Technical Field

This invention relates to a mill suitable for producing milled particles from a millable material, to an apparatus for milling a combustible millable material to produce a combustible fuel, to a system for providing a combustible fuel to a burner, and to a process for producing a combustible fuel. The invention also relates to a feed composition controller for supplying millable material and a gas in a predetermined ratio to a mill.

Background Of The Invention It is known to reduce the particle size of granular material by attrition, for example in a hammer mill, ball mill, roller mill or disk mill. Typically, known milling methods produce a range of particle sizes and shapes. Known mills are subject to limitations in the fineness of the milled particles that can be produced economically. In particular, it has hitherto been difficult to produce very fine particles in a disk mill when the material to be milled is relatively hard, owing to the tendency for known disk mills to clog when the milled particles are very small, for example in the micron range.

In addition, the milled particles produced by a mill are often transported in a flowing airstream. When the milled particles are of an uneven or irregular shape and have a range of sizes, the particles transported in an airstream do not behave coherently which can lead to difficulties and inefficiencies in the system for transporting the milled material.

One application of milling to produce finely divided particles suspended in an airstream is the reduction grinding of coal to produce fuel for combustion. Reserves of oil and natural gas are much more limited than reserves of coal, and a suspension of coal in air which is sufficiently finely divided so that it will combust in air similarly to namral gas or liquefied petroleum gas (LPG) is an attractive fuel. However, it has hitherto been difficult to produce coal particles sufficiently small and in a uniform ratio of coal particles to air so as to permit a suspension of coal in air to be used as a practical fuel replacement for natural gas or liquefied petroleum gas. Particles of sufficiently small size and having substantially no sharp corners or edges behave coherently in a moving gas stream. Consequently, the composition of a suspension of such finely divided particles in a travelling gas stream is much easier to control than a suspension of uneven or irregular particles. Control of the composition of a suspension of finely-divided combustible material such as coal is particularly important when the suspension is used as a fuel. When coal particles in an airstream are sufficiently small they burn rapidly with relatively little radiant heat generated. Larger particles produce more radiant heat when they burn. Irregular particles tend to agglomerate in some regions of a burner and either behave as larger particles or burn unevenly. In either event, the result is the production of larger quantities of nitrogen oxides than occurs when

a coherent stream of very small particles is burnt, with attendant adverse environmental consequences.

Accordingly, there is a need for a method of milling a millable material so as to produce very fine milled particles.

Objects Of The Invention

It is an object of this invention to provide an improved mill for producing milled particles from a millable material. Further objects of the present invention are to provide an apparatus for milling a combustible millable material to produce a combustible fuel, a system for providing a combustible fuel to a burner, and to provide a process for producing a combustible fuel. It is a still further object of the present invention to provide a feed composition controller for supplying a predetermined ratio of a gas and a millable material to a mill.

Disclosure Of The Invention

According to a first embodiment of the present invention there is provided a mill for producing milled particles from a millable material, the mill comprising: a first rotatable milling means having a first milling means milling surface; a second, optionally rotatable, milling means having a second milling means milling surface; means to support said first and second milling means such that said first and second milling surfaces are substantially opposed to one another; and a housing to house said first and second milling means; said first and second milling means being operatively arranged with respect to one another and said housing to define a milling region between said milling surfaces capable of milling said millable material to produce milled particles, said housing having an outlet for said milled particles adjacent said milling region, and at least one of said milling means having an inlet for said millable material into said milling region.

Typically, in this embodiment, the means to support the first and second milling means is the housing.

According to a second embodiment of the present invention there is provided a disk mill for producing milled particles from a millable material, the disk mill comprising: a first axially rotatable disk having a first milling surface; a second optionally axially rotatable disk having a second milling surface; and means to support the first and second disks such that the first and second milling surfaces are substantially opposed to one another; the first and second disks and the means to support defining a region having an inlet and an outlet about which region the disks are operatively associated with one another so as to be capable of milling the millable material to produce milled, particles.

