Campfire coals will work but won’t be of the highest quality but I don’t think that matters on small scale. You can get some mileage if you put them while still glowing in a steel pail or something and dowse it with water.

It’s a far cry from a steam vessel but it certainly can’t hurt.

My biochar episode: https://fountain.fm/episode/kG7hlNnH9S4kBMNPKeW9