Study On The Influence Of Supersaturated Water Vapor Environment And Magnetized Water On The Characteristics Of Fine Particle Growth

The coal combustion is a major energy resource and that fine particles exhausted from coal-fired power plants are one of the main sources of PM_2.5 in China.However,traditional dusting devices are far less efficient in collecting fine particles in the range of 0.1-1?m.Currently,a significant research route to remove PM_2.5 has been put forward to enlarge the sizes of fine particles via physical and chemical process before particles entering into the dust removal devices as the pretreating technical way.Heterogeneous vapor condensation is a simple process and also an efficient method to enlarge particle diameters as a pretreating applied technology.Combined with the characteristics of coal-fired fine particles and flue gas,it is of great importance to study the fine particle growth by vapor condensation.Supersaturation has major effects on particle growth by vapor condensation.In our study,the modified two-dimensional model of heat and mass transfer with variable physical parameters was employed.The influences of physical parameters,sections of growth tube,carrier gases and supersaturation achieving methods on characteristics of supersaturation profiles were studied.The results show that the effect of the temperature on the heat and mass transfer coefficients has great influence on the supersaturation achievement.The higher level of supersaturation can be obtained by heating the gas flow.And the multi-section growth tube is beneficial to offset the decrease of supersaturation in the latter stage of one-section growth tube.Additionally,the efficient supersaturated environment achieved is closely related to the whole length of the growth tube,the sections,temperature differences and achieving methods.For better supersaturation achievement,it is suitable to warm the flow for the smaller Le of carrier gas but cool the flow for larger Le of carrier gas.To obtain more stable and higher level of supersaturation profiles,experimental studies are conducted on the characteristics of particle growth in two-section growth tube?TGT?.The results indicate that the particle enlargement is obviously improved from both methods in TGT.With warming the flow in the extended section,the mean size of final droplets is amplified to 3.2?m when the temperature of first section is 323K.On the same condition,the mean size is enlarged to 3.7?m with cooling the flow in the second section.It is worth noting that when the lower temperature level is presented in the first section,the warmer extended section is beneficial to the improvement of particle growth.Moreover,for the higher temperature of the first section,the facilitated particle enlargement is favored by the cold water in the second section.Because of the flue gas emitted from coal-fired power plants containing different compositions,the thermal and mass diffusivity of water vapor in the multiple components are investigated.Additionally,the effects of different component concentrations on the supersaturation profiles and characteristics of particle growth are also studied.The results demonstrate that the content of CO₂ has the most obvious influence on the heat and mass transfer coefficients.When the volume content of CO₂ from 10%increased to 50%,the thermal diffusivity is decreased by 23%and the mass diffusivity of water vapor is declined by 9%.With the increase of CO₂ content,the flatter supersaturated environment could be achieved and the mean value of supersaturation is improved.In TGT,warming the flow is beneficial to improve the mean diameter of particles when the content of CO₂ improved,but it does not contribute to the particle growth with cooling the flow.To clarify the relationship between the supersaturated environment and particle growth furtherly,the process of heat and mass transfer between the gas and particle phase was analyzed to predict the particle size distribution in the actually supersaturated environment for poly-disperse particles.The simulated results indicate that the effective evaluation of flow temperature around particles has great influence on the particle growth prediction.Meanwhile,at higher level of supersaturation,the deviation between the values of prediction and the experimental results is relatively larger.And the increase of particle concentration results in the remarkable movement to the scope of small size particles.With the method of warming the flow,the enlargement of particle is strongly promoted with the increase of wetted walls temperature.For the effective particle growth in the low level of supersaturation,the way of improving the enlargement of fine particles in the supersaturated environment constructed with magnetized water?MW?has been proposed.The effect of MW on the wettability of coal-fired fine particles and characteristics of particle enlargement was investigated.Furthermore,the value of critical supersaturation and nucleation rate were also calculated on the condition of MW for theoretical analysis.The results demonstrate that the surface tension of the water reduces after treated in the magnetic field.It is also related to the magnetic field strength and the magnetized time.It also reveals that the contact angles with MW decline obviously comparted with non-magnetized water.It indicates that wettability of fine particles is improved on the condition of MW.Additionally,it could be inferred that the particles have a better enlargement at lower level of supersaturation.Meanwhile,on the same condition,the particle concentration has stronger effect on the particle growth with magnetized water.Numerical calculation results demonstrate that the critical supersaturation is obviously lower and nucleation rate is elevated when the water magnetized.That will be beneficial to the nucleation of particles.The surface tension is obviously lower at the same concentration when the wetting agent solution is magnetized.The same decrease of contact angles with magnetized solution is presented.Compared with the non-magnetized solution,the good particle enlargement is presented when the concentration of magnetized solution is relatively lower.