According to a third embodiment of the present invention, there is provided an apparatus for milling a combustible millable material to produce a combustible fuel, comprising: a first axially rotatable disk having a first milling surface; a second optionally axially rotatable disk having a second milling surface; means to support the first and second disks such that the first and second milling surfaces are substantially opposed to one another, the first and second disks and the means to support defining a region having an inlet and an outlet; and means to supply a pressurised gas to the inlet of the region, wherein the disks are operatively associated with one another about the region so as to be capable of milling the combustible millable material to produce a suspension of the milled combustible material in the pressurised gas.

According to a fourth embodiment of the present invention, there is provided a system for providing a combustible fuel to a burner, said system comprising: a mill of the first embodiment; means to supply a pressurised combustible or combustion-supporting gas to the inlet of said milling region of said mill, said first and second milling means being operatively associated with one another about said milling region so as to be capable of milling said combustible millable material to produce a combustible fuel comprising a suspension of said milled combustible material in said pressurised gas; and means operatively associated with said mill to conduct said combustible fuel to said burner.

According to a fifth embodiment of the present invention, there is provided a process for producing a combustible fuel, comprising supplying a combustible material and a pressurised gas to a system of the fourth embodiment, milling said combustible material in said milling region to a sufficiently fine size for the milled combustible material to be suspendable in said pressurised gas to produce a combustible fuel, and removing said combustible fuel from said system. According to a sixth embodiment of the present invention there is provided a process for producing a combustible fuel, comprising supplying a combustible material and a pressurised gas to an apparatus of the third embodiment, milling the combustible material in the region defined by the first and second disks and the means to support, to a sufficiently fine size for the milled combustible material to be suspendable in the pressurised gas, and removing a suspension of milled particles of the combustible material in the pressurised gas from the apparatus.

Typically, in the apparatus of the third embodiment, the system of the fourth embodiment and the process of the fifth and sixth embodiments, the combustible fuel is suitable for combustion in a natural gas or LPG burner.

In the mill of the present invention, the second milling means or disk may be fixed, or it may be vibratable, or it may be axially rotatable. Typically, the second milling means or disk is axially rotatable.

In one form of the mill of the present invention, at least the first milling means has an axis of radial symmetry and is rotatable about that axis. Typically, in this form of the mill, the second milling means also has an axis of radial symmetry. More typically, the first and second milling means are arranged with respect to one another so that their axes of radial symmetry are substantially collinear. Generally, in this form, the first and second milling means have substantially equal diameters. Generally, in the mill of the present invention, the milling means or disks are substantially circular in plan view and have a diameter in the range of from 10 cm to 1 m, depending on the application for the mill and the desired rate of production of milled material.

Typically, a mill of the present invention comprises means to axially rotate the rotatable milling means, operatively associated with the milling means. Where both milling means are rotatable, they may rotate in the same or opposite directions. Generally, the first and second milling means are rotatable in opposite directions. The milling means may be rotatable at the same or different speeds. Typically in operation the milling means rotate at different speeds in opposite directions. In such case, the first milling means may rotate faster than the second milling means, or the second milling means may rotate faster than the first milling means.

Generally, the first milling means or disk comprises a plurality of radially extending vanes at its periphery. Typically, each vane extends from the milling means or disk in a plane substantially parallel to its axis of rotation. Typically, the milling region is in communication with the space or spaces between at least one pair of adjacent vanes, so that milled particles discharged from the milling region enter the space(s) between the vanes. It will be appreciated that the number and spacing of the vanes will vary depending on the size of the milling means or disk. Typically, the vanes are evenly spaced, at intervals of from about 0.5 cm to about 10 cm. The number of vanes around the periphery of the milling means or disk is typically from about 10 to about 200, more typically from about 20 to about 100, even more typically from about 25 to about 80, still more typically from about 30 to about 50.

