Charcoal briquettes into biochar?

I know that BBQ charcoal briquettes aren't good for biochar because they aren't purely carbon. However, could they be put into a retort and have all the other stuff gassified out of them to then BECOME the perfect char for making into true biocha?

I wouldn't feel safe that 100% of the undesirable stuff would off-gas or burn off in the process.

If I had a bunch of extra briquettes around, I'd probably use them to fuel the retort to char some known clean inputs.