OCCURRENCE OF EXPLOSIVE GASES IN COAL MINES.

By N. H. DARTON.

INTRODUCTION.

This report presents the results of an investigation begun by the Government in the summer of 1907, the investigation being started under the immediate supervision of Dr. J. A. Holmes and continued under him as director after it was transferred to the Bureau of Mines in 1910, the field studies being completed in the spring of 1912. The purpose of the investigation was to obtain information on the origin of the inflammable gases in coal and the conditions under which they occur. It was especially intended to ascertain whether there was any relation between the occurrence of gas and the structural or other geologic features of the coal beds. To this end many months were spent in mines and much time was devoted to the examination of mine maps, bore-hole records, and other data made available by the kindness of various coal companies. Two fields of work were selected—one in the northern anthracite basin of Pennsylvania, where the beds are considerably flexed, and the other in the southern part of the bituminous coal field of Illinois, where the beds lie nearly horizontal.

In order to take advantage of results of previous investigations of the same general subject, an extended examination was made of reports of various investigators in Europe and America. A digest of the information obtained from these publications is included in the first part of this report, which constitutes an introduction to the discussion of the conditions governing the occurrence of inflammable gas in coal beds.

THE GASES IN COAL AND THE CONDITIONS CONTROLLING THEIR ESCAPE.

GENERAL CONSIDERATIONS.

All coal contains in its pores and crevices some inflammable gas. This gas is given off when the coal is in the ground, during mining, and for a long time after the coal has been taken from the mine. Peat and marsh muck have the same property; hence the name "marsh gas," which indicates a source in organic material in marshes, is given to methane, the principal inflammable gas in coal.

The amount of gas in coal, the relative proportions of its constituents, and the volume given off vary greatly. In some coals and in some areas the amount of inflammable gas evolved is so small that the mines are classed as "nongaseous." Probably, however, no coal is entirely free from inflammable gas, and if only a small amount now comes from any given coal most of the original content has been lost previously. On the other hand, some coals give off several times their volume of gas that contains a large proportion of methane. In the anthracite region of Pennsylvania there are many mines, from each of which every minute more than 2,000 cubic feet of methane, or nearly 3,000,000 cubic feet every 24 hours, escapes into the air at the mouth of the upcast.

At ordinary temperatures most coals yield an equal volume of gas, or 20 cubic feet to the ton; some yield five times as much, and samples of a few coals have yielded gas at the rate of 150 cubic feet per ton. A volume greater than 100 cubic feet per ton of coal (20 to 25 cubic feet) is, however, relatively rare. The proportion of methane in the gases given off also varies greatly, but from most coals is in excess of 75 per cent.

Inflammable gas is liberated from coal in nearly all mines, but in most shallow mines the volume given off is small, and even in some deep mines of certain districts only moderate ventilation is necessary to keep methane from accumulating sufficiently to be detected with a safety lamp. Unfortunately, however, gas is given off unevenly from place to place in a bed, and in many mines large volumes occur unexpectedly. Wherever inflammable gas is allowed to accumulate an explosion may happen. Formerly all coal-mine explosions were attributed to ignited gas, but now it is known that in large explosions coal dust is the main explosive agent. Most gas explosions are small, but they are frequent in gaseous mines, and many of them cause fatal burns. Most afterdamp, or air vitiated by the products of an explosion, contains dangerous constituents, the most poisonous of which is carbon monoxide.

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CHARACTER OF MINE GASES.

Only a brief outline of the character of the principal gases in mines is given, as the subject is treated in many books. Methane (CH) is the most important of the mine gases on account of the fact that mixtures of methane and air in certain proportions are explosive. Carbon dioxide (CO2) is also a common constituent of mine gases even when methane is absent, but only in exceptional instances is the proportion sufficient to be of practical importance. Carbon monoxide (CO) rarely occurs in mines except as a product of incomplete combustion. Oxygen and nitrogen, in the form of air and in proportions other than those in which they occur in air, are contained in gases extracted from coal, and in coal-mining operations large volumes of these gases, as air, are made to pass through the workings to dilute and remove the mine gases. In places mine air carries small amounts of various hydrocarbon compounds besides methane, and hydrogen and hydrogen sulphide occur locally in certain mines.

METHANE.

As expressed by its chemical symbol, CH,, methane is a compound of 1 atom of carbon and 4 atoms of hydrogen. It is a colorless, odorless gas, and at ordinary temperatures is chemically inert. On ignition, it unites with oxygen to form carbon dioxide and water vapor. In places it is given off nearly pure by the coal but in most mines it is mixed with more or less carbon dioxide, nitrogen, and oxygen. Its specific gravity in comparison with air is 0.55314; that is, it is slightly more than half as heavy. Seemingly it is not poisonous when mixed with sufficient oxygen to support respiration. Some poisonous effects that have been ascribed to the presence of methane in large quantities are undoubtedly those of suffocation, possibly being increased in certain instances by some poisonous gas. Many miners work all day in air containing 1 to 4 per cent of methane without evidence of injurious effects. An experiment by Haldane ↳ is on record in which he breathed a mixture of 20 per cent methane and 80 per cent oxygen for 5 minutes without any unfavorable results and a mouse kept in the mixture for 40 minutes was not affected.

CARBON DIOXIDE.

