Chapter LV.
ILLUMINATING GAS.
304. Source.—The three main elements in combustion are O, H, C. Air supplies O, the supporter; C and H are usually united, as hydro-carbons, in luminants and combustibles. H gives little light in burning; C gives much. The fibers of plants contain hydro-carbons, and by destructive distillation these are separated, as gases, from wood and coal, and used for illuminating purposes. Mineral coal is fossilized vegetable matter; anthracite has had most of the volatile hydro-carbons removed by distillation in the earth; bituminous and cannel coals retain them. These latter coals are distilled, and furnish us illuminating gas.
Experiment 129.—Put into a t.t. 20 g. of cannel coal in fine pieces. Heat, and collect the gas over H2O. Test its combustibility. Notice any impurities, such as tar, adhering to the sides of the t.t., or of the receiver after combustion. Try to ignite a piece of cannel coal by holding it in a Bunsen flame. Is it the C which burns, or the hydrocarbons? Distil some wood shavings in a small ignition-tube, and light the escaping gas.
305. Preparation and Purification.—To make illuminating gas, fire-clay retorts filled with coal are heated to 1100 degrees or more, over a fire of coke or coal. Tubes lead the distilled gas into a horizontal pipe, called the hydraulic main, partly filled with water, into which the ends of the gas-pipe dip. The gas then passes through condensers consisting of several hundred feet of vertical pipe, through high towers, called washers, in which a fine spray Fig. 60. Gas Works.
A, furnace; C, retorts containing coal; T, gas-tubes leading to B, the hydraulic main; D, condensers; O, washers, with a spray of water, and sometimes coke; M, purifiers-ferric oxide or lime; G, gas-holder. In C remain the coke and gas carbon. At B, D, E, and O, coal tar, H2O, NH3, CO2, and SO2 are removed. At M are taken out H2S and CO2.of water falls, into chambers with shelves containing the purifiers CaO or hydrated Fe2O3, and finally into a gas-holder, whence it is distributed. At the hydraulic main, condensers, washers, and purifiers, certain impurities are removed froth the gas. Coke is the solid C residue after distillation. Gas-carbon, also a solid, is formed by the separation of the heavier hydro-carbons at high temperature, and is deposited on the sides of the retort.
Coal gas, as it leaves the retort, has many impurities. It is accompanied with about 3 its weight of coal tar, 1/2 its weight of H2O vapor, 1/50 NH3, 1/20 CO2, 1/20 to 1/50 H2S, 1/300 to 1/600 S in other forms. The tar is mostly taken out at the hydraulic main, which also withdraws some H2O with other impurities in solution. The condensers remove the rest of the tar, and the H2O, except what is necessary to saturate the gas. At the main, the condensers, and the washers, NH3 is abstracted, CO2 and H2S are much reduced, and the other S compounds are diminished. Lime purification removes CO2 and H2S, and, to some extent, other S compounds. Iron purification removes H2S. Fe2O3 + 3 H2S = 2 FeS + S + 3 H2O.
The FeS is revivified by exposure to the air. 2 FeS + O3 = Fe2O3 + 2S. It can then be used again. H2S, if not separated, burns with the gas, forming H2S03, which oxidizes in the air to H2SO4; hence the need of removing it. CO2 diminishes the illuminating power.
306. Composition.—Even when freed from its impurities coal-gas is a very complex mixture, the chief components being nearly as follows:—
Percent Diluents, having little C, give H 45) very little light. Notice the small CH, 41) diluents. percentage of luminants, or light- CO 5 ) giving compounds, also the proportion C,HB 1.3) of C to H in them. C,H6 1.2)luminants. CZH4 2.5) Cannel coal contains more of C02 2) impurities. the heavy bydro-carbons, CnH2n, N, etc. 2) etc., than the ordinary bituminous 100 coal. Ten per cent of the coal should be cannel; naphtha is, however, often employed to subserve the same purpose, one ton of ordinary bituminous coal requiring four gallons of oil.