Effect Of Steam On Properties Of Nano CaO/Al₂O₃ Sorbent In Ca-looping Process

Hydrogen is an important industrial gas and a clean energy source for the future.The new technology of hydrogen production by reactive sorption enhanced reforming(ReSER)is actually the result of combining Ca-looping technology with methane steam reforming reaction.Ca-looping technology is the technology for removing carbon dioxide from flue gas,and the sorbent is the core of Ca-looping technology.The nano-CaO/Al2O3 sorbent is the preferred high-temperature CO2 sorbent for Ca-looping technology because of its fast adsorption rate,large adsorption capacity and low-cost advantage.Because the reaction sorption enhanced methane steam reforming reaction system contains a large amount of steam and the sorbent regeneration process may also entrain steam,therefore,it is important theoretical and practical significant to study the reactivity and stability of the nano-CaO/Al2O3 sorbent in carbonation and calcination in Ca-looping process with steam.Firstly,laboratory nano-CaO/Al2O3 sorbent was used to study the effect of steam on the reaction rate and sorption capacity stability of nano-CaO/Al2O3 sorbent in the carbonation stage by a laboratory-scale fixed-bed reactor at 600?.The results showed that when the carbonation stage contained 20-60%steam,compared with the no steam carbonation,the 20th cycles sorption capacity of the sorbent increased from 5.6 mol/kg to 7.4 mol/kg,and the sorption capacity stability increases by about 25%.It is because steam in the carbonation stage can promote CO2 to pass through the product layer CaCO3,and further react with the embedded active CaO to increase the carbonation conversion rate.Secondly,the effect of steam on the reaction rate and sorption capacity stability of nano-CaO/Al2O3 sorbent in the calcination was studied at 800?.The results showed that the introduction of 2.5%-50%steam at 800? can shorten the regeneration time of nano-CaO/Al2O3 sorbent by 33%.However,only in case of the steam concentration less than 5%is favorable for the sorption capacity stability of the sorbent.The stability of sorption capacity after 20 cycles was increased by 5%.However,as the concentration of steam increased,the stability of the sorbent decreased obviously.Morever,the reasons for the effects of steam on the stability of nano-CaO/Al2O3 sorbents have been studied by testing the changes in morphology,microstructure,material composition and elemental dispersion of the nano-CaO/Al2O3 sorbent before and after Ca-looping process under different steam conditions.The results included the effects of active component CaO,Al2O3 and calcium-aluminum reaction:In the calcination,the growth of CaO grains is significant.The steam concentration in calcination below 5%is beneficial to the stability of the sorbent.The sorbent stability is improved mainly because the low concentration of steam has few influences on the growth of CaO and steam could shorten the regeneration time and reduced the thermal sintering of the sorbent.The increase of steam concentration in the calcination,the more significant the sintering growth of CaO grains,so the high steam concentration calcination is not conducive to the stability of the sorbent.The phase transition of Al2O3 occurs at 750 C.Therefore,since the temperature is lower than 700? in the carbonation,the steam has no effect on Al2O3.However,the temperature in the calcination is higher than 750 C.The steam promotes the Al2O3 phase transformation and sintering,resulting in the decrease in the specific surface area and pores of the sorbent.Moreover,the higher the steam concentration,the more serious the decrease in the specific surface area and pore volume of Al2O3.The reaction temperature of CaO and Al2O3 to form calcium aluminate is about 1000?.However,the temperature will decrease to 800 C because of the presence of more than 10%steam concentration in the calcination.The higher steam concentration acceleration of the formation reaction of calcium aluminate.The reaction consumed the active component CaO and the additive Al2O3 resulting in a decrease in the stability of the sorbent.However,if the steam concentration in the calcination is less than 5%,the reaction will not be induced.Finally,a laboratory-scale fluidized bed reactor was used to study the effect of steam on the mechanical strength of sorbent.The operation condition is that carbonation in 600?,calcination in 800? and a fluidization number of 40.The sorbent attrition test results were analyzed by attrition ratio,SEM and particle size distribution characterization.The results showed that the steam in the carbonation stage had little effect on the attrition of the sorbent.The attrition ratio in the end of the 20 cycles was 9.3%,the spalling attrition mainly;the steam in the calcination stage had a great influence on the attrition of the sorbent,and the attrition rate of the 20 cycles is 13.5%.Through SEM and EDS analysis,it was found that the place where spalling and fracture occurred was mainly at the site of CaCO3 aggregation,indicating that CaCO3 is a place where the sorbent is easily broken.