Post-combustion CO2 capture by formic acid-modified CaO-based sorbents

• The use of formic acid to reactivate sorbents for Ca looping has been explored.
• Dilute aqueous solutions enhance performance better than concentrated ones.
• Vapor phase treatments are ineffective due to the low conversion of sorbents to Ca formate.

The performance of CaO-based sorbents modified with formic acid in both its liquid and vapor phase has been investigated for high-temperature post-combustion CO2 capture in calcium-looping cycles. The treatment of limestone with aqueous solutions containing 10 or 30 vol% formic acid was found to promote crystal growth. By contrast, higher acid concentrations produced smaller crystals. However, all sorbents modified by acid solutions had almost identical reductions of 44% and 46% in surface area and pore volume (determined by N2 adsorption), respectively, relative to the parent material. Despite the low porosity, limestone (fine powder) treated with 10% acid solution displayed the highest CO2 capture capacity in the first cycle with a capture of 0.6 g CO2/g sorbent compared to 0.49 g/g for untreated powder material. By 20 cycles, the modified sorbent still captured 67.4% more CO2 than the natural sorbent captured under similar conditions. Relatively low concentration formic acid solution improved the CO2 capture capacity of CaO-based sorbents better than treatment with acid vapor due to the limited acidification achieved by vapor phase treatment.