Modern Gasworks Practice

GAS-MAKING AND OTHER COALS 373 5. W. Pair,1 as a result of mimerous experiments conducted with. Illinois coals, arrived at the following principal conclusions :— (a) The oxidation of coal is continiious over a wide range of temperatures, beginning, with freshly-mined coal, at ordinary temperatures. (6) At low temperatures initial oxidation is due to the presence in the coal substance of certain unsaturated compounds having a marked avidity at ordinary temperatures for oxygen. (c) Although the slow oxidation is exothermic, it does not, when air is freely circulated, cause any material self-heating of the coal round about, until a temperature of from 60° to 80° C. is reached. (<Z ) The self-heating at any temperature is more rapid the finer the state of division of the coal. (e) The presence of moisture increases the Chemical reactivity of the coal-air system at any temperature. (f) At a temperature of about 135° C. in air the constituents of coal beginto oxidize more freely with the evolution of carbon dioxide and water, a state of affairs which. must be regarded as highly dangerous. (g) The point at which oxidation becomes autogenous or self-propellant lies somewhere below 200° to 270° in air. As soon as this temperature is reached there will be a rapid rise to the ignition point of the coal, which in general is above 350° C. Bacon and Hamor 2 are of the opinion that the subject of spontaneous firing of coal requires more inquiry from a fundamental standpoint. They state that the entire process is essentially one of “ surface combustion ” ; but between the initial absorption of oxygen by the coal substance and its final expulsion in the products (water and oxides of carbon) an entire series of complex phenomena may be involved. Means of Prevention Many divergent views have been expressed as to the most effective means of preventing spontaneous ignition in coal Stacks. One authority may be found strongly to advocate ventilation, while o thers suggest that free access to the air actually causes fire. Again, the free use of the hosepipe is recommended by some, whereas it is pointed out, on the other hånd, that the use of water is likely to be followed by serious results, especially in. confined stores. Primarily, then, it is necessary to find out the conditions actually favourable to combustion. In. the first place, there seems little doubt that there is far greater liability in hot weather than in cold. With regard to the question of ventilation, many opinions have been expressed ; and some few years ago it was quite a regulär practice to resort to such expedients as perforated iron pipes, wickerwork baskets without bottoms, also elaborate ventilating skafts of brick and wood, so as to ensure the passage of a good strong current of air through the lieap. Great caution, however, is necessary, for it must be borne in mind that when once a fire has been started it will not continue to burn unless the supply of oxygen is maintained ; and very often 2 University of Illinois, Bulletins Nos. 17 and 46. 3 J. Soc. Chem. Ind., Vol. xxxviii, 12, p. 161 T.