GB2189806A - Briquette hardening - Google Patents

Abstract

Shaped briquettes for use as a smokeless or smokey combustible solid fuel are manufactured by firstly compressing a mix of agglomerated solid fuel particles and a binding agent such as resin and starch or a lignosulphonate to produce "green" briquettes of low compression strength. The green briquettes are then treated in a fluidised bed dryer, (such as in a horizontal vibrating fluidised bed dryer, with air at elevated temperature - e.g. 120 DEG -220 DEG C) for a period of time sufficient to bring about partial or complete hardening and curing of the green briquettes. If further hardening and curing treatment is required to increase the compression strength of the final product to an acceptable level, a retort oven or another fluidised bed dryer may be used. A process is also disclosed using a static fluidised bed dryer with sand as a heat transfer agent instead of a horizontal vibrating fluidised bed dryer.

Description

SPECIFICATION Agglomerated solid fuel briquettes, and improved briquetting process The present invention relates to an improved process for the preparation of combustible solid fuel materials in the form of shaped briquettes or ovoids comprising compressed aggromeratedfuel particles held together bya 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.

Known processes for the production of solid fuel briquettes can conveniently be divided into two main types on the basis oftemperature.

Cold processes involve the use of chemical binding systems wherein a cure-active agent or hardener is added to accelerate curing and to result in a briquette of acceptable compression or crushing strength to satisfy handling and market requirements. An example of such a process is described in International PatentApplication No. WO 84/04534 which employs a lignosulphonate binder with sodium dichromate as a cure-active agent. The main disadvantage of lignosulphate binding systems in cold processes is that binder degradation may occurto produce volatile sulphoxides such as SO2 and S03 resulting in the undesirable formation of sulphuric acid.These breakdown products create objectionable fumes, often offend local environmental legislation, require extra equipment involving higher costs to trap them, and can cause corrosion ofplantand buildings.

Processes involving heat treatment in an oven to bring aboutcuring of the binder are more commonplace but in order to be economical, curing time and all energy inputs should be carefully controlled to produce optimum results. In a known procedure, briquettes or ovoids are manufactured from solid fuel of small particle size such as anthracite duff, i.e. of dust or powder grade, by combining such material with lignosulphonate as a binding medium, compressing the mixture to produce so-called "green" briquettes of low crushing strength, which are then hardened and cured in a conveyor oven where they are subjected to a temperature profile of up to 220"C with a residence time of one to two hours. The conveyor belt or mesh in the oven may be in excess of 30 m in length so that such apparatus takes up considerable plant space.

Conveyor ovens tend to have high unit fuel costs, because of significant heat losses, and are comparatively expensive in terms of capitai cost.

Also there is a risk of combustion of the green briquettes and inert gas may have to be used inside the oven to prevent this, which adds to production costs. Occasional fires are difficu It to deal with and result in increased down time and maintenance.

However, the main disadvantage of conveyor heat treatment is that the green briquettes which are static tend to be heated and dried out more on exposed surfaces than on unexposed surfaces in spite of attempts to turn over the briquettes during drying using scraper blades, such as the inclined flights in a rotary agitator arrangement. The affect of uneven heating, drying and curing is to give a product which has a variable crushing strength, which may result in cracking, breaking or "oystering" during cooling and subsequent handling and storage, and generaliy reduces the quality of the product.

Bitumen or coal tar pitch has been used as a binder in the manufacture of briquettes but bituminous binding systems suffer from many disadvantages.

Much highertemperatures of about 450-600"C are required to bring about complete volatilization of the bitumen so as to produce an acceptable smokeless briquette, which adds greatly to energy input costs.

Alternatively a slow heat process may be used to reduce the risks of charring and coke formation at highertemperatures, but again this is expensive. If the bitumen content is less than about 4%, which would be required to produce a smokeless fuel, the compression strength of the briquettes may not be sufficientto renderthem suitable as a solid fuel without the addition of a hardening agent in the binding system. Furthermore, bitumen is becoming increasingly scarce, resulting in higher raw material costs.

