Rolling-Bed Design Allows Low Temperature Drying
For example, in a dry state, organic waste products are usable as alternative fuel or can be transformed to charcoal, but they normally have high initial moisture content and need long residence times during drying. The resulting products often are non-uniform in shape and size, contaminated, sensitive to heat, prone to ignition or emit volatiles.
For drying green waste, the use of either rotary drum dryers, belt dryers or fluid bed dryers has been state of the art. But those types of dryers may have the disadvantages of insufficient product-residence times, partial overheating of the solid, inability to use low temperature secondary heat or poor solid mixing.
To avoid such problems, Almo Process Technology Inc., Middletown, Ohio, has developed the rolling bed dryer.
“The use of low temperature waste heat for drying becomes possible, says Tom Schroeder, Almo president. Waste heat is abundant, especially at many recycling companies and municipalities, he says.
In the rolling bed dryer, the non-uniform bulk material is steadily and homogeneously mixed, staying in the dryer for long retention times, despite the equipment's compact design, Schroeder says. "Even irregular shredded organic waste or unwieldy bulk like branches and tree cuttings, bark and leaves or unevenly cut wood chips is conveyed by the dryer in a reliable way,” he says.
Almo installed the first dryers at Topell Energy in The Netherlands, where two of the units convert wood waste into charcoal by a pyrolysis process called torrefaction. The charcoal is pelletized to be stored, transported, and then used at power stations to make electricity.