Multi-spin mixer for particulate coal supply conduit

A mixer having two or more concentric cylindrical flow channels is placed in a supply conduit for a combustion chamber to mix airborne particulate coal prior to entering a manifold supplying four parallel branch conduits. Vanes are mounted in the mixer channels for imparting spin to the coal/air flow. The spin direction in one channel is opposite the spin direction of the adjacent channels(s). A turbulence-producing transition section is located downstream of the mixer.

FIELD OF THE INVENTION

This invention relates to systems for supplying airborne particulate coal to the combustion chamber of a coal-fired boiler of the type used to generate steam for turbines in an electric utility plant, and more particularly to a mixer device for reducing or eliminating non-uniform flow rates in parallel supply conduits located between a pulverizer and a combustion chamber.

BACKGROUND OF THE INVENTION

It is well known to feed combustion chambers for turbine generator boilers with airborne particulate coal; structures for carrying out this function are commonly found in electric utility plants throughout the United States and Canada. It is common in these systems to use a main supply conduit to receive particulate coal from a pulverizer/classifier. It is also common to divide the main supply conduit into several parallel branches which are connected to spaced points around the combustion chamber.

A problem which arises in systems of the type described above is ensuring that the branch conduits exhibit at least approximately equal coal flow rates so that the fireball in the combustion chamber is stabilized as to size and location within the combustion chamber. The flow of particulate coal through parallel branch conduits of different lengths and configurations tends to be unstable and inherently non-uniform. Many devices have been created to deal with this problem; see, for example, U.S. Pat. Nos. 5,873,156, 6,055,914, 6,186,079, 6,257,415 and 6,234,090.

SUMMARY OF THE INVENTION

The present invention is, according to one aspect, a mixer for use in a coal-fired combustion chamber supply conduit, typically the main supply conduit downstream of a pulverizer, the effect of which is to promote uniformity in the rate of flow of airborne particulate coal from the main supply conduit to the various branches of a parallel branch feed system. In general, the invention comprises a mixer comprising a plurality of substantially concentric walls, typically but not necessarily cylindrical and made of a wear-resistant material such as steel or a steel alloy, defining at least two substantially concentric annular flow channels receiving airborne particulate coal from a source such as a pulverizer/classifier. The two channels may be referred to as “inner” and “outer” channels but it is to be understood that there may be three, four or more such concentric channels in a particular embodiment. In the case of three channels, they are referred to as “inner,” “intermediate” and “outer” channels. A first plurality of circumferentially spaced vanes are located in the outer flow channel and are oriented to impart a clockwise spin to the airborne particulate coal flowing therethrough. A second plurality of circumferentially spaced vanes are located in the inner flow channel to impart a counterclockwise spin to the airborne particulate coal flowing therethrough.

In the above description as well as throughout this document, the terms “clockwise” and “counterclockwise” are used only in a relative sense to make it clear that the flow in one of the annular flow channels spins or rotates around the axis of the supply conduit in a direction which is opposite to the spin or rotation of flow in the adjacent annular flow channel or channels.

The mixer may be fabricated as an integral part of the supply conduit or made in the form of an insert which can be removed for servicing or replacement.

In the preferred embodiment hereinafter described in detail, there are three or more annular and concentric flow channels defined by cylindrical walls and consisting of at least an outer flow channel, an intermediate flow channel and an inner flow channel. The cross-sectional areas of all of the flow channels are at least approximately the same. To achieve this, the radial spacing between the walls of the outermost flow channel is less than the radial spacing between the walls of the innermost flow channel. The vanes in these channels are located in an overlapping fashion so there is no straight path for coal particulates to follow through the mixer.