In the mill of the present invention, the milling surfaces may typically be flat, concave, convex, or comprise a conical projection or a conical depression, or a combination of more than one of these. Typically, at least one of the milling surfaces comprises at least a portion of the curved surface of a right circular cone. More typically, at least one of the milling surfaces comprises at least two portions of the curved surfaces of different right circular cones. Even more typically, both milling surfaces comprise at least two portions of the curved surfaces of different right circular cones, the

portions being a projection in one milling surface and a depression in the other milling surface. The angle made by such conical surfaces with the base of the cone depends on the diameter of the milling surface, but is generally between 0.5° and 45°, more generally between 1° and 30°, even more generally between 2° and 20°. Usually, one milling surface is substantially complementary or approximately complementary in shape to the other milling surface. Where the two milling surfaces are substantially complementary in shape, the width of the milling region, that is, the distance between the milling surfaces, is substantially the same everywhere. More typically, the milling surfaces are only approximately complementary in shape, the shapes of the surfaces being such that the milling region defined between them is widest at its centre, where the millable material enters the milling region, and narrows taperingly at least part of the way towards its periphery.

It will be appreciated that the width of the milling region narrows when the mill is operated, owing to thermal expansion of the materials of the mill. Accordingly, sufficient clearance is allowed between the milling surfaces or disks before the mill is operated, to provide for adequate clearance when the mill has reached operating temperature. Typically, in operation the width of the milling region is from 100 microns to 15 mm at its centre, more typically from about 1 mm to about 10 mm, even more typically from about 1 mm to about 5 mm, still more typically about 2.2 mm. Typically, in operation the width of the milling region at its periphery is from about 10 microns to 1 mm, more typically from about 20 microns to 500 microns, even more typically from about 50 microns to about 100 microns, still more typically about 70 microns.

Generally, in the mill of the present invention, the milling surfaces comprise a plurality of annular segments which may be of the same or different materials. Typically, the annular segments are independently removable from the milling means of which the milling surface is a part. The number of annular segments in each disk may be from 1 to 20. Typically, the number of annular segments is from 2 to 5, more typically 2. Generally, each annular segment of one milling surface is opposed to an annular segment of the other milling surface. Typically, the opposed segments are of different materials and/or of different roughness so that different frictional forces are exerted on a particle which contacts both milling surfaces. Materials suitable for use in the milling surfaces are generally known in the art. Such materials include carborundum, tungsten carbide, boron carbide, boron nitride, garnet, emery, quartz or other forms of silica, corundum or other forms of alumina such as fused alumina or sintered alumina, zirconia, zirconia/alumina and diamond. The roughness of the surfaces of the annular segments is selected to suit the desired particle size of the milled particles. Generally, the annular segments are arranged in each milling means or disk so that the segments having the roughest surfaces are innermost, with the other annular segments arranged so that each segment has a surface which is less rough than, or as rough as, the segment immediately

inside it. Typically, the roughness of the surfaces of the annular segments ranges from the equivalent of about 20 grit to the equivalent of about 10,000 grit. More typically, the roughness of the surfaces ranges from the equivalent of about 46 grit to the equivalent of about 5,000 grit, in gradations from the innermost segment to the outermost segment, for example in gradations of 46, 60, 80, 100, 120, 150, 180, 220, 280, 320, 400, 500, 600, 1200, 2400 and 5000 grit.

Typically, the width of the milling region is adjustable by axially displacing one of the disks relative to the other by an axial displacement means operatively associated with one or both of the milling means or disks. In this way, the particle size of milled particles produced by the mill may be adjusted to a desired value. Typically, the axial displacement means is a screw adjustment abutting one or both of the milling means or disks. Alternatively, the milling means or disks may be moved relative to each other hydraulically, electrically or magnetically. In a preferred form of the present invention, a mechanism for axial displacement of one of the milling means or disks is operatively associated with a device for measuring the particle size of the milled particles leaving the mill. Thus, if the particle size of the milled particles is greater than a desired preset value the axial displacement means can be caused to operate so as to narrow the space between the milling surfaces until the desired particle size is obtained. Conversely, if the measured particle size is too small, the displacement means can be caused to widen the space between the milling surfaces.

