COAL Of all known fuels, coal (q. v.) is by far the most important. In spite of developments in the use of oil and water power it still provides three-quarters of the world's total energy supplies. Further, the reserves of coal, which according to recent estimates may be expected to last some 1,000 years at the present rate of consumption, are far above those of any other combustible. Even in America, the most important petroleum consuming country in the world, coal accounts for some 7o% of the energy produced, natural oil and natural gas for 25% and water power for only 5%.
Coal is a stratified mineral which has been formed by the action of decay, heat and pressure upon accumulations of vegetable, and woody, or cellulosic, matter laid down in bygone ages. It varies widely in composition and properties. While the proportions of the elements present in any particular variety of coal and its behaviour under various forms of treatment are known, the fundamental causes of the differences between the different types have not yet been placed upon a really rational basis. A knowl edge of the true constitution of coal, which is being sought by many research workers, could not fail to be of the greatest assist ance to those responsible for the proper utilization of a vast but irreplaceable store of fuel.
The chemical composition of solid fuels (moisture free) is given by Butterfield on p. 89o; it will be seen that the transition from cellulose to anthracite is marked by an elimination of oxygen, and a corresponding increase of carbon.
Regnault and Gruner were among the first to formulate methods for the classification of coal, which they grouped according to its flaming characteristics and the nature of the coke residue. S. W. Parr of the University of Illinois has proposed a classifica tion based upon the heat value of the true coal substance "Unit Coal," free from ash or sulphur, which is given by the formula Total B.Th.U. — 5000 S where S and A are the percentages ofI — (1.08A+0.55 S) sulphur and ash respectively. One of the best methods of classi fication is that of Seyler, in which coals are divided according to their hydrogen content into five groups which are further sub divided according to the carbon content. This classification has been used by the British Geological Survey in publications on the coals of South Wales.
For carbonization purposes one of the most important prop erties of bituminous coals is their ability to form coherent coke and the extent to which they swell or shrink during this process. This behaviour forms the basis of a useful subclassification of such coals. Illingworth and others have classed coals according to the temperatures at which certain constituents decompose.
In 1920 the British Government Fuel Research Board, which is responsible for the Physical and Chemical Survey of the National Coal Resources, appointed a committee to examine methods of analysis, including (I) proximate and (2) ultimate analysis, (3 ) determination of caking index and (4) measurement of calorific value. The following methods of analysis have been adopted: (1) Proximate Analysis. This consists of the determination of moisture, ash, volatile matter and fixed carbon in a sample of coal ground to pass a standard sieve of 6o meshes to the inch and air dried.
(a) Moisture:—This is given by the loss in weight of one to two grammes of coal, when heated for one hour at a temperature of 105-110° C. Coals specially liable to oxidation should be heated in a current of dry nitrogen.
(b) Ash:-1 to 2gm. of the coal in a platinum or silica dish is heated gradually in air to about Boo° C. When combustion is com plete the residue is cooled and weighed.
(c) Volatile Matter:—igm. of the coal is heated for 7 minutes at a temperature of about 925° C in a platinum crucible of special shape closed by a well fitting lid. The loss of weight is taken conventionally to represent the volatile matter.
Fixed Carbon:—This value is obtained by subtracting the sum of the percentages of ash, volatile matter and moisture from loo.
(a) Ultimate Analysis. This consists of the determination of the proportions of the various constituent elements of the coal. The carbon and hydrogen are determined by the combustion of 0.2 of a gram of coal in a current of oxygen. The products of combustion are passed over copper oxide at a temperature of 800° C and then over granular lead chromate at a temperature of 600° C to absorb sulphur compounds. The carbon dioxide and water produced are weighed separately and from these weights are calculated the percentages of carbon and hydrogen in the coal. Nitrogen is determined by digesting one gram of coal with sulphuric acid according to the method of Kjeldahl. The nitrogen is thereby changed into ammonia which after distillation is titrated with standard solutions. The sulphur is determined by conversion into sulphate by heating the coal gently with a fusion mixture of lime and magnesia ("Eschka's method").
(3) Caking Index. This is used as a rough measure of the power which a coal possesses of being converted by heat in the absence of air into a coherent mass of coke. It is obtained by heating a mixture of coal and sand in various proportions in a covered cruci ble, the caking index being the maximum proportion of sand to coal which allows of the residue being strong enough to bear a 5oogm. weight with the production of not more than 5% of loose powder.
