GB2053958A - A process for the preparation of coal for coking - Google Patents

Abstract

A process for the preparation of coal for coking, comprising reducing the coal to small grains, preheating the small-grained coal to a temperature exceeding 100 DEG C, and treating the small grained coal with a dehydrogenization medium in a reaction chamber with sulphur as the dehydrogenization medium and removing the sulphuretted hydrogen produced from the reaction chamber.

Description

SPECIFICATION A process for the preparation of coal for coking The invention relates to a process for the preparation of coal for coking, in which the coal is reduced to small grains, the small-grained coal is preheated to a temperature exceeding 1 000C, preferably exceeding 1400C, and is treated with a dehydrogenization medium in a reaction chamber.

The term "coal" also includes coal mixtures, and other carbon-containing materials that can be coked.

In the procedures of this general kind known in practice the dehydrogenization medium is oxygen.

This is not without drawbacks, for even careful dehydrogenization with oxygen reduces the baking properties of the treated coal. The following details may be given of the state of the art and the physics and chemistry that are involved: in the production of high-temperature cokes the yield of coke essentially depends upon the volatile constituents of the coal used. The volatile constituents are defined as the proportion of dry coal that escapes when the coal is heated in a laboratory test to 900--9500C in the absence of air. Before natural gas became generally available the volatile constituents possessed a value that was greater than that of the coke.The devaluation of the volatile constituents commenced about 20 years ago and has now progressed so far that in relation to its calorific value the value of the volatile constituents amounts to about 60% compared with that of coke. Under these circumstances the demand to increase the yield of coke at the expense of the volatile constituents has become ever more pronounced. It could be done with mixtures containing larger amounts of low-voiatility coal and carbon-containing materials such as coke dust and petroleum coke.

However only limited use can be made of this possibility because the coking quality, which is indicated by high breaking strength and a low proportion of fines, diminishes too rapidly. Also in the last two decades supplies of bituminous coals of medium volatility and non-bituminous coals of low volatility diminished, whilst on the other hand supplies of high-volatility coals increased, so that limits to the reduction of the volatile constituents have been set by the raw materials supply quite apart from the quality of the coke. The developments described resulted in a high level of activity in process engineering which can be described under the headings of preheating, hot briquetting, and moulded coke, but only those processes which provide for preheating of the input mixture of coal to about 2000C and retain chamber ovens as the coking installation have achieved any degree of favour.None of these processes offers an increased coke yield. On the contrary, with some of them the coke yield falls considerably. Their advantage therefore lies solely in the possibility of converting lower-grade coal into coke, in which process however this coal must be subjected to quite special pretreatments.

When the chemical structure of the coke forming substances in the coal is investigated it is found that these structures are based on condensed ring systems with side-chains of different lengths. As regards the thermal decomposition of these types of hydrocarbons during the coking process it can be stated that as the temperature increases the side-chains of the ring systems are the first to split up, followed by the hydrogen. If one wishes to hinder the splitting up of side-chains on heating, that is, the formation of hydrocarbons that appear later amongst the volatile components, then before the full thermal stresses are applied a reaction must take place which removes the hydrogen and thereby reduces the "volatility" of the compound.The processes of this generic kind, in which the coal is treated with oxygen at temperatures in excess of 1 500C, come into this category. The formation of water, and also that of carbon monoxide and dioxide can be observed during this treatment. As a very considerable disadvantage a very evident reduction or even elimination of the baking properties of the coals can be observed, which is to be attributed not only to decomposition of the organic material by oxidation but also to incorporation of oxygen in the "coal molecule" by which polar groups, not fusible groups, appear.

In contrast with this it is the basic object of the present invention to dehydrogenize the coal molecule before the actual coking, with the aim that the fusion properties of the coal should be impaired as little as possible and that during coking the transformations of the material should take place in such a way that few organic substances become volatile.

According to the present invention, there is provided a process for the preparation of coal for coking, comprising reducing the coal to small grains, preheating the small-grained coal to a temperature exceeding 1 000C, and treating the small-grained coal with a dehydrogenization medium in a reaction chamber with sulphur as the dehydrogenization medium and removing the sulphuretted hydrogen produced from the reaction chamber.