Typically, in a mill of the present invention the milling means having the inlet for the millable material further comprises means located adjacent the inlet operatively associated therewith for urging the millable material into the milling region. Generally, the urging means is an impeller or an auger. More generally the urging means is an impeller.

The milling means may be rotated by any suitable means. Typically, the milling means each comprise a shaft which is driven by a motor, for example an air motor or an electric motor. Generally, one shaft is hollow and has an inlet and an outlet, the outlet communicating with the milling region, and the inlet provided for supply of the millable material to the mill. Typically, one milling means is adapted to rotate at from 100% to 0% of the rotation speed of the other milling means; more typically, from 99.9 to 50% of the rotation speed of the other milling means. Even more typically, one milling means is adapted to rotate at from 99.5 to 80% of the rotation speed of the other milling means. Still more typically, one milling means is adapted to rotate at from 97 to 85% of the rotation speed of the other milling means. The speed of rotation of the milling means depends on the nature of the millable material and on the desired particle size, and typically ranges from 500 to 3000 revolutions per minute (rpm). More typically, the rotation speed is from 2000 to 2900 rpm, even more typically from 2600 to 2900 rpm.

In the apparatus, system and process of the third to sixth embodiments, the combustible material is typically coal or coke, more typically clean coal. Even more typically, the combustible material is clean coal in granules of up to about 10 millimetres in size. Generally, in the mill of the present invention, air or other gas is admitted to the mill together with the millable material. The gas supply may be pressurised, for example to a pressure of from about 5 kPa to about 2000 kPa. Typically, the gas supplied in the mill of the first or second embodiments, or the apparatus, system and process of the third to sixth embodiments, is air. When the mill is operated, vanes on the periphery of the first milling means or disk, when present, together with the pressure of the air supply, if applied, produce a pressurised airstream leaving the mill, containing finely divided milled particles of the millable material when the mill is operated. Where present, vanes on the periphery of the first milling means or disk also cause a pressure drop across the milling region which, together with the centrifugal forces acting on the millable material due to the rotation of the milling means, causes milled material to be discharged from the milling region and helps to prevent clogging of the mill.

It is preferred that where electric motors drive the milling means or disks, the motors should be pressurised and sealing means should be provided so as substantially to prevent ingress of milled particles and air into bearings or other moving parts of the motors. When the millable material is coal or similar combustible material, the pressurised airstream containing milled coal particles which leaves the mill can be suitable for supply directly to a namral gas or LPG burner for combustion, for example in an electricity generating plant or steam generating plant. Typically, the pressure of such a pressurised airstream containing finely milled coal particles is from about 690 kPa to about 1720 kPa. In such a case, the stream of coal particles and air leaving the mill is suitable for combustion in conventional burners without any modification to the burners. Thus, such a stream of coal particles may be used as a synthetic fuel as a substitute, or a partial substitute, for natural gas or LPG. It is possible to operate conventional gas-fired plants interchangeably on such a synthetic fuel and a gas fuel, or on a mixture of a gas fuel and a coal particle stream from a mill of the present invention. Suitably, where electric motors are used to drive the mill of the present invention so as to produce a synthetic fuel of finely divided milled coal particles in air, the motors are sealed pressurised motors. The motors may be externally cooled, but it is preferable for the heat generated by the motors in operation to be used to preheat the air supplied to the mill, or to preheat the synthetic fuel produced by the mill before it is supplied to the combustor.

Typically when the mill of the first and second embodiments and the apparatus and system of the third and fourth embodiments are operated, contact of the millable material with the milling surfaces causes the millable material to be rotated in at least two directions. Generally, the mill of the first and second embodiments and the apparatus and

system of the third and fourth embodiments produce milled particles which have substantially no sharp corners or edges. Usually, the milled particles are rounded. More usually, the milled particles are approximately spherical.

