Industrial Alkaline Waste Carbonation: Challenges and Opportunities. The case of Municipal Solid Waste Incineration Ashes (MSWIA)

The urgent need to mitigate carbon dioxide (CO₂) emissions and address climate change has propelled the exploration of innovative approaches for carbon capture and storage (CCS). Among these, the carbonation of Industrial Alkaline Wastes (IAW) emerges as a promising avenue. In the design of a carbonation process, the primary challenges revolve around enhancing the CO₂ absorption rate while minimizing energy consumption and water demand. This holds true irrespective of the nature of the gas stream—whether it be flue gas, pure CO₂, or even air. Beyond the storage of CO₂, the main co-benefit of IAW carbonation lies in the value added through waste stabilization, facilitating its reutilization.

The study seeks to contribute to the optimization of carbonation reactors by providing a comprehensive understanding of the relevant factors influencing both the CO₂ absorption rate and the waste stabilization during MSWIA aqueous carbonation in open systems. The investigation considers Ca(OH)₂, Mg(OH)₂, and MSWIA carbonation individually, with a focus on free Ca and Mg oxides/hydroxides as the main phases providing cations for carbonation to occur. The detailed exploration of operational parameters aims to guide the design of efficient strategies, addressing the critical question of "How can an IAW carbonation reactor be optimized to achieve maximum CO₂ absorption, high yields, and stable byproducts?". The investigated operational parameters include:

• Ca(OH)₂, Mg(OH)₂, MSWIA initial concentrations;
• CO₂ flow rate;
• CO₂ concentration in the gas stream;
• Temperature;
• NaCl concentration (salinity);
• NaSO₄ concentration as accumulating impurities;
• MSWIFA concentration as accumulating impurities;
• Mixing system, with a comparison of bubbling (pipe), sparger and porous stone diffusor;
• Ball-milling of MSWIFA for particle size reduction;
• One-step vs two-step process (mineral extraction through pH-swing).

An experimental dataset, based on batch experiments, was collected using high-precision gas flow sensors to measure the percentage of flowing CO₂ absorbed by the reactor under a wide range of operational conditions. Leaching tests were carried out according to the EN 12457-2 standard on solid waste. Solid-liquid phases characterisation was conducted using XRPD with Rietveld refinement, SEM-EDS and ICP-MS. A computational set comprising equilibrium and dissolution kinetic models was developed using Phreeqc to interpret CO₂ absorption vs. time patterns as well as the pH dependence of the MSWIA leaching.

We acquired numerous relevant findings:

• Ca/Mg (hydr-)oxides dissolution is widely considered as the main rate-controlling step for IAW carbonation over CO₂. Using a CO₂ diffusion systems, we have shown that increasing the CO₂-water interfacial surface area by reducing the size of the bubbles causes a cascade of kinetic acceleration of dissolution.
• The average NaCl seawater concentration, 3.5 wt.%, optimizes the CO₂ absorption rate.
• By employing CO₂ sparger/porous stone diffusion alongside a 3.5 wt.% NaCl concentration, it becomes feasible to achieve an absorption rate exceeding 90 % for 2 L/min of CO₂ when using a solution with 7.5 wt.% Ca(OH)₂ in just 1 kilogram of water.

These insights pave the way for more energy-efficient and environmentally sustainable IAW reactor designs with the potential for widespread application in carbon capture and storage efforts.