AU2018282405A1 - Industrial apparatus - Google Patents

Abstract

INDUSTRIAL APPARATUS The invention provides a pulveriser mill having a port ring 124 located around the circumference of a grinding ring of the mill for common rotation therewith. The port ring has openings separated by vanes or lands. One or more blanking elements removably located within respective openings prevent or reduce air flow therethrough whilst other openings are left open. Direction of Rotation

Description

INDUSTRIAL APPARATUS

Field [0001] This invention relates to industrial apparatus, namely a pulveriser or grinding mill, in which pieces of a material are pulverised into a finer particulate form. The invention relates particularly, but not exclusively, to a mill in which coal is pulverised into a powder form which is conveyed to combustion apparatus e.g. of a power station.

Background [0002] In particular the invention concerns a mill having a lower grinding ring, which may be a part formed with an annular depression. Grinding elements are sandwiched between the lower grinding ring and a top member, which may have an annular depression facing an annular depression in the grinding ring. The grinding ring and the top member are moveable relative to one another. The grinding ring and the top member are typically ring-shaped; the terms “grinding ring” and “top ring” may hereinafter be used.

[0003] Typically the required relative movement between the grinding elements and the lower grinding ring is achieved by driving the grinding ring, while the top ring is held against rotation. The grinding elements, which are typically steel balls or rollers, are not driven. They may be fixed in position, or free to precess.

[0004] The mill with which the invention is concerned is of the type having a rotating port ring generally as described in EP 0507983A. Such a port ring is provided, between the periphery or circumference of the grinding ring and an inclined liner (which may also be called a skirt, or gusset) carried by the wall of the mill. There is provided an annular passage or “throat”, just outboard of the grinding ring. Air flows upwardly through the port ring. The port ring has inner and outer annular walls, between which there is a plurality of spaced-apart, inclined, vanes, defining openings, inclined to the vertical, through which air can flow. The port ring rotates with the grinding ring and the vanes impart a desired vector to the generally upwards air flow.

[0005] In the embodiment described in EP 0507983A, around its 360 degree extent the port ring may define only openings and the through-thicknesses of the vanes. That is to say there is in

2018282405 20 Dec 2018 effect an annular passage separated into individual openings only by the through-thicknesses of the vanes.

[0006] The inner and outer annular walls of the port ring are fixed. The gap between them, in which the vanes are located, cannot be varied.

[0007] The size of the gap is selected to provide an optimal air flow rate, which assures efficient advancement of coal fines towards the combustion apparatus. Control of air flow rate is of critical importance in a mill. Too high an air flow rate for a given throughput gives an increased risk that non-combustible materials may be carried forward to the combustion apparatus, along with desired coal fines. Too low an air flow rate, and the coal fines are not all carried to the combustion apparatus, leading to inefficient operation.

[0008] The rotating port ring of EP0507983A is an excellent and successful mill feature but it is not optimal with coal sources which give rise to incomplete grinding; especially with coal sources which contain inclusions of rock. In such circumstances some unground pieces may be too big to fall through the port ring, and back into the material to be fed to the grinding zone, or scrapped.

[0009] To improve the discrimination of the mill - the ratio between the desired pulverised material passing onward to combustion and the rejection undesired material - it was proposed in WO 2009/115828 to provide port ring openings separated by lands serving as obstructions to the flow of air from beneath the grinding ring to above the grinding ring. The lands were welded over what would otherwise have been openings.

[0010] The engineering solution represented by WO 2009/115828 has been successful but it will be appreciated that the lands are a fixed solution without scope for adjustment on commissioning or later, in use.

