STAGES IN THE FORMATION OF COAL
Peat represents the first stage in the metamorphosis of vegetable matter into coal. It may contain 85% water when cut from a well-drained bog. The moisture has to be reduced to 25-30% before it can be burnt.
LIGNITE (BROWN COAL)
This represents the second stage in the formation of coal. There are many kinds, and it may contain 40% water when mined. The poorer qualities are only suitable for gas production and for distilling, to form tar oils. Lignite usually disintegrates on exposure to the air.
This is the third stage. The coal is black, relatively brittle and burns with a smoky yellow flame. It is stable to air, and its moisture content is about 4%-6%. There are two main types, 'caking' and 'non-caking', with many grades in between.
Caking coals are rich in hydrocarbons. These coals swell and fuse together when heated. They soften into a pasty mass which needs a strong draught and constant working when used for raising steam. They are excellent for gas manufacture, and were used a great deal for this when 'coal gas' was in full - scale production.
Non-caking coals, also known as steam coals, are much better for steam raising and industrial use since they burn freely with a fair draught and have no tendency to cake.
This is the final stage in coal formation. Anthracite is a hard, dense and brittle black coal but it is clean to the touch, and no vegetable tissues are visible. It contains very little volatile matter. It ignites with difficulty and with a short flame but it has a very high heating value. It needs a strong draught for efficient combustion.
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