Othertypes of binding systems used in the manufacture of smokeless briquettes include sulphite lye, olefin polymers and copolymers, thermo emulsions,Wafex (Trade Mark), silicones, etc. It is also known to use a system combining starch and a natural or synthetic resin. All of these systems require heat treatment to bring about curing and hardening.

The present invention seeks to provide an improved "hot" process for the manufacture of solid fuel briquetteswherein raw material costs, capital plant costs, and energy input costs are optimised and hardening/curing time is significantly reduced in an effort two reduce overall production costs. It is also an aim of the invention to provide a briquette, in particular a smokeless briquette, which has been substantially uniformly cured and hardened to an acceptable compression strength.

While it is known to dry granular or pulverulent material, or material of yreaterthan 25 mm particle size, with a vibrating fluidised bed dryer, for example as described in British Patent Specification No.

1,308,775,fluidised bed dryers in general have not been used orthoughtto be suitable for use a briquetting process. We have now surprisingly foundthatso-called green briquettes,which may have a compression or crushing strength of as low as 10 pounds, or less, are capable of withstanding vibrating movement and ambient gas pressure and temperature during their passage through a vibrating fluidised bed dryertoyield substantially uniformly hardened briquettes, wherein the compression strength may be increased in several minutes by a factor of 5to 10 or more.One would not have expected the briquettes to have gained any significantstrength bythisshorttreatment particularly as oven treatment known from the prior art indicates a comparable hardening time of one or two hours. Furthermore, we have found that process parameters can be varied to result in a practically negligible amount of cracking, breaking or oystering of briquettes hardened by this treatment. It is also interesting that much lowertemperaturesthan prior art processes bring about partial or complete curing and hardening ofthe briquettes. Moreover, we have found that more rapid curing and hardening treatment results in no objectionable amounts of volatile sulphur compounds being evolved when a lignosulphonate binder system is used.

According to the invention there is provided 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 ofthe particulate material and the binding medium to form low strength "green" briquettes, and of subjecting the green briquettesto partial or complete hardening and curing treatmentwith fluid atelevated temperature in a fluidised bed dryer. The preferred fluid is air. Preferably the temperature is in the range of about 1 200C to about 220"C, most preferably 140 C, andthe dryer is preferably a horizontal vibrating fluidised bed dryer.

Suitably, residence time of the green briquettes undergoing hardening and curing treatment in the fluidised bed dryer is less than about one hour.

Advantageously, the residencetime of the green briquettes in the fluidised bed dryer is from about 2 minutes to about 20 minutes, preferably about 10 minutes, wherein an additional hardening and curing treatment step is performed, preferably in a low temperature oven.

The low temperature oven is preferably a tumbling retort oven, and the temperature of the oven is preferably within the range of about 11 00to about 1800C, preferably about 140"C.

The invention also provides a smokeless solid fuel briquette manufactured by the process herein described characterised by a compression strength of at ieast 200 pounds, preferably about 450 pounds to about 550 pounds. Advantageously, the combustible particulate material comprises anthracite duff, wherein the particle size is preferably less than about 100 microns. The binding medium preferably comprises a synthetic resin and starch, each in a proportion of about 1 to 3% by weight, preferably 1.5% byweight, ofthe particulate material/binding medium mix.

A preferred briquetting process in accordance with the present invention and solid fuel briquettes produced bythis process will now be described in further detail with reference to the accompanying drawings, wherein: Figure lisa diagrammatic representation of a briquetting process in accordance with the invention, Figure2 is a side elevation of vibrating fluidised bed dryersuitablefor use in the process, and Figures 3a, 3b and 3c are respectively a schematic side elevation, end elevation and plan view of a retort oven suitable for use in the process.