[0011] Also in WO 2009/115828 variable openings are described, provided in the annular inclined liner. As mentioned above this is a part carried by the wall of the mill. It is located outside the port ring. Such variable openings were proposed in addition to the openings in the port ring. Variable openings were intended to be opened and closed by closure parts moving circumferentially on a support track, moved under mechanical control. However a pulveriser mill

2018282405 20 Dec 2018 is an aggressive and abrasive environment and engineering solutions which involve relative movement of parts in use can be problematic. For reasons of controllability and practicality it would be preferable to employ only the port ring for control. It would also be desirable to provide a simpler engineering solution, which is centred on the port ring, and which permits adjustment after the mill has been constructed, for example during mill commissioning, or subsequently. Further, it would be desirable to provide such a solution which is simple to adjust, and without need for an operative to enter the mill.

Object of the Invention [0012] It is an object of the present invention to substantially satisfy at least one of the above desires.

Summary of Invention [0013] In accordance with a first aspect of the present invention there is provided a pulveriser mill having a rotatable grinding ring and a rotatable port ring around the circumference of the grinding ring, wherein the port ring defines, around its 360 degree extent, a plurality of openings provided to allow air to flow from beneath the grinding ring to above the grinding ring; the mill comprising one or more blanking elements located within respective opening(s) to prevent or reduce air flow therethrough whilst other openings are left open; wherein the mill has a mill wall which has one or more access doors closing respective access aperture(s) permitting access to the port ring, and wherein the or each blanking element is of size to pass through the access aperture(s).

[0014] The use of a blanking element or blanking elements of size to pass through an existing aperture in the mill wall thus giving access to the port ring means that the blanking element(s) may be employed in the constructed mill, to attain optimal air flow conditions. Blanking element(s) may be inserted into port ring opening(s) as required. Preferably the blanking element(s) are removable. Thus blanking element(s) may also be removed from port ring opening(s) as required.

[0015] To effect easy location and removal the blanking element(s) may be bolted in place. In one embodiment the or each blanking element may be bolted to the outer annular wall of the port

2018282405 20 Dec 2018 ring (the port ring having inner and outer annular walls, suitably with a plurality of spaced-apart vanes between the inner and outer annular walls). In an alternative embodiment the blanking elements(s) may be bolted to the inner annular wall. In an alternative embodiment the blanking elements(s) may be bolted to vane(s) of the port ring. A single bolt is likely to be sufficient for the or each blanking element, but the use of two or more bolts to secure a blanking element to one or more of the outer annular wall, the inner annular wall and the two vanes (which together define a given opening) is not excluded.

[0016] Alternatively the blanking element(s) may be welded in place, preferably by means of one or more welds which may be easily ground out, so that the or each blanking element may still be removed and re-inserted relatively easily. For example a fillet weld may be provided between the port ring and the blanking element. The fillet weld is preferably a stitch (intermittent) weld to facilitate removal of the blanking plate. In one embodiment the blanking element(s) may be welded to the outer annular wall of the port ring. In an alternative embodiment the blanking elements(s) may be welded to the inner annular wall. In an alternative embodiment the blanking elements(s) may be welded to vane(s) of the port ring. A single weld (including a stitch weld) is generally sufficient to secure the or each blanking element, but the use of two or more welds to secure a blanking element to one or more of the outer annular wall, the inner annular wall and the two vanes (which together define a given opening) is not excluded.

[0017] One use of blanking element(s) is on commissioning of a mill or on a series of mills. Blanking element(s) may be inserted and removed as required and operation checked with the grade of coal to be employed. Once performance is optimal blanking element(s) may be left in place. Further mills to be commissioned for operation under the same conditions and yet to be assembled may employ blanking element(s) in an identical arrangement or could if preferred have welded plates over the openings which correspond to the openings closed by blanking elements in the first mill. However if blanking elements are used in all mills they offer the advantage of possible removal (or insertion) in operation, for example when a coal feedstock changes.

[0018] It is preferred that the blanking elements are made of steel.

2018282405 20 Dec 2018 [0019] Suitably a blanking element has a head and a body, at an acute angle to each other. The body is located within the opening to be partly or (preferably) completely blocked, and may be used to secure the blanking element to the port ring. The head acts as the obstruction, in partially or wholly obstructing the respective port ring opening. Suitably the head and the body are welded together. A buttress or support member may be located between them to provide extra stability.

