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

Complete Combustion

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.