Fuel and refactory materials

Front matter

(lass TP3I8

Book -S4-

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Contents

'CHAP.

I. Combustion II. Heating Power of Fuels

III. Fuels — Wood, Peat, Coal

Chapter I

Combustion. — Heat is required in almost all metallurgical operations, and is always obtained directly or indirectly by the combustion of substances called fuels. A consideration of the phenomena of combustion and of the nature and methods of using fuel is therefore essential as an introduction to the study of metallurgy.

Chemical combination is almost invariably attended with the evolution of heat, and when this is sufficient to raise the combining bodies or the products of combination to the temperature at which they evolve light, combustion is said to take place. Combustion may therefore be defined as vigorous chemical combination, attended with the evolution of light.

In practice the combination is always between a combustible or fuel and the oxygen of the air, which is therefore said to support combustion.

Conditions which favour Combustion. — In order that combustion may begin, the fuel must be brought in contact with the air at a suitable temperature ; and in order that it may continue, this temperature must be kept up, a supply of oxygen must be maintained, and the products of combustion must be removed.

As a rule, the larger the surface of the combustible in contact with the air, the more readily will combustion take

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place, and as gases, by the mobility of their molecules, allow of the largest possible amount of contact, they, if combustible, usually burn very readily.

If a combustible gas be allowed to escape from a tube into the air at a temperature at which it will burn, combustion takes place with great facility ; for as the gas comes into the air diffusion takes place, the gas molecules are brought into close contact with the oxygen molecules, and they combine, forming a zone of combustion surrounding a core of gas, and thus producing a flame.

If a combustible gas be thoroughly mixed with air and a light be applied, combustion takes place almost instantly through the whole mass, travelling very rapidly from particle to particle ; heat is suddenly evolved, great expansion results, and an explosion of more or less violence takes place.

Liquids do not burn readily in mass, for the air cannot penetrate them, and there is therefore only contact at the, comparatively small, surface of the liquid. There are apparent exceptions to this, due to the fact that most liquids are volatile, and combination therefore takes place near the liquid surface, between the vapour and the air. If a combustible liquid be broken up into a fine spray by a steam or air jet, it will burn almost exactly as if it were a gas, and will form an explosive mixture with air.

Combustible solids usually burn readily when in pieces of such a size as to allow ready access of air, and at the same time exposing a large surface of contact. If the lumps be too large the contact surface is too small and combustion is hindered, and if the substance be in a powder the air will be unable to penetrate, and therefore there will still only be a small surface of contact. If a finely -powdered solid fuel be projected at a high temperature into air, it burns very rapidly, almost exactly in the same way as a gas, and such a powder may even form an explosive mixture with air. It is quite certain that many colliery and other explosions, if not entirely due to, are at any rate very much intensified by the presence of coal or other combustible dust in the air.

Many examples of the influence of contact surface are familiar. A piece of charcoal of large size will burn readily, because being very porous air can find its way into it, and thus provide a large contact surface, while a large lump of anthracite, not being porous, will hardly burn at all. Paper and wood-shavings are employed to light a fire, though they have almost the same composition as the wood, because, owing to their thinness, they expose a large surface to the air, and ignite readily. These and such materials as light fabrics are very combustible, but books in which the paper leaves are pressed closely together, and bales of fabrics, are very difficult to burn ; and when a warehouse has been burned, whilst all the loose goods are destroyed, it is quite common to find bales of the same materials only singed on the outside. Heavy beams of wood make fireproof floors, and saw-dust or coal-dust thrown on a fire will often extinguish it.

Proportion of Combustible. — In order that combustion may take place, the combustible and air must be present in — within certain limits — definite proportions. This is not so noticeable in the case of solid or liquid fuels, or of gas burned in a flame, because, owing to the circulation set up in the air, the proportions to some extent adjust themselves. It is, however, well seen in the case of mixtures of gases. If coal-gas and air be mixed in certain proportions a violently explosive mixture results ; but there may be a large quantity of gas in the air, enough to be detected by the smell and to produce chemical and physiological effects, and yet the mixture will not explode on applying a light. The presence of a large quantity of inert matter in a fuel may much hinder or even prevent combustion, whilst the presence of a comparatively small quantity of carbon dioxide in the air will prevent it supporting combustion. Professor Clowes has recently shown that air which contains about 4 per cent of carbon dioxide, the oxygen being reduced by a like amount, will extinguish ordinary combustibles, such as candles or oil flames. 1 Marsh-

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gas must be mixed with at least 17 times its own volume of air to form an explosive mixture.

