Studies On Synthesis Of Nano CaCO₃by Reactive Crystallization From Phosphogypsum And Its Application For CO₂Capture

Nano-CaO based CO2sorbent has low decomposition temperature, fast reactive sorption rate and high sorption capacity. It plays an important and effective role in CO2separation from flue gas or syngas and hydrogen production from a reactive sorption enhanced reforming. However, nano-CaO based CO2sorbents suffer from high cost and the rapid decay of CO2sorption capacity during multiple carbonation-calcination reaction cycles, which is the key problem restricting industrial application. Phosphogypsum is a by-product generated from the wet-acid process for phosphoric acid manufacture, which leads to the environment pollution and the waste of resources. Therefore, the studies on using phosphogypsum as calcium precursor for synthesis of nanoCaO based CO2sorbent with low cost and improving its durability during multiple carbonation-calcination reaction cycles have important theoretical and practical significances in CO2emission control technologies and sorption enhanced hydrogen production.In this thesis, as the raw material, phosphogypsum was used to prepare nano-CaCO3and its crystallization kinetics of nano-CaCO3was studied. Then the nano CaCO3prepared, which was adopted as calcium precursor, was mixed with magnesium sol to prepare nano-CaO/MgO CO2sorbent. The Influence of impurity in phosphogypsum on nano-CaCO3particle size and sorption performance of nano-CaO based CO2sorbent was studied. At last, and the influence of carbonation-calcination cycle condition and the mechanism of self-reactivation and durability of nanoCaO based CO2sorbent were studied. Main results of this thesis have been obtained as followings:The formation of nano-CaCO3in gas-liquid-solid three phases during piping CO2into the slurry mixed with ammonium hydroxide and phosphogypsum was investigated. The influence of reaction time, reaction temperature of5-60℃, CO2flux of96-354mL·min-1, stirring rate and soluble impurities on the CaSO4conversion, CaCO3content, CaCO3particle size, CaCO3morphology and CaCO3crystal form had been studied. Results showed that the increase of reaction time, temperature, CO2fluxes and stirring rate or removal of soluble impurities favored increase of CaSO4conversion. The interaction of temperature and CO2flux affects the content and size of CaCO3. Under the optimization condition of temperature30℃and CO2flux of 251mL·min-1without soluble impurities, the CaCO3sample included CaCO3content of83wt.%with an average diameter of73nm.Crystallization kinetics of CaCO3prepared from phosphogypsum in one step gas-liquid-solid three phases were studied. In this thesis, the mechanism of CaCO3preparation was studied and based on the crystal population balance equations and Laplace transform, the crystallization kinetics of CaCO3including nuclear and growth kinetics were developed. Under the conditions of temperature of20-40℃and CO2flux of138mL·min-1and251mL·min-1, the nuclear index n and growth index g were optimized and estimated as0.6and0.2respectively. The activation energy of nuclear and growth reaction was-62.1kJ·mol-and70.3kJ·mol-1respectively. Increasing CO2flux favored both of nuclear reaction and growth. Increasing temperature decreased the nuclear reaction rate and increases the growth reaction rate. The average relative deviation of the CaCO3crystallization rate was about20%between the predicted value and experimental value.A porous nano-CaO/MgO based sorbent was prepared using MgO as a support in order to increase the sorption capacity and durability. The magnesium sol prepared by reacting MgO slurry with citric acid was added to nano-CaCO3slurry, and the mixture was calcinated to obtain the nano-CaO/MgO based sorbent. The influence of MgO content on the structure and sorption performance of the resulting sorbent was studied. The pore radius and specific surface area of the sorbent increased with MgO content increasing. The sorbent exhibited superior sorption performance during carbonation-calcination cycles and maintained a good durability at the calcination temperature, thus being an interesting candidate for future work.Nano-CaO based CO2sorbent was prepared by using nano-CaCO3as calcium precursor. Nano-CaCO3was prepared from phosphogypsum and mixed with MgO to prepare nano-CaO based sorbent. The influences of CaCO3particle size and impurity on the decomposition temperature, sorption activity and durability were studied. The effects of MgO content on the durability of nano-CaO based sorbent were also studied. Decreasing in CaCO3particle size and impurity favor decrease of decomposition temperature and durability of nano-CaO based sorbent. The decomposition temperature of CaCO3sample with73nm is lower10℃than commerce nano-CaCFO3. Nano-CaO/MgO sorbent prepared by using it as the precursor exhibited0.2gCO2·g-1sorbent after50cycles.The mechanism for self-reactivation of nano-CaO-based CO2sorbents was investigated. Nano-CaO, nano-CaO mixed with MgO, and nano-CaO coated with CaTiO3were prepared as samples of nano-CaO based CO2sorbents. Influences of carbonation-calcination conditions on the phenomenon of self-reactivation were investigated. Results showed that increasing in sorption temperature and time favored self-reactivation and improvement of durability of nano-CaO based sorbent. And self-reactivation was precluded at high calcination temperatures. Nano-CaO based CO2sorbent maintained conversion above50%for over50cycles of carbonation at700℃for10min; conversion with micro-CaO significantly reduced to14%after30cycles. A new grain-pore-core model was proposed for self-reactivation in nano-CaO based sorbents. This model considered the change of grain and structure of the sorbent during both pre-calcination and the subsequent carbonation-calcination cycles.