Modern Gasworks Practice

HIGH TEMPERATURE CARBONIZATION OF COAL 385 the thickness of the modem heavy charge the heat penetrates to the interiør of the mass comparatively slowly; accordingly hydrocarbons and gases of a primary-nature are continuously being evolved until the central cool core is carbonized. through. Portions of these primary gases, however, only exist momentarily. They tmdergo degradation into simpler compounds on coming into contact with. the heated coke, and are further decomposed whilst travelling along the free space to the outlet of the retort. For this reason it is advantageous to distinguish between not only primary and secondary products, but also between primary, secondary and tertiary efiects. That is to say, to consider the ultimate substances obtained to consist of:— (a) True primary products escaping further degradation. (b) Secondary products resulting from decomposition due to passage through the hot coke mass. (c) Tertiary products resulting from the further degradation of secondary products owing to the exposure of the latter to radiant heat and to hot surfaces during their passage from the retort. It is the hydrocarbons of botli the saturated and unsaturated series which are chiefly affected by these further influences, but oxygen-bearing substances and nitrogen compounds also undergo some change. In addition to being present in the coal substance, oxygen is derived in some quantities from the moisture in coal, the latter varying from 4 to 10 per cent. This moisture is in the main detrimental to the process of carbonization, in that it tends to reduce the temperature of the retort owing to the large quantity of latent heat absorbed in converting the water into steam. In the earlier stages the oxygen of the coal, as distinet from that emanating from the moisture, is evolved largely as oxi'des of carbon, whilst on combination with the hydrogen some portion, will give rise to water. Colman is of the opinion that the oxygen is originally evolved as volatile compounds of carbon, hydrogen and oxygen, and that these on further heating are decomposed into carbon monoxide, carbon dioxide and steam. Whatever the manner of formation, of the oxides of carbon, however, it is quite evident that the carbon dioxide, during its passage through the outer coke layers, tends to combine with the red-hot carbon, undergoing reduction to carbon monoxide. In this way secondary reaction is responsible for an inerease in the combustible gases at the expense of an inert and useless diluent. The greater the temperature of distillation the more prouounced will this interchange be. Of the oxygen present in coal the greater portion combines with hydrogen to yield steam, about 25 per cent, exists as oxides of carbon, and a small proportion is found as compounds of hydrogen and carbon, such as the phenols, etc., in the tar. So far as the steam derived both from the original moisture in the coal and from the direct. combination of hydrogen and oxygen is concerned, a large proportion of this will eventually condense out as water, whilst some portion will accoimt for the formation of water gas during its passage through the heated coke. It will be seen, accordingly, that the importance attached in the past to the “ unoxidized hydrogen ” content of a coal was mucli over-estimated. for an appreciable portion c c