GB2196643A - Improvements in processes for producing agglomerated solid fuel briquettes - Google Patents

Abstract

A process is described for producing solid fuel briquettes in which green briquettes of agglomerated combustible particulate material and a binding agent are partially or completely cured in a vibrating fluidised dryer by a fast air flow (acting as said fluid) through the bed at elevated temperature. Thus the air flow is maintained sufficiently high that air feed temperatures of between 140 DEG C and 320 DEG C an be used without the briquettes self heating.

Description

SPECIFICATION Improvements in processes for producing agglomerated solid fuel briquettes The present invention relates to an improved process for the curing of combustible solid fuel materials in the form of shaped briquettes or ovoids comprising compressed agglomerated fuel particles held together by a suitable binding agent, and in particular to smokeless solid fuel briquettes having a low volatile fuel content. The invention is thus mainly concerned with solid fuel briquettes manufactured from low volatile coal particles, preferably anthracite duff. However, the process is not limited to the production of smokeless briquettes, nor to the use of coal particles or the like as a starting material.

One known process for curing briquettes uses conveyor ovens. These are relatively large units, for example a 10 tonne per hour may be 6 to 8 ft. wide and 100 to 150 ft. long. The total processing time may be 1 to 2 hours resulting in a fuel briquette hold up i.e. the amount on the conveyor at any one time, of 10 to 20 tonnes, this presents a considerable fire risk particularly as these units show a tendency to catch fire. Conveyor ovens are expensive to buy, install and operate and are also expensive to repair after fires.

In Patent Application No. 26667/85 there ds described a process for producing solid fuel briquettes comprising agglomerated combustible particulate material and a binding medium wherein the process includes the steps of compressing a mix of the particulate material and the binding medium to form low strength "green" briquettes, and of subjecting the green briquettes to hardening and partial or complete curing treatment with fluid at elevated temperature in a fluidised bed dryer. Preferably the temperature is in the range of about 1200C to about 220"C, most preferably 140"C, and the dryer is preferably a horizontal vibrating fluidised bed dryer. Thepreferred mean air flow rate described in that Patent Application is 148 cu.ft./min. per square ft.

of fluidised bed deck.

The present invention is based on the discovery that by using a vibrating fluidised bed and faster air flows than hitherto proposed, very fast rates of curing can be obtained, this results in much smaller units (e.g. two units each 4 ft. by 22 ft. for 10 tonnes per hour) residence times of 10 to 15 minutes and hold up masses (i.e. the masses on the fluidised bed decks at any one time) of 1.5 to 2.5 tonnes.

The fast air flow rate also results in a reduced tendency for the briquettes to catch fire as will be explained hereinafter. This together with the smaller hold up of briquettes makes the units much safer with respect to the outbreak of fires.

Whilst the briquettes can be completely cured in a vibrating fluidised bed, in further development of the invention, the process can also be carried out in two stages whereby the briquettes are heated, dried and partially cured in the vibrating fluidised bed and then held at an elevated temperature in a simple holding device, e.g. a hopper.

Partially cured briquettes will slowly oxidise and self heat from quite low temperatures, sometimes as low as 50"C, provided the air flow through the system is relatively low. This effect can be exploited in the second stage to raise the temperature of the briquettes to any desired level (temperatures in excess of 800"C are readily achievable) to produce briquettes free of volatile matter (for low smoke emission during burning) and in which the solid components are partially fused to give a strong briquette.

Background of the invention During curing there are two overlapping processes or zones:- 1. Heating and drying 2. Chemical curing - (polymerisation or cross linking).

Heating and drying requires heat-energy input which is expensive in terms of equipment and fuel input. Curing only requires the maintenance of an acceptabie or useful temperature, this is associated with much lower equipment and fuel costs. The equipment costs can be greatly reduced if heat can be transferred into the briquettes rapidly and safely. It is known to those in the briquetting manufacturing field that it is difficult with existing processes to heat and dry the briquettes rapidly without running the risk of setting them on fire.