Temperature of Combustion. — For combustion to take place a certain temperature, varying with the nature of the combustible, is necessary. A mixture of hydrogen and oxygen in explosive proportions will remain inert for any length of time until a portion of the mixture is raised to about 1100° F. (590° C), when ignition will at once take place. Coal-gas ignites in air at a red heat. Many of the metals are not acted on by dry oxygen at ordinary temperatures, but if they be heated to redness some of them burn, as in the case of magnesium, with great brilliance. On the other hand, phosphorus ignites at such a low temperature that for safety it is always kept under water or otherwise protected from contact with the air, and some substances have such affinity for oxygen that they take fire on coming in contact with it.

The temperature at which combustion can take place varies also with the condition of the combustible. Lead and iron can be obtained in such a fine state of division that they take fire spontaneously in air at ordinary temperatures. 1

Continuous Combustion. — In some cases when a substance has been ignited it will continue to burn, in others it will go out as soon as the external source of heat is removed. This depends on the relationship which exists between the heat evolved by combustion and the temperature of ignition. If the heat evolved be sufficient to maintain the temperature above the ignition point the combustion will continue, if not it will cease.

Combustibles and Supporters of Combustion. — The fuel or substance which burns is usually called a combustible, and the oxygen of the air is called a supporter of combustion. These terms, though convenient, are not strictly correct, except in so far as they indicate an accident of position. Combustion is a mutual action in which both substances play an equal part, which being combustible and which supporter of combustion depending on circumstances.

I These fine powders may be allotropic modifications of the metals.

When a solid combines with a gas, the gas surrounds it, and is therefore regarded as a supporter of combustion. When a mixture of a combustible gas, such as hydrogen, and air is exploded it is impossible to say that either is the combustible « rather than the other ; but when gas is burnt at a jet the flame is surrounded by the excess of air, which is therefore called a supporter of combustion. It is quite simple to arrange experiments so as to burn air in coal-gas or oxygen in hydrogen, and thus reverse their usual positions.

Complete and Incomplete Combustion. — All combustibles in common use are composed chiefly of carbon (C), usually combined with hydrogen (H), oxygen (0), and sometimes small quantities of other elements, but the carbon and hydrogen are always the valuable constituents.

When a combustible burns, the combustion may be either complete or incomplete. It is complete when all the combustible constituents are oxidized to their highest state of oxidation, and it is incomplete when any fuel is either left unconsumed, or passes away combined with less oxygen than the maximum with which it is capable of combining.

In the case of hydrogen, there is only one compound that can be formed — water, H 2 ; and therefore, if the combustion be incomplete, some of the hydrogen must remain unconsumed.

In the case of carbon, the highest state of oxidation is carbon dioxide, C0 2 ; but there is also another oxide, carbon monoxide, CO, which contains, for the same amount of carbon, only one-half as much oxygen. When carbon is incompletely burned, therefore, either carbon may be left unconsumed, or carbon monoxide may be formed and pass away with the products of combustion. Both carbon dioxide and carbon monoxide are colourless gases, so that it is often not easy to decide whether combustion is complete or not.

The combustion of carbon commences at a comparatively low temperature ; at about 750° F. (400° C.) the product is almost entirely carbon dioxide ; as the temperature rises the rate of combustion increases, and the proportion of carbon

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monoxide formed increases till, at 1830° F. (1000° C), the product is almost entirely this gas, which, if the air supply be sufficient, is rapidly burned to carbon dioxide. It is for this reason that carbon at low temperatures simply smoulders, whilst at very high temperatures it burns with a flame.