Firstly, a feedstock is prepared by grinding, if necessary, solid fuel to a powder grade of a particle size of about3 mesh (0.25 inch) and less; typically, 80% would pass through a 10 mesh screen wherein the feedstock particle size would be in the range of 3 mesh (0.25 inch) to about 270 mesh (0.002 inch).

Anthracite is preferred, on account of its comparatively low volatile content, which may be in the form of a washed duff or dust to begin with having been screened to remove oversize particles, or which may undergo grinding treatmentto reduce particle size. Any standard grinding plant may be used, for exampie a British Geoffrey Diamond (Trade Mark) grinder.

The particulate feedstock material is transferred to a feed hopper A. This material may have a high moisture content, for example greaterthan 15%, and is thus transferred to a dryer B in orderto reducethe moisture content to less than 10%, preferably 5 to 6%. The dryer 3 is a staticfluidised bed dryer, such as a Buell (Trade Mark) dryer, operating art a temperature of about 60 C.

The dried particulate feedstock is then combined with a binding medium in a proportion of about 1.5% by weight of a phenolic resin such as phenol formaldehyde resin,for example Ciba Geigy AEROPHEN 0807 or 1719 (Trade Mark) and about 1.5% by weight of starch such as conventional industrial starch,for example C.P.C. Ltd. AMIJEL (Trade Mark). The components are treated in standard pug mill C which includes a vertical rotary mixerarrangementto produce an intimate mixofthe particulate material and the binding medium which is then transferred to a storage mixer D pending furthertreatment.

The dried and mixed feedstock is then fed to a standard briquetting roll press E, such as a Condeur (Trade Mark) roll press to produce discrete ovoids or briquettes of compressed particulate material.

However, since the binding medium has notyetbeen cured these briquettes, called "green" briquettes, are of low compression strength in the region of 10 pounds and are thus relatively fragile and unsuitable for mechanical handling or stacking. Curing is necessary to activate a hardening reaction within the binding system and heat treatment is required to bring this about.

The green briquettes are transferred to a horizontal vibrating fluidised bed dryer F of a type described in British Patent No. 1,308,775, such as a TRAFFIELD (Trade Mark) dryer. Such a dryer is illustrated in Figure 2, which comprises a traversing deck 1 and a plenum chamber 2 separated from one another by a gas-pervious membrane 3, mounted on springs Sforvibrating movement on base frame 4. A motor 6 causes the necessary vibrating action. Green birquettes are loaded onto the traversing deck 1 by means of chute 7. A burner 8 with fan unit 9 driven by motor 10 supplies hot air to the plenum chamber 2, wherein air is sucked into the burner 8 by primaryfan 11. Dust control classification hoods 12 may befitted above the unitwith extractorfans (not shown).

The traversing deck 1 area may for example be 4.25m x 1.80m(14ft. 6ft.)andhotairata temperature of 1 20"C to 220"C is blown through the gas-pervious membrane 3 at a rate of, for example 1163 m3/min. (12500cm. ft./min.). Usable airflow is 5 to 100 cu.ft./lb, of briquettes, but expressed thus, 10 to 60 cu.ft./lb. would be the preferred rate. When the green briquettes enterthe dryer F, they are carried along the length of the traversing deck 1 in a "bed" of hotair,forward motion being imparted bythe horizontal vibratory movement of the deck 1.It is surprising that material of such low strength does not breakup undersuch vigorous action, butthe reason why disintegration does not occur is thought to be due to the fact that the green briquettes move along the deck in a hopping action moving less than one millimetre at a time, such that the forces repeatedly applied to the briquette surfaces through collision with each other and with the deck surface are so infinitesimally small that the briquettes gradually harden and retain their integrity.