In practice the heat obtainable from a fuel is always less than in theory, since losses due to imperfect combustion and to heat carried away in the flue gases, in clinkers and in ashes, can never be entirely avoided. Further, in fuels containing hydrogen the calorific value measured in the calorimeter and used generally for scientific purposes (the "gross" calorific value) is higher than that obtainable under working conditions (the "net" calorific value) by an amount equal to the latent heat of vaporization of the water formed. Examples of the magnitude of the differences are : Calorific Value (B.Th.U. per pound) Gross Net Methane (CH4) . . . . . . 23,830 21,450 Acetylene (C2H2) . . . . . . 21,460 20,700 Hydrogen (H2) . . . . . . 57,790 Carbon Monoxide (CO) For coal containing 5% of hydrogen the difference between the gross and net calorific values is about 3%.
The more important methods of research which have been employed in investigating the constitution of coal may be briefly enumerated as (a) extraction by solvents; (b) action of reagents, e.g., controlled oxidation, hydrogenation, chlorination, methyla tion, etc.; (c) carefully regulated destructive distillation; (d) microscopic examination of thin sections or etched surfaces; and (e) examination by X-rays. These methods have all contributed useful information.
One of the most recent investigations has been to determine the effect of hydrogen under high pressure upon the coal substance, by which it has been found possible to confer caking properties on coals which are normally non-caking. Even anthracite has yielded to this treatment, while the substance of certain types of coal has been converted into a mineral oil. By the extraction of coal with benzene under high pressure, F. Fischer in Germany and W. A. Bone in England have been able to separate the con stituents in which the caking properties of a coal appear to reside. Similar results were obtained by S. W. Parr in America by the use of phenol and xylene as solvents. Bone obtained a consider able yield of mellitic and other benzene carboxylic acids in the oxidation products of the residue from the benzene extraction, and concluded that a considerable proportion of the coal substance possesses a six carbon ring structure, each carbon of the ring being connected to other carbon atoms.
According to R. V. Wheeler bituminous coal consists essentially of insoluble ulmins in which organized plant tissues are dispersed. By mild oxidation, e.g., with hydrogen peroxide or with air at I oo–I 50° C the ulmins are rendered soluble in alkali and may thus be separated from the organized plant remains. Fossil plant cuticles and other tissues have been recognized in this residue. The ulmins when oxidized by dilute nitric acid yield oxalic, succinic, picric and pyromellitic acids, indicating that the ulmin molecules consist of benzenoid groupings linked together by such structures as pyrol and furan or their derivatives. The results of the destructive distillation of coal have proved difficult of inter pretation from the point of view of the constitution of the coal substance, owing to overlapping of the various decomposition processes taking place. Examination of the oils produced by hydrogenation of coal seems to confirm the views of Bone and of Wheeler on the six carbon ring structure of the coal substance. By microscopic study of thin sections Dr. Marie Stopes has identified four main ingredients in British banded bituminous coal which she has designated vitrain, clarain, durain and fusain. Thiessen on the other hand considers that American coals contain three main ingredients which he terms mother of coal, attritus and anthraxylon. C. A. Seyler has applied the methods of metallog raphy to the microscopical examination of coal surfaces and has been able thereby to identify directly many forms of plant tissue in the coal.
The structural basis of wood is cellulose, of which the simplest form may be taken as The composition of wood tissue has been expressed by Schultze and Schappe as "an aggregate of cellulose and a lignone complex" namely 5 or, in percentage composition, C =49-66, H=5.74, 0=44.6o. The water content of green wood ranges from 5o% to over Z00% of the weight of dry fibre ; in other words wood may contain as much as two parts of water to I part of fibre, according to species, the position in the tree and age. On air-drying this is reduced to some 15% to 20%, or still further by artificial drying; dried wood, however, may under certain conditions re-absorb moisture.
Both the inflammability and the calorific value of wood are greater in the soft resinous varieties such as pine, fir and spruce, than in hard woods like oak or elm, the calorific value after air drying varying from 6,50o to 9,00o B.Th.U. per lb. The tem peratures attained, however, are comparatively low owing to the high moisture and hydrogen content. On this account raw wood is unsuitable for metallurgical operations, but before the days of coke manufacture large quantities of wood charcoal were used in the iron industry.
Peat varies from a light spongy material mainly composed of sphagnum moss in the upper layers to a dense brown more humified substance at the bottom of thick bogs. In its natural state it contains from go% to 95% of water but by draining this may be reduced to 88% or 91%. The "water ratio" (the ratio of water to dry peat substance) of peat containing 95% water is 19:1, that of peat containing 90% water 9:1: the latter will thus contain twice as much solid matter although its water content has only been reduced by 5%. The moisture may be reduced by (a) air-drying; (b) evaporation by heat with partial heat re covery; (c) mechanical pressure; or (d) electrical osmosis. Many methods involving combinations of two or more of these have been tried, but as yet air-drying is used in all schemes working on a commercial basis. By air-drying the water may be brought down to about 25%, the calorific value then being about 7,00o B.Th.U. Peat thoroughly dried by heat takes up about 16% of water on exposure to air; it is thus useless to dry it artificially below this limit. The density of air-dried cut peat is about o.5, that of macerated machine-cut peat 0.85 to 1•0. For industrial purposes I tons of air-dried peat is about equivalent to a ton of coal, but owing to its lower density nearly 32 times the storage space is required.
