Enhancement Of Nano-CaO Carbonation Properties On Hydrogen Production By Steam Methane Reforming

Hydrogen is an important petrochemical raw material and is also the clean energy carrier of future.Reactive sorption enhanced reforming(ReSER),which uses nano-CaO as CO2 sorbent,is a highly efficient hydrogen production technology.Compared with hydrogen production with traditional methane steam reforming,ReSER technology has obvious advantages,which can not only reduce the reaction temperature,improve methane conversion and hydrogen concentration,but also can save energy and shorten the process.In the ReSER system,the reaction characteristics of nano-CaO are the key to the enhancement of steam methane reforming.Therefore,the study on improving of CO2 sorption properties of nano-CaO,and its relationship to the enhancement of hydrogen production is important for the industrial application of ReSER technology.First of all,the effect of nano-CaO carbonation reaction characteristic on the hydrogen production from steam methane reforming was analyzed by simulation.The relationship between the CO2 sorption capacity of nano-CaO and the enhancement was simulated by COMSOL Multiphysics software.The results showed that,in the ReSER system,the enhancement of hydrogen purity could be improved by increasing the CO2 sorption rate of sorbents.However,there was no linear correlation between the enhancement and CO2 sorption rate and there exist an upper limit of the enhancement factor.The effect of sorption capacity of nano-CaO on the steam methane reforming mainly reflected in the duration of enhancement,that is a higher sorption capacity can prolong the enhancement duration.Secondly,the cage-like nano-CaO with different diameter was prepared by carbon spheres template method and its sorption performance was studied.The results showed that the cage-like nano-CaO exhibited higher sorption rate and sorption capacity than commercial nano-CaO.The optimized cage-like nano-CaO with a diameter of 1.62?m exhibited the theoretical maximum sorption capacity of 0.786 gCO2/gCaO under a carbonation reaction temperature of 600 ? and 20%CO2.And the maximum sorption rate of the cage-like nano-CaO was 1.44 times of commercial nano-CaO.In order to improve the cyclic adsorption stability of cage-like nano-CaO,Zr-modified nano Ca-based sorbent was prepared by zirconium addition.Studies showed that the optimized sample,which with Ca/Zr molar ratio of 5,retained a CaO conversion of 76%after 30 cycles,whereas the conversion of cage-like nano-CaO without zirconium modification remained only 20%after 30 cycles.In addition,compared with the prepared Mg and A1 modified cage-like nano Ca-based sorbent,the Zr-modified cage nano Ca-based sorbent exhibited better cyclic stability.What's more,zirconium modification and cage-like structure played a significant synergistic effect in improving the cyclic stability of sorbent.In addition,according to the carbonation reaction characteristics of cage-like nano Ca-based sorbent,a new criterion for carbonation reaction section distinguishment was proposed.The Boltzmann equation was used to fit the kinetic equation of the cage-like sorbent in the rapid reaction section.The results showed that the relative error of the kinetic equation was 5.78%,and the activation energy was 26.66 kJ/mol,which was 3.54 kJ/mol lower than that of commercial nano-CaO.Finally,the optimized Zr-modified nano Ca-based sorbent were used to enhance the reaction of hydrogen production via steam methane reforming.Based on the transport phenomenon and three reactions,the model was simulated and verified by experiments.It was found that calculated methane conversion deviation for the model was 5.15%and the cage-like Ca-based sorbent had a higher hydrogen enhancement than commercial nano-CaO,especially at 650 ?,5 bar,with a S/C molar ratio of 3.5 and the GHSV of 700 h-1,the enhancement factor of the cage-like nano Ca-based sorbent was 2.02 times of that of commercial nano-CaO sorbent.The simulation results of enhancement under different reaction conditions showed that the increase of S/C molar ratio was beneficial to the conversion of methane and molar fraction of hydrogen under the same reaction condition.A methane conversion of 99.8%and a molar fraction of hydrogen of 99.9%could be obtained under the condition of S/C molar ratio of 4,650 ?,1 bar.Increase of pressure was not conducive to methane conversion and hydrogen purity.The reaction temperature or the S/C molar ratio could be improved in order to achieve a better reaction performance under high pressure.The response surface shape of the enhancement factor under different S/C molar ratio was convex,and a maximum enhancement factor of 67.2%could be obtained when S/C molar ratio of 3,temperature of 611 ? and pressure of 5 bar.In order to study the effect of CaO carbonation reaction on the enhancement process,COMSOL Multiphysics software was used to analyze the transient behavior.Concentration and temperature changes along the axial bed in the reaction process were simulated.In addition,the existence and variation of the enhanced reaction front was discovered and it was believed that its formation and migration depended on the change of carbonation reaction.And the enhancement on hydrogen production was mainly concentrated in the enhanced reaction front.The phenomenon of the low temperature region and the moving of high temperature peak were found by analysis of the axial temperature change with time.