| The oxy-fuel technology in the CFB is a type of clean coal combustion technologywhich is recently developed and has the advantages of both CFB and the oxy-fueltechnology. But the oxy-fuel combustion process at the end of the heating surface of theCFB may produce more ash deposition than the air combustion process. And the morelower quality of the coal, the more severe situation of the ash deposition. The ashdeposition does great harm to not only the safe operation of the boiler and the increasingeconomic costs, but also to the reducing heat transfer and thermal efficiency of theboiler. How to reduce the effects of the ash deposition problems on the operation of theboiler is the research focus which the experts and scholars at home and abroad paidattention to.In order to do some research on the ash deposition under the oxy-fuel combustion atthe end of the CFBB, we firstly start from the fly ash deposition characteristics andfocus on the influence of operation parameters on the fly ash deposition and then the ashdeposition model is set up, including the fly ash particles transport model, the fly ashcollision rate model, fly ash adhesion rate model and the growth model of the fly ashparticles. Then carried on the preliminary exploration and research on the ash depositionprocess, and the research work mainly includes the experiment research and numericalcalculation.Experimental research. Firstly, the fly ash deposition test bench is established, whichis used to simulate the tail flue of the boiler. And then one sampling probe which hasbeen patented is used to simulate the heating surface of the tail flue of the boiler byadjusting the probe surface temperature to make it closer to the working conditions ofthe superheater, rehearter and economizer in the tail flue. The test work is mainlyincluding the measurement of the temperature field, the research on the heat transfercharacteristics on the probe surface and the influence of the probe diameter and the fluegas flow rate on the ash deposition, which make a preparation for the later numericalcalculation.Numerical calculations. Fly ash deposition process is very complex. In order tofacilitate the calculation, the paper simplifies the deposition process, and only do someresearch work on the transport process, collision process, the adhesion process and thedeposition process of the fly ash particles. And then the fluent software is used to makea numerical simulation of the ash deposition under the oxy-fuel combustion in CFB. Wefirstly determine the parameters for the numerical calculation, set the initial conditionsand boundary conditions and choose the standard κ-ε turbulence models for steady-state simulation of continuous phase flow field. And then we select the DPM model to makea coupling calculation between the discrete phase and continuous phase. We last use theuser-defined function (UDF) to make the programming for the models to do someresearch on the ash deposition. The main content of the work is to study the influence ofdifferent particle sizes, different flue gas flow rate, different probe diameter anddifferent first and second wind flow rate on the ash deposition. Research results showthat the collision rate is positively correlated with the particle sizes, the adhesion rate isshowing a negative correlation relationship, and the capture rate commonly presentspositive correlation. Fly ash deposition rate and deposition amount are also positivelyrelated with the particle size. The collision efficiency is positively related with the fluegas flow rate, and the adhesion efficiency and capture efficiency have a negativelycorrelation with it. Fly ash deposition rate is positively related with the flue gas flowrate, and deposition amount is a negative correlation relationship. The influence ofdifferent probe diameters on the fly ash deposition process is mainly manifested in thecollision efficiency, adhesion efficiency and capture efficiency into the positivecorrelation relationship with the probe diameter. The fly ash deposition amount anddeposition rate are positively related with the probe diameter. The fly ash depositionrate and the collision efficiency are positively correlated with the first and second windspeed, while the deposition amount, the adhesion efficiency and capture efficiencypresent a negative correlation with them.The comparison results between the numerical calculation and experiment. Numericalsimulation of temperature field is very similar with the actual measured result, whichmeets the requirements. Under the different smoke gas and probe diameter, we carriedout a comparative analysis about the particle deposition amount and relative depositionrate between the numerical calculation and test results, and found that there is bigdifference between the test results and numerical calculation. The most important reasonis that the results of numerical calculation is based on the2-d model, and the actualtesting process is based on3-d actual model, and this makes both can’t make acorresponding accurate comparison. But due to the same changing trend, the test resultsstill can be used to judge the trend of the changing of the numerical results, and thisprovides a validation criteria for numerical calculation. |
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