In principle the sulphur may be used in any desired form, e.g. as powder, as vapour or as gas, and it may be added before, during or after the preheating. According to the preferred method of performing the invention the sulphur is brought into contact with the coal in a finely divided state.

In the simplest case this takes place by bringing the sulphur into contact with the coal in the form of vapour, possibly in the presence of an inert gas.

The coal can be in the static condition and in this respect similar to a filter bed, percolated by sulphur in the vapour state and/or by a stream of carrier gas bearing a fine-grained sulphur powder.

in a preferred method of carrying out the process according to the invention, the coal is vibrated into a fluidized condition and the fluidized bed so formed is percolated by sulphur in the vapour state and/or by a stream of carrier gas bearing along fine-grained sulphur powder with itself. The grain size of the coal should preferably be below 3 mm. In order to control the dehydrogenization accurately it is preferred that the coal should first be preheated, namely to a temperature of 140 to 2500 C, and that the preheated coal is then treated with the sulphur.

Preferably the treatment is with sulphur in the form of vapour at a temperature in excess of 444.60C. The dosage of sulphur is governed by the desired degree of dehydrogenization. It has proved effective to introduce sulphur into the coal in a quantity which, based on the coal, is 8 to 10% per 0.5% of hydrogen in the coal. The sulphuretted hydrogen drawn off can be reduced to sulphur once more, and the finely-powdered sulphur so obtained can be re-used for dehydrogenization.

According to the invention elementary sulphur is added to the coal or mixtures of coal to be coked, a process which must hitherto have met with complete rejection, for it was the accepted view amongst experts that an increase of sulphur in the coking coal would lead to high sulphur in the coke. This may very well have been true when thinking of the old methods of coking in which the coal reached the ovens in a damp condition.

Current preheating techniques offer a possibility of the solution which presents itself in the form according to the invention in which the coal, or mixture of coal, to be coked is, during or after its preheating preferably to 150 to 2500C, partly or completely treated with elementary sulphur. By means of the process according to the invention the sulphur reacts with the hydrogen of the hydrocarbons in the coal to form sulphuretted hydrogen and leaves the carbon with free valencies which make other compounds possible which, surprisingly, chiefly exhibit aromatic behaviour. The results of dehydrogenization have a very favourable effect on the coke yield. This can be seen by comparing the volatile constituents with the hydrogen content and the coke yield (see Ruhr Coals Handbook 1969, pp. 61 and 206).A reduction of the hydrogen content of about 0.5% reduces the volatile constituents of coal by about 10%, for example from 37% to 27%. This means that there is an increase of about 10% in the coke yield, a figure that opens up economic vistas that are of the greatest significance. In addition to the increase of coke yield by dehydrogenization of part of the coal the baking performance of the coal remains adequate and can be maintained by control during the period of treatment, for the sulphur cannot, like oxygen, penetrate inconveniently deeply into the coal grains, thus the properties of the coal within the grains are not changed. In addition the invention makes use of the fact that the preheating of the coal is accomplished by normal coke-oven plant.Quite apart from the coking process, temperatures are there available in the range between 1 50 and 2500C. This however is the temperature range in which elementary sulphur dehydrogenizes hydrocarbons of the coal in the manner described.