Typically, the mill of the present invention produces milled particles having a diameter of up to one millimetre. More typically, the milled particles have a diameter of up to 100 microns, even more typically up to 20 microns and still more typically less than one micron, for example from 0.001 microns to 1 micron or from 0.01 microns to 0.5 microns. Hitherto, it has been difficult or impossible to produce approximately spherical milled particles of these sizes with a mill. Typically, the apparatus, system and process of the third to sixth embodiments produce a combustible fuel containing milled particles of combustible material having a diameter of up to 20 microns, more typically up to 1 micron. Even more typically, the milled particles have diameters in the range 0.005 microns to 1 micron, still more typically from 0.01 microns to 0.5 microns. Particles of these sizes are sufficiently small to be capable of forming suspensions in a pressurised gas such as air.

In a preferred form of the present invention, the mill comprises control means on the gas inlet to the mill which is capable of controlling the ratio of inlet gas to millable material supplied to the mill. One application of such a control means is to maintain the composition of the gas stream containing milled particles which leaves the mill outside of the explosive range when the milled particles are combustible and the inlet gas contains oxygen.

Thus, according to a seventh embodiment of the invention, there is provided a feed composition controller for supplying a predetermined ratio of millable material and gas to a mill of the first or second embodiment, comprising: metering means for the millable material; gas metering means; and means for supplying the metered millable material and the metered gas to the mill, wherein the metering means for the millable material and the gas metering means are operatively associated so as to provide an amount of the millable material and an amount of the gas in a predetermined ratio, to the supply means.

Generally, the gas supplied to the feed composition controller of the seventh embodiment is air.

Generally in the feed composition controller of the seventh embodiment the metering means for the millable material comprises a plurality of spaces of equal volume which are adapted to be successively fiUable by the millable material and then dischargeable into the mill supply means. Similarly, the gas metering means typically comprises a plurality of spaces of equal volume which are adapted to be successively fillable by gas and then dischargeable into the mill supply means. In each metering means the spaces are suitably defined by a plurality of separators. Suitably, the metering means

are adapted for rotation of the spaces from a filling region of the metering means to a discharging region. Typically, in the feed composition controller of the seventh embodiment, the rotation of the spaces is achieved by connection of the assembly of separators and spaces to a rotatable shaft. The predetermined ratio of the millable material to the gas is then determined by adjusting the speed of rotation of one or both of the metering means, or by selecting metering means of appropriate size, or both. More typically, the assembly of separators and spaces of each metering means is connected to a common rotatable shaft, so that each rotates at the same speed. This provides a substantially constant ratio of millable material to air from the feed composition controller, the ratio being determined by the relative sizes of the metering means for the millable material and the gas metering means. Suitably, the rotatable shaft is driven by a motor. Most typically, the speed of the motor is adjustable in accordance with the output rate of the mill which is supplied by the feed composition controller of the seventh embodiment. In this way the mill can be operated so that the rate of supply of millable material to the mill is matched to the rate of production of milled material from the mill.

When the millable material is a combustible material and the gas supplied to the mill contains oxygen, the feed composition controller of the seventh embodiment can be used so as to arrange that a mixmre of gas and finely divided milled particles of combustible material which is produced by the mill is sufficiently deficient in oxygen to prevent spontaneous combustion of the mixmre, thereby providing for safe operating conditions. Suitably, when the combustible material is coal and the gas is air, the mass ratio of air to coal supplied by the feed composition controller of the seventh embodiment is approximately 3:1.

Brief Description Of The Drawings Figure 1 is a partially sectioned side view of a preferred mill in accordance with the present invention.

Figure 2 is a partially sectioned plan view of a housing portion of the mill shown in Figure 1.

Figure 3 is a schematic diagram of a preferred feed composition controller for use in conjunction with the preferred mill shown in Figure 1.

