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The New Zealand Railways Magazine, Volume 2, Issue 9 (January 1, 1928)

Theory of Combustion

page 36

Theory of Combustion.

In order to fire an engine properly the fireman need not necessarily know anything about the theory of combustion. He may have learned to apply the principles without knowing the reason for so doing. In fact many first rate firemen do not understand anything about these principles. There are, however, certain fundamental facts that should be borne in mind when endeavouring to explain to firemen the necessity of doing certain things to get proper results. To produce heat in a locomotive firebox three conditions are necessary—and only three.

First. There must be a supply of coal.

Second. There must be a plentiful supply of air.

Third. The air and the coal must be brought together at a temperature at which they will burn.

Although through long experience a good fireman acts in a very skilful manner, it is certain that an intelligent knowledge of the theory of combustion will secure better results. Every driver and fireman should pay great attention to the study of combustion and endeavour to appreciate what is taking place in the firebox, tubes and smokebox.

Every fireman is aware that the air passing through the damper doors and firebars causes rapid combustion of the heated fuel, that gases are given off which are burned by mixing with the air that comes through the firehole door. He is also aware that in this way heat is produced which is passed to the water in the boiler through the firebox sheets and through the tubes, and that the heating of the water produces steam.

Now a man who wishes to become a good engineman will not rest here. He will seek to understand what is taking place when coal is shovelled into the firebox. It is in the interest of fuel economy that supervising officers should help the staff to grasp the essential facts of the theory of combustion. We will endeavour to show what combustion is, and what goes on in the firebox, and the best methods to be adopted by the fireman to get the best results.

Combustion Visualised.

The steam locomotive is a heat engine—heat being its one and only source of power. The theory of combustion is the understanding of heat generation and transfer.

Combustion is a chemical combination or reacting that produces heat, and heat is a form of energy due to molecular vibration or motion.

To understand this we must try and get a mental vision of the processes of chemical combination and we must be able to picture a structure of matter that is capable of molecular vibration. We will endeavour to do this as simply as possible.

Coal as a Fuel.

We already know that coal consists of fixed carbon, volatile matter, moisture and ash.

The term “fixed” carbon is used to distinguish that part of the carbon that remains unmixed, chemically, with any other substance from the carbon that is contained in the volatile matter in chemical combination with hydrogen. These mixtures of hydrogen and carbon contained in the volatile matter are known as hydro-carbon; which, when the coal is heated, is driven off in the form of a gas, or of a semiliquid tarry substance.

Carbon is the chief constituent of coal. If a piece of wood is charred, partially burnt or heated in a retort, it is converted into charcoal which consists almost entirely of carbon. Carbon is also produced by lighting a match.

The moisture or water in coal is made up of hydrogen and oxygen. The ash usually contains some of the clinker and honeycomb forming elements, sulphur and iron, which is so often a source of trouble to the fireman. In addition there may be chemical compounds known as the “oxides” of silica, aluminium, calcium and magnesium.

Carbon and hydrogen are the “fuel” elements contained in coal, or any other form of fuel. Sulphur has a low heat value and is an undesirable impurity. The chemical combination of the fuel elements—carbon and hydrogen, with oxygen from the air, is called combustion, and combustion results in changing chemical energy into a form of energy which we know as heat.

All this does not, however, make the “how” and “why” of combustion and heat clear.

A body will burn and give out heat when it unites with oxygen. This is what the carbon in coal does when burnt in a firebox. The oxygen is supplied by the air which is a mixture of 23 parts oxygen and 77 parts nitrogen in every 100 parts by weight.

page 37

The nitrogen takes no part in combustion. Combustion is known as a chemical combination, but the cause of chemical combination has always been, and still is, more or less a mystery. Any explanation of the mechanism of combustion is therefore not only rather difficult to make but is also open to question.

When carbon burns combustion can take place in two ways. Combustion takes place by the uniting together of very minute particles or “atoms” of substances. These minute particles have different weights—each carbon atom weighs 12 and each oxygen atom 16, as compared with the atom of hydrogen (the lightest known substance)—the weight index figure of the latter being taken as 1.

In the incomplete combustion of carbon, each atom unites with one atom of oxygen and the product is carbon monoxide. In the complete combustion two atoms of oxygen combine with one atom of carbon and this is called carbon dioxide.

If sufficient air is supplied the carbon will be burnt to carbon dioxide gas at the bottom of the box; if insufficient air is supplied it will be immediately burnt to carbon monoxide gas, the complete combustion of the fuel taking place, in the latter case, above the fire.

If sufficient air is supplied the carbon will be burnt to carbon dioxide gas at the bottom of the box; if insufficient air is supplied it will be immediately burnt to carbon monoxide gas, the complete combustion of the fuel taking place, in the latter case, above the fire.

