History. The nuisance created by coal smoke seems to have been recognized in London as early as the reign of Queen Elizabeth; but it is only in more modern times that the question has come to be regarded as one of real practical importance, and even yet it is far from receiving that general attention which it demands. In 1785 the first smoke-abating invention was patented by James Watt, who, as the inventor of the steam-engine, is responsible for so many boiler fires and so much consump-tion of coal. In 1815 Cutler patented the first would-be smokeless grate for domestic purposes; and his principle of feeding underneath was afterwards adopted by Dr Neil Arnott in a grate which has now been in use in one form or another for more than half a century. There is now a vast number of such inventions, good and bad. In 1819 the attention of parliament was directed to the question, and a select committee was appointed "to inquire how far persons using steam-engines and furnaces could erect them in a manner less prejudicial to public health and comfort." This committee gave an encouraging report. In 1843 another select committee recommended the introduction of a bill prohibiting the production of smoke from furnaces and steam-engines. In 1845 yet another select committee reported that such an Act could not in the existing state of affairs be made to apply to dwelling-houses. The Acts of 1845 and 1847 followed as the results of these inquiries; and since then there has been much legislation brought to bear on factories and railways. The results have been most beneficial; but very much still remains to be done. One is apt to think that, because steam-engines and fac-tories consume individually much more coal than dwelling-houses, they alone are responsible for the smoke nuisance, forgetting how greatly the dwelling-houses outnumber the factories. In reality there is little doubt that domestic fires are mainly responsible for the smoky condition of the atmosphere of our towns; and they for the most part continue to evolve smoke undeterred by legislation or scientific invention. In 1881, however, a movement was commenced by the National Health Society and the Kyrle Society, which resulted in a great smoke-abatement exhi-bition being held at South Kensington. At the close of the exhibition a national smoke-abatement institution, with offices in London, was incorporated by authority of the Board of Trade.
Combustion of coal. A knowledge of the nature of coal and of the chemical changes that it undergoes when burnt is essential for an understanding of the smoke problem. More detailed in-formation on these points is given under COAL, where the several varieties are described. For the purposes of this article coals may be classified as smoke-producing and smokeless, the former including all those varieties most commonly used as fuel. The elementary constituents of such coals are carbon (generally about 80 per cent. of the whole), hydrogen, nitrogen, oxygen, and sulphur; and they also contain a varying quantity of earthy impurity or ash. The process which occurs in a coal fire consists of two dis-tinct operations. The first, which requires a comparatively low temperature and is independent of the presence of air, is one of destructive distillation, and is similar to that which occurs in the retorts of gasworks. It results in the decom-position of the coal, and in the rearrangement of its con-stituent elements and the formation of the following substances:(1) hydrogen, marsh gas, carbonic oxide, olefiant gas, benzine, other hydrocarbons of the type of marsh gas or of benzine, water,all of which are either gaseous at the temperature at which they are formed or capable of being converted into gas at somewhat higher temperatures, and all of which are combustible except the water; (2) ammonia and other compounds of nitrogen, and certain compounds of sulphur, which are also volatile and combustible; (3) coke, which consists of carbon (and ash) and is non--volatile but combustible. It is these products of distilla-tion, not the coal itself, that burn, in the strict sense of the word; and this second process requires the presence of air and also a much higher temperature than the first. If the combustion is perfect, the only products are (1) water-vapour, (2) carbonic acid, (3) nitrogen, and (4) sulphurous acid, the first of which contains all the hydro-gen originally present in the coal, the second all the carbon, the fourth all the sulphur, while the nitrogen is liberated as such together with the very much larger volumes of nitrogen derived from the air which has supplied the necessary oxygen. All these products of com-bustion are discharged through the chimney.
Two things are necessary for the ensuring of such complete combustion, viz., an adequate, but not too large, supply of air, properly administered, and the maintenance of the requisite temperature. In practice, however, these conditions are never perfectly fulfilled, and consequently the combustion of coal is always more or less imperfect and gives rise to a complex mixture of vapours. This mixture contains not only the combustion products already mentioned but also the following unburnt or partly burnt distillation products:(5) hydrogen, (6) hydrocarbons, (7) carbonic oxide, which contains a lower proportion of oxygen than carbonic acid, (8) unburnt carbon in a very finely divided state,and also considerable volumes of unused air.
