Pulverizer mill high performance classifier system

Field of the invention

This invention relates to pulverizer mills, e.g., mills that are used for the crushing of large pieces of coal into smaller coal particles. More particularly, this invention relates to a dust separating system known as a classifier which is designed to segregate large, partly ground coal particles from smaller, completely ground particles within a pulverizer mill.

Background of the invention

Pulverizer mills are commonly used for crushing large coal pieces into small particles which are required for conventional coal fired boilers. A common type of pulverizer mill includes a flat or dished grinding bowl or table which is attached to and driven by a vertical spindle and three (3) large rollers or wheels which rotate around separate shafts as the bowl rotates with the vertical spindle. Large coal particles are introduced onto bowl and are crushed as they are captured between the rollers and the bowl. An air stream (known as primary air flow) passing upwardly around the bowl carries the crushed coal particles upward into the classifier through the classifier vanes and then out of the mill to the boiler through an outlet pipe or pipes.

Occasionally large coal particles are swept up and out though the outlet pipe due to the high velocities of the primary air flow inside the top of the classifier. This is an undesirable characteristic of all coal pulverizers. In order to minimise the amount of large coal particles which are swept out of the mill, a cone-shaped classifier has been used in all prior art designs for receiving partly crushed coal particles and for separating large particles which must be crushed further from the fine particles (which are desired). The interior surfaces of all prior art classifiers are smooth. Although the classifier is an integral part of all vertical spindle mill designs, it is not as effective as desired in many instances. Consequently, the grinding capacity of a mill can be limited because of the inherent inefficiencies of present classifier designs. Or conversely, the large amounts of unburned coal found in the ash of many typical boilers reduces the efficiency of said boilers and increases the operating costs of the users.

In GB-A-2132920 there is described a pulverizer mill having a classifier with a plurality of fixed vanes attached to the upper end of the inner conical wall of the mill, for imparting a rotational motion to the air-solids stream, and at least three pivotable deflector vanes downstream thereof, at the top of the classifier.

There has not heretofore been described a classifier system having the advantages provided by the present invention.

Summary of the present invention

In accordance with a first aspect of the present invention there is provided an improved classifier system for a pulverizer mill (e.g., a mill of the type used for crushing coal into fine particles). The mill has a housing and the improved classifier system is characterised by a volume-increasing spacer assembly located at or near the top or bottom of the mill housing.

Preferably the spacer assembly is located at or near the top of the mill housing, and it is hereafter referred to as the outlet turret extension. This extension is positioned in such a manner so as to increase the overall height of the existing coal pulverizer mill. By increasing the overall height of said pulverizer mill, the volume is thus increased as well. This increase in volume will improve the efficiency of coal particle separation within the housing of the pulverizer mill. This outlet turret extension is normally cylindrically shaped (although other shapes may exist if the existing pulverizer mill housing so dictates), the length of which is determined for each individual coal pulverizer mill.

The improved classifier system may also include an intermediate classification liner, an optional feature of a pulveriser mill of the present invention, attached circumferentially to the interior surface of the mill housing high above the grinding elements (or grinding zone). The intermediate classification liner provides a converging-diverging orifice assembly which extends around the interior surface of the housing between the grinding zone and the classifier. This intermediate classification liner redirects the upwardly moving and turbulent primary air flow towards the center of the pulverizer mill. This redirection of the primary air flow will result in a large loss of upward momentum in the bigger partly ground coal particles, causing them to fall back into the grinding zone without passing through the classifier. This new method of particle separation is referred to herein as intermediate classification.

The classifier system may include curved classifier vanes at the upper end (and inlet) of the classifier, and preferably (but not necessarily) the vanes extend downwardly to a point below the air inlet to the classifier. The curved classifier vanes greatly enhance the spin of the air flow entering the upper end of the classifier, although larger flat vanes may also be used.

The classifier system may include spin initiator vanes located on the interior surface of the pulverizer housing located immediately below the inlet to the upper end of the classifier or immediately below the lower end of the classifier vanes. The spin initiator vanes represent a new design feature not believed to be found in any prior art proposal. Suitably the spin initiator vanes are located parallel to each other and are tilted at an angle in the range of about 30° to 45° relative to the vertical plane. The spin initiator vanes effectively control the upwardly flowing and turbulent primary air flow within the upper region of the mill housing. The spin initiator vanes re-direct the air flow, causing a strong clockwise or counter-clockwise motion of the primary air flow, depending upon the specific mill design. This turning of the primary air flow greatly increases the efficiency of the classifier vanes.

By carefully controlling the primary air flow in the upper part of the mill housing, enhanced particle separation can be realised. Large coal particles lose their upward momentum due to the re-directed air flow and fall back down into the grinding zone. Also, the air flow is effectively turned prior to entering the classifier vane section of the pulverizer mill. This greatly enhances the performance of the classifier vanes and, thus, the entire classifier section.

The coal particles, which are carried out of the mill by the primary air flow, are much more finely ground when compared to prior art classifier designs. This system is as easy to retrofit as any conventional replacement static classifier and much less expensive than dynamic or rotating classifiers which are currently available. Also no additional power requirements are needed for auxiliary drive motors or other associated equipment which may be necessary with rotating classifiers. This system of the invention greatly reduces the amount of unburned (wasted) coal which ultimately must be purchased by the user of the pulverizer mill.

In accordance with a further aspect of the present invention, there is provided a method of constructing a pulveriser mill as defined above as the first aspect of the invention, comprising the step of retro-fitting the spacer assembly to an existing mill. When items such as the intermediate classifier section, curved classifier vanes and spin initiator vanes are present, they may likewise be retro-fitted.

