https://www.researchgate.net/figure/Several-types-of-coal-Coal-2010_fig13_270897248
https://www.facebook.com/102450918361981/photos/a.102458548361218/102465448360528/?type=3
https://www.pmfias.com/coal-formation-of-coal-types-of-coal-peat-lignite-bituminous-coal-anthracite-coal/
https://www.americangeosciences.org/critical-issues/faq/what-are-the-different-types-of-coal
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