How is biochar production as a CDR method, which is good, different from crop residue burning, which is bad? Does it have something to do with oxygen availability?
Deliberate biochar production differs from open burning in two important ways.
- The first is oxygen availability as you said. Limiting the amount of oxygen means far more of the biomass ends up as solid carbon rather than CO2.
- The second is particulate pollution. Burning biomass always produces particulates, but open burning is worse than burning in a controlled manner (in an incinerator, power station, or even wood-burning stove). This leads to severe effects on air quality, up to and including smog for huge distances downwind of the source. Compared to incinerating, biochar production causes still less particulate pollution if done properly. Field methods (similar to the charcoal-burning of old) may still release a lot.
Let's compare biochar production from wood with burning wood:
- 1000 tonnes of biochar sequester 3000 tonnes of CO2 and the production of said biochar creates 5000 MWh of energy as heat, requiring 10625 solid cubic meters of wood
- 10625 solid cubic meters of wood would produce 21356 MWh of energy as heat if directly burned, but if wood is burned, it retains approximately zero of the carbon dioxide, but the carbon dioxide can be captured in gaseous form and stored if you happen to have a depleted gas field within pipeline reach
So it's a matter of whether you want more energy or carbon capture in solid material. In some cases, it may be you can capture the carbon dioxide from wood burning, transport it via pipelines and store it in a depleted gas field. Also a benefit of energy is that there are still areas that are cold in winter and heat is being produced by combusting coal. Wood burning can offset coal burning.
However, capture, transport and storage of gaseous carbon dioxide is expensive, so in most cases, if wood is burned, the carbon dioxide is released to the atmosphere. Also the cases where wood burning offsets coal burning are getting rarer and rarer every year, and in 5 years we most likely are in the situation that no area in developed nations is heated by coal, but rather heat is being produced using heat pumps utilizing renewable or nuclear electricity (and large-scale heat storage is used if the electricity is intermittent renewable to provide heat during sun-free and wind-free hours). Therefore, I would argue that today the most useful choice is to capture as much carbon dioxide as solid material as we can. Thus, this would favor biochar production. You still get some energy as heat from it, but much less than if wood was directly burned.