Stochastic Solution Of Population Balance Modeling And The Research On High-efficiency Removal Of Particulate Matter From Coal Combustion

Inhalable particles or particulate matter (PM or PM10), whose aerodynamic diameters are less than 10μm, have been the serious problem of air pollution. Because of the micromechanism of PM and the complicated physicochemical process of its formation, growth and evolution, little information relating with theoretical model and quantitative description was reported, not only about its sampling and analysis of the physicochemical characteristic, or its formation mechanism and micro-scale dynamic evolution during coal combustion, but also about its nonlinear and nonsteady evolution in natural environment or the