COKE 397 



and residuum, with the result that the former bodies are so increased in comparison to the 

 humus and residuum that a good coke results, although rather too porous and bulky. In 

 the fourth class, the proportion of resin and hydrocarbon bodies has reached the right 

 ratio as compared with the humus and residuum, and the best coking coal is obtained. 

 Bituminous coals of the kind classified by Gruner may therefore be looked upon as an agglom- 

 erate of humus and the degradation products of these bodies down to carbon, luted and 

 protected by resin bodies and their derivatives; steam coal and anthracite as the degradation 

 products of humus which has nearly completed its decomposition owing to the small quantity 

 of resin bodies in the original vegetation; cannel coal as consisting mainly of resin bodies, 

 derived from accumulations of seed spores which, having been brought down by streams, 

 have mixed with sedimentary earthy matter, and are therefore often high in ash. 



Such a theory has an important bearing upon the consideration of the thermal value of 

 coal as well as its other properties, as it is manifestly the ratio of the coal constituents and 

 not the percentage of the elements in the coal that governs these. In all 

 Effect of attempts to calculate the calorific value from the percentage composition it is 

 ofl^Aermaf f un ^ that, with coals poor in oxygen, such as steam coal or anthracite, there 

 va/ue. ' is a very fair agreement between the calculated value and the value as deter- 

 mined in a bomb calorimeter, but that directly the coal contains much over 5 

 per cent of oxygen, the value as determined rapidly rises above that calculated. 



This is due to the humus bodies in the coal still retaining the endothermic character of 

 the cellulose and lignose from which they were derived, and it is found that whilst the calcu- 

 lated heat values for the carbon, resin bodies and hydrocarbons in the coal are in close agree- 

 ment with the determined values, an artificially prepared pure humus will have its calorific 

 value as determined 16 or 17 per cent higher than when calculated from its percentage 

 composition. It is clear therefore that direct determination in a bomb calorimeter is the 

 only accurate method for determining the thermal value of coal. 



The proportions of the constituents in the coal also govern the nature of the products 

 when it is subjected to carbonisation. The humus bodies during carbonisation yield a 

 large proportion of the gaseous products, and under the influence of heat show 

 Effect of no s ;g n o f melting, but begin to break up at about 300 C. The decomposition 

 composition Decomes more rapid as the temperature rises. Water distils over in the early 

 f P frj? stages; the tar is thin and poor in quantity, and the gases up to 600 C. consist 

 satioa. ' f hydrogen, methane and carbon dioxide, with smaller quantities of carbon 

 monoxide and traces of other saturated hydrocarbons. The decomposition 

 can be completed below 800 C.; but if the temperature is run up to 1000 C. the carbon 

 dioxide is reduced in quantity by the action on it of the red hot carbon. Carbon monoxide 

 increases correspondingly, whilst hydrogen and methane are still evolved. 



The decomposition of the humus is also largely affected by the rate of heating. If slowly 

 heated, a large proportion of the oxygen is given off in combination with hydrogen as water 

 vapour; whilst if quickly raised in temperature, more combines with carbon to form carbon 

 dioxide and monoxide. The residue shows no sign of caking, whilst, like the naturally 

 formed residue mother of coal it requires a large proportion of cementing material to 

 make the particles cohere. The resin bodies and hydrocarbons which form the cementing 

 portion in the coal melt between 300 and 320 C. ; and if a coarsely powdered sample of the 

 coal becomes pasty or semi-fluid at this temperature, it is a strong inference that the coal 

 will coke on carbonisation a fact noted by C. Anderson. About these temperatures also 

 the resin bodies and hydrocarbons begin to decompose. 



The resin bodies at low temperature yield saturated hydrocarbons, unsaturated, chiefly 

 hexahydrides or napthenes, together with some oxygenated compounds; whilst the hydro- 

 carbons yield paraffins and liquid products all these primary constituents undergoing fur- 

 ther decomposition at slightly higher temperatures. The liquids produced begin to distil 

 out as tar vapours, and leave behind with the residuum pitch, which at 500 C. forms a mass 

 already well coked together if the residuum from the humus is not too great. The coke 

 formed is soft; but if the heat be raised to 1000 C., the pitch residue undergoes further 

 decomposition, yielding gas and leaving carbon, which binds the mass into a hard coke. 



Coke and " smokeless" fuels. The efforts]of theJSmoke Abatement Society in England 

 have led to many attempts being made to introduce smokeless fuels like coke for domes- 

 tic consumption, and there is no doubt that if a fuel of the character of " Coalite " 

 easy to light and burning with a bright cheerful combustion could be produced 

 economically and sold at a reasonable price, it would not only command an enormous 

 sale, but, in conjunction with gas fires, would go far to solve the problem of smoke 

 prevention. The combustion of bituminous coal as a national domestic fuel brings 

 in its train consequences so objectionable that every effort should be made to bring* 

 about a more rational state of things. 



In order to overcome the trouble of smoke whilst using solid fuel, many forms of grate 

 have been suggested, the most successful of them being dependent upon the principle of 



