You don’t need to convert it to coal. Use it to build houses, furniture, and other things.
I am currently building a wooden house this way. Wooden frame, wooden exterior, wooden floors, even wood-based insulation (https://huntonfiber.co.uk/). The isolation has the shortest life span and it is expected to last at least 60 years.
But left out to rot and yeah, the fungus and bacteria will ultimately consume the wood and release CO2 as a byproduct.