Basic Research On Ultra-low Nitrogen Emission Technology For Circulating Fluidized Bed Coal Combustio

As one of the mainstream technologies in the field of power and heating generation in China,circulating fluidized bed（CFB）coal fired boiler has been widely used in industry because of its advantages of wide fuel applicability,wide range of load regulation,low and uniform temperature distribution in furnace,wide fuel particle size distribution and easy to realize low nitrogen combustion.During the burning of coal in circulating fluidized bed boiler,nitrogen oxides（NO_x）will be produced,which will not only promote the generation of photochemical smog,ozone hole,acid rain and other air pollution or harm after being discharged to the atmosphere,but also play a role in promoting the formation of haze as the main precursor of nitrate generation in PM_2.5air pollutant.In recent years,with the large-scale development of CFB boilers and the demand of"deep peak shaving"to absorb renewable power,CFB is facing higher technical challenges in ultra-low nitrogen emission technology.In order to ensure that the NO_xemission of flue gas meets the national ultra-low emission standards of coal-fired boilers in real time during the actual operation process of circulating fluidized bed boiler,usually the flue gas denitrification technology,such as selective catalytic reduction（SCR）and selective non-catalytic reduction（SNCR）are superimposed upon the use of low nitrogen combustion technology in the furnace.Both denitrification technologies use ammonia or urea（ammonia generated after decomposition）as reducing agent to react with NO_xto convert it into N₂.For the realization of NO_xultra-low emissions,ammonia or urea must be used too much,this leads to serious problems such as flue dust collecting equipment corrosion and heat exchanger,which directly affect the system of the boiler thermal efficiency and security,meamwhile,excess ammonia getting into atmosphere（ammonia escape）,not only pollute the environment,also provide ammonia source for formation of secondary particles from SO₂and NO_x,and then promote the generation of condensable particulate matter and PM_2.5emission in the atmosphere.Therefore,the research and development of advanced circulating fluidized bed low-nitrogen combustion technology is an important frontier subject to promote the theory and technology innovation of CFB coal combustion and has very important scientific significance and application value,which could be realized by regulating the fuel characteristics,optimizing the organization way of combustion reaction and conditions,reducing the generation of NO_xconcentration to ultra-low emission levels in the process of coal firing of circulating fluidized bed boiler,and solving problems of NO_xemissions and serial issues caused by it from the source.This thesis takes this as the research objective,selects the coals with the same particle size range as the circulating fluidized bed boiler as the experimental samples,uses the advanced micro fluidized bed experimental device,studies the basic law of nitrogen release,migration and reduction in the circulating fluidized bed boiler under the pyrolysis,gasification and combustion reaction conditions.The main research contents include:combustion characteristics of particles in typical dense phase zone and typical dilute phase zone in CFB boiler;Gaseous nitrogen release characteristics of coal particles with different sizes in pyrolysis,gasification and combustion atmosphere;Reduction characteristics and mechanism of NO by semi-coke size change.The research results provide basic data support for the research and development of ultra-low NO_xemission technology in CFB.Firstly,the combustion characteristics of two typical coal particles with particle size distribution of 1.70～3.35mm and 0.12～0.23mm were tested in the temperature range of790～900℃by using a micro-fluidized bed system.The dynamic combustion characteristics of coal particles were obtained successfully.Two main stages of coarse-particle combustion,volatile combustion and in-situ char combustion,were presented.The kinetic characteristics of typical coarse-particle coal were studied by isothermal method,and the activation energy parameters of combustion were determined.It is found that the volatiles release and combustion are rapid reactions,and the combustion rate of granular coal is controlled by the combustion process of in-situ char.The results of kinetic analysis show that the intrinsic kinetic activation energies corresponding to volatile combustion and in-situ char combustion are 107.2k J/mol and 143.9k J/mol,respectively.Secondly,the release regularity of four major gaseous nitrogen products,HCN,NH₃,NO and NO₂,under pyrolysis,gasification and combustion reaction conditions of bituminous coal and anthracite with three different particle size distributions was studied,and the release path of primary gaseous nitrogen products during fluidized bed combustion was summarized.The results show that the dynamic release sequence and type of volatile nitrogen and coke nitrogen can be detected in real time,and the release amount of HCN and NH₃is mainly affected by the change of pyrolysis,gasification and combustion reaction atmosphere.Gaseous nitrogen of pyrolysis products mainly comes from volatiles.The release of HCN and NH₃in combustion reaction is obviously related to temperature,while the release of various gaseous nitrogen in gasification reaction fluctuates little with temperature.It is found that the release characteristics of NH₃can be used as an important feature to distinguish the release of volatile N from semi-coke N.Finally,on the basis of mastering the combustion characteristics of fluidized bed granular coal and the main gaseous nitrogen release law,the feasibility of effectively reducing NO_xgeneration by controlling the size of coal char was studied.Combined with real-time flue gas analysis,the reduction characteristics and mechanism of NO（the most important component of flue gas NO_x）by semi-coke with five particle sizes were quantitatively studied.It is found that the reduction ability and mechanism of NO are related to the particle size of semi-coke:with the increase of particle size,the dominant time of NO reduction reaction is longer and the reduction ability is stronger.The analysis shows that coarse char has a large reactivity to reduce NO,and a large amount of CO release is beneficial to the reduction of NO.Meanwhile,char has a strong catalytic effect on the reduction of NO by CO.The research shows that the ultra-low emission of NO_xcan be achieved under the condition of efficient combustion by regulating the particle size of semi-coke in the reaction process,rationally organizing the coal combustion process,and enhancing the competitiveness of the semi-coke reduction NO and the semi-coke oxidation reaction.This study result shows that reasonable designed fluidized bed boiler structure,optimize the air distribution in the fluidized bed,science organization combustion temperature field distribution,realize the circulating fluidized bed boiler NO_xoriginal low nitrogen emissions is feasible,the results in deepening the understanding of the mechanism of NO_xformation and inhibition in the process of coal has a certain scientific value.It is also of great significance to the development of new circulating fluidized bed ultra-low nitrogen oxide emission technology.