The article they link about pyrolysis is worth a read too. The main source of CO2 emissions from cement production is cooking down limestone into lime IIRC. I was curious how much energy is used to turn the biomass into the end product and what waste is generated. It's a bit too detailed for me to understand, but the process ends up with 15-25% biochar (the stuff they're promoting in this article), some potentially useful byproducts, and some regular combustion pollution.