The presence of H2O

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“The presence of H2O also influences the reaction between magnesium and CO2, and it is one of the issues for practical application because flue gas from coal-fired power plants contains water vapour. Duan and Sorescu showed both the initial sorbent (MgO) and Mg(OH)2 are capable of forming MgCO3 in the presence of H2O during the carbonation process. Fig. 19 shows the sorption temperature was lower than the transition temperature (Ttr) and thus, the carbonation process is dominated by Mg(OH2) + CO2 ↔ MgCO3 +H2O, whereas the carbonation of MgO occurs when the sorption temperature was higher than the Ttr [112].”

“This was attributed to the reaction between MgO and Mg(OH2) at the transition temperature: MgO + H2O = Mg(OH2) and controlled by the water vapour pressure. During regeneration, CO2 will be released from MgCO3 and form Mg(OH2) instead of MgO in the presence of water vapour at low system temperature. In addition, a significant 80 % conversion of MgO into MgCO3 occurs in the presence of H2O (∼10 %) occurred at 623 K [113]. A mechanism for MgO carbonation in the presence of water vapour has been proposed [114]. The formation of CO32– and H+ ions occurs when the MgO was surrounded by water. Free Mg2+ ions react with CO32– to form MgCO3. This indicates that water vapour acts as a medium for improved chemisorption interactions between MgO and CO2. This phenomenon was in agreement with a study carried out by Ding et al., as shown in Fig. 20. The CO2 uptake of MgO increases from 0.82 to 3.42 mol kg–1 when the relative humidity increases from 30 to 70 %, respectively [60]. However, a high water content in the sorbent system will require high energy due to the sensible heat [112].”


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