September 15, 1916. THE COLLIERY GUARDIAN. 513 THE OCCLUSION OF GASES BY COAL.* By J. Ivon Graham, B A., B.Sc. The permeability of coal to air or gas is a matter of much interest and practical importance in coal mining, and a. knowledge of this property is essential to a com- prehension of the process of spontaneous heating in coal and the production and discharge of firedamp and black- damp. The solubility of gases in coal is also a matter of considerable practical and great scientific interest. In the experiments on the permeability of coal to gases the following method was adopted : Thin slabs of coal were sawn off from a large lump by means of a fine metal hack saw. Some of these slabs were cut in the v direction of the cleavage of the coal, others at right angles to this. A number of these slabs were obtained from different kinds of coal, having no visible traces of cracks. Pieces of glass -tubing, to 1 in. in diameter, were sealed on to each side of the slab, being placed 'directly opposite to one another. In order to obtain an airtight seal, the following method was adopted : The end of the glass tube, previously warmed in a blow-pipe flame, was dipped into molten bitumen (such as is used for electrical insulation purposes), and a small quantity of bitumen in an almost molten condition was thus obtained on the end of the tube. The latter was then quickly pressed against the coal slab, and held rigidly until the bitumen became quite hard. Successive thin layers of seccotine were then applied round the junction and allowed to. dry. The second tube was then sealed on to the opposite side of the slab in a similar fashion. It was found that by this method a practically airtight seal could be obtained (sufficiently tight to hold a vacuum for several days). In the case of a slab of “ softs,” it was found almost impossible to obtain a slab free from cracks, owing to the fragile character of this coal. With cannel, also, cracks developed very readily. The majority of the results obtained deal with slabs from ” hard ” coal. The permeability of these slabs to air, hydrogen, carbon dioxide, and methane was ascertained, the quan- tity of each gas passing through a definite thickness and ■across a definite area being recorded in Table I. Coal slab. Air- Hards at right angles to cleavage Hards in direc- tion of clea- vage Cannel at right angles to cleavage Softs in direc- tion of clea- vage Carbon dioxide— Hards in direc-. tion of clea- vage Hards at right angles to cleavage over 17 hours Hards at right angles to cleavage over 243 hours Softs in direc- tion of clea- vage Hydrogen— Hards at right < angles to cleavage 1. Hards at right < angles to cleavage II. Hards at right angles to cleavage III. Methane— Hards at right angles to cleavage I. for first 66 hours Hards at right angles to cleavage II. for next 99 hours Table I. ga ■«a Area, in sq. mm. Pressure difference of gas on two sides of mercury in mm. Volume of gas passed per hour in cu. cm. 4’5 . .. 3’2 . 730 . 0’05 7 . .. 3’2 730 . 0’05 7 . .. 2’7 700 . 0’09 10 . .. IT . 700 . 0’95 crack (?) 7 . .. 3’2 .. 811 at commence- .. ment, 450 at end . 0’004 3 . .. 3 .. 700 at commence- .. ment, 655 at end . 0'003 3 . .. 3 .. 770 at commence- ment, 690 at end 0’003 10 . ..11. .. 730 at commence- .. ment, 650 at end . 0’6 crack (?) 4’5 .. 3’2 . .. 700 at commence- .. . ment, 3 0 at end . 0’4 4’5 . .. 3’2 . .. 730 at commence-.. ment, 620 at end . 0’4 3 . .. 3 . .. 730 at commence- .. ment, 625 at end . O’ 4 3 . .. 3 .. 760 at commence- .. ment, 753 at end . 0’006 3 . . 3 . .. 760 at commence- .. . 