[Terrapreta] Terrapreta Digest, Vol 11, Issue 70

[Nikolaus Foidl]

Dear Gerhard!

I only heat the Wood up to 280 degrees Celsius .In my atmospheric kiln this
takes some 3 hours. ( Gas outlet temperature from the gas tube). I still
don't know how many time it will take on my new high pressure kiln. The
torrefied wood is very brittle as most of the hemi cellulose is already gone
and the cellulose is partially charred. Try with a range of temperature lets
say from 240 to 320 degrees. The arrow hardening effect is because you get
in a very short time a complete elimination of cellulose and hemi cellulose
in the outer layer and then the lignin left at about 300 to 350 degrees gets
over the melting point closing all pores and insulating the wood toward the
inside. After cooling down the lignin will harden out and render very
abrasive resistant.( same effect you get in the piston -pellet presses when
you pelletize sawdust).If you are below this melting point then the lignin
just gets stripped of the cellulose and hemi cellulose and the reminding
lignin structure is brittle.

Best regards Nikolaus