Sulphur, phosphorus and silicon, the other principal combustible
elements, are only of limited application as fuels. The
first is used in the liquidation of sulphur-bearing rocks. The ore
is piled into large heaps, which are ignited at the bottom, a
certain proportion, from one-fourth to one-third, of the sulphur
content being sacrificed, in order to raise the mass to a sufficient
temperature to allow the remainder to melt and
run down to the collecting basin. Another application
is in the so-called “pyritic smelting,” where
ores of copper (q.v.) containing iron pyrites, FeS2,
are smelted with appropriate fluxes in a hot blast,
without preliminary roasting, the sulphur and iron
of the pyrites giving sufficient heat by oxidation to
liquefy both slag and metal. Phosphorus, which is
of value from its low igniting point, receives its only
application in the manufacture of lucifer matches.
The high temperature produced by burning phosphorus is in
part due to the product of combustion (phosphoric acid) being
solid, and therefore there is less heat absorbed than would be the
case with a gaseous product. The same effect is observed in a
still more striking manner with silicon, which in the only special
case of its application to the production of heat, namely, in the
Bessemer process of steel-making, gives rise to an enormous
increase of temperature in the metal, sufficient indeed to keep
the iron melted. The absolute calorific value of silicon is lower
than that of carbon, but the product of combustion (silica)
being non-volatile at all furnace temperatures, the whole of
the heat developed is available for heating the molten iron,
instead of a considerable part being consumed in the work of
volatilization, as is the case with carbonic oxide, which burns
to waste in the air.
Assay and Valuation of Carbonaceous Fuels.—The utility or value of a fuel depends upon two principal factors, namely, its calorific power and its calorific intensity or pyrometric effect, that is, the sensible temperature of the products of combustion. The first of these is constant for any particular product of Calorific power. combustion independently of the method by which the burning is effected, whether by oxygen, air or a reducible metallic oxide. It is most conveniently determined in the laboratory by measuring the heat evolved during the combustion of a given weight of the fuel. The method of Lewis Thompson is one of the most useful. The calorimeter consists of a copper cylinder in which a weighed quantity of coal intimately mixed with 10–12 parts of a mixture of 3 parts of potassium chlorate and 1 of potassium nitrate is deflagrated under a copper case like a diving-bell, placed at the bottom of a deep glass jar filled with a known weight of water. The mixture is fired by a fuse of lamp-cotton previously soaked in a nitre solution and dried. The gases produced by the combustion rising through the water are cooled, with a corresponding increase of temperature in the latter, so that the difference between the temperature observed before and after the experiment measures the heat evolved. The instrument is so constructed that 30 grains (2 grammes) of coal are burnt in 29,010 grains of water, or in the proportion of 1 to 937, these numbers being selected that the observed rise of temperature in Fahrenheit degrees corresponds to the required evaporative value in pounds, subject only to a correction for the amount of heat absorbed by the mass of the instrument, for which a special coefficient is required and must be experimentally determined. The ordinary bomb calorimeter is also used. An approximate method is based upon the reduction of lead oxide by the carbon and hydrogen of the coal, the amount of lead reduced affording a measure of the oxygen expended, whence the heating power may be calculated, 1 part of pure carbon being capable of producing 3412 times its weight of lead. The operation is performed by mixing the weighed sample with a large excess of litharge in a crucible, and exposing it to a bright red heat for a short time. After cooling, the crucible is broken and the reduced button of lead is cleaned and weighed. The results obtained by this method are less accurate with coals containing much disposable hydrogen and iron pyrites than with those approximating to anthracite, as the heat equivalent of the hydrogen in excess of that required to form water with the oxygen of the coal is calculated as carbon, while it is really about four times as great. Sulphur in iron pyrites also acts as a reducing agent upon litharge, and increases the apparent effect in a similar manner.
The evaporative power of a coal found by the above methods, and also by calculating the separate calorific factors of the components as determined by the chemical analysis, is always considerably above that obtained by actual combustion under a steam boiler, as in the latter case numerous sources of loss, such as imperfect combustion of gases, loss of unburnt coal in cinders, &c., come into play, which cannot be allowed for in laboratory experiments. It is usual, therefore, to determine the value of a coal by the combustion of a weighed quantity in the furnace of a boiler, and measuring the amount of water evaporated by the heat developed.