Smoke. Usually the name "smoke" is applied to this vaporous mixture discharged from a chimney only when it contains a sufficient amount of finely divided carbon to render it dark-coloured and distinctly visible. The quantity, however, of this particular ingredient is apt to be overrated. It always bears an extremely small proportion to the vast volumes of water-vapour, carbonic acid, and nitrogen with which it is mixed; it probably never amounts, even in the worst cases, to 3 per cent. of the weight of the coal from which it is formed; and its importance, reckoned in terms of so much fuel wasted, is certainly not greater than that of the unburnt hydrogen and hydrocarbons. It is per-haps best to use the name "smoke" for all the products of imperfect combustion (5 to 8) which are avoidable, as contrasted with the necessary and unavoidable ingredients (1 to 4) of the mixture. The problem of smoke abate-ment is thus seen to resolve itself into the problem of the production of perfect combustion.
Advantages of efficient smoke abatement. The first advantage to be gained by the solution of this problem is an important saving in fuel. It has been calculated that at least twice as much coal is used in boiler fires and six times as much in domestic fires as is theoretically required for the production of the effects obtained. A considerable portion of this loss is due to causes other than those that can be treated of here, and some is cer-tainly unavoidable; but there is no doubt that much of this enormous waste could be prevented by improved methods of combustion, such as would solve the smoke problem. The second advantage to be looked for is a great gain in cleanliness and public convenience. Not only would there be an end to sooty chimneys but the atmo-sphere of towns would no longer be polluted as it is now by the discharge of unburnt carbon, whose total quantity is enormous, though the amount contained in any given puff of smoke is very small. The "London fog" " would be a thing of the past,not because fogs would become any less frequent than now in London and other large cities, but because they would lose their distinctive char-acter of grimy opacity. It is often stated that these fogs are caused by the smoke that blackens them; but this is an error. The combustion of coal is certainly responsible for their existence, but it is the sulphur of the coal (oxidized ultimately to sulphuric acid), and not the carbon, that is the active agent. And so long as coal is burnt at all this manufacture of sulphuric acid and of fogs must continue; it is not to be got rid of by improved methods of combustion, though the character of the fogs may be materially altered for the better. The evil effects of town air on plant life and human lungs, also often attributed to preventible smoke, are in like manner due to this non--preventible sulphuric acid. The great gain in cleanliness, however, that would follow the abolition of smoke cannot be overrated.
Methods of smoke abatement. The methods that have been suggested for the abolition of smoke may be divided into two great classes, viz., those that seek to attain this end by improving the appliances for the burning of bituminous coal, and those that propose to abolish its use and substitute for it some other kind of fuel. The proposals of the first class may be divided into those applicable to domestic purposes and those applicable to boiler fires and other large-scale operations. Those of the second class may be divided according to the nature of the fuel which they suggest. The innumerable inventions of the first class depend for their success (so far as they are successful) on the attention bestowed on the scientific requisites for complete combustion, viz., a sufficient but not too great supply of air, the thorough admixture of this air with the products of the destructive distillation of the coal, and the maintenance of a high temperature within the fire. In our old and crude methods the facts which most militate against the attainment of these desiderata are(l) that large masses of fresh fuel are continually being thrown on at the top, which cool down the fire just at that point where highest temperature is required; (2) that the products of the distillation of this fresh fuel, heated from below, do not get properly mixed with air till they have been drawn up the chimney; (3) that unduly large volumes of cold air are continually being sucked up through the fire, cooling it and carrying its heat away from where it is wanted, and yet without remedying the second evil. In the improved methods regularity of supply of both fuel and air is sought so as to maintain a steady evolution of distillation products, a steady temperature, and a steady and complete combustion. In many cases it is sought to warm fresh air before it enters the room by a regenerative system, the heat being taken from the escaping gases which would otherwise carry it up the chimney; and in some cases the air which feeds the fire is heated in the same way.
Domestic stoves and grates. -- tests. We cannot here discuss the merits of individual inventions; but we may summarize the chief results of the tests applied at the South Kensington Exhibition. These tests, for domestic grates and stoves, included a chemical ex-amination of the chimney gases, observations of the "smoke-shade" as indicating the proportion of unburnt carbon, and a record of the amount of coal burnt, of the rise of temperature produced, of the radiation, and of the amount of heat lost by being carried away through the chimney. Domestic grates and stoves were divided into six classes as follows:(1) open grates having ordinary bottom grids and upward draught; (2) open grates having solid floors (adapted for "slow combustion") and upward draught; (3) open grates fed from below,supplied with fresh fuel beneath the incandescent fuel; (4) open grates fed from the back or from the sides or from hoppers; (5) open grates having downward or backward or lateral draught; (6) close stoves. Each of these classes was sub-divided according as the apparatus was "air-heating" or "non-air-heating," i.e., according as an attempt was or was not made to save heat on the regenerative principle. This attempt does not appear to have been distinctly successful in any class except the fifth; indeed the evidence of the tests as a whole is rather against the air-heating principle. The following table gives the average results of tests for each class and sub-class as regards general rise of tempera-ture and radiation per pound of coal and smoke-shade. The figures under the last head refer to a standard of shades ranging from 0 (smoke imperceptible) to 10 (black and dense). It was found in practice that the results of this smoke-shade test were in general accord with those of the chemical examination of the chimney gases. The letters "a" and "n" in the first column signify air-heating and non-air-heating respectively, the average results for the whole class being given before those for each sub-class. All the experiments were made with Wallsend coal, a fair representative of the bituminous coals.
