Burn rate per what? Coal surface area? Define the question better.
I feel like this is going to be one of those classic "it depends" engineering questions. Is it on an open flat plate, on a grate, enclosed boiler, forced convection draft, natural draft, airflow rate, energy content of the coal, the size of pieces the coal is broken in to...
Many many variables that could cause 100% or more changes in total "burn rate" that can't just be tableized, even for a general best guess per surface area.
Something may exist in an older HVAC/power plant engineering handbook somewhere in the world though.