January 25, 191 2] 



NATURE 



433 



acid, &c., occur in much larger quantities than in coal tar, 

 and there are also present quantities of polyhydric phenols 

 or other esters of the type met with in coal tar, which 

 form resinous masses difficult to investigate. The pitch 

 left as a residue amounts to about 40 per cent, of the tar, 

 and is of very fine quality, owing to the practical absence 

 of free carbon. 



When low-temperature coke that has been formed with 

 the evolution of 5000 cubic feet of 22 candle-power gas, as 

 measured by the No. 2 Argand, is further heated to a 

 high temperature, it evolves nearly as much gas as it did 

 before, and the composition of this gas is approximately — 



Hydrogen ... ... ... ... ... 71-13 



Saturated hydrocarbons ... ... ... 18-26 



Unsaturated hydrocarbons ... 0-52 



Carbon monoxide ... ... ... ... 6.30 



Carbon dioxide ... ... ... ... ... 2-09 



Nitrogen ... ... ... 1-70 



The gas is practically non-luminous when burnt alone, 

 but has a heating value of 447 B.Th.U. gross. 



It seems clear from these experiments that in the dis- 

 tillation of coal for gas there are three distinct sources 

 which give the final product : — 



(i) The primary gases evolved from the coal, and dis- 

 tilled out as the advancing temperature travels through the 

 mass. 



(2) The gas evolved by the decomposition of the heavy 

 tar or pitch left in the coking mass, 



(3) The gas produced by secondary actions, and contact 

 of the primary gases and vapours with the hot coke and 

 walls of the retort. 



It now becomes possible to trace, roughly, the actions 

 taking place in the destructive distillation of a gas coal. 

 Up to about 450° C. the products are chiefly primary : — 



"Carbon — 



H amus bodies 



Water ... 



Carbon monoxide 



Carbon dioxide... }-Gases 



Hydrogen 



Methane 



Resin bodies and hydrocarbons 



r Methane 

 I Ethane 



1 Propane 

 I Butane 



'oke and pitch — 



Watery tar 



About 400° to 450° C. 

 saturated hydrocarbons 



f Pentane 



Hexane 



Heptane 



Octane 



Liquids^ Nonane 



Hexahyd rides 

 Oxygenated hydrocarbons 

 like cresylic acid and 

 more complex bodies 



the secondary actions start, the 

 split up into unsaturated and 

 simpler members of the saturated series, the hexahydrides 

 shed hydrogen and give aromatic hydrocarbons, the tar 

 thickens and alters in character, and synthetic actions 

 start, cresol and hydrogen form more toluene, carbolic acid 



aid carbon yield carbon monoxide and benzene. About 



)O0° C. the degradation of all the hydrocarbons and other 

 oxygenated bodies is proceeding, and finally the mixture of 

 the results of endless actions and reactions yields us the 

 high-temperature gas and tar, the mixture being diluted 

 with the carbon monoxide, hydrogen, and methane yielded 

 by the decomposition of the pitch residues in the soft coke, 

 which leaves the hard coke behind. 



VV'ith a good Durham coal, capable of yielding 11,000 

 cubic feet of gas and 10 gallons of tar per ton when dis- 

 tilled under ordinary conditions at a temperature of about 

 1000° C, it is found that on carbonising at 600° C. it 

 vields only 5000 cubic feet of gas, but 22 gallons of tar 

 pi^r ton, and the residue, on continuing the distillation at 

 1000° C, yields a further volume of 4500 cubic feet of 



>is, but no tar, so that removing the gas first formed and 

 the tar vapour from the secondary reactions induced by 

 high temperature has reduced the gas yield by some 1500 



ubic feet per ton, and increased the tar yield by 12 gallons; 



nd an examination of the tar shows that the 12 gallons 

 ,:4asified by direct distillation at high temperature consist 

 of the lighter portions of the whole of the tar, which at 

 this temperature is capable of producing 1200 cubic feet 

 of gas, leaving 300 cubic feet to represent the volume 



NO. 2204, VOL. 88] 



gained by degradation of gaseous hydrocarbons and de- 

 position of free carbon, of which the high-temperature tar 

 contains 25 to 35 per cent. 



