Modern Gasworks Practice

618 MODERN GASWORKS PRACTICE residue remains. It has been stated that the reaction takes place on lines such as the following— 2 C6H5NH2 + CS2 = (C6H5NH)2CS + H2S. The carbon disulphide is thus precipitated in the form of a solid, namely, thio-carbanilide. The outstaiiding difficulty would appear to be the loss of the amino compounds by volatilization. This loss may, however, be curtailed by washing the gas with weak acids, but the cost of the process renders it prohibitive for use on a working scale. The Liming of Coal The treatment of coal by adding lime to it in the retort is by no means novel, for it was attempted in the earliest days of gas lighting. The use of lime for this purpose did not, however, become anything like general, and any advantages which may result from the process seem to have been lost sight of for a long time, until. in 1882, W. J. Cooper was granted a patent relating to the treatment of coal in this way. Cooper claimed substantial advantages for his system, amongst otters an enhanced illuminating power, a better coke, less impurities, and more ammonia.. Present-day interest in coal-liming is chiefly attached to the method as carried on at Cheltenham Gas Works, and with which the names of R. 0. and J. Paterson are particularly associated. Liming at Cheltenham was first practised in the early ’eighties, and in spite of the repeal of the sulphur restrictions in 1906, the process, though dropped for a time, was reintroduced in 1910. In its modern form the system consists in admitting to the coal, just prior to its entering the breaker, a small proportion of caustic lime. In Order that the liming effect may be uniform a small steam jet is allowed to play on the coal as it falls from the breaker. In this way the lime is fixed to the surface of the coal, giving each small particle of coal a regulär coating. A further quantity of steam, introduced at the overhead retort-house hoppers, completes the fixing process. So far as results are concemed, it is of interest to note that at Cheltenham the sulphur compounds were reduced to a steady average of about 21J grains per 100 cubic feet of gas. The yield of gas per ton of coal advanced by over 800 cubic feet, whilst the sulphate of ammonia sliowed an increase of 1-88 1b. per ton of coal. The quantity of lime employed varies from 1-5 to 2 per cent, of the weight of coal car-bonized, and it is estimated that of the total quantity 75 per cent, remains behind in the coke. The intennixture of lime with coal is adopted on gasworks, not with a view to increasing the make of gas and other products, but primarily for the purpose of curtailing the sulphur compounds in the finished gas. The männer in which. this object is attained is not wholly understood, the reaction, both. chemically and physic-ally, taking place on extremely complex lines. It has been suggested that the addition of lime is followed by a decrease of sulphur compounds in the gas, but a corresponding increase may be noted in the sulphur contents of the other products, chiefly the coke. Possibly the presence of the lime merely curtails the formation of CS 2, the gaseous sulphur being evolved almost wholly as sulphuretted hydrogen.