0’005 ment, 690 at end These results are very remarkable. They show that, contrary to what is usually supposed, solid coal is extremely airtight, and lets very little air or gas through, even with a driving pressure of a * whole atmosphere. Carbon dioxide passes through the slab much more slowly than hydrogen, and to a smaller extent the same may be said to be true of methane. Carbon dioxide and methane being both very soluble in coal, the former especially so, it was thought that this question of solu- bility might affect the results for the initial stages of the flow of gas through the slab—that is, until the coal at different points became saturated at the pressure of gas finally existing at such points. In the light of experi- ments on the solubility of methane in coarse coal (T% to | in. particles), the mass of coal between the two tubes should become saturated in 60 hours at the most. It was found, however, that the rate of flow for the first 66 hours is practically the same as for the following 100 hours. A similar result was obtained for carbon dioxide. All these figures for the rate of flow of gases through a coal slab are so small that they should be looked upon as rather giving the order of permeability than as abso- lutely accurate results. Much more work remains to be done in order to investigate this question thoroughly. * From a paper read before the Institution of Mining Engineers. If one considers a section of a working face of coal 3 square centimetres in area (that is, the area of the experimental slab), calculating from the results obtained, it would take about twenty years for 10cu.cm, of air to penetrate to a depth of 5 yds. Practically speaking, the amount of oxygen that would get further than a few inches of the external surface of coal would be nil, owing to the very slight permeability of coal, and the fact that the difference in oxygen pressure outside and inside a lump of coal will never be more than about a fifth of an atmosphere. The quantity of oxygen, therefore, which can permeate any distance into a perfectly solid pillar of coal is negligible; hence, when through any cause a sufficient supply of oxygen or air can get to the central part of such a pillar, rapid oxidation may take place, even after the pillar has been standing for a great many years. Haldane and Meachcm have recorded the fact that in the thick coal of South Staffordshire, at a depth of about 2,000ft., many serious heatings and fires occur in the walls of main roads driven through solid coal. The present experiments make it easy to understand why these fires are liable to occur many years after a road has been made. They also show the importance of pre- venting any standing face, rib, or pillar of solid coal from being broken down by pressure. As solid coal is almost impermeable to air or gas, it is evident that changes in barometric pressure can have no appreciable influence on the emission of firedamp or blackdamp from -solid coal. Another fact which is at once explained is that in comparatively shallow bore- holes in solid coal very heavy gas pressures are met with. These pressures were discovered, and accurately observed, in various seams by Sir Lindsay Wood.* The results of subsequent observations in other countries are collected in a recent paper by N. H. Darton.f If the coal is not fissured, the gas cannot escape through the solid coal, and the pressures (up to 461 lb. per square inch, or 32 atmospheres, in Sir Lindsay Wood’s obser- vations) which gradually accumulate in a complete gas- tight borehole are thus readily intelligible. It has hitherto been generally supposed that the fire- damp which is given off from coal is contained in pores in the coal, and probably liquefied under great pressure. This cannot be the case in coal mines since the critical temperature of methane is 95-5degs. Cent. (Dewar), at which temperature a pressure of 50 atmospheres is required for liquefaction to take place. Any pores, how- ever, which exist in coal are certainly very minute, for the specific gravity of very finely-divided coal is hardly different from that of larger masses. In Part II. of his paper on “ The Absorption of Oxygen by Coal,” + Mr. Winmill described an experiment in which fine coal dust appeared to have a specific gravity of 2*6 when determined by displacement in nitrogen. In the light of the results detailed in this paper, the high result obtained for the specific gravity was due to the solubility of nitrogen in the coal. For the purposes of this investigation, the specific gravities of the various samples of coal have been determined by the method of displacement in the toluene (a liquid which appears to thoroughly ‘‘ wet ” the coal dust)j after pumping all gas out of the dust sample. The