Figure 4 is a schematic representation of a system for combusting a combustible material including a mill of the invention, including an arrangement for adjusting the particle size of milled particles produced by the mill.

Best Mode Of Carrying Out The Invention As seen in figure 1, the mill 1 comprises electric motors 15 and 25 which drive disk assemblies 10 and 20 contained in housings 16 and 26 respectively. Bolts 24 passing through holes in the housings 16 and 26 clamp the housings 16 and 26 together.

The lower disk assembly 10 includes milling segments 11 and 12 which are fixed to a support consisting of a base plate 13 and a shaft 17. Similarly, the upper disk assembly 20 includes milling segments 21 and 22 fixed to a support which consists of a receptacle 23 and a shaft 34. Each of the segments 11, 12 and 22 is silicon carbide with its exposed face shaped by diamond dressing at high speed, and segment 21 is of tungsten carbide or silicon carbide, diamond milled to suit a fine clearance to the segment 11. The segments 11, 12, and 21, 22 are attached to the base plate 13 and receptacle 23, respectively, by high temperature epoxy adhesive. The supports of the upper and lower disk assemblies 20, 10 hold the milling segments 11 and 12 in opposition to milling segments 21 and 22 respectively.

The shaft 34 is in the form of a hollow tube, with the hollow portion tapering from its upper end 50 to its lower end, which connects with a milling region 30 formed between the lower faces of the milling segments 21 and 22 and the upper faces of the milling segments 11 and 12. The milling region 30 has an inlet 31 at its inner end and tapers from the inlet 31 towards its outer end where it forms a narrow outlet 32. The lower end of the hollow centre of the shaft 34 communicates with the inlet 31 of the milling region 30. The end of the shaft 17 of the lower disk assembly 10 projects into the hollow centre of the shaft 34 and terminates in an impeller 52 adjacent the inlet 31 of the milling region 30. Air and millable material is admitted to the upper end 50 of the shaft 34, by a controller (not shown) as schematically shown in Figure 3.

As best seen in figure 2, the lower disk assembly 10 is provided with a number of vanes 35 around its outer periphery. The vanes 35 project into an annular space 36 surrounding the circumference of the disk assemblies 10 and 20. The annular space 36 is in communication with the outlet port 37 of the mill. The outlet 32 of the milling region 30 opens into the annular space 36 and is positioned below the upper edge of each of the vanes 35.

Bearings 45 are provided between the shaft 34 of the upper disk assembly and the housing of the motor 25. An annular seal 39 is provided between the upper surface of the disk assembly 20 and the lower surface of the housing 26, and a further seal 43 is provided to the bearing 45. The lower disk assembly 20 is provided with a bearing carrier 44 which can slide up and down in the housing 16 for adjustment of the width of the milling region 30 between the milling segments 11, 12 and 21, 22. The bearing carrier 44 is provided with a pin 41 and slot (not shown) to prevent its rotation. The bearing carrier 44 is also provided with a bearing 42 which is lubricated with namral graphite. Bearing 42 may be either prepacked or injected. The width of milling region 30 is adjusted by a screw setting (not shown) beneath the lower motor 15. Pressure seals 38 and a labyrinth seal 40 are provided between the bearing carrier 44 and the housing 16, and between the disk assembly 10 and the housing 16, respectively. A nipple 48 is provided for pressurising the assembly of housings 16, 26.

