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August, 1951

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Coal is derived mainly from plant material which has undergone progressive decay over long periods of time and has been subjected to bacterial action and to changes of temperature and pressure.

Being of vegetable origin, coal can be considered as an organic material, but it also contains varying amounts of inorganic matter derived from the mineral matter of the original plants, from the shale, clay, pyrites, etc., of associated minerals, and possibly from infiltered waters. The mineral matter in coal which is in intimate association with the coal substance, and which is not removable by any ordinary cleaning process, is known as inherent mineral matter and includes that which was associated with the original plants. The mineral matter extraneous to the coal substance is known as adventitious.

Of the three main constituents of coal — carbon, hydrogen and oxygen — carbon predominates, being 75-95 per cent., depending on the type of coal. There is a progressive change from lignite, through bituminous coal to anthracite, with increasing percentage of carbon and decreasing percentage of oxygen.

When viewed by the naked eye, various distinct dull and bright bands can be observed in bituminous coal. Stopes¹ identified four bands, which were named vitrain, clarain, durain and fusain. Vitrain has a highly reflective surface, and in thin sections looked at under the microscope by transmitted light appears as a red jelly with only a very faint structure of the original plant cells. Clarain resembles vitrain to some extent, but has no conchoidal fracture, and is less reflective. Under the microscope, spores and plant remains can be seen in a clear transparent medium. Clarain is regarded as a mixture of vitrain and durain. Durain is in solid bands with a matte, even greyish, surface of close texture. It contains a large number of organised plant remains, and also inorganic sediment. Fusian is fibrous in appearance, with a surface sheen. It is very friable and resembles charcoal.

These different bands do not represent physical and chemical entities. Vitrain has some degree of homogeneity and is considered to be a fundamental part of the substance.

Little is known of the chemical constitution of the organic matter of coal. Indirect evidence shows that as coalification proceeds from lignite to anthracite, there is a progressive increase in aromatic structure. No substance of polycyclic structure has yet been isolated directly from coal.

Minerals associated with coal are classed as shales or clays, pyritic material, and carbonates. These occur as layers between the coal hands, as enclosures or nodules, and as intrusions at right angles to the bedding planes in the coal.

The minor constituents of coal fall into certain categories :—

1. Those in larger quantity and of special importance, such as nitrogen and sulphur.
2. Those in smaller quantity which may have a significant influence in the subsequent use of the coal, such as chlorine, phosphorus, fluorine and arsenic.
3. Trace elements, such as germanium, gallium and boron.

Nitrogen

Nitrogen is found in all coals and amounts to 1-2 per cent. It is derived mainly from the proteins or other nitrogen-containing compounds of the original plants, but a portion may come from animal proteins. No definite information is available of the form of the nitrogen compounds in coal. Solvent extraction is not very successful in removing nitrogen products. High-vacuum distillation at a low temperature² gives a small quantity of nitrogen bases. Several workers^{3 4} by heating coals at varying temperatures and comparing the behaviour of the nitrogen with that which occurs when heating known nitrogen compounds, have postulated the presence of an amino group or a substituted amino group in coal, but whether they exist as such in the original coal or, as is more probable, are secondary products produced by heating the coal is not known. Treatment with sulphuric acid⁵ has shown that 5 per cent. of the original nitrogen of the coal can be extracted; mono-amino acids and amides are present in the extracts. By oxidation by the Kjeldahl method⁶ nicotinic acid has been said to be isolated, giving evidence of a heterocyclic-ring system in coal. Oxidation experiments at the Fuel Research Station⁷ using alkaline hypobromite, showed the presence of a small quantity of 1 : 3 : 5 tribromo-aniline, indicating association of a part of the nitrogen with a carbocyclic ring. There still remains considerable work to be done before a definite answer can be given as to the constitution of the nitrogen compounds in coal.