The ash in peat varies considerably, say from 1% to 8%, usually increasing with depth. The nitrogen likewise increases with the depth from I% to 2%. The proximate analysis of a good Irish peat, air-dried, may be taken as : moisture, 20.2; organic volatile matter, 49-5; fixed carbon, 26.8; and ash, 3.4. Peat burns readily with a smoky flame and a characteristic odour. The ash is powdery and light except in certain varieties high in ash through the inclusion of sand, etc. Peat is largely used for domestic pur poses and forms a fuel suitable for boiler firing either in the briquetted or pulverized form ; it has also been used in gas pro ducers, and the coke from carbonized peat forms a suitable fuel for small producers such as are sometimes used for motor trans port purposes.
Lignite is intermediate in its properties between peat and bi tuminous coal, containing when dry some 6o to 75% of carbon and a variable proportion of ash. Raw lignite conforms to two types, brown or amorphous and black or pitch-like; it is char acterized by a high water content which may amount to as much as 6o% in the brown varieties. On weathering a proportion of this moisture is given up, and a disintegration or crumbling of the material occurs which reduces its value as a fuel. Lignite tends also to disintegrate during combustion ; hence the losses through the grate are relatively high. It does not store well, it is un economic to transport long distances and it is liable to spon taneous combustion. Against these drawbacks, many of the beds lie close to the surface and are of great thickness, sometimes over roof t. ; they are, therefore, easily worked, and the cost of pro duction is low.
The U.S. Geological Survey restricts the term lignite to brown coals of woody or amorphous structure, classifying the bituminous or black lignites as sub-bituminous coal. The latter, however, retain some of the more characteristic features of lignites; in particular, a high moisture content-usually averaging from r o to 25%-and a tendency to disintegration on air-drying which differentiates them from true bituminous coals.
Recently the possibility of producing economically a third type of coke by carbonization at about 600° C has attracted consid erable attention. This "low temperature coke" retains some 8% or 9% of volatiles compared with only r % or 2% in gas or metal lurgical coke, and so can be ignited almost as easily as coal and burns freely in open grates, producing a pleasing glowing fire. It thus forms an excellent smokeless substitute for household coal; but it is lighter and more friable than ordinary cokes. The gas yield is less from low temperature carbonization than from high but the tar yield is increased.
From one ton of coal the yields of coke are approximately as follows : Metallurgical by-product coke . . 13 -14 cwt.
Gas coke . . cwt.
Low temperature coke . . . . . . 14 -15 cwt. Since the heating of the ovens is usually effected by producer gas made from a proportion of the small coke, the amount of coke available for sale will be less than these figures.
The following analyses are typical of cokes produced by the three methods under conditions of proper quenching. If the coke has been badly quenched or stored in the open the moisture content will be much greater.
Low Metallurgical Gas coke Ternpera coke tare cokeProximate Analysis Horizontal Vertical Moisture . . . 0.7 0.9 0.6 2.o Volatile matter . . 2.6 2.9 3.5 7.5 Fixed carbon . . 88.2 86-5 86.4 80.o Ash . . . 8.5 Ultimate Analysis (dry coke) C . . 88•o 85.8 85.4 78.7 H . . . . o•5 o•6 o•8 2.5 N . . . . I.o 1.2 1.2 1.5 S . . . . 0.9 I.9 I.8 I.0 O . . . . . 0.9 0.6 I.o 5.6 Ash . . . . 8.7 9'9 10•7 Structure of Coke.-Recent work has shown that the physical characteristics of coke have an important bearing upon its suitabil ity for different uses. The cell structure of coke appears to be due to the formation of bubbles during the fusion of certain por tions of the coal substance and their subsequent perforation. Its exact nature depends largely upon the conditions of heating, the type of coal dealt with, and the fineness to which it is ground prior to carbonization. Thus the dense coke made from finely divided coal of suitable coking properties consists of minute cells of uniform structure ; but dense cokes can be produced even from strongly swelling coals by blending or by pressure during carbon ization, usually effected by preventing expansion by walls, etc.
