Coal is a solid fuel created from deposits of ancient vegetation that have undergone a series of metamorphic changes resulting from pressure, heat, submersion, and other natural formative processes occurring over a long period of time. Because of the different combinations of these processes acting on the vegetation and the widely differing forms of vegetation involved, coal is a complex and nonuniform substance.
Coal is often classified according to the degree of metamorphic change to which it has been subjected into the following categories:
2. Bituminous coal
3. Semibituminous coal
The formation of peat represents a precoal stage of development. After increased pressure and time, lignite begins to form. The next stage is semibituminous coal, followed by bituminous coal and finally anthracite. Each of these stages is characterized by an increase in the hardness of the coal. Anthracite is the hardest of the coals. Graphite represents a postcoal developmental stage and cannot be used for heating purposes.
Anthracite is clean, hard coal that burns with little or no luminous flame or smoke. It is difficult to ignite but burns with a uniform, low flame once the fire is started. Anthracite contains approximately 14,400 Btu/lb and is used for both domestic and industrial heating purposes. Its major disadvantage as a heating fuel is its cost.
Anthracite is divided by size into several different grades. Each of these grades (e. g., egg size, buckwheat size, pea size) is suitable for a specific size of firepot. They are described in greater detail in Chapter 3 of Volume 2 (“Coal-Firing Methods”).
Bituminous coal is softer than anthracite and burns with a smoky, yellow flame. A great amount of smoke will result if it is improperly fired. The term bituminous coal actually covers a whole range of coals, many of which have widely differing combustion characteristics. Some of the coals belonging to this classification are hard, whereas others are soft.
The available heat for bituminous coal ranges from a low of
11,0 Btu/lb (Indiana bituminous) to a high of 14,100 Btu/lb (Pocahontas bituminous). The heat value of the latter approximates that of anthracite (about 14,400 Btu/lb); however, unlike anthracite coal, it is available in far greater supply, a factor that makes it a very economical solid fuel to use.
Semibituminous coal is a soft coal that ignites slowly and burns with a medium-length flame. Because it provides so little smoke, it is sometimes referred to as smokeless coal.
Lignite (sometime referred to as brown coal) ignites slowly, produces very little smoke, and contains a high degree of moisture. In structure it is midway between peat and bituminous coal. Lignite contains approximately half the available heat of anthracite (or about 7400 Btu/lb) and burns with a long flame. Its fire is almost smokeless, and it does not coke, a characteristic it shares with anthracite.
Lignite is considered a low-grade fuel, and its caloric value is low when compared with the other coals. Moreover, it is difficult to handle and store.