GAS [COAL] (FR., Gaz d la HouWe ; GER., Steinkohlengas).
In the present article, it is proposed to give a brief resumd of the chemistry and general physics of the manufacture and purification of coal-gas, avoiding, as far as possible, descriptive details of the mechanical appliances by which such manufacture and purification are practically carried out, accounts of the various appliances used at a gas-works, belong more properly to the domain of engineering, and the reader desirous of acquainting himself with that branch of the subject is referred to Spans" Dictionary of Engineering' and its ' Supplement,' in which all the desired information will be found.
The production of illuminating-gas from coal may be considered as a rearrangement of the elementary constituents of the coal, under the influence of heat, this rearrangement being so conducted as to afford the particular products which are most valuable to the gas-maker. Apart from the composition of the original coal. there are many conditions which affect the quality and general character of the gas produced, chief among which may be mentioned the following, viz. :— (1) whether the coal has beeu carefully stored, or exposed to a long continuation of atmospheric changes ; (2) whether the coal at the time of distillation is wet or dry; (3) the temperature at which the distillation is carried out ; and (4), the general perfection of manufacturing details and subsequent purification. Briefly stated, the conditions most favourable to success may be thus
enumerated :—The coal should be of good quality, rich in hydrogen, and with a low proportion of sulphur and ash ; it should he used for gas manufacture as soon as possible after its removal from the pit, or be carefully stored under cover until required; at the time of its distillation, it should be as dry as possible, and the temperature used should be a full cherry-red beat, approaching to whiteness ; the gas should be removed from contact with the heated retort as soon after its genera tion from the coal as can be conveniently effected; and lastly, the purification of the gas should be efficiently carried out, more especially with regard to the removal of carbonic acid, sulphuretted hydrogen, and ammonia. In discussing the general character of the physical and chemical changes which take place from the manufacture of the gas to its final storage, the subject naturally divides itself into three beads, viz. :—(1) The changes which occur during the distillation of the coal; (2) the character of the crude gas ; and (3) the subsequent purification of the crude material. The discussion of tho subject under the three divisions named will now be proceeded with.