That will hold a 100 pounds of coal and will burn either Bituminous or Anthracite. The little holes in the top of the stove are for secondary air. Some of these models have the secondary air openings without any type of damper control. In that case it is designed to admit a pre determined amount of air to burn off hydrocarbons and to provide the tiny bit of secondary air needed later in the combustion cycle.
The main goal is to have the ash pit door as tight as possible and to install a pipe damper about 2 feet beyond the exhaust collar in the stove pipe.
This will provide plenty of cheap and reliable heat for your green house.