Carbon Dioxide Capture from Atmospheric Air Using Sodium Hydroxide Spray

TLDR

This study explores capturing CO₂ directly from ambient air, offering a way to address diffuse emissions and past releases. The authors aim to assess the feasibility of a sodium hydroxide spray contactor for air capture by estimating its cost and energy use per ton of CO₂. They built a prototype to measure CO₂ absorption, energy consumption, and water loss, and used a numerical drop‑collision model to predict full‑scale operating parameters and cost. Results show that NaOH spray capture is technically feasible with off‑the‑shelf components; drop coalescence lowers efficiency but energy remains manageable, water loss can be mitigated, and the full‑scale cost ranges from $53 to $127 per ton, reaching $96 in the base case and $53 with 50 µm droplets.

Abstract

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.