Modern Gasworks Practice

 THE COMPLETE GASIFICATION OF COAL 771 the water gas thus formed rises through the coke and passes through the column of coal. As at the time of formation the water gas is at a temperature of 800° to 1,300° C., itcokesthe coal and liberates the volatile products. Passing through, the coal, the gases are gradually cooled, and leave the top of the producer at a temperature of 100° to 300° C. Thence they are successively conveyed to the condensers, washers, purifiers, and gasholder. The periods of run and blow vary with. the kind of coal employed, the average being three minutes blow with. five to ten minutes run. As the operation of coking absorbs a great deal of heat, and carniot be sustained with. the heat of incandescent water gas, air must be introduced during gasification. The superheated air is conveyed into the coking zone through two pipes, fitted with valves. The air oxidizes part of the water gas with the formation of carbon dioxide and water, and as both of these reactions are exothermic the requisite amount of heat is supplied for carbonization. Owing to the diluent efiect of nitrogen and CO2 it is, of course, necessary to limit the amount of air to the minimum just necessary for carbonization. The composition of Tri-gas obtained from a mixture of coal and lignite has been given above. It is interesting to note, however, that when a good quality coal alone is employed the composition of the gas obtained approximates to the following :— Composition of Tri-Gas (Good Quality Coal) Hydrogen__________________________50 per cent. Methane . . . . . . , . . 10 „ CnHm.............................1 Carbon monoxide ......... 30 „ Carbon dioxide ......... 5 „ Nitrogen .......... 4 „ The “ Double-Gas ” Plant This plant is generally known as Smith’s modification of the “ K. & A.” patents. In the original “ K. & A.” plant the system entails the use of two distinet generators, which are run through in series and blown through in parallel, thus complying with the necessity of arranging for a deep fuel-bed during the run and a shallow fuel-bed during the blow. In the latest type (Fig. 464) of plant the same procedure is arranged for, but there is only one generating vessel, instead of the two, thus reducing Capital outlay and minimizing heat losses. The outside stell, being of an oval cross-section, is divided into two parts by a plate suspended from the centre and reaching within a short distance of the base. The spaces on either side of this mid-feather form the two generators, whilst room is also found for special compartments, known as regenerators. It will be noticed from the sectional sketch that eacli generator is made up of two distinet portions, namely, the larger round coke receptacle at the base, whilst the tipper tapered portion is D-shaped and forms, among other duties, a charging pouch for the coke or coal. It will be seen that at the upper end of the lower generator a series of nostril holes is arranged