Molecular Design And Preparation Of High-efficiency Magnesium-based CO₂ Absorbers And Their Temperature-changing Absorption Properties

Carbon dioxide capture and storage technology(CCS)is considered the most effective measure for the reduction of carbon dioxide emission into the atmosphere,and it is of great importance of efficient CO₂ absorbent in the CCS technology.For CO₂ capture from industrial flue gas at the medium temperature(200-500?),the Mg-based absorbent is mainly applied for pressure swing absorption process,which is energy-consumping,therefore,the materials designed for the thermo-swing process is highly pursued,however,some disadvantages including slow absorption rate,ease of sintering and poor cycle stability of Mg O-based absorbent exist.To solve these problems,we have developed a series of magnesium oxide-based CO₂ absorbents to improve the absorption rate and stability in thermo-swing absorption processes.In this thesis,various preparation strategies of Mg O-based absorbents were studied,and some characterization methods has been used to characterize their physical and chemical properties,finally dynamic CO₂absorption-desorption cyclic test was performed to evaluate their performance.At the same time,the relationship between the structures and compositions of the absorbent was found to determine the active Mg O precursor,and finally the absorption rate and stability of the magnesium-based absorbent was improved by designing two-dimensional structure to facilitate the CO₂ absorption or nitrogen doping for good recyclability.The following work has been done below:1.Different counter ions including NO₃^-,Cl^-and SO₄^2-led to different double salts phases,i.e.Na₂Mg(CO₃)₂,Na₃Mg(CO₃)₂Cl,and Na₆Mg₂(CO₃)₄SO₄,respectively,and the latter two made the sorbents lose or decrease the absorption capability.As for the preparation temperature,higher temperature usually led to more hydromagnesite phase and less eitelite phase in the sorbents,finally decreased absorption capacity;different feeding orders have been investigated at room temperature,adding sodium carbonate into Mg precursor solution will lead to a uniform nano-sheet-like absorbent,with sodium-magnesium double salt and hydromagnesite intergrowing together,and fast absorption rate and better stability were obtained;while the reverse feeding order leads to more sodium-magnesium double salt phase,which contributes to high initial capacity while the obvious decrease in absorption capacity could be found.Finally a morphology evolution has also been observed as the aging time increase.2.The effect of each component(sodium-magnesium double salt,hydromagnesite,sodium nitrate)in the precursor of the absorbent was studied,and it was found that sodium-magnesium double salt(Na₂Mg(CO₃)₂)was the only origin of active Mg O,which has been explained from the molecular level,i.e.,the calcined structure of the sodium-magnesium double salt shows a special layered assembly mode…[2Na⁺-CO₃^2--Mg²⁺-O^2-]_n…,here Mg O is difficult to become a dense phase with Na₂CO₃ working as a dispersion agent.Meanwhile,in this structure,both Na and Mg cations are adjacent with O atoms and carbonate group,which can also be regarded as a monolayer distribution of Na O and Mg CO₃.The strong alkalinity of Na O is the main reason to increase the absorption rate of absorbent.3.Since Na₂Mg(CO₃)₂ is the only origin of active Mg O,and easy to form bulk solid,we designed and prepared Na₂Mg(CO₃)₂ with a two-dimensional layered structure in order to increase absorption rate.Using methylcellulose M450 as a structure-directing agent,a sodium-magnesium double salt was prepared as a precursor of carbon dioxide absorbent.When the suitable amount of methylcellulose M450(1.0 g),Na₂Mg(CO₃)₂ with uniform nano-sheet can be obtained.This is due to the planar structure of methylcellulose M450 and the hydrogen bonding between two stacked M-450 layers.The absorbent absorbs at 305°C for 1 h at a CO₂ concentration of 16.7 vol%,and the CO₂ absorption capacity can reach 19.6 wt%,and it has the fastest absorption rate.An equilibrium absorption capacity of 14.56 wt%can be reached in about 10 minutes.4.N-doped Mg O-based CO₂ absorption materials were synthesized by simple precipitation method combined with high temperature pyrolysis assisted by melamine.It is explored that nitrogen is present in the absorbent in the form of a trace amount of Na OCN.which can enhance the alkalinity of the absorbent.High melting point of Na OCN(550 ^oC)is above the absorbent working temperature,which makes it a dispersant during the CO₂ absorption-desorption process of the stability improvement.After 9 dynamic absorption-desorption cycles,the optimum adsorbents with 5%nitrogen doping showed good cycled stability.5.The preparation process of the absorbent by precipitation requires filtration step,and the content of soluble components such as nitrate remaining in the filter cake is uncontrollable,resulting in poor repeatability of absorbent preparation.Due to the melting-solidification cycle of nitrate in the absorption-desorption process,excessive amounts will cause the agglomeration,and there is still controversy about the component to promote absorption,so it is chanlleging to synthesize absorbents with controlled compositions.Using simple material mixing method,the materials have been successfully prepared by cheap raw materials without nitrate as the counter ion,with sodium carboxymethyl cellulose as the directing agent,and rotary evaporation has been used for drying.The layered adsorbent has the best CO₂ adsorption performance.The static equilibrium absorption capacity is 22.8 wt%;while even in dynamic absorption without holding time,the CO₂ capacity can reach and maintain at9.7 wt%after 20 dynamic absorption-desorption cycle tests,showing good cycle stability.This study will provide necessary support for the industrial preparation of Mg O-based CO₂ absorbents with enhanced absorption rate and stability.