If one of those those blocks get lit on fire, their burning time will be similair when being used as fuel inside a furnace.

Burning coal wil produce Sulfur dioxide (SO2) in the proccess

I don't want to mess with the properties of blocks like that. Also, we're not adding a bunch of different gases. Just the ones listed here (Minus Afterdamp): https://en.wikipedia.org/wiki/Damp_(mining)