The Crucible of Flame

I believe there is something not quite right with the simulators calculations of this Essence. It ALWAYS thinks it should be used. I don’t see good results with it and the other sims have it ranked much lower.

Not enough info to help you here, what spec, what fight type?

If it’s the major essence you’re referring to that is something you specify when you set BiB up, along with where you choose your talents and if you want single target or mythic+ recommendations.
If you want to see how the different essences rank have a look at the guides, it’s a link on the top right of the page.