Is there a simple assay to determine the elemental carbon content of biochar? A little bit of background. I make charcoal for use as biochar: putting carbon into the ground where it retains water and soluble nutrients. It also provides a good habitat for the friendly soil bacteria and fungi, along with low-tech carbon capture and storage for the masses.

I would like to know how much carbon is going back into the ground, and be able to say with confidence what difference I'm making to my carbon footprint, but the two – at least – unknown factors are the ash content of the char, and the water content from quenching and dust abatement sprays. I can say with confidence that a kilo of dry char will retain 3 kilos of water, which makes it difficult to judge by the look and feel.

I'm looking for a simple assay which will tell me with reasonable confidence how much elemental carbon a sample contains.

FWIW, my understanding of chemistry is rusty: I've got an A-level (the UK university entrance qualification from 1976) but my degree is in Physics, and my background is in engineering. Thanks for any help.
Assuming your charcoal is mostly:


water (evaporates)
carbon (burns)
ashes (does neither)


I would :


weigh a sample of your charcoal to get the mass $\ce m_0$
dry it in an oven at low-ish temperature (50°-80° or more, whatever doesn't burn your charcoal) for a few hours.
weigh it again to get $\ce m_1$
burn the sample in air as thoroughly as possible
weigh what's left to get $\ce m_2$


Now you can calculate :


mass of water (+ other volatiles) = $\ce m_0$ - $\ce m_1$
mass of carbon (+ other non-volatile burnables) = $\ce m_1$ - $\ce m_2$
mass of ashes = $\ce m_2$