254894 Analysis of CO2 Sorption/Desorption Kinetic Behaviors and Reaction Mechanisms with Steam On Li4SiO4
The greenhouse effect and energy crisis are currently two of the most important problems worldwide. Hence, CO2 capture technologies are required to decrease CO2 emissions to the atmosphere and improve energy efficiency. They can be used in the direct separation of CO2 from the high-temperature exhaust gases from power plants, particularly in in situ CO2 sorption enhanced fuel steam reforming processes, which aims to produce pure hydrogen at lower temperature. The key point of a CO2 sorption enhanced reaction system is to develop a satisfactory solid CO2 captor with a large capacity, high selectivity, good cycle performance, and proper kinetic behaviors at relatively high temperatures. Thus far, several materials, such as hydrotalcite-like materials, CaO-based sorbents, and lithium-containing materials, have been proposed for CO2 capture. Among these materials, lithium orthosilicate (Li4SiO4) is considered to be one of the most potential materials. Li4SiO4 has been reported to adsorb CO2 more than 30 times faster than Li2ZrO3 and is lighter and cheaper than Li2ZrO3. The mass uptake on Li4SiO4 due to CO2 adsorption is almost 50% greater than the weight change for Li2ZrO3. It has also been used in sorption enhanced fuel steam reforming experiments, where it has shown an evidently promoting effect on the process.
Furthermore, in the methane steam reforming system, the existence of H2O can not be ignored. Thus, the effect of water on the CO2 sorption properties is very important for the application of the sorbents. Essaki and coworkers observed a beneficial effect of adding water when absorbing CO2 on Li4SiO4 at room temperature. Esther also investigated the CO2 uptake profiles over Li4SiO4 and Li2ZrO3 at 525°C and 10% of CO2 with and without water addition. They also found the same trends of the positive impact of steam on the capture kinetics. These results make Li4SiO4 to be more practical in sorption enhanced steam reforming system. However, the reasons for the beneficial effect of steam addition on the CO2 absorption kinetics of the CO2acceptors are not clear yet. These important phenomena need to be explained by kinetic mechanism.
Meanwhile, the investigation of the reaction system mechanism and design requires a suitable kinetic model for the CO2 sorption/desorption process in Li4SiO4. However, only a few studies have reported relative models for Li4SiO4.
In this work,the CO2 sorption/desorption kinetic behaviors on Li4SiO4 were analyzed. The theoretical compositions of the sorption/desorption reactions were calculated using FactSage. The sorption/desorption process on Li4SiO4 was investigated by comparing the shrinking core, double exponential, and Avrami–Erofeev models. The Avrami–Erofeev model fits the kinetic TG experimental data well and, together with the double-shell mechanism, clearly explains the sorption/desorption mechanism. For the sorption process, the reaction occurs rapidly on the particle surface and forms a double-shell structure consisting of Li2CO3 and Li2SiO3 firstly. The process is limited by the rate of the formation and growth of the product crystals. With the increase in double-shell thickness, the limiting-step becomes the diffusion control when the conversion of Li4SiO4 α>0.1 below 650 °C and when α>0.84 at 700 °C. The whole desorption process is found to be controlled by the rate of the formation and growth of the Li4SiO4 crystals. Furthermore, the influence of steam on the CO2 sorption process was theoretically analyzed. It has been observed that the presence of steam enhance the mobility of Li+ and therefore the rate of diffusion control stage.
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