Biomass carbon removal & storage

Anaerobic digester exploration

Anaerobic digesters (ADs) break down biogenic waste such as manure, food scraps, and crop residues to produce biogas and digestate. Biogas, a mixture of methane and CO₂, can be upgraded to renewable natural gas (RNG). RNG is considered by many to be beneficial for the climate because it transforms waste into a fuel that can replace fossil natural gas. However, the exact climate impact depends on several factors, such as what would have happened to the methane in the absence of the project and the rate of methane leakage. If the methane would have been released into the atmosphere, then its use is beneficial to the climate. Conversely, if the methane would not have existed (i.e., it’s intentionally-produced) or could have been flared, then even a small amount of methane leakage reduces the climate benefits and could even result in net climate harm since methane has a global warming potential around ~28 times greater than CO₂ over 100 years.

One approach to CDR is to capture and store CO₂ from biogas, which is currently vented as part of the RNG upgrading process. Our concern with this approach is two factors resulting in low CDR efficiency: (1) emissions from digestate decomposition and (2) leaks of intentionally-produced methane. For the former, approximately 50% of the carbon in biomass entering the AD is converted to biogas; the other half ends up in liquid and solid digestate. The decomposition of that digestate, even if aerobically to CO₂ rather than anaerobically to methane, reduces the net negativity of this pathway. For the second point, many ADs co-digest crop residues that would have decayed aerobically to CO₂. AD co-digestion releases some amount of methane to the atmosphere that wouldn’t have existed otherwise. If methane leaks are high enough, then the CO₂e of those emissions could be comparable to the amount of CO₂ captured and stored from the biogas. This is why biogas upgrading + CCS is not in scope, as noted above.

To address these concerns, we are looking for new AD approaches that (1) minimize emissions from digestate and (2) produce products other than methane, thus yielding higher CDR efficiency by reducing the potential for methane leaks. This could be, for example, ADs where electricity, propane, or volatile fatty acids are produced onsite and as much of the carbon in the biogas and digestate is captured as possible.

Modular BECCS

Because waste biomass tends to be distributed, we’re looking to continue supporting modular approaches to BECCS that can take advantage of small and diverse sources of biomass while remaining both economical and efficient. Potential projects might include systems that can be transported from place to place for on-site processing of biomass, or low-capex systems that are fast and cheap to stand-up at new sites. We’re particularly interested in technologies that can handle agricultural residues, which might be difficult to use economically with current BECCS technologies because of low density and high transportation costs. The metrics we are looking for include very low cost (to allow for more budget for biomass transport), high CDR efficiency, and a co-benefit beyond CDR.