Modern Gasworks Practice

CHAPTER XXI THE COMPLETE GASIFICATION OF COAL- When coal is treated in the ordinary manner in retorts the volatile matter is gasified, ab out 70 per cent, of the weight of the coal remaining behir.d in the fixed form as carbon and ash. In many cases the fixed residue is wholly or partly treated with steam for the production of a leaner gas (“ blue ” water gas) which is eventually admixed. with the higher quality straight coal gas. Such a method involves two entirely distinet processes and two separate plants differing considerably in structural features. In the early days of water-gas production an attempt was made to combine in one plant the two separate processes. At that time, however. progress was rendered difficult owing to the universal institution of illuminating power standards, while mechanical and structural knowledge had not reached a stage of development sufficient,ly advanced to overcome practical defeets. The recognition of caloriflc power, the official approval of the leaner types of gas, and the introduction of the therm basis of selling have, however, completely changed the situation, with the result that to-day the most efficient method of gas production must be regarded neither as that which yields the greatest number of cubic feet of gas per ton nor the system which yields the ricliest form of gas, but that which prodnees the cheapest British thermal unit. It has been recognized, though not definitely proved, that a British thermal unit, whatever Company it keeps, will always yield a definite duty on combustion. In other words, a gas containing 500 B.Th.U. per cubic foot will, when consumed, do twice the work of a gas having a calorific power of 250 B.Th.U. per cubic foot. Degree of concentration of heat units, in faet, does not affeet the proclivities of individual units. Comparison on such a basis is, of course, dependent upon the supposition that all gases, whatever their thermal concentration, are bumed in such a manner that the whole of their potential heat energy is liberated. By the complete gasification of coal is meant the treatment of the coal under such conditions that no solid residue other than ash remains. Complete gasification. may, of couise, be conducted in two distinet ways, namely, the treatment of the coal in two stages for the production firstly of straight coal gas and secondly of “blue ” water gas in distinet plants, the two gases being subsequently mixed either in the erude or purified state. The alternative is to employ plant of special design in which the above two processes are carried out simultaneously. From the standpoint of gas production the two methods may be contrasted as follows:— 756