Effects Of Coupling Fluidization/Reaction Processes On Structure & Reactivity Of Iron Based Oxygen Carrier In Coal Chemical-Looping Combustion

Coal chemical-looping combustion (CLC) is a promising coal combustion technology to reduce CO2 emission for the inherent CO2 separation feature. In coal CLC process, oxygen carriers (OC) circulate the whole system by reacting in complex atmosphere at high temperature, realizing the full combustion of coal and the high efficiency CO2 capture. However, reactivity and physical structure of the oxygen carrier will be gradually deteriorated under the complex fluidization and reaction conditions. Natural clay attapulgite was introduced into preparing Fe2O3-based compound oxygen carriers with high reactivity and structural stability through different methods. The performance of the prepared oxygen carriers and the variation of their reactivity and structure were explored in a fluidized bed reactor. The main findings are summarized as follows:(1) Fe2O3/ATP oxygen carriers were prepared through mechanical mixing method, impregnation method and sol-gel method. For the catalytic effects of ATP on the coal conversion process, both the reaction rate peaks of the coal pyrolysis process and the char gasification process came out 10 min earlier than that in the coal gasification process. Attrition resistance of the Fe2O3 oxygen carriers was enhanced by supported on ATP, and attrition rates of the prepared Fe2O3/ATP oxygen carriers decreased by about 70% than the pure Fe2O3 oxygen carrier. A synergistic effect may exist between ATP and Fe2O3 which promoted the coal conversion. The CaO contained in ATP catalyzed the coal conversion process by enhancing its conversion rate; ATP improved the reactivity of the oxygen carriers by modifying their structures, which accelerated the coal conversion by reducing the inhibition of the syngas.(2) The U-Fe4ATP6 oxygen carrier prepared by sol-gel method exhibited a more significantly catalytic effect on the coal conversion process with an initial carbon conversion rate of 0.168 min-1; its large surface area reaching 4.92 m2/g, ensured the high reactivity of the oxygen carrier with the average CO2 concentration of 98.9% in the outlet gas. The catalysis of the U-Fe4ATP6 oxygen carrier was deactivated in 60 cycles for the conversion of CaO into Ca2SiO4. After 60 cycles, the light deactivation of pore structure and surface area for agglomeration resulted in a slight reactivity decrease of U-Fe4ATP6 oxygen carrier. To ensure the high combustion efficiency in coal CLC, the value of fo tH2 must be higher than k95 (tH2>t95), indicating a surface area larger than 1.72 m2/g is necessary for an iron-based oxygen carrier used under homothetic conditions.(3) Attrition in fluidized bed was the main reason for the particle size distribution variation of the U-Fe4ATP6 oxygen carrier. Attrition rate for the U-Fe4ATP6 oxygen carrier at 900℃ was much lower than that at ambient temperature. The reactions between syngas and the oxygen carrier promoted the conversion of CaO into Ca2SiO4. Coal ash contributed mainly on the contraction in surface area of the U-Fe4ATP6 oxygen carrier by reducing the pores whose size was smaller than 10μm. In CLC circulation, the ash which mainly changed the structure of the oxygen carrier should be separated with the oxygen carrier in real coal CLC process by proper method for the long life-time of the oxygen carrier.