WO2004063315A1

WO2004063315A1 - Handling of coal fines

Info

Publication number: WO2004063315A1
Application number: PCT/ZA2004/000002
Authority: WO
Inventors: Jacobus Petrus Smit
Original assignee: Brickett Technologies Limited
Priority date: 2003-01-09
Filing date: 2004-01-07
Publication date: 2004-07-29

Abstract

A method of handling coal fines which includes the steps of forming the fines into a plurality of particles of a given size, placing the particles into a container and sealing the container.

Description

HANDLING OF COAL FINES

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to the handling and processing of coal fines.

[0002] Coal fines, generated at coal mines and coal plants for diverse reasons, represent a substantial source of energy which cannot easily be tapped because of problems which are associated with handling and burning of the material.

[0003] It is possible to bum coal fines in a specially adapted boiler or furnace but such devices are expensive for small applications and not necessarily in widespread use.

[0004] Coal fines cannot be used in conventional industrial chain grate boilers, or furnaces which are the industry norm for burning coal. The coal fines, in suspension, can be explosive and are blown out of the boiler system if introduced directly into chain grate boilers or furnaces. In an attempt to overcome this problem it is known to make use of a binder which is mixed with the fines or dust and to compress the mixture into briquettes. The briquettes can be handled more readily and when introduced into a fire box of a boiler, burn satisfactorily. There are however at least two problems associated with the use of a briquette of this kind.

[0005] In the first instance the coal fines are hygroscopic and absorb moisture from the atmosphere or when they come into contact with water. When this occurs the briquette disintegrates and the briquette crumbles and reverts to coal fines.

Secondly, the cost of the briquette is substantially increased.by the addition of binder to overcome the problem of water absorption and mechanical handling. Sufficient Dinαer must De added to the briquette to impart to the briquette adequate mechanical strength to enable it to withstand the effects of loading into vehicles, transport, discharge, stock piles and the like. Generally the strength of the briquette increases as the quantity of binder increases and it is found that, for practical reasons of the kind mentioned, the percentage binder in a briquette must be significant to enable the briquette to possess adequate mechanical strength properties. The inclusion of an adequate quantity of binder, however, materially increases the price of a briquette and this can have the result that the briquette is no longer cost-competitive compared to conventional coal.

[0006] Huge stacks of coal fines exist throughout the coal mining areas in most countries. These stacks have significant environmental effects and, at the very least, must be covered and rehabilitated to guard against pollution of air, water and soil by the coal dust.

SUMMARY OF INVENTION

[0007] The invention provides a method of handling coal fines which includes the steps of forming the fines into a plurality of particles of a given size, placing the particles into a container and sealing the container.

[0008] The fines may be formed into the particles using any appropriate technique. This technique may involve the application of pressure in a compacting process so that the fines are formed into particles of the required size. Pressure alone may be used at this stage but it may be necessary, according to requirement, to make use of one or more binders. [0009] The binder may be selected from starch, molasses, lime, cement, suitable fibres or any other appropriate material.

[0010] The quantity of binder which is used is important. Sufficient binder should be used to ensure that the particles have adequate strength to withstand subsequent mechanical handling, before use, but on the other hand the quantity of binder which is used should not be so high that the cost associated therewith makes the use of the binder prohibitive.

[0011] A binder which .holds significant potential in this regard is the pith or the spongy core of sugarcane, a by-product of paper mills and sugar mills. This material is fibre-like and possesses substantial strength. It has a lower calorific value than coal fines but its volatile material content may be of the order of 73% compared to a figure of 23% for coal fines. This means that the pith ignites quicker than coal when it is introduced into a boiler. The pith also improves the structural strength of the briquette for it acts as a reinforcing material

[0012] Use may be made of briquetting, extruding, pelletizing or similar techniques to form the fines into the particles.

[0013] The particles may be of any suitable size fraction and this, in turn, may dictate the nature of the process used for their formation. It has been found that a suitable size of particle is from 8mm to 28mm.

[0014] The particles may be placed into a container of any suitable type and preferably the container is in the nature of a flexible bag. The bag may be water impervious or it may include a water impervious liner. [0015] The method may include the step of sealing the bag.

