Experimental Study Of CO₂Sequestration From Flue Gas By Direct Aqueous Mineral Carbonation Under Low-Medium Pressure Conditions

"Greenhouse Effect", which is mainly caused by CO2emissions, has attracted world wide attention because of its impact to human survival and living environment. Nowadays, as the world’s largest coal producer and consumer, China has faced an enormous pressure to reduce the CO2emissions into the atmosphere. CO2sequestration by mineral carbonation, a new option for CO2storage which has great potential for development, has attracted many scholars’attentions. In this paper, three typical natural silicate minerals which are rich in calcium or magnesium, wollastonite, serpentine and olivine were used to do a systematic research on directly sequestration of CO2from coal-fired flue gas by direct aqueous mineral carbonation under low-medium pressure conditions. Through these studies, we would expect to provide theoretical basis for the large-scale industrial applications of CO2sequestration by mineral carbonation and geological sequestration.Firstly, the carbonation reaction characteristics of wollastonite were studied. The thermodynamics and experiments results confirmed the feasibility of CO2sequestration by direct aqueous mineral carbonation of wollastonite from coal-fired flue gas. The results also show that, with the increase of reaction temperature, decrease of particle size and addition of buffer solution, the carbonation conversion of wollastonite increased significantly. Heat-treatment of wollastonite did not promote the mineral carbonation reaction process. For pure CO2and SO2simulated flue gas conditions, the carbonation conversion increased with the increase of reaction pressure. But the carbonation conversion decreased with the increase of reaction pressure for NO simulated flue gas and oxygen-enriched combustion flue gas conditions. Overall, the carbonation conversions of wollastonite in the four gas conditions were:pure CO2gas conditions> oxygen-enriched combustion flue gas conditions>SO2simulated flue gas conditions> NO simulated flue gas conditions.Secondly, the carbonation reaction characteristics of serpentine were also studied by detailed theoretical and experimental analyses. The results show that it was possible to sequestrate CO2directly from coal-fired flue gas by direct aqueous mineral carbonation of serpentine. As the reaction temperature increased, the carbonation conversion of serpentine increased firstly and then decreased. For pure CO2gas and SO2simulated flue gas conditions, the mineral carbonation conversion of serpentine increased with the increase of reaction pressure. While for NO simulated flue gas and oxygen-enriched combustion flue gas conditions, as the reaction pressure increased, the carbonation conversion of serpentine slightly increased firstly and then showed a decreasing trend. Heat-treatment of serpentine minerals, addition of NaHCO3buffer solution and decrease of mineral particle size can effectively enhance the mineral carbonation conversion. As a whole, the carbonation conversions sequence of serpentine in the four gas conditions were similar to wollastonite,.Then, the direct aqueous mineral carbonation reaction characteristics of olivine were also included in this paper. The results show that although the thermodynamic studies results confirmed CO2sequestration by direct aqueous mineral carbonation of olivine from coal-fired flue gas was feasible, but the experimental studies showed a much lower reactivity of olivine which suggested that using olivine for CO2sequestration by direct aqueous mineral carbonation under low-medium pressure conditions was difficult to achieve a satisfactory carbonation conversion. For pure CO2gas and oxygen-enriched combustion flue gas conditions, the carbonation conversion of olivine increased as the reaction temperature rose. While for SO2simulated flue gas and NO simulated flue gas conditions, with the reaction temperature rising, the carbonation conversion of olivine increased firstly and then decreased. As the reaction pressure increased, the carbonation conversion of olivine increased under pure CO2gas and SO2simulated flue gas conditions but slightly decreased under NO simulated flue gas and oxygen-enriched combustion flue gas condiitons. NaHC03buffer solution added in the reaction system and the mineral particle size reduced can appropriately improve the carbonation conversion of olivine. Heat-treatment would reduce the reaction reactivity of olivine particles. In general, the carbonation conversions of olivine were slightly higher under pure CO2gas conditions than in the other three gases conditions and the highest carbonation conversions of olivine obtained in this paper was only18.4%.Finally, the carbonation reaction characteristics of wollastonite, serpentine and olivine were compared and the carbonation reaction mechanism of direct aqueous mineral carbonation with SO2and NO existed was revealed. An initial carbonation model was also been built. The comparison results showed that wollastonite had the highest carbonation reaction activity among these three silicate minerals, followed by serpentine and the reaction activity of olivine was the lowest. Meanwhile, based on this comparison results, wollastonite was chosen as the raw material for the thermodynamics and experimental research of mineral carbonation in CO-supercritical water system. The experimental results show that wollastonite can be used to react with CO2to form a stable carbonate during CO-supercritical water reaction. Increasing of reaction temperature and CO initial pressure can improve the carbonation conversion of wollastonite. As wollastonite added in the CO-supercritical water system, the reaction can be accelerated in a certain extent. The amount of wollastonite added in the CO-supercritical water system and the reaction time set should consider to balance the efficiency of carbonation conversion and hydrogen production.