Structure-performance Relationship And Sorption Mechanisms Of Alkali Metal Salt-promoted Solid Sorbents For Intermediate-and High-temperature CO₂ Capture

Environment and energy are key issues of the development of human civilization.The anthropogenic emission of greenhouse gases,especially CO2,contributes greatly to global warming since the Industrial Revolution.The CO2 capture,storage and utilization technology is considered one of the most effective strategies for carbon emission reduction.As regards CO2 capture,solid sorbents have demonstrated great potential for practical applications,owing to the advantages such as wide availability,low cost and good operability.Unfortunately,however,solid sorbents have some drawbacks,e.g.,low rate for CO2 capture and poor stability in multiple cycles.To this end,the present dissertation designed and prepared a series of alkali metal salt-promoted solid sorbents for intermediate-and high-temperature CO2 capture,which exhibited good CO2 capture performance in terms of both rate and stability due to the promoting effect of molten alkali metal salts on CO2 capture.The structure-performance relationship of different sorbents was systematically explored;moreover,their CO2 sorption mechanisms were probed.First,alkali metal nitrate and carbonate salts(AMS)-promoted MgO-CaCO3 sorbents were prepared using a solvent thermal-deposition technique.The effect of Mg/Ca molar ratio on the structure-performance relationship of sorbents was investigated.It was demonstrated that the fast formation of CaMg(CO3)2 with aid of molten AMS increased the CO2 sorption rate and the cyclic stability.The sorbent with a Mg/Ca molar ratio of 95/5 showed the best CO2 sorption performance among all MgO-CaCO3 materials.Second,a series of alkaline earth metal carbonate AeCO3(Ae=Ca,Sr or Ba)-doped AMS-promoted MgO sorbents were prepared using a precipitation-deposition method.The effects of type and content of AeCO3 on the structure-performance relationship of sorbents were studied.The results showed that CaCO3 or BaCO3 participated into the CO2 sorption of MgO with the fast formation of CaMg(CO3)2 or BaMg(CO3)2 to enhance the sorption rate,while SrCO3 acted as an inert barrier to separate and delay the sintering of MgO particles and thus improve the cyclic stability of sorbents.An increase in the AeCO3 content was helpful for increasing the CO2 sorption rate,but unfavorable for the CO2 sorption capacity.Third,a series of NaNO3-promoted CdO-MgO sorbents were prepared by a precipitation-deposition method.The effect of Cd/Mg molar ratio on the structure-performance relationship of sorbents was investigated.It was shown that both CdO and MgO participated in CO2 sorption with the aid of molten NaNO3,with fast formation of CaMg(CO3)2 that contributed to the enhanced sorption rate.In particular,the sorbent with a Cd/Mg molar ratio of 1/1 had the highest rate.In addition,the decomposition process of CaMg(CO3)2 was divided into low-and high-temperature stages,corresponding to the formation of CdO-MgCO3 and CdO-MgO,respectively;the low-temperature decomposition stage was beneficial to the sorbent stability.As a consequence,we for the first time put forward a novel Cd-Mg looping(CdO+MgCO3+CO2(?)CdMg(CO3)2)for CO2 capture and thermochemical energy storage.It was demonstrated that this novel looping can not only convert a low concentration CO2 in integrated gasification combined cycle power plants into pure CO2,but also realize stable energy storage and release in intermediate-temperature thermochemical energy storage.Next,a series of Na/K-doped Li4SiO4-based sorbents for high temperature CO2 capture were prepared by sol-gel method.The effects of type and content of Na/K dopant on the structure-performance relationship of sorbents were studied.It was shown that the presence of active Li3NaSiO4 on the surface of Na-doped Li4SiO4 was beneficial for enhancement of sorption rate,while the formation of eutectic Li2CO3-K2CO3 molten carbonates on K-doped Li4SiO4 during CO2 sorption promoted the ion migration and CO2 diffusion and consequently gave rise to an improved cyclic stability.The sorbent with a Na/Si or K/Si molar ratio of 10/1 presented good CO2 capture performance.In addition,the Na,K-codoped Li4SiO4-based sorbent possessed both fast kinetics and better cyclic stability.Finally,based on the above investigations,a series of MgO-and CaO-based sorbents were prepared from dolomite(rich in Mg and Ca)with the aid of magnesite and limestone,and applied to intermediate-and high-temperature CO2 capture,respectively.For MgO-based sorbents,CaCO3 took part in the carbonation of MgO,thus improving both rate and stability of sorbents,while for CaO-based sorbents,inert MgO delayed the sintering of CaO particles.An optimal MgO-or CaO-based sorbent was combined with a Ni-MgAl2O4 catalyst in a layered configuration in a fixed bed reactor for integrated CO2 capture and conversion process.Preliminary experiments demonstrated the feasibility of this process,which can convert CO2 captured in pre-and post-combustion systems into CH4 and syngas(CO and H2),respectively.In summary,this dissertation has systematically studied alkali metal salt-promoted solid sorbents for intermediate and high temperature CO2 capture,which is expected to provide new ideas for carbon neutralization.