Migration Characteristic Of Alkali Metal Sodium During High-Alkali Coal Combustion

Ash related problems of fouling and slagging are easily occurring during high alkali coal combustion in boilers.Study on the migration and transformation behavior of alkali metals in furnace is helpful to understand the mechanisms of fouling and slagging,and realize the safe and economical utilization of high alkali coal during boiler operation.Based on literature review,this paper carried out experimental study,theoretical model investigation and numerical simulation to study the sodium migration charactistic during CFB combustion and develop an on-line detection technique for gaseous sodium species.The simulation part included the following contents:developing a sodium migration and deposition model,validating in a 30 k W CFB system,proposing a two-dimensional equivalent calculation method,influence parameters analysis,and application in a 300 MW CFB boiler.Experiment study on gaseous sodium measurement were presented.By these researches,it could provide suggestions for the safe,stable and efficient combustion of high alkali coal in CFB system.?1?Firstly,a sodium migration model during CFB combustion was introduced.The model was developed based on the gas-solid hydrodynamics,coal combustion and heat transfer from the three-dimensional integrated numerical boiler model for CFB combustion?Com-CFD-CFB-model?.The sodium migration model consisted several sub-models,including alkali metal release,homogeneous and heterogeneous reactions,alkali vapor condensation,particle deposition and shedding process.By coupling with the Com-CFD-CFB-model,the sodium migration prediction in CFB boilers was realized.?2?For verification,the sodium migration model was applied to calculate the Zhundong coal combustion in a 30 k W CFB test rig.By simulation,reasonable three-dimensional distributions of gas-solid field,temperature profile and combustion products were shown.The sodium distributions in gas,ash and deposits were obtained.By comparing with experimental results,the developed model was validated.What's more,the sodium deposition rate by condensation,inertial impaction and thermophoresis around the deposition probe were displayed and analyzed.?3?This paper proposed a two-dimensional equivalent calculation method based on the three-dimensional sodium migration model,in order to reduce the calculation time and improve the calculation efficiency.Accuracy of the two-dimensional model was validated in the 30 k W CFB test rig,and the calculation time was decreased by 75%when compared with the three-dimensional simulation.Furthermore,the two-dimensional model was used to investigate the influences of operation parameters on sodium transformation and ash deposition.Results show that increasing the furnace temperature will enlarge the gaseous sodium emission rate,as well as the sodium/ash deposition rate.Higher excess air ratio results in higher gaseous sodium emission rate,but reduce the sodium/ash deposition rate.Increasing the secondary air ratio could reduce the gaseous sodium emission rate,but induces higher sodium/ash deposition rate.Lower temperature of the probe surface leads to higher sodium/ash deposition rate.?4?This paper carried out a numerical calculation for the Midong 300 MW CFB boiler,in order to verify the applicability of the sodium migration model in large-scale CFB furnace.By simulation,distributions of Na Cl?g?and Na2SO4?g?in the furnace were displayed.The sodium deposition rates by condensation,inertial impaction and thermophoresis in waterwall and suspended surfaces were shown and analyzed.What's more,the net deposition rate of ash particles in waterwall and suspended surfaces were obtained.Then,tendency of fouling and slagging in different areas of CFB furnace could be predicted and judged.Calculation results of the gas-solid flow,furnace temperature and gaseous species were in agreement with the measured data.?5?This paper also designed a detection probe for on-line measurement of gaseous sodium in flue gas,which was based on the surface ionization?SI?principle.By the calibration experiments,feasibility of this method was verified,and a linear correlation between the detected voltage signals and the vaporizing rates of sodium was obtained.Then,the detection probe was applied to measure the released sodium content in flue gas for the prepared Zhundong coal samples?raw coal,washed coal,and coal with sodium additives?during the heating process.Results show that,for raw coal samples,the sodium release rates are fitted well by a quadratic curve in the temperature range of 335-575?.When the addition ratio reaches 3%of the addition coal samples,the maximum sodium-release rate shifts to low temperature,indicating that the Na-holding capacity of char is saturated.It is found that Cl^-has a greater influence on sodium release when comparing with SO₄^2-.