The combustion of hydrocarbons is much more complex. If the- combustion be quite complete, the products are water and carbon dioxide ; but if it be incomplete, what products will be formed depends on circumstances. Many hydrocarbons dissociate or split up into simpler hydrocarbons with separation of hydrogen, as, for instance, ethylene, C 2 H 4 , which goes partly to hydrogen and acetylene, C 2 H 2 . The hydrogen burns to water, and the acetylene, partially escaping as such, imparts a most unpleasant odour to the products of combustion of incompletely-burnt hydrocarbon gases. The carbon will be burned to carbon dioxide, or a mixture of this and carbon monoxide. Under some conditions carbon may also be separated by dissociation in the solid form as soot.

Incomplete combustion of any kind always means considerable loss of heat.

Conditions of Complete Combustion. — In order to ensure complete combustion, three things are essential : the air supply must be sufficient, the air must be brought into intimate contact with the fuel, and the temperature must be kept up to ignition point until combustion is quite complete. Either insufficient air supply or too rapid cooling is the usual cause of incomplete combustion.

Flame. — Fuels burn in very different ways. Some, as for instance charcoal at low temperatures, burn with a glow, evolving but little light ; others, such as the metal magnesium, burn with a very brilliant light, but with no flame ; others, like hydrogen, burn with a non-luminous flame ; and lastly, some, like coal-gas, burn with a bright luminous flame. All combustible gases — and gases only — burn with a flame. There are some apparent exceptions to this, but they are only apparent ; and whenever a solid or liquid seems to burn with a flame, it is because it is converted into gas either before or during combustion. " Flame is gas or vapour, the surface of contact of which with the atmospheric air is burning with the emission of light " (Percy). As combustion only takes place at the surface of contact, the flame must be hollow.

A simple flame is one in which there is only one product of combustion, and a compound flame is one in which there are two or more. Almost all flames used in the arts are compound, the only examples of simple flames being those of hydrogen and carbon monoxide.

Simple Flame. — As an example of a simple flame a jet of hydrogen burning in air may be taken.

As the hydrogen escapes from the jet it displaces the air and then diffuses into or mixes with it, and in the space where

the gases mix combustion takes place. In the centre of the flame, therefore, will be a core of unburnt gas, outside will be the air, and between is the zone of combustion, or the flame.

Compound Flames. — With compound flames the reactions are much more complex, and it is often difficult to determine the exact changes which take place. The compound flames of most practical importance are those obtained by the combustion of various hydrocarbon gases. The fuel may be used in the form of gas from a burner, in the solid form as a candle, or in the liquid form as in the case of the burning oils. In the last two cases the combustible is drawn up by the wick by capillary attraction, gasified by the heat

Fig. 1.— Gas Flame.

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evolved during combustion, and the gas is burned ; indeed, candles and lamps may be regarded as combined gas producers and burners.

The structure of a compound flame is much the same as that of a simple one. In the centre is a core of unburnt gas, outside is the air, and between the two is the zone of combustion or the flame ; but this zone is much more complex than in a simple flame, the combustion in the inner portions being usually incomplete.

Luminosity of Flame. — Most of the flames produced by hydrocarbons are more or less luminous, and the cause of this luminosity has given rise to a vast amount of discussion. Clearly the luminosity is not due to temperature alone, for a hydrogen flame, especially when formed by a mixture of hydrogen and oxygen, is intensely hot, but almost non-luminous ; and it is quite possible to burn coal-gas, e.g. in the Bunsen burner, in such a way as to give a very hot, but non-luminous flame.

Davy suggested that the luminosity of flame was due to the separation of particles of solid carbon by incomplete combustion within the flame, which, being heated to intense whiteness, evolve light. This theory has been generally accepted, and in its favour many facts may be quoted :

1. If solid matter be introduced into a hot, non -luminous flame, light is evolved. A cylinder of lime, for instance, placed in an oxy-hydrogen flame gives the brilliant lime-light; a mantle of thoria and ceria suspended in a non-luminous Bunsen flame is the Welsbach incandescent burner now so largely used for illumination purposes.

2. If a cold surface be held in an ordinary luminous flame it becomes covered with a black deposit of solid carbon or soot.

3. Many substances which burn with the formation of solid products of combustion, e.g. magnesium or zinc, give an intense white light.

