[Terrapreta] Economics of biochar

[andrew]

On Monday 31 December 2007 13:08, andrew wrote:
> I have collected a sample by
> scraping and 3kg wet has reduced to kg air dry, I'll carbonise
> this and then ash it just to see what the energy balances look
> like with inevitable soil contamination.

Just in case anybody was following this, it's a bit UK centric but 
here goes:

I was looking at "other" woody residues that would normally be left 
to rot on the ground to consider their possible worth as a carbon 
sink.

As I had been working shredding standing invasive species 
(rhododendron) within a 60 year old pine plantation I collected a 
small 3kg sample of the arisings. I estimated that this represented 
30Tonnes/ha of material.

I then dried, charred and ashed the sample to see what the yields 
were, bear in mind I expected fairly heavy soil/stone contamination. 
In fact I only found 2 small stones in the ash.

The figures:
Wet sample    2.971kg
air dry       1.556kg
oven dry      1.220kg
char [1]      0.323kg
ash           0.210kg

[1] there was some loss of dm in the charring as the tlud burn went 
into a combustion phase and the carbonising was completed in a 
biscuit tin retort in the wood stove, I would estimate the max 
temperature to be 600C and the fixed carbon >80%

This shows that the mid winter harvested shredded material and forest 
litter had a wet basis water content of ~60% and the ash free yield 
of char was 11% of total dry matter.

Given the high ash/soil contamination and reduced to a per kg basis 
it looks like there was a total energy of 15.4MJ/kg oven dried matter 
of which 2.97MJ was left in the char. The process gave off 12.5MJ of 
energy which would have been adequate to supply the drying and 
heating to pyrolysis temperature of between 3 and 5 times this 
amount of wet material depending on process efficiency and process 
temperature. Lower temperature would retain more energy in the char, 
increase the gross amount of lower fixed carbon char but reduce 
process energy available.

Now whether it would be economic to do this in order to sequester 
just over 1 tonne of 80% fixed carbon char/ha whilst handling 30 
tonnes of material is another matter.

AJH