Final answer:
As the rank of coal increases, the proportion of carbon in it increases, leading to a higher carbon content in higher-grade coal like anthracite compared to lower-grade coal like lignite. This results in higher CO₂ emissions per unit of energy when higher-ranked coal is burned.
Step-by-step explanation:
The question pertains to the variation in carbon content as the rank of coal increases. In the formation of coal, there is a progressive metamorphism that increases the carbon content of coal as it transforms from lignite (lower grade) to anthracite (higher grade).
This process involves the loss of volatile components like water and methane, leading to a higher concentration of carbon in higher-ranked coals.
Carbon Content in Different Types of Coal
Lignite coal, which has a higher oxygen content and a hydrogen to carbon ratio of about 1.0, is considered the lowest grade of coal. As coal matures and becomes bituminous and then anthracite, the proportion of carbon increases, while the oxygen content and the hydrogen to carbon ratio decrease.
Anthracite coal, which has the highest carbon content and the lowest oxygen and hydrogen components, has the highest energy content of the coal types.
CO₂ Emissions and Coal
The higher carbon content in higher-ranked coals means that these coals, when burned, release more carbon dioxide (CO₂) per unit of energy compared to lower-ranked coals. This is why anthracite is considered more carbon-intense than other forms of coal, such as lignite or sub-bituminous coal.
The carbon intensity of coal is a crucial factor in understanding its environmental impact, as coal has historically been the dominant CO₂ producer among fossil fuels, contributing significantly to atmospheric CO₂ levels, as evidenced by scientific data and reports.