| Calcium looping(CaL)technology is one of the most promising technologies in the field of high temperature CO2 capture.The synthetic nano calcium-based CO2 adsorbent is the application trend of CaL technology because of its high sorption capacity,fast sorption rate and so on.However,the loss of the sorption capacity has become a bottleneck problem for the industrial application of the adsorbent.Therefore,in this paper,commercial nano-CaCO3 powder was used as calcium precursor to prepare nano calcium-based adsorbent.Experiments were carried out to study the CaCO3 and CaO grain growth and the pore structure evolution characteristic of the adsorbent during CaL process.The deactivation mechanism of the adsorbent was put forward including the CaCO3 thermal grain growth mechanism and the dynamic loss of pores mechanism,and modification strategies were proposed referred to the deactivation mechanism.The research content has very important theoretical significance and application reference value for improving the cyclic sorption capacity durability of the adsorbent.The main research contents and results are shown as follows:(1)Based on the experimental results,the grain growth mechanism of adsorbent deactivation was proposed.The effects of "nano-CaCO3 thermal grain growth","nano-CaO thermal grain growth" and "CaO-CaCO3 cyclic chemical transformation" on the reactive grain growth of nano CaO in calcium looping process were innovatively studied by inert atmosphere protection method at 750-850?.The results showed that the activation energy of the grain thermal growth of nano-CaCO3(105kJ/mol)was lower than that of nano-CaO(213kJ/mol),which meant that the main reason for the larger grain size of CaO after regeneration process was the thermal growth of nano-CaCO3 at high temperature.In addition,the chemical transformation process could increase the contact of nano-CaCO3 grain surface,which could increase the thermal grain growth rate of nano-CaCO3 by 19.4%.Therefore,the grain growth mechanism of adsorbent deactivation was the thermal growth of nano-CaCO3 grain during CaL process.(2)Based on the experimental results,the pore structure evolution mechanism of adsorbent deactivation was proposed.The pore volume change of the nano-CaO adsorbent at 1-100nm was measured during 10 CaL cycles.The results showed that the mesoporous volume at 2-50nm decreased obviously.Therefore,the pore structure evolution mechanism of adsorbent deactivation was the dynamic loss of mesopores during CaL process,which could be called the deactivation mechanism of dynamic loss of pores.(3)According to the deactivation mechanism of dynamic loss of pores,the stability of the initial adsorbent skeleton was improved by skeleton hardening to improve the sorption durability.The adsorbent skeleton was hardened by heat-pretreatment method at 800?-930?,and then self-reactivation was triggered to recover the sorption capacity.The influence of composition and preparation process on the skeleton hardening effect of adsorbents was systematically explored.The results showed that the solid-phase reaction between TiO2 dopant and CaO could strengthen the skeleton hardening effect of adsorbent,and the durability of the adsorbent doping TiO2 is 20.0%higher than that of the adsorbent doping MgO during 20 CaL cycles.Compared with the sol mixing preparation method,the adsorption phase reaction preparation method with better micro-dispersity could increase the initial carbonation conversion by 21.3%.There were optimal values of the dopant content and the heat-pretreatment condition(molar ratio of Ca to Ti was 16,heat-pretreatment temperature and time were 900? and 1 h),and the effect of skeleton hardening would become worse if the optimal value was deviated.The deactivation mathematical model of carbonation conversion-cycle number was established.In this model,the definition of overall average deactivation rate kt and the overall stable carbonation conversion Xr-t were the average attenuation rate of the overall carbonation conversion and the stable carbonation conversion through infinite cycles integrated the fast and slow carbonation segment of the adsorbent,which could be used to evaluate the cyclic sorption durability and the stable sorption capacity of adsorbents,respectively.For nano CaO/CaTiO3 adsorbent with the best skeleton hardening effect,at carbonation temperature of 600?,kt value was 0.206 and Xr-t value was 0.115,at carbonation temperature of 700?,kt value was 0.383 and Xr-t value was 0.503.(4)According to the deactivation mechanism of dynamic loss of pores,in situ pore-formation by chemical looping reaction was proposed innovatively to compensate lost pores dynamically and improve the cyclic sorption capacity durability.Transition metal oxides were introduced as pore-formation agents.The dynamic pore-formation process was realized by chemical looping reaction of H2 reduction and CO2 oxidation.The influence of the doping content and the type of agent on the dynamic pore compensation effect were systematically explored.The results showed that the dynamic pore compensation effect of the agent was affected by both vapor releasing amount and anti-sintering ability.The sintering of the agent would be intensified by the excessive doping content or poor thermal stability of the agent,which would inhibit the effect of dynamic pore compensation.NiO was the best pore-formation agent.NiO with 21.3wt.%doping content owned the best pore-formation ability(increasing the specific surface area by 18.8m2/g)and pore-formation stability(remaining 62.8%conversion for reduction of NiO by H2 after 15 cycles).For nano CaO-NiO/Al2O3 adsorbent with in situ pore-formation,at carbonation temperature of 600?,kt value was 0.332 and Xr-t value was 0.516,at carbonation temperature of 700?,kt value was 0.309 and Xr-t value was 0.612.Comparing two modification methods for dynamic loss of pores,the in situ pore-formation method by chemical looping was better than the skeleton hardening method for improving the durability of the adsorbent.(5)The decrease of ambient temperature could reduce the thermal grain growth rate.According to the deactivation mechanism of the thermal growth of nano-CaCO3 grain,it was innovatively proposed to enhance the regeneration rate by improving the thermal conductivity of the adsorbent,so as to decrease the regeneration temperature,slow down sintering degree during regeneration and improve the sorption durability.The results showed that doping 15.4wt%BeO could increase the regeneration endothermic rate by 12.3%,increase the regeneration reaction rate by 10.1%and reduce the regeneration operating temperature by 50?.For nano CaO-BeO/Al2O3 adsorbent with heat conduction enhanced regeneration,at carbonation temperature of 600?,kt value was 0.505 and Xr-t value was 0.362,at carbonation temperature of 700?,kt value was 0.323 and Xr-t value was 0.513.(6)According to the deactivation mechanism of the thermal growth of nano-CaCO3 grain,it was innovatively proposed to in situ remove regeneration product CO2 by introducing H2 reduction reaction to enhance the regeneration rate,decrease the regeneration temperature,so as to slow down sintering degree during regeneration and improve the sorption durability.The results showed that the 50vol.%H2 could convert nearly half of CO2 produced by CaCO3 decomposition,which would make regeneration rate to be nearly doubled,and the regeneration operating temperature was reduced by 70?.For nano CaO/Al2O3 adsorbent with reactive enhanced regeneration,at carbonation temperature of 600?,kt value was 0.481 and Xr-t value was 0.416,at carbonation temperature of 700?,kt value was 0.322 and Xr-t value was 0.534.Compared with the heat conduction enhanced regeneration method,the reactive enhanced regeneration method reduced the regeneration temperature by 20? more,leading to better cyclic durability improvement.The research results showed that compared with decreasing regeneration temperature,the effect of dynamic pore compensation on the cyclic durability improvement of nano calcium-based adsorbent was more significant.What's more,bi-enhanced regeneration process coupling two modified strategies of in situ pore-formation by chemical looping and reactive enhanced regeneration could inhibit the thermal grain growth and dynamic loss of pores together,leading to the best improvement effect of cyclic sorption durability of the adsorbent.The minimum kt value of 0.281 and the maximum Xr-t value of 0.693 could be obtained at carbonation temperature of 700?. |
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