In operation for producing a synthetic fuel, electric motors 15 and 25 are pressurised via nipple 48 and sealed. When operated, they rotate the disk assemblies 10 and 20 in opposite directions at different speeds. The rotation speed of the faster disk is approximately 2900 rpm, and the rotation speed of the other disk is from about 2800 to about 2450 rpm. Clean coal in granules from about 2 millimetres to about 10 millimetres in size is admitted together with air at approximately 690 kPa to the hollow centre of the shaft 34 through its upper end 50. The admitted coal granules fall to the lower end of the shaft 34 and are met by the impeller 52, thereby being driven into the inlet 31 of the milling region 30 between the milling segments 11, 12, 21, 22 of the disk assemblies 10 and 20. The different rotational speeds of the disk assemblies 10, 20 result in different frictional forces being applied to the upper and lower sides of the coal granules, resulting in a net force in one direction or the other on the granules. As a result, the coal granules tend to move towards the periphery of the disk assembly and acquire axial and radial components of velocity. Additionally, through contact with the milling segments 11, 12 and 21, 22 the coal granules are reduced in size and attain a relatively rounded shape. By virtue of the rotation of the disk assembly 10 to which the vanes 35 are attached, the vanes 35 move through the annular space 36, causing a pressure difference between the annular space 36 and the outlet 32 of the milling region 30. This pressure difference, together with the centrifugal forces experienced by the particles in the mill, causes the milled particles of coal to be discharged from the outlet 32 into the annular space 36, and helps to prevent clogging of the mill. A milled particle discharged from the outlet 32 passes into one of the spaces defined between two adjacent vanes 35 and is swept around the annular space 36. The mixmre of milled coal particles suspended in air, at a pressure of about 830 kPa, leaves the mill via the outlet port 37. Figure 3 is a schematic diagram of a feed composition controller suitable for use in conjunction with the preferred mill shown in Figure 1. As seen in Figure 3, the feed composition controller 100 consists of a solids volume control device 117 and a gas volume control device 124. Devices 117 and 124 are of similar construction, which will be described with reference to the solids volume control device 117. The solids volume control device 117 is of the rotating- vane type and consists of a number of equally spaced vanes 121 which project radially from a shaft 120 and which are contained in a housing

(not shown) which is slightly wider than the width of the vanes 121 and the shaft 120 to which they are attached. The shaft 120 passes through the walls 129 of a throat 122 which may be connected to the main inlet 50 of the mill shown in Figure 1. Seals 118, 119 are provided to prevent loss of gas or millable material from the solids volume control device. The housing of the volume control device 117 has a mouth 116 which opens into the base of a bin 110, and diametrically opposed to the mouth 116 is a second opening into the throat 122. The gas volume control device 124 is of a generally similar construction, but is built to generally closer tolerances, so that gas is precisely metered by

the device and escape of gas between the ends of the vanes and the inner surface of the housing (not illustrated) is minimised.

The solids volume control device 117 is provided with granular particles of millable material from bin 110, which is filled with the millable material by the solids feed pipe 111 using a conventional double valve control 112. A gas inlet 113 and a gas outlet 114 are provided to the bin 110. The gas outlet 114 connects with a gas pipe 123 via a filter 115 to prevent entrained particles of millable material from being carried out of the bin 110 thereby bypassing the solids volume control device 117. The other end of the gas pipe 123 passes through the wall 129 of the throat 122 of the feed composition controller 100.

The gas volume control device 124 is provided with a supply of gas from the gas inlet 125, and is connected to the gas inlet 113 to the bin 110 by means of a gas pipe 126. The shaft 120 drives the gas volume control device 124 as well as the solids volume control device 117, and is caused to rotate by motor 130. Gas loss from the housing of the gas volume control device 124 around the shaft 120 is prevented by the seals 127, 128.

In operation for the supply of a controlled mixmre of pressurised air and coal to the mill shown in Figure 1, clean coal of granular size from about 2mm to about 10mm is charged into the bin 110 via the double valve control 112 and die solid feed pipe 111. The bin 110 is pressurised to about 690 kPa by the air supplied via gas inlet 113. The motor 130 rotates the shaft 120 so that successive spaces between adjacent vanes 121 pass the mouth 116 of the housing of the solids volume control device 117 and fill with coal granules and air. Further rotation of the shaft 120 causes the successive filled spaces containing coal granules to move from their filling position to an emptying position where the housing of the solids volume control device 117 opens into the throat 122 of the controller 100. Still further rotation causes the emptied spaces to return to mouth 116 position for refilling.