In addition to the chemical tests already described which are applicable to any solid fuel, certain special tests have been devised for the investigation of other properties of coke. Those most usually relied upon are determinations of specific gravity (apparent and true), and porosity. Strength is measured by the "shatter test," in which 5olb. of gin. screened coke are dropped four times from a height of 6f t. on to an iron plate and the propor tion broken determined. A limit of r% has been adopted as the maximum allowable sulphur content in blast furnace coke. Reac tivity to carbon dioxide is also of importance, and is receiving much attention by research workers.
Charcoal is a brittle and porous material retaining the original shape of the wood while its microstructure preserves the vegetable cell structure. By far the greater portion of the world's s&pply of charcoal is used for heating and cooking. Other uses are for the manufacture of gunpowder, absorbent and decolourizing agents, heat insulating materials and for hop drying, case-harden ing steel, etc. As a fuel it is frequently used in gas producers. Good charcoal is deep black in colour and breaks with a bright fracture. It rings when struck and burns without smoke. In the dry state the apparent specific gravity varies from about • I i for a soft wood to .20 for a hard wood. The true specific gravity of charcoal substance varies between i and 2. The carbon content increases with the temperature at which it is prepared, being about 65% for 250° C, 96% for 1,500° C; the loss of weight involved at the high temperatures however, is very serious, the percentage yield falling from about 50% at 25o° C to 15% or less at 1,500° C. Charcoal is comparatively low in ash and varies from 12,000 to 14,000 B.Th.U. in calorific value. The charcoals prepared at low temperatures are exceedingly inflammable, imperfectly charred pieces often being used in Europe for kindling purposes.
Briquetting forms a means of utilizing quantities of small coal, which, though of good quality, is unsaleable, or of coals which are deficient in caking power and are therefore unsuitable for coke making or for burning in the fine state. Special processes are re quired for absolutely non-coking materials, such as anthracite duff, since these disintegrate on the fire through the melting out and burning of the pitch. Germany occupies the premier place in the briquetting industry, using mainly brown coal ; but briquetting is also carried on in France, Austria, the United States and \Vales. Coal briquettes are used on railways and steamers, and for in dustrial purposes; but lignite briquettes which do not stand trans port well are chiefly used as household fuel in the neighbourhood of their manufacture.
Briquettes are usually of rectangular shape varying in weight up to about 281b., but smaller egg-shaped or ovoid briquettes are produced in roll presses for domestic purposes. They should be uniform in composition and weight, of high calorific value, strong, waterproof and able to bear transport or storage without disinte gration. The density should be about 1.3, the same as that of the lump coal. The ash content where pitch or a similar binder is used is less than that of the original coal, which should be small; but when inorganic binders such as clay or lime are adopted the non-combustible constituents of the binder are added to the ash of the coal. The average ash content of a commercial briquette of good quality is about 7%. The calorific value is affected by the binder, but in pitch briquettes the calorific value is slightly greater than that of the raw coal.
Briquetting under pressure either at ordinary temperatures or at temperatures high enough to cause incipient fusion of some of the constituents, is sometimes adopted as a preliminary process in the carbonization of blends of finely ground coal. It not only aids the production of a homogeneous fuel, but simplifies the retorting process.
Similar investigations are in train in other countries.
Substantial economies in the use of coal might be effected by an improved distribution of the various available grades and to this end a wider adoption of the method of buying to specification would be advantageous. It is not easy to lay down suitable speci fications for widely differing purposes; but in Great Britain gas is now sold on a thermal basis and great interest is being taken in the feasibility of selling coal by some analogous method which would take account not only of calorific value, but also of other prop erties.
Since the World War the world's annual production has remained more or less stationary at 1,200 million tons, a figure roughly the same as the maximum reached in the period immediately preced ing the war. By far the greater proportion of this coal is of the bituminous type, anthracite and lignite each accounting for approximately Io% of the total. The production of lignite is, however, increasing very rapidly.
Data are not available to show accurately the proportions of coal which are used for different purposes but the following per centage figures give an estimate of distribution, and at the same time show how prevalent is the burning of coal in its raw state:— Railways, 20%; Domestic Consumers, 15%; Metallurgical Coke, 15%; Electricity Production, 5%; Gas Manufacture, 5%; and Industrial and other Uses, 40%.