4. Luminous flames when examined with a spectroscope give a continuous spectrum.

5. , When sunlight is reflected from hydrocarbon flames, it is polarized exactly in the same way as light reflected from solid carbon particles suspended in air.

On the other hand, Frankland contended that — at least in many cases — luminosity was due to the presence not of solid particles but of very dense gases or vapours. In support of this view it may be urged :

(1) That the luminosity of many flames is much increased under pressure, even a flame of hydrogen becoming luminous at high pressures, and that under reduced pressures the luminosity of ordinary luminous flames is much reduced.

(2) That many substances, e.g. phosphorus and arsenic, burn with a very luminous flame, though at the temperature of combustion the products are gaseous. Hydrogen produces water having a vapour density of 9 (H =1), and the flame is non-luminous. Phosphorus produces phosphoric anhydride, P 2 5 , having a density of 71, and the flame is luminous ; and arsenic, which produces arsenious oxide, As 4 6 — vapour density 198 — also burns with a luminous flame. Therefore whether the flame will be luminous or not seems to depend on the density of the products of combustion.

(3) Soot is not pure carbon, but always contains hydrogen ; and further, the fact that soot is deposited does not prove that it existed as such in the flame, as it may have been produced by the decomposition of dense hydrocarbons present.

(4) Gases under great pressure give much more complex spectra than under ordinary conditions, becoming banded, and ultimately tending to become continuous.

For these reasons it has been urged that luminosity of ordinary hydrocarbon flames may be due to the presence of very dense hydrocarbons, which under suitable conditions split up into carbon and hydrogen or light hydrocarbons, which are then burned.

Profs. Lewes' and Smithells' Researches. — Professors V. B. Lewes and A. Smithells investigated the question of the luminosity of coal-gas and similar flames, and

FUEL whilst their work confirms the view that the luminosity is due to separated carbon particles, it has thrown fresh light on the reactions within the flame by which these are separated and the conditions under which luminosity can be produced.

Prof. Smithells describes x the structure of an ordinary luminous gas flame as consisting of four parts, which, for convenience, may be taken in the inverse order to that in which they are given by him :

(4) A dark inner core or region, consisting principally of unburned gas, mingled with some products of combustion which have diffused in from the surrounding parts (c in Fig. 2).

(3) A yellow luminous portion, marking the region in which hydrocarbons are undergoing decomposition, the heat producing the dissociation being largely derived from the outer zones (a).

(2) An inner light-blue portion, visible at the base of the flame (d) ; and

(1) An outer sheath or mantle (b) ; these parts (1 and 2) corresponding to the outer and inner flame-cones of the Bunsen burner, and marking the region where the gas is undergoing combustion in presence of excess of air.

The explanation which has usually been given of the phenomena of luminosity is something like this : The gas coming into the air is at first mixed with only a very limited supply of air ; the hydrocarbons cannot be completely burned, therefore the hydrogen burns, forming water, and the carbon is liberated and heated to incandescence by the heat evolved by the combustion of the hydrogen.

This description has been shown to be incorrect in at least

Fig. 2.— Candle Flame.

two points : (1) the order of combustion, and (2) the source of the heat.

1. The hydrogen does not burn first, in the case of methane at any rate ; the reaction being, as pointed out by Dalton, CH 4 + 20 = CO + H 2 + 2H — water, carbon monoxide, and hydrogen being thus formed. But as carbon monoxide and water can mutually decompose each other, CO + H 2 = C0 2 + 2H, a further reaction may take place till the system attains equilibrium, the conditions of which, " according to

Dixon, are expressed by the coefficient -^ ^ = 4. This is subject to certain conditions of temperature and dilution." 1 These reactions, however, probably take place only to a small extent in the inner luminous part of the flame.

The source of the heat, therefore, is to be looked for, not altogether in this partial combustion, but in transmission from the outer zone, where the temperature is very high.

2. The constituent of coal-gas to which the luminosity is mainly due is ethylene, C 2 H 4 , and perhaps some of its higher homologues. This at high temperatures (1500 - 1800° F.) splits up, yielding acetylene and methane, 3C 2 H 4 =C 2 H 2 + CH 4 , the acetylene then polymerizing into more complex hydrocarbons. At higher temperatures (above 2130° F.) no polymers are formed, but only acetylene, and at that temperature methane also dissociates, yielding acetylene and hydrogen, 2CH 4 = C 2 H 2 + 2H, so that all the hydrocarbons present will have split up into acetylene and hydrogen. At still higher temperatures acetylene itself splits up into carbon and hydrogen, this change taking place at about 2430° F.