It may be taken, in round numbers, that when 11,000 

 cubic feet of gas are obtained per ton of such a coal, 

 45 per cent, of the volume is from the low-temperature 

 distillation, 42 per cent, from the residues left in the low- 

 temperature coke distilled at a high temperature, and 

 13 per cent, from the various secondary actions and tar. 



When tested by the No. 2 Argand, the low-temperature 

 gas has a candle-power of 22 ; the gas from the pitch in 

 the coke is neai-ly non-luminous, giving not more than 2^^ 

 to 3 candles, but when mixed with the rich gas in the pro- 

 portions of 45 of the latter to 41 of the former gives a 

 16 to 17-candle gas ; whilst the gas from the tar and the 

 secondary actions is lo-candle gas when tested alone, and 

 when mixed with the others brings down the total candle- 

 power to 14 to 15. 



In the new methods of carbonisation, where the makes 

 approximate to 13,000 cubic feet, the improvement found 

 is entirely due to the free escape without over-heating of 

 the products from the first two-thirds of the coal 

 carbonised, whilst the extra volume is obtained from the 

 complete degradation of the products from the remaining 

 third, and that this is so is shown by the methane in the 

 gas. In all fair coal gas in which there has been no 

 over-degradation, even if you have been getting 11,500 

 cubic feet per ton from light charges, the methane will be 

 about 34 to 35 per cent, of the gas ; but notice the pi^>- 

 ducts of the new carbonisation, and you will find plenty 

 of samples with only 28 per cent., and some even lower. 



In considering the ultimate effect of pushing tempera- 

 tures to the highest possible extent, it is as well to con- 

 sider the amount and value of the gas and coke that could 

 possibly be obtained from an ordinary coal. 



If we took a coal of the composition — 



Carbon ... ... ... ... ... ... 80 



Hydrogen 5 



Oxygen 10 



Nitrogen and ash , 5 



and were to carbonise it in an inverted vertical retort with 

 the coal fed in by a ram at the bottoni, so that the gas 

 and vapours had to traverse a column of 10 feet of coke 

 at 1000° C, contact would decompose all the gaseous and' 

 volatile compounds to hydrogen and carbon monoxide, and 

 we should obtain — 



Coke 



Hydrogen 

 Carbon monoxide 



i5"27 cwt. 



22,400 cubic feet = 77*87 per cent. 

 6,366 cubic feet = 22 13 per cent. 



28,766 cubic feet 



that is, you would rather more than double the volume of 

 gas ; but it is a non-luminous gas of the same thermal 

 value (gross) as water gas, and not worth more than the 

 3id. to 4(i. a thousand that you could make water gas at 

 by one of the newer processes. 



Now this is exactly what is done in all the new pro- 

 cesses directly the cool passage for escape of the primary 

 gas gets tar-logged, and the gas from the remaining coal 

 is driven through the red-hot coke and along the sides of 

 the retort. Under these conditions some 3500 cubic feet 

 of 22-candle gas are obtained, whilst there is a free, cool 

 passage for its escape ; and after that is closed the remain- 

 ing coal yields by complete degradation of the tar and 

 hydrocarbons 9500 cubic feet of hydrogen and carbon 

 monoxide. , j 



The limit of volume in gas-makmg, if not already 

 reached, is fast being approached; economics are day by 

 dav getting more difficult to make; coni is not likely to 

 cheapen ; and all this means that the chance of consider- 

 able reduction in the price of gas is getting less and less. 

 If the price of gas could be reduced in our large cities to 

 the price charged at Widnes, the consumption of gas could 

 be economically incren.sed, and for power and heat gas 

 would hold an unassailable position ; but this can never 

 be done in existing circumstances, because even if the gas 

 could be made at the necessary price, the increased output 

 of coke would outrun the demand. If, however, the com- 

 panies would only livo up to their titles of " Gas Light 

 and Coke Compani' u-stow as much care on the 