In a similar manner the percentage of carbon dioxide in the flue gases corresponding to the theoretical proportion of air can be calculated. The equations, however, tell us nothing regarding the manner in which gases are given off from the raw material and interact among themselves and with oxygen and water vapour before the final products of complete combustion are formed. A complex mixture is involved containing simple gases such as hy drogen and carbon monoxide, in addition to molecules of greater complexity which are less directly acted upon by oxygen than the simpler ones. The combustion of the heavier hydrocarbons pre sents great difficulty since the action of heat causes them to form still denser aggregates. Even the combustion of such a compara tively simple gas as methane has been shown by Bone to proceed in stages involving the entry of successive atoms of oxy gen. Methyl alcohol is first formed, but this is subsequently oxi dized to an unstable compound which breaks up into formalde hyde and steam. Under certain conditions of slow combustion further stages are involved before complete oxidation is effected, but under high temperature conditions the formaldehyde is de composed into carbon monoxide and hydrogen, which then burn directly to carbon dioxide and steam.
Combustion of Carbon in the Lower Layers of a Furnace. —It is not definitely known whether solid carbon burns directly to carbon dioxide or whether it proceeds first to carbon monoxide and so to Three possible reactions are involved : The older theory which is still widely quoted is that the carbon is first burnt to which is subsequently reduced to carbon monoxide by contact with heated coke.
By circulating air or oxygen at temperatures from loo° C to 900° C over purified wood charcoal Rhead and Wheeler were led to the conclusion that and CO are formed simultaneously from an unstable "physicochemical complex" C„O,, which is the first product of the action of oxygen on carbon. Langmuir's ex periments appear to show that the first gaseous product of the interaction between carbon and oxygen is carbon dioxide, as he was unable to detect the presence of carbon monoxide. On pass ing upwards the two oxides in the presence of carbon react one upon the other in accordance with the reversible reaction C---CO2 ±2CO, the final products being formed in definite relative pro portions which depend upon the conditions, such as temperature and concentration of the various gases present, and also upon whether sufficient time has elapsed for equilibrium to be estab lished. In the reaction quoted above, equilibrium is established at 85o° C, when the percentages of CO, and CO are 6.23 and 93.77 respectively; at 1,200° C when they are 0.06 and 99.94. Under furnace conditions the gas leaving the incandescent bed will thus be almost entirely composed of carbon monoxide. With coal fir ing this carbon monoxide is consumed together with the gases of distillation by the secondary air, but even a coke fire, although smokeless, may give off carbon monoxide if the supply of sec ondary air should be deficient or badly distributed.
Distillation takes place in the upper layers of an orr:nary fire, and in order to burn the gas and smoke "secondary air" must be supplied at some point above the fire bed. The solid coke burns in the lower portions of the fire, "primary air" for this purpose pass ing through the bed from under the grate. Thus combustion both of solid carbon and of volatile matter is involved. Special devices are sometimes used to force the volatile products through the in candescent coke; for example, by down draught in which the flue gases are drawn away below the grate, or by downward combus tion in which the raw fuel is introduced at the bottom of the fire. In practice air in excess of that theoretically necessary for com plete combustion is always required. In good boiler practice 5o% of excess air is about the minimum amount necessary to ensure absence of smoke and fairly complete combustion. Too much air is undesirable, since by cooling down the gases it may retard co?- . bustion and aggravate the production of black smoke, and will increase heat losses. The best furnace conditions are obtainable with mechanical methods of continuous firing in which the air supply is under more exact control than can be obtained when the firing door has to be opened at frequent intervals for hand charging.
The ores used may be either oxides or carbonates of iron. The reactions occurring are complex; in the lower portion of the fur nace near the tuyeres the fuel is burnt by the blast to carbon monoxide, which effects the reduction of the ore in the upper portion of the shaft, where spongy iron impregnated with carbon and carbon monoxide is produced by a series of reactions of which the following is the resultant 2Fe+3CO3. This spongy iron is melted as it passes through the lower layers and can be drawn off from time to time through tapping holes. Lime stone is added to the charge in order to assist in fluxing.
Blast furnace practice has been greatly influenced by the work of Sir Lowthian Bell, who in 1872 applied the laws of mass action to the reactions in the blast furnace, and came to the conclusion that the practical limit for the ratio of CO to CO, in the gases leaving the furnace could not be reduced below 2•0. In modern blast furnaces a ratio of 1.7 has been reached, but in average practice a ratio of 3.o is more common. Bell's argument, however, remains valid, although imperfections in scientific measurement affected the accuracy of his calculation. It will thus be seen that fuel economy in iron and steel practice depends essentially upon finding uses for the combustible gases (10o B.Th.U.) which are necessarily rejected in large quantities from the shaft. In modern furnaces these may be used either for heating the blast, or for power and heat requirements in the subsequent treatment of the pig iron. Concentration of coke ovens, blast furnaces and steel making in the same works enables the fuel consumption per ton of finished steel to be reduced to a minimum, from 3o to 33cwt. or less of coal per ton of finished steel.