The various hydrocarbons, as is well known, burn with very different degrees of luminosity. The flame of methane, CH 4 , is very slightly luminous, that of ethylene, C 2 H 4 , is more luminous, whilst that of acetylene is intensely brilliant. That the luminosity is not due merely to the amount of carbon which the combustible contains is shown by the fact that in equal volumes of gas ethylene and acetylene contain

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the same weight of carbon, whilst benzene, C 6 H 6 , which burns with a luminosity much inferior to that of acetylene, contains, in the gaseous condition, three times as much carbon.

Neither is the luminosity due to temperature alone, as has been already shown.

It is now possible to form some idea of what actually takes place in a luminous gas flame. The inner blue core must be regarded as altogether unburned gas, for in it no combustion is taking place, but the temperature is rising, much heat being received by radiation from the outer zones of the flame, and the hydrocarbons being to some extent dissociated. As the temperature rises, dissociation takes place to a greater extent, till all the hydrocarbons, or nearly so, are converted into acetylene, and then the acetylene itself undergoes dissociation. This dissociation evolves heat, and at once brings the separated carbon up to vivid incandescence. As the products of these reactions pass outwards they are burned, the temperature therefore rising to the edge of the flame, and the combustion is completed in the very hot but feebly luminous mantle.

The luminosity, therefore, seems to depend not so much on the actual amount of carbon contained in the gas, or on the temperature, as on the readiness with which the gases present form acetylene or some other hydrocarbon which will similarly dissociate.

The luminosity of an acetylene flame is much reduced by the admixture of other gases, even though they themselves are combustible and evolve a large quantity of heat ; as, for instance, hydrogen and carbon monoxide. The presence of such gases not only reduces the luminosity of the flame, but enormously raises its dissociation temperature, which is the point at which luminosity begins ; thus —

Percentage of Acetylene. Hydrogen. Temperature of Luminosity.

90 10 1650

80 20 1830

10 90 3090

The above " acetylene theory " of the chemical reactions on which luminosity of flame depends is not now generally accepted. 1 In any case the reactions are complex.

Non-luminous Combustion. — If coal-gas, or any other gas which usually gives a luminous flame, be burnt in such a way that excess of oxygen penetrates into every part of the flame, and the acetylene burns before it can undergo dissociation, the flame will be non-luminous. Thus, if a gas flame be turned very low it is non-luminous, so also is candle flame when the wick is very short. The best means of obtaining a non-luminous flame is the Bunsen burner.

The Bunsen Burner. — This burner consists of a tube, usually about f " in diameter and 3J" long, though the size may be varied within wide limits. Gas is admitted to the bottom of the tube, and just above the jet by which the gas enters are holes for the admission of air. The air (usually 2J volumes) mixes with the gas, and the mixture burns with a hot non-luminous flame.

" When a Bunsen burns under normal conditions it has a bluish central zone, but if the air supply be largely in excess of that required for non-luminous combustion, the flame becomes smaller and fiercer with the formation of a green central zone." 2

The cause of the non-luminosity of the flame has usually been attributed to the more perfect combustion of the hydrocarbons due to the excess of oxygen in the interior of the flame. Professor Lewes has shown that this is not by any means necessarily the case, as nitrogen, carbon dioxide, and other inert gases also prevent luminosity, dilution, as already remarked, very much retarding the dissociation of acetylene, and therefore the production of luminosity. It is this dilution which is generally efficient in preventing luminosity in the " Bunsen " ; but if the air supply be too large then oxidation takes place rapidly, and the inner cone changes in appearance. The temperature of the flame is a little higher when the diluent is air than when it is nitrogen.

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Air.

Nitrogen.

\ inch above burner

54°

30°

C.

.

Tip of inner cone .

Centre of outer cone

Tip of outer cone

Side of outer cone level with tip of inner cone

.