fact that the figure for the specific gravity of coal determined by displacement in hydrogen at 100 degs. Gent, closely approximates to that obtained by the toluene method,' confirms the results obtained by the latter. Mr. Win- mill’s results for the solubility of carbon dioxide were evidently calculated after taking the specific gravity of coal dust as equal to 2'6. However, if calculations are made from the true specific gravity, they will fall into line with the experimented results given in this paper. The experiments made by the author on the determin- ation of the solubility of methane in different samples of coal, for fine and coarse coal samples, and for samples at different temperatures, afforded material for curves connecting the amount absorbed with the concentration of the gas over the dust; and these results are tabulated in Tables II. and III. in order that the solubilities at the same partial pressure of gas may be easily compared. Table 11.—Methane Dissolved PER 100 Grammes of Coal in Cubic Centimetres AT 0 degs. Cent, and 760 mm. Pressure. Sample Ground to Pass a 200-mesm Sieve. centage Barnsley Barnsley Welsh A y]|d^Cannel haids. softs, steam. Mother of coal. or gas. seam. 10 ... 22’6 . .. 26’5 . .. 32’7 ... 19’8 ... 15’6 . .. 11 5 20 ... 44 T . .. 52 2 .. .. 63’4 ... 39’2 ... 30’4 . .. 22’7 50 ... 101’7 . .122’5 .. .. 142’5 ... 92’2 ... 71’5 . .. 54’5 100 ... 184 0 . 219’5 .. . 252’0 ... 166’5 ...134’5 . ..100’0 — ■ — ■■ -I- .— I— ■■ — Per cent. of ash in sample... 4’7 .. .. 2’8 .. 4’7 ... 6’2 ... 1’9 . .. 3’2 Specific gravity 1’31. .. 1’285.. 1’31... 1’325 ... 1’305 1’55 The absorption of oxygen by coal must not be con- fused with the phenomena with which the present paper deals, namely, the “ solubilities ” of gases in coal. The former is the result of definite chemical action, and it is not possible to extract from the coal the oxygen that has been added—it has become definitely absorbed by the coal substances to form some oxidation product. In the case of the solubilities of gases, however, the action is reversible, and the total quantity of gas added may be pumped out again from the coal by suitable means. This has been done in the case of soft coal and mother of coal. In the latter case— Total quantity of CH4 added = ................ 102’3 c.c. ,, ,, „ pumped out = between 102 & 103c c. A similar result was obtained for the soft coal, the * Trans. N. E. Inst. “ The Pressure of Gas in the Solid Coal,” by Lindsay Wood: 1881, vol. xxx., p. 163. | “ The Occurrence of Explosive Gases in Coal Mines.” Ame"ican Bureau of Mines, Bulletin No. 7*2. I Trans. Inst. M. E., 1915, vol. xlviii., p. 503. Table III.— 1’he Solubility of Differs :nt Gases in the same Coal Sample THROUGH A 20U-MEbH SIEVE AT 30 degs. and 100 DEGS. Cent. Results given in Cubic Centimetres Measured at 0 degs. Cent, and 760 mm. Pressure. Percenta ve of gas. / y Mo. 2w 50. 100. Methane— At 30 degs. Cent 18’9 ... 37'1 ... 85 9 ... — At lOo degs. Cent 5 4 ... 10’5 ... 25’0 .. 47’6 Carbon monoxide— At 30 degs. Cent. ... 8’1 ... 16’1 ... •33’3 ... 712 At 100 degs. Cent 1’6 ... 3’2 ... 8’0 ... 15’9 Carbon dioxide— At 30 degs. Cent 154 ... 275 ... 529 ... 800 At 100 degs. Cent 18 ... 35’2 ... 81’7 ... 148 Nitrogen — At 30 degs Cent 6’0 ... 12’1 ... 29’6 ... 57'6 At 100 degs. Cent IT ... 2’1 ... 5’5 ... 11’5 Hydrogen— At 3u degs. Cent 0’7 1’45 ... 3’50 ... 6’81 At 100 degs. Cent 0’35 ... 0’75 ... 1’95 ... 3’8 Percentage of methane (CHJ. 10 50 100 amount it was possible to extract being found equal to that originally added. The solubility of oxygen—as distinct from the ordinary (chemical) absorption by coal—has also been determined for a sample of oxidised coal dust, it being, of course, impossible to determine the solubility in unoxidised coal directly. The sample used had been oxidised for about 14 days at 100 degs. Cent., and even after this a slight- absorption at 30 degs. Cent, still took place. For this, however, a correction would be made, and through this it was found that oxygen was soluble in that oxidised sample of coal dust (through a 200-mcsh sieve) to the extent of about 40 c.c. per 100 grm. of dust when the percentage of oxygen