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.

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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 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.