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viously described. The remarks of Mr. Charles Hunt,' the late examiner in gas manufacture, on this subject, will be of interest:
• There is good ground for the opinion that the best allround results are obtained from small charges with moderate, as distinguished from very high, temperatures. It has generally been assumed that the deficient yield of tar, which usually accompanies the use of a high carbonizing temperature, is fully compensated by increased production of gas. In the author's experience, however, the highest production of gas has been accompanied by the largest yield of tar and ammoniacal liquor."
Mr. Hunt then proceeds to quote Mr. Wright's experiments showing the gain resulting from the employment of the higher temperature in the table on p. 57, remarking that the results obtained in the laboratory of the Windsor Street Gas Works, Birmingham, with very high yields of gas, showed a falling off in illuminating value. The experiments were made on 56 lbs. of North Staffordshire gas nuts carbonized in a small iron retort, at temperatures ranging from 1400° F. to 1835° F. found that the best results, that is, the greatest number of “ candles per ton,” were obtained at a temperature of 1600° F.
Mr. Hunt, commenting on the results of these experiments, then states that he “ does not wish it to be supposed that he considers the best the temperature which gives the best result in a small iron retort suitable for the retort-house, where the coal is to be burnt off in a fixed time, viz., four to six hours, but he is of opinion that the lowest temperature which will suffice for this is to be preferred. For while the best all-round results are to be obtained by carbonizing at a fairly high temperature, it is
“Minutes of Proceedings, Institution of Civil Engineers," vol. cxvii., Part III.
essential that the gaseous products should be enabled to pass fully away without encountering in the ascension pipe any absorbent of hydrocarbons, such as thick tar. When obstruction occurs to any extent in the shape of stopped pipes, and with high temperature these are seldom entirely absent, the loss thus occasioned is likely to turn the scale in favour of a lower temperature."
The best temperature for obtaining a good yield of gas of fair illuminating power, from Newcastle coal, is that of a dull orange, which is supposed to be about 2010° F. Different kinds of coal, whether cannel or caking, give off their
gas in different times, even when subjected to the same conditions as regards temperature and size of retort. Cannel, as a rule, is more easily distilled than caking coal, as it yields the whole of its gas in about one-sixth less time.
In the case of cannel coal the first hour of the charge produces the most gas, while in the case of caking coal, the second hour is usually the most productive, the difference in the behaviour of the two kinds of coal in this respect being usually set down to the fact, that in the case of caking coal the amount of moisture present is converted into steam during the first hour, and this abstracts a large portion of the heat.
THE EFFECTS OF TEMPERATURE UPON THE PRODUCTION
HE yield and quality of the products obtained in the
destructive distillation of coalwill greatly vary, according to the temperature of the retorts; with the amount of coal carbonized; with the manner in which it is deposited in the retorts; with the period of distillation; and with a number of other factors.
We will first discuss the effects of temperature on the residual “ tar.”
The quantity, and more especially the quality, of the tar greatly depends upon the temperature at which the coal is distilled. Low temperatures with 9,000 cubic feet of gas per ton of coal will yield with some coals 16 gallons of tar, whilst high temperatures will yield but 9 gallons, with about 11,000 cubic feet of gas from the same coal."
If’ the temperature be a comparatively low one mostly such hydrocarbons are formed as belong to the paraffin (methane) series, having the general formula C, H,n + 2 along with olefines, C, H, n.
The lower members of these series are liquid, the higher ones are solid.
“They are always accompanied by oxygenized derivatives
1 Davis, “ Journ. Soc. Chem. Ind.," 1886, p. 5. 2 Lunge, “Coal-tar and Ammonia,” p. 17.
of the benzene series (phenols), but of these the more complicated ones predominate.
“ Liquid products prevail, and among the watery ones acetic acid (which is again a compound of the fatty series) is paramount. Of course also permanent gases are always given off, though in comparatively small quantity.
“If, on the other hand, the coal has been decomposed at a comparatively high temperature, the molecules are grouped quite differently. Whilst the olefines and members of the acetylene series still occur more or less, the hydrocarbons of the paraffin series disappear almost entirely, and from them are formed, on the one hand, compounds much richer in carbon, on the other hand, more hydrogenized bodies. The latter always occur in the gaseous state; hence the gas so produced contains methane, or marsh-gas, CH,, and free hydrogen, as principal constituents, and is very much increased in quantity. The carbon thus set free is partly deposited in the retorts themselves, and then occurs in a very compact graphitoidal form ; another portion of the free carbon occurs in a state of extremely fine division in the tar, and forms a constituent of the pitch or coke remaining behind from tar distilling; another portion contributes to the formation of compounds richer in carbon, belonging to the aromatic series, all of which are derived from benzene, C. H..
At the same time the action of heat effects further molecular · condensations, usually with separation of hydrogen, by which process compounds of a higher molecular weight areformed, as naphthalene, anthracene, phenanthrene, chry
The never-absent oxygen must also in this case cause the formation of phenols, but here phenol proper, or carbolic acid, C. H, (OH), predominates, whilst cresol and the other homologues are diminished in quantity, and the bioxy-benzenes, as well as their methylated derivatives disappear altogether.”
Shortly expressed, at low temperatures we obtain less permanent gases and more methylated derivatives (toluene, xylene, higher phenols); at a higher temperature more permanent gases, together with benzene, naphthalene, and anthracene, and considerably more free carbon is formed.
We are indebted to Mr. L. T. Wright, F.C.S., for some valuable information on the effect of temperature in modifying the quantity and quality of the tar produced in the destructive distillation of coal.
Mr. Wright found that the tar produced is affected to an even greater extent than the gas by alteration in the temperature at which the destructive distillation of the coal is conducted. Mr. Wright found that an increase of temperature in the process of distillation not only augmented the volume of gas given off, but it lowered the weight of tar, though it doubled the amount of free carbon in the tar, the specific gravity of the latter being increased. At lower temperatures the specific gravity of the tar produced is also lower. Experimenting with a caking coal, Mr. Wright obtained the following results:
Yield of gas per ton.
6,600 7,200 8,900 10,162 11,700
An analysis of the tar showed that the increase of the specific gravity was due to the increase in the quantity of pitch, which rose from 28.89 to 64:08 per cent., while the NH3, naphtha and light oils, steadily fell in quantity, the creosote and anthracene oils following in the same direc- · tion, but to a smaller extent. It is generally known that although the actual specific gravity of the tar obtained is