(768c) Continuous Vacuum Drying of Wood Chips for Residential/Commercial/Industrial Heating Applications

AIChE Annual Meeting – Minneapolis 2013

Title: Continuous Vacuum Drying of Wood Chips for Residential/Commercial/Industrial Heating Applications      

Authors: Jeffrey A. Marquis, B. Wayne Bequette, John E. Okonski

Abstract: Wood Chips are an abundant, renewable and affordable fuel source throughout the northeast United States. Decreasing the moisture content from approximately 50 wt-% to 20 wt-% nearly doubles the heating value per unit mass of final product¹. Dried woodchips can be burned in high efficiency woodchip boilers ranging in size from 15 kW to 500 kW². This work examines an unconventional continuous screw conveyor vacuum drying system. The decreased boiling point of water at elevated vacuum levels is exploited to expedite the water removal rate.

Existing technologies for wood chip drying such as air drying, kiln drying and rotary drying are slow (taking up to months of processing) expensive, and do not scale well^2,3. The processes examined allows for the fast and efficient removal of excess moisture from wood chips.  A series of bench-scale batch experiments provided the necessary insight into required design parameters for a pilot-scale system: Wall temperature, T_wall, vacuum pressure, P_vac, and residence time, τ. The resulting design parameters were T_wall = 90°C, P_vac = 28 inHg, τ = 15 minutes.

A 100 lb dry mass per hour pilot scale system has been designed and constructed based upon the insight gained from the bench-scale testing. It is expected that this pilot-scale system will allow for fast, efficient, and inexpensive drying of wood chips over competing technologies.

References:

1)      T. M. Maker, Biomass Energy Resource Center, Montpelier VT (2011).

2)      www.evo-world.com, May 2, 2013

3)      G. Lianbai, Drying Technology, 25, 463-469 (2007)

4)      S. Danielsson, A. Rasmuson, Drying Technology, 20, 1427-1444 (2002)