town gas vs coal gas

town gas and coal gas both are nouns.

Word	Noun	Adjective	Verb	Adverb
town gas	Yes	No	No	No
coal gas	Yes	No	No	No

As nouns, coal gas is a hypernym of town gas; that is, coal gas is a word with a broader meaning than town gas:

town gas: coal gas manufactured for domestic and industrial use
coal gas: gaseous mixture produced by distillation of bituminous coal and used for heating and lighting

town gas (noun)	coal gas (noun)
coal gas manufactured for domestic and industrial use	gaseous mixture produced by distillation of bituminous coal and used for heating and lighting