When coal is carbonised, nitrogen is distributed among the products. The results obtained vary according to the temperature, the method of distillation, the type of coal, and the rate of heating. Nitrogen is first liberated as ammonia between 315° C. and 400° C. ; the evolution continues with rising temperature, up to about 8000 C., when the ammonia ceases. The percentage of the nitrogen of the coal evolved as ammonia is small, amounting to 11-17 per cent. ; the greatest portion is left in the coke and amounts to 50-80 per cent. The remainder is found in the tar as tar bases, or in the gas as hydrogen cyanide and nitrogen. Under certain conditions during carbonisation hydrogen can increase the proportion of nitrogen evolved as ammonia^{4 8}, but its action is limited. The nitrogen left in the coke is in a very stable form and can only be liberated as ammonia by gasification of the coke. It has been shown^{9 10} that the carbon-nitrogen ratio in the coke remains constant during gasification, indicating that the nitrogen is combined with the carbon and cannot be liberated without the carbon. Advantage is taken of this in the Mond system of gasification, in which air saturated with steam, is passed through the fuel bed. Yields of ammonia are obtained equivalent to 70 per cent. of the original nitrogen of the coal.

About 3 per cent, of the nitrogen of coal is found in the tar from carbonisation, mainly as bases such as pyridine, and 1-1.5 per cent. of the nitrogen is found as hydrogen cyanide in the gas, or as ammonium cyanide in the ammonia liquor.

Sulphur

The next most important element is sulphur. This occurs not only in the organic material but also in the inorganic material, or mineral matter, of coal.

It is important at this stage to distinguish between the mineral matter of coal and the ash of coal. Coal ash is the residue remaining after coal has been burnt, and is very different from the original mineral matter from which it was formed. When heated in air, the shales lose water, iron sulphide is converted to the oxide, the carbonates are converted to oxides with loss of carbon dioxide chlorine, fluorine and arsenic may be volatilised alone or in conjunction with other elements. In some cases the carbonates react with the sulphur dioxide from the pyrites to form sulphites and sulphates. It is for this reason that, to obtain comparable results, great care has to be taken in the determination of the ash of coal.

The inorganic sulphur of coal exists mainly as pyrites and marcasite, with some sulphate, usually calcium sulphate. The organic sulphur may be derived partly from the sulphur compounds of the original vegetation from which the coal is formed, and partly from the action of sulphur bacteria on iron sulphate and organic matter.

Attempts have been made to extract from coal organic compounds containing sulphur. Traces of mercaptans and disulphides have been found, but the main sulphur compounds cannot be extracted. There is some indication that the organic sulphur compounds and the nitrogen compounds are part of the fundamental composition of coal¹¹.

Iron pyrites is the main inorganic sulphur compound. It can occur as layers in the coal bed, as nodules and lumps, or as minute particles dispersed throughout the mass of coal. The pyrites may have been formed by the reduction of iron sulphate by organic matter. Another possible mode of formation is by the interaction of hydrogen sulphide, evolved during decay of the vegetable matter, with iron salts.

The total sulphur of British coal ranges from 0.5 to 4.0 per cent. If the sulphur is in the pyritic form in layers or nodules it may he possible to reduce the sulphur content of coal by mechanical cleaning, either in dry cleaners or wet washers.

Before 1939 there was a limited recovery of lump coal pyrites, amounting to 4,000 to 5,000 tons per annum. This was used for the production of sulphuric acid, but it only provided about 1 per cent. of the total production — a negligible amount. During the war there was a demand for an increased supply of sulphur-containing materials of home production suitable for the manufacture of sulphuric acid. In addition to increasing the amount of lump pyrites recovered, plant was installed at eight collieries¹² for the recovery of coal-pyrites fines. The production of coal pyrites was increased to 10,000 tons of lump and 25,000 tons of fines per annum, equivalent to 3 per cent. of the sulphuric-acid industry’s total requirements. One advantage of coal pyrites is that it is relatively free from arsenic.

When coal is burnt, the major portion of the sulphur is converted to oxides of sulphur, mainly sulphur dioxide. It is estimated that the total atmospheric pollution, calculated as sulphur dioxide, amounts to about 5 million tons a year, of which one-fifth is from domestic appliances.

The removal of oxides of sulphur from chimney gases involves complicated washing equipment and is very costly, and no marketable product is recovered. Plants were installed at two power stations in London — Battersea and Fulham — and for a time at Tir John in South Wales. The cost before the war was two to three shillings per ton of coal burnt, but to-day’s cost has been estimated at nearer ten shillings a ton. A cheaper process is required, one that would be applicable, not only to large boiler installations, but also to smaller ones, and one that would produce a marketable product, preferably sulphur.

