Final answer:
Coal forms from plant material that undergoes a series of processes over millions of years, starting as peat and, with compaction and carbonization, turning into lignite, subbituminous, bituminous, and ultimately, anthracite coal, increasing in carbon content and energy yield.
Step-by-step explanation:
Coal forms through a sequence of biological and geological processes starting with the accumulation of plant material, primarily from the dense swamp forests of the Carboniferous Period. Over time, this organic material becomes buried and transformed through biological decay and chemical changes, a process known as humification. The initial product of this decomposition is peat, a partially decayed substance that is often used as a fuel after drying.
As geological processes bury the peat deeper, increasing temperature and pressure begin to compact this material further, driving off moisture and volatile compounds—a phase known as carbonization. This leads to the formation of lignite, the lowest rank of coal, which can then transform into higher grades of coal such as subbituminous and bituminous with continued pressure and heat. Ultimately, the highest grade of coal, anthracite, is formed under the greatest amounts of heat and pressure, exhibiting the highest carbon content and calorific value.
The stages of coalification, starting from peat and progressing to lignite, subbituminous, bituminous, and finally anthracite, represent the increasing 'rank' of the coal, which corresponds to its energy content and the environmental impact of its use. Burning bituminous coal, common in the United States, can release sulfur dioxide (SO2), contributing to acid rain due to the presence of pyrite. Clean coal technology and measures like using low sulfur coal and installing scrubbers are methods to reduce these environmental impacts.