[0016] Preferably the bag has an upper inlet through which the particles are placed into the bag and a lower outlet from which particles may be discharged, when necessary, from the bag. The inlet and the outlet may be closed in any appropriate way to seal the bag and its contents. If desired the bag can be vacuum-sealed.

[0017] If the bag includes a liner then the inlet and the outlet may be closed by heat-sealing the liner. It is also possible, however, to close the inlet and the outlet by means of any appropriate closure devices such as flexible ties.

[0018] The bag may be adapted to be handled by mechanical means. To this end the bag may include one or more handles or formations to facilitate lifting and handling of the bag.

[0019] Although there is no limitation on the size of the bag a suitable size which inter alia represents a compromise between cost and handling ability is a bag with a volume of the order of 1 m³. A bag of this type can hold approximately 1 ton of the particles.

[0020] The use of the bag means that the particles are not directly subjected to adverse mechanical handling effects nor exposure to moisture, which otherwise would result if the particles were not enclosed in a bag. Thus it is possible to reduce or even do away with the binder which is normally employed in a coal fines briquetting process. As the bag is sealed the ingress of moisture into the particles or briquettes is substantially eliminated. The briquettes therefore maintain their integrity until the point of use. [0021J it Tails within the scope of the invention to coat the particles or briquettes with a suitable material so that, upon burning, the particles do not immediately disintegrate. In most chain grate boilers and furnaces, the coal particles are brought to the fire box using a steel conveyor. The particles ignite before being discharged from the conveyor and as they burn they turn into ash. If the burning process has not been completed there is a tendency, brought about inter alia by the vibratory forces of the conveyor, for the semi-ash material, before the burning process has been completed, to fall through gaps in the conveyor. By coating the particles with a suitable material such as styrogel-t this tendency can be resisted to a significant extent which means that a more thorough burning process takes place.

[0022] The invention also extends to a combustible product which includes a flexible container with a sealed, upper inlet and a sealed, lower outlet, and a plurality of particles inside the container, each particle being formed from compacted coal fines.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The invention is further described by way of example with reference to the accompanying drawings in which:

Figure 1 illustrates a container from the side and in cross section which is used in the method of the invention, Figure 2 is flowchart of various steps carried out in the method of the invention; and

Figure 3 illustrates the manner in which briquettes of coal fines may be used in a chain grate boiler or furnace . DESCRIPTION OF PREFERRED EMBODIMENT

[0024] Figure 1 of the accompanying drawings illustrates a container 10 which is used in the method of the invention.

[0025] The container includes an outer bag 12 which is made from a suitable material such as woven polypropylene and which has a fitted waterproof plastics liner 14.

[0026] The bag 12 is roughly cubic in shape and may be of the order of 1 x 1 x 1 meter. This gives the bag a volume of 1 m³. These values are merely illustrative and are by no means limiting.

[0027] At each of its upper four corners the bag has respective slings or loops 16 which are sized so that forks of a fork lift truck or any other appropriate lifting device, not shown, can be engaged therewith. The loops are sufficiently strong so that the bag and its contents can be lifted by the loops.

[0028] At a central location on an upper surface 18 the bag has a filling opening 20 which can be closed, when required, in any appropriate way, for example by using a suitable flexible tie 22. Similarly, at a lower side 24 of the bag a discharge opening 26 is provided. This opening can be closed using appropriate means, for example by using a flexible tie 30.

[0029] Figure 2 illustrates various steps in the method of the invention. Coal dust 34 from any suitable source, eg. from a waste stack at a coal mine, is dried, using conventional techniques, in a step 36. The dried coal dust is then placed into a suitable bulk container 38 and transported, by road or rail (step 40), to a central plant at which the particles are formed (step 42).

[0030] The particles may be formed into briquettes or pellets of a given size fraction, eg. in the size range of 8mm to 20mm. Again these values, although preferred, are given only by way of non-limiting examples. Any suitable technique can be employed for forming the briquettes but generally it can be said that the briquettes are formed by the application of pressure which moulds the coal fines into particles of a suitable size. Pelletizing or extrusion techniques can also be used to form the particles.

[0031] Under certain conditions the particles can be formed by the application of pressure alone. In most instances however it will be necessary to make use of a binder which helps to maintain the integrity of the particles. The binder may be of any kind known in the art and for example may comprise molasses, cement, starch or lime. As indicated hereinbefore an appropriate binder is the sponge of sugarcane. This material is fibre-like and has a significant volatile material content which assists in combustion and ignition of the particles at the ultimate point of use.

