For the beehive kiln, I've been using firewood. Three layers per pile (three logs tall/block or 24 firewood per stack (216 firewood per fire)) I've found is the cheapest you can fuel it, given that firewood is renewable and there's no need to dip into precious charcoal as it offers no benefit beyond a slightly faster start time but with a 2 day firing time regardless of fuel, firewood wins. Hope this helps others!