Suspension of the green briquettes in thefluidised environment results in each individual briquette being subjected to an even surface temperature and promotes uniform evaporation of moisture from the entire surface area of the briquettes. Depending upon the residence time in thefluidised bed dryer F, and on the type of binder employed, partial or complete curing of the binder may occur. However, a residence time oftwo to ten minutes, preferably four minutes using the starch and resin binding medium described above, is sufficient to produce a briquette which is partially cured, but in which the compression strength is increased by a factor of five to ten to allowfurther mechanical handling.For example, afterfour minutes treatment at about 1 900C the moisture content is reduced by a further 2-3%, preferably to about 5.5%, and the compression strength is increased to about 100 pounds. Ideally, continuous throughput of green briquettes during treatment in the vibrating fluidised bed dryer F is at a rate of 1 Otons per hour. However, batchwise operation or slowerthroughput at a lower temperature of about 140 C and longer residence time of not more than one hourwould bring about satisfactory hardening and complete curing treatment.

If the briquettes are only partially cured after treatment in the vibrating fluidised bed dryer F, they are then transferred to the top of one or more vertical or inclined tumbling retort ovens G wherein the briquettes are exposed to temperatures within the range 1 100Cto 1800C, preferably 1200Cto 1600C which is thus a low temperature oven treatment. The residence time in the retort oven is more than 10 mins. and can be upto 3 hours, but is preferably30to 80 mins. The resulting briquette has a compression strength of, for example, 450 pounds. To meet the relevant British Standard, compression strength should be in excess of 300 pounds.

It will be appreciated that any subsequent treatment may be used to bring about complete curing, for example, additional vibrating fluidised beds, conveyor ovens or rotary calciners.

The briquettes which have been treated in the retort oven G are transferred to a cooling conveyor H, wherein the briquettes cool to room temperature but curing may still take place during cooling down.

Cooling may be assisted by cold airfans. This process could instead be accelerated by using lower temperature fluidised bed treatment. Provided that the moisture content is evenly distributed throughoutthe briquette structure when the briquettes reach the final cooling down step, there should be no danger of cracking or oystering at this stage or in subsequent storage. The finished smokeless anthracite briquettes are transferred from the cooling conveyor H to a stock storage area I.

Figures 3a to 3b illustrate a typical retort apparatus wherein partially cured briquettes are fed to the top of an inclined retort 20a for example via a spiral chute which tends to reduce the risk of chipping or damage. The briquettes are supported between perforated plates or grids 21 in plenum chamber 22 through which hot air is blown from fan 23 to a collection hood 24 from where it is conducted away and preferably re-cycled. Airflow can be reversed if necessary. An orthodox vibrating discharge unit 25 is used to control the rate of discharge from the base of the unit. Level switches in the top of the retort control the rate of discharge to ensure that the plenum chamber is maintained full of briquettes.

Figures 3a and 3b show a second unit 20b, but any numberofunitscan be used in continuous processing.

The briquetting process described above gives a product strength which is up to 25% greater than that of briquettes produced buy a conventional process using a travelling belt oven, and overcomes the problems associated with non-uniform drying. The fragile green briquettes can be quickly transformed into briquettes which can be safely handled in subsequent processing without the risk of breakage or damage of the briquette structure. The use of a comparatively low cure temperature binding system such as resin with starch allows the whole hardening and curing procedureto be conducted at temperatures of less than 220"C above which the danger of combustion becomes acute.Thus, energy input costs are significantly lower than in higher temperature processes, and/or processes requiring much longer dwell time in ovens. The fluidised bed dryers B and F are less proneto plant failure as compared with travelling belt ovens on account of the smaller number of moving parts, and the capital cost is also less.

In an alternative process, a staticfluidised bed dryer is used instead of a vibrating fluidised bed dryer at process step F.

Sand or a similar inert particulate heat transfer agent may be held in fluid suspension in the static fluidised bed chamber while the green briquettes are passed through the chamber batchwise or continuously and removed either by physically moving the briquettes with the aid of a scoop or by allowing them to flow through. The bed may for example be heated by heated air, electrical heaters immersed in the bed or strapped to the outside, or by burners submerged in the bed.

