Studies On The Preparation And Sorption Capacity Durability Of Nano CaO-Based CO₂ Adsorbent By Coating TiO₂

CaO-based CO2 reactive adsorbent has been identified as the most promising candidate because of its stoichiometric reactive sorption capacity, low cost, and the abundance of its natural precursors. A CO2 capture process that employs a solid CaO-based reactive adsorbent plays an important and effective role in the CO2 capture for many industrial processes, such as CO2 separation from flue gas or syngas, hydrogen production from a reactive sorption enhanced reforming, chemical heat pumps systems etc.. However, CaO-based CO2 adsorbents suffer from the rapid decay of CO2 sorption capacity during multiple carbonation-calcination reaction cycles, which is the key problem restricting industrial application. Therefore, the studies of the decay reasons and of improving the durability of CO2 sorption capacity during multiple carbonation-calcination reaction cycles have important theoretical and practical significances both for energy saving hydrogen production technologies and CO2 emission reduction technologies in the energy and environment fields.In this thesis, nano CaCO3 was adopted as the precursor to prepare TiO2 surface coated nano CaO-based CO2 adsorbent. The mothodes and mechanism to improve the sorption capacity durability of nano CaO-based CO2 adsorbent were studied. Main results of this thesis including the following four aspects have been obtained:(1) Decay mechanism analysis of nano CaO-based CO2 adsorbent sorption capacity during multiple carbonation-calcination reaction cycles. At first, the grain growth characteristics of nano CaO in 750-900℃,10-240min calciantion were studied by X-Ray Diffractomer(XRD) and calculated by Scherrer equation. And a grain growth model for nano CaO was formed related to calcination temperature and time. Second, the changes of nano CaO adsorbent micro-structural properties in one carbonation-calcination cycle were investigated on nitrogen physical sorption in liquid N2 and scanning electron microscope(SEM). Results showed that the sorption capacity decay of nano CaO was the result of the reduction of the specific surface area caused by nano CaO sintering and carbonation-calcination reaction. In sintering process, nano CaO grain size significantly extended with the increase of calcination temperature and calcination time, and approached an asymptote as isothermal calcination proceeded. When the calcination temperature increased, nano CaO grain grew faster, and the the grain size was larger, nano CaO tended to be nonporous and resulted in the decrease of specific surface area; Alternatively, pore blockage caused by CaCO3 product layer formation and pore closure during sorption reaction also resulted in the reduction of surface area of nano-CaO.(2) Preparation of TiO2-coated nano CaCO3 powder. According to the principle of Ti(OC4H9)4 hydrolyzed to form TiO2, two methods include hydrolyzing deposition and micro-scale hydrolysis were adopted to prepare TiO2-coated nano CaCO3 powder. XRD and transmission electron microscopy (TEM) were used to characterize components and coating morphology respectively, and X-ray spectroscopy (XPS) spectrum was used to test and calculated compact factor defined by author. The coating preparation conditions on the impact of TiO2 layer compactness were detailed examined. Results showed that TiO2-coated nano CaCO3 was successfully prepared by hydrolyzing deposition, and the coating layer with about lOnm thickness was anatase TiO2. Micro-scale hydrolysis could effectively prepare TiO2 coating layer with various compactness defined by author. When TiO2 content increased from 5wt% to 20wt%, the data of compact factors A were increased from 0.81 to 1.89, and when hydrolysis temperature elevated from 20℃ to 70℃, the data of compact factors A reduced from 1.03 to 0.77.(3) Study on the sorption capacity durability of TiO2 coated nano CaO-based CO2 adsorbent. The sorption capacity in carbonation-calcination reaction cycles were measured by thermalgravimetric analyzer (TGA) under the conditions of 600℃, 20%CO2/N2, 10min sorption and followed by 730℃, N2, 10min decomposition. From the aspects of adsorbent materials, calcination atmosphere and calcination temperature, CO2 sorption capacity durability of TiO2 coated nano CaO-based CO2 adsorbent were studied. First, CO2 sorption capacity within 30 cycles of TiO2 coated nano CaO-based CO2 adsorbent prepared by hydrolyzing deposition and micro-scale hydrolysis processes were investigated. Results showed that TiO2 coated nano CaO-based CO2 adsorbent prepared by micro-scale hydrolysis method had better durability of CO2 sorption capacity. Under the optimum preparation conditions, with 10wt% TiO2 content and the hydrolysis temperature at 20℃, the sorption capacity of TiO2 coated nano CaO-based CO2 adsorbent maintained at 5.2mol/kg. This was 15.4% higher than the nano CaO-based adsorbent prepared by hydrolysis deposition method and 58.8% higher than nano CaO derived from nano CaCO3 decomposition. Second, TiO2-coated nano CaCO3 were pretreated in N2 atmosphere, and ≥850℃ calcination. The sorption capacity of TiO2 coated nano CaO-based CO2 adsorbent was increased with the number of cycles (in some extent) called self-reactivation phenomenon. Analysis of self-reactivation mechanism shows that the specific surface area of TiO2 coated nano CaO-based CO2 adsorbent increases with the number of cycles during self-reactivation process, and improved sintering resistent ability caused by TiO2 coating layer makes sorption capacity remained stable.(4) Sintering resistent mechanism analysis of TiO2 coated nano CaO-based CO2 adsorbent. The sintering characteristics of TiO2 coated nano CaO-based CO2 adsorbent were investigated from two aspect of grain size and particle morphology in multiple carbonation-calcination cycles. Results showed that compared with nano CaO-based CO2 adsorbent derived from uncoated nano CaCO3(85-93nm), the grain size of TiO2 coated nano CaO-based CO2 adsorbent in multiple cycles was smaller(75-78nm), and the particle size abserved with SEM hardly grown up, showing excellent sintering resistent properties. The sintering resistent mechanism of TiO2 coated nano CaO-based CO2 adsorbent is as follows. (i) The introduction of the thermal stabe TiO2 coating layer separates CaO particles and hinders the coalescence of adsorbent particles, makes the particle hardly grown up; (ii) The distribution ofTiO2 in the nano CaO triple grain boundaries can weaken the driving force of grainboundary movement, restrict the increase of CaO grain size in multiple cycles.Thereby, the sintering resistent ability of nano CaO-based CO2 adsorbent is improved. The development of nano CaO-based CO2 adsorbent modified with TiO2 coatingand research results on the sorption capacity durability and sintering resistentmechanism provide the knowledge of improving sorption capacity durability of nanoCaO-based CO2 adsorbent.