Part 29

Coal is never definitely crystalline, the nearest approach to such a structure being a compound fibrous grouping resembling that of gypsum or arragonite, which occurs in some of the steam coals of South Wales, and is locally known as "cone in cone," but no definite form or arrangement can be made out of the fibres. Usually it occurs in compact beds of alternating bright and dark bands in which impressions of leaves, woody fibre and other vegetable remains are commonly found. There is generally a tendency in coals towards cleaving into cubical or prismatic blocks, but sometimes the cohesion between the particles is so feeble that the mass breaks up into dust when struck. These peculiarities of structure may vary very considerably within small areas; and the position of the divisional planes or cleats with reference to the mass, and the proportion of small coal or slack to the larger fragments when the coal is broken up by cutting-tools, are points of great importance in the working of coal on a large scale.

The divisional planes often contain small films of other minerals, the commonest being calcite, gypsum and iron pyrites, but in some cases zeolitic minerals and galena have been observed. Salt, in the form of brine, is sometimes present in coal. Hydrocarbons, such as petroleum, bitumen, paraffin, &c., are also found occasionally in coal, but more generally in the associated sandstones and limestones of the Carboniferous formation. Gases, consisting principally of light carburetted hydrogen or marsh gas, are often present in considerable quantity in coal, in a dissolved or occluded state, and the evolution of these upon exposure to the air, especially when a sudden diminution of atmospheric pressure takes place, constitutes one of the most formidable dangers that the coal miner has to encounter.

Classification.

Anthracite.

The classification of the different kinds of coal may be considered from various points of view, such as their chemical composition, their behaviour when subjected to heat or when burnt, and their geological position and origin. They all contain carbon, hydrogen, oxygen and nitrogen, forming the carbonaceous or combustible portion, and some quantity of mineral matter, which remains after combustion as a residue or "ash." As the amount of ash varies very considerably in different coals, and stands in no relation to the proportion of the other constituents, it is necessary in forming a chemical classification to compute the results of analysis after deduction of the ash and hygroscopic water. Examples of analyses treated in this manner are furnished in the last column of Table I., from which it will be seen that the nearest approach to pure carbon is furnished by anthracite, which contains above 90%. This class of coal burns with a very small amount of flame, producing intense local heat and no smoke. It is especially used for drying hops and malt, and in blast furnaces where a high temperature is required, but it is not suited for reverberatory furnaces.

Bituminous coals.

The most important class of coals is that generally known as bituminous, from their property of softening or undergoing an apparent fusion when heated to a temperature far below that at which actual combustion takes place. This term is founded on a misapprehension of the nature of the occurrence, since, although the softening takes place at a low temperature, still it marks the point at which destructive distillation commences, and hydrocarbons both of a solid and gaseous character are formed. That nothing analogous to bitumen exists in coals is proved by the fact that the ordinary solvents for bituminous substances, such as bisulphide of carbon and benzol, have no effect upon them, as would be the case if they contained bitumen soluble in these re-agents. The term is, however, a convenient one, and one whose use is almost a necessity, from its having an almost universal currency among coal miners. The proportion of carbon in bituminous coals may vary from 80 to 90%--the amount being highest as they approach the character of anthracite, and least in those which are nearest to lignites. The amount of hydrogen is from 4½ to 6%, while the oxygen may vary within much wider limits, or from about 3 to 14%. These variations in composition are attended with corresponding differences in qualities, which are distinguished by special names. Thus the semi-anthracitic coals of South Wales are known as "dry" or "steam coals," being especially valuable for use in marine steam-boilers, as they burn more readily than anthracite and with a larger amount of flame, while giving out a great amount of heat, and practically without producing smoke. Coals richer in hydrogen, on the other hand, are more useful for burning in open fires--smiths' forges and furnaces--where a long flame is required.

Gas coal.

