Roles of double salt formation and NaNO_3 in Na_2CO_3-promoted MgO absorbent for intermediate temperature CO_2 removal

摘要

Absorption and desorption of carbon dioxide on Na_2CO_3-promoted MgO have been studied at temperatures compatible with warm gas cleanup (300-470 °C) from a pre-combustion syngas. The absorbents are synthesized through the formation and activation ofthe precipitate resulting from the addition of sodium carbonate to an aqueous solution of magnesium nitrate. The absorbent, which comprises MgO, Na_2CO_3 and residual NaNO_3 after activation, forms the double salt Na_2Mg(CO_3)_2 on exposure to CO_2. Thethermodynamic properties of the double salt, obtained through computational calculation, predict that the preferred temperature range for absorption of CO_2 with the double salt is significantly higher compared with MgO. Faster CO_2 uptake can be achieved as a result of this higher temperature absorption window. Absorption tests indicate that the double salt absorbent as prepared has a capacity toward CO_2 of 15wt.% (3.4mmol CO_2/g absorbent) and can be easily regenerated through both pressure swing andtemperature swing absorption in multiple-cycle tests. Thermodynamic calculations also predict an important effect of CO_2 partial pressure on the absorption capacity in the warm temperature range. The impurity phase, NaNO_3, is identified as a key component in facilitating CO_2 absorption by these materials. The reason for reported difficulties in reproducing the performance of these materials can be traced to specific details of the synthesis method, which are reviewed in some detail.