Coal is basically solidified crude oil -- decayed plant matter, compressed over long, geological time. For cooking purposes, burning coal differs from burning charcoal in that exposure to a coal fire can poison your food with petroleum by-products. Coal can be used to fire a stove or an oven, but only if the food is in a completely separate environment from the fire.
"Indirect heat" as in barbecue terminology is not indirect enough when the fuel is coal.
In coal-fired pizza ovens, which are still used in the Northeast US, and not to confuse them with wood-fired hearth ovens. A coal fire is much hotter than a wood fire and would be great for getting the thermal mass of a big ceramic oven up to temperature, but the firebox and exhaust are completely isolated from the cooking chamber.
Humphrey's DownEast Beast W/BBQ Guru