residue vs spent grain

residue and spent grain both are nouns.

Word	Noun	Adjective	Verb	Adverb
residue	Yes	No	No	No
spent grain	Yes	No	No	No

As nouns, spent grain is a hyponym of residue; that is, spent grain is a word with a more specific, narrower meaning than residue:

residue: matter that remains after something has been removed
spent grain: the residue of malt and grain which remains in the mash-kettle after the mashing and lautering process.

Other hyponyms of residue include crackling, greaves, ash, cotton cake, cottonseed cake, dottle, brewer's grain, draff.

residue (noun)	spent grain (noun)
something left after other parts have been taken away	the residue of malt and grain which remains in the mash-kettle after the mashing and lautering process.
matter that remains after something has been removed