Why does energy density generally increase with coal rank?

• A. Higher energy density is achieved by higher moisture content.
• B. Energy density increases with coal rank because higher rank has higher carbon content and lower moisture.
• C. Higher rank coal has more impurities which increases density.
• D. Energy density is unrelated to rank.

Answer: (B)

Energy density per unit mass increases with coal rank because higher-rank coal has more carbon and much less moisture.Carbon is the main source of energy in coal—more carbon means more energy released when burned. Moisture, on the other hand, borrows energy to heat up and evaporate, but it doesn’t contribute to the useful energy output. So when moisture drops as rank rises, a larger fraction of the coal’s mass actually contributes to heating value. As a result, higher-rank coal (like anthracite) delivers more energy per kilogram than lower-rank coal (like lignite). Impurities can vary, but the dominant effect is the combination of higher carbon content and lower moisture with increasing rank.