Research On Gas-solid Flow And Pollutants Generation Characteristics Of A Coal-gangue-fired Supercritical CFB Boiler

Circulating fluidized bed boiler technology is developing in the direction of large capacity,supercritical and ultra-supercritical parameters.This technology can make comprehensive use of low calorific value coal such as gangue produced by coal preparation in coal industry.This paper takes the 660MW supercritical circulating fluidized bed boiler proposed by a power plant in Shuozhou,Shanxi Province as the research object,the furnace is designed to mix coal with gangue and washing medium coal.Because the gangue-fired 660MW supercritical fluidized bed is rarely used in China,in order to ensure its performance after commissioning,this paper starts with the fuel characteristics,the layout and structure of the key component,and simulates and predicts the characteristics of gas-solid flow and pollutants in the bed,systematic research on several problems brought to the boiler due to burning coal gangue and large-scale.Based on the HSC Chemistry software,the thermodynamic equilibrium calculation of the coal samples mixed with two kinds of gangue,the distribution characteristics of sulfur-containing phases under various working conditions such as different temperature,atmosphere,calcium-sulfur ratio,iron-sulfur ratio and oxygen-carbon ratio were discussed,and the influence of the composition characteristics of ash in gangue on sulfur-fixed products was analyzed.The results showed that the presence of sulfur-containing compounds were significantly different under the oxidation and reducing atmospheres.The former is mostly SO2 and sulfate and the latter is mostly H2S and FeSx.Through the calculation of specific working conditions,it is verified that under the reducing atmosphere,the fundamental reason why the sulfur phase is mainly represented as the copolymer of CaO,Al and Si instead of CaS is that the content of Al and Si in coal samples is high;it is proposed to use O/C ratio to replace excess air coefficient,which is more reasonable for thermodynamic equilibrium calculation.On the 3MW circulating fluidized bed thermal test furnace,industrial tests have been carried out on the design coal and verification coal of 660MW real furnace,and the characteristics of combustion stability,burnout,desulfurization in the furnace,NOx generation and ash formation were mastered,which provided an important basis for the design and operation of the real furnace data.The results show that the coal slagging index Rz of the two tests coals is 0.913 and 0.796 respectively,and the slagging tendency is slight.Successful ignition can be achieved by using under-bed hot flue gas for ignition and putting test coal into the furnace at bed temperature of 435?,and the combustion-supporting fuel can be cut off smoothly at 710?.The fly ash and bottom slag have high carbon content,relatively poor burnout characteristics and poor bursting performance,therefore the coal particle size should be reduced.It is suggested that the median diameter(d50)can be set at 1.1mm and the maximal diameter(dmax)less than 8mm.The ash-slag ratio data obtained under the condition of test bed can be used as a reference for the selection of auxiliary equipment.When the bed temperature of the test coal ?was 840??920?,the self-desulfurization rate was 20.4%?31.6%,and the desulfurization efficiency in the bed was 88%?95%.The limestone is too fine,resulting in a high Ca/S ratio.Attention should be paid to this problem during furnace operation.NOx production concentration is relatively low,and the denitration rate after SNCR operation can reach more than 50%,ensuring that NOx emission is less than 100mg/Nm3.In order to solve the problem of uneven distribution of multiple separators in the large-scale process of CFB boiler,the material deviation of different layout schemes was studied by Euler-Euler method.The design of the different structures of a single separator was optimized to ensure the performance of the separator to deal with the burning out of mixed burned gangue.Chemical reaction dynamics is combined with CFD to discuss the influencing factors in the SNCR process in the separator and the multiple elementary reactions in it.The research shows that shifting the two intermediate separators by 1.5m is the optimal layout based on the initial design scheme,and the material deviation between each separator is the smallest.The composite performance of the separator's fractional efficiency and resistance was evaluated with an indicator Y=?/?,the optimal structural scheme is to offset the inner cylinder by 180°on the basis of the initial design.At this time,the Y value is the largest,Y=3.38.The removal of NO by SNCR process tends to rise first and then fall with the increase of system temperature.The maximum denitrification rate is reached at 950?.At higher temperature,the concentration of active roots such as OH radicals increased,which will promote the oxidation of NH3 and affect the NO Removal.The numerical simulation of the Euler-Euler gas-solid multi-phase combustion process was carried out on a 3MW test bed,a user defined function UDF describing multi-phase combustion was compiled,the developed simulation method was verified by the test results of the test bed.The results show that EDC_G method is more suitable to describe combustion and chemical reaction in CFB boiler bed than ED_FR method.NH3 reaches equilibrium before HCN in the height direction of the furnace.The consumption route of CNO is mainly to generate NO through the reaction of R13 and O2.The reduction of N2O and NO is dominated by heterogeneous reactions.This paper simulated the initial design of 660MW supercritical CFB boiler for gas-solid multiphase flow and combustion,predicted the velocity,particle concentration and temperature distribution in the bed,studied the influence of secondary air design parameters on the mixing and friction problems in the bed,and discussed the influence of different operating parameters on the generation of pollutants.The study shows that the vertical velocity of the solid phase at different cross-sections along the height of the bed presents a "ring-core structure".The temperature level in the bed is around 1100K.The penetrability of the three different jet flow directions is greatly different,and 30°horizontal upward is the best.Increasing excess air coefficient,primary air rate,or reducing the ratio of upper and lower secondary air will reduce the concentration of SO2 at the outlet,and also affect the generation of NO and N2O.