[0020] Suitably the port ring openings are generally rectangular in horizontal cross-section (in which case the “length” is the straight length of the opening; orthogonal to the radial width), or are arcuate, preferably following the curvature of the circumference of the port ring (in which case the “length” is measured along the “hoop direction” thereof). The body of the blanking element could be of matching shape; that is planar or curved. However in either case, as a practical matter the body of the blanking element can be planar.

[0021] All measurements and definitions based thereon given in this specification are made with reference to the horizontal plane and/or as viewed from above in plan view.

[0022] The port ring preferably has spaced apart vanes, and therefore inclined openings. The vanes preferably have upper and lower ends, and are preferably oriented at an angle in the range of 20° to 40° relative to a vertical axis of the mill, in a manner such that the lower ends lead, in the direction of rotation of the grinding ring, and the upper ends trail.

[0023] When the vanes of the port ring are oriented at an angle, as described above, it is necessary for the body of a blanking element to be of shape to locate within the resulting oriented port ring opening. This may be achieved by the body being narrower than the opening. However the body is preferably approximately as wide as the opening (though able to slide into the opening easily), as the body then provides an anchoring function. In such embodiments the body is secured to the head at an appropriate oblique angle to allow the body to fit within the inclined opening while the head is in its correct position, generally in the plane of the end of the port ring.

[0024] Preferably the shape of a blanking element is determined by the geometry of the openings within the port ring. Preferably the openings are angled to the vertical, as determined by the inclination of the vanes or walls of the port ring. The body of the blanking element fits within the

2018282405 20 Dec 2018 inclined opening, and when it does so the head of the blanking element is in the plane of an end of the port ring.

[0025] Preferably the body of the blanking element(s) is longer than the outer annular wall of the port ring. This facilitates removal of the blanking element(s) as well as allowing for the possibility of securing the blanking element(s) in place by a simple and readily removable fillet weld or stitch weld.

[0026] Preferably when the body of the blanking element(s) is longer than the outer annular wall of the port ring its region which projects beyond the outer annular wall may have securement means to receive a removal tool. The securement means may comprise a hole or eye.

[0027] Preferably the head of a blanking element is located at the upper end of the port ring, generally in the plane of the upper end of the port ring. Correspondingly, when the body of a blanking element is longer than the outer annular wall of the port ring, its region which projects beyond the outer annular wall projects downwards.

[0028] The size of the blanking element(s) depends on the dimensions of the port ring openings. However for guidance it may be stated that the maximum dimension of the blanking element(s) preferably does not exceed 40 cm, and preferably does not exceed 30 cm (the maximum dimension generally being the diagonal measurement across the body). In contrast the aperture(s) in the mill wall are larger, typically being square opening(s) of at least 50 cm by 50 cm.

[0029] Preferably the port ring has from 12 to 64 openings (preferably 16 to 48) of which from 2 to 12 (preferably from 4 to 8) contain blanking elements.

[0030] Preferably a blanking plate is of size to locate within one opening only.

[0031] The mill could just have one access door, and the port ring could be turned to different positions, as needed for the fitment of the blanking elements. However preferably the mill has more than one access door; often there are three, or four. An operative can do all the work needed to remove and insert blanking plate(s) from outside, working though the access opening. There is no need to enter the confines of the mill.

2018282405 20 Dec 2018 [0032] The invention may be applied to mills in which a port ring is of the type described in EP 0507983A, which does not have lands, and to mills in which a port ring is of the type described in WO 2009/115828, which does have lands. The easy insertion and removal of blanking elements, through an already-available access door without dismantling of a mill or entering a mill, is of value in attaining optimal flow conditions whatever the starting port ring design.

