Modular Solvent-Free Carbon Removal: A Solid–Sorbent Reactor for Sequential Direct Air Capture and Flue Gas Decarbonization

Scaling carbon removal to gigatonne-per-year levels requires capture technologies that are simultaneously efficient, environmentally clean, and deployable across heterogeneous emission sources, from dilute atmospheric CO2 (∼420 ppm) to concentrated industrial flue gas (1–33 vol %). Established amine- and hydroxide-based systems typically employ circulating liquid capture media and cyclic regeneration, making solvent handling, regeneration energy, and capture-medium degradation central to process complexity and economics. Here, an alternative solvent-free carbon capture process is demonstrated that addresses both source types within a single sequential platform. A composite sodium hydroxide–sodium percarbonate sorbent captures ambient CO2 via humidity-activated sodium peroxide (Na2O2) formation, fully converting NaOH to Na2CO3 with a capacity of 0.545 gCO2 gNaOH–1. The resulting carbonate is subsequently converted to bicarbonate under 15 vol % CO2 through water-mediated trona-phase growth, and is stacked in a modular reactor to achieve complete sequential capture at 0.612 gCO2 gsorbent–1. The end product, sodium bicarbonate, may be stored or used as feedstock for downstream electrochemical and chemical syntheses, offering a complementary solvent-free pathway for sequential carbon capture.