PROCESS OF MANUFACTURE. When the coal is placed in the retort the volatile matter is driven off by the exposure to heat, rapidly at first and then more and more slowly. The reactions taking place in the retorts are complex and not per fectly understood; but in general they consist of the decomposition of the coal into coke and heavy hydrocarbons, and the breaking down of the latter into lighter hydrocarbons, with the set ting free of hydrogen and marsh gas; and when the breaking down is carried too far, of solid carbon, which is deposited on the interior of the retort. Reactions also occur between some of the nitrogen and hydrogen, the hydrogen and sulphur, and the carbon and nitrogen by which comparatively small amounts of ammonia, sul phureted hydrogen, and cyanogen are formed. The coke which is left in the retort is com posed almost entirely of uncombined carbon, with a percentage of ash dependent upon the amount of ash in the coal. The extent to which the hydrocarbons are broken down increases with the temperature at which the retorts are main tained, and therefore the volume of the gas pro duced increases, and its illuminating value de rise above the guide framing, their stability being maintained by the weight of the lower lifts which they carry. In some cases, usually those of comparatively small holders, the guide fram ing has been completely done away with, the guiding being performed either by means of spiral guides fastened to the inside of the tank wall, and to the inner surfaces of the sections of the holder, or by means of wire ropes, creases, with each increase in the temperature of the retorts. The product of illuminating value and quantity is, however, up to a certain point, greater with high than with low heats, and the retorts are usually heated to a temperature of from 2000° to 2300° F. The gas produced is deteriorated in illuminating value if exposed to prolonged contact with the hot walls of the re tort, and to reduce the extent of this contact to a minimum the volume of the charge of coal should be as large as possible in proportion to the size of the retort. The existence of a pres sure in the retort also increases the contact be tween the gas and the walls, and it is to avoid this, as well as leakage of gas through minute cracks in the, clay, that the pressure is taken off the retort by means of the exhauster. In the United States the length of charge or time the coal is left in the retorts is usually four hours, the heat and the weight of charge being so proportioned that the gas is all driven off in this time. In England the length of charge is
usually five to six hours.
The gas leaving the retort is a mixture of permanent gases, principally hydrogen, marsh gas, and carbonic oxide, with some carbonic acid, nitrogen, sulphureted hydrogen, ammonia and cyanogen, hydrocarbon vapors that can be re tained in the gas by proper treatment and are the most important light-giving constituents, and hydrocarbon vapors that cannot be retained in the gas when it is reduced to ordinary tem peratures. The problem to be solved in the cooling of the gas, which begins as soon as it enters the standpipe, is the removal of these heavy vapors in such a manner as to leave in the gas a sufficient quantity of the lighter vapors to fully saturate it at the minimum temperature and maximum pressure to which it is to be sub jected in the future. After the gas has been cooled it is necessary to remove the ammonia and sulphureted hydrogen, and in some cases the carbonic acid and cyanogen are also taken out.
The heaviest of the vapors condense in the hydraulic main, forming tar, which must not be allowed to rise to the level of the lower edges of the dip-pipes, since if brought into intimate contact with the gas it will absorb the lighter hydrocarbon vapors. For this reason it is also necessary that the heavy tar, that is not de posited in the hydraulic main, should be re moved from the gas before it is cooled, and this is done by the friction tar-extractor. The lighter tar is then condensed out by the cooling effected in the condensers. This cooling should be done very gradually to avoid the 'condensation of vapors that should be retained in the gas. As the gas cools, some of the water-vapor, with which it is saturated, condenses and absorbs a portion of the ammonia, forming ammoniaeal liquor. The tar and ammoniacal liquor thus formed in the hydraulic main and the condensers are run off through suitable drains into wells. The portion of the ammonia that still remains in the gas when it leaves the condensers is removed in the washer and scrubber. By using weak ammoniacal liquor as the washing liquid in the first stages of the scrubbing, the ammonia is made to combine with carbonic acid and sul phureted hydrogen, the resulting liquor being an aqueous solution of carbonate, sulphide, and various other salts of ammonia.