The excess of hydrogen in a coal, above the amount necessary to combine with its oxygen to form water, is known as "disposable" hydrogen, and is a measure of the fitness of the coal for use in gas-making. This excess is greatest in what is known as cannel coal, the Lancashire kennel or candle coal, so named from the bright light it gives out when burning. This, although of very small value as fuel, commands a specially high price for gas-making. Cannel is more compact and duller than ordinary coal, and can be wrought in the lathe and polished.

TABLE I.--_Elementary Composition of Coal_ (the figures denote the amounts per cent).

+----------------------------------------------------------------------------------------+----------------------+ | | Composition | | | exclusive of Water, | | | Sulphur and Ash. | +----------------------------+--------+-------+------+-------+------+------+------+------+-------+------+-------+ | |Specific| |Hydro-| |Nitro-| Sul- | | | |Hydro-| O. | | Localities. |Gravity.|Carbon.| gen. |Oxygen.| gen. | phur.| Ash. |Water.|Carbon.| gen. | and N.| +----------------------------+--------+-------+------+-------+------+------+------+------+-------+------+-------+ |_Anthracite._ | | | | | | | | | | | | | 1. South Wales | 1.392 | 90.39 | 3.28 | 2.98 | 0.83 | 0.91 | 1.61 | 2.00 | 93.54 | 3.39 | 3.82 | | 2. Pennsylvania | 1.462 | 90.45 | 2.43 | 2.45 | .. | .. | 4.67 | .. | 94.89 | 2.54 | 2.57 | | 3. Peru | .. | 82.70 | 1.41 | 0.85 |10.35 | 3.75 | 0.94 | 97.34 | 1.66 | 1.00 | +----------------------------+--------+-------+------+-------+------+------+------+------+-------+------+-------+ |_Bituminous Steam and Coking Coal._ | | | | | | | | | | | | 4. Risca, South Wales | | 75.49 | 4.73 | 6.78 | 1.21 |10.67 | 1.12 | 86.78 | 5.43 | 7.79 | | 5. Aberdare, " | .. | 86.80 | 4.25 | 3.06 | 0.83 | 4.40 | 0.66 | 92.24 | 4.51 | 3.25 | | 6. Hartley, Northumberl'd | .. | 78.65 | 4.65 | 13.36 | 0.55 | 2.49 | .. | 80.67 | 4.76 | 14.5 | | 7. Dudley, Staffordshire | 1.278 | 78.57 | 5.29 | 12.88 | 1.84 | 0.39 | 1.03 | 1.13 | 79.70 | 5.37 | 14.9 | | 8. Stranitzen, Styria | .. | 79.90 | 4.85 | 12.75 | 0.64 | 0.20 | 1.66 | .. | 81.45 | 4.92 | 13.63 | +----------------------------+--------+-------+------+-------+------+------+------+------+-------+------+-------+ |_Cannel or Gas Coal._ | | | | | | | | | | | | | 9. Wigan, Lancashire | 1.276 | 80.07 | 5.53 | 8.08 | 2.12 | 1.50 | 2.70 | 0.91 | 85.48 | 5.90 | 8.62 | |10. Boghead, Scotland | .. | 63.10 | 8.91 | 7.25 | 0.96 |19.78 | .. | 79.61 |11.24 | 9.15 | |11. (Albertite) Nova Scotia | .. | 82.67 | 9.14 | 8.19 | .. | .. | .. | 82.67 | 9.14 | 8.19 | |12. (Tasmanite) Tasmania | 1.18 | 79.34 |10.41 | 4.93 | 5.32 | .. | .. | 83.80 |10.99 | 5.21 | +----------------------------+--------+-------+------+-------+------+------+------+------+-------+------+-------+ |_Lignite and Brown Coal._ | | | | | | | | | | | | |13. Cologne | 1.100 | 63.29 | 4.98 | 26.24 | .. | 8.49 | .. | 66.97 | 5.27 | 27.76 | |14. Bovey Tracy, Devonshire | .. | 66.31 | 5.63 | 22.86 | 0.57 | 2.36 | 2.36 | .. | 69.53 | 5.90 | 24.57 | |15. Trifail, Styria | .. | 50.72 | 5.34 | 33.18 | 2.80 | 0.90 | 7.86 | .. | 55.11 | 5.80 | 39.09 | +----------------------------+--------+-------+------+-------+------+------+------+------+-------+------+-------+

