I disagree with the folks who are concerned about the ashes in compost. Often
in here and other places I read about food contamination in a compost piles.
But, compost piles are excellent places for food waste processing.
The balance of the material in the ashes are also good compost candidates.
Composting is a remediation technique for a wide range of contaminates and BBQ
ashes are really tame things compared to TNT.