LOW-TEMPERATURE CARBONIZATION. By the low-temperature carbonization of coal is understood its partial coking at temperatures ranging from 500 to 600° C, with the con tain oils—obtainable to a much less extent under the high tempera tures above those named—as high even as r,000° Care some times spoken of as "low-temperatures" in this connection.
The late Vivien B. Lewes, in connection with the carbonization of coal regarded— "Low temperature" as ranging between 400° and 500° C.
"Medium temperature" as ranging between 500° and 900° C.
"High temperature" as above goo° C.
At gas works the coal is carbonized at temperatures ranging between 'Jo° and 1,300 degrees C. The original low-temperature process for the manufacture of "coalite" was based on the car bonization of coal at a temperature of 420 degrees C (788° F).
The objects which the kw-temperature carbonization of coal aims at securing are mainly the production of—(a) a smokeless fuel of high calorific value which can be burned in the domestic hearth or for the raising of steam, and (b) the production of cer tain oils—obtainable to a much less extent under the high tempera ture carbonization as applied in gas works and in the manufacture of metallurgical coke, in which processes the volatile hydro carbons are entirely extracted, but only to a small extent in the form of oils, whereas in the semi-coke resulting from low-tempera ture carbonization upwards to 12% may remain in the coke (see COKE).
In order fully to comprehend the difference between low and high temperature carbonization and the character of the respective products obtainable thereunder, it is necessary to study the effects on bituminous coal when treated to a rising temperature in the process of coking. Thus, at 1 oo° C (212° F) the removal of the moisture content of the coal begins and continues up to 260° C (500° F). Intumescence or swelling of the coal takes place at to 400° C. "Intumescence" is a characteristic peculiar to some bituminous coals, being particularly evident in the case of coking coals; steam coals do not intumesce.
In most coals the transformation of a great part of the coal from an insoluble to a soluble condition begins at 232° and continues to C (i.e., such substances as tar are soluble; coke
is not) : and it is only after such transformation has taken place that the distillation of the oil begins. The extraction of the oils from the coal covers a very short range of temperature. Beyond C practically no oil is distilled, but under "oils" are not included tars, pitches, naphthalenes, anthracenes or the phenols, all of which, with similar combinations, are produced beyond this point of temperature. Below 400° C there is very little decom posed tar left in the residual coke, so that, according to Prof. V. B. Lewes, between 400° C and 42o° C may be taken as the temperature at which coke forms. The yield of ammonia increases with the more complete gassification of the coal; that is to say, the higher the temperature the more complete the carbonization.
Comparing the results obtained by low-temperature carboniza tion with those due to high-temperature carbonization—under low temperature carbonization there is obtained (a) a lower volume of gas but a gas which is much richer than that obtained under high-temperature conditions. The calorific value of a "low temperature" gas as compared with that obtained under high temperature conditions in gas works with a given coal will be as about r,000 B.Th.U. to 450 B.Th.U.: (b) a higher yield of tar of a lower specific gravity than is obtainable under high-temperature carbonization : (c) a low yield of benzene and its homologues, owing to the fact that the tar formed under low-temperature con ditions contains a large proportion of paraffin, hydrocarbons and little benzene and toluene : (d) a low yield of ammonia com pounds: and, (e) the tar produced under high-temperature condi tions, though rich in the aromatic hydrocarbons (benzene, toluene, etc.), is so poor in the paraffins as to render these practically negligible ; consequently, under a process of high-temperature car bonization the light (fuel) oils are in great part lost.