BACKGROUND TO THE INVENTION

THIS invention relates to the combustion of solid fuel and is concerned with an efficient method of burning such fuel and a burner for burning such fuel efficiently. The solid fuel may be coal, coke, anthracite, wood, peat, or other solid fuel in lump or similar form.

At present for burning solid fuel "underfed" burners are used. Such burners have a floor grate through which solid fuel is supplied to a burning zone either by a screw feeder or a chain feeder. This type of burner has a number of defects, for example, the grate tends to burn away in use, combustion is generally poor, and the burner must be cleaned frequently which cleaning involves extremely uncomfortable work. It is also known to burn pulverised solid fuel such as coal powder, in a rotary burner. One problem with such burners is that they involve an exacting technique requiring finely controlled conditions. Also starting up is difficult and requires liquid or gaseous fuel to bring the burner up to the required temperature for igniting the powder and such fluid fuel is also required to maintain combustion. Moreover the installation must be large and small burners cannot be constructed on this principle. Also the heat produced is not easily controllable over a wide range. The burner is expensive per se and further expense is also incurred in providing machinery for pulverising the coal to powder and large capacity air compressors are required for producing air for combustion. Finally it may be mentioned that the exhaust is "dirty" insofar as it contains considerable fine ash so that this type of installation is not suitable for direct air heating and apparatus, such as scrubbers, are required for cleaning the exhaust.

This invention seeks to provide a burner for solid fuel which will be efficient, simple and reliable in construction, and easily controllable with regard to the amount of heat produced. It is also preferred that the burner shall be self-cleaning.

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided a method of burning a solid fuel including the steps of feeding a solid fuel to a cylindrical burner, arranging the burner so that as the burner 4s rotated the fuel moves along the length of the burner,rotating the burner to cause the fuel to form a mass that tumbles over itself, and feeding combustion air to the fuel through the walls of the burner at desired locations at the base of the mass of fuel.

Preferably the principal axis of the burner is inclined, so that fuel fed into one end of the burner moves along the length of the burner as the burner rotates, and the residue remaining after the fuel has been burnt is automatically discharged from the other end of the burner. The angle of inclination will be very small, and may be between 0,5 and 5 degrees.

The quantity of heat generated by the burner can be controlled by varying the volume of fuel fed to the burner, or by varying the rotational speed of the burner, or by varying the volume of combustion air supplied to the burner or by varying the angle of inclination.

According to another aspect of the invention there is provided a burner for solid fuel comprising: a cylindrical outer casing; a cylindrical inner casing forming an annular space with the outer casing; generally longitudinally extending elements arranged in the annular space between the inner and outer casings and dividing the annular space into a plurality of longitudinally extending compartments; a plurality of ports extending through the inner casing at longitudinally and circumferentially spaced positions thereof to connect each compartment with the interior of the burner, each port being inclined with respect to a radius of the cylindrical burner casings and the ports all being inclined in the same sense; means supporting the burner for rotation about an axis; means to rotate the burner in a direction such that the radially innermost end of each port trails the radially outermost end thereof; controllable means for selectively admitting combustion air to each of the compartments at a desired location; and means for feeding solid fuel particles or lumps into the burner, the burner being arranged so that the fuel fed in travels along the length of the burner as the burner is rotated.

The principal axis of the burner preferably slopes downwardly from the inlet for solid fuel to the opposite end of the burner, so that as the burner rotates fuel progresses from one end of the burner to the other and clinker or ash is discharged automatically from the lower end of the burner. The degree of slope of the axis of the burner will depend on the type of fuel used, the rotational speed of the burner, the volume of combustion air fed to the burner and so on, but generally the slope will be of the order of 0,5 to 5 degrees,preferably 1 or 2 degrees. In certain cases it may be possible for the burner axis to be horizontal, if the fuel in the burner forms a pile of continuously decreasing height towards the outlet. This could have the same effect as inclining the burner axis.

Further aspects of the invention are that hot air, e.g. for drying purposes, can be forced axially through or be drawn axially through the burner; or that a heat exchanger can be provided inside or outside the burner, the heat exchanger being an air-to-air type if "clean" hot air is required or an air-to-liquid type if hot liquid is required, e.g. if the burner is used as a boiler. The burner may be supported on an axial shaft or be provided with external riding rings which rotate on rollers.

Further features and advantages of the invention will become apparent from the following description of an example of the invention made with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

• Figure 1 shows a schematic longitudinal section through an example of a burner of the invention;
• Figure 2 shows a cross-section through the burner of Figure 1 on the line II-II; and
• Figure 3 shows a cross-section through part of the burner of Figure 1 on the line III-III.

DESCRIPTION OF A PREFERRED EMBODIMENT

The burner shown in the drawings comprises an outer cylindrical shell 10 and a radially inner cylindrical shell 12 spaced from the outer shell 10 and secured thereto by partition wall members 14 extending longitudinally and parallel to the principal axis of the cylinder. The partition walls 14 divide the annular space between the two shells into a plurality of separate compartments 16. A lining 18 of a suitable refractory material is applied to the inner wall of the inner shell 12. Ports or tuyeres 20 connect each compartment 16 at a number of places along its length to the interior of the burner. As shown in Figure 2 each port 20 is inclined with respect to a radius or a normal of the burner and with respect to the direction of rotation R so the innermost end of each port 20 trails behind the radially outer-most end of the port; this is to ensure that during rotation solid fuel lumps or particles in the burner do not move from the interior of the burner to the compartments 16. Each port 20 is formed by casting a suitable metal tube, such as rectangular section tubing, into the refractory lining and connecting the interior of each tube with one of the compartments 16. The outer shell 10 is fitted with riding rings 22 so that the burner can be supported on rollers, not shown. While shown exaggerated in the drawings, the axis of the cylinder slopes at about 1°. Each end of the burner is in sliding, sealed contact with an end plate 24 or 26. The end plate 24 is at the discharge end of the burner and is formed with an opening 25 through which clinker and ash may be discharged. The plate 26 is at the inlet end of the burner and is fitted with a coal feed 28, which may be of any suitable type such as a chain feeder, screw feeder and the like.