These properties are most highly developed in the substance known as jet, which is a variety of cannel found in the lower oolitic strata of Yorkshire, and is almost entirely used for ornamental purposes, the whole quantity produced near Whitby, together with a further supply from Spain, being manufactured into articles of jewellery at that town.

Caking coals.

When coal is heated to redness out of contact with the air, the more volatile constituents, water, hydrogen, oxygen, and nitrogen are in great part expelled, a portion of the carbon being also volatilized in the form of hydrocarbons and carbonic oxide,--the greater part, however, remaining behind, together with all the mineral matter or ash, in the form of coke, or, as it is also called, "fixed carbon." The proportion of this residue is greatest in the more anthracitic or drier coals, but a more valuable product is yielded by those richer in hydrogen. Very important distinctions--those of caking or non-caking--are founded on the behaviour of coals when subjected to the process of coking. The former class undergo an incipient fusion or softening when heated, so that the fragments coalesce and yield a compact coke, while the latter (also called free-burning) preserve their form, producing a coke which is only serviceable when made from large pieces of coal, the smaller pieces being incoherent and of no value. The caking property is best developed in coals low in oxygen with 25 to 30% of volatile matters. As a matter of experience, it is found that caking coals lose that property when exposed to the action of the air for a lengthened period, or by heating to about 300° C., and that the dust or slack of non-caking coal may, in some instances, be converted into a coherent coke by exposing it suddenly to a very high temperature, or compressing it strongly before charging it into the oven.

Lignite.

Lignite or brown coal includes all varieties which are intermediate in properties between wood and coals of the older formations. A coal of this kind is generally to be distinguished by its brown colour, either in mass or in the blacker varieties in the streak. The proportion of carbon is comparatively low, usually not exceeding 70%, while the oxygen and hygroscopic water are much higher than in true coals. The property of caking or yielding a coherent coke is usually absent, and the ash is often very high. The specific gravity is low when not brought up by an excessive amount of earthy matter. Sometimes it is almost pasty, and crumbles to powder when dried, so as to be susceptible of use as a pigment, forming the colour known as Cologne earth, which resembles umber or sepia. In Nassau and Bavaria woody structure is very common, and it is from this circumstance that the term lignite is derived. The best varieties are black and pitchy in lustre, or even bright and scarcely to be distinguished from true coals. These kinds are most common in Eastern Europe. Lignites, as a rule, are generally found in strata of a newer geological age, but there are many instances of perfect coals being found in such strata.

Ash of coal.

By the term "ash" is understood the mineral matter remaining unconsumed after the complete combustion of the carbonaceous portion of a coal. According to Couriot (_Annales de la société géologique de Belgique_, vol. xxiii. p. 105) the stratified character of the ash may be rendered apparent in an X-ray photograph of a piece of coal about an inch thick, when it appears in thin parallel bands, the combustible portion remaining transparent. It may also be rendered visible if a smooth block of free-burning coal is allowed to burn away quickly in an open fire, when the ash remains in thin grey or yellow bands on the surface of the block. The composition of the ashes of different coals is subject to considerable variation, as will be seen by Table II.

Sulphur in coal.

