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Calcium oxide (CaO) has been investigated as a high-temperature performance sorption material for carbon capture due to its high carbon dioxide (CO2) capacity, low cost, and environmental benignity. Unfortunately, commercialized application of CaO materials has been hindered by severe capacity loss during cycling operations. In an attempt to overcome these problems, this study investigated the effects of adding zirconia-based ceramic dopants and coating sorbents with highly crystalline zirconium dioxide (ZrO2) layers derived from the hydrolysis of zirconium butoxide to control the growth of ZrO2 crystals. Uncoated natural Cadomin delivered a high CO2 capture capacity of more than 13 mol CO2/kg, while the zirconia-coated sorbents showed excellent cycling stability corresponding to a maximum capacity of retention of 79% after 21 cycles at high temperatures. More importantly, the highly crystalline ZrO2-coated sorbents doped by zirconia-based sorbents exhibited a significantly lower decay capacity compared to the uncoated sorbents, which could be attributed to the formation of a perovskite structure that is resistant at high temperatures and a high-temperature-resistant ceramic layer through the solid-state reaction of the highly mesoporous crystalline ZrO2 layer with CaO. These results shed light on the development of sorbents with special structures and superior chemical and mechanical properties for high-performance calcium looping or regenerative calcium cycle processes. [ABSTRACT FROM AUTHOR]

In this study, a series of core-shell structured spheriform CO2 sorbents were synthesized by using CaO-based pellets as cores and different mesoporous metal oxides (e.g. alumina, ceria, and yttrium-stabilized zirconia) as shells through a repeated wet impregnation coating process. Cyclic CO2 capture performance of the obtained sorbents was investigated using a thermogravimetric analyzer. Among all the core/shell sorbents under study, the pellets coated with a layer of alumina exhibit the best performance in the retention of CO2 uptake over 20 cycles with the lowest activity loss of only 30.4 %, attributed to the existence of the thermal stable porous alumina shell which prevents the sintering and the aggregation of the CaO grains. Moreover, the attrition study using an air-jet apparatus and a standard test method reveals that such sorbents exhibit enhanced attrition resistance due to the protection of the porous shell providing them with a great potential for application in fluidized bed conditions. [ABSTRACT FROM AUTHOR]

Alkali release was studied in a 10 kW th chemical looping pilot operated with a Linz-Donawitz (LD) slag oxygen carrier (OC) and three biomass fuels. Experiments were performed at three temperatures and in three operation modes: chemical looping combustion (CLC), chemical looping gasification (CLG), and oxygen-carrier-aided combustion (OCAC). Gas-phase alkali release was measured with a surface ionization detector (SID). Fuel reactor (FR) gas-phase alkali emissions increased with the temperature. This occurred as a result of increased evaporation of KCl and enhanced decomposition of alkali salts during char conversion. Air reactor (AR) alkali emissions were lower than in the FR and independent of the operating temperature. In comparison of operating modes, CLC and CLG modes resulted in similar gas-phase alkali emissions due to the similar extent of char conversion. In contrast, operation of the reactor system in OCAC mode resulted in significantly lower levels of gas-phase alkalis. The difference in alkali emission was attributed to the steam-rich atmosphere of CLC. The effect of steam was further investigated in CLC and OCAC tests. Lowering steam concentrations in CLC operation resulted in lower gas-phase alkali emissions, while introducing steam to the FR during OCAC operation resulted in higher alkali emissions. It was concluded that steam likely enhances gas-phase K release through a reaction of K 2 CO 3 within the fuel char with steam to produce KOH(g). Solid sampling and analysis for K content was used along with SID measurements to develop a K mass balance for the reactor system. Mass balance results for the straw pellet fuel tests showed that LD slag OC absorbs approximately 15-51% of fuel K, 2.2% of fuel K is released to the gas phase, and up to 3.4% of fuel K is captured in the AR fly ash. The residual 40-80% of fuel K was determined to leave the FR as K-rich fly ash., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)