Charcoal: Economics of carbon sequestration
from Carbon Commentary
Besides all the benefit of improvement to soil, the carbon sequestration itself already worth the effort:
A tonne of good quality biochar has an energy value of about 28 gigajoules (GJ), slightly less than the best quality coal. (Pure black carbon is about 32 GJ/tonne.) Standard coal costs about £1.50 per GJ. If a power station operator is prepared to pay the coal-equivalent price, biochar is worth about £42 per tonne in the UK.
Burning a tonne of biochar will produce about three and a half tonnes of CO2. (Pure carbon would generate 3.667 tonnes.) The current price of CO2 in the European Emissions Trading Scheme (ETS) is about £16, meaning that sequestering 3.5 tonnes ought to be worth approximately £56. Since £56 is greater than £42, the economic logic suggests that we should hold the carbon in the soil rather than burning it. This is before considering the secondary climate change benefits of reduced fertiliser use and lowered nitrous oxide emissions. [my emphasis]