Smoke and Smoke Prevention

SMOKE AND SMOKE PREVENTION. Smoke and its Production.—Smoke is a general term applied to the visible exhalations from burning materials. The character of smoke varies according to its source, which may be a waste heap, a bon fire or a cigarette; but the smoke which is produced in by far the largest quantities, and which on account of its injurious effects has become a serious economic problem, is that associated with the general use of raw bituminous coal as a fuel.

Nearly all fuels consist essentially of carbon, hydrogen, oxygen and nitrogen, in various proportions and variously combined. In addition they usually contain a little sulphur, while in solid fuels varying amounts of incombustible mineral ash are also incorpo rated. If complete combustion were always attainable no fuel would emit smoke, the final products in such an ideal case being limited to carbon dioxide, water vapour and free nitrogen, all quite innocuous gases, and invisible unless the water vapour con denses to a cloud of steam. There would, however, if sulphur were present, also be produced small quantities of sulphur dioxide gas, which, although also invisible, has a pungent smell, and in contact with air and moisture tends rapidly to be converted into a corrosive acid; while the mineral constituents would remain un burned in the form of ash.

To achieve such finality it is necessary only that a fuel should be brought into contact with enough air for full oxidation while maintained at a temperature sufficiently high for combustion to take place. These conditions, although apparently simple, are by no means easy to realise, and in practice some proportion of a fuel always eludes complete combustion. The unburned products vary widely both in amount and in composition according to the nature of a fuel and the manner of its use, being in some circum stances inappreciable, in others very large. They are moreover not necessarily in the form of smoke, since with insufficient air carbonaceous materials may emit gaseous intermediate products such as carbon monoxide and unsaturated hydrocarbons; but whether or not smoke is produced, imperfect combustion is al ways indicative of thermal loss. The heat which a given weight of carbon liberates in oxidising to carbon monoxide, for instance, is less than one-third that generated by its complete combustion to carbon dioxide.

Thorough admixture with air is relatively easy to secure in the case of gaseous fuels, which in properly constructed and properly adjusted burners produce neither smoke nor other unburned prod ucts in appreciable quantity. An inadequate air supply, however, or the chilling or smothering of the flames, may result in the evo lution of unburned gaseous products, including carbon monoxide and oxides of nitrogen, both highly poisonous; or in extreme cases may even cause the deposition of soot.

Smoke from Solid Fuels.

Owing to the relatively high density of solid fuels, the problem of bringing them into contact with sufficient air for complete oxidation is greatly intensified, and, even with an air supply far in excess of that theoretically required, perfect combustion cannot in practice be counted upon. Certain forms of solid fuel, however, such as anthracite, which is naturally nearly free from volatile matter, or coke, from which the volatile matter of the original coal has been artificially ex tracted, can be burned, if not completely, at least without smoke emission. With bituminous coals, on the other hand, smoke pro duction to a greater or lesser degree, according to the circum stances, is practically unavoidable; for such coals are subject to decomposition at temperatures below the ignition point, with the evolution of combustible gases and condensable tarry vapours. These are of so complex a character, and under the action of heat are subject to such complicated chemical changes, that although the more readily ignitible constituents may burst into spasmodic flames, others almost inevitably escape unburned. Coal smoke consists of such unconsumed distillation products, in asso ciation with carbon and tarry matter condensed by premature chilling of flame, together with dust and ash entrained by the upward rush of hot air and gases from the grate. Some of this settles on the walls of the flue as soot ; the remainder is carried out through the chimney into the atmosphere with the excess air and gaseous products of combustion, both burned and unburned.