I think you would fire the furnace with a carbon neutral source, like wood. I think you would fire it with the very vegetation that you are carbonizing. I think they just restrict the amount of air they let in to the chamber.
Also I believe the proposed solution is to apply the char to soil to condition it.
I did a quick google of "pyrolysis carbon sequestration" and then "biochar"
I would say that most of the carbon sequestration ideas I have heard of sound much more crazy than pyrolysis of vegetation.
Personally, I would never support carbon sequestration anyway. I would say I am against purposeful human intervention of climate. If we have messed things up so far by accident or out of ignorance, imagine how bad things will be when we mess with climate for politics.