The process of the present invention offers significant advantages when using other binding systems. It has been found thatwhen lignosulphonates are used, very low amounts if any of sulphoxides are given off due to the much more rapid curing time achieved.

Thefluidised bed unit brings about the curing stage much more rapidly and at much lower temperatures than conventional processes and it is believed thatthese much milderconditionsarethe reason for the greatly reduced decomposition of the lignosulphonates. In the test examples below, Example 6 illustrates a process using a lignosulphonate binder system.

This lowtemperature curing can be followed bya high temperature treatment if thins is required for any reason. As the briquettes are relatively strong after the low temperature cure, the retort curing system would be most suitable forthis second curing.

If the lowtemperature cured product cannot be classified as a smokeless fuel it has value as a smokeyfuel. Furthermore, ifthe product is not acceptably waterproof it has value as a prepacked fuel where the packaging provides the necessary protection against water.

There is a range of lignosulphonates that can be used, differing in the metal or cation component of the molecule.

The two most interesting lignosulphonates are: 1) Ammonium lignosulphonate, as this introduces very little ash into the briquette.

2) Calcium lignosulphonate, as this is probably the cheapest form, but does introduce some ash.

While these two lignosulphonates are used and recommendedforbriquette manufacture, there are many otherforms available, which include sodium, magnesium, and aluminium lignosulphonate, but not all are completely suitable for briquette manufacture.

There is an overlap in the starch-resin and lignosuiphonate binder systems in that lignosulphonates can replace the starch in the former system. Whilst this is known, this gives a particularly advantageous system when used in a process according to the present invention including a vibrating fluidised bed unit and retort. This mixed system yields green briquettes having good green strength, the vibrating fluidised bed treatment results in a fast and effective cure, and the resin results in the final briquettes having good strength and water resistant characteristics.

Processes according to the present invention will now be further illustrated with referencetothe following test examples: Example 1 Using a one ton batch ofwashed anthracite duff of N.C.B. origin, and phenol formaldehyde resin (1.5% W/W) [Ciba GeigyAEROPHEN (Trade Mark)] and starch (1 W/W) [C.P.C. Ltd. AMIJEL (Trade Mark)] as binders, the strength of green briquettes produced from the roll press was 8 pounds. The batch was cured in afluidised bed dryerfora period of two hours using hot air at 4000 cu.ft/min. at 1800C, followed by cold cooling airfor 17 minutes to give an empty band temperature of 55"C. The final average compression strength was around 400 pounds, and the cured briquette had good handling and appearance characteristics.

Example 2 Green briquettes of similar composition to those of Example 1 were produced with a compression strength of 10 pounds, moisture content 9.25%, and fed to a TRAFFIELD (Trade Mark) horizontal vibrating fluidised bed dryer. These cured well at 1 900C for one hour with 15 minutes cooling treatment.

Example 3 Similar treatment as Example 2, but moisture content 8.5% and cooling for 10 minutes. Good end product.

Example4 Same treatment as Example 3, but curing at 140"C and cooling for 15 minutes. Very good end product, made well, negligible cracking.

Example 5 In a test using the apparatus described above with reference to Figure 1,the process was conducted ata continuous throughput rate of 10tons perhourand starting materials as described in Example 1.

Briquettes of good appearance were produced with a compression strength of about 475 pounds.

Example 6 Small green briquettes, made using 5% ammonium lignosulphonate as binder, were fed to a vibrating fluidised bed using air at a temperature of 1 60 C. The feed rate to the vibrating fluidised bed was controlled to give a residence time of 10 minutes. The resulting briquettes were hard and strong and quite suitable for normal handling and transport. During this trial, no noticeable amounts of sulphur compounds were evolved. However, similar trials carried out under prior art conditions at 240"C and 280"C with residence times of one hour resulted in noticeable amounts of sulphur compounds in the off gases from the plant.