The present invention enables the heat to be introduced more rapidly and greatly reduces the heating and drying time, whilst at the same time, increasing the safety of the system, i.e.

decreases the risk of fires within the equipment.

Briquettes can catch fire during the curing process the fundamental reason being that partially cured briquettes can and do oxidise and liberate heat at relatively low temperatures, certainly as low as 50"C.

However, in accordance with the invention briquettes can be quite safely processed on the vibrating fluidised bed at temperatures in the 170"C to 300"C range without igniting but will self heat and ignite from temperatures as low as 60"C if held in a low air flow system. Indeed, briquettes that have quite safely been processed and dried at 180/200 C and then cooled to 60"C, will, if packed in a confined volume with limited and low air flow, oxidise and generate heat and reach temperature in excess of 700"C in less than 12 hours.

Thus briquettes heated in a fast air flow in accordance with the invention do not self heat and ignite, whilst the same briquettes readily ignite in a slow air flow.

The mechanism is based, as demonstrated in the graph of Fig. 1, on the fact that heat is lost from a self heating system at a rate that is a linear function of temperature (curve A), whilst self heating being a chemical reaction is an exponential function of temperature (curve B).

If the process is operated at temperatures below point C more heat will be removed than is generated and the system will not ignite (in the conventional sense). The slope of A depends on the rate of air flow. At low air flow (curve D), the rate of heat removal is low and the point at which the system can self heat and run away, point E, is much lower. Thus, in one trial, point C was above 200"C but point E was below 60"C. By operating high flow rates the safe upper working temperature of the system was greatly increased, particularly during the second curing zone when the briquettes were hot and dry.

The high air flow rates used in accordance with the invention increase the rate of heat transfer to the briquettes and helps to reduce the total processing time as will now be explained by comparison with the prior art.

In known very large conveyor ovens, the air flow rates are low and this results in heat transfer coefficients of about 3 BTUs/ft2/hour/ F or often much less, whereas in the vibrating fluidised bed, heat transfer coefficients of 6 BTUs/ft2/hour/ F or more are often achieved.

These high air flows provide a further advantage and a disadvantage that can be counteracted.

In a low flow system the air gives up a large proportion of its heat to the briquettes as it passes through them. Thus, in one prior art system, air is fed in at 200"C and leaves at 80"C resulting in a mean temperature (in a typical cross flow system) of 140"C. In this system, it was necessary to heat the briquettes to 130"C, the net mean margin to transfer the heat (relatively) was then only 10"C. Whilst this temperature difference is adequate, it results in a low rate of heat transfer and a long residence time, typically 90 minutes.

In the system of the present invention, air can be advantageously fed in at 240"C and only gives up a small proportion of its heat and emerges at typically 160"C. This results in a mean temperature of 200"C and a net mean margin over the briquettes of 70"C. (for a briquette temperature of 130"C).

As heat transfer is more or less directly proportional to the heat transfer coefficient and the temperature difference, it can be seen that the relative times for the curing stage for the two examples quoted above are, typically 90 minutes for the old low flow system and 3 10 90 ) < -)c 6.4 minutes for the high flow system 6 70 These fast curing times have been confirmed in practice where curing times of 3 to 30 minutes have been achieved.

In the high air flow system, disadvantageously the air only gives up a small proportion of its heat and if allowed to escape from the system the thermal efficiency would be very low. Thus in the low flow case, the thermal efficiency was:200 - 80 i.e. 60% 200 In the fast flow:- 240 - 160 i.e. 33 /0 240 This loss of efficiency can be avoided by taking off the gases, reheating them and recycling them to the vibrating fluidised bed. This allows efficiencies as high or even higher than the low air flow system to be obtained.

Alternatively, the air can be passed through a number of fluidised beds in a counter current flow to the briquettes, this can result in very high thermal efficiency, or a combination of the two techniques can be used.

Thus, the high air flow system in accordance with the invention results in: 1. High heat transfer coefficient.

