The New Zealand Railways Magazine, Volume 3, Issue 3 (July 2, 1928)
Theory of Combustion — (Continued)
The subject of weathering cannot be gone into without involving the subject of spontaneous combustion.
Owing to market conditions and the crowding of transport facilities the storing of coal has become more and more of a necessity in recent years. Users of coal are compelled to have on hand a reserve supply that will enable them to carry on without interruption in case of truck shortage, or other unforseen circumstances. For this purpose the amount of coal stored will vary, from a few hundred, to several thousand tons.
Most bituminous coal will ignite spontaneously if placed in large heaps. Moreover, it suffers disintegration (more or less) during storing and handling.
At ordinary temperature, the atmosphere oxidation of the coal is going on slowly all the time, and continues with increasing rapidity at higher temperatures until, in many cases, it results in ignition. Any heating of the coal in the stock represents so much heat loss which will not be available for the boiler. Most of the atmosphere oxidation does not produce any sensible heating, but it results, all the same, in an appreciable loss of heat units in the coal.
The changes taking place in stored coal may be divided into two classes: (1) oxidation of pyrite, marcasite and other inorganic constituents; and (2) the direct oxidation of the organic matter of the actual coal. To the change in the inorganic matter most of the visible changes are due. The iron sulphide changes into sulphate of iron and sulphuric acid, and the latter in its turn unites with the calcium and magnesium carbonates, almost invariably present in coal, to form gypsum and magnesium sulphate. All such changes result in a large increase in volume and a marked disintegration of the coal. They will also, in many instances, bring about a considerable increase in the weight of the coal unless removed by the leaching action of water.
The conclusion is that stored coal may increase in weight, but that its heating value decreases most rapidly during the first week after mining, and continues to decrease more and more slowly for an indefinite time. The losses due to disingration of the coal and to spontaneous ignition are of greater importance than any change of weight or heating value. The larger the size of coal stored the less liable is it to fire spontaneously.
Storage of coal under water will prevent disintegration of the coal to a very large extent, and will absolutely prevent spontaneous ignition. Apart, however, from the advantage mentioned of storing coal under water, there seems to be little in favour of any particular method of storing coal.
I do not know of any case during the last 27 years on the South African Railways where we have had a case of any of our stored coal firing, although some time ago there was a pretty bad case of firing on the Cornelia Mines at Viljoen's Drift. The firing was due possibly to spontaneous combustion of the fine coal dust in the mine chambers.
Things for Enginemen to Remember.
1. The combustible matter in coal consists chiefly of carbon, but also of hydrocarbon and page 35 a little sulphur. (Hydrocarbon is carbon and hydrogen joined together, and are the chief light giving portions in coal gas.)
2. When coal burns the carbon and other combustible parts join into the oxygen in the air, and are said to burn.
3. The oxygen used in the firebox is admitted through the firehole door, the damper doors, and the firebars, being drawn in by the blast.
4. When carbon burns completely a gas called carbon dioxide (or carbonic acid gas) is formed. It consists of 32 parts by weight of oxygen, and 12 parts by weight of carbon.
5. When the carbon is not completely consumed another gas is formed called carbon monoxide, which consists of 12 parts by weight of carbon, joined to 16 parts by weight of oxygen.
(If a further supply of oxygen is available the carbon monoxide will burn into a blue flame and form carbon dioxide. If oxygen in sufficient quantities is not available, the carbon monoxide will escape up the chimney and a great deal of the heat will be lost necessitating the burning of more coal to produce the same amount of steam.)
6. The amount of air required to complete the combustion of 11b. of coal (assuming it to be pure carbon) is 12lbs. by weight. This, at ordinary temperature, has a volume of 156 cubic feet—leaving the firebox at a high temperature its volume is increased to 500 cubic feet.
Installed at the New Workshops in the Hutt Valley, Wellington
(Photo by A. P. Godber)
Top: Modern Loudon Planing Machine. The machine is electrically controlled, and its return speed is very high. Bottom: 36 in. Bullard Vertical Turret Lathes. A feature of these machines is that both turning and boring can be done in one operation.
7. It is not correct to regard coal as wholly combustible matter. All coal contains some incombustible matter, which falls into the ashpan as ashes, is carried as dust into the smokebox, or remains in the firebox as clinker.
8. Too much air must not be admitted into the firebox. Air above what is required for combustion of the coal is heated up as it travels to the chimney, and carries away with it heat that could be used to make steam. This means that a certain amount of heat is given out by every pound of coal, and if part of this is used for warming up a quantity of air (which passes away hot), this portion must be lost, and, in addition, this needless air, increasing as it does the quantity of gases which are heated up, causes the temperature of the flame to be lower than it otherwise would be, Loss of heat, and therefore of coal, results because the hotter the gases the more freely they part with some of this heat to the water—providing, of course, they are not allowed to escape into the atmosphere at too high a temperature. The escape of the gases in this way is caused by the admission of too much air.
An insufficient supply of air affects the working of the boiler, for in the first place, there is a tendency for the fire to smoke (owing to particles of carbon not coming into contact, especially when hot, with the oxygen necessary for their combustion), then there is a great danger of the carbon being burnt only to carbon monoxide and being allowed to escape in that state up the chimney. Moreover, some of the hydrocarbons may also pass up the chimney in an unburnt state.
The admission of the correct quantity of air into the firebox may be determined thus:—If an additional supply of air be admitted into the firebox when carbon monoxide, hydrocarbons and smoke are escaping up the chimney, the smoke will be the last to be consumed. When, therefore, the last traces of smoke have disappeared it is fair to assume that combustion is as near right as possible. The ideal state of the fire is that in which no smoke is given off.
(To be concluded.)