Modern Gasworks Practice

HIGH TEMPERATURE CARBONIZATION OF COAL 383 (2) Evolution of hydrocarbons of the paraffin series ceases approximately at temperatures above /OO C1. (3) Ethane, propane, butane, and liigher members of the paraffin series form a large percentage of the gases evolved at temperatures below 450° C. From these facts Burgess and Wheeler drew the conclusion that coal contains a compound which undergoes decomposition at temperature above 700° C., and yields hydrogen as its principal gaseous product; and they held the same compound to be responsible for those hydrocarbons of the paraffin series making their appearance at low temperatures. Taylor and Porter,1 as a result of a mimber of experimeirts with American coal, have shown that a high-grade bituminous coal of the gas-making type decom-poses by heat primarily into paraffin hydrocarbons, and a completfely altered non-volatile residue, with. small quantities of water, carbon dioxide, and carbon monoxide. The three latter products are the first produced, although in small quantity. From some other types of bituminous coal, they are produced in greater relative quantities th.an from the gas-coal type. Complex and varied secondary reactions induced by superheating the hydrocarbons, water vapour, and carbon dioxide are of great importance in industrial high-temperature carbonization. The products of low-temperature carbonization from coal of the Pittsburgh type on an industrial scale at about 427° to 482° C. will consist of a rich gas aniouiiting to 0-6 to 0-7 cubic feet per pound of coal, and a large yield of oil or tar, comprising 10 to 12 per cent, of the coal. This tar consists chiefly of paraffin hydrocarbons, is very low in (or possibly entirely devoid of) benzene and naphthalene derivatives, and practically devoid of free carbon. The gas will contain 6 to 7 per cent, of un-saturated hydrocarbons and 20 to 25 per cent, of ethane and its higher homologues, and consequently will liave a high calorific and illuminating value. The tar may be either redistilled or subjected to cracking processes, so as to pföduce light oils whose yield will be greater than, and probably at least double, that obtained by high-temperature carbonization. Tliese investigators also suggest the following hypothesis in connection with the Constitution of coal. All kinds of coal consist of cellulosic degradation products more or less altered by the processes of aging, together with derivatives of resinous substances in different proportions, also more or less altered. These substances are jmany in number, and closely graded into one another in their nature and com-position. They all undergo decomposition on moderate heating. Some, however, decompose more rapidly than. otters at the lower temperatures. The less altered cellulosic derivatives decompose more easily than the more altered derivatives, and also more easily than the resinous derivatives. The cellulosic derivatives on moderate heating decompose so as to yield water, carbon dioxide, carbon monoxide, and hydrocarbons, giving less of the first three products the more mature and altered they are. The resinous derivatives, on the other hånd, decompose on moderate heating, so as to yield principally the paraffin liydrocarbons, with probably hydrogen also as a direct decomposition product. 1 U.S. Bureau of Mines, Paper No. 140.