2. High temperature difference between air and briquettes.

3. Much wider temperature range for safe working without self ignition.

4. Much shorter curing time.

This results in small low cost units in which the hold up of briquettes can be reduced from 15 to 20 tonnes to 1 to 3 tonnes. This smaller inventory further reduces both the chance of fires and the scale of any fire and indeed makes the extinguishing of fires a very simple and inconsequential affair. Fires in conveyor ovens have often caused substantial damage to the plant resulting in long shut down times.

Whilst the technology described above allows complete heating, drying and curing in a vibrating fluidised bed system - consisting of one or more beds in series, parallel or combination of series and parallel, it is also proposed as a development of the invention that the vibrating fluidised bed can be used as the first stage in a combined vibrating fluidised bed - hopper/retort system as described below.

Two stage curing system As mentioned earlier, the overall curing stage consists of a heating/drying stage and an overlapping chemical reaction stage.

The first stage requires the input of energy in the form of hot air, while the latter merely requires the maintenance of a specified temperature - time relationship. This will be different for different binders. For the starch-phenol formaldehyde resin system any of the time temperature relationships would be suitable - (or intermediate relationships) for the curing stage.

1. 4 minutes at 130"C ) These are internal 2. 8 minutes at 120"C ) temperatures of the 3. 16 minutes at 1 100C ) briquettes, the applied air temperature will be much higher.

Some curing will occur during the heating and drying and some during the cooling. The temperature-time relationship can be varied by adding catalyst or inhibitor and by changing the precise form of the resin but it will be recognised by those skilled in the art that for any systema time-temperature relationship of the above type will bring about the curing of the resin.

By using the vibrating fluidised bed unit to heat, dry and partially cure the briquettes, and raising them to a sufficiently high temperature - the curing process can then be completed in a simple holding vessel. This holding vessel may be operated continuously or batch wise. The holding vessel will be very much simpler and less expensive than the vibrating fluidised bed and very much cheaper than a conveyor oven.

On an industrial scale using hoppers containing say from 1 to 20 tonnes, the rate of -heat loss is very low and the temperatures can readily be maintained for the desired time without.

introducing heat. However, for some systems heat can be supplied if necessary.

Using such a system the temperature profile of the briquettes would take the form shown in Fig. 2.

For many systems a residence time of 10 to 15 minutes in each stage would be typical but the time in the hopper can be prolonged without disadvantage.

For a starch-resin system temperature A would be in the range 100-150"C, for lignosulphonate 180-230 C but these temperatures can be varied to suit the requirements of the end product. Thus, for example, if an all starch or lignosulphate system is to be waterproofed by.

painting or packing in water impervious bags, the very strong briquettes can be obtained at relatively low hot air temperatures 130-170"C - in such cases the second stage may be so short it is achieved ip the conveying and cooling stage.

An example of a two stage process is now given. In this example, the briquettes were heated and dried in a vibrating fluidised bed for 8 minutes using hot air at 1 700C. The outer layer of a typical briquette was more or less cured to a depth of 0.3 inches leaving a core of about 0.5 inches diameter soft and incompletely cured. The briquette lost some heat during the transfer from the vibrating fluidised bed to the hopper but has a mean temperature of 90"C in the latter unit. After two hours it was found that the briquettes were completely cured.Their relative crushing strengths were: Into vibrating fluidised bed 8 Ibs From vibrating fluidised bed 140 to 200 Ibs From Hopper 300 to 400 Ibs For an industrial process, preferably the briquette temperature into the hopper would be higher and the residence time shorter (e.g. 10 to 20 minutes) and such a process will now be described.

These are prior art methods of producing briquettes in which the briquettes are raised to temperatures in the 400 to 600"C range or even higher. In these processes, the high tempera ture brings about the removal of the smoky volatile components of the fuel and can also bring about fusion within the briquettes that results in a very strong and stable product. These processes tend to be very expensive, often requiring inert gas system and special metal equipment that can carry the briquettes through the hot zone.

In the proposed process the briquettes pass through the vibrating fluidised bed system to yield a reasonably strong briquette, this stage is operated under conditions of low fire hazard.

