From carbon neutral to carbon negative: a theoretical bioenergy and CO ₂ removal retrofit at Ngāwhā geothermal power station

For countries like New Zealand with high renewable electricity generation (≥80%) but large emissions per capita, traditional decarbonisation methods are limited or costly. However, carbon dioxide removal (CDR) technologies can drive multiple sectors of the economy across the net-zero barrier through negative CO₂ emissions. We argue that the key to scaling up CDR begins with the preponderance of New Zealand's geothermal and biomass resources. New Zealand has a proud and innovative history with geothermal energy, currently producing ∼20% of the country's electricity. Modern advances in geothermal energy have demonstrated that it is peerless amongst renewable energy-sources in the ability to facilitate onsite CDR by repurposing existing wells. We examine a theoretical bioenergy retrofit at the Ngāwhā geothermal plant to increase capacity by 1 MWe, with as much biogenic CO₂ as permissible dissolved in reinjectate. Forestry residues are the feedstock of choice due to their abundance in the Far North. We show that up to 15.9 ktCO₂/year can be removed effectively from the atmosphere. Only 6% of the Far North's forestry residues are required to achieve this. Under 2024s emissions trading scheme (ETS), the annual revenue of CDR ($0.79 million) could exceed that of new electricity ($0.47 million).