1911 Encyclopædia Britannica/Smoke

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SMOKE (from O. Eng. smēocan, to smoke, reek, cf. Dutch smook, Ger. Schmauch, probably allied to Gr. σμύχειν), the vapour or volatile matter which escapes from a burning substance during combustion, especially the visible vapour produced by the burning of coal, wood, peat or vegetable substances generally. In this article the various legislative and other measures recommended or adopted for the abating of the nuisance caused by the excessive production of smoke are dealt with. For smoking of tobacco see Tobacco and Pipe, and for opium-smoking Opium.

Smoke Abatement. — The nuisance created by coal smoke seems to have been recognized in London since 1306, when a citizen was tried, condemned and executed for burning “sea cole” in the city of London; but it is only in more modern times that the question has been regarded as one of real practical importance. In 1785 the first smoke-abating invention was patented by James Watt, and in 1800 a mechanical stoker was patented by Robertson. 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 1819 a parliamentary 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.” 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 Public Health Act 1875 contains the statutory law as to the emission of smoke and applies throughout the country, except to London and a few large provincial towns such as Manchester, Liverpool, Sheffield, Leeds, Bradford and Nottingham, where smoke nuisances are controlled by special local acts. The law applying to the Metropolis is identical with that which governs the country at large, and is contained in the Public Health (London) Act 1891.

Section 91, sub-section 7, of the Public Health Act 1875 enacts: “Any fireplace or furnace which does not, as far as practicable, consume the smoke arising from the combustible used therein, and which is used for working engines by steam, or in any mill, factory, dyehouse, brewery, bakehouse or gaswork, or in any manufacturing or trade process whatsoever”; and sub-sec. 8, “any chimney (not being the chimney of a private dwelling-house) sending forth black smoke in such quantity as to be a nuisance, shall be deemed to be a nuisance liable to be dealt with summarily in manner provided by this act.”

A further clause provides that for the purposes of sub-sec. 7 the offence is not merely the emission of smoke, but the use of a fireplace or furnace “which does not as far as practicable consume the smoke,” and this enables a technical defence to be raised which in practice has been found to destroy the efficacy of sub-sec. 7. Under sub-sec. 8 the mere fact of sending forth black smoke in such quantity as to be a nuisance is an offence, unless it be emitted from the chimney of a private dwelling-house. This sub-section is therefore always resorted to by sanitary authorities who initiate prosecutions for smoke nuisances. It has been decided that where black smoke issued from a chimney several times a day for varying periods the magistrate was justified in finding that the smoke issued in “such quantity as to be a nuisance,” although it was not shown that any particular person, or property, was injuriously affected thereby (South London Electric Supply Corporation v. Perrin (1901) 2 K.B. 186). It has also been held that smoke need not be injurious to health in order to be a nuisance (Gaskell v. Bayley, 30 L.T.N.S. 316). It therefore follows that the issue of black smoke from ordinary factory chimneys is per se a nuisance. From a practical point of view, however, it is often found difficult to identify exactly the colour of the smoke, the appearance of which varies in accordance with the position of the observer, and the light behind or in front of the smoke. To aid inspectors various smoke charts and instruments have been devised, none of which is wholly satisfactory. The best chart is the Ringlemann smoke scale, made by ruling black lines at right angles on a white background. It has six shades, numbered 0-5, obtained by graduating the thickness of the lines.

The difficulty of accurately defining the colour of smoke has led to a movement, initiated by the London County Council, for securing the deletion of the word “black” from the Public Health Act, so as to leave to magistrates the duty of deciding a question of fact — whether the smoke complained of constituted a nuisance. The Nottingham Improvement Act 1874 (sec. 74) contains the most efficacious provisions in regard to smoke nuisances which are to be met with in England. It enables steps to be taken in cases where the engines or furnaces are not suitable, and if they are properly constructed, but negligently used, it enables the fireman or other responsible employee to be fined.

