Low-Temperature Carbonization

Slow Development.

The development of the low-tempera ture carbonization industry in Great Britain, America and the continent of Europe has been very slow, in fact it can hardly as yet be termed an industry, though an enormous amount of research work has been carried out in connection therewith, and its litera ture probably exceeds that bearing on the making of metallurgi cal coke, which has for a great many years been a prominent branch of industry. The reason is probably due to the fact that whereas in the latter there was ready to hand a market for the coke and for the by-products—sulphate of ammonia, tar, pitch, ben zene, etc.—in the case of low-temperature carbonization the markets have in some respects to be created.

In order that low-temperature carbonization processes may be profitable the resultant semi-coke, which constitutes about 7o to 8o% of the coal used in the process, must fetch a price at least equivalent to the purchase price of the coal, that is to say, must sell per ton at a higher price than that of the coal purchased, leav ing the by-products to pay the cost of working and to provide a profit—a fairly narrow margin. But the British public have been slow to realize the advantages of the semi-coke as a domestic fuel and prefer adhering to the wasteful use of coal to provide them with a cheerful flaming fire instead of paying the higher price for the hotter, smokeless, and almost flameless semi-coke.

Internal and External Heating.

The processes of low temperature carbonization may be divided into two main classes, namely those dependent on internal heating and those dependent on external heating, the former giving a large production of gas of comparativly low calorific value and consequently a com paratively small production of semi-coke, the latter a compar atively small volume of gas of high calorific value and a large yield of semi-coke. The coal used in most of the processes is in the form of slack, but in others it is in the form of dust. In some of the processes the retort is stationary, in others it rotates; in some the charge is stationary, in others it moves down vertical retorts by gravity, and in others again it moves with the rotation of the retort. In some of the low-temperature processes the through-put, as in the case of the metallurgical coke and the gas making industries, is intermittent, in others it is continuous.

A process of "double carbonization" i.e., high temperature fol lowing on low temperature, has been tried in the United States. In this process, the residual matter from the low-temperature carbonization retort is briquetted and the briquettes subjected to high-temperature carbonization to secure a high yield of ammonia and benzene compounds not available from the low-temperature carbonization. In some few of the processes which have been tested a semi-coke is obtained which is sufficiently hard and com pact to allow of its being used in metallurgical processes; but usually it is too soft, though eminently suitable for domestic use both in the open grate, stoves, and for central heating. Of course the make of semi-coke with any given process varies chiefly with the coal used, but may be taken to he from 12.5 to 15cwt. per ton of coal charged. Likewise, the make of tar oil varies in accordance Washed and refined oils Tar calculated to gallons per ton of dry coal with the chemical composition of the coal. The greater the hydrocarbon content in the coal the greater the yield of tar oil. The make of tar oil, refined oils, tar acids and pitch obtainable therefrom as obtained by the low-temperature treatment in the case of three Yorkshire coals and a mixture of two of them by the Fuel Research Board at their research station near. Greenwich

and given in the Board's report for the years 1920-21 is of interest. See table in previous column.

The following oils were specially separated from the Dalton Main Tar 2: Taking the Mitchell Main Coal the result may be expressed in percentages thus:— Analysis Moisture . . . 0.95 Volatile matter less moisture 0•95 Fixed carbon . . . . 62.41 Ash . . . . . 5.25 A high class and very bituminous coal.

Carbonized at 600° C.

Make of Coke, 77.88% or 15.58 cwt. per ton of coal carbonized. Volatiles in Coke, ii.81.

Yield of Gas, 3,44o cubic feet; B.T.U. per cu.ft., 1,090.

Therms per ton of coal, Yield of Tar (dry), 17.0 gallons per ton of coal.

Yield of Liquor, 26.4 gallons per ton of coal.

Ammonia sulphate, lb. per ton of coal, 8.6.

Motor Spirit, 1.63 gallons per ton of coal.

advantages of Semi-coke.

Great advantages would accrue were the fuel used for raising steam in the production of power and in domestic heating to be the semi-coke procured by the low temperature distillation of coal instead of the raw coal largely used. The atmosphere of towns would he polluted to a much less degree with consequent improvement in health, and the waste of national resources through the burning of the volatile hydrocarbons con tained in coal would be prevented. At least 35 million tons are annually devoted to domestic use, and assuming 15 gallons of fuel oil suitable for naval purposes if there were no fuel to he used for domestic purposes other than low-temperature semi-coke, there would be derivable in the manufacture thereof two million tons of fuel oil per annum for the Navy, which though less than the War requirements is considerably more than the peace require ments; and the motor spirit produced would amount to about ioo million gallons. Incidentally the national output of coal, presum ing there to be no saving of fuel effected, would have to be increased by at least io million tons per annum.

Reference has already been made to some of the oils of the paraffin group contained in the tar which can be used as fuel oil either for raising steam or for internal combustion or "cracked" to produce petrol and added to the spirit which is derived, chiefly, from stripping the gas, though some of it is available in the tar. This light spirit can, when refined, be used direct in motor cars or as an admixture to petrol; or, as it has "anti-knock" properties, be mixed advantageously with standard petrol. The tar acids— phenols and cresols—are also of value; they can be burned to raise steam, and Dr. Spilker has shown that primary tar distil lates of high phenol content can be used successfully in Diesel engines; but they have a use beyond this. Phenol (or carbolic acid) is used in large quantities in the manufacture of fieric acid and other explosives, also in the manufacture of dyes. The cresols (methyl phenols) which are produced in the distillation of coal both by the high-temperature and the low-temperature carbonization processes, are used as a disinfectant, as a sheep dip, for creosoting timber, and in the manufacture of "hakerlite" (an insulating material largely used in electrical transformation and generator construction) and other synthetic resins.

(See also LIGNITE, S.V. COAL.) (R. R.)