Carbon dioxide (CO2) is of general occurrence in coal, and is a common constituent of mine air, but the proportion in most mine air is less than 1 per cent. It is without odor or color and is more than half again as heavy as air, its specific gravity being 1.529. It is

a Much information is given by Beard, J. T., Mine gases and explosions, 1908, 402 pp. For a résumé of data relating to gases and methods of analysis see Burrell, G. A., and Siebert, F. M., The inflammable gases in mine air: Tech. Paper 39, Bureau of Mines, 1913, 24 pp.; the sampling and examination of mine gases and natural gas; Bull. 48, Bureau of Mines, 1913, 50 pp.

b Haldane, J. S., Investigation of the occurrence, properties, and composition of black damp: Trans. Fed. Inst. Min. Eng., vol. 8, 1894-95, p. 549.

neither explosive nor poisonous but is incapable of supporting life or combustion. In places it accumulates in pits and dug wells. A person descending into such a pit may be suffocated if the proportion of oxygen is much less than 10 per cent.

CARBON MONOXIDE.

Carbon monoxide (CO) is rarely an ingredient of gases issuing from the coal and when it exists in mine air is generally a product of incomplete combustion, whether that of an explosive, of coal or wood in a mine fire, or of explosions of coal dust. It is the principal toxic agent in afterdamp. It is a colorless, odorless gas slightly lighter than air, strongly explosive and extremely poisonous. Air containing a very small proportion of carbon monoxide will cause death especially if the proportion of oxygen is diminished. The amount that can be endured by a person depends mainly on his vitality and the length of time he breathes the gas, but no one can survive more than a few moments in air containing 0.5 per cent. The effect is cumulative and half this proportion would be fatal if breathed for a long time. Removal from the poisoned atmosphere and vigorous long-continued artificial respiration with plenty of air, or better, oxygen, may save a person who has succumbed. The chances of recovery are, however, much less than in suffocation by carbon dioxide or other inert gas.

NITROGEN AND OXYGEN.

Nitrogen is almost invariably a constituent of the gas emanating from coal, in some mines constituting 90 per cent of the gas. In most mines it is mixed with oxygen in various proportions. Air consists of a mixture of approximately 79 per cent nitrogen, 21 per cent oxygen, and 0.03 per cent carbon dioxide, but air in mines loses much of its oxygen by absorption by coal and by various oxidation processes. Air containing less than 7 per cent of oxygen will not support human life.

FIRE DAMP.

The term fire damp is applied to mixtures of methane and atmospheric air; generally to those mixtures that are inflammable or explosive or nearly so. As stated above, pure methane is not explosive because a large volume of oxygen is required for its combustion; neither is a mixture of air and a small percentage of methane explosive. Generally fire damp contains 10 to 30 per cent of methane, for the gas as it emerges from the coal is mixed with more or less air, but in places where the air has been displaced, the proportion of methane may rise much higher; 99.5 per cent was found at a pit in Westphalia in 1894 and 1895. The gas extracted from fresh samples of coal, "blower gas," and gas accumulations in cavities in the roof where little or no

fresh air has had access are much richer in methane than is ordinary fire damp.

The ignition and explosion of fire damp are subjects treated in various books on mine ventilation and mine gases, and their discussion is not within the scope of the present investigation. However, a few general properties may well be restated here. It is generally agreed that a mixture of methane and air containing 9 per cent methane causes the most violent explosion, and Burrell and Siebert a have found that mixtures containing less than 5.5 per cent or more than 12.8 per cent will not explode, the explosive power being greatest when the proportion of methane is between 8 and 12 per cent. The limit of inflammability is reached at about 30 per cent, the mixture burning quietly between this proportion and explosibility.

The limits of explosibility are materially affected by the presence of carbon dioxide and other gases and by coal dust in suspension. The factor of air movement is also of moment. Hydrocarbon gases other than methane increase the explosibility of the mixture, as does also carbon monoxide, but these gases are of infrequent occurrence. Carbon dioxide and nitrogen greatly reduce the explosive range, the most explosive mixture becoming inert when diluted with about 15 per cent of either or both of these gases. A mixture of 1 volume of methane and 9.55 volumes of dry air ignites at 1,200° F., the calculated temperature of the combustion being 3,900° F. The pressure developed by such a mixture when it is ignited in a closed space is 102 pounds to the square inch, which explains why a gas explosion in a mine does so much damage. Probably in a confined space in a mine all the methane does not burn completely in the explosion, for the proportion of oxygen may vary and some of the gas is forced in advance of the explosion wave (and hence is not burned) by the violence of the initial explosion. The presence of coal dust, especially the inflammable dust of bituminous coal, is also most important.

NATURE OF GASES IN COAL BEDS.

Many analyses have been made of the gases given off in the mine by coal and by "blowers" and "feeders." Methane is the predominant gas in most places, but its proportion is exceedingly variable, in general ranging from 75 to 99 per cent of the gases as given off. Carbon dioxide is an almost invariable constituent, and the propor

a Burrell, G. A., and Siebert, F. M., The inflammable gases in mine air: Tech. Paper 39, Bureau of Mines, 1913, 24 pp.

b Clement, J. K., The influence of inert gases on inflammable gaseous mixtures: Tech. Paper 43, Bureau of Mines, 1913, 24 pp.

c Mallard and Le Chatelier, Recherches expérimentales et théoriques sur la combustion des mélanges gazeux explosifs: Annales des mines, vol. 4, 1883, pp. 274-378.

d Rice, G. S., The explosibility of coal dust: Bull. 20, Bureau of Mines, 1912, 204 pp., 14 pls.; Coal-dust explosions, Miner's Circular 3, Bureau of Mines, 1911, 22 pp.; Frazer, J. C. W., Hoffman, E. J., and Scholl, L. A., jr., A laboratory study of the inflammability of coal dust: Bull. 50, Bureau of Mines, 1913, 58 pp.