Brief description of the drawings

The invention is described in more detail hereinafter with reference to the accompanying drawings, wherein like reference characters refer to the same parts throughout the several views and in which:

• Figure 1 is a side elevational, cut-away view of a pulverizer mill which includes one embodiment of the improved high performance classifier system of the invention;
• Figure 2 is a perspective view of one embodiment of a cylindrical extension member known as the finned cyclone classifier section;
• Figure 3 is a perspective view of a segment of one embodiment, showing spin initiator vanes which in this embodiment are integral with the outlet turret extension in accordance with the invention;
• Figure 4 is a perspective view of a preferred embodiment of a cesta-curved classifier vane which is useful in this invention; and
• Figure 4a is a top view of the classifier vane shown in Figure 4.

Detailed description of the invention

With reference to Fig. 1, showing a high performance classification system, all components of the said classification system are constructed of a steel material, either mild steel or of a wear-resistant type. Further, said components may be protectively lined or covered with abrasion-resistant ceramic tiles of numerous descriptions. Also said components may be protectively lined or covered with welded overlays of high-alloy wear-resistant material. In Fig. 1, a spacer assembly of the present invention, in the form of an outlet turret extension 1, is located at the top of the existing pulverizer mill housing 10. This outlet turret extension acts as a volume-increasing device which may be located either at the top of or at the bottom of any existing coal pulverizer mill housing. The specific design of the coal pulverizer mill will dictate the location and installation method of the outlet turret extension. Typical installation methods include a bolted and flanged arrangement or simply a weld-in modular design. The outlet turret extension will be constructed with a cross-sectional shape which corresponds to the existing pulverizer mill housing. This shape may be cylindrical, hexagonal, or any other shape utilized by coal pulverizer manufacturers. Note that the outlet turret extension is also shown in Fig. 3.

Also shown in Fig. 1 is a spin initiator means 2, an optional feature of a pulveriser mill of the present invention, comprising a plurality of evenly-spaced spin initiator vanes, which are oriented at 30° to 45° to the vertical. The spin initiator vanes are normally welded to the interior surface of the coal pulverizer mill housing. However in certain installations, as shown in Fig. 3, the spin initiator vanes may be combined with and secured to the outlet turret extension. This will minimise the installation difficulties and costs for the end user of the coal pulverizer mill. Although both drawings depict the spin initiator vanes and the outlet turret extension as an integral unit these devices may, in fact, be installed as separate units in the high performance classification system.

Fig. 1 also depicts an intermediate deflector liner 3, an optional feature of a pulveriser mill of the present invention. The intermediate deflector liner is a circumferentially-built converging-diverging orifice assembly. Also constructed of a steel material as described above, this liner assembly may be bolted or welded to the interior surface 10A of the existing pulverizer mill housing. As dictated by the individual pulverizer mill design, the intermediate deflector liner will be constructed with upwardly and downwardly sloping surfaces which are oriented at 30° to 60° to the horizontal plane. Thus, the total developed angle between the two sloping surfaces would be in the range 60° to 120°. The components of the intermediate deflector liner may be designed and built as a single unit or may be designed as separate smaller segments for easier installation.

Another feature of the high performance classification system shown in Fig. 1 are cesta-curved classifier vanes 4, an optional feature of a pulveriser mill of the present invention. The cesta-curved classifier vanes feature is one of the preferred aspects of this design and will increase the efficiency of the coal particle separation in the top region of the interior of the classifier cone. However, flat or planar classifier vanes may be utilized with only a slight degradation of the high performance classification system's performance. The flat classifier vanes will reduce costs and are easier to construct from a wear-resistant material. The cesta-curve of the classifier vanes is unique to this high performance classification system. Note that this classifier vane design is also shown in Fig. 4.

A finned cyclone classifier section 5, an optional feature of a pulveriser mill of the present invention, is also shown in Fig. 1. A detailed view of one embodiment of this extension member is shown in Fig. 2. The interior surface of this section of the classifier is rough by design. It may be thought of as being similar to the corrugations found in certain types of cardboard construction. This roughened surface area, which consists of a plurality of spaced and radially inward projecting structures 5A, may have a variety of different designs. The details shown in Fig. 2 represent a piece of steel sheet which has been folded and bent into the shape drawn. Other construction methods may include the welding or fastening of steel bars, which in themselves may be of a variety of shapes, to the inside surface of a cylindrical body. The essence of this design feature is that the projections and the increased surface area provided by this roughened interior surface will much more rapidly slow the movement of any large coal particles which may come into contact with it. The roughened interior surface may be in the nature of vertical bars, slanted bars, discontinuous bumps or bars, or combinations of any of these, to disturb the surface flow of the circulating air and coal particle stream. In another embodiment the high performance classifier system may include a classifier cone outlet extension, as is known in the art. This outlet extension is useful in the control of partly ground coal particles, in that these partly ground particles may be more accurately returned to the grinding zone of the pulverizer. This outlet extension may normally be constructed from a mild or wear-resistant steel material. This outlet extension will, in many cases, enhance the control of the coal fineness by increasing the efficiency of the crushing of the already partly-ground coal particles.

The cone outlet may also include an adjustable restriction ring 7 which may be used to control the primary air flow in such a way that this air will not flow into the lower end of the cone or upwardly through the interior of the high performance classifier, thus reducing the efficiency of the system. The ring 7 defines an annular opening at the lower end of the cone.

Other variants are possible without departing from the scope of this invention.