[Terrapreta] White Coal

[andrew]

On Thursday 08 May 2008 03:12, Michael Bailes wrote:

> It's called 'white coal' - it may be a byproduct combined with
> agricultural waste or a cleaner coal product.   This may be an
> accurate definition of it, "*White coal* is a form of fuel
> produced by drying chopped wood over a fire. It differs from
> charcoal which is carbonised wood. White coal was used in England
> to smelt lead ore from the mid-sixteenth to the late seventeenth
> centuries. It produces more heat than green wood but less than
> charcoal and thus prevents the lead evaporating. 

It sounds a bit like torrefied wood, that is wood that has been 
heated to super dry and incipient pyrolysis but little offgas has 
been removed other than VOCs and a bit of acetic acid, so it retains 
nearly all the calorific value of the wood but in less mass. Fire 
brands (brown ends) are largely torrefied wood.

Green wood (with all its moisture present) burns in the range 800C to 
1200C, dry wood reaches 1600C and charcoal over 2000C. With lead 
boiling at 1740C you can imagine that using dry wood was a simple 
means of controlling peak temperature.

AJH