What conditions during the formation of coal may reduce its quality for industrial use?

Question

Quizplus · Accepted Answer

Clay and silt washing periodically through the swamp that formed the coal can introduce impurities like ash and sulfur that reduce the quality of the resulting coal. High heat and pressure are actually necessary for coal formation, and flooding or the type of plants that formed the coal are not necessarily indicative of lower quality.

What Conditions During the Formation of Coal May Reduce Its