煤化学链燃烧载氧体研究进展

Recent advances in oxygen carriers for chemical looping combustion of coal

煤的化学链燃烧是清洁煤燃烧的重要技术之一。化学链中载氧体的使用可以避免煤和空气直接接触,从而避免氮氧化物等污染物的产生并提高能量转化效率。一般来说,煤的化学链燃烧有2种反应途径:煤气化化学链燃烧和氧解耦化学链燃烧;不同反应途径将极大影响载氧体组分以及结构设计。详细论述了2015-2020年煤化学链燃烧中固态金属载氧体的研究进展,包括铁基、锰基、铜基、镍基、硫酸钙以及其他复合金属载氧体。总结了不同金属载氧体的优缺点、反应路径、气-固和固-固反应机理、金属与载体的相互作用以及载氧体失活原理。铁基载氧体被广泛应用于气化化学链燃烧中,但单一铁基载氧体的反应速率较低。适量添加碱金属或碱土金属可以提升载氧体的反应活性。锰基载氧体在化学链燃烧中具有两面性:一方面可以在高温缺氧气氛中释放气态氧,另一方面也可以与还原性气体发生气-固反应。通过使用惰性载体以及碱金属添加剂可以提高锰基载氧体的机械强度和氧解耦能力。含铜载氧体具有出色的氧解耦能力和反应活性而被广泛关注,然而铜及其氧化物低熔点所带来的金属聚集导致载氧体的失活问题亟需克服。研究发现使用铁、锰和铜矿石制得的载氧体具有良好的反应性能。硫酸钙载氧体具有较好的反应活性,但煤的化学链燃烧时潜在的二氧化硫和硫化氢副产物需要引起重视。镍基载氧体虽然在煤的化学链燃烧中反应性能较好,但硫毒化、成本较高和环保性能不佳等缺点导致近年来镍基载氧体的研究较少。新型双金属或多金属载氧体可以同时结合2种金属的反应特性,从而显著提高载氧体的整体反应活性。基于载氧体的研究现状,对未来的发展方向提出了4点建议:结合2种煤的化学链燃烧机理设计新型氧解耦辅助化学链燃烧载氧体;发展新型材料和金属组分的载氧体;利用冶金工业废料制得载氧体;开发新型结构的载氧体。

Chemical looping combustion of coal is one of the most vital technologies for clean coal combustion.A metal-based oxygen carri⁃er in chemical looping process is applied to avoid direct contact between the coal and air,which can eliminate the production NOx pollu⁃tants and improve energy conversion efficiency.In general,there are two reaction pathways for chemical looping combustion of coal:in si⁃tu gasification chemical looping combustion(iG-CLC)and chemical looping with oxygen uncoupling(CLOU).The difference in combustion pathway will greatly affect the composition and structure design of oxygen carriers.The research progress of solid metal-based oxygen carriers in coal chemical looping combustion from 2015 to 2020 was discussed in detail,including iron-based,manganesebased,copper-based,nickel-based,calcium sulfate,and other composite oxygen carriers.The advantages and disadvantages of differ⁃ent metal oxygen carriers,reaction path,gas-solid and solid-solid reaction mechanism,interaction between metal and carrier and deactiva⁃tion principle of oxygen carrier were summarized.Iron-based oxygen carriers are widely applied in iG-CLC system,but the reaction rate of single iron-based oxygen carrier is low.The addition of alkali and/or alkaline-earth metals can improve the reactivity of the iron-based oxygen carriers.on the one hand,manganese-based oxygen carrier can release gaseous oxygen in high temperature and anoxic atmosphere;and on the other hand,the manganese-based oxygen carrier can oxidize reducing gas such as CO and CH4.By applying inert support mate⁃rials and alkali metal promoters,the mechanic strength and oxygen decoupling ability can be further boosted.Copper-based oxygen carrier has attracted much attention due to good oxygen uncoupling behavior.However,the deactivation of oxygen carriers due to metal aggrega⁃tion caused by low melting point of copper and its oxides needs to be overcome.Besides,several research works observed that using iron,manganese and/or copper ore as oxygen carriers can improve the reactivity.The reactivity of calcium sulfate oxygen carrier is excel⁃lent,but the possible side-products in coal chemical looping combustion such as sulfur dioxide and hydrogen sulfide need to be addressed.Nickel-based oxygen carrier has good reaction performance in chemical looping combustion of coal,but there are only a few recent studies because of the disadvantages of nickel-based oxygen carrier,such as sulfur poisoning,comparably higher cost,and environmental-un⁃friendly.Novel bimetallic and multi-metallic oxygen carriers can combine the reaction characteristics of the two metals at the same time,thus significantly improving the overall reactivity of the oxygen carriers.Based on the recent advances of oxygen carrier research,this re⁃view suggests four possible areas to explore in the future:1,combining iG-CLC and CLOU pathways to design novel chemical looping as⁃sisted by oxygen uncoupling(CLOU)carriers;2,developing oxygen carriers with novel support material and metal composition;3,utili⁃zing metallurgical waste as raw material to synthesize oxygen carriers;4,designing oxygen carriers with special structures,including coreshell and skeleton structures.