Although steam-engines and factories consume individually much more coal than dwelling-houses, they alone are not responsible for the smoke nuisance, for there is little doubt that domestic fires are mainly responsible for the smoky condition of the atmosphere of our towns, for they continue to evolve smoke undeterred by legislation. In 1881, however, a movement was begun by the National Health Society and the Kyrle Society, which resulted in a smoke abatement exhibition being held at [ 276 ] South Kensington. At the close of the exhibition a national smoke abatement institution, with offices in London, was formed.

In the United Kingdom the subject takes an important place in the programme of the Royal Sanitary Institute, whilst the Coal Smoke Abatement Society is devoted to improving the prevailing conditions, especially in the Metropolis, and has organized a number of exhibitions and conferences on the subject. Several smoke abatement committees exist in the provinces.

A knowledge of the nature of coal and of its combustion is essential for an understanding of the smoke problem. For the purposes of this article coals may be classified as smoke-producing or bituminous, and smokeless, the former including all those varieties most commonly used as fuel. The elementary constituents of such coals are carbon (generally about 80%), 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 distinct operations. The first, which requires a comparatively low temperature and is independent of the presence of air, is one of destructive distillation, similar to that which occurs in the retorts of gasworks. It results in the decomposition of the coal, and the formation of the following substances: — (1) hydrogen, marsh-gas, carbon monoxide, ethylene, benzene, other hydrocarbons of the paraffin and benzenoid series, 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 nitrogenous compounds 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 distillation, 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) carbon dioxide, (3) nitrogen and (4) sulphur dioxide, the first of which contains all the hydrogen 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. These products are discharged through the chimney.

Two things are necessary for ensuring 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) carbon monoxide, (8) unburnt carbon in a very finely divided state, and also considerable volumes of unused air.

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, carbon dioxide and nitrogen with which it is mixed; it probably never amounts, even in the worst cases, to 3% 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 perhaps 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). The problem of smoke abatement is thus seen to resolve itself into the problem of the production of perfect combustion.

The solution of this problem would lead to 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 certainly unavoidable; nevertheless, much of this enormous waste could be prevented by improved methods of combustion. Another advantage is the gain in cleanliness and public convenience; not only would there be an end to sooty chimneys, but the atmosphere of towns would no longer be polluted by unburnt carbon, whose total quantity is enormous, though the amount contained in any given puff of smoke is very small. The “London” or “pea-soup” fog would be avoided, not because fogs would become any less frequent than now in London and other large cities, but because they would lose their distinctive grimy opacity.

An investigation of London fogs was made in 1901-1903 by the Meteorological Council with the assistance of the London County Council, from which it appeared that 20% of fogs were entirely due to smoke, and that in every case the density and duration of fogs was enormously added to by smoke.

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. 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 improved. 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. Sixteen million tons of coal are annually used in London for heating purposes, and it has been shown by Dr Rideal that, as the sulphur content of this coal ranges from 1 to 2%, there is diffused in the air of the metropolis from half a million to a million tons of sulphuric acid every year. The extent to which smoke and fog affect life and injure property is, perhaps, a matter of opinion. It has, however, been proved that the death-rate enormously expands in foggy weather, and the Hon. Rollo Russell has made a careful calculation showing the extra cost which the smoke nuisance annually imposes upon London. The figure at which he has arrived is £5,470,000, including damage to buildings, fabrics and works of art.

The amount of coal consumed each year in the country was calculated by the Royal Commission on coal supplies to amount to 160,000,000 tons, of which 36,000,000 or 19.2% are consumed for domestic purposes, and 53,000,000 tons are used in ordinary factories. Thirteen million tons are taken by railways, 15,000,000 by gasworks and 28,000,000 tons by the iron and steel industries.

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 the old and crude methods the facts which most militate against the attainment of these desiderata are — (1) that large masses of fresh fuel are thrown on at the top, which cool down the fire where the 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.

