A New Direct Air Capture System Operating on Low Grade Heat Generated from Geothermal Plants

While technologies to control the emissions from stationary sources are available (these are feasible but expensive). It is not practical to remove CO2 when it is produced by small generators and transportation systems (e.g., cars, trucks, and airplanes). This, Direct Air Capture (DAC) the only option to control the CO2 concentration in the atmosphere. Because of its low concentration, recovering CO2 from ambient air requires much more energy per ton of CO2 removed than the systems used to remove it from flue gas. Low concentrations of CO2 in air also means that very large air handling systems are needed to bring sufficient amounts of CO2 in contact with the capture system. This generates a very high parasitic power load; in a DAC system the energy needed to circulate the air far exceeds the energy required to separate it from the air. Integrating DAC with a process that generates low grade heat (or waste heat) and/or circulates large volumes of air may reduce the cost of capture. Geothermal power plants reject 87-90% of their source heat from the hot brine as low-grade waste heat to the environment. TDA Research Inc. (TDA) is developing a new sorbent-based DAC system, integrated with a geothermal power plant, to utilize the waste heat and make use of the large amount of air that is driven through the power cycle’s condensers to drive the DAC process. TDA’s sorbent consists of a high surface area support grafted with amine functional groups that adsorb and remove CO2 from the air. A temperature swing adsorption (TSA) cycle is employed to utilize the waste heat from the geothermal plant to regenerate the sorbent. The Phase I objective was to develop and demonstrate that a DAC system can be integrated with a geothermal plant. Under representative DAC conditions, we showed sorbent stability over 1,000+ thermal cycles, with high CO2 uptake (> 2%-w), high capture efficiency > 80%, and the rapid cycling needed to make an economical bed size. The sorbent is not only stable under humid conditions, but improves in the presence of water. The proposed reactor design achieved adequate uniform heating within a reasonable cycle time as shown in both our modeling and experiments. The Phase I results met or exceeded the key requirements for both the DAC sorbent and contactor. In the Phase II, we will scale-up and economize production of the sorbent. We will evaluate the impact of daily and seasonal fluctuations in operating conditions with parametric testing, and continue the sorbent stability testing through 2,000 cycles. We will continue to improve the integrated contactor design via CFD and reduce the pressure drop through the sorbent bed. In addition, we will design and fabricate a prototype system for a proof-of-concept durability evaluation (500 h minimum) and carry out a high-fidelity techno-economic analysis (TEA) and life cycle analysis (LCA). The new technology will reduce the cost of CO2 capture from dilute sources (including air) at low cost. Hence, it will find immediate use in various carbon capture applications.