We recently had a go at making Biochar at the nursery.
Here’s what Biochar International have to say about Biochar

International Biochar Initiative

Sustainable biochar is a powerfully simple tool to fight global warming. This 2,000 year-old practice converts agricultural waste into a soil enhancer that can hold carbon, boost food security, and discourage deforestation. Sustainable biochar is one of the few technologies that is relatively inexpensive, widely applicable, and quickly scalable.

Biochar is a solid material obtained from the carbonisation of biomass. Biochar may be added to soils with the intention to improve soil functions and to reduce emissions from biomass that would otherwise naturally degrade to greenhouse gases. Biochar also has appreciable carbon sequestration value. These properties are measurable and verifiable in a characterisation scheme, or in a carbon emission offset protocol.

Our experiment worked well. We made the charcoal using a traditional earth clamp.
About one third of the wood didn’t burn but we ended up with over four buckets of charcoal.
The next step is to crush the charcoal then ‘charge’ it by leaving it in a compost tea or pile.