Climate Impact of LCNs at Scale
Here is the raw chemical composition of biomass to remind ourselves what we are working with.
The important point here is that biomass on average is about 50% carbon by mass. This makes things easy to track for carbon purposes: for any Xkg input of biomass, half of that mass is carbon. That is what we are working with.
Of that mass, about half is fixed carbon in the structural lattice of the biomass, and about half is mixed across the large cocktail of volatile molecular forms. The fixed carbon is what, to an approximation, is available for charcoal making, thus why we usually say the top- end practical yield rate for biochar is about 25% of the original biomass (note importantly that this is 50% of the original carbon amount).
Any yields above this 25% of the original carbon are working via the recondensation of volatiles/tars and usually produce dirty char. Yes, you can get to a higher carbon yield, but it is unlikely that you would want to work with the result— especially in soils.
Which brings us to the core answer of carbon flows through biomass thermal conversion with biochar co-product, and thus what is available for carbon sequestration in soil . . .
The chart below shows the mass balance/flow of carbon in biomass thermal conversion–from raw biomass through gaseous and solid carbon outputs–across a range of biochar yield rates. The gaseous carbon is what ends up as CO2 in the final accounting. We may get to it through one-step combustion or a multistep gasification with the following engine, but the raw carbon output in gas form is the same.
The importance of Biochar for Carbon Drawdown
Note again that we usually discuss biochar yield in relation to the original biomass basis, not the original carbon basis. Thus when we say our biochar yield rate is 5%, that is 10% of the original input carbon. If we are running at the top practical yield rate of 25%, that is 50% of the original carbon. I in biochar, we are working with much more of the actual carbon in biomass than we often realize.
So the simple literal answer to “how carbon negative is biomass thermal conversion with biochar?” is this carbon basis yield rate to biochar. This is shown above in both % carbon and absolute mass carbon yield to biochar across the common operating range.
Currently the PP30 v2 is a 10% yield rate from biomass carbon to biochar (or 5% on biomass basis). The future swirl hearth will move this up to around 30% carbon yield. And, and our plans for a dedicated biochar- making chartainer will do the standard 50% yield rate of input carbon (or 25% biochar output vs biomass input).