Modern Gasworks Practice

384 MODERN GASWORKS PRACTICE The more mature bituminous coals with good coking properties contain a large percentage of resinous derivatives, and their cellulosic constituents have been highly altered. The younger bituminous and sub-bituminous coals consist chiefly of cellulosic derivatives much less altered than those in the older coals. They undergo a large amount of decomposition below their fusion points, and (partly for this reason) many of them do not coke. THE ELEMENTS IN CO AL AND THEIR RELATION TO CARBONIZATION Of the elements in coal, carbon is present in the greatest proportion, partly fixed and partly capable of volatilization. In the ordinary way from 15 to 20 per cent, of the total is present in the “ free ” state, and the actual distribution is somewhat as follows :— Distribution of Carbon in Coal (a) As hydrocarbons ........ 8 per cent, of total carbon. (&) As carbon monoride and dioxide ..... 2-3 „ „ „ (c) As tar .......... 8 „ „ „ (rf ) Remaining in coke ........ 80-85 „ „ „ (e) Other sources, such as “scurf” ..... 1 „ „ „ From the gasmaking point of view the carbon and hydrogen are the two most-important constituents. The hydrogen passes ofl partly in combination with oxygen as steam, and partly combined with carbon to form hydrocarbons of both the saturated and unsaturated series. By far the largest quantity, however, is found as free-hydrogen in the gas. A certain proportion of the hydrogen remains behind in the coke, but with the modern tendency to conduct carbonization at extremely high temperatures, little can withstand the final stages of the period, and the amount so remaining seldom exceeds 0-5 per cent. Although the solid residue left behind in the retort contains a greater proportion, of carbon and less hydrogen than the original coal, the reverse is the case with the volatile portion which shows a preponderance of hydrogen. The Constitution of this volatile portion largely depends upon the temperature prevailing. As a general rule the volume of permanent gas obtained is in direct ratio to the temperature employed, whilst the quality varies inversely with the temperature. Of the carbon and hydrogen compounds, those evolved at the lower temperatures of carbonization are mostly condensible at ordinary temperatures, and the permanent gas obtained consists mainly of methane, ethane and ethylene ; that is to say, hydrocarbons of the paraffin and olefiant series. In addition, a small proportion of hydrogen due to secondary reaction will be present. The liquid producta will be largely of a parafiinoid nature. It is during the early stages of carbonization that the richer constituents are evolved, and as the temperature throughout the entire mass gradually rises to the present limit of about 1,100° C. the volatile products are composed mainly of methane and hydrogen, the proportion of the former diminishing and of the latter increasing as the period proceeds, until finally little more than the residual hydrogen and nitrogen of the coke remain to be expellecL Owing to