Ph.,' 1886, vi. vol. viii. p. 271), were made in the more elaborate calorimeter of Faber and Silbermann. Nixon's Coal.-Scheurer-Kestner (Soc. I. Mul.,' 1888, vol. lviii. In 'Soc. I. Mul.,' 1891, p. 517, Scheurer-Kestner states that all his previous results are too high. Prof. H. Fritz ('Dingler's J.,' 1876, vol. ccxix. p. 185) gives the following list : The following table is a summary of C. Grove and W. Thorpe's (1889, p. 54) analyses of 111 samples of fuel; it contains only the highest and lowest values, and clearly shows how much their compositions vary. H. Poole gives very exhaustive information about coals of the world, his tables covering about forty-two pages, of which, however, English coal, coke, &c. cover about two pages. The object of determining the heating power of a fuel is undoubtedly to obtain a correct measure of its commercial value; but heating power is not everything, and there are other qualities, such as liability to smoke, to flame, to cake, to ignite easily and even spontaneously, to break up into dust when exposed, and various others. Burning Qualities. Amongst these one of great importance is as to whether a coal will ignite easily or not. Thus Cannel coal can be lit by applying a match, while anthracite can only be kept alight under carefully regulated conditions of draught, &c. The success of explosives depends on a good knowledge of this subject; thus we find that sulphur, which ignites at a low temperature, has to be added to gunpowder in larger or smaller quantities, according as to whether it has to explode quickly or slowly. More recently it has been discovered that the temperature at which wood is converted into charcoal materially affects the temperature at which it ignites, and that where the process is carried out gently and over a long period, as is the case with wooden beams in houses placed close to flues, ignition ultimately takes place spontaneously. Gun cotton ignites so readily that it could not be used for ammunition until it was discovered that an admixture of camphor or nitro-glycerine raised this temperature, and also retards the burning process; chloride of tin still further reduces its liability to ignite. Some curious phenomena have been noted. Thus phosphorus, which ignites at a temperature of 140° F. in an ordinary atmosphere, will not burn if placed in absolutely pure oxygen. A mixture of hydrogen and chlorine gas ignites at a very low temperature if illuminated by actinic rays, but no amount of heating, short of redness, will effect their chemical combination in the dark. With hydrocarbons the igniting temperature depends on the chemical composition; thus benzine ignites at about 500° F., while some of the dense oils will quench red-hot iron without igniting. Generally speaking hydrocarbons prevent both quick and spontaneous combustion. More precise information on igniting temperatures would greatly assist the understanding of fuel combustion. Much be learnt about the behaviour of various coals by consulting the following papers:- Memoirs of the Geological Survey of Great Britain (London, 1848, vol. ii. p. 539); Report of Sir H. De la Bêche and Dr. Lyon Playfair. may Influence of Air Pressure on Combustion.-Frankland's experiments (1877, p. 863) show that, contrary to the accepted views, increased pressure (not draught) does not accelerate combustion, but that it increases the luminosity and smokiness of flames. At twenty atmospheres air-pressure the hydrogen flame gives a perfectly continuous spectrum, while the carbonic oxide flame is equally luminous at fourteen atmospheres. An ordinary candle gives hardly any light when burnt on the summit of Mont Blanc, but smokes under pressure. Temperatures of Flames. When no heat is lost by radiation or convection the entire quantity generated during combustion must have been used to warm the products, raising their temperature sufficiently to make them glow, and the result is a flame. Its temperature is calculated as follows:- If n pounds of air are supplied for every pound of carbon burnt, then the weight of the products will be n + 1, the heat evolved will be 14,540 cals. per pound of fuel, and if is the specific heat of the gases, the temperature of the flame 14540 T= o. (n+1)' The Specific Heat of a Gas can only be accurately estimated if the specific heats of individual substances are known. The following small list contains all the necessary data There is some uncertainty as to the specific heat of steam, but as it does not enter very largely into the composition of burnt products, the value 4805 will be accepted. J. MacFarlane Gray (Phil. Mag.,' 1882, vol. xiii. p. 337) estimates it at 379 (see p. 65). Air being composed of 208 volumes of oxygen and 79-2 volumes of nitrogen, or by weight 23 oxygen and 77 nitrogen, its specific heat is 23 x 2157 + 77 × ·2434 = 100 2375, as stated in the table. The quantity of oxygen required to consume 1 lb. of carbon is = 2-667, and this carries with it 8.92 lbs. of nitrogen. The calculation is therefore as follows: So that the specific heat of this mixed gas is 2355. But the amount of air supplied, viz. 11 6 times as much as the carbon, is only just sufficient for perfect combustion; in practice it is above 15 and sometimes even over 30. By adding 7.413 lbs. of air to the above the products of combustion are increased to 20 lbs., and the specific heat would be raised to 2360. Coal contains both hydrogen and some moisture, and, as the specific heat of steam is about twice as great as that of dry air, the average value is somewhat higher than the above. However for practical purposes the value 237 is sufficiently accurate. = Flame Temperature. It is now possible to determine the temperature T of the flame with the help of the previous formula. Let n + 1 20, then T = 3,080° F. above t, the mean temperature of the air and fuel. Roughly speaking, the furnace temperature multiplied into the ratio of products to coal (= n + 9) is 61,000° F.; but this is true only down to a limit of about n = 15. Below this point the combustion of the coal is not a perfect one, and instead of 14,540 only 4,340 calories are evolved. Therefore, unless much hydrocarbon gas or smoke is produced we have the following three conditions : When n = 68, carbonic oxide and nitrogen are the only products; the initial temperature is 2,350° F. + t. When n is greater than 6-8 and less than 15, the products con- Above this point the temperature grows less and less, as stated When 1 lb. of hydrogen is being burnt, 8 times its weight of oxygen is required, which is accompanied by 26.8 lbs. of nitrogen, making a total of 35.8 lbs. of products of combustion. These consist of 9 lbs. of steam and 268 of nitrogen. The specific heat of this 9 × 4805 x 26·8 × ·2434 35.8 mixture is, say, = 303 (or 278). The heat evolved being 52,860 cals., the temperature of this flame is 4,873° F. + t. For such cases in which n + 1 exceeds, say, 50, divide 247,000 by (7.7+n) and the quotient is the temperature. The flame temperature of a compound containing carbon and hydrogen would of course depend on some as yet undiscovered law, but a fairly good guess can be made by taking a mean of the values just found. Roughly T= 61,000 C + 247,000 H Of course the actual temperatures of flames are decidedly lower than those found by this formula, because loss of heat takes place by radiation during the process of combustion.' Moisture in the atmosphere seriously affects boiler performances. |