Charcoal is the product of burned wood, with a few other additives. As wood ash is compostable, this is also true of other wood products, including charcoal. There are 2 conditions to this, though. First, the charcoal itself should be basic wood. Beware of preservatives and other additives to make the product 'burn,' as these may not be environmentally compatible. For example, some charcoal burns the wood, leaving only a ‘binder’ chemical as the ash that is not suitable for composting.