Oh, I guess they use some kind of chemical war in specific case.
However, it needs confine space, so I do understand and even imagine for tunnel (in case of siege). But it was not as main weapons but as countermeasures to specific threats. There is no Ypres before Industrial mass production was available.
Sulphur burns oxygen. It used when you open a keg of vine to stop the oxidation when you finished to take a limited quantity and close the plug of it.

The problem faced by not industrial nations would be the production, collection and storage.