Carbon dioxide emissions from cement kiln flue gas constitute a significant contributor to global climate change and environmental pollution. As global emission reduction targets progress, research and application of carbon capture, utilization, and storage (CCUS) technologies have increasingly become critical strategies for mitigating CO₂ emissions. Among these technologies, the calcium cycle method demonstrates substantial potential due to its low cost, high efficiency in CO₂ trapping, and broad applicability, particularly in the context of cement kiln flue gas. However, traditional calcium-based materials, such as limestone, face challenges including high precalcination temperatures and resource wastage, highlighting the need for more efficient and sustainable alternatives.

Calcium carbide slag, an industrial by-product, has shown significant promise as a material for the calcium cycle method. It contains a high proportion of reactive CaO, along with inert components such as alumina and magnesium oxide. These inert components enhance resistance to sintering after preburning. However, repeated use of calcium carbide slag leads to pore structure collapse, particle agglomeration, and sintering, severely diminishing its adsorption performance and posing a significant barrier to practical application.

To address these challenges, various modification techniques have been explored to improve the absorption efficiency and sintering properties of calcium carbide slag, as well as to mitigate the impact of SO₂ and NO_x in flue gas on CO₂ absorption.

Organic acid modification has been identified as an effective strategy to enhance the adsorption performance of calcium carbide slag. Through reactions with calcium hydroxide to form organic calcium salts, pyrolysis at high temperatures releases small molecules that disrupt the original structure of the slag. This process increases porosity, specific surface area, and CO₂ adsorption capacity. Different organic acids, due to their distinct molecular structures, yield variations in the molecular weight of organic calcium salts and the types and quantities of small molecules released during pyrolysis. Consequently, modifying calcium carbide slag with specific organic acids allows tailoring of its pore structure to enhance CO₂ adsorption performance.

Doping modification technology is another effective approach. By incorporating various substances into the slag, the properties of calcium carbide slag can be optimized through different chemical reaction mechanisms. Inert materials serve as structural frameworks, inhibiting CaO grain migration and growth to improve anti-sintering performance. Doping with oxygen-deficient materials facilitates CO₂ diffusion and O^2- migration, while potassium and sodium salts increase defect concentrations in the CaCO₃ product layer, enabling more efficient Ca²⁺ migration and enhanced CO₂ absorption.

Hydration processes also play a critical role in influencing sintering. While CaCO₃ typically decomposes at high temperatures, introducing water vapor during calcination reduces the partial pressure of CO₂, promoting its conversion to CaO. Water vapor also shortens decomposition residence time and slows sintering, delaying sintering deactivation and improving stability. By optimizing calcination temperatures and water vapor concentrations, the CO₂ adsorption performance and stability of calcium carbide slag can be enhanced over multiple cycles.

In the context of cement kiln flue gas treatment, SO₂ and NO_x pose additional challenges to the CO₂ trapping performance of calcium carbide slag. SO₂, being strongly acidic, preferentially reacts with CaO to form dense CaSO₄ layers, which accumulate over cycles, diminishing adsorbent activity. Research suggests that specific modification methods or adjustments in the absorption sequence (e.g., absorbing CO₂ before SO₂) can mitigate these effects. Regarding NO_x, calcium carbide slag inherently lacks reductive properties and cannot remove NO_x through traditional calcium cycle methods. Doping with reducing substances, such as copper, iron, and other metal oxides, can enable NO_x reduction by promoting reactions that convert NO_x into harmless nitrogen and oxygen.