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How do coal rank and heat value change from lignite to bituminous coal?
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coal utilization
Cross-regenerative coke oven. (A) Cross section, showing the alternating arrangement of flue walls and ovens; (B) longitudinal section, showing (left) a series of combustion flues in a single flue wall and (right) part of a long, slotlike oven.
Coal rank is determined by the degree of coalification, a process driven primarily by increasing temperature during burial. As coal matures from lignite to subbituminous and finally to bituminous coal, its physical and chemical properties change significantly.12
The heat value, or calorific value, of coal generally increases as its rank increases. This progression reflects a transition in composition:32
- Lignite: As the lowest rank, lignite has the lowest heat value, typically ranging from 6,300 to 8,300 Btu/lb (14.65 to 19.30 MJ/kg). It contains high levels of moisture and retains a woody structure.32
- Subbituminous Coal: This intermediate rank is harder and contains less water (typically 10 to 25 percent) than lignite. Its heat value is higher, generally ranging from 8,300 to 11,500 Btu/lb (19.30 to 26.74 MJ/kg).23
- Bituminous Coal: This higher-rank coal possesses a greater heat value than both lignite and subbituminous coal, often exceeding 11,500 Btu/lb (26.74 MJ/kg). It is more compact and often exhibits agglomerating characteristics.32
Because lower-rank coals have lower heat values, larger quantities must be burned to produce the same amount of energy compared to higher-rank bituminous coal.2