The optimum curing conditions appear two be as follows: Starting green briquettes: Compression strength 10 pounds Moisturecontent-8.5% Curing parameters: Curing Time -1 hour Curing temperature 140'C Cooling Time -15 mins.

It will be readily appreciated that the briquetting process of the present invention may be employed using a range of raw materials and binding systems to give acceptable smokey briquettes, or smokeless briquettes meeting strict official requirements.

Claims (24)

1. Aprocessforproducing solidfuei briquettes comprising agglomerated combustible particulate material and a binding medium wherein the process includes the steps of compressing a mix ofthe particulate material and the binding medium to form low strength "green" briquettes, and of subjecting the green briquettes to partial or complete hardening and curing treatment with fluid at elevated temperature in a fluidised bed dryer.

2. A process as claimed in claim 1, wherein the fluid is air.

3. A process as claimed in claim 1, wherein the elevated temperature is in the range of about 1 20"C to about 2200C.

4. A process as claimed in claim 3, wherein the fluidised bed dryer is a horizontal vibrating fluidised bed dryer.

5. A process as claimed in claim 1, wherein the fludised bed dryer is a staticfluidised bed dryer.

6. A process as claimed in claim 5, wherein an inert particulate heat transfer agent is suspended in the fluidised bed dryer.

7. A process as claimed in claim 6, wherein the heat transfer agent is sand.

8. A process as claimed in any one of the preceding claims wherein the binding medium comprises a resin and starch.

9. A process as claimed in any of claims 1 to 7 wherein the binding medium comprises lignosulphonate.

10. A process as claimed in any of claims 1 to 7 wherein the binding medium comprises a resin and lignosulphonate.

11. A process as claimed in claim 4claim 5, wherein the residence time ofthegreen briquettes undergoing hardening and curing treatment in the dryer is less than about one hour.

12. A process as claimed in claim 4, wherein the residence time of the green briquettes undergoing hardening and curing treatment is from about 2 to 20 minutes, further comprising an additional hardening and curing treatment step.

13. A process as claimed in claim 12,whereinthe additional hardening and curing treatment step is performed ion a low temperatu re retort oven, at a temperature within the range of about 11 00C to about 180"C.

14. A process substantially as hereinbefore described with reference to Figure 1 ofthe accompanying drawings.

15. A process substantially as hereinbefore described with reference to any one of the Examples.

16. An agglomerated solid fuel briquette whenever manufactured by a process as claimed in any of claims 1 to 15.

17. Asmokelesssolidfuel briquettewhenever manufactured by a process as claimed in any one of claims 1 to 15 comprising agglomerated anthracite duffand a binding medium consisting of a resin and starch characterised buy a compression strength of about 200 pou nds to about 550 pou nds.

18. Apparatus for producing solid fuel briquettes comprising agglomerated combustible particulate material and a binding medium,which apparatus comprises in combination a mixerto mix the particulate material and the binding medium, a press to form low strength "green" briquettes from the said mix, and a fluidised bed dryer to subjectthe green briquettesto partial or complete hardening and curing treatment.

19. Apparatus for producing solid fuel briquettes, as claimed in claim 18, comprising a grinderto pulverize combustible feedstock material, and a dryerto dry the particulate material, priorto mixing said material with the binding medium.

20. Apparatus for producing solid fuel briquettes, as claimed in claim 18 orclaim 19further comprising means to subject partially hardened and cured briquettes to additional hardening and curing treatment.

21. Apparatus for producing solid fuel briquettes, as claimed in claim 20 wherein said means comprises a retort oven.

22. Apparatus for producing solid fuel briquettes, as claimed in claim 20 wherein said means comprises a fluidised bed dryer.

23. Apparatusforproducing solid fuel briquettes, as claimed in claim 21 ,with a retort oven arrangement substantially as described herein with reference to Figures 3a to 3c of the accompanying drawings.

24. Apparatusforproducing solid fuel briquettes in accordance with a process as claimed in any one of claims 1 to 15, substantially as hereinbefore described.