Modern Gasworks Practice

382 MODERN GASWORKS PRACTICE THE DECOMPOSITION POINT OF COAL In considering the question as to the temperature at which coal, when subjected to heating, commences to decompose, it is necessary to distinguish between the evolution of the occluded gases (which may be evolved at a temperature so low as 100° C.) and the volatile products arising from actual decomposition. An enormous amount of research has been carried out in connection with the initial decomposition of coal, but it is only proposed to consider here the later theories which. have been propounded. Burgess and Wheeler1 heated bituminous coals in a vacuum, and showed the following to be the sequence of events:—Occluded or Condensed gases (unex-tractable at ordinary temperatures) continued to be removed in small quantity up to 150° or 200°. These gases were mainly the paraffin hydrocarbons, the higher members of the series predominating. A decomposition-point was reached at about 200° C., for there was then. a copious evolution of water (small quantities of ggs being also evolved), and water continued to.make its appearance at successive stages in the distillation up tos 50° C. (the.high.est temperature reached in the experiments). The gases evolved during the period of most rapid formation of water contained a high percentage of the oxides of carbon. The decomposition of some sulphur-containing organic compound occurred between 200° C. and 300° C. This decomposition began at about 270° C., and was approximately completed at 300° C. ; simultaneously with. the hydrogen sulphide resulting from the decomposition the gases evolved contained a considerable proportion of the higher olefin.es; the evolution of these did not, however, cease or diminish. until a temperature of 350° C. was reached. Liquids other than water began to distil at about 310°, at which. temperature a thin reddish-brown oil appeared. A decomposition point occurred at about 350°, when a rapid evolution of gas and much viscid oil was formed. Decomposition continued with increasing rapidity as the temperature was raised. Many other investigations may be quoted in support of the faet that the first products of decomposition are invariably water and the two oxides of carbon. As these substances are the first decomposition products of celluloses and ligno-celluloses Stopes and Wheeler2 point out that the natural conclusion is to consider the cellulosic constituents of the different fuels to be responsible for tlieir first decomposition products. It is also found that as we pass from fuels containing a high, proportion of oxygen to those with a high carbon content decomposition becomes more difficult. As regards the decomposition of coal the principal facts brought to light3 are as follows:—■■ ( 1) With all coals, whether bituminous, semi-bituminous, or anthracite, there is a well-defined decomposition-point, at a temperature lying between 700° and 800° C., which corresponds with a marked increase in the quantity of hydrogen evolved. With bituminous coals the increase in the quantity of hydrogen evolved falls off at temperatures above 900°, but with. anthracite coals it is mairrtained up to 1,100° C. 2 Trans. Chem. Soc., vol. 105 (1914), p. 131. 3 The Constitution of Coal, 1918, p. 9. 3 Stopes and Wheeler, loc. eit.