Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/99

Rh GAS 89 gas-coal that can be made from chemical analysis, the really satisfactory test being actual experiment. According to H. Fleck, the coal most available for gas—making should contain to every 100 parts of carbon 6 parts of hydrogen, of which 4 parts are available for forming hydrocarbon compounds. It is desirable that coal used for distillation in gas retorts should be as far as possible free from sulphur, that in the case of coking coal the amount of ash should be small, and the proportion of oxygen should also be low, since that ele- ment abstracts hydrogen to form injurious watery vapour. The amount of ash present, however, in the best forn1s of Scotch cannel is large ; and consequently the resulting coke, if the residue can be so called, is of comparatively little value. Unless coal can be stored in sheds which protect it from the weather, it ought to be used as soon as possible after being raised, rain a11d sunshine being detrimental to its gas—mak- ing qualities. The following table exhibits the chemical analysis and gas-yielding properties of a few of the principal and typical examples of coal for gas-making :- Oomposition of Cools usrcl in (z"as-Making. 2 i: =3 Variety of Coal. S 2 a 2 3’ __5'_- 3, 5-3 -3 -E‘ :3 :2. >. ,5; E : 6 m :2 35; 5 -7: Newcastle Pcureth Gas-Coal 824'! 482  0'86 1111 0'79 lllaydon Main, Tyncside .... .. 6'88 7806 5'80 185 2'22 312 8'9-I "“L“‘_i,‘]‘,‘:""‘~ “$“‘°“‘“'“‘°"' 6-19 33-25 5-75  0-36 5-06 3-43 vl;‘,;:lll Cannel..................... 5'65 8-907 5'71   7'82 2'40 .Iold-Lceswood Green Cannel  7751 8'47  0'71 632 Boglicad Canncl .... .. S t 12:63 6310 8-91  0'96 7'25 19-78 .a-ma Brown Shale 33,,‘ 9-33 00-44 7-54 1-36 0-34 10s4 1-2-93 lielty om st-am .... .. J -  70-50 5-03  0-94 11-03 2-25 Products of the Iﬁstillation of 1 ton of Coal. Illuminat- Cub. feet Lbs. of Lbs. of AI“::1‘0?]fi a ing power of Gas. Coke. Tar. Li um, of Gas in q ' Candles. Newcastle Cannel  9,883 1,426 983 60'0 25'2 '1gan Canncl .  10,850 1,332 2183 161'6 19'-1 Boghead Cannel  13,334 715 733'3 ml. 46'2 When the bowl of an ordinary clay pipe is ﬁlled with small fragments of bituminous coal, luted over with clay and placed in a bright ﬁre, immediately smoke is seen to issue from the stalk which projects beyond the ﬁre. The smoke soon ceases, and if a light is then applied to the oriﬁce of the stalk, the issuing gas burns with a bright, steady ﬂame, while a proportion of a black, thin, tarry liquid oozes out fron1 the stalk. After the combustion ceases there is left in the bowl of the pipe a quantity of char or coke. This simple operation is, on a small scale, an exact counter- part of the process by which the destructive distillation of coal is accomplished in the manufacture of gas. The pro- ducts of the distillatory process classed in the gas-works as gas, tar, and ammoniacal liquor, with a solid residue of coke, are in themselves mixtures of various deﬁnite chemical compounds ; and as may be evident from the following list, these substances are very numerous and complex :— Products of the I)ist£llatz'on of Coal at high-red heat. I. Illuminating Gases. II. Components of Tar. Acetylene, Cgll-_» ............. .. Bcnzol, (‘-EH6 .......... Ethylene, C2114 ........... .. Toluol. C,—ll3 .......... ... Liquid P*'0P)'l0n0v («s”a Cnmol, C9II,._. ....... .. I hydrocarbons. gutylelng. gal; ............. .. Cymol, c,,,n,, ....... .. cnzo, ,5 G ................... .., Naphthalin, C H Naphthalin, own, .......... V"1’°“""- Anthracene, C‘l,o,II8,o...l Solid "_!"d1‘°S0". H ................... Pyrene, Cwllw ....... .. ( hydrocarbons. Light carburetted hydro- . Crvscne C H ., 1)] ts. . I8 1. I-ICE. CH, ...................... .. I ‘ “en ' Carbolic acid, C;,ll,,('). Carbonic oxide, C0 (‘arbonic acid. C02 . Ammonia, NH3 Cyanogen, C2.'.. ........ .. Cresylic acid, C,Il8 Rosolic acid, C20IIlG03- Pyridine, C5H_.,N. Aniline, C¢;ll,N. Bisnlphidc of car‘oon, CS2 Iii Im- Picolinc, c,,n,N_ S‘11Ph111'0t1ed hrdrozzen H25 purities. Lutidinc, C,-lI,,N. 10,};-:ex1.0, ............. .. .. Collidine, c,,n,,x. I iogen, Ix ..... .. Lcucoline, C,I{,N. Aqueous vapour, II2 I ll. Ammoniaral Liquor. Amm nl l h t ‘I ' Ammonium carbonate, 2N H..C0,_ (3, um i::1y1’:zpnii)l(i2Tlll)I1i,(_:l{I(:.h'I‘CS. .. sulphydrate, I'll.,.lIS. ,, chloride, NIi,.Cl. IV. Coke and Ash in Retort. The proportions in which coal yields these products may be indicated by the case of u cannel givin 1: off 11,000 feet per ton of gas of a density of 01300. From 100 parts of such a coal there would be yielded- Gns 99-25 Tar .............................................................. .. 8'50 Ammonia water ............................................ .. 9'50 Coke 50-75 The proportions, however, and even the nature of these pro- ducts of distillation are greatly modiﬁed by the temperature at which the distillation is effected, a low red heat yield- ing a small proportion of non-condensible gas but a large amount of heavy hydrocarbon oils, whence the distillation of shales and coal in the par-afﬁn manufacture is conducted at a low red heat. By excessive heat, on the other hand, the compounds evolved become simpler in their chemical constitution, carbon is deposited, pure hydrogen is given off, and the gain in amount of gas produced is more than counterbalanced by its poverty in illuminating properties. Of the gases and vapours which pass out of the retorts in a highly heated condition, some portion, consisting of tarry matter and ammoniacal liquor, precipitates almost immedi- ately by simple cooling, and other injurious constituents must be removed by a system of puriﬁcation to which the gaseous products are submitted. What thereafter passes on as ordinary gas for consumption still contains some percentage of incombustible matters—aqueous vapour, oxygen, nitrogen, and carbonic acid. The combustible portion also is separable into two classes, viz., non-luminous supporters of combustion, and the luminiferous constitu- ents,-—the former embracing hydrogen, marsh gas (light carburetted hydrogen), and carbonic oxide, while the latter includes the hydrocarbon gases acetylene, ethylene (oleﬁant gas or heavy carburetted hydrogen), propylene, butylene, and vapours of the benzol and naphthalin series. Formerly it was the habit to regard the proportion of heavy carburetted hydrogen (ethylene and its homologues) as the measure of the illuminating power of a gas. It has, however, been pointed out by Berthelot that the proportion of such compounds in some gas of good luminous qualities is exceedingly small ; and in particular he cites the case of Paris gas, which, according to his analysis, contains only a mere trace of acetylene, ethylene, and other hydrocarbons, with 3 to 3'5 per cent. of benzol vapours. Subsequent ex- periments of Dittmar have proved that a mixture of pure ethylene and hydrogen burnt in the proportion of 3 volumes of hydrogen to 1 of ethylene yields little more light than ordinary marsh gas, while benzol vapour to the extent of only 3 per cent. in hydrogen, gives a brilliantly luminous ﬂame. Frankland and Thorne have more recently deter- mined the illuminating power of a cubic foot of benzol vapour burnt for 1 hour in various combinations, with the following results :- With hydrogen it gave the light of ................... .. 6971 candles. ,, carbonic oxide ,, .................. .. 73'38 ,, ,, marsh gas ,, ................... .. 9245 ,, ,, ,, (second series) ,, ................... .. 93'94 ,, Thus it is highly probable that the illuminating value of coal-gas depends much n1ore on the presence of benzol vapour than on the proportion of the heavy gaseous hydro- carbons,.and the estimation of benzol in the gas is a point which has hitherto been comparatively neglected. In view of the inference that the presence of benzol vapour is so intimately related to illuminating power, the fact observed by Dittmar that water readily and largely dissolves it out of any gas mixture is of great consequence. When ben- zolated hydrogen containing 6 per cent. of benzol vapour was shaken up with water, the percentage of the vapour was found on analysis to be reduced to le)s{s than 2. . — I2
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