Characteristics Of Coal-fired PM_2.5 Nucleation And Growth Under Supersaturated Water Vapor

PM_2.5,which is emitted from coal combustion,is one of the major source of air particle pollution,however,the traditional dust removal equipment is far less efficient for these particles collection,especially for the fine particles range from 0.1-1?m.A great amount of coal-fired fine particles enter into ambient air which not only makes great loss for national economic development but also causes damage for human health.Therefore,it has become an urgent problem to collect the coal-fired fine particles with high efficiency.Vapor condensation is one of the most promising pretreatments for particle enlargement and sub sequent removal,especially for the high temperature and humidity of the flue gas,hence,the study of the characteristics of nucleation and condensational growth of coal-fired fine particles has important applied value.Firstly,this paper have investigated the chemical compositions and the wettability of coal-fired fine particles,then the relationship between wettability and typical chemical compositions of coal-fired fine particles was discussed,also,the effect of carbon residue on particle wettability was analyzed.In order to improve the wettability of coal-fired fine particles,surfactant Tween 20 was applied and the contact angle was reduced successfully.The results showed that the worse wettability of initial particles,the higher wettability improvement.In addition,the wettability of different kinds of particles presented discrepancy with the increase of surfactant addition.For the sake of direct visualization of the process of vapor nucleation and condensation on fine particle's surface,the superhydrophobicity/particle was made to realize the spatial control in the vapor nucleation and condensation on fine particle's surface.Vapor condensation on single a few particles' surface were experimentally studied,and the nucleation model of vapor nucleation on particle's surface was confirmed.The results showed that the vapor condensation on single particle's surface was seem like that was proposed by Fletcher,the embryo appeared at some point of particle's surface randomly;nevertheless,the embryo was always appeared at the interface of particles when vapor condensation on several particles' surface;the fine particle nucleation model was identified preliminarily.In order to avoid using the uncertain kinetic constant,Kc,growth kinetics of droplet formed on single coal-fired fine particle was studied based on these two mechanisms:direct addition of water molecules from the vapor and surface diffusion of adsorbed water molecules on the particle.The results showed that particle diameter,vapor supersaturation,temperature,droplet radius and particle's surface wettability had effect on droplet growth,the growth rate of droplet formed on different coal-fired fine particles' surface was predicted with this kinetic model.The experimental platform was set up for direct measurement of particle size after its growth,the characteristics and the effects on three coal-fired fine particles and a biomass-fired fine particles growth under supersaturated water vapor were researched.The results showed that initial particle size and wettability had an effect on particle growth,supersaturation was the most important effect on its growth,particle number increase would aggravate the vapor competition among particles which would lead to a decline of supersaturation level in the growth tube,and then had a negative effect on the particle enlargement;however,residence time increase had a positive effect on its growth.The average diameter of the fine particles emitted from power plant boiler could be enlarged above 2 ?n when the maximum supersaturation reached 1.324 in the growth tube.The surfactant was added into the hot water and it could highly improve the particle enlargement,especially for the small range particles,the wettability and initial particle size had an effect on particle enlargement,the larger of initial particle size and the better of wettability,the better of enlargement performance.The average diameter of particles after growth increased with the increase of surfactant addition,however,for the particles with great amount small particles,the increase of wet agent addition could increase the final average diameter,but decrease the enlargement performance of small particles.A new technique of gas circulation coupled with vapor condensation was proposed,and the effect on particle enlargement of this technique was experimentally studied.Comparing with the particle enlargement in the original growth tube,this technique could highly improve the particle enlargement even under low supersaturated environment.The circulation point,proportion,supersaturation and particle concentration had an effect on this technique,under the optimum condition,the average diameter of particles could increase from 2.171 ?m in origin growth tube to 6.156 ?m with initial diameter at 0.130?m.