Carbon Dioxide Capture from Atmospheric Air Using Sodium Hydroxide Spray

In contrast to conventional carbon capture systems for power plants and other large point sources, the system described in this paper captures CO₂ directly from ambient air. This has the advantages that emissions from diffuse sources and past emissions may be captured. The objective of this research is to determine the feasibility of a NaOH spray-based contactor for use in an air capture system by estimating the cost and energy requirements per unit CO₂ captured. A prototype system is constructed and tested to measure CO₂ absorption, energy use, and evaporative water loss and compared with theoretical predictions. A numerical model of drop collision and coalescence is used to estimate operating parameters for a full scale system, and the cost of operating the system per unit CO₂ captured is estimated. The analysis indicates that CO₂ capture from air for climate change mitigation is technically feasible using off-the-shelf technology. Drop coalescence significantly decreases the CO₂ absorption efficiency; however, fan and pump energy requirements are manageable. Water loss is significant (20 mol H₂O/mol CO₂ at 15 °C and 65% RH) but can be lowered by appropriately designing and operating the system. The cost of CO₂ capture using NaOH spray (excluding solution recovery and CO₂ sequestration, which may be comparable) in the full-scale system is 96 $/ton-CO₂ in the base case, and ranges from 53 to 127 $/ton-CO₂ under alternate operating parameters and assumptions regarding capital costs and mass transfer rate. The low end of the cost range is reached by a spray with 50 µm mean drop diameter, which is achievable with commercially available spray nozzles.