over the dust equalled 100. The solubility of nitrogen in the same sample was found to be equal to approximately 35 c.c. for 100 per cent, nitrogen per 100 grm. of dust. The figure for the solu- bility of nitrogen in unoxidised coal for similar concen- tration was 57*6. It is highly probable, therefore, that the solubility of oxygen in unoxidised coal will be some- where about 65 c.c. in pure oxygen per 100 grm. of dust. The influence of the coarseness of the dust has also been determined. A sample of hard coal ground to pass a | in. mesh sieve, but not passing a to in. mesh sieve, was fr-eed from all fine dust. Part of this sample was then powdered up to pass a 200-mesh sieve. The solu- bility of methane in the fine and coarse samples was then determined, each sample having been heated to 95 degs. Cent, for several hours to drive off hydrogen, and the coarse sample being further exhausted by means of a sprayed mercury pump for about 10 days, to ensure that all gas had been given off. In the case of the coarse sample, the gauge reading only became constant after about 50 hours, when employ- ing a concentration of 90 per cent, of gas. A reference to Table II. will show that, for different samples of coal, -the solubility is of much the same degree, although the actual amount dissolved does vary. This difference is fairly marked in the case of the South Wales sample (a very gassy coal) and the Scotch sample, which originally contained very little gas. Table III. shows there is a considerable variation in the degree of solubility of various gases, and that this diminishes in every case with rise of temperature. For the less soluble gases the amount dissolved is, practically speaking, directly proportional to the pressure or concen- tration of the gas over the coal. In other words, Henry’s law for the- aqueous solution of gases is obeyed by coal. For the more soluble gases—carbon monoxide, methane, and carbon dioxide—however, the ‘‘-amount dissolved— concentration of gas ”—curves deviate somewhat from a straight line, this deviation being the more evident the greater the solubility of the gas. Table IV.—Cubic Centimetres Dissolved in 100Gr\mmes of Coal at 30 degs. Cent. (1). . (2). Coal passing a Same sample 2-meshbut as (!) ground remaining in to pass a 1 (2)* 10-mesh sieve. 200-mesh sieve. 17 ... 2*2’6 ... 75 79 ... 101’7 ... 77 144 ... 184 ... 78 In these cases it will be seen, from reference to the tables, that the amount dissolved at low concentration of gas is proportionally greater than that dissolved at higher concentration. Sir Lindsay Wood’s investigations on the gas pressures in coal seams gave pressures of from 2 to a maximum of 32 atmospheres, so that the quantities of methane which may be dissolved in coal underground are enormous. The theory that the gas is simply contained in fires in the coal is evidently quite untenable, since it follows from the foregoing data that coal even at a temperature of 30 degs. Cent, may take up at atmospheric pressure over three times its volume of methane. The solubility in coal of other gases besides methane renders intel- ligible the surprisingly large amounts of those gases obtained by Bcdson* by means of a vacuum pump. The fact that coal dissolves nearly its own volume of carbon monoxide is of some practical interest, for it is evident that coal which has been for some time in contact with air containing carbon monoxide will give this gas off to a current of pure air, and so become a source of unsus- pected danger to rescuers. On the other hand, air con- taining freshly-formed carbon monoxide will gradually 1 >se it in passing slowly over coal. This may help to explain the unexpected survival of men and horses after explosions and fires, and the survival of many of the mice living in the zone of an explosion. The solubility of gases in finely-powdered shale-dust has also been determined. In every case, however, this has been found to be small compared with the solubility in coal dust. For example, carbon dioxide only dissolves to the extent of a twenty-fifth of the quantity dissolving in an equal weight of coal dust. * Trans. N. E. Inst., 1887-88, vol. xxxvii., p. 245.