Besides the damage caused by corrosion, and damage to materials by the oxides of sulphur emitted when coal is burnt, sulphur in the coal is also responsible for the formation of deposits on the generator tubes and superheaters of large boilers fired by mechanical stokers, at power stations. The bonded deposits consist of a matrix of sulphates or acid sulphates of sodium and potassium, in which are embedded particles of ash carried forward from the boiler fire. The inner layers of these deposits may contain 50 to 70 per cent. of acid sulphates, whereas the outer layer deposited under hotter conditions contains 20 to 40 per cent, of sulphates. Deposits on economiser tubes may contain up to 50 per cent. of sulphates. Further reference to these deposits will be made when chlorine, phosphorus, and boron are considered. In the cooler parts of the boiler, such as the air preheaters, acid deposits may occur which contain 50 per cent. sulphur trioxide as sulphates and sulphuric acid. Conditions may be so bad that tubes or plates of air heaters last only a few months before a replacement is required.

On carbonisation of coal, the sulphur is distributed between the coke, tar, ammonia liquor and gas. A typical distribution is :—

In	coke	60	per	cent.
,,	ammonia liquor as sulphide	10	,,	,,
,,	gas as H2S	27	,,	,,
,,	gas as CS2	2	,,	,,
		100

The main sulphur compound evolved on carbonisation is hydrogen sulphide, but other sulphur compounds in the gases are carbon disulphide, carbon oxysulphide, and thiophen. Carbon¹³ disulphide and carbon oxysulphide are secondary products. These residual sulphur compounds are objectionable in town gas as they form sulphur dioxide on combustion, causing corrosion of gas fittings, or sometimes causing a deleterious effect on processes which use gas heating, and where the products come in contact with the waste gases, as in steel and glass furnaces. Thiophen can be removed from gas by washing the gas with oil in scrubbers, or by use of active carbon. Where works have benzole-recovery plants, the thiophen content of the gas is reduced. Carbon disulphide and oxysulphide can be converted to hydrogen sulphide by catalysts¹⁴. Where complete benzole recovery is practised, about one-third of the organic sulphur compounds are removed in an oil-washing process, and about two-thirds in an active carbon process when the carbon is fresh.

Phosphorus

There are a number of elements in coal whose proportion is very variable, and yet they may have a profound influence on the use to which the coal is put. One instance is phosphorus. The amount present in coal is from 0.001 to 0.1 per cent. The source of the phosphorus is thought to be largely the associated minerals in the form of phosphates¹⁵, often in conjunction with fluorine, e.g., calcium fluorapatite 3 Ca₃ (P0₄)₂. CaF₂. When coal is burnt in boilers, phosphorus compounds can be volatilised and selective condensation in the boiler system can yield deposits containing 20 per cent. of phosphorus from coals containing only 0.05 per cent. of phosphorus. Work at the Fuel Research Station has shown that reducing conditions and temperatures higher than 1,600° C. are necessary for the volatilisation of "combined" phosphorus, so that where coals rich in phosphorus are burnt in certain types of stoking plant in boilers, these phosphate deposits may be found, particularly on generator, superheater and economiser tubes. Where coals rich in phosphorus are burnt in the pulverised form, the combustion approaches more nearly fully oxidising conditions and only loose dusts are formed. A phosphatic deposit, although not as common as a sulphatic one, is much more difficult to remove. It forms a hard shell which cannot be removed by mere water soaking as with sulphatic deposits, as the iron and aluminium phosphates, of which the deposit mainly consists, are insoluble in water. The loss of time caused in cleaning boilers at electricity generating stations may be considerable and under bad conditions due to deposits may amount to several months each year for each boiler.

The phosphorus, when coal is carbonised, is retained in the coke, and if the coke is used for metallurgical purposes, the phosphorus is found subsequently in the iron and steel, with detrimental effects. The phosphorus content of coke for certain metallurgical purposes is limited to under 0.02 per cent.

Drawings and Photographs accompanying the article