If there is a plentiful supply of air 12 parts by weight of carbon will unite with 32 parts by weight of oxygen and a non-inflammable gas called carbon dioxide is produced. (Complete combustion).

If the supply of air be limited, only half this amount of oxygen may be taken up and carbon monoxide will be formed which is inflammable and capable of taking up more oxygen to form carbon dioxide.

This is how to consider it:—

  • Weight Carbon = 12.

  • Weight Oxygen = 16.

Incomplete Combustion.

  • Carbon = 12 parts by weight

  • unites with oxygen = 16 parts by weight

forming carbon monoxide = 28 parts by weight. This gives off only 3–10ths of the heat in the fuel. Now we know 2 atoms of oxygen are required to complete combustion with one atom of carbon bring in more air, and you get—

  • carbon monoxide = 28 parts by weight

  • unites with oxygen = 16 parts by weight

  • 44 parts by weight

which gives off the remaining 7–10ths of the heat in the fuel.

Complete Combustion.

Carbon = 12 parts by weight, uniting with, two atoms of oxygen = 32 parts by weight, forming carbon dioxide = 44 parts by weight, which releases all the heat contained in the fuel. In burning to carbon-monoxide, carbon gives out only 3–10ths as much heat as it does in completely burning to carbon-dioxide and therefore if, through not admitting enough air through the firehole door, carbon dioxide only is formed, about 7–10ths of the heat is lost, or about 7 lbs. of coal out of every 10 consumed in this way are wasted. Young firemen who like to fill the firebox with green coal then close the firehole door and sit down, think of this!

Heat can also be wasted by admitting too much air; this will be explained later on. (See remarks under Nitrogen). To completely burn 1lb. of carbon the two 2–3rd lbs. of oxygen contained in 121bs. of air are required, and this air, at the ordinary temperature, would measure about 156 cubic feet about 12½ foot square.

We have already pointed out that coal does not wholly consist of carbon. The best coal has about 80 per cent, carbon, the remainder consisting of hydrogen, nitrogen, sulphur, ash and water.

The hydrogen is partly united to the oxygen and these together are given off as water vapour when the coal is burnt. Another portion of the hydrogen is given off in the form of hydrocarbon vapours and this produces the luminous flames. When the boiler is being fired without the blower on, or without the exhaust steam discharge, the hydrocarbons can be seen coming from the chimney as a yellowish smoke.

At the high temperature of the firebox, when running, these hydrocarbons are inclined to split up into carbon and hydrogen and if sufficient oxygen is not present to completely burn the carbon, some of it escapes unburnt, causing a black smoke. This is waste of coal and money. A good fireman does not allow his smoke stack to belch forth black smoke, a poor or lazy fireman does.

If properly burnt, both the carbon and the hydrogen unite with the oxygen of the air, the former in the manner described above and the page 38 latter forming water which passes off in the form of steam. (See fig. 2.)

The heat given off by hydrogen in burning is much greater than that given off by an equal weight of carbon, but the amount of hydrogen in coal is small and some is already united with the oxygen in the coal and therefore gives out little or no heat when the coal is burnt.

Sulphur Ash and Clinker.

The sulphur in coal is usually united with iron in the iron pyrites. The sulphur burns out but the iron is left behind and tends to run together forming clinker. Clinker, of course, as all firemen know, spreads across the fire-bars and if not removed prevents the air from passing up through the fire-grate, and steam is not produced. Some coals do not clinker, but fall into the ashpan in a state of powder. This ash is of an earthy nature and consists principally of silica, aluminium, calcium and magnesium.


The nitrogen in coal is very small and plays no part in combustion; it passes up the smoke stack when the coal is burnt carrying with it some heat.

Of great importance is the large volume of nitrogen which has to pass through the fire-box in the air required to burn the coal. This will be understood when you remember that to burn 1lb of coal requires 12 lbs of air of which 9 lbs is nitrogen.

Fig. 2

Fig. 2

This nitrogen does not itself burn, but it reduces the rate of combustion and having to be heated up with the other gases in the firebox it absorbs a lot of heat in doing it, and of course this is waste.

The temperature at which the gases leave the smoke stack is often over 800 deg. F. and to heat 9lbs of nitrogen to this temperature will take about 1–9th of a lb of coal.

You will see then how necessary it is that only the amount of air required for burning the fuel is allowed to pass through the firehole and damper doors owing to the loss arising from the heat carried away by any superfluous air which may be drawn in.

Moisture in Coal.

All coals contain water which has to be evaporated in the firebox and converted into steam. This carries away useful heat. It has been proved that 1–25th part of the total heat given out by coal in burning may be carried away in the steam formed by the evaporation of the water contained in the coal and that formed by the combustion of the hydrogen.

The presence of water, volatile hydrocarbon and earthy matter mechanically mixed in with the fixed carbon, accounts for the ease with which a lump of coal disintegrates and breaks up into smaller pieces when heated.

(To be continued.).