The composition of the ash of true coal approximates to that of a fire-clay, allowance being made for lime, which may be present either as carbonate or sulphate, and for sulphuric acid. Sulphur is derived mainly from iron pyrites, which yields sulphates by combustion. An indication of the character of the ash of a coal is afforded by its colour, white ash coals being generally freer from sulphur than those containing iron pyrites, which yield a red ash. There are, however, several striking exceptions, as for instance in the anthracite from Peru, given in Table I., which contains more than 10% of sulphur, and yields but a very small percentage of a white ash. In this coal, as well as in the lignite of Tasmania, known as white coal or Tasmanite, the sulphur occurs in organic combination, but is so firmly held that it can only be very partially expelled, even by exposure to a very high and continued heating out of contact with the air. An anthracite occurring in connexion with the old volcanic rocks of Arthur's Seat, Edinburgh, which contains a large amount of sulphur in proportion to the ash, has been found to behave in a similar manner. Under ordinary conditions, from 1/8 to ¼ of the whole amount of sulphur in a coal is volatilized during combustion, the remaining ¾ to 7/8 being found in the ash.

TABLE II.--_Composition of the Ashes of Coals._

+----------------------+-------+--------+--------+-------+---------+-------+---------+----------+--------+ | | | | Ferric | | | |Sulphuric|Phosphoric| | | |Silica.|Alumina.| Oxide. | Lime. |Magnesia.|Potash.| Acid. | Acid. | Total. | +----------------------+-------+--------+--------+-------+---------+-------+---------+----------+--------+ | True Coals. | | | | | | | | | | | Dowlais, South Wales | 39.64 | 39.20 | 11.84 | 1.81 | 2.58 | .. | .. | 3.01 | 98.08 | | Ebbw Vale, " | 53.00 | 35.01 | .. | 3.94 | 2.20 | .. | 4.89 | 0.88 | 99.92 | | Königsgrube, Silesia | 55.41 | 18.95 | 16.06 | 3.21 | 1.87 | 2.05 | 1.73 | 0.36 | 99.64 | | Ohio | 44.60 | 41.10 | 7.40 | 3.61 | 1.28 | 1.82 | 0.59 | 0.29 | 100.69 | | | | | | | | | | | | | Lignites. | | | | | | | | | | | Helmstadt, Saxony | 17.27 | 11.57 | 5.57 | 23.67 | 2.58 | 2.64 | 33.83 | .. | 97.13 | | Edeléney, Hungary | 36.01 | 23.07 | 5.05 | 15.62 | 3.64 | 2.38 | 12.35 | .. | 98.12 | +----------------------+-------+--------+--------+-------+---------+-------+---------+----------+--------+

Water in coal.

The amount of water present in freshly raised coals varies very considerably. It is generally largest in lignites, which may sometimes contain 30% or even more, while in the coals of the coal measures it does not usually exceed from 5 to 10%. The loss of weight by exposure to the atmosphere from drying may be from ½ to ¾ of the total amount of water contained.

TABLE III.--_Composition of Fuels (assuming Carbon = 100)._

+------------------------------+-------+---------+---------+-----------+ | | | | |Disposable | | |Carbon.|Hydrogen.| Oxygen. | Hydrogen. | +------------------------------+-------+---------+---------+-----------+ | Wood | 100 | 12.18 | 83.07 | 1.80 | | Peat | 100 | 9.85 | 55.67 | 2.89 | | Lignite | 100 | 8.37 | 42.42 | 3.07 | | Thick Coal, S. Staffordshire | 100 | 6.12 | 21.23 | 3.47 | | Hartley Steam Coal | 100 | 5.91 | 18.32 | 3.62 | | South Wales Steam Coal | 100 | 4.75 | 5.28 | 4.09 | | American Anthracite | 100 | 2.84 | 1.74 | 2.63 | +------------------------------+-------+---------+---------+-----------+

Origin of Coal.

