Research On Control Mechanism Of Char-Nitrogen Conversion For Layer-Burning Boilers

In China,layer-burning boilers are important equipment for industrial steam and winter heating.The Clean Heating Plan for Winter in North China（2017-2021）which promulgated in 2017 clearly stipulated that the super-low emission of NO_x for the heating boilers will be achieved in 2021.Therefore,how to control the NO_x emission from layer-burning boilers,improve the economy and system stability of super-low emission of NO_x has become a hot issue.Grate-fired process has a remarkable partition combustion characteristics,and there are three typical control processes:chemical power,transition,and diffusion.How to improve combustion efficiency and reduce NO_x emission of layer-burning boilers,in the condition of wide screening coal,is the difficulty of the problem,and also the key to technological breakthrough.For this reason,the technical routes of combustion optimization,local flue gas recirculation and nitriding char reburning are put forward.The key problems of each unit technology and a“low NO_x combustion technology for industrial boilers”are studied as follows:Firstly,in the small-scale one-dimensional fixed-bed system,pyrolysis char of Shenhua bituminous coal was taken as the research object,and the char was mixed with quartz sand.By changing the char/O₂ ratio,the trend of oxidation layer and reduction layer during the char grate-fired process was analyzed.The migration path and transformation rule of char nitrogen were studied.The surface temperature of windward（the oxidation layer）and leeward（the reduction layer）of char layer was measured on-line by infrared thermal imager.The results show that,the complete oxidation layer determined the stratification of char layer,the reduction layer determined NO emission and the transformation of char nitrogen.The heat transfer of char layer and the gasification reaction of char-CO₂ determined the variation trend of surface temperature of oxidation layer and reduction layer.Secondly,the secondary gasification of pyrolysis char of Shenhua bituminous coal was carried out in a horizontal tube furnace test system（gasification medium was CO₂）,and the gasified char with different conversion rates in diffusion control zone was obtained.The effect of gasification reaction in the reduction layer on char nitrogen conversion and char combustion were studied.By correlating the pore structure and graphitization degree of char with the reactivity of char-NO,the intrinsic relationship between the conversion of char nitrogen and the physicochemical properties of char was clarified.The rules of NO reduction by gasified char and the conversion from char nitrogen to NO were obtained.The effect of CO₂ gasification on the reaction characteristic parameters of char were also investigated.It is found that,the pore structure and graphitization degree of char can be improved by CO₂ gasification;the reduction ability of NO can be enhanced in inert atmosphere,and the conversion rate of nitrogen can be increased under combustion conditions;the long-term CO₂ gasification was not conducive to the stable combustion of char.Thirdly,aiming at the characteristics of low O₂ concentration and high O₂ flux of primary air in the condition of local flue gas recirculation,an experimental method was proposed,in which the O₂ flux was taken as the key factor.The effects of the O₂ flux,concentration of CO,CO₂ and recycled NO on the oxidation-reduction layering and char nitrogen conversion during char grate-fired process were studied.Based on the mass ratio of oxidation layer and reduction layer,the relationship between the promotion and inhibition of NO emission by different components of flue gas during char grate-fired process was compared and analyzed.The results show that,the increase of O₂ flux can reduce the proportion of reduction layer and promote the NO emission;the increase of CO and CO₂ concentration can reduce the NO emission of the oxidation layer,but has little effect on the whole char layer;the existence of recycled NO can effectively reduce the NO emission.Then,according to the characteristics of ash layer on the surface of large particle char during grate-fired process,the effect of ash layer growth on the combustion process of two kinds of high ash pyrolysis char（Neimenggu char and Shuozhou char）was investigated by one dimensional combustion.The trend of combustion conditions of char after ash layer peeling was deduced.By studying the char nitrogen conversion of high ash chars under different combustion conditions,the effect of ash layer peeling on NO emission was obtained.The results show that,the effect of ash layer peeling can be equivalent to the increase of reaction temperature,O₂ concentration and NO concentration;the effect of reaction temperature and O₂ concentration on conversion rate of char nitrogen for different chars was not consistent,and the effect of external NO on char-NO emission was significant.Then,an industrial test was carried out on a 29MW chain boiler with advanced air distribution.The effects of local flue gas recirculation on boiler load and NO emission were studied.The results show that,a proper amount of flue gas in NO.2 air chamber can increase the boiler load by about 3MW,reduce NO emission by 26.96%.Finally,by means of urea water solution immersion drying,and following inert atmosphere high temperature treatment in the horizontal tube furnace test system,through adjusting urea mass and high temperature treatment time,the pyrolysis char of Shenhua bituminous coal was modified by urea nitriding.The content of nitrogen element in char,the surface/interior enrichment of nitrogen element,and the evolution of pore structure of char were analyzed.The mechanism of urea nitriding modification on NO reduction by char under different reaction conditions was studied.The results show that,all nitriding chars had higher NO reduction ability than original coke in inert atmosphere.The difference of NO reduction ability of nitriding chars was mainly due to the coupling effect of three factors,namely,the increase of C（N）,the surface enrichment of N,and the blockage of pore structure.