The invention relates to improvements in dryers, and more particularly to a rotary dryer of the type which may be used for drying bulk material.
More particularly, the invention relates to dryers of the type which may be used in papermaking plants for drying hog fuel or bagasse with waste heat and direct fired dryers used for difficult to dry materials such as pulp mill sludge. The interest in drying systems for drying materials to be used as fuel results from the fact that dry fuel increases boiler efficiency and fuel BTU value. In the case of sludge, drying is beneficial in preparing the wet material as fuel or to substantially reduce its bulk, thereby reducing or eliminating costly landfill requirements. If drying of fuel is done in the boiler, much of the energy of combustion which could be used to generate steam is consumed by the drying process of incoming fuel. Because boilers are inefficient dryers, the effect on performance is dramatic in that efficiency declines substantially where the fuel must be dried within the boiler. In the case of sludge, drying in the boiler is extremely disruptive to the boiler operation.
In the boiler, wet fuels require large amounts of excess air to sustain combustion. The excess air, combined with the water vapor generated during drying reduces boiler efficiency and furnace temperature. The colder furnace produces less steam and more particulate emissions as unburned fuel passes up the stack.
Drying fuel results in more BTU's per pound of fuel as well as an increased boiler efficiency. Drying also allows the effective control of fuel moisture content as the moisture varies by season and with the composition of the raw material. The consistency of the fuel also contributes to more efficient and predictable boiler performance.
Fuel dryers have been used heretofore which are heated by waste heat and by direct firing, but the efficient and effective transfer of heat from the hot gases to the wet material varies greatly depending on the material properties and on the dryer loading.
It is accordingly an object of the present invention to provide an improved dryer such as may be used for drying hog fuel, sludge or bagasse with waste heat and by furnace direct firing wherein a unique and improved transfer of heat from the hot gases to the wet material is accomplished.
A further object of the invention is to provide a rotary dryer wherein the contents are tumbled or moved in a unique manner so as to increase the efficiency, capacity and effectiveness of heat transfer to the material even at low feed rates.
A further object of the invention is to provide an improved flight system within a rotary dryer wherein panels are provided arranged so that improved heat transfer results through controlling gas swirl and mixing, and wherein flights are provided having metal surfaces which effectively increase heat transfer through conduction.
Another object of the present invention is to provide an improved rotary drum center flight design which has a significant return effect on material passing through the dryer to significantly counteract the gas flow forces increasing retention time and drying efficiency on difficult to dry materials such as sludge.