From this table the following facts, among others, may be deduced:(a) the air-heating principle has not been applied with success except in class 5 ; (b) close stoves (class 6) are superior to open grates (total average of classes 1-5) in respect of freedom from smoke and of general heating effect, but they are greatly inferior in radiating power,a deficiency which partly explains their unpopularity in the United Kingdom ; (c) the "slow-com-bustion" principle gives a high radiation factor, but is otherwise not successful; (d) the class of air-heating grates with downward, backward, or lateral draughts is, on the whole, most efficient.
Boiler fires. Much attention has been devoted for many years to the question of how to work boiler fires, both for locomotives and for fixed appliances, with the least possible production of smoke and the greatest possible evaporative power. Here the desiderata are essentially the same as in the case of domestic fires, viz., adequate admixture of the com-bustible vapours given off by the coal with the necessary air and the maintenance of a high temperature; and the principles involved are consequently also the same, though the appliances are necessarily different. These improve-ments may be all classed under one or other of two heads, according as the mode of supplying the fuel or the mode of supplying the air is the subject of the improvement. These two kinds of improvement may of course be com-bined. The article FURNACE may be consulted; see also STEAM-ENGINE, sect. "Boilers."
Mechanical stokers. In the old forms of furnace fresh fuel, as it is wanted, is supplied by hand labour, the furnace doors being opened ca and large quantities of coal thrown in. One result of this is the inrush of great volumes of cold air, which, aided by the equally cold fuel, lowers the general temperature of the furnace. Mechanical stokers meet this difficulty by supplying the coal regularly in small quantities at a time. They may be divided into those which deliver the coal at the front and gradually push it backward, those which scatter it generally over the surface of the grate, and those which raise it from below so that the products of its dis-tillation pass through the already incandescent fuel. The mechanism by which these results are attained is often of a complex nature.
Air-supply. It is generally recognized that air cannot be efficiently supplied to the furnace if admitted only in front, and accordingly there have been many plans devised for supply-ing it also at the back. In some cases currents of air are induced by steam-jets; but this plan has not proved very successful. The best inventions are on the regenerative principle. In them the air, before entering the furnace, is made to circulate through chambers heated externally by the products of combustion, and, having thus acquired a high temperature and absorbed heat that would, other-wise have been lost, is admitted through openings at the bridge. Many of these appliances are almost absolutely smokeless, and they are much in use.
Substitutes for bituminous coal. Anthracite. The advocates of the total or partial disuse of smoke--producing coals are variously in favour of the following substitutes-anthracite, coke, liquid fuel, and gas.
For some purposes anthracite and other coals containing a high percentage of carbon may be, and have long been, advantageously used as fuel. They yield a much smaller percentage of distillation products than ordinary coals, and produce no smoke, or almost none. But they are difficult to ignite, and in small fires difficult to keep burning; they give very little flame, and are comparatively expensive, so that they are under considerable disadvantage as compared with the usual kinds of coal. Many of the grates and stoves exhibited at South Kensington were specially devised for burning anthracite, and some of them are decidedly successful; but it is not likely that anthracite will ever take the place of bituminous coal to any great extent in the British Isles. There the great coal-fields undoubtedly are the natural sources of fuel, and no pro-posal involving a complete neglect of this fact can ever be successfully carried out.
Coke. This remark, however, does not apply to the use of coke and of gas, which are themselves made from coal. Coke is produced in large quantities both for its own sake and as a bye-product in the manufacture of gas for lighting purposes, and is largely used in various kinds of furnace. It gives no smoke; but it resembles anthracite also in being but ill adapted to use in open grates on account of the difficulty of ignition and the absence of flame (see FUEL).
Liquid fuel. In America, where natural petroleum is obtained in such enormous quantities, the experiment has been made of using it as the source of heat for boilers. A jet of superheated steam (at about 600° Fahr.) is blown into the hot combustion chamber and the oil and air enter mixed with it. The results are said to be excellent,the fire smokeless and the efficiency high. The residue from coal-tar, after the naphtha and light oils have been recovered from it, can also be advantageously used in this way. The chief disadvantage attending the use of liquid fuels such as petroleum seems to lie in the fact that they are some-what dangerous, fatal accidents having occurred in America; and the range of their application is necessarily limited. To use them for the heating of houses is of course quite out of the question.
