循环流化床锅炉超低氮氧化物排放理论与实践

循环流化床(CFB)能够燃用低热值燃料,在我国广泛应用。其重要优势是无成本的低氮氧化物(NOx)原始排放,这是缘于CFB较低且均匀的燃烧温度及其固有的燃烧还原性气氛。随着中国燃煤电站污染排放要求日益严格,CFB燃烧原始NOx排放浓度超过了排放限制的最新要求。笔者理论分析了CFB燃烧过程,根据CFB燃烧条件下NOx生成与还原的途径,认为可以通过气固流态的优化调控NOx生成与还原反应,进一步降低NOx的原始排放。进而提出流态优化的工程实现途径:提高床质量、减少粗颗粒床存量、增加循环量。详细讨论了该技术路线的基本原理:床质量提高、粗颗粒床存量减少以及循环量增加,可显著强化燃烧过程中的密相区和稀相区的还原性气氛、减少NOx生成,并在稀相区乃至分离器中加强对生成的NOx的还原,配合合理的床温和风配比,使CFB锅炉在不采用烟气脱硝条件下,实现NOx低于50 mg/m3。该技术设想的关键点经实验室验证后,在150、260和560t/h CFB锅炉上进行了工程实践。运行效果表明,通过流态优化后,NOx排放显著下降,可达到NOx原始超低排放;同时,未见由此导致的燃烧效率显著降低;这些原始超低排放工程案例涵盖了烟煤、贫煤和无烟煤。通过流态设计优化降低NOx排放浓度的技术路线为CFB锅炉NOx控制提供了一条新途径。

Theory and practice of ultra-low NOx emission in circulating fluidized bed boilers

Circulating Fluidized Bed(CFB)boiler,featured by the low quality coal as the fuel,is widely used in China.One of the CFB combustion advantage is its lower NO x emission without cost due to its lower and uniform combustion temperature of the alone circulating loop as well as its unique characteristics that the combustion atmosphere is reduced.With the increasingly strict emission requirements for coal fired power plant in China,the original NO x emission of CFB combustion exceeds the emission limit of the updated National Emission Regulation.Therefore,the CFB combustion processing was theoretically analyzed,according to the NOx generation and reduction paths under the condition of CFB combustion.It is believed that it is possible to decrease the NOx original emission in advance by controling NO x generation and reduction and using the optimized gas-solid two-phase flow pattern.Furthermore,the engineering method to optimize the gas-solid two-phase flow pattern was suggested,which was to improve the bed quality,decrease the course bed inventory and increase the circulating flow rate.The principle of this technology was discussed,including the improvement of bed quality,the decreasing of the course bed inventory and the increasing of the circulating flow rate.It can significantly enhance the reduced atmosphere to decrease NOx generation in both dense zone and dilute phase area and promote the reduction of NO x in freeboard even in cyclone.Besides,the reasonable bed temperature and air supply should be considered.The NOx emission of this technology was expected less than 50 mg/m^ 3 without flue gas denitrification.After the key points were confirmed in laboratory one by one,this technique was proved to be effective in CFB boilers with capacity of 150,260 and 560 t/h.The operation results show that the gas-solid two-phase flow pattern optimization decreases the NO x emission obviously and achieves ultra lower NOx emission.In the same time,no obvious decrease of combustion efficiency is found.These engineering cases of original ultra-low emission include bituminous coal,lean coal and anthracite coal.This technical method of reducing NO x emission concentration through flow design optimization provides a new approach to control the NOx emission in CFB boilers.