Simultaneously, the rotation of the shaft 120 causes the gas volume control device 124 to rotate, sweeping pressurised air supplied from the gas inlet 125 in a controlled quantity which is proportional to the quantity of coal discharged from the solids volume control device 117, via gas pipe 126 to the bin 110. From the bin 110, gas pipe 123 equalises the pressure either side of the solids volume control device 117, so that the volume of air swept by the gas volume control device 124 enters the throat 122 of controller together with the measured amount of coal granules. Thence the coal granules and the air are supplied to the mill shown in Figure 1. The solids volume control device 117 and the gas volume control device 124 are sized so that a desired ratio of air to coal granules enters the mill in order that the stream of finely milled coal particles and air exiting the mill is not in the explosive range.

Figure 4 is a schematic representation of a system for combusting a combustible material including a mill of the first embodiment, including an arrangement for adjusting the particle size of milled particles produced by the mill. The mill assembly is represented by upper and lower disks 405, 406 rotatable respectively by motors 410, 411 via shafts 417, 418. The disks are housed in housing 408. Upper disk motor 410 is equipped with an inlet 415 which communicates through motor 410 and shaft 417 with an opening (not shown, but corresponding with shaft 34 seen in Figure 1) passing through disk 405 to enable combustible material and gas to be provided to milling region 425. Housing 408 is equipped with outlet 426 which communicates via conduit 427 with particle sampler 420 and thence via conduit 435 to burner 430. Conduit 438 connects particle sampler 420 with particle size measuring device 440 which provides particle size information to comparator 450 via electrical connection 445. Comparator 450 has an electrical input signal 452 which is adjustable manually and has an associated dial 453, and is connected by electrical connection 455 to servo motor 460. Servo motor 460 is connected by a drive or drives 465, 466 (shown dotted in Figure 4) to at least one of screw adjusters 475, 476.

In operation, air and milled particles produced by the mill represented in Figure 4 emerge from opening 426 in housing 408 and pass through conduit 427 into particle sampler 420. A small proportion, typically less than 0.1 % by weight, of the particles emerging from the mill are sampled by particle sampler 420 and the remainder pass through conduit 435 into burner 430 where the combustible material is burned. The sampled particles pass from sampler 420 through conduit 438 into particle size measuring device 440 which continuously measures the distribution of particle sizes in the sample. A signal representing the average particle size of the milled particles in the sample is generated by device 440 and conducted via wire 445 to comparator 450 where it is compared to the preset input signal 452. Input signal 452 is adjustable, a number corresponding to input signal 452 being displayed on associated dial 453. A signal representing the difference between the two signals provided to comparator 450 conducted by wire 455 to servo motor 460. Depending on whether servo motor 460 adjusts the position of one or both of disks 405, 406, the signal provided to servo motor 460 causes one or both of drives 465, 466 to rotate, and thereby rotate one or both screw adjusters 475, 476 which move one or both disks 405, 406 so as to change the width of milling region 425. When the particles emerging from the mill are larger than a preset or desired value, the signal from particle size measuring device 440 to comparator 450 is larger than the preset input signal 452. The resulting signal provided from comparator 450 to servo motor 460 causes drive or drives 465, 466 to rotate one or both of screw adjusters 475, 476 so as to narrow the width of milling region 425. This process continues until the size of the milled particles entering particle sampler 420 is equal to the preset or desired value.

The desired setting of the input signal may be determined by calibrating the adjustment of the input setting as follows. Conduit 438 is temporarily disconnected from particle sampler 420 and particles of the milled material sampled by sampler 420 are collected manually. The particle size of the sampled particles thus collected, and the reading on dial 453 are noted and plotted on a graph of particle size against dial reading. The setting of input signal 452 is the changed by adjusting dial 453, and the above procedure is repeated. In this way, a graph relating the reading on dial 453 to the particle size of milled material acmally produced by the mill at each setting may be obtained, from which the necessary dial reading for any desired particle size may be inferred.