[0033] Preferably, however, the mill of the present invention has a port ring with openings and lands, the latter being fixed obstructions. Blanking element(s) of the present invention are employed to facilitate further restrictions as needed to achieve optimisation. Preferably such a port ring defines, around its 360 degree extent, a plurality of openings which are separated by lands, the openings permitting air to flow from beneath the grinding ring to above the grinding ring when permitted to do so by blanking elements, and the lands serving as fixed obstructions to the flow of air from beneath the grinding ring to above the grinding ring, wherein the aspect ratio of the openings (length divided by radial width) is in the range from 1:1 to 3:1.

[0034] In this type of pulveriser mill having openings and fixed lands there are typically from 16 to 30 openings, more preferably from 18 to 24 openings. There are typically from 16 to 30 lands, more preferably from 18 to 24 lands.

[0035] In this type of pulveriser mill having openings and fixed lands:

[0036] preferably the openings in total occupy at least 90 degrees of the 360 degree extent of the port ring, preferably at least 120 degrees, most preferably at least 160 degrees [0037] preferably the openings in total occupy 270 degrees of the 360 degree extent of the port ring, preferably up to 240 degrees, preferably up to 200 degrees.

[0038] preferably the lands in total occupy at least 90 degrees of the 360 degree extent of the port ring, preferably at least 120 degrees, most preferably at least 160 degrees.

[0039] preferably the lands in total occupy up to 270 degrees of the 360 degree extent of the port ring, preferably up to 240 degrees, preferably up to 200 degrees.

2018282405 20 Dec 2018 [0040] preferably the aspect ratio of the openings is in the range from 1:1 up to 2.5:1, most preferably from 1.2:1 up to 2.1:1. The aspect ratio of the openings may be defined herein as the (mean) length divided by the (mean) width in the radial direction.

[0041] The above definitions of ‘openings’ include free openings and openings which contain blanking elements.

[0042] In accordance with a second aspect of the present invention there is provided a method of optimising an existing pulveriser mill having a rotatable grinding ring and a rotatable port ring around the circumference of the grinding ring, wherein the port ring defines, around its 360 degree extent, a plurality of openings provided to allow air to flow from beneath the grinding ring to above the grinding ring, wherein one or more blanking elements located in respective opening(s) prevent or reduce air flow therethrough whilst other openings are left open; wherein the mill has a mill wall which has one or more access doors closing respective access aperture(s) permitting access to the port ring, and the blanking element(s) are introduced into the mill through the access aperture(s) and inserted into port ring opening(s).

[0043] Blanking plate(s) may be introduced and removed, without an operative needing to enter the mill, during commissioning of the mill. The mill may be tested with different arrangements of blanking plate(s) until optimal mill operation is achieved. Likewise, after the mill has been commissioned adjustment can readily be effected, for example when a different coal source is used.

[0044] Insertion and/or removal via the access aperture(s) avoids any need to dismantle the mill. In fact, dismantling the mill is such a substantial operation that most operators would prefer not to do so, but instead would accept non-optimal operation. Achieving optimisation with minimal down time and avoiding dismantling are important advantages of the invention. A further important advantage is that operatives can carry out the required operations from outside the mill.

[0045] In accordance with a third aspect of the present invention there is provided a method of operating a mill of the present invention as defined above, in which the mill has been optimised on commissioning or is adjusted subsequently, in service, utilising blanking elements selectively in port ring openings.

2018282405 20 Dec 2018 [0046] Preferred features of the second or third aspect are any of the features defined as being necessary or desirable features of the first aspect.

Brief Description of Drawings [0047] The invention will now be further described, by way of example, with reference to the accompanying drawings, in which:

[0048] Fig. 1 is a schematic side sectional view of the grinding part of a first type of known pulveriser mill, in operational condition;

[0049] Fig. 2 is a schematic expanded side sectional view of a side region of a similar known pulveriser mill;

[0050] Fig. 3 is a plan view from above of the region shown in Fig. 2;

[0051] Fig. 4 is a schematic drawing showing the arrangement of vanes and openings, in the region shown in Figs. 2 and 3;

[0052] Fig. 5 is a plan view from above of a peripheral region of a second type of mill;

