| Title | Carbon negative geothermal: Theoretical case study for biogenic CO2 removal at Ngawha power station |
|---|---|
| Authors | K. Titus, D. Dempsey, R. Peer, F. Hanik |
| Year | 2023 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | geothermal, biomass, CO2, retrofit, economics |
| Abstract | Geothermal energy is a mature and base load source of low carbon electricity in New Zealand, and the only one that can facilitate onsite carbon dioxide removal (CDR). Here, we investigate the potential for bioenergy hybridisation and biogenic CDR using existing reinjection wells at Ngawha geothermal field. The proposed designs would produce increased electricity generation and negative CO2 emissions. Bioenergy hybridisation is achieved with biomass combustion that directly increases the enthalpy of production fluid before or after a separator. Mass-energy balance determines the optimal biomass burn rate to sustain turbine delivery enthalpy in the face of resource decline. We calculate the potential CO2 emissions that can be sequestered via dissolution in the reinjection line. Finally, we estimate highlevel economic indicators for this retrofit. Our findings suggest that it is possible to increase plant capacity by 1 MWe through combustion of 24 kt/year of forestry residues. The cost of new electricity generation could be competitive with conventional geothermal projects at CDR investment prices as low as $75/tCO2. Conversely, the cost to remove biogenic CO2 through this process could range between $77-154/tCO2 depending on the configuration, which is more cost effective than most direct and indirect atmospheric carbon removal schemes (Fasihi et al., 2019). Monetized on international markets, CDR revenues could reach USD $3.9 million per year, which adequately covers anticipated biomass fuel costs of USD $2.1 million per year. Applications of CDR technologies in New Zealand can decarbonise hard to abate emissions from agriculture, steel and aviation. CDR via geothermal reinjection wells makes use of onsite infrastructure and mitigates CO2 buoyancy risks, allowing for cost-effective and secure storage. In addition, hybridisation with bioenergy could alleviate or delay the need to drill more wells to maintain plant capacity. This can be important if fluid production is nearing a geothermal field’s maximum allowable consent. |