Modern Gasworks Practice

GAS-MAKING AND OTHER COALS 357 tion as the process advances. The carbon, it must be understood, increases, not synthetically, but merely owing to the evolution of the remaining constitucnts ; which. faet is best illustrated by recalling that the volume of the final substance amounts to only one-tenth. to one-sixteenth of the original vegetation from which. it was formed. Cellulose, one of the main constituents of vegetable tissues, may be considered as the basis from which. the gradual evolution of coal eau be convcniently followed. It has been shown that the carbonic acid exhaled by animal life and resulting from all forms of combustion is regenerated into oxygen by plant life coupled with. the agency of the sun. Expressed chemically in its most simple form, the nature of this extremely involved action is somewhat as follows :— 6 CO2 + 5 H2O = C6H10O5 (cellulose) + 6 O2. Experimental results in connection with the artificial conversion of cellulose indicate that it undergoes changes on the following lines :— 4 C6H10O5 = C2iH16O2 (coal-like residue) + 3 CO2 + 12 H2O. Although. however, this may be true for artificial conversion, we know that in all natural conversion methane is produced; thus, considering pure cellulose, humus matter would be given by th.e decomposition, and the reaction would approach the following :— 6 (C6H10O5) = C21H20O8 (humus) + 8 CO2 + 7 CH4 + 6 H2O. Bergius says that the differentiation of the various types of coal is not due to the time occupied in their formation, or to the prevailing temperature, but to marked inerease in the pressure ; and he supports his theory, and shows it to be in conformity with. geological facts, by pointing out ih at anthracite has been formed where. the earth’s erust has become folded, thus squeezing the layers of ■carbonaceous matter to a terrific extent. Peat, lignite, bituminous coals, and anthracite form a series further and further removed from vegetation and wood in composition. and character. The brown colouring of the lignite is a transition stage to the black of the coal, and whilst the vegetable structure disappears the specific gravity increases uniformly, until (as is now not generally disputed) the final stage of conversion is arrived at by the formation of graphite containing 100 per cent, of carbon. A point of importance to note is that a portion of the total carbon, is in such a state of combination that it is capable of being volatilized, but this proportion decreases as the formation proceeds. It is for this reason that steam coals and anthracites are of no use to the gas engineer, owing to their indifferent capabilities as regards gas yields and coke. Steam coals are inter-mediate in composition between bituminous coals and anthracite, and should not be confused with the latter. In average bituminous coals, as used for gas-making, the volatile carbon amounts to from 15 to 20 per cent, of the total carbon, and is of extreme importance, in that it accounts on distillation for the most valuable constituents of the gas and tar.