GB2138441A - Briquetting coal - Google Patents

Abstract

Coal briquettes for use as feedstock in coal gasification processes can be made from coal fines of up to 6.5 mm particle and a binder which comprises tars or pitches which may be obtained from organic components condensed and separated from a gas produced by the gasification of coal with steam and oxygen. The briquettes may also contain fluxes especially if they are employed as feedstock for ash-slagging gasification processes.

Description

SPECIFICATION Improvements relating to the briquetting of coal This invention relates to the coal feedstocks for use in coal gasification processes and more particularly to the production of briquettes which comprise coal fines.

In the early development of coal gasification techniques lump coal was utilised as the feedstock since for techniques based on fixed bed reactors only limited amounts of coal feed below a certain size can be tolerated. These small sized coal particles are known as 'fines' and generally have a particle size of less than about 6mm. Thus, these fixed bed reactor systems could employ either only special high grade coal feeds or the coal had to be screened to separate out the fines. It was therefore not convenient to use run-of-mine (ROM) coal.

Improvements in the gasification techniques have allowed for more fines to be employed in the coal feed. However, these improvements are offset by the fact that modern mining techniques produce ROM coals which contain a very high proportion, eg. some sources claim well over 50% of fines.

To alleviate this problem, the technique of binding coal fines has been proposed. For example, our published UK Patent Specification GB-A-2079780 discloses mixing coal fines with a calcareous hydraulic binder such as Portland Cement. The mixture is moistened, shaped and then hardened. Organic binding agents for coal fines agglomeration, pelleting or briquetting eg.

tars, pitches, asphalts and bitumen, are also known and their use is described in, for example, UK Patent Specification No. 1410649 and UK Patent Specification GB-A-2040308. The properties and disadvantages of both pitch and bitumen are well known. In particular, care has to be taken to select materials having the correct physical properties to enable them to be used for the manufacture of briquettes having, for example, good crush strength and good storage properties.

In the published specification of GB-A-2061317, there is described coal briquettes which are employed as feedstock for a fixed bed gasification reaction and wherein coal particles and ash are compressed together to form small briquettes: the coal and ash particles have to be comminuted to size of less than 1 mm, preferably less than 0.2 mm. At least 50% of the coalash mixture should preferably have a particle size of less than 0.1 mm and more preferable at least 30% should be less than 0.06mm. It is also required that the mixture comprise 10-30% by weight of the ash. It may be concluded from this approach that briquetting of fine coal particles is very difficult as it is not desirable to recycle an inert by-product back through the gasification stage with the consequential loss of overall plant thermal efficiency unless it was necessary.The mixture to be briquetted may additionally contain from 0.5 to 5% by weight of tar or pitch which it is stated is used for water-proofing purposes.

In the gasification of coal, a condensible material is by-produced along with the main-product synthesis gas. This condensible material has an aqueous component derived from the steam gasifying agent and an organic component, derived from the coal and which comprises oils and tars. These oils and tars comprise a valuable source of hydrocarbons from which synthesis gas can be produced.

We have unexpectedly found that the tars obtained from these synthesis gas condensates and the pitches distilled therefrom are particularly useful for briquetting coal fines from the whole ROM coal.

Additionally to the value of recycling the condensate material back to the gasifier, the tars and pitches can be used as binder for ail fines material below 6.5mm mesh and they will produce briquettes having very good initial strength. Thus, soon after manufacture, briquettes can be added with lump coal into the gasifier coal feed system. This minimises the need for storage and conditioning areas for briquettes.

The present invention provides a coal briquette comprising that portion of run-of-mine coal of a particle size up to 6.5mm and a binder comprising a tar or pitch. The binder may be obtained with advantage, by condensing and separating organic components from a gas produced by the gasification of coal with steam and oxygen, and the briquettes of the invention may be employed in such a gasification process.

In a preferred embodiment where the briquettes are to be used as feedstock for a gasification reaction a fluxing agent may be incorporated therein. The flux may be crushed slag or frit obtained during the ash-slagging gasification of coal, a limestone such as Dolomite or even blast furnace slag.

The flux is added in a slagging-type gasifier in order to improve the flow characteristics of the produced slag making it easier to remove. The viscosity of the slag is a function of the Silica Ratio (SR) of the coal ash, where SiO2 SR = x X 100 SiO2 + Fe2O3 + 1.43 Fe + CaO + MgO + 1.11 FeO (the oxides being expressed as percentages of the ash).

High SR values will adversely affect the operation of the gasifier since the slag is more viscous and higher temperatures are required to maintain sufficient mobility of the slag. In addition to increasing the cost of operation and decreasing the overall thermal efficiency, the use of such high temperatures may have serious effects on the equilibrium composition of the product synthesis gas.

For good slag flow characteristics, the SR should preferably be not greater than about 48.

Thus, for a coal with a very high refractory ash content of, eg. 20%, a limestone flux addition 0.4kg of flux for every kilogram of ash may be required. Alternatively where the alimentation rate of the total coal feed, i.e. lump coal together with briquettes, is known, the briquettes may be manufactured such that the flux for the total coal feed is contained in the briquettes.