[0053] Fig. 6 is a side sectional view of a region of the mill also shown in Fig. 5, showing the arrangement of lands and openings within the port ring;

[0054] Fig. 7 is a perspective view of a blanking element, illustrating the present invention;

[0055] Fig. 8 is a front elevation of the blanking element of Fig. 7;

[0056] Fig. 9 is a perspective view generally from below of the blanking element of Figs. 7 and 8, about to be inserted into an opening of a port ring;

[0057] Fig. 10 is an elevation from above of the blanking element of Figs. 7 and 8, partially inserted into the opening;

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2018282405 20 Dec 2018 [0058] Fig. 11 is an elevation from above of the blanking element of Figs. 7 and 8, fully inserted into the opening;

[0059] Fig. 12 is a perspective view generally from below of the blanking element of Figs. 7 and 8, inserted into the opening.

[0060] Detailed Description Figs. 1-4 show a first prior pulveriser mill generally in accordance with EP 507983A. The mill has a driven, lower steel grinding ring 2 (which is alternatively called a grinding member, or a grinding wheel, in this art). Grinding ring 2 has an upwardlyfacing annular groove 4, in which a plurality of grinding elements 6, e.g. steel rollers or balls, are located. Above the grinding elements is located a fixed (non-rotating) steel top ring 8, which has a downwardly-facing annular groove 10, aligned with the annular groove 4 of the grinding ring

2. Therefore the arrangement is like a ball race, with the grinding elements 6 free to precess within the oppositely-directed grooves 4, 10. This apparatus is all housed within a cylindrical wall 12. The wall has in its lower region, close to the level of the grinding ring 2, three access doors 14 located equiangularly, each closing a respective square access aperture 16 approximately 60 cm by 60m in size.

[0061] This type of pulveriser mill is used in a highly demanding environment, to crush coal into fines (powder) to be combusted. The coal fines are carried upwardly by an air current, towards the combustion apparatus.

[0062] Around the grinding ring 2 is a narrow throat 22 and in the throat 22 there is provided a port ring 24 (Fig. 2). This rotates as one with the grinding ring 2, to impart a desired movement to the upwardly-directed air, which carries the coal fines to the combustion apparatus.

[0063] The port ring 24 comprises a plurality of spaced-apart vanes 26. The vanes 26 are welded between spaced-apart support rings 28 and 30 which are inner and outer circumferential walls of the port ring. Preferably the inner and outer support rings 28, 30 of the port ring 24 are short sections of vertical concentric cylinders. The vanes 26 are inclined. The angle of inclination of the vanes is in the range of 20 degrees to 40 degrees from vertical. Preferably the angle of inclination is 25-30 degrees. The upper ends of the vanes are tilted in a direction opposite to the direction of normal rotation of the grinding ring (that is to say, the tilt of the vanes is such that the upper ends trail the lower ends when the grinding ring is rotated). In Fig. 3

2018282405 20 Dec 2018 the top edge of a vane is indicated as 26a; 26b denotes the projection of the inclined frontal face of a vane, visible from above due to the inclination of the vane; and the lower edge of a vane is indicated as 26c. Inner support ring 28 may be secured to the periphery of the grinding ring by means of bolts 32 or by welding, for example.

[0064] An annular mill liner 34 extends downwardly from the inside wall 35 of the mill body, to which it is preferably secured, towards the upper and outer edge of the port ring. Then the mill liner extends vertically downwardly to within about 1 cm of the upper edge of the outer member 30 of the port ring. The angle of inclination of the mill liner is typically between 30 degrees and 60 degrees, to the wall of the mill body (i.e. to the vertical).

[0065] Particles produced by the crushing or pulverising process are carried upwardly by means of air passing through the port ring 22. Air flows upwardly in a nearly vertical manner with minimal swirling or spinning. As a result, the crushed particles are lifted upwardly in a smooth and efficient manner.

[0066] Figs. 5-6 show a second prior pulveriser mill, generally in accordance with WO 2009/115828.

