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Decomposition of carbonates in capture of carbon dioxide from ambient air Campbell, James Stephen

Abstract

Direct Air Capture (DAC) is a technology for absorbing and concentrating CO₂ from air for geological sequestration or utilization. Capture is possible using alkali hydroxide solutions, forming alkali carbonates, with regeneration using lime or hematite. The CO₂-release step in each regeneration process requires high temperature, leading to high cost. A better understanding of the kinetics could reduce temperatures and facilitate integration with sustainable energy. In lime regeneration of KOH(aq), CaCO₃(s) decomposes forming CaO(s) and CO₂(g) (+178 kJ.mol⁻¹). In this thesis it was found that all CO₂ was released by 780°C during thermogravimetric analysis (N₂(g) flow rate of 60 ml.min⁻¹, heating rate of 20 °C.min⁻¹, particle size <5 µm and sample mass ~3 mg). A novel Accumulation Model was derived to describe the kinetics. Results indicated that the temperature of lime regeneration is tightly constrained by accumulation of reaction-generated CO₂(g) in the particle pores and sample interstices. In hematite regeneration of NaOH(aq), Fe₂O₃(s) reacts with Na₂CO₃(s), forming NaFeO₂(s) and CO₂(g) (+164 kJ.mol⁻¹). Ball-milling reduced particle size and mixed reactants intimately such that all CO₂(g) was released by 760°C (1:1 mole ratio) or 680°C (16:1 mole ratio). 90 % of the sodium was extracted as NaOH(aq) by hydrolysis of NaFeO₂(s). A Contact Point Model was used to describe the kinetics in terms of temperature, conversion, mole ratio, particle size and sample compactness. Results indicated that, with correct pre-treatment of the reactants, hematite regeneration could operate at lower temperature than lime regeneration. Even greater temperature reductions might be achieved using alternative reagents. A multi-criteria decision analysis was used to create a shortlist of seven oxides based on their acidity, solubility, toxicity and price. Each of the seven oxides was ball-milled with Na₂CO₃(s) and heated. Bixbyite (Mn₂O₃) showed most promise, reacting with Na₂CO₃(s) (1:1 mole ratio) forming NaMnO₂(s) and releasing all CO₂(g) by 650°C (+132 kJ.mol⁻¹). 80 % of the sodium was recovered as NaOH(aq) by hydrolysis of NaMnO₂(s). Bixbyite regeneration may therefore allow substantial reduction in the overall cost of Direct Air Capture using NaOH(aq).

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