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Numerical Simulation of Atomization and Evaporation Processes in Multi-Spouted FGD Reactor

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Tutor: GaoJiHui
School: Harbin Institute of Technology
Course: Thermal Power Engineering
Keywords: evaporation characteristics,spray drying,flue gas desulfurization,droplet collis
CLC: X701.3
Type: Master's thesis
Year:  2008
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Abstract:
Water fed into the reactor is one common process of the semi-dry FGD technique. For the typical semi-dry, slurry atomization is the way to feed water in to reactor, then reacting with the flue gas. SO2 is solidified in the slurry droplet in the form of sulfate. According to recent researches, SO2 is mainly removed under the condition that droplet surface contains sufficient liquid water. The content of liquid water is affected by droplet evaporation. Thus, controlling slurry droplet evaporation is of significance for steady operation and optimizing FGD technique.The thesis numerically simulated the spray drying processes of the slurry in MS-FGD(Multi-Spouted Flue Gas Desulfuriztion) reactor, employing the CFD (Computational Fluid Dynamics) code FLUENT. The spray model and its submodel were analyzed at length. Some factors relating with the model were found and studied. The characteristics of the nozzle, distribution characteristics of liquid, temperature and humidity of gas were got under different inlet conditions.In order to simulate the spray and evaporation processes accurately, the spray model and droplet impinging wall mode were firstly studied. The results show that the simulation results of droplet collision model are influenced by the calculation conditions, such as grid size, time step, number of particle stream and initial droplet diameter. Considering the conditions of computer hardware and software complexly, choosing the appropriate calculation parameters can raise the simulation precision. The droplet impinging wall models offered by FLUENT could not describe the droplet impinging wall phenomenon in tower. Basing on Watchters and C.Weiss’s experimental results, the model was modified, and the Weber Number was used to determine the action of droplet impinging wall. The Weber Number of droplets adjoining the wall is lower than 80, and the action of impinging droplets is sticking on the wall.Characteristics of pressure nozzle were influenced by atomization pressure, nozzle diameter, liquid Viscosity and surface tension. The dimension analysis methods were adopted to establish the dimensionless relations between droplet Sauter diameter of nozzle and the factors above. The unknown parameters of the relationship function were solved by the least squares. The simulation results show the result of the Non-dimensional relationship has a good consistency with numerical simulation result, and the max. relative error is 12%. Through the researches of the nozzle characteristics, it is useful for us to controlling the nozzle diameter in below.On the basis of research above, the relative humidity and evaporation rates in the first-class absorber were studied for different flow conditions, such as flue gas temperature, inlet flue gas flow rate, atomization pressure and inlet flue gas relative humidity. The results indicate that with the increase of flue gas relative humidity of inlet, liquid concentration increases by correspondingly in the first-class absorber. Flue gas temperature, flue gas flow rate and atomization pressure speed up the evaporation rate and lower liquid concentration. Flue gas flow rate and atomization pressure have a great influence on the expansion of droplet profile of nozzle.
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