The inorganic impurities found in Indian coal are often naturally integrated into the coal structure, making their removal quite challenging. The original plant materials, which eventually transformed into coal, were transported by rivers and deposited as fluvial, lacustrine, or deltaic sediments. As a result, these plant materials became mixed with clay and other mineral matter, leading to coal with a higher ash content.

In contrast, some coal is formed in situ, where the vegetation grew and was buried at the same location without much transportation. In such cases, the plant material was less contaminated by external minerals, producing coal with lower ash content and relatively higher purity.