Emissions accounting for biomass energy with CCS

To the Editors —

Sanchez et al.¹ provide a viable technological roadmap for using biomass energy with carbon capture and storage (BECCS) in the western United States¹. However, they oversimplify emissions accounting by assuming a zero or negative carbon emissions factor. Accounting for total lifecycle emissions is perhaps the greatest challenge in deploying biomass (in solid, gaseous, or liquid form) to reduce carbon emissions^2,3.

When utilized to generate electricity, emissions sinks and sources for biomass occur in two different sectors. As plants grow, they take up CO₂ and store it. When combusted, the stored CO₂ is released and contributes to emissions. Accordingly, counting the emissions factor for biomass electricity generation as zero, or negative in the case of BECCS, causes double-counting of emissions on a cross-sectoral basis⁴. These accounting challenges persist when developing national or international carbon control regimes⁵. Further, in a carbon-constrained world, both biomass producers and electricity generators will have competing claims concerning monetization of their low-carbon attributes.

Sectoral accounting is further complicated by the timing of emissions in the biomass electricity lifecycle. Power generation releases CO₂ that was previously sequestered — and an implicit assumption made by Sanchez et. al. is that harvested biomass provides room for re-growth and sequestering of released emissions. This assumption, however, raises two problems.

First, if regrowth does not occur, net emissions will increase, even if CCS confines the majority of emissions. Measurement and verification are needed to ensure biomass is regrown and net negative emissions actually occur.

Second, the rate of CO₂ uptake from biomass fuel sources varies considerably. Trees — the dominant source of utility-scale biomass fuel today — grow over decades with different CO₂ uptake rates at different ages and across species. Ricke and Caldeira recently found that the climate impact of CO₂ emissions could occur in as few as 10 years⁶. The CO₂ released by uncontrolled biomass burning can thus contribute to short-term radiative forcing before CO₂ is sequestered by regrowth.

The concerns we raise suggest that additional, nuanced, and refined research is needed to improve our understanding of carbon flows in BECCS, develop efficacious legal regimes for CO₂ emissions reduction ownership, and design successful monitoring regimes for biomass regrowth. Only then can the future role of bioenergy and BECCS be more fully contextualized and appreciated.