Feasibility of Na-based thermochemical cycles for the capture of CO2 from air—Thermodynamic and thermogravimetric analyses

Three Na-based thermochemical cycles for capturing CO2 from air are considered: (1) a NaOH/NaHCO3/Na2CO3/Na2O cycle with 4 reaction steps, (2) a NaOH/NaHCO3/Na2CO3 cycle with 3 reactions steps, and (3) a Na2CO3/NaHCO3 cycle with 2 reaction steps. Depending on the choice of CO2 sorbent – NaOH or Na2CO3 – the cycles are closed by either NaHCO3 or Na2CO3 decomposition, followed by hydrolysis of Na2CO3 or Na2O, respectively. The temperature requirements, energy inputs, and expected products of the reaction steps were determined by thermodynamic equilibrium and energy balance computations. The total thermal energy requirement for Cycles 1, 2, and 3 are 481, 213, and 390 kJ/mol of CO2 captured, respectively, when heat exchangers are employed to recover the sensible heat of hot streams. Isothermal and dynamic thermogravimetric runs were carried out on the pertinent carbonation, decomposition, and hydrolysis reactions. The extent of the NaOH carbonation with 500 ppm CO2 in air at 25 ◦C – applied in Cycles 1 and 2 – reached 9% after 4 h, while that for the Na2CO3 carbonation with water-saturated air – applied in Cycle 3 – was 3.5% after 2 h. Thermal decomposition of NaHCO3 – applied in all three cycles – reached completion after 3 min in the 90–200 ◦C range, while that of Na2CO3 – applied in Cycle 1 – reached completion after 15 min in the 1000–1400 ◦C range. The significantly slow reaction rates for the carbonation steps and, consequently, the relatively large mass flow rates required, introduce process complications in the scale-up of the reactor technology and impede the application of Na-based sorbents for capturing CO2 from air.