If the supply of air be too small then a luminous point appears at the tip of the inner cone.

It does not follow because the flame is non-luminous that combustion is complete ; it frequently happens that the ingress of air has cooled the gases below ignition point, and not " inconsiderable portions of methane, carbon monoxide, acetylene, and even hydrogen escape unburnt," both from non-luminous and luminous flames.

Professor Lewes thus describes the structure of an ordinary luminous flame, and the actions which render it non-luminous in the Bunsen :

" 1. The inner zone, in which the temperature rises from a comparatively low point at the mouth of the burner to about 1000° C. (1830° F.) at the apex of the zone. In this portion of the flame various decompositions and interactions occur, which culminate in the conversion of the heavier hydrocarbons into acetylene, carbon monoxide being also produced.

"2. The luminous zone, in which the temperature ranges from 1000° C. (1830° F.) to 1300° C. (2370° F.). Here the acetylene formed in the inner zone becomes decomposed by heat with liberation of carbon, which at the moment of separation is heated to incandescence by the combustion of the carbon monoxide and hydrogen, thus giving luminosity to the flame.

"3. The extreme outer zone. In this part of the flame, the combustible gases meeting air, combustion takes place, making this the hottest part of the flame ; but towards the outer part of this zone, combustion being practically completed, the cooling and diluting influence of the entering air renders a thin layer of the flame non-luminous, finally extinguishing it. This description of a luminous flame is of necessity far from complete."

" The various actions which tend to cause the loss of luminosity in a Bunsen burner may be summarized as follows :

"1. The chemical activity of the atmospheric oxygen which causes loss of luminosity by burning up the hydrocarbons before they, in their diluted condition, can afford acetylene.

" 2. The diluting action of the atmospheric nitrogen, which, by increasing the temperature necessary to bring about the partial decomposition of the hydrocarbons, prevents formation of acetylene, and in this way will by itself cause non-luminosity. In the normal Bunsen flame it acts by doing this until destruction of the hydrocarbons by oxidation has taken place.

"3. The cooling influence of the air introduced, which is able to add to the general result, although the cooling is less than the increase in temperature brought about by the oxidation due to the oxygen of the air.

"4. In a normal Bunsen flame the nitrogen and the oxygen are of about equal importance in bringing about non-luminosity ; but if the quantity of air be increased oxidation becomes the principal factor, and the nitrogen practically ceases to exert any influence."

The amount of air supplied to the ordinary Bunsen is quite insufficient to support combustion without the air outside.

Propagation of Flame. — If a long glass tube, closed at one end, be taken and filled with an explosive mixture, say of coal-gas and air, and a light be applied at the open end, the flame will run down the tube with a definite and usually measurable speed, combustion not taking place instantaneously, but the ignition being transmitted from molecule to molecule at a comparatively slow rate. The speed at which the flame travels is called " the speed of propagation of the

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flame," and combustion thus taking place has been called an " explosion of the first order."

If, instead of a closed tube filled with gas, a tube open at both ends be used, and the mixture be made to flow through it — if the gas be lighted so that the ignition has to travel in the opposite direction to that in which the gas is flowing, its speed of transmission will be reduced, and will be the difference between the speed of propagation and the rate at which the gas is flowing, if the former be greater than the latter. If the rate of flow of the gas be very slightly in excess of the speed of propagation of the explosion, the flame will remain just at the mouth of the tube, and if it be much greater there will be a more or less long flame. Deville made a series of most interesting experiments on the rate of propagation of flames, and it is to his work, and that of Bunsen, that we owe most of our knowledge of the subject. He burnt a mixture of two volumes of carbon monoxide and one volume of oxygen — the gases therefore being almost exactly in the proportions required for complete combustion — at a jet having an area of 5 square millimetres. A flame 70 to 100 mm. high was formed, which consisted of two portions, an inner cold core, 10 mm. high, and an outer flame zone. It is obvious that in this case the inner core was not due to the absence of oxygen for combustion, but to the fact that the gases were travelling forward at such a speed that the ignition could not travel backwards, and ignite the mixture in the tube ; and no doubt had the rate of flow been diminished the flame would have grown smaller and ultimately lighted back.