Sulfur is a type of dust added by Thermal Foundation. It can be obtained as a byproduct of processing Coal or various Nether-based materials in a Pulverizer or an Induction Smelter. Sulfur is also sometimes dropped by mobs that are immune to fire or Lava (mostly mobs from the Nether). This can be disabled in the mod's configuration file.
Sulfur is a critical component in the creation of Pyrotheum Dust and, ultimately, Blazing Pyrotheum. The material can also be used together with Niter to create Gunpowder. Lastly, it can be used to craft Flux Capacitors in Thermal Expansion.