Understanding the time-varying CO2 emissions of carbon-negative emissions conversions

The climate crisis is likely to require carbon dioxide removal in addition to emissions mitigation. One of the most-scaled removal techniques at the present is biochar, which is produced by pyrolyzing biomass to stabilize the remaining carbon. Most commonly, biochar is proposed as a positive solution that can use waste biomass as its input. The conversion efficiency for this process ranges from roughly 20-50% of the carbon in the initial biomass. The remaining carbon is lost in the form of CO2, bio-oil, and syngas. The most common practice is to combust these products, converting all the carbon to CO2. The historic precedent is to consider the CO2 produced from the combustion of biomass to be neutral or not counted. As a result, the production of biochar can lose most of its carbon during production and still be considered an immediate carbon removal. This thesis compares the emissions fate of waste biomass used for biochar production with a counterfactual of leftover waste residues left on site. This analysis suggests that the current crediting methodology may be over-crediting by 78%. These modeled results are highly dependent on a few specific input parameters that have varying levels of confidence. At a minimum, this thesis suggests that the biochar industry should carry out more work to justify its fundamental assumption of biogenic emission neutrality.