The briquettes of the present invention may be manufactured from any size of coal fines up to 6.5mm, a size distribution from 2 to 6.5 mum mesh being perfectly acceptable. The briquettes may additionally contain coal particles obtained from ROM of greater than 6.5mm mesh but ground down to a size of less than 6.5mm.

The moisture content should desirably be less than 5% w/w and, if necessary, the coal fines should be dried to this level prior to briquetting.

The coal briquettes of this invention are characterised in having good initial mechanical strength, i.e. straight from the press. The strength of the briquette is dependent on the amount of tar or pitch employed. Satisfactory initial strengths are obtained using pitch of 60"C softening point in an amount greater than 3% w/w to less than 9% w/w when briquetting at normal temperature of about 90"C, binder levels of from 5-9% w/w being preferred. Lighter pitches can be briquetted to the same strength at lower temperature but at standard briquetting temperatures, a slightly higher range of pitch concentrations will be required. When briquetting with heavier pitches at standard briquetting temperatures, a lower range of pitch concentrations may be required.In tests using tar, it was noticed that a longer delay time was required to allow the product to gain adequate strength for feeding to a gasifier.

Although, according to a preferred embodiment, tars and pitches obtained from the gasification of coal, are desirably incorporated into the briquettes of the invention, such briquettes may be produced employing any pitch ie. the heavier product from any distillation of a tar. The softening point of the pitch may range from 30"C to 90"C.

The briquettes of the invention can be produced using conventional techniques and apparatus.

The invention will be illustrated with reference to the following example.

4.064 tonnes of Markham Main coal (of size 5 cm and under) were screened through a 6mm mesh. About 40% of the initial coal feed was obtained as fines. The fines were dried to a water content of 5% w/w and were found, on analysis, to have an ash content of 15%.

205 litres of a Westfield tar was distilled to produce 60"C Softening Point (Ring 8 Ball method) Pitch. The pitch was crushed to O.Smm particle size and mixed with portions of the treated fines to produce mixtures in which the pitch contents were 3,5,7 and 9% w/w respectively.

The mixtures were then pressed on a double roller briquetter to produce briquettes 50 X 41 X 23mm, weighing between 40 to 30g.

For comparison a Manvers Coal was crushed to less than 2mm mesh mixed with 7% w/w of the pitch and bnquetted Immediately after manufacture and at predetermined intervals therefore the briquettes were subjected to crush and impact tests. The results of the tests are given in the following Tables.

Table 1 illustrates results of the Crushing Strength test whilst Table 2 gives results of the impact test. The impact test was performed by dropping 30 briquettes once, immediately after manufacture, and three times, five minutes later onto a steel plate from a height of 1 83m and determining, as a percentage, the number which were undamaged.

Table 1 - Crushing Strength (Kg)

Binder TimConc'n (%) 7/Manvers 3 5 7 9 (Comparison) Initial 10 15.4 10.4 11.35 7.3 5 17.7 24.1 32.7 41.0 43.6 10 21.8 46.3 42.7 52.7 89.9 15 22.25 46.3 43.6 72.6 104.9 30 22.25 47.2 50.0 87.1 110.8 60 22.7 49.0 50.0 - Table 2 Impact Test - % undamaged

NM No. of Impacts Binder Conctc (%) \ 1 3 3 37 0 5 56 13 7 77 17 9 93 70 7/Manvers 93 57 As can be seen from the results above, that, as the briquettes cooled and hardened, they gained strength rapidly over the first ten minutes after manufacture. Full strength was gained after an hour.

Although this trend is apparent from an analysis of the crush-strength results, there was little difference in the impact test results between freshly made briquettes and those ten minutes after manufacture.

It can also be seen from the results given above that to make a briquette directly from fines, of strength equivalent to that of a briquette made with 7% w/w pitch in the finer crush coal, a pitch level of about 9% w/w is required.

Claims (8)

1. Coal briquettes comprising that portion of run-of-mine coal of a particle size up to 6.5 mm and a binder comprising a tar or a pitch.

2. A briquette as claimed in Claim 1 wherein said tar or pitch is obtained by the condensation and separation of organic compounds from a gas produced by the gasification of coal with steam and oxygen.

3. A briquette as claimed in Claim 1 or Claim 2 in which the softening point of the pitch ranges from 30"C to 90"C.

4. A briquette as claimed in one of the preceeding claims in which the fluxing agent is included.

5. A briquette as claimed in Claim 4 in which a fluxing agent is added in an amount to reduce the Silica Ratio (as hereinbefore defined) to not more than 48.

6. A briquette as claimed in Claim 4 or Claim 5 in which the fluxing agent is a blast furnace slag, a limestone or a frit obtained from the slag produced during the ash slagging gasification of coal.

7. A briquette as claimed in any one of the preceding claims wherein said tar or pitch is present in an amount of from 3 to 9% by weight of the composition of the briquette.

8. A briquette as claimed in any one of the preceding claims in which the particle size of the coal is from 2 to 6.5mm.