[0032] Once the particles or briquettes are formed they are placed into a plurality of containers (step 44). Each container is preferably of the kind shown in Figure 1. The briquettes, designated 46 in Figure 1 , are directed to a hopper or funnel 48 which is positioned to discharge into the filling opening 20 of the container. The discharge opening 26 is closed. Once the bag has been filled the filling opening is closed by means of the tie 22. It is pointed out that the use of the ties 22 and 30 is optional for the liner 14 could be heat sealed and, if necessary, air could be extracted from the liner to provide a vacuum seal for the briquettes 50 inside the container.

[0033] The container 10 is of a size which lends itself to being mechanically handled. It is desirable in this respect to make use of generally available equipment such as fork lifters or the like. The mass of the bag and the briquettes 50 is not unreasonably high, for example of the order of 1 ton.

[0034] The containers with their contents can be transported by rail or road (step 56 in Figure 2) to various points of use (step 58).

[0035] Figure 3 illustrates somewhat schematically the manner in which a container of the briquettes is charged into a fire box 60 of a boiler. The container 10 is elevated, using a mechanical device, so that the discharge opening 26 is positioned above a funnel 62. The opening is opened and the briquettes 50 fall under gravity action into the funnel which directs the briquettes onto a sloping chute 64. The briquettes slide down the chute, under gravity action, onto a chain or steel conveyor 66 which extends through a charging opening 68 into the fire box 60. The speed of the conveyor is controlled, using techniques which are known in the art, to feed the briquettes (designated 70) at a desired rate into the fire box. The briquettes ignite and burn on the conveyor and ash 72 produced thereby is discharged into a bin 74.

[0036] An important aspect of the invention lies in the protection which is given to the briquettes 50 by the container 10. As the briquettes are containerised they must, of necessity, be handled in modular form by mechanised means. This means that the individual briquettes are not subjected to shocks of the same severity as what would be encountered if the briquettes were loose ie. not packaged. The quantity of binder which is used in the briquettes can therefore be reduced significantly and in certain instances no binder at all will be required. The reduction in the binder quantity does weaken the briquettes but, as stated, this is more than compensated for by the protection which is afforded by the bag 12.

[0037] As the bag is sealed the possibility of the briquettes absorbing water, and then disintegrating or crumbling, is practically eliminated. This factor also means that the binder requirement is reduced.

[0038] To ensure that the briquettes 70 are fully combusted while on the conveyor and to prevent the semi-combusted briquettes from falling through gaps in the conveyor, the individual briquettes may be coated after the step 44 with an appropriate material such as styrogel-t which substantially preserves the integrity of the briquettes until they are fully combusted.

[0039] The sequence of steps shown in Figure 2 is illustrative and is not limiting. If the coal source 34 is significant and close to the point of use then the coal dust, after being dried in the step 36, can immediately be formed into particles or briquettes in the step 42 with the steps 38 and 40 being dispensed with.

Claims

1. A method of handling coal fines which includes the steps of forming the fines into a plurality of particles of a given size, placing the particles into a container and sealing the container.

2. A method according to claim 1 wherein pressure is applied to the fines in a compacting process so that the fines are formed into particles of the given size.

3. A method according to claim 1 or 2 wherein a binder is used in the formation of the particles.

4. A method according to claim 3 wherein the binder is selected from starch, molasses, lime, cement and suitable fibres.

5. A method according to claim 3 or 4 wherein the binder is the spongy core of sugarcane.

6. A method according to any one of claims 1 to 5 wherein the particles have a size of from 8mm to 28mm.

7. A method according to any one of claims 1 to 6 wherein the container is a flexible bag.

8. A method according to any one of claims 1 to 7 which includes the step of discharging the particles from an outlet in the container for combustion in a boiler or furnace.

9. A combustible product which includes a flexible container with a sealed, upper inlet and a sealed, , lower outlet, and a plurality of particles inside the container, each particle being formed from compacted coal fines.

10. A product according to claim 9 wherein each particle includes a binder selected from starch, molasses, lime, cement, suitable fibres and the spongy core of sugarcane.