Biochar as carbon sequestration?

I am VERY ignorant of this topic (in fact I might not even have placed my question in the correct forum! Mods, feel free to move it) but I am trying to understand more. Today I saw a LinkedIn post by Chris Magwood, who wrote the excellent book "Making Better Buildings". In the post he was talking about a new commercially available biochar that he was excited about because of its carbon sequestration properties when used as an element in concrete.

As I understand it, carbon sequestration means using carbon for something productive rather than releasing it into the atmosphere as a byproduct of energy production. Biochar is appealing to him because the carbon in it is retained as a solid that can be used in building materials.

But...isn't biochar made by creating a lot of heat? Doesn't that heat tend to release a lot of carbon?

Presumably I am misunderstanding something and my ignorance is showing...but I'm here to learn.

Good question, Ned, and you're not the only one who has asked it. Woody biomass, which is the most common feedstock for biochar production, is about half carbon by dry weight. Burning biomass, or complete combustion, converts all that carbon into CO2 (possibly some into CO and CH4, but that would be incomplete so we're ignoring it). Pyrolysis is similar to burning but limits the oxygen so that half the carbon remains instead of being turned to carbon dioxide.

Because pyrolysis can be done in what is essentially a fire, it leads some people to think that it uses energy, or that it causes emissions. It's more of an "interrupted" or incomplete form of combustion called gasification, and there is enough energy in the feedstock to drive the process with heat left over. What's important is that the temperature is high enough that the carbon left behind is in a form that won't further oxidise or be consumed by microbial activity. The emissions are CO2 that was taken out of the atmosphere by the plants or trees while they were alive and growing, so it's just part of the cycle.

So, we send half of the carbon on its continuing journey and lock the other half up in a form that lasts hundreds to thousands of years, which is the sequestration value. The added benefits of water retention, aeration, fertility, and homes for beneficial soil life are the icing on the cake and these can add carbon-storing properties to the environment that biochar is added to. Plus, if we use the spare heat of pyrolysis to do something that was previously done by fossil fuels, we're displacing an energy input and reducing those emissions as well.