When we subject coal to an elementary organic analysis we simply obtain the total amount of carbon, hydrogen, oxygen, nitrogen, sulphur, and ash which it contains, as given in the tables above, but we know little or nothing of the way in which these elements are combined. "There can be but little doubt, however, that it is correct to look upon it as carbon which contains, either mixed with it or combined with it, hydrocarbon compounds of much the same character as the paraffins, and also compounds of carbon, hydrogen, and oxygen, of somewhat similar kind to the woody fibre from which it has sprung, while other compounds containing nitrogen, and probably of more complexity, are also present."

This theory has been recently corroborated by an investigation conducted by Mr. W. M'Connell, of the Durham College of Science, and reported in the "Journal of Gas Lighting" for January 30th, 1894.

One piece of information which has an important bearing on the suitability of a coal for gas-making purposes is obtained from an elementary analysis of the same, viz., the percentage of unoxidized hydrogen, by which is meant hydrogen unaccompanied by an equivalent of oxygen, which is obtained by deducting one-eighth of the percentage of oxygen from the percentage of hydrogen.

According to Professor Wanklyn' coal is very poor for

"Gas Engineers' Chemical Manual," p. 13.

gas-making purposes when the unoxidized hydrogen is less than four per cent., and it is exceedingly rich when the unoxidized hydrogen exceeds five per cent. The following is an example of what is meant.

The reason why the percentage of unoxidized hydrogen plays such an important part in the choice of a coal is, that when the coal is carbonized, every 16 parts of oxygen present unite with 2 parts of hydrogen to form 18 parts of water, so that it is only the hydrogen over and above that required by the oxygen, that is available for the production of hydrocarbons.

In addition to an elementary analysis, much useful information relative to the suitability or non-suitability of any particular coal for gas-making purposes, may be obtained from a "proximate" analysis, which tells us the moisture, specific gravity, percentage of volatile matter, ash, and sulphur. The amount of moisture in coal is of importance. When Newcastle coal is carbonized, water is given off with the gas to the amount of about twelve gallons per ton, but the whole of this is not due to the chemical combination of the hydrogen and oxygen in the retort, but a portion of it (varying in the case of Newcastle coals from two to four gallons per ton) is due to the moisture held in mechanical combination by the coal. The presence of this moisture in

the retort is prejudicial for many reasons, as it reduces the heat of the retorts, increases the CO2 in the gas, and liberates a portion of the sulphur, which would otherwise remain behind in the coke. The amount and nature of the ash, or clinker, is a very important factor in the choice of a coal. The ash in coal is purely inorganic, and generally consists of a variable mixture of silica, alumina, oxide of iron, and lime, the colour varying in Newcastle coals from a light cream to a brick red colour. The red colour is due to oxide of iron, which indicates the presence of iron pyrites, and consequently of much sulphur in the coal. Coals of this description, i.e., coals yielding a red ash, destroy the furnace bars. Another important feature in the nature of the ash is whether a thick, pasty slag, or a thin, fluid slag is formed in the furnace. If a liquid clinker be the result, then the coke will be more easily worked in the furnace, and consequently be better adapted for keeping up the heats in the retorts.

The following is an analysis of the ash of a Newcastle. coal as given in Newbigging's "Handbook: "

The sulphur which is always present in coal, exists in many forms, generally combined with iron in the form of iron pyrites, sometimes as CaSO4, and more rarely in combination with organic elements. In common coals the sulphur that is present in the form of iron pyrites alone

affects the gas manufacturer, and in this condition it is either invisible, or it is seen in the form of nodules, or in the yellow laminations known as brasses."

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The formula for iron pyrites is FeS2, and when it is heated in the atmosphere of hydrogen which exists in the retort, it loses half of its sulphur, which goes off into the gas as sulphuretted hydrogen (SH,), carbon disulphide (CS), and other sulphur compounds; but if moisture is present to any extent in the coal, the water acts upon the other atom of sulphur and sets the latter free also. It should be noted that the sulphur in caking coals generally is found as iron pyrites, while the sulphur in cannel coal occurs in some organic form. This organically combined sulphur is nearly all expelled in the process of carbonization, and unites very readily with carbon at the temperature usually met with in the retort, to form the impurity CS2.

The amount of sulphur given off from a coal during distillation is not the same under all conditions, as it greatly depends upon the temperature at which the coal is distilled. It is with high heats, and at the end of the charge, that most of the sulphur, especially in the form of CS, is given off. For an account of the gas-producing powers of the various descriptions of coal, the student should consult Newbigging's "Handbook," or King's "Treatise."

CHAPTER II.

CARBONIZATION

The construction of a retort or oven best adapted for the destructive distillation of coal-The setting of retorts and the construction of retort furnaces—The principles of combustion and their application to the working of retort-furnaces.

TH

HE process by which gas is obtained from coal, is a chemical operation known as destructive distillation, and technically called carbonization. The chemical changes which occur during the destructive distillation of coal are both numerous and interesting, and as it is very necessary for those engaged in the manufacture of gas to have a correct knowledge of the nature of such changes, it will be advisable, before proceeding further, to say a few words concerning what takes place in the retort when the coal is destructively distilled.

The greater part of the hydrogen of the original coal passes off, partly in combination with oxygen as aqueous vapour (HO), and partly combined with carbon as marshgas (CH), and olefiant gas or ethylene (C2H), together with smaller quantities of acetylene, benzol, and other hydrocarbons, while a portion passes off in the free state. The nitrogen of the coal is driven off, combined with hydrogen in the form of ammonia, and with carbon in the form of cyanogen; and the sulphur which is present in the original coal in the form of iron pyrites or "brasses," is evolved in two forms, viz., as sulphuretted hydrogen (SH), and as carbon bisulphide (CS), and according to some authorities as SO2.