I think there should be a required subject in school science classes called "Principles of Coal Combustion". In this course, the instructor would repeat over and over "The combustion air must all enter the chamber from beneath, and flow up and through the grates on which a solid bed of fuel sits". While drawing on the chalkboard with directional arrows showing the air path.
I have tried to explain this to woodburners over and over, and they give me vague nods and blank uh....'s. They later tell me "the coal wouldn't burn". GEEZ LISTEN TO ME, I repeat "The combustion air must..."