[0067] The overall arrangement is similar to that described with reference to Figs. 1-4, in its grinding apparatus, and in that a rotating port ring is provided. Like the port ring described with reference to Figs. 1-4, the port ring 124 has a series of vanes 126, mounted to the grinding ring (not shown) at its circumference. The vanes are mounted and inclined as described above, except that they are not evenly spaced. Each vane is welded in place such that the space 140 to one side of it is longer, in the hoop direction, than the space 142 on the other side of it. The longer spaces 140 are blanked off by lands 144, welded to the upper edges of the respective vanes 126, and to the upper edges of the support rings 128 and 130, over the spaces 140. Thus, only the other spaces 142, forming fixed openings or ports, and defined by the more closely spaced vanes, are available for the through-flow of air. In this embodiment the ratio of the lengths of these spaces in the hoop direction (space 140 to space 142) is approximately 1.5 to 1. It will be apparent that approximately one-half of the annular extent of the port ring 124 has been rendered unavailable for air flow - see Fig. 6.

2018282405 20 Dec 2018 [0068] It is highly desirable to keep air speed at an optimum level and, at least approximately, to maintain the available area for the throughput of air. To achieve this, the port ring 124 is wider than the port ring shown in Figs. 1 to 4. The port ring, and in particular the openings in the port ring, are approximately 2.5 times as wide as they would have been, had the lands not been used in this embodiment.

[0069] The arrangement described permits large pieces of unground spoil, such as rock, to fall through the port ring of Figs. 5 and 6, and back into material to be fed into the grinding zone, or into scrap, instead of accumulating on the port ring, as would have happened previously.

[0070] The invention will now be described, by way of example, with reference to Figs. 7-12.

[0071] The overall arrangement is similar to that described with reference to Figs. 5-6, in its grinding apparatus, and in that a rotating port ring is provided, with alternate openings and lands. However in the present invention blanking elements are employed to close selected openings in the port ring. Blanking elements may be inserted or removed as required, to achieve highly efficient operation.

[0072] A blanking element 200 is steel and has a head 202 and a body 204. These are welded together at an acute angle - about 60 degrees in this embodiment, to conform to the geometry of the openings to be closed. A supporting strut or buttress 206 is located between the head and the body. The head is generally rectangular. The body is generally rhomboid and is set obliquely to the head, to conform to the inclination of the openings. Thus the blanking element 200 can be pushed into an opening in the port ring, as shown by the progressive drawings Figs. 9-11 with the body spanning the opening, until the head is aligned with, and substantially blocks, the opening at its upper end (Fig. 11), [0073] As can be seen in Fig. 11 the head does not form a tight fit in the opening. This would not be practical and is not needed. In a milling operation any gap is soon filled by ground materials.

[0074] It will be seen that the body has a through-hole 210 between its central region and the head 202. The outer annular wall of the port ring has a through-hole 212. Through-holes 210 and 212 are used for bolting the parts together. A bolt head can be seen at 214. Of course, the use of bolts means that the parts may be released again if needed, and the blanking element removed. In

2018282405 20 Dec 2018 an alternative embodiment stitch welding, easily removable by grinding, may be employed between the blanking element and the outer annular wall, where the blanking element projects from the outer annular wall.

[0075] In this embodiment it is easiest for an operative to secure the blanking element to the outer wall of the port ring. In other mills it may be easiest for the operative to secure the blanking element to the inner wall of the port ring, or to a vane. It is considered adequate to secure a blanking element using a single bolt or weld (including stitch weld) but the use of two or more bolts, or two or more welds, to one of more of the outer annular wall, the inner annular wall, and the two vanes (which together define an opening) can be carried out if thought necessary.

[0076] It will be further seen that the body has a second through-hole 220 adjacent to its end region which is distal from the head 202. The purpose of this through-hole is to facilitate removal. A tool may be inserted into through-hole 220 and used to pull out the blanking element.

