Hey, I can answer this one. I did my M.Sc on amending soils with biochar.

I'm not sure what Stockholm is doing, though.

To get good recovery of biochar, you need a pretty controlled environment. It's also energy intensive to make; you need a heat source that's not going to produce CO2, ideally, and cut into the benefit that you'd get by sequestering carbon in the first place. Like if it takes 1 ton of waste wood fuel to make 1 ton of biochar, it's pretty much a wash. You could use green energy I suppose, like it you had a hydro grid set up to fire whatever furnace you're cooking the stuff in, but that's not always available or scalable (in the sense you can do it everywhere).

You CAN make biochar crudely, and we've been doing so for centuries, but the rub is that your recovery quickly drops and decreases in recovery are increases in CO2 produced. I was toying with this idea to manage non-merchantable timber on mines, and using the char as a soil amendment.

Suppose everything DOES work the way we want, though. Good recovery, cheap energy, people onboard. We now have a mountain of char and no place to put it. You could go underground mines). Stockpiling it aboveground would a nightmare and massive fire hazard... Etc. Etc.

I like using char to store carbon and it'll stay for a long time, but you can't just capture it at the source like you can with other forms of carbon capture and storage. Those other methods don't work well either... At least with char you have a solid to manage rather than stuffing gas underground and hoping the earth doesn't fart.

Like a lot of things, char use like you envision is 'we would if we could but we can't so we wont'.