Modern Gasworks Practice

70 MODERN GASWORKS PRACTICE In brief, to take an exaggerated analogy, all attempts at working the producer on conditions approaching those of the water-gas generator (necessarily an inter-mittent operation, owing to the large thermal losses during gas-making) are to be condemned. A point well wortb. consideration in connection with the operation of retort producers is the effect of gas composition on efficiency, and the relative values of hydrogen and carbon monoxide as gaseous fuels. For instance, the calorific powers of hydrogen and carbon monoxide are practically identical, hence, at first sight, there seems little need to consider the character of the combustible gases so long as, in total, they conform to the usual figure. Thus we might have two distinet gases with identical proportions of combustibles. In one gas the combustibles might consist wholly of carbon monoxide, and in the second gas there might be nearly equal proportions of carbon monoxide and hydrogen. From the calorific power standpoint there would be nothing to choose between the two ; but, when the question of flame radiation is considered, it must be borne in mind that the radiative power of a CO gas flame is, per unit volume of gas burnt, nearly 2| times that of a hydrogen flame of the same size. Hence, it lias come to be recognized in many of the industries in wliich producer gas is employed that the best results are dependent upon a high proportion of carbon monoxide and a low percentage of hydrogen. [The practical working of producers and economy of fuel are further discussed in Chapter IV.] HORIZONTAL BENCHES : GENERAL CONSIDERATIONS Direct-fired furnaces, in spite of their inefficiency, are still to be found (par-ticularly as a stand-by) in country works of moderate size. Their chief recommenda-tion lies in the faet that in case of emergency spare units can be put under working conditions in før skorter time than is the case with gaseous settings, thus avoiding the necessity of keeping the latter under slow fires. The original type, however, with. air supplies entirely uncontrolled, is now giving way to a more efiicient design operated on semi-gaseous lines. This principis is sliown in Kg. 17, from which it will be seen that the air entering under the grate is controlled by dampers, so that a portion of the coke is burnt to CO instead of wholly to CO2. A further supply of air (also under control) is then admitted above the fuel, so that complete com-bustion takes place. In this way the supply can be regulated within finer limits than is the case when the whole is admitted under the grate, and the excess of air in the flue gases can be reduced by more than one-half. Works making up to 100 or 120 million, cubic feet per annum usually employ the 10-foot stop-ended retort, and where shovel charging is in vogue this system is undoubtedly the most suitable. A later introduction, however, is the 10-foot through bench seen in Fig. 18; whilst typical stop-ended settings are shown in Figs. 19 and 20. The single through beuch has the advantage that discharging machinery may be used ; and, the back-end of the retort, being close up to the retort-house wall, the coke can be pushed direct into the yard. A modem favouritc for