| Title | CARBON NEGATIVE GEOTHERMAL: THEORETICAL COMBINED GEOTHERMAL BECCS INJECTION CYCLE |
|---|---|
| Authors | K.A. Titus, D. Dempsey, R. Peer |
| Year | 2021 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | geothermal, hybrid, biomass, dissolved CO2, BECCS, forestry waste, reinjection, non-condensable gases |
| Abstract | Geothermal energy is a mature and established technology in Aotearoa New Zealand. In 2019, it provided 17% of total electricity generation. However, power production from geothermal resources is often limited by heat transfer and conversion efficiencies, which are especially restrictive for low-temperature resources. For almost a century, there has been interest and research in superheating geothermal fluid with an ancillary fossil fuel boiler to improve efficiencies. Hybrid geothermal-solar and geothermalbiomass plants have been considered as carbon neutral solutions in a similar vein. The latter is of particular relevance in New Zealand due to the colocation of the Taupo Volcanic Zone (TVZ) with a large forestry industry. This work assesses the feasibility of using a biomass boiler coupled with carbon capture and storage in geothermal power generation in New Zealand. The superheating of geothermal fluid with bioenergy has been adopted in other parts of the world, and has been shown to result in an increase in energy output when retrofitting existing power stations. Coupling bioenergy with carbon capture and storage (BECCS) technologies at geothermal plants is a pathway for net carbon negative generation. Since geothermal plants typically require reinjection wells as part of reservoir pressure management, part of the infrastructure for reinjection of CO2 is already present. A simple systems model was constructed to explore end-member energy cycles and determine, per unit mass rate of geothermal fluid, both the biomass fuel requirements for superheating and associated CO2 emissions. The model also quantifies the proportion of CO2 that can be dissolved in the condensate streams for reinjection. As this CO2 originates as atmospheric carbon that is locked in the biomass fuel during growth, the result is a carbon sequestering energy cycle (net carbon negative). We apply the model to a theoretical geothermal doublet producing at 150 to 195 °C with a condensing turbine and optional wellhead separator. We showed that superheating the separated steam with biomass can yield electricity gains of about 50% and full emissions capture. If no wellhead separator is used and total production fluid is superheated, power output is increased two orders of magnitude, with emissions capture exceeding 30%. With only 1 kg/s of geothermal fluid flow, this energy cycle could sequester almost 2 kT of CO2 per annum for each MWe generated. However, understanding the challenges of large-scale dissolved CO2 injection remains a key uncertainty in determining the viability of carbon negative geothermal cycles. |