The hot briquettes are then passed to a hopper or a retort where controlled amounts of air are allowed to enter. The rate and amount of air flow is sufficient to achieve the desired rate of heat generation but insufficient to affect any significant cooling. Using this technique briquette temperatures of from 130 to 800"C can readily be achieved.

Whilst the proposed process can be operated as a batch process, it is particularly effective as a continuous process.

It has been demonstrated that briquettes made using starch resin will, with controlled amounts of air, self heat from about 50"C and reproducibly self heat from temperatures of 60-80"C. It will be appreciated that is no upper limit to the temperatures that can be reached. Briquettes of the above type will self heat from 60"C to 700"C in less than 12 hours and from 100 to 500"C in less than three hours.

The parameters for the fluidised bed processing in accordance with the invention and using fluidised beds which are for example 22 ft. long and 4 ft. wide can be summarised as follows: For a nominal 10 t.p.h. unit two beds would be used in series.

Depth of briquettes on the beds 3 to 15 inches (typically 9-12") Residence time 5 to 30 minutes (typically 15 minutes) Feed air temperature 140 to 320"C (typically 250"C) Air flow 20,000 to 100,000 c.f.m. (typically 70,000 c.f.m.) c.f.m./lb product 55-270 (typically 180/190) The briquettes could be made from anthracite duff and the binders could be Starch Starch - Phenol Formaldehyde resin Sodium or Ammonium Lignosulphonate Sodium or Ammonium Lignosulphonate plus Phenol Formaldehyde resin Sugar or Molasses Sugar or Molasses plus polymerisation and/or cross linking agents Unsaturated polyester resin systems In previously proposed versions of the vibrating fluidised bed the exhaust duct from the hood above the bed was often located at the feed end of the bed.This can result in fine material present in the bed being carried in an opposite direction to the flow of briquettes. This allows fine material to build up in the bed and interfere with its function.

If hoods of the form shown in Fig. 3 are used in which the off gases flow in the same direction as the briquettes then the build up of fine material is prevented.

The efficiency of the vibrating fluidised bed can be further enhanced by drilling the perforations in the deck plate at an angle so that the air is discharged in the direction of the briquette flow.

This not only further aids the removal of fines but also makes a useful contribution towards driving the briquettes in the desired direction. Perforations drilled at angles to the vertical form from 10 to about 30 are both useful and convenient to produce.

In addition to the high air flow playing a significant part in ensuring good heat transfer, the actual vibration on the fluidised bed also enhances or increases the rate of heat transfer.

Claims (8)

1. A process for producing solid fuel briquettes wherein green briquettes comprising agglomerated combustible particulate material and a binding medium are hardened and at least partially cured by fluid at elevated temperature in a fluidised bed dryer, wherein said fluidised bed dryer is a vibrating fluidised bed dryer and a fast air flow through the dryer is used as said fluid.

2. A process according to claim 1, wherein said briquettes are completely cured in said dryer.

3. A process according to claim 1 or 2 wherein the rate of said air flow is sufficiently high that it can be fed to the bed at a temperature between 140"C and 320"C without the briquettes self heating.

4. A process according to claim 3, wherein said temperature is about 240"C.

5. A process according to any preceding claim, wherein the briquettes have a residence time of 5 to 30 minutes in the fluidised bed dryer, the temperature of the air fed to the bed is between 140"C and 320"C and the air flow is of the order of 115-565 cu.ft. per minute per square ft. of fluidised bed deck and of the order of 55-270 cu.ft. per minute per pound weight of briquette.

6. A process according to claim 1, wherein said briquettes are only partially cured in said dryer and are then held at an elevated temperature in a holding device such as a hopper.

7. A process according to claim 6, wherein the rate of said air flow is sufficiently high that it can be fed to said bed at a temperature between 1300C and 1700C.

8. A process according to claim 5 or 6, wherein controlled amounts of air are allowed to enter said holding device, the rate and amount of air flow being sufficient to achieve the desired rate of heat generation but insufficient to effect any significant cooling.