Leveraging Industrial Side-Streams and Carbonate Chemistry for Sustainable CO₂ Capture

Doctoral thesis, 2026

As environmental research seeks to accelerate the transition away from fossil-based industries, it is essential to adopt a diverse range of decarbonization strategies. One promising approach for CO₂ capture is the utilization of industrial side streams in inorganic carbonation. Inorganic carbonation is a versatile process involving the reaction of CO₂ with alkali or alkaline earth metal hydroxides, toward the formation of metal carbonates. Thus, alkaline wastes and side streams, found in abundance at industrial sites, could have potential as low-cost CO2 absorbents. In Sweden, the two major industrial sectors –pulp and paper and metallurgy– both generate substantial amounts of alkaline by-products.

The present work focuses on the investigation of carbon capture via carbonation with 6 different absorbents: black liquor and green liquor dregs (pulp and paper residues), three steel slags (tunnel kiln slag, ladle furnace slag, and electric arc furnace slag), and NaOH-ethanol solutions. Direct aqueous carbonation experiments were conducted with mixtures of 15 or 30% CO2 balanced with N2 in the case of the side-stream materials, while the NaOH-ethanol solutions were investigated in direct air capture. Experiments were conducted at ambient pressure and the uptake of CO2 by each absorbent system was monitored. Key operating parameters, such as reactor configuration, particle size, and temperature were investigated.

Black liquor contains dissolved NaOH, which reacts with CO₂ to form soluble Na₂CO₃ and NaHCO₃. Black liquor samples prepared from soda-pulping presented absorption capacity similar to that of a 3 wt% pure NaOH solution. The other materials derive their alkalinity primarily from Ca-based compounds, such as hydroxides and silicates, which react with CO₂ towards the precipitation of CaCO₃. The accessibility of Ca-bearing phases and solubility of Ca2+ significantly affect the carbonation performance. Thus, reducing particle size by crushing or stirring and decreasing the solid-to-liquid ratio had a positive impact on the yield of carbonation. In the case of NaOH-ethanol solutions their performance in direct air capture is primarily mass-transport controlled. The use of a stirred reactor led to nearly complete removal of the CO2 in the air passing through the reactor.

Overall, this thesis demonstrates the versatility and simplicity of inorganic carbonation as a carbon capture strategy, highlighting several promising pathways. Further research is required to develop a complete process tailored to each individual system and to enable their successful industrial scale-up.