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TJNITED STATES NAVAL INSTITUTE.
WOI. WI. 1880. No. XIII.
NAVAL INSTITUTE, A N N A POLIS.
THE VALUATION OF COAL.
By the valuation of coal is meant the estimation by experiment of its value as a calorific agent, and it will be admitted that the discovery of some method which will readily give accurate and reliable results, and which will enable us to avoid the costly and prolonged test of actual use—a test which may involve great waste and prove very vexatious—is a great desideratum.
In valuing a coal the estimation of the calorific power is of the first importance, yet there are other characteristics of the fuel to be considered, which will render it more or less suitable for the use to which it is to be put, and which should not be overlooked in an examination of and in deciding upon its fitness. These are the nature of its ash, the readiness with which it burns, the presence of sulphur, and, when the coal is for naval use, the loss by attrition; and in this paper it will be the aim, after briefly stating the properties and composition of coal, and describing some of the means proposed for estimating its calorific
power, to allude to the methods employed in the estimation of these Secondary properties.
Since the heat developed by a fuel depends upon the union of the carbon, hydrogen, and other combustible constituents which it contains, with the oxygen of the air, and since also the heat produced by the combustion of measured quantities of each of these substances in oxygen has been determined with great accuracy, it would appear a simple thing to determine the calorific power of a coal by subjecting it to an elementary analysis and calculating from the weights of the elementary substances obtained the heat produced by its combustion; and this method has been followed to a considerable extent in the past, but it has been found in practice to give very erroneous results, some of the sources of which we will consider.
We have in coal a substance whose composition is very variable and very complex; but, as we have no proximate analysis of coal, it is not possible to make this directly apparent, and therefore we must prove . the truth of the statement in another way. The following table gives the results of the ultimate analysis of several varieties of coal, and exhibits the variability in its ultimate composition.
An ultimate analysis, however, gives us little real knowledge of the character of a coal, for, as a few experiments will show us, the substances we have determined do not exist wholly in an elementary condition in it. Let us first examine the coal by subjecting a weighed quantity in a confined space to the action of a rarefied atmosphere and heat. We shall find that a considerable amount of gas is evolved from the coal, that this gas is a mixture of compound gases, and that in our ultimate analysis we have estimated their constituents as simple substances. The following table gives the results of some of Mr. Thomas’ analyses made in the way described : —
#: Percentage Composition of Gas.
o CO, CO CH, C.H" | O ... N Lignite, Bovey, . 114.3 96.74 2.80 0.46 Cannel, Wigan, . 350.6, 9.05 | – || 77.19 || 7.80 | – || 5.96 Jet, Whitby, . o 30.2|10.93|||—|| C.H.0%|86.90 — 2.17 Bituminous coal, S. Wales, 55.9|36.42 O.80 62.78 Semi-bituminous, “ 73.6 | 12.34 — | 72.51 — 0.64 || 14.51 Steam coal, “. . .218.4 5.46 — 84.22 — 0.44 9.88 Anthracite, & 4 |; 2.62 — 193.13 — I — 4.25
If, in addition to this, we heat the coal in closed vessels, out of contact with the air, if the coal be other than anthracite we shall find that in addition to the gases evolved, as given above, the coal will yield a large number of substances, solid, liquid, or gaseous, which exist already formed in the coal, or which are produced by the action of heat on substances existing in the coal, and there will be left behind a mass of coke. We may thus prove the complex composition of the coal, but our methods of analysis do not yet admit of our estimating these constituents.
However, our ultimate analyses have shown that carbon is the most important element present, and it is probable that it exists to a large extent in the coal in a free state. Let us consider what would result if we were to estimate the calorific power of the carbon present from a simple determination of the percentage of free carbon. Carbon is one of the elementary substances which exists in several allotropic or unlike states. In all of these its chemical properties are precisely the same, though its physical properties are widely different. These differences are believed to be due to the difference in the arrangement of the atoms in the molecules. Among other differences Favre and Silbermann have found that their heats of combustion differ considerably, increasing inversely as the density, as the following table, embodying their results, shows.
