Results of analyses of gas given off by coal samples from bore hole in Hart-Williams mine, Benton, Ill.

These results throw no light on the volumes given off; moreover, considerable air was taken in when the samples were placed in the can. The figures, however, show that methane was evolved, probably in different amounts, from the three samples and that some of the oxygen had been absorbed.

RELATION OF METHANE EMANATION TO DEPTH.

It is well established that in most areas where the coal is near the surface little gas remains in it, and in general up to a moderate depth the deep mines are more gaseous than shallow ones. The escape of gas from the coal to the surface has been in progress for an extremely long time and even from deep-seated beds there has been some loss in this way. An important factor affecting the rate of escape is the relative permeability of the strata overlying the coal, and the degree of permeability depends not only on the texture of the rock, but also on the extent to which faulting and joint crevices are present. A bed of gaseous coal near the surface overlain by sandstone would be likely to lose much of its gas, whereas a cover of shale or clay would greatly impede the escape of gas.

Interstitial moisture also impedes the movement of gas through the strata, but the extent of its influence is not well known. Fracturing of the strata is also an important factor, for the gas travels along fault planes and joint cracks. A coal bed lying several hundred feet below the surface is usually overlain by alternations of the clays and sandstones that compose the coal measures. As much of this material is only slightly permeable, there is with increasing depth a proportionate general increase in the amount of gas confined in the coal; but, as shown by figure 33, the relation is not a close one and this increase ceases below a certain depth. The table on page 224 shows the relative volumes of methane given off by mines of southern Illinois, arranged in order of their depth, and also the amount in relation to daily coal production from each mine.

FIGURE 33.-Relation between depth of mines and amount of methane per ton

of coal mined in southern Illinois.

Total methane emanation from southern Illinois mines arranged in order of their depth.

PROBABLE INCREASE OF GAS IN DEEPER OR LARGER MINES.

A question of practical importance considered in the investigations in Illinois concerns the amount of gas to be expected in the deeper parts of the field, as yet undeveloped. The danger of gas and the added expense that must be incurred in combating large volumes of it are important considerations affecting the development of the

100

91

8

1,500

96

3

1,450

86

11

1,000

80

0

80

66

1,200

761

11

800

58

3

1,200

coal in many areas. The results of this investigation show that the deeper mines are not likely to find more gas than has been encountered in those of moderate depth. It is found that below 150 or 200 feet beneath the surface depth is not the principal factor affecting the volume of gas, but that variations in the character of the coal cause differences in the amount given off. This regional variability also indicates that a mine with low gas volume must not be conducted with a false sense of security, for a gaseous district may be entered at any time and the gas become dangerous on very short notice. Accordingly equipment should be provided capable of coping with sudden developments of greatly increased volumes of gas.

Another important consideration is whether or not there will be an important increase in gas volume as the workings become larger and the area of exposed coal surface consequently increases. Several of the companies have already extended their mining operations over nearly all of the area owned by them, and the increase of total gas volume appears not to have increased proportionately to the area worked. This result is due to the fact that the larger part of the gas has been given off from freshly exposed coal in the working faces during the mining so that the old pillars do not contribute any large proportion of the total methane emanation. As mining progresses in the larger properties the disposition will be not to extend the workings to great distances, but to move to the adjoining area and sink a new shaft, as the item of long underground haulage is an important one in the cost of production.

SUMMARY OF CONDITIONS OF OCCURRENCE OF

METHANE.

Although many observations have been made as to the conditions under which explosive gases occur in coal and are given off by it, some of the causes of the variations in the character, amount, and pressure of such gases are still unknown. In this chapter are summarized the principal facts developed by the investigations in Europe and this country, including the author's own observations in Pennsylvania and Illinois.

COMPOSITION OF GAS GIVEN OFF BY COAL.

The gas given off by coal in mining as well as in laboratory experiments under different conditions shows considerable range in composition. Although methane is the principal component its proportion varies not only in gases in the mine but in gases extracted from coal samples under various conditions. In most cases the gas given off by coal and by blowers contains more than 80 per cent of methane, and in places the methane is nearly pure. Carbon diox47004°-Bull. 72-15-15

ide, nitrogen, and oxygen, with proportions varying greatly, are the other principal ingredients of the gas from fresh coal. The variations in composition doubtless indicate a complex chemical process in the development of the coal, which is further indicated by the great variation of volume of contained gases in the coal from bed to bed and from place to place in the same bed. These variations have not as yet been connected with visible characteristics in the coal, and they appear to be entirely independent of the chemical constitution of the coal, reported as "fixed carbon," "volatile combustible matter," etc.

The gas given off by coal at different stages of exposure and in fragments of various sizes also shows great variability. In the mine there is considerable difference between the composition of gas from blowers and from the fresh face and that from old pillars and ribs, and in the laboratory the degree of pulverization and the duration of tests materially affect the composition of the gases obtained. Doubtless air absorption with resulting oxygen combination has much to do with this phase of the problem.

MANNER OF ESCAPE OF GAS FROM COAL.

Seemingly gas escapes from coal by percolating through the pores or interstices between the coal grains and also through fissures great and small. The principal controlling factors under these conditions therefore are, (1) texture or size of grain, (2) extent and size of fissures, and (3) amount of pressure forcing the gas outward. Gas escapes most rapidly when the mining operation reaches a fissure or a body of shattered coal that constitutes a reservoir under high pressure and when such a body of coal is thrown out in such large quantity that much of the gas is given off at once. Outbursts of this character have caused numerous accidents, some with considerable loss of life. Very careful study of these outbursts has not revealed any definite law of occurrence and no dependable means has been found for detecting their proximity or for preventing them. They happen most often in zones where the strata are crushed or disturbed or in places where the coal thickens and thins, but they are not connected with any peculiarities of the coal that are discernible according to our present knowledge. Bodies of softer, more porous coal inclosed by harder coal are naturally very likely to be a source of outbursts, for they may contain a large volume of gas dammed back by the hard coal. The likelihood of increased gas emanation and accumulation from squeezes and other local movements in mines is well recognized, for ordinarily these accidents not only cause deep fissuring but interfere with ventilating currents.