The mixer may optionally be combined with other, downstream turbulence-causing features as hereinafter described.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first toFIG. 1, a conventional coal pulverizer/classifier10is shown to have a central vertical coal inlet supply conduit12for feeding lump coal into the pulverizer/classifier in controlled quantities. The pulverizer/classifier10comprises a main outlet supply conduit14which, in the illustrated embodiment, is concentric with the inlet supply conduit12but substantially larger in diameter. Some pulverizers have side feed features in which case the conduit12serves as a center outflow channel with or without vanes. Alternatively, it can be blocked off. A mixer insert16is located in the outlet supply conduit14as better shown inFIGS. 2-4. The outlet supply conduit14merges into a frustoconical transition section18which acts as a manifold to supply airborne particulate coal to four parallel branch conduits20,22,24,26which are arranged in the fashion disclosed in my prior patents listed above to supply the four corners of a combustion chamber28which is associated with a boiler for supplying steam to the turbine of an electrical power generator. The transition section18may be straight-sided; i.e., substantially cylindrical.

Referring toFIGS. 2-4, the mixer insert16is mounted in the main outlet supply conduit14by means of flanges and other mechanical assemblies not shown in detail and comprises a pair of radially spaced cylindrical walls29and30to define a first annular channel32for the upbound flow of particulate airborne coal. A third cylindrical wall34concentric with the wall30and the conduit12is mounted in coplanar and surrounding relationship to the wall30to define a second annular channel36. A fourth cylindrical wall38is mounted concentric and coplanar with the walls29,30and34to define a third annular channel40for airborne particulate coal. In the arrangement shown inFIG. 2, channel32is referred to as the “inner” channel, channel36is referred to as the “intermediate” channel, and channel40is referred to as the “outer” channel. Obviously, these names can be interchanged or varied according to how many concentric channels there are in a particular embodiment of the invention. To fit the mixer16around the conduit12, it may be necessary to make the mixer16in two mirror-image sections and bolt them together around conduit12. The walls29,30,34and38may be slightly frustoconical if desired.

As better shown inFIGS. 3 and 4, vanes42are welded between walls29and30to impart a clockwise rotation or spin to the airborne particulate coal flowing through the upbound channel32. Vanes44are mounted such as by welding between the walls30and34to impart a counterclockwise spin to the upbound airborne particulate coal flowing through channel36. Vanes46are mounted such as by welding between the walls34and38to impart a clockwise spin to the upbound airborne particulate coal flowing through channel40. It will be understood that the terms “clockwise” and “counterclockwise” are used in a relative sense. The vanes as shown inFIGS. 3 and 4are all angled relative to a longitudinal axis sufficiently to overlap in plan or projected view so as to eliminate any straight-through paths for the airborne particulate coal flowing through the channels32,36and40. Annular kicker plates52are mounted on the tops of the walls30,34and38to deflect the airborne particulate coal back inwardly toward the center of the assembly. The plates52are optional.

As best shown inFIGS. 2 and 3, radial spacing between wails29and30is greater than the radial spacing between the walls30and34, and the radial spacing between walls30and34is greater than the spacing between walls34and38. The spacing is arranged in such a fashion that the cross-sectional areas of the channels32,36and40are approximately the same.

As best shown inFIG. 2, dentillated steel plates47,48and50are mounted in the transition section18to create turbulence and additional mixing in the airborne particulate coal which emerges from the mixer insert16. The plates47are mounted essentially in parallel to the flow axis. Plates50are mounted around the outside wall of the transition section18. Radial plates48, also of a dentillated design, are disposed on plates53running radially outwardly from the plates47. These plates may be arranged in various fashions as is more completely described in my previously issued patents as set forth above. The plates47,48,50and53are an optional feature of the illustrated embodiment; i.e., the mixer16can be used with or without the additional turbulence-causing plates in the transition section18.

In operation, lump coal is gravity fed through the inlet supply conduit12to the pulverizer/classifier10which operates in a conventional fashion. Pulverized coal is carried upwardly in an air stream through the main supply conduit14into the mixer insert16where the opposite sense spins are imparted to the three divided concentric annular flow quantities by the vanes42,44and46disposed in the channels32,36and40. The spinning airborne particulate coal then encounters the transition section and the various means47,48and50therein where it is turbulently intermixed before entering the four parallel branch conduits20,22,24and26. Those conduits supply the four corners of the combustion chamber or “firebox”28of the turbine boiler.