Coal is the result of the transformation of woody fibre and other vegetable matter by the elimination of oxygen and hydrogen in proportionally larger quantity than carbon, so that the percentage of the latter element is increased in the manner shown in Table III., given by J. Percy, the mineral matter being also changed by the removal of silica and alkalis and the substitution of substances analogous in composition to fire-clay. The causes and methods of these changes are, however, not very exactly defined. According to the elaborate researches of B. Renault (_Bulletin de la Société de l'Industrie minérale_, 3 ser. vol. xiii. p. 865), the agents of the transformation of cellulose into peaty substances are saprophytic fungi and bacterial ferments. As the former are only active in the air while the latter are anaerobic, the activity of either agent is conditioned by variation in the water level of the bog. The ultimate term of bacterial activity seems to be the production of ulmic acid, containing carbon 65.31 and hydrogen 3.85%, which is a powerful antiseptic. By the progressive elimination of oxygen and hydrogen, partly as water and partly as carbon dioxide and marsh gas, the ratios of carbon to oxygen and hydrogen in the rendered product increase in the following manner:--

C : H C : O Cellulose 7.2 0.9 Peat 9.8 1.8 Lignite, imperfect 12.2 2.4 " perfect 12.6 3.6

The resulting product is a brown pasty or gelatinous substance which binds the more resisting parts of the plants into a compact mass. The same observer considers Boghead coal, kerosene shale and similar substances used for the production of mineral oils to be mainly alteration products of gelatinous fresh water algae, which by a nearly complete elimination of oxygen have been changed to substances approximating in composition to C2H3 and C3H5, where C : H = 7.98 and C : O + N = 46.3. In cannel coals the prevailing constituents are the spores of cryptogamic plants, algae being rare or in many cases absent. By making very thin sections and employing high magnification (1000-1200 diameters), Renault has been enabled to detect numerous forms of bacilli in the woody parts preserved in coal, one of which, _Micrococcus carbo_, bears a strong resemblance to the living _Cladothrix_ found in trees buried in peat bogs. Clearer evidence of their occurrence has, however, been found in fragments of wood fossilized by silica or carbonate of lime which are sometimes met with in coal seams.

The subsequent change of peaty substance into coal is probably due to geological causes, i.e. chemical and physical processes similar to those that have converted ordinary sediments into rock masses. Such changes seem, however, to have been very rapidly accomplished, as pebbles of completely formed coal are commonly found in the sandstones and coarser sedimentary strata alternating with the coal seams in many coalfields.

The variation in the composition of coal seams in different parts of the same basin is a difficult matter to explain. It has been variously attributed to metamorphism, consequent upon igneous intrusion, earth movements and other kinds of geothermic action, greater or less loss of volatile constituents during the period of coaly transformation, conditioned by differences of permeability in the enclosing rocks, which is greater for sandstones than for argillaceous strata, and other causes; but none of these appears to be applicable over more than limited areas. According to L. Lemière, who has very fully reviewed the relation of composition to origin in coal seams (_Bulletin de la Société de l'Industrie minérale_, 4 ser. vol. iv. pp. 851 and 1299, vol. v. p. 273), differences in composition are mainly original, the denser and more anthracitic varieties representing plant substance which has been more completely macerated and deprived of its putrescible constituents before submergence, or of which the deposition had taken place in shallow water, more readily accessible to atmospheric oxidizing influences than the deeper areas where conditions favourable to the elaboration of compounds richer in hydrogen prevailed.

The conditions favourable to the production of coal seem therefore to have been--forest growth in swampy ground about the mouths of rivers, and rapid oscillation of level, the coal produced during subsidence being covered up by the sediment brought down by the river forming beds of sand or clay, which, on re-elevation, formed the soil for fresh growths, the alternation being occasionally broken by the deposit of purely marine beds. We might therefore expect to find coal wherever strata of estuarine origin are developed in great mass. This is actually the case; the Carboniferous, Cretaceous and Jurassic systems (qq.v.) contain coal-bearing strata though in unequal degrees,--the first being known as the Coal Measures proper, while the others are of small economic value in Great Britain, though more productive in workable coals on the continent of Europe. The Coal Measures which form part of the Palaeozoic or oldest of the three great geological divisions are mainly confined to the countries north of the equator. Mesozoic coals are more abundant in the southern hemisphere, while Tertiary coals seem to be tolerably uniformly distributed irrespective of latitude.

Sequences of carboniferous strata.