The Composition of Coal
The two types of coal in use today are Anthracite (Welsh Coal) and Bituminous Coal. Bituminous Coal is the more common although its composition varies from pit to pit and seam to seam. The crew of the locomotive must adapt to the type of coal being used to achieve the best results.
Combustion requires Heat, Air, Space and Time. In the 1860‘s the Midland Railway introduced several features which resulted in improved combustion within the boiler. These included the brick arch (which increased the space available for combustion by extending the path of the volatile gases), the rocking grate and the fire hole door (improved air flow into the firebox).
Air is admitted into the firebox from two main points. Primary Air (Bottom Air) is admitted through the grate and Secondary AIr (Top Air) enters through the firehole door. There is also a third source of air known as Supplementary Air which travels through the gaps around the fire doors.
An even fire built up and well burned through (left) would give good results, maintaining steam at high pressure whilst the engine is being heavily worked. Air is being admitted through the front damper and firehole door. The smoke at the chimney should be a light grey showing that the correct amount of air is being mixed with the volatile gases resulting in complete combustion. No smoke at the chimney indicates too much air and the coal is burnt through. It is time to fire again. However, too much coal will result in smoke.
Smoke does not make Steam!
Black smoke indicates a lack of air and unburnt volatiles escaping to atmosphere. A fire of the kind shown (right) gives very bad results in maintaining steam, causes a great loss of heat and much smoke! The large mound of coal is cooling the bed of the fire and prevents an even flow of air through the fire grate.
The size of coal also effects combustion. If coal is broken into pieces no larger than a man‘s fist, then a greater surface area is exposed and the fire burns better.
Here, an example of bad firing is shown (left). The fire is built up under the brick arch and the area which is normally the hottest part of the fire is the coolest. Air cannot flow easily, the brick arch is cooler than it should be and its effectiveness as a radiant is impared. Good steaming results cannot be obtained.
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