Adelaide Hills Biochar Initiative

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by Brian Lewis - May 18th, 2010.
Filed under: Articles.

DON’T WASTE THAT FALLEN TREE!

Whenever a tree falls down or is cut down the usual practice is to either burn the remains or convert it to wood chips for use as landscaping mulch. Unfortunately when this happens all the ‘work’ that the tree has done over its lifetime in capturing and storing carbon from the atmosphere is completely undone! Typically about half of the wood is carbon by weight. (Ref. 1).

When burning the tree all the carbon is released back into the atmosphere as carbon dioxide and even though this is a carbon neutral process overall the reality is that because of the time discrepancy between the capturing of the carbon (perhaps over 100 years) and the release of the carbon (say 1 hour) the atmosphere is a lot worse off very quickly.

After converting to wood chips the carbon is also released back into the atmosphere but over a longer period (a few years) during the de-composition and rotting of the wood chips. These slow emissions could be as carbon dioxide or even worse as methane.

Q: So what can be done to prevent this?

A: Carbonise the tree remains and sequester the resulting carbon.

Q: How can this be done?

A: Use a high efficiency carboniser to convert most of the carbon previously locked in the tree into biochar.

Q: How much carbon will be captured? And how much greenhouse gas will be averted?

A: For every tonne of tree about 0.5 tonne will be carbon. The efficiency of a good carboniser is 85-90% so that means you can get about 0.45 tonne of carbon per tonne of tree. For every tonne of carbon captured 3.66 tonnes of carbon dioxide (see note 1) is diverted from the atmosphere (less any carbon dioxide generated in the process). So for every tonne of tree about 1.5 tonnes of greenhouse gas will be averted.

Reference:
1. Neil Sampson. Monitoring and measuring wood carbon. Colorado SWCS Conference on Carbon as a potential commodity, Denver, Dec. 4, 2002.

Note 1. Molecular weight (MW) of carbon dioxide (CO2) is MW of carbon plus 2 x MW of oxygen. MW of carbon = 12; MW of oxygen = 16; so MW of carbon dioxide is 12 + (2 x 16) = 44.
Then as MW of carbon dioxide divided by MW of carbon is 44/12 or 3.66 it follows that the weight of carbon dioxide averted is 3.66 times the weight of carbon that is captured.

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