Bituminous coal does not create creosote, but as others have said when
burned without enough secondary air forms a coal tar substance which
at room temperature is a solid but when heated is sticky. In my boiler
it forms in the upper area and will burn off with a hot fire,in the summer
usually the fire is not hot enough to burn it up. This coating (I Believe)
actually protects the steel of the boiler from the air that causes the
rusting of the steel.
This coal tar is actually a condensate of the bituminous coal smoke
when it contacts a cooler surface in the heating unit. In my boiler the
smoke is pulled down through the hottest part of the fire where it burns
up and produces heat. A great deal of heat is contained in the smoke
which are the volatile gases driven from the coal, and are wasted as
objectionable smoke if not consumed in the fire,thereby sending some
of the heat value up the flue, wasted.
These gases will not burn in the flue and have to be burned,with extra air,
in the hottest part of the fire.They will condense on the flue if exhausted
before complete combustion has taken place,but are hard to ignite in the
flue (low heat and oxygen) when only coal is burned.