Name the four coal ranks in order of increasing metamorphism and typical energy density.

• A. Lignite, sub-bituminous, bituminous, anthracite
• B. Anthracite, bituminous, sub-bituminous, lignite
• C. Lignite, sub-bituminous, anthracite, bituminous
• D. Lignite, sub-bituminous, bituminous, anthracite

Answer: (D)

The concept being tested is how coal changes with metamorphism and how its energy content increases as it becomes more mature. As coal is subjected to greater heat and pressure, its structure becomes more carbon-rich and less moisture and volatile matter remain. Lignite is the least mature form with the lowest carbon content and the lowest energy density. Sub-bituminous coal has more carbon and a higher energy density than lignite. Bituminous coal contains even more carbon and delivers more energy per mass and is widely used for electricity. Anthracite is the most mature, with the highest carbon content and the highest energy density, burning cleaner and hotter.

So the four coal ranks in order of increasing metamorphism and typical energy density are lignite, sub-bituminous, bituminous, anthracite. Sequences that place the highest-rank coal first or mix up the order would imply a nonincreasing metamorphism, which doesn’t fit how coal progresses.