The addition of sulphur to the coal can take place in various ways, but it should be carried out from the point of view of the maximum possible degree of fine division. This means that the sulphur dehydrogenizes the coal most uniformly if it is applied in the form of vapour or in a solvent medium. It is also an advantage to add the sulphur when the coal is already dry and hot, so that the reaction can take place without delay. The quantity of sulphur that has to be added depends upon the degree of dehydrogenization, which in turn depends upon the degree of breakdown of the volatile constituents that is required.If 0.5% of hydrogen is to be removed from a highly volatile coal, which in most cases will be quite adequate to bring the coal into the bituminous range, this means removing a quantity of 5 kg H2 from 1000 kg of coal. 5 kg H2 corresponds to 2.5 kmol H2, which on the assu-mption of complete transformation according to the equation H2 + S 4 H2S can be decomposed by 2.5 x 32 kg S = 80 kg S. This amount of sulphur appears to be a very high specification. It is nevertheless less significant to the extent that, according to the preferred method of carrying out the invention, the sulphuretted hydrogen formed is reconverted into sulphur, e.g. in a Claus oven, so that a sulphur cycle is present and only the losses that occur have to be replaced. These losses are to be found in the coal and they are released during coking.They can be removed by the normal sulphur-wash of the coke-oven gas. The dehydrogenization reaction speed essentially depends upon the extent of comminution of the coal and the temperature during the encounter of the sulphur with the coal. This means that it is always most advantageous to dehydrogenize at the upper range of working temperature mentioned. At a temperature exceeding the boiling point of sulphur, 444.60C, the sulphur in the sulphur vapour is so ready to react that it is converted into sulphuretted hydrogen by dehydrogenization of the coal at the very first instant of contact. Any locally excess quantities of sulphur can condense on the coal, which although it is preheated is relatively cold compared with the sulphur vapour, but they still react sufficiently rapidly. Expressed in the language of coking practice the dehydrogenization process is like a material of non-bituminous character being added to the coal. By this process the closest possible blending of the non-bituminous material with the coal is brought about in addition to a reduction of the volatile constituents and the increase of coke yield running in parallel therewith. As a result it is possible to produce, without any additions, hard, large-sized coke from highly volatile coals.

Claims (20)

1. A process for the preparation of coal for coking, comprising reducing the coal to small grains, preheating the small-grained coal to a temperature exceeding 1000C, and treating the small-grained coal with a dehydrogenization medium in a reaction chamber with sulphur as the dehydrogenization medium and removing the sulphuretted hydrogen produced from the reaction chamber.

2. A process as claimed in Claim 1 , wherein the preheating temperature exceeds 1400 C.

3. A process as claimed in Claim 1 or Claim 2, wherein the sulphur is used in a powder form.

4. A process as claimed in Claim 1 or Claim 2, wherein the sulphur is used in a vapour form.

5. A process as claimed in Claim 1 or Claim 2, wherein the sulphur is used in a gaseous form.

6. A process as claimed in any one of Claims 1 to 5, wherein the sulphur is added before preheating.

7. A process as claimed in any one of Claims 1 to 5, wherein the sulphur is added during preheating.

8. A process as claimed in any one of Claims 1 to 5, wherein the sulphur is added after preheating.

9. A process as claimed in any one of Claims 1 to 8, wherein the sulphur is brought into contact with the coal in a finely divided state.

10. A process as claimed in any one of Claims 1 to 8, wherein the sulphur is brought into contact with the coal in the form of vapour.

11. A process as claimed in Claim 10, wherein the vapourized sulphur is present in an inert gas.

12. A process as claimed in any one of Claims 1 to 11, wherein the coal is treated in a static condition and is percolated by the sulphur.

13. A process as claimed in any preceding Claim, wherein the coal is vibrated into a fluidized condition and the fluidized bed so formed is percolated by sulphur in the vapour state and/or by a stream of carrier gas carrying along finegrained sulphur powder.

14. A process as claimed in any preceding Claim, wherein the coal grain size is less than 3 mm.

1 5. A process as claimed in any preceding Claim, wherein the coal is preheated to a temperature of 1 400C to 2500C and the preheated coal is then treated with the sulphur.

1 6. A process as claimed in Claim 15, wherein the preheated coal is treated with sulphur in the form of vapour at a temperature in excess of 444.60C.

17. A process as claimed in any preceding Claim, wherein the sulphur is introduced into the coal in a quantity which, based on the coal, is 8 to 10% per 0.5% of hydrogen in the coal.

1 8. A process as claimed in any preceding Claim, wherein the sulphuretted hydrogen drawn off is reduced to sulphur and the sulphur obtained is used once more for dehydrogenization.

1 9. A process for the preparation of coal for coking substantially as hereinbefore described.

20. Coke produced by the method of any preceding Claim.