GB2196642A - Agglomerated solid fuel briquettes - 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 microwave oven at elevated temperature (e.g. 120 DEG C-300 DEG C) for a short period of time sufficient to bring about partial or complete hardening and curing of the green briquettes. There is also disclosed apparatus for such manufacture.

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 aggromerated 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.

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

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 Patent Application 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 occur to 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 of plant and buildings.

Processes involving heat treatment in an oven to bring about curing 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 ligno-sulphonate 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 capital 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 difficult 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 generally 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 higher temperatures of about 450-600"C are reqired 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 higher temperatures, 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 sufficient to render them 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.

Other types 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.

Our earlier British Patent Application No.

8526667 describes 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 partial or complete hardening and curing treatment with fluid at elevated temperature (preferably about 120"C to about 220"C) in a fluidised bed dryer, such as a vibrating or static fluidised bed dryer with air as the fluidising medium.The residence time of the green briquettes undergoing hardening and curing treatment in the fluidised bed dryer is generally less than one hour, but an additional hardening and curing treatment step is usually required (e.g. in a low temperature oven or retort) when the residence time in the fluidised bed dryer is about 10 minutes.

This process offers considerable advantages over the previously known conveyor oven treatment, in terms of energy input costs and throughput time.

We have now found that a further reduction in overall throughput time, along with possible further savings in energy input costs, may be achieved in a process for producing solid fuel briquettes using microwave technology.

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 of the 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 at elevated temperature in a microwave oven. Preferably the temperature is in the range of about 120"C to about 300"C.

The microwave oven is suitably adapted for a continuous or batchwise throughput of green briquettes and comprises a generally horizontal bed to support a layer of briquettes from two to twelve deep.

Suitably, residence time of the green briquettes undergoing hardening and curing treatment in the microwave oven is less than about ten minutes.

Advantageously, the residence time of the green briquettes in the microwave oven is from about 2 minutes to about 6 minutes, preferably about 4 minutes, dependent upon the depth of the briquettes located on the bed of the oven.

The invention also provides a smokeless solid fuel briquette manufactured by the process herein described characterised by a compression strength of at least 300 p.s.i., preferably about 450 p.s.i. to about 550 p.s.i. 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% by weight, of the particulate material/binding medium mix. Alternatively, conventional lignosulphonate binders may be used alone or in combination with a resin.

Firstly, a feedstock is prepared by grinding, if necessary, solid fuel to a powder grade of a particle size of about 100 microns, or alternatively 0-10 mesh. 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 treatment to reduce particle size. Any standard grinding plant may be used, for example a British Geoffrey Diamond (Trade Marl < ) grinder.

The particulate feedstock material is transferred to a feed hopper. This material may have a high moisture content, for example greater than 15%, and is thus transferred to a dryer in order to reduce the moisture content to less than 10%, preferably about 8.5%. The dryer may be a static fluidised bed dryer, such as a Buell (Trade Mark) dryer, operating at 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 which includes a vertical rotary mixer arrangement to produce an intimate mix of the particulate material and the binding medium which is then transferred to a storage mixer pending further treatment. The moisture content at this stage must not be too low as this assists the mixing process.

The dried and mixed feedstock is then fed to a standard briquetting roll press, such as a Condeur (Trade Mark) roll press to produce discrete ovoids or briquettes of compressed particulate material. However, since the binding medium has not yet been cured these briquettes, called "green" briquettes, are of low compression strength in the region of 10 p.s.i.

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 then transferred to a microwave oven facility of industrial size.

The green briquettes in the microwave oven are subjected to an even surface temperature which promotes uniform evaporation of moisture from the entire surface area of the briquettes. Depending upon the residence time in the microwave oven, and on the type of binder employed, partial or complete curing of the binder may occur. However, a residence time of two to six minutes, preferably four minutes using the binding medium described above, is sufficient to produce a briquette which is totally or partially cured, and in which the compression strength is increased to 200 to 500 Ibs in a microwave oven with a level of three briquettes or a bed depth of about eight inches. The briquettes are dried and cured to give a compression strength greater than 200 Ibs. Ideally, continuous throughput of green briquettes during treatment in the microwave oven is at a rate of 10 tons per hour.

The direct process described above, gives a product strength which is up to 25% greater than that of briquettes produced by conven tional processes using a traveling conveyor belt oven and other similar methods of transferring heat to the briquettes for the purposes of drying and curing.

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 procedure to be conducted at tem peratures of less than 220"C, above which the danger of combustion becomes acute. Thus, energy input costs are significantiy lower than in higher temperature processes, and or processes requiring much longer dwell time in ovens.

Processes according to the present invention will now be further illustrated with reference to the following test examples: EXAMPLE 1 The procedure described above was carried out using small amounts of green briquettes in a Sharp R2240E domestic model microwave oven at a level of three deep in a time of four minutes. The final average compression strength is around 350 Ibs compression, and the cured briquettes have 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 Ibs, moisture content 82% having been fed into a Sharp R2240E microwave oven at a level of two deep for two minutes compression strength of 300 Ibs was obtained.

EXAMPLE 3 Similar to Example 2, with green briquettes being stacked four deep, produced a product with compression strength 400 Ibs, and excellent appearance.

EXAMPLE 4 Similar to Example 3, with green briquettes being stacked four deep, with a residence time of five minutes in the microwave oven-compression strength of 400 Ibs.

Claims (11)

1. 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 partial or complete hardening and curing treatment in a microwave oven.

2. A process as claimed in claim 1, wherein the elevated temperature is in the range of about 120 C to about 300"C.

3. A process as claimed in claim 2, wherein the microwave oven is adapted for batchwise treatment of green briquettes.

4. A process as claimed in claim 2, wherein the microwave oven is adapted for continuous processing.

5. A process as claimed in any preceding claim wherein the green briquettes are stacked from about two to about twelve deep on the bed of the microwave oven.

6. A process as claimed in any preceding claim wherein the residence time of the green briquettes undergoing hardening and curing treatment in the microwave oven is from about two to about ten minutes.

7. A process as claimed in any of claims 1 to 6 wherein the binding medium comprises lignosulphonate.

8. A process as claimed in any of claims 1 to 6 wherein the binding medium comprises a resin and lignosulphonate.

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

10. An agglomerated solid fuel briquette whenever manufactured by a process as claimed in any of claims 1 to 9.

11. Apparatus for producing solid fuel briquettes comprising agglomerated combustible particulate material and a binding medium, which apparatus comprises in combination a mixer to mix the particulate material and the binding medium, a press to form low strength "green" briquettes from the said mix, and a microwave oven to subject the green briquettes to partial or complete hardening and curing treatment.

1 2. Apparatus for producing solid fuel briquettes, as claimed in claim 11, comprising a grinder to pulverize combustible feedstock material, and a dryer to dry the particulate material, prior to mixing said material with the binding medium.