276 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



LSeptekbfii, 



Tabie V. — S/iotcinff the Amount of Various Suhitances produced by the 

 destructive Distillation of certain Welsh Coals. 



It has been for some time asserted, that the evaporative value of 

 a bituminous coal is expressed by the evaporative value of its 

 coke, the heat of combustion of its volatile products proving- in 

 practice little more than that necessary to volatilise them. If 

 this supposition were even near the truth, the most useful practical 

 results might follow from it. By a larger and better applied system 

 of pas manufacture, the volatile products of distillation might be 

 made useful not only for the purposes of illumination, but also for 

 domestic heat, and the residual coke miglit be used with an equal 

 economy in our manufactures* ; thus preventing the emission of 

 tliat smoke which at present is so destructive to the comfort of 

 our large cities. It is easy from analysis to examine whether the 

 duty performed by the coal is to be attributed to its fi.xed ingre- 

 dients or coke, by estimating the work which the latter is capable 

 of performing. This may be done by subtracting the amount of 

 ashes in the coal from its amount of coke (Table III.) and esti- 

 n\ating the remainder as carbon. This carljon multiplied by its 

 lieating power, ISStJS, and divided by 965-7 or the latent heat of 

 steam, indicates the iiunil>er of pounds of water which the coke by 

 itself could evajiorate, witliout tlie aid of the combustible vola- 

 tile ingredients of tlie coal. These results are placed in column 

 K, of Table VI., in juxta-position with the actual work done by 

 the coal, and it will be seen, that notwithstanding several striking 



* In this ease it wnuld not be necessary to carry on tbe process of distillation so far 

 aa at pieaeul, as tbe residual toke would be more combustible uud tbe gases jjurer. 



exceptions, which might have been expected, they on the whole 

 ■show that the work capable of being performed by the coke alone, 

 is actually greater than tliat obtained by experiments with the 

 original coal. 



The whole system of manufacturing coke is at present very im- 

 perfect. Besides losing the volatile combustible sub.stances, whic'i 

 under new adjustments might lie made of much value, an immense 

 quantity of ammonia is lost Iiy being thrown into the atmosphere. 

 Ammonia and its salts are daily becoming more valuable to agri- 

 culture, and it is their comparative high price alone which 

 prevents their universal use to all kinds of cereal cultivation. By 

 a construction of the most simple kind, the coke ovens now in use 

 might be made to economise much of the nitrogen which invariably 

 escapes in the form of ammonia. As an inducement to this 

 economy, we have appended to Table VI two columns (H. and I.), 

 showing the quantity of ammonia (N H.,), and its corresponding 

 quantity of commercial sulphate (N H, O, SO,.), which each 

 100 lb. of the respective coals may be made to produce. AVhen it 

 is remembered, that the price of sulphate of ammonia is about 

 £13 per ton, or that 100 tons in coking is capable of producing, on 

 an average, about 6 tons of this salt, its neglect is highly repre- 

 hensible. 



By the preceding data, the actual value of the coals will be con- 

 trasted with that which is theoretically possible, supposing their 

 combustion proceeded under circumstances which prevented any 

 loss of heat. The actual duty obtained by a pound of coal from 

 the boiler employed may be easily expressed by the number of 

 pounds raised to the height of one foot. This result may readily 

 be obtained by the simple formula — 



Wi) X 965-7 X 782 = x, 



W representing water, of which ij pounds are evaporated by a 

 pound of coal. This formula is deduced from the fact that 

 T? pounds of water multiplied by 965-7,* or the co-efficient for the 

 latent heat of steam at 212\ indicates the number of pounds of 

 water wliich would be raised 1° Fahrenheit ; and the number 782 

 arises from experiment on the mechanical force denoted by the 

 elevation of a pound of water 1° Fahrenheit; that force being 

 equal to 712 lb., raised to the height of one foot, according to the 

 careful experiments of M. Joule, on the friction of oil, water, 

 and mercury. 



The theoretical value of the coals, with reference to the number 

 of pounds of water which one pound of fuel will convert into 

 steam, is obtained by the formula — 



/C203268\ / H - A X 62470 \ _ 

 V 965"^7 / "^ V 965-7 j - ^ '' 



in which C is the quantity of carbon, H the quantity of hydrogen 

 in a unit of fuel, and h the quantity of hydrogen corresponding to 

 the oxygen contained in the coal. These multiplied by their 

 heating powers, according to the results of Dulong, and divided 

 by the latent heat of steam, indicate the number of pounds of 

 water that can be converted into the latter by a pound of coal. 

 The numbers thus obtained can be changed into the expression of 

 mechanical force, by the previous formulae. 



The result of these calculations are thrown into Table VI. 



The best Cornish engines are stated to raise 1,000,000 lb. to the 

 height of one foot, by every pound of coal consumed ; so that only 

 about ij of the artmit force generated becomes availaViIe, or only -rr 

 or -f'j of the force theoretically possible, is applied in practice. 

 The various experiments made on boilers, with regard to the eva- 

 j)orative power of coal, have not given very uniform results. 

 Smeaton, in 1772, with one pound of Newcastle coal, evaporated 

 7-88 lb. of water from 212^ ; Watt, in 1788, came to the conclusion 

 that 8-62 lb. of water might be evaporated by the same quantity of 

 coal; and later (in 1810), Wicksteed found tliat 1 lb. of Merthyr 

 coal could be made to evaporate 9-493 lb. of water fi-om 80°, which 

 is equal to 10-746 lb. from 212°. In some experiments made on 

 the boiler of the Loam's engine, at the United mines, in Cornwall, 

 each pouiul of coal was found, by a trial of si.x months, to evapo- 

 rate 10-29 lb. of water from 212^, this being the reduction of the 

 result given, viz., that 234,210 cubic feet of water at 120' were 

 evaporated by 700 tons of coal. Statements have indeed been 

 made that 14 lb. of water have been evaporated by 1 lb. of coal 

 burned in Cornish boilers ; but as this is the utmost quantity 

 theoretically possible, it is difficult to conceive that it has been 

 realised in practice, even in the best-constructed steam-engines. 



* The co-efficient for the latent heat of stMm at 212° Is generally taken at lOOO", but 

 the above number in frsm tbe recent experiments 01 Kegnuult on this subject, as glveu 

 In Table 1. 