Gaseous fuel. Of all the schemes and inventions for the abatement of smoke that one which proposes to distil coal in one opera-tion, and to burn the products of the distillation in another and quite separate operation, is without doubt the most thoroughly scientific; and to it, rather than to patent grates and furnaces, we must look for the ultimate solution of the question. Many arguments may be adduced in favour of gas-heating as opposed to coal-heating, the most important of which are here briefly given. (1) Coal gives, on distillation, not only gas and coke, which are both good heating agents, but intermediate products, many of which are of commercial value; these include ammonia, benzine, carbolic acid, anthracine, &c. As science advances the value of coal-tar will probably be enhanced by further dis-coveries; already it gives the raw material for the preparation of numberless beautiful dyes, of antiseptics, and of some drugs, and quite lately a substance described as an admirable substitute for sugar has been prepared from it. All these intermediate products are now, according to our barbarous methods of burning coal, used simply as fuel. (2) Gas can be laid on in pipes to any spot, can be lit or turned out at any moment, and can be so managed that less heat is frittered away and more applied to the specific object than in the case of coal-burning. (3) It produces no smoke and leaves no ash or cinder, so that cleanliness is attained and much labour and expense are saved. (4) The coke produced during the preparation of the gas has uses of its own as solid fuel and for other purposes. (5) As has been already said, sulphur is an ingredient of all coals, and sulphuric acid is one of the necessary results of burning them, not to be got rid of by "smoke abatement." Coal gas, however, can to a great extent be freed from sulphur compounds, and it is possible that the purification methods in vogue may hereafter be improved, so that we have here a means, if any exist, of curing the chief evils of our present system,injury to our respiratory organs, production of fogs, and destruction of vegetation in towns. The principal disadvantage of the proposal is to be found in the high cost of coal gas, which now varies generally from 3s. to 4s. per 1000 cubic feet, whereas it has been calculated that it would have to cost not more than 1s. or at most 1s. 6d. to compete success-fully with coal. There is no doubt, however, that the cost might, and it probably will, be brought down to this, as the high rate is due to causes not inherent in the nature of things. Sir William Siemens proposed that two sets of mains should be laid in English towns, one for heating and one for lighting gas, and showed that the first and last portions of every preparation of gas are possessed of very low illuminating power, but if collected apart would do excellently for heating purposes, while the rest would be improved for lighting. It is probable, however, that electricity will ultimately drive gas out of the field as an illuminating agent and that it will then be relegated to its true place as a heating agent. When that is done coal will no longer be burnt as a whole, but only those of its products (gas and coke) which are good for heating and for nothing else.
Meanwhile, ordinary coal gas has already, expensive as it now is, been largely applied to certain purposes, notably to cooking stoves and other domestic requirements, to gas--engines (in which the generation of steam is unnecessary), and to bakers ovens; and these inventions are calculated materially to diminish the smoke nuisance. In order to obtain an economical gas capable of being generated on the spot and used for operations on a large scale, Sir W. Siemens devised a gas-producer in which coal is partially burnt in a limited atmosphere and is wholly converted into gaseous products (chiefly carbonic oxide), only the ash being left. This "producer-gas" is a weak fuel, being largely diluted with atmospheric nitrogen, and is therefore in-applicable to domestic purposes ; but for many others it suits admirably, one of the best examples of its application being Siemenss own regenerative gas furnace for melting steel (see SIEMENS). Other gas-producers have been patented, and the cost of the gas so made is as low as 4d. per 1000 cubic feet, or even less. It is probably, however, but a temporary substitute for true coal gas. In the use of this latter we shall, without doubt, find the true scientific solution of the smoke-abatement problem. As an example of what gaseous fuel can do, it may be mentioned that in Pittsburgh in Pennsylvania the furnaces are now being fed by natural oil gas and that that city, once one of the dirtiest of manufacturing towns, is be-coming one of the cleanest.
Literature.The specifications of patents may be consulted. See also C. W. Williams, The Combustion of Coal and the Prevention of Smoke (London, 1858); W. W. Barr, Practical Treatise on the Combustion of Coal (Indianapolis, 1879); Official Report of the Smoke-Abatement Committee (London, 1882) ; Smoke-Abatement Exhibition Review (London, 1882); and papers and discussions in the Journal of the Society of Chemical Industry, 1881 and following years. (O. M.)
The above article was written by: David Orme Masson, M.A., D.Sc.; Professor of Chemistry, Melbourne Univ., from 1886; acted as assistant to Prof. Ramsay, Bristol, 1880; author of papers on chemistry in the transactions of various learned societies.