Substance. Product. Units of Heat. Density. Wood charcoal, CO, 8080 1.500 Gas-retort carbon, ( & 8047 1.885 Native graphite, & 4 7797 2.300 Artificial graphite, C & 7762 -*. Diamond, ( & 7770 3.530
* “These figures point to the conclusion that the heat of combustion of an elementary substance depends not only on its chemical constitution but also upon its physical state before combustion. It varies not only with the nature of the atoms but also with the manner in which they are grouped together. We cannot deduce the calorific power of graphite from that of charcoal, nor that of the diamond from either. If, then, the mere fact that a substance is composed of pure carbon is not sufficient to determine its heat of combustion, it is not reasonable to suppose that the like information can be acquired in the case of So complex a substance as coal, by a calculation based only on a knowledge of the quantities of carbon, hydrogen and oxygen which it contains.” These substances exist in the coal in a state of combination, the Compounds of the various elements being mixed together. Hence, when they are burned these compounds must be broken up before they can unite with the oxygen of the air, and, as a general rule, heat is absorbed by the analytical process, and consequently the true heat of the combustion of the coal will be less than the calculated result. Should the compounds, however, be of such a nature that their decomposition is attended with an evolution of heat, the true heat will be greater than the calculated. Another source of error is due to the fact that the calorific power of hydrogen was determined when that substance was in the gaseous state. Now hydrogen would certainly exist in the coal in a solid or liquid state, and, during the process of combustion, would be converted into a gas. We know that if a solid or liquid is converted into a gas, heat is absorbed. “Therefore, even if the assumption that the ‘available' hydrogen is not combined with any of the other elements present in the coal were correct, the calculations themselves would be open to objection, since the hydrogen in its conversion to the gaseous state would absorb heat. Hence, in assuming that the calorific power of solid hydrogen is, like that of gaseous hydrogen, 34,462 units, we commit an error, the existence of which we are certain of, while we are totally ignorant of its magnitude.” Experimental proofs are not wanting to confirm the doubts which theory suggests as to the accuracy of this method of calculation. Two physicists, Scheurer-Kestner and C. Meunièr, have made a long series of experiments on the heat of combustion of coal. They analyzed numerous specimens, calculated their calorific power by the ordinary # “Coal: Its History and Uses,” Prof. Thorpe; page 243. # Ann. phys. et chim, 4 ser, t. xxi, et. xxvi.
rules, and then made direct experiments to determine their heat of combustion. A comparison of the numbers obtained by calculation and observation proved that they did not agree. Thus in the case of two coals, one from Ronchamp and the other from Creusot, which contained almost precisely the same proportions of carbon, hydrogen, and oxygen, the calorific powers, instead of being identical, were 9,117 and 9,622 respectively. The difference between the real and calculated calorific powers amounted in some instances to as much as fifteen per cent. In the case of two specimens of coal from England, and several from France, the calculated heat of combustion was too small. In that of six kinds of brown coal from France and Germany it was too large, while experiments on several different coals from Russia proved that in these cases the discrepancies between calculation and experiment were comparatively unimportant. It is evident then, that in order to determine the calorific power of a coal with precision we must resort to direct experiments, and that we cannot trust to the calculations based on the elementary composition of the coal. To determine this factor with accuracy we must use the delicate calorimeters employed by the physicist, and at the same time estimate the amount of incombustible matter present. But such precise results are not necessary for the examination of coal for use in the generation of steam ; coarser methods will yield results which are sufficiently accurate for this purpose, some of which we will consider. Thomson has devised a calorimeter which has sometimes been used for determining the calorific power of coal. It consists of a thin, copper cylinder placed inside another, of similar material, which is perforated with holes at the bottom and furnished with a stopcock at the top. The coal to be examined is finely powdered and mixed with ten to twelve times its weight of a mixture of three parts of potassic chlorate and one of potassic nitrate, and this mixture, which will burn out of contact with the air, is then placed in the inner cylinder and the whole submerged under a known weight of water. As the mixture burns, the hot gases bubble up through the holes and warm the waters, until the combustion is completed, when the stopcock is opened and the water flows in to fill the vessel. The heat of combustion is deduced from the elevation of temperature of the vessel and water. The quantities of coal and water employed are so adjusted as to make the calculation extremely simple. It has been shown, however, by Dr. Percy, that there is an error in this method, due to the fact that the bubbles of gas which escape are not completely cooled when passing