In a research upon the heating power and other properties of coal for naval use, carried out by the German admiralty, the results tabulated below were obtained with coals from different localities.
| Slag left in Grate. | Ashes in Ashpit. | Soot in Flues. | Water evaporated by 1 ℔ of Coal | |
| Westphalian gas coals | 0.33–6.42 | 2.83–6.53 | 0.32–0.46 | 6.60–7.45 ℔ |
| Do. bituminous coals | 0.98–9.10 | 1.97–9.63 | 0.24–0.88 | 7.30–8.66 |
| Do. dry coals | 1.93–5.70 | 4.37–10.63 | 0.24–0.48 | 7.03–8.51 |
| Silesian coals | 0.92–1.30 | 3.15–3.50 | 0.24–0.30 | 6.73–7.10 |
| Welsh steam coals | 1.20–4.07 | 4.07 | 0.32 | 8.41 |
| Newcastle coals | 1.92 | 2.57 | 0.35 | 7.28 |
The heats of combustion of elements and compounds will be found in most of the larger works on physical and chemical constants; a convenient series is given in the Annuaire du Bureau des Longitudes, appearing in alternate years. The following figures for the principal fuel elements are taken from the issue for 1908; they are expressed in gramme “calories” or heat units, signifying the weight of water in grammes that can be raised 1° C. in temperature by the combustion of 1 gramme of the substance, when it is oxidized to the condition shown in the second column:
| Element. | Product of Combustion. | Calories. | |
| Hydrogen | Water, H2O, condensed to liquid | 34,500 | |
| ”as vapour | 29,650 | ||
| Carbon— | |||
| Diamond | Carbon Dioxide, CO2 | 7,868 | |
| Graphite | ” ” | 7,900 | |
| Amorphous | ” ” | 8,133 | |
| Silicon— | |||
| Amorphous | Silicon Dioxide, SiO2 | 6,414 | |
| Crystallized | ” ” | 6,570 | |
| Phosphorus | Phosphoric pentoxide, P2O5 | 5,958 | |
| Sulphur | Sulphur dioxide, SO2, gaseous | 2,165 | |
The results may also be expressed in terms of the atomic equivalent of the combustible by multiplying the above values by the atomic weight of the substance, 12 for carbon, 28 for silicon, &c.
In all fuels containing hydrogen the calorific value as found by the calorimeter is higher than that obtainable under working conditions by an amount equal to the latent heat of volatilization of water which reappears as heat when the vapour is condensed, though under ordinary conditions of use the vapour passes away uncondensed. This gives rise to the distinction of higher and lower calorific values for such substances, the latter being those generally used in practice. The differences for the more important compound gaseous fuels are as follows:—
| Calorific Value. | ||
| Higher. | Lower. | |
| Acetylene, C2H2 | 11,920 | 11,500 |
| Ethylene, C2H4 | 11,880 | 11,120 |
| Methane, CH4 | 13,240 | 11,910 |
| Carbon monoxide, CO | 2,440 | 2,440 |
The calorific intensity or pyrometric effect of any particular fuel depends upon so many variable elements that it cannot be determined except by actual experiment. The older method was to multiply the weight of the products of combustion by their specific heats, but this gave untrustworthy Caloric intensity. results as a rule, on account of two circumstances—the great increase in specific heat at high temperatures in compound gases such as water and carbon dioxide, and their instability when heated to 1800° or 2000°. At such temperatures dissociation to a notable extent takes place, especially with the latter substance, which is also readily reduced to carbon monoxide when brought in contact with carbon at a red heat—a change which is attended with a large heat absorption. This effect is higher with soft kinds of carbon, such as charcoal or soft coke, than with dense coke, gas retort carbon or graphite. These latter substances, therefore, are used when an intense local heat is required, as for example, in the Deville furnace, to which air is supplied under pressure. Such a method is, however, only of very special application, the ordinary method being to supply air to the fire in excess of that required to burn the fuel to prevent the reduction of the carbon dioxide. The volume of flame, however, is increased by inert gas, and there is a proportionate diminution of the heating effect. Under the most favourable conditions, when the air employed has been previously raised to a high temperature and pressure, the highest attainable flame temperature from carbonaceous fuel seems to be about 2100°–2300° C.; this is realized in the bright spots or “eyes” of the tuyeres of blast furnaces.
Very much higher temperatures may be reached when the products of combustion are not volatile, and the operation can be effected by using the fuel and oxidizing agent in the proportions exactly
