An air CO2 capture system based on the passive carbonation of large Ca(OH)2 structures

Abstract

Direct Air Capture (DAC) requires contacting a vast flow of air with a functional surface, which must be accommodated in a large and costly CO₂ capture device (i.e. at least 0.2–0.4 m³ of the reactor volume per t_CO2 per year). We propose in this paper a low-cost alternative that involves contactor volumes that are one or two orders larger, but require only the passive CO₂ carbonation of purpose-built porous structures of Ca(OH)₂. Such low-cost materials can be manufactured from natural limestone and/or from recycled carbonated structures by using oxy-calcination technologies, and then simply stacked in such a way as to leave gaps for air to pass through. On the basis of an analysis of the rate controlling factors of the carbonation reaction, we employed as the structural element sintered Ca(OH)₂ plates with an area of 2 × 2 m² and 3 cm thick with a porosity of 0.5, which can be fully carbonated in about 6 months. The cost of CO₂ captured from air is estimated to be between 140 and 340 $ per t_CO2 depending on assumed cost values for fuel, land use, structural materials manufacture & transport, and process and project contingencies.