失活石灰石自活化增强循环捕集CO_(2)特性

Self-reactivation of spent limestone to enhance its CO_(2) capture capacity in multiple cycles

钙循环工艺是一种低成本高效率捕集CO_(2)技术,运行过程需不断补充新鲜吸收剂并排出失活吸收剂,实现失活钙基吸收剂原位资源化利用具有重要意义。为研究颗粒状石灰石失活后自活化特性,运用双固定床反应器制备了失活石灰石,分析了自活化后石灰石碳酸化转化率随循环次数的变化规律,采用XRD、SEM、N_(2)吸附等分析测试手段探究了自活化提高失活石灰石循环捕集CO_(2)性能机理。结果表明,失活石灰石置于环境中可吸收空气中水分生成Ca(OH)_(2),吸水率φ达100%后,继续吸水生成氢氧化钙水合物,极限吸水率为130%。不同程度自活化后的石灰石循环捕集CO_(2)性能均有不同程度提高,随吸水率变化呈线性升高趋势。与分析纯CaCO3相比,失活石灰石对吸水率变化更敏感,随吸水率升高其循环捕集CO_(2)性能提高更快。吸水率为130%时,自活化后石灰石循环捕集CO_(2)性能甚至优于新鲜石灰石。微观结构分析结果显示:新鲜石灰石因高温烧结而失活过程中,CaO晶粒尺寸由41.9 nm长大至72.2 nm,孔隙结构发生坍塌阻塞,比孔容和比表面积显著降低。经过自活化,煅烧后的石灰石中CaO晶粒尺寸降低,原本密实的表面重新生成孔隙结构;吸水率为130%时,晶粒尺寸降至35.1 nm,比孔容和比表面积分别恢复至新鲜石灰石的70.5%和107.6%,特别是10~100 nm孔隙得以再生,因此失活石灰石循环捕集CO_(2)性能恢复。虽然自活化过程会加剧失活石灰石颗粒磨损速率,但吸水率100%的自活化石灰石磨损导致直径每小时减小量仅为颗粒直径的0.55%。综上所述,自活化后的失活石灰石完全可替代新鲜石灰石,作为补充钙基吸收剂用于钙循环捕集CO_(2)。

Calcium looping process is a kind of cost-effective and high-efficiency CO_(2) capture technology.A certain amount of fresh calcium based sorbent should be supplemented to the process and corresponding amount of spent sorbent is discharged.It is of significant to realize in situ reutilization of such a quantity of spent calcium based sorbent.A dual-fixed bed reactor was employed to prepare the spent limestone and investigate the cyclic carbonation conversions of spent limestone after self-reactivation.XRD,SEM and N_(2) adsorption analysis were employed to discuss the mechanism that self-reactivation improves the CO_(2) capture capacity of spent limestone.The results show that when placed in a constant humidity environment,spent limestone can absorb the H_(2)O in the air to form Ca(OH)_(2) and go on to form calcium hydroxide hydrate when the value of water absorptionφis 100%.The limit value forφis 130%.CO_(2) capture capacity of the spent limestone is dramatically enhanced after self-reactivation,which is proportional toφ.Compared with CaCO 3,the CO_(2) capture capacity of spent limestone is much more sensitive toφ.Whenφis 130%,the CO_(2) capture capacity of the spent limestone after self-reactivation is even higher than that of the fresh limestone.The microstructure analysis results show that due to the sintering occurred under high temperature calcination,the CaO grain size in calcined limestone increases from 41.9 nm to 72.2 nm.The micro-pores are blocked.The pore volume and BET surface area decrease dramatically.After self-reactivation,the CaO grain size in the calcined limestone obviously decreases,and plenty of pores are regenerated on the surface.Whenφis 130%,the CaO grain size decreases to 35.1 nm,the pore volume and BET surface area are recovered to 70.5%and 107.6%those of the calcined fresh limestone,especially the pore volume that distributed in 10-100 nm is regenerated dramatically.Therefore,the CO_(2) capture capacity of spent limestone can be enhanced by self-reactivation.The self-reactivation process can accelerate the attrition rate of spent limestone.However,even whenφis 100%,the diameter of the sorbent particle only decreases by 0.55%per hour.To sum up,the spent limestone after self-reactivation can sufficiently be used as supplementary sorbent instead of fresh limestone in calcium looping process for CO_(2) capture.