In this paper, an improved engineering model and analysis techniques are presented of the fly ash particles coagulation with droplets in Venturi tubes of industrial scale, based on spatial variation of the collection efficiency of particles on individual droplets, accounting for spray water flow polydispersity and particle size distribution, which allows to define optimal droplets size and spray water flow rate at different modes of thermal power units operation. The model is validated with the published experimental data on the wet Venturi scrubbers operations at the thermal power plants (TPP) in Ukraine, Russia and Kazakhstan. The results of calculations demonstrate good coincidence with experimental data. The influence of the boiler load and spray water consumption on an efficiency of the TPP Venturi wet scrubbers was studied. The simulation results have shown that moderate increment of spray water consumption up to 0.24 L/m 3 allows increasing of the Venturi wet scrubber's efficiency without threat of reaching the dew point in flue gas flow, thus preventing potential corrosion of the power plant equipment downstream. The calculations confirmed that without retrofit of wet scrubbers installed at Ukrainian power plants it will not be possible by these stations to meet European requirements on the allowable levels of emissions from the coal firing power plants.
A mathematical model is proposed for the scavenging process of the dispersed particles by droplets in a wet scrubber under excess spray density in Venturi tube within kinematic approach of the interaction of particles in countercurrent gas-dispersed flows, which refines the existing engineering model, taking into account the spatial size variation of the droplets, due to their coagulation with wet slurry droplets and uncaptured particles entering a wet scrubber from the Venturi tube. The results of calculations with the adopted mathematical model showed that in case of possibility to organize the spraying of a gas flow in a scrubber with 300–500 micron droplets aerosol at a specific spray density of about 1 liter/m3, a 1–2 meters wide layer of droplets ensures effective absorption of both uncaptured PM2.5 solid particles, and the slurry droplets from the Venturi tube. The ejection of the slurry droplets into a wet scrubber from the Venturi tube, and the associated increase in the size of the scrubber droplets due to coagulation with slurry droplets, does not noticeably affect the efficiency of the dusted gas stream cleaning. An adopted mathematical model was applied to calculate the capture of particles by droplets in cylindrical and conical scrubbers. Due to the increase in a residence time of the droplets upon increased velocity of the countercurrent gas flow, the efficiency of gas cleaning from dispersed particles in a conical scrubber appears to be higher than in a cylindrical scrubber. However, with an increase in the spray density above 2 liter/m3 and with droplet diameters greater than 1000 microns, the efficiency of the conical scrubber decreases, which is associated to an increase in the escape of a significant proportion of massive drops to the walls with a reduction in the scrubber reactor cross-section. Bibl. 21, Fig. 4.
The radical increase in the density of spraying the flue gas stream in wet Venturi scrubbers allows to significantly increase the efficiency of these dust precipitators to the level of compliance with the European requirements for dust emissions. Such a shift in the operating mode significantly affects the nature of the processes of heat and mass transfer and has feature sthat are important to consider when reconstructing and designing wet gas cleaning plants. The mathematical modeling of the process of flue gas cleaning from fly ash particles in wet Venturi scrubbers in the conditions of excess spraying is performed, the dynamics of the main thermophysical parameters of the heterogeneous flow in a medium with variable moisture content and the influence of the droplet diameter on the efficiency of wet scrubbers are investigated. The problem of changing hydrodynamic resistance of a wet scrubber at different gas flow spraying densities has been studied; the effect of the input fly ash particles distribution on the result of dust cleaning is estimated. Bibl. 26, Fig. 5, Tab. 1.
The mathematical modeling is performed of the efficiency of flue gas cleaning from fly ash particles of coal-fired thermal power plants, upon installation of a preliminary flue gas cleaning system that consists of a louvered dust concentrator and a battery cyclone, with the recirculation of flue gas from the battery cyclone outlet to the electrostatic precipitator pre-chamber. Based on the available experimental data for the fractional composition of fly ash downstream the boilers of coal-fired TPPs, the size distribution functions were calculated, of fly ash particles at each stage of the preliminary dust-cleaning process, as well as concentrations and modified particle size distributions, to be further used as the input data for designing options and scope of the reconstruction of existing electrostatic precipitators. Bibl. 13, Fig. 3.
The question of influence of the electric wind on efficiency of dispersed particles removal from a flue gas stream in electrostatic filters has not been studied well enough. Estimates are given of the role and influence of electric wind on the particle trapping processes in industrial electrostatic precipitators, as well as the results of experiments and calculations using aplied computational fluid dynamics packages. The results of mathematical modeling of the speed of dust particles of different diameters under the action of electric wind in the inter-electrode space of a corona discharge are presented; and the effects of turbulence of a gas flow on the particles capturing, the influence of the near-wall jet and the probabilistic nature of the removal of solid particles from the dusted exhaust gas flow are evaluated. Ref. 18, Fig. 5.
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