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Vacuum Pyrolysis.

The Université de Sherbrooke during 1981-1985, and the Université Laval, both in Canada, developed the so-called vacuum pyrolysis . The process merely is a combination of slow (solids) and fast (gas) pyrolysis conditions. Course solids are heated relatively slowly to temperatures higher than that of slow pyrolysis, while the gas is removed from the hot temperature zone relatively quickly by applying a reduced pressure of 2 to 20 kPa in the process. An attempt to commercialize it was carried out by Pyrovac International by the end of the nineties. In the concept, biomass material was conveyed over horizontal grates, which were heated indirectly by a mixture of molten salts composed of potassium nitrate, sodium nitrite and sodium nitrate. The salt itself was heated by a gas burner fed with the non-condensable gases produced by the pyrolysis process. Limitations in heat transfer required the bed of particles to be agitated, but obviously internal heat transfer limitations cannot be avoided. A 3.5 t/hr demonstration plant for bark pyrolysis was erected in Jonquiere Quebec, Canada, and taken into operation in 1998. Pyrovac’s operation were ended in June 2002, where after the process equipment and buildings were purchased by a third party from the bankruptcy in 2003. A group of investors led by US-based NewEarth Renewable Energy planned to restart the plant in 2010 for industrial production of torrefied wood, of wood charcoal and of bio-oils, but no further news was provided later on.