be hot flue gas that is tempered with air or other material that is at a lower temperature than the combustion gas in yet another related embodiment a purpose built boiler or combustor can be used to heat the working fluid the compacted product usually in flake or pellet form is transferred to a vessel where feed particles can be efficiently contacted with the unsaturated working fluid in a preferred embodiment that works with coal or other heat sensitive applications the drying vessel is an indirect rotary dryer indirect rotary dryers transfer heat into the wet compressed material in two ways first heat is transferred by convection this is accomplished by passing hot working fluid over the wet material second heat is transferred by conduction by contacting the wet compressed material with a hot surface shell of the rotary dryer both sources of heat evaporate water in other applications that work with materials that are not heat sensitive a direct rotary dryer may be used direct dryers use heat supplied by the hot working fluid alone and do not heat the material by conduction the working fluid used in a direct dryer is typically hotter than the working fluid used in an indirect rotary dryer in another embodiment unsaturated air can be directed across a stockpile of compacted material fans or natural convection can used to accelerate the air to increase the rate of drying in another embodiment material can be conveyed on.