The feedstock biomass could vary from sawdust to agricultural residue, but the goal is to use biomass with no other use or value such as corn straw and leaves. The first step of the process is to pre-prepare the biomass and feed it to the fast pyrolyser consistently - which can be quite technologically challenging.
Bio-oil is produced by heating the biomass up to about 500°C in a few seconds without any oxygen. This process is called fast pyrolysis and results in liquid oil with high carbon content.
An advantage of the fast pyrolysis - it’s somewhat easier to make bio-oil than full gasify to biochar - is that the equipment needed is less complicated, smaller and can be made in modular, transportable units, reducing the need to transport bulkier biomass to it. This gives this technology and edge in terms of scalability.
Bio-oil is transported and injected into injection wells (disused oil wells), where the bio-oil sinks and solidifies in place for permanent storage. The bio-oil has much lower energy density than fossil oil as it’s heavily oxygenated, so the chance of it ever being pumped up again and used as fuel is really small.
Every carbon credit represents 1 ton of net carbon removal. The actual carbon dioxide equivalents in the bio-oil sequestered are higher, typically 1.43 tonnes, to also compensate for things like the production of bio-oil (0.17 tonnes), transport of biomass feedstock and bio-oil to the injection site (0.20 tonnes) and the injection itself (0.06 tonnes).
There are a few environmental impacts that need to be considered, which are particulate and NOx emissions from pyrolysis and increased road traffic and the potential for seismic activity. Seismic activity could be increased by lubricating the interfaces between rock layers in the injection wells; not at current volumes but if this solution was to scale up massively. Geological sequestration capacity could become a constraint at larger scales. This is subject to ongoing research, and should not be seen as a dealbreaker in these early stages.