[0077] The mill does not have to be disassembled to modify the port ring engineering, and an operative does not have to enter the mill. All that is required is to remove an access door, which all mills have (or remove more than one, if present and if wished) to expose an access aperture, and insert blanking plates by simple bolting or welding, as described above, working from outside the mill. The port ring may be turned to fit further blanking elements from the same access aperture; or the mill may have multiple access doors, which may be used. The mill can be trialled with different numbers or configurations of blanking plates, until excellent performance is achieved. If conditions subsequently change - for example if a different source of coal is used - the operation can be repeated, with minimum downtime.

[0078] It can be seen best in Fig. 11 that the port ring in this embodiment has welded lands and remaining openings, as described in WO 2009/115828. In another embodiment the port ring could be the simpler type described in EP 0507983A, that is, not having lands. In either case the principle of achieving improvement or optimisation by using blanking plates, inserted or removed as required without any need to dissemble the mill or enter the mill, is a valuable one.

Claims (13)

1. A pulveriser mill having a rotatable grinding ring and a rotatable port ring around the circumference of the grinding ring, wherein the port ring defines, around its 360 degree extent, a plurality of openings provided to allow air to flow from beneath the grinding ring to above the grinding ring; the mill comprising one or more blanking elements located within respective opening(s) to prevent or reduce air flow therethrough whilst other openings are left open; wherein the mill has a mill wall which has one or more access doors closing respective access aperture(s) permitting access to the port ring, and wherein the or each blanking element is of size to pass through the access aperture(s).

2. A pulveriser mill as claimed in claim 1, wherein the or each blanking element has a head and a body, at an oblique angle to each other; the head acting as a means of partially or wholly obstructing the respective port ring opening; and the body being located within the opening, and used to secure the blanking element to the port ring.

3. A pulveriser mill as claimed in claim 1 or 2, wherein the openings of the port ring have upper and lower ends.

4. A pulveriser mill as claimed in claim 3, wherein the upper and lower ends are oriented at an angle in the range of 20° to 40° relative to a vertical axis of the mill, in a manner such that the lower ends lead, in the direction of rotation of the grinding ring, and the upper ends trail.

5. A pulveriser mill as claimed in claim 3 or 4, wherein the body of a blanking element is of shape to locate within the inclined port ring opening; this being achieved by the body being secured to the head at an oblique angle.

6. A pulveriser mill as claimed in any proceeding claim, wherein the or each blanking plate is removably secured within its opening in the port ring.

7. A pulveriser mill as claimed in any proceeding claim, wherein the port ring has from 12 to 64 openings of which from 2 to 12 contain blanking elements.

2018282405 20 Dec 2018

8. A pulveriser mill as claimed in claim 7, wherein the port ring has from 16 to 48 openings of which from 4 to 8 contain blanking elements.

9. A pulveriser mill as claimed in any proceeding claim, wherein the body of the blanking element/s) projects beyond the outer annular wall of the port ring.

10. A pulveriser mill as claimed in claim 9, in which the region of the body which projects beyond the outer annular wall has securement means to receive a removal tool.

11. A pulveriser mill as claimed in any proceeding claim, wherein the port ring has openings and lands, which are fixed obstructions, and blanking elements are employed to facilitate further restrictions as needed to achieve optimisation.

12. A method of optimising an existing pulveriser mill having a rotatable grinding ring and a rotatable port ring around the circumference of the grinding ring, wherein the port ring defines, around its 360 degree extent, a plurality of openings provided to allow air to flow from beneath the grinding ring to above the grinding ring, wherein one or more blanking elements located in respective opening(s) prevent or reduce air flow therethrough whilst other openings are left open; wherein the mill has a mill wall which has one or more access doors closing respective access aperture(s) permitting access to the port ring, and the blanking element/s) are introduced into the mill through the access aperture/s) and inserted into port ring opening/s).

13. A method of operating a mill as claimed in any proceeding claim, in which the mill has been optimised on commissioning or adjusted subsequently, in service, utilising blanking elements selectively in port ring openings.