The Modification Of CaO-based Sorbents Used For Sorption Enhanced Methane Steam Reforming

Sorption enhanced methane steam reforming（CH₄-H2_O reforming） is an innovative technology for hydrogen production. In this new process, CO₂ derived from reforming reaction was removed in-situ by CO₂ sorbents to shift the reaction equilibrium and enhance the reaction process, thus to produce high concentration of hydrogen in one step. The technology can successfully solve the problems of high energy consumption and complex process that the traditional hydrogen production technology has, thus it has essential research value. CO₂ sorbent is the key point of the technology.Calcium oxide（CaO） based sorbents were widely used in sorption enhanced methane steam reforming due to it’s high theory CO₂ sorption capacity, low costs and extensive source, however there still exists two aspects of issues: one is that during the carbonation reaction, the pores of sorbent particles were gradually filled by CaCO₃, which caused the CO₂ diffusion difficult and thus the low CO₂ sorption capacity of CaO based sorbents; the other one is that the CaO based sorbents always suffered from the rapid loss of reactivity in cyclic utilization, it is derived from the sintering of CaO grains which lead to the decrease of sorbents surface area and pore volume. Therefore, it is the key point for the industrial application of sorption enhanced methane steam reforming to develop CaO based sorbents that have high CO₂ sorption capacity and strong cyclic sorption stability. In this paper, the carbonation reaction characteristics and kinetics were deeply analyzed, and it is observed that during carbonation reaction the diffusion of CO₂ in sorbents particles has a significant effect on CO₂ sorption capacity. Improving sorbents surface area and pore volume, reliefing the pore blocking of sorbents particle and enhancing the stability of them during the cyclic carbonationregeneration are the main strategies to enhance the CO₂ sorption capacity and cyclic sorption stability of sorbents.In this paper, the CaO sorbents were doped by Al aiming to improve the sorbents cyclic sorption stability, firstly. Also, the effect of Al additive amount on the sorbents sorption performance was investigated and the mechanism for the enhanced sorption capacity of Al-doped sorbents was in-deep analyzed. Then, the abtained optimal sorbent was separately pretreated by steam and CO₂ to improve its CO₂ sorption capacity further. Finally, the as-synthesized CaO-based sorbent was primarily used for CO₂ sorbent in sorption enhanced methane steam reforming. The main results and conclusions obtained in this paper were as follows:（1） The as-synthesized CaO-based sorbents all possessed strong cyclic sorption stability, moreover, proper amount of Al addition can obviously enhance its CO₂ sorption capacity. Among the as-synthesized sorbents, the sorbent with CaO/Al2O3 mass ratio of 9:1 had the highest CO₂ sorption capacity. During 20 loops of operations, its CO₂ sorption capacity just decreased from 4.65 mmol CO₂/g sorbent to 4.11 mmol CO₂/g sorbent which is remarkably superior to 2.13 mmol CO₂/g sorbent of the as-synthesized pure CaO sorbent.（2） The excellent CO₂ cyclic sorption stability of as-synthesized sorbents was ascribed to the original structure of small surface area and pore volume which formed after the high temperature calcination of sorbents preparation process. The enhanced CO₂ sorption capacity of Al-doped sorbents was due to the uniformly dispersion of inert compound Ca₃Al₂O₆, which decreased the expansive degree of sorbents particle during the carbonation reaction and thus provided many pores for CO₂ diffusion.（3） The effect of inert compound content on sorbents CaO utilization was theoretically analyzed through grain reaction model. The results showed that during carbonation reaction, the inert compound greatly relieved the pore blocking of sorbents particle caused by CaCO₃ and enhanced CO₂ diffusion, thus significantly improved the utilization of CaO in sorbents.（4） Steam pretreatment and CO₂ pretreatment of CaO-based sorbents made its CaO grain size obviously decreased and surface area and pore volume increased thus enhanced the sorbents CO₂ sorption capacity. CaO-based sorbents pretreated by steam at 300 oC and pretreated by CO₂ at 650 oC possessed the best CO₂ cyclic sorption performance. The cyclic sorption stability of these sorbents during 15 loops operations slightly decreased but finally the CO₂ sorption capacity remained stable at the value 20% higher than the value of untreated sorbent.（5） Based on the thermodynamic analysis results of sorption enhanced methane steam reforming, the reaction conditions of lower pressure, temperature at 500⁶50 oC, higher H₂O/CH₄ ratio is beneficial for hydrogen production. Considering the energy consumption, the reaction condition of 1 atm, 600 oC and 4 for H₂O/CH₄ ratio was chosen for sorption enhanced methane steam reforming. 98.0% for H2 concentration and 98.0% for CH₄ conversion was obtained in the first operation of sorption enhanced methane steam reforming using whether CaCO₃ or CaO/Ca₃Al₂O₆ for CO₂ sorbent. The prebreakthrough periods of the process both lasted for 40 min. Under the same reaction condition, the thermodynamic equilibrium H2 concentration and CH₄ conversion for traditional methane steam reforming are just 75.9% and 85.4%, respectively. The cyclic H2 production efficiency using CaO/Ca₃Al₂O₆ is obviously higher than using CaCO₃ as CO₂ sorbent. CaO/Ca₃Al₂O₆ sorbent possessed superior cyclic sorption performance in cyclic sorption enhanced methane steam reforming process.