Air is introduced into the compartmentsl6 from an annular manifold 30. The manifold itself is fed with air from a blower 32. The manifold is divided into two sections 30a and 30b divided by walls 31, one of which includes a butterfly valve 33. The manifold section 30a communicates with compartments 16 feeding ports 20 which open under the burning mass of coal (compare Figures 2 and 3). If the valve 33 is closed, there is no air outlet from the other compartments 16, and flames will therefore be prevented from passing along the other ports 20. Under certain conditions it may however be desirable to pass air through all the ports 20, and the valve 33 can be opened to allow air to bleed from section 30a to section 30b and into the central space of the burner through compartments 16 and ports 20.

In use, the burner operates as follows.

The burner is started up by feeding a small quantity of the solid fuel to be burnt, through the feed 28. This fuel is ignited using a suitable torch or lance. The fan is switched on.When the fuel is burning sufficiently a motor, not shown, is actuated to cause the burner to rotate. The burning coal forms a mass which tends to move up one side of the inner refractory lining 20 as shown in Figure 2. At the same time air for combustion is forced through the compartments 16 and ports 18 to emerge below and pass through the burning mass of fuel. Further fuel is now added to the burner either continuously or intermittently until the mass of burning fuel extends along the entire length of the burner. Naturally, near the inlet the fuel will be largely unburnt, while near the outlet the fuel will have been completely burnt. As the burner continues to rotate and more fuel is fed in through the feed 28 the fuel progresses along the burner until it is discharged through the opening 25. The rate of burning of the fuel is controlled by selectively controlling the blower 32 to control the volume of air through each of the inlets 30 and by opening or closing the valve 33. The rate of progression of the fuel along the burner is controlled by the slope of the burner and the rotational speed. The volume of heat generated can be controlled by varying the amount of fuel fed to the burner and by varying the volume of combustion air.

As will be appreciated the burner of the invention will find many applications. Where it is required to produce air for drying, e.g. crops, where a slight amount of ash contamination is acceptable, then air is simply admitted through openings 34 in the plate 26. This air may be forced through the opening 34 or may be drawn through the burner through an outlet 38. If clean air is required, then this is simply achieved by using an air-to-air heat exchanger inside or outside the burner. Where hot liquid is required, e.g. if the burner is used as a boiler or water heater, then an air-to-liquid heat exchanger is used.

Some of the features of the burner of the invention are now enumerated. Firstly the burner is exceptionally efficient, because the fuel mass is slowly rolled or turned over while burning in the presence of a localised stream of air. Operation of the burner is continuous, convenient, and automatically self-cleaning. There is efficient heat transfei to any desired medium because there is very little loss of heat through the refractory lining 18. Control of the heat generated and temperature inside the burner is easily accomplished. The burner will burn any size or type of solid fuel or waste material so that it will be highly economical to use and it will be able to use any available fuel in almost any form. The port system may confine the supply of combustion air to the area where due to rotation, the mass of fuel is concentrated. As the furnace tube revolves, compartments 16 communicate with the combustion zone and are in turn supplied with air from the manifold section 30a. The refractory lining operates under favourable conditions in that no particular region of the lining is continually in contact with the burning fuel. The refractory lining is a poor conductor of heat so that there is minimal heat loss. Also heat passing through the lining warms the incoming combustion air, thus improving combustion efficiency. The burner can be rotated very slowly and so will have a long life in that abrasion of the refractory lining will be minimal. Power consumption for operating the burner is minimal. Wear and maintenance are minimal. Fire bars and chain grates as used conventionally, are eliminated. The unit is simple in principle and operation, easy to understand, and compact in relation to the quantity of heat produced. If a heat exchanger is used then this will be well placed for efficient heat pick-up both from the burning fuel and from the refractory lining.

While a preferred embodiment of the invention has been illustrated and described, it will be appreciated that many changes can be made in the structure and structural interrelationships shown without departing from the principles of the present invention. In this respect, for example, the partition walls need not necessarily be parallel to the longitudinal axis but may be slightly helical. Also any suitable type of feeder system for the coal may be used, e.g. a feeder belt, a vibratory feeder, a rotary valve, a screw feeder, etc. and such feeders may be independent or rotating with the machine and may be either manually or automatically controlled. The burner may rotate on a central main shaft or stub axles connected to the end plates 24 and 26 instead of the riding rings 22 as shown. Also as shown in dashed lines in Figure 1 a breech or outlet fitting 40 may be provided at the discharge end of the burner. The breech comprises a substantially annular housing which rides on bearings or bearing shoes 42 on the outer casing 10 and has a clinker outlet 44. The use of such a breech may be preferred rather than using a fixed end plate 24 in which event axial expansion and contraction of the burner might cause sealing problems. Moreover the use of such a breech, when the burner is used for supplying hot air to a drier, assists in connecting the drier and provides a convenient mounting for a feed chute 46 to the drier.