GASES IN MINES AND COAL. Gases of several kinds are given off by rocks and es pecially by coal in mining. Some of these gases are poisonous and some are explosive so that fresh air has to be forced through most under ground workings to ventilate them. More or less oxygen is also consumed and carbonic oxide is produced by men and animals in the mine, the decay of timbers, explosives, oxida tion of coal, etc. Ordinarily the principal gas that emanates from freshly cut coal is methane, CH., popularly known as marsh gas, which when mixed with air is the highly explosive "fire damp." The following is a list of the prin cipal mine gases, their composition and specific gravity.
Oxygen and nitrogen in mines are mostly from air, but considerable oxygen is absorbed by the coal, lowering the oxygen ratio from its normal one (approximately 79 per cent nitrogen and 21 per cent oxygen). Carbon dioxide i occurs in moderate amounts in most mines, but in some cases and locally it is present in large volume. Carbon monoxide is rarely an ingre dient of the gases in coal and when present in mines it is generally a product of incomplete combustion, whether that of explosives, coal or wood, or explosion of gas or coal dust. It is a colorless, odorless gas slightly lighter than air, strongly explosive and extremely poisonous.
All coal contains in its pores and crevices some of the inflammable gas methane, which is given off when the coal is in the ground, during mining and in diminished volume for a long time after mining. The amount in the coal varies greatly in different beds and localities, and in some mines the amount is so small that they are classed as °non-gaseous.* In some such cases most of the original gas has been lost, especially in the shallower workings. In the anthracite region of Pennsylvania there are several mines giving off 2,000 cubic feet of meth ane a minute, or nearly 3,000,000 cubic feet a day, which is brought to the surface by the ventilating system. This would be sufficient to supply heat and illumination for a city of 100,000 inhabitants. It is estimated that the total out flow of the as in the Northern Anthracite Basin alone is 31,000 cubic feet a minute, 43,640,000 cubic feet or 1,000 tons a day.
At ordinary temperatures most coal yields at least an equal volume of gas or 20 cubic feet to the ton. Some coal yields five times as much as this and unusual samples have yielded at the rate of 150 cubic feet per ton of coal. The proportion of methane in the gases also varies hut from most coal mines it is in excess of 75 per cent, the remainder being carbon dioxide, oxygen and nitrogen.
Origin.— Methane in coal undoubtedly is a product of the decomposition or molecular rear rangement of organic matter during its trans formation into coal by putrefactive bacilli. The
process may have continued for some time but is now no longer in progress in the coal. Pos sibly, however, the diminution of pressure in the coal by erosion of strata or even by mining may effect disassociation of some as yet unrecognized intermediate compounds with resulting emanation of more or less additional methane.
Probably different degrees of putrefaction had much to do with variations in the amount of methane in coal. Renault has studied the process in detail and discovered remains of the various organisms which cause putrefaction in vegetal accumulations. It was found that the process varied with the nature of the materials and their rate of deposition, and accordingly the resulting gases vary in character and propor tions. The stage of progress when the vegetal matter was buried by sand or mud and the nature of the cover had great influence. Natu rally all these conditions varied from place to place so that the variation in the volume of methane now found in the coal was largely caused early in its history. Of course there has been more or less subsequent escape of the gas, especially where overlying strata are porous or have been thinned by erosion. Ordinarily there is more methane in anthracite than in bitu minous coal and if this gas was developed when the coal was formed it is a mistake to suppose that in anthracite most of the volatile matter has been °baked out* and escaped through porous rocks and joints. It is im possible to conceive of the loss of the heavy hydrocarbons of the so-called °volatile matter* and the retention of several volumes or any of the highly volatile methane: As shown above it is most improbable that any notable amount of the methane developed after the °metamor phism.* Fire Damp.— The term fire damp is applied to mixtures of methane and air in which the proportion of the former is from 4 to 30 per cent. Pure methane is not explosive because a large amount of oxygen is required for its combustion, and ordinarily when its proportion falls below 5 per cent the danger of explosion is passed. A proportion of 9% per cent of methane causes the most violent explosion and Barrel! and Siebert have found that mixtures containing less than 5.5 per cent or more than 12.8 per cent will not explode. The limit of inflammability is near 30 per cent, the mixture burning quietly between this proportion and ex plosibility (12.8 per cent). However, propor tions of methane above 4 per cent are highly dangerous because a slightly increased rate of emanation might bring the proportion up to the explosive figure and mixtures containing more than 12.8 per cent may quickly become di luted with sufficient air to make them explosive.