Tests applied at the South Kensington Exhibition of 1882 and in recent years by the Coal Smoke Abatement Society acting in conjunction with the Office of Works, for domestic grates and stoves, have included a chemical examination 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 are divided into six classes: — (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 is subdivided according as the apparatus is “air-heating” or “non-air-heating,” i.e. according as an attempt is or is not made to save heat on the regenerative principle. The following conclusions, among others, have been arrived at: — (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 [ 277 ] they are greatly inferior in radiating power; (c) the “slow-combustion” principle gives a high radiation factor, with a lower consumption of fuel, but is otherwise not successful; (d) the class of air-heating grates with downward, backward, or lateral draughts and with a large surface of fire-brick for radiating heat is, on the whole, most efficient (see Heating).

In boiler fires, both for locomotives and for fixed appliances, the desiderata are essentially the same as in the case of domestic fires; the principles involved are consequently also the same, though the appliances are necessarily different. These improvements 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 combined.

In the old forms of furnace fresh fuel, as it is wanted, is supplied by hand labour, the furnace doors being opened 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 “coking” stokers, which deliver the coal at the front and gradually push it backward; “sprinkling” stokers, which scatter it generally over the surface of the grate; and “underfeed” stokers, which raise it from below so that the products of its distillation pass through the already incandescent fuel. The mechanism by which these results are attained is often of a complex nature.

It is generally recognized that air cannot be efficiently supplied to the furnace if admitted only in front, and accordingly many plans have been devised for supplying it also at the back and sides. In some cases currents of air are induced by steam-jets; but this plan has not always proved successful. The inventions on the regenerative principle are more generally satisfactory. 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 otherwise have been lost, is admitted through openings at the bridge. Many of these appliances are almost absolutely smokeless, and they are much in use, as they have been shown to effect great economy in coal consumption.

It must not be forgotten, however, that with the use of trained stokers a high degree of boiler efficiency is reached by hand-firing alone. Indeed, it has been proved by actual tests that, when pitted against untrained men, skilled stokers have raised the thermal efficiency of their plant by over 16%, without creating smoke nuisances. In Germany stokers are trained under careful state supervision, and similar work has been started at the Borough Polytechnic Institute by the London County Council.

The advocates of the total or partial disuse of smoke-producing coals are variously in favour of anthracite, coke, electric power, liquid fuel or gas.

In some factories, such as malting works, 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 without forced draught; 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 grates and stoves have been devised for burning anthracite for domestic heating, and some of them are successful and economical; but, in view of the national prejudice in favour of a bright and open fire, it is not likely that anthracite will ever replace bituminous coal to any great extent in the British Isles, where the great coal-fields undoubtedly are the natural sources of fuel.

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 by-product in the manufacture of gas for lighting purposes, and is largely used in various kinds of furnaces. It gives no smoke; but it resembles anthracite also in being but ill adapted for use in open grates on account of the difficulty of ignition and the absence of flame.

One of the most notable features of the smoke abatement movement in recent years has been the manufacture of smokeless fuels capable of being readily and satisfactorily burnt in ordinary household grates. The use of such fuels is growing and will, in conjunction with the enormous expansion in the use of gas-cookers and heating appliances, do much to eliminate smoke nuisances from private houses. Over 750,000 gas-cookers are in use in the metropolis alone, and their aggregate effect in preventing the emission of smoke from kitchen chimneys must be very great.

Liquid fuel or natural petroleum, which has come into exceptional prominence during recent years as a heating agent, owes its success to its relatively smokeless combustion and high efficiency. The same applies to gaseous fuel, which includes in addition to ordinary coal gas other mixtures of gases which burn with a high heating value and with no deleterious vapours or smoke (see Fuel: Liquid and Gaseous) . Electricity is now also being largely utilized in factories for power purposes, and is thus bearing its share in solving the problem of smoke abatement.

See Official Report of the Smoke Abatement Committee (London 1882); W. C. Popplewell, The Prevention of Smoke (1901); W. Nicholson, Smoke Abatement (1905); also the publications of the London Coal Smoke Abatement Society; Booth and Kershaw, Smoke Prevention and Fuel Economy (1904); Reports of the Laws in certain Foreign Countries in regard to Emission of Smoke from Chimneys (Foreign Office Return), Cd. 2347 (1905); London Fog Inquiry (1901-1902) (Reports to and by the Meteorological Council).

(O. M.; L. W. Ch.)