fossil fuel vs coal

fossil fuel and coal both are nouns.

Word	Noun	Adjective	Verb	Adverb
fossil fuel	Yes	No	No	No
coal	Yes	No	Yes	No

As nouns, coal is a hyponym of fossil fuel; that is, coal is a word with a more specific, narrower meaning than fossil fuel:

fossil fuel: fuel consisting of the remains of organisms preserved in rocks in the earth's crust with high carbon and hydrogen content
coal: fossil fuel consisting of carbonized vegetable matter deposited in the Carboniferous period

Other hyponyms of fossil fuel include gas, natural gas, crude, crude oil, fossil oil, oil, petroleum, rock oil.

fossil fuel (noun)	coal (noun)
fuel consisting of the remains of organisms preserved in rocks in the earth's crust with high carbon and hydrogen content	a hot fragment of wood or coal that is left from a fire and is glowing or smoldering
	fossil fuel consisting of carbonized vegetable matter deposited in the Carboniferous period