furnace enclosure or flues downstream of the exit opening, where appropriate temperatures of the flue gas may be presented at certain load ranges which require NOx reduction. In some embodiments of the invention, certain features of the invention may sometimes be used to advantage without a corresponding use of the other features. Accordingly, all such changes and embodiments properly fall within the scope of the following claims.