The Spontaneous Ignition of Coal.

The Spontaneous Ignition of Coal.

At the recent meeting of the British Association, Professor Vivian Lewes read a paper upon the spontaneous ignition of coal, which brought about an instructive discussion, an abstract of which we take from an English contemporary.

自从贝采里乌斯第一次建议热evolved by the oxidation of the pyrites in coal might have an important bearing upon spontaneous ignition, this has been accepted as the popular explanation, although the question has been contested. Coal most liable to ignition, however, often contains as little as eight per cent of pyrites, mostly FeSa and rarely more than two per cent, and even if this amount were concentrated in one spot and oxidized rapidly, instead of during months and years, the total rise of temperature would be inadequate to cause ignition, which requires a temperature of 700 degrees to 870 degrees Fahr., the latter for Welsh steam coal. It is acknowledged that only finely divided pyrites can be dangerous ; large masses do not oxidize appreciably, nor do heaps of pyrites free from carbonaceous matter. Professor Lewes regards the true explanation as partly physical and partly chemical. Charcoal condenses gases upon its surface, freshly won coal absorbs from one and one-lialf to three times its volume of oxygen from the air, and there is a circulation of the air through the interstices of the mass. This absorption is slow at low temperature, but rapid over 100 degrees Fahr. The oxygen absorbed is, after some time, given off as carbonic oxide and water vapor, and becomes more active in heaps by compression, and by elimination of the nitrogen. Carbon powder in zinc cases ignites at 250 degrees Fahr. within a few hours. The effect of mass is well illustrated by statistics. For cargoes of under 500 tons, ignition cases amount to one-quarter per cent only ; in cargoes of over 2000 tons they rise to nine per cent. The ports also have their influence. European ports are pretty safe, the Cape of Good Hope knows of many cases. The effect of moisture is to swell and break up the coal, and in this respect moisture is injurious in the presence of pyrites also ; moisture increases the capacity for absorption, as Percy’s tables show. Ventilation with cool air would be a remedy, but is impossible on board the fast liners where bunker fires are, Mr. Lewes asserted, not infrequent though not admitted, especially when the boilers are close to the bunkers. It might be objected that, if this were so, the seat of fire should be near the bulkhead, whilst it is usually found near the centre and outside ; but Professor Lewes showed by a little sketch that the openings towards the deck are not above one another, So that whilst the charring starts near the bulkhead, as has been proved, the ignition takes place some way off, where the air has access. Sir Lowthian Bell claimed competency to speak, as he has been burning 2000 tons of coal a day for the last twenty years and more. He took up the one argument of Mr. Lewes that the pyrites might cause sufficient rise of temperature locally, though it could not heat the whole mass sufficiently, and he mistrusts his pyrites still ; moreover, why did ignition never start at the top ? Professor Vernon Harcourt also thought that Mr. Lewes had made his good case too strong by speaking of the ignition temperature of coal, whilst it was a question of sulphur. Mr. Bauermann questioned that the oxidation of sulphur would yield anything but acid compounds, but reminded the section that pulpy lignites, when pressed, ignite far below their supposed ignition temperature, in which he was supported by Dr. Bedson, who had experimented with the gases given off by heated coal dust. Mr. Spiller believed less in Dr. Bedson’s resins and paraffins than in organic sulphur compounds, which would become dangerous on exposure to air ; the moisture would act as a starter, and the physical conditions would be of great influence. Professor M’Leod wished to settle experimentally what kind of bodies caused ignition, whether a gas or a sulphur hydro-carbon, which might be extracted. Sir Frederick Bramwell put the best question, whether Mr. Lewes was going to give a supplementary paper how to prevent ignition. Mr. Lewes had a means ; he suggests a second bulkhead nine inches from the first, sea water to be circulated between the two by one pump, so that the coal would be practically waterjacketted, a remedy which he also recommends for powder magazines. As regards the chemistry, he fancies bituminous compounds more than sulphur combinations, and does not favor an oxidation of gases. The fire originated at the bottom and not at the top, because there was the greatest mass to keep the heat in. Dr. Bedson probably had a much higher temperature inside his ovens.