钙基脱硫剂对循环流化床NO_x排放影响研究进展

Research progress of the effects of Ca-based sorbents on the NO_x reaction in circulating fluidized bed boilers

近年来提出的燃煤电厂大气污染物超低排放标准对CFB锅炉的污染物治理提出了更高的要求。为解耦CFB锅炉高效脱硫和低NO_x排放之间的矛盾,需要更加深入了解钙基脱硫剂对含氮反应的影响。目前普遍认为,石灰石脱硫和NO_x排放2个过程,通过各含钙化合物相互转化和对含氮反应选择性催化构成一个整体,而影响其中任一环节的因素都可能使最终NO_x排放量发生变化。笔者围绕CFB燃烧条件下NO_x基本反应,各含钙化合物间相互转化规律和表面选择性含氮催化反应性,以及烟气组分、温度和配风等影响因素,阐述了钙基脱硫的影响。通常,投放钙基脱硫剂会导致NO_x排放量升高,N_2O排放量降低,这是多方面因素共同作用的结果,如CaO催化挥发分氮氧化生成NO、促进CO等还原性气体氧化消耗、促进N_2O分解等。而脱硫产物CaSO_4及CaS等也对NO还原表现出明显的催化活性,实践发现基于钙基脱硫和喷氨脱硝的同步脱硫脱硝技术是可行的。未来还需进一步定量研究石灰石脱硫与NO_x排放之间的相互作用规律,并在此基础上开发一体化脱硫脱硝技术,以期实现CFB锅炉NO_x和SO_2的双超低排放。

Circulating fluidized bed( CFB) boilers face sever challenges due to the ultra-low emission rules proposed in recent years in China.In order to decouple the contradiction between high efficiency desulfurization and de-NOx combustion,it is necessary to deeply understand the impacts of Ca-based sorbents on nitrogen reaction.Most research shows that the limestone desulfurization and NOx emission are closely related due to the mutual conversion among different calcium compounds and the selective catalytic relativity to several nitrogen reactions.Any affecting factor of these processes may also have significant effects on the NOx emission.The basic nitrogen reactions under CFB combustion conditions,the mutual conversion rules of calcium compounds,the nitrogenous selective catalytic reactions,and some other factors of these processes such as flue gas composition,temperature,air distribution,etc.were reviewed,thus the influence of calcium-based desulfurization on NOx emission were thoroughly discussed.Normally,the use of limestone leads to higher NOx emission and lower N2 O emission.It can be explained by many factors.For example,the Ca O particles can obviously catalyze the oxidation of volatile nitrogen to NO,the oxidation of CO which weakens the reducing atmosphere,and the decomposition of N2 O.Nevertheless,some calcium compounds can also significantly catalyze the NO reduction,such as Ca SO4 and Ca S.It is proved in practice that the simultaneous desulfurization and denitrification( SDD) based on limestone and SNCR or SCR is effective in CFB boilers.It is necessary to further investigate the interaction between limestone desulfurization and NOx emission quantitatively in the future,and further develop the new generation of SDD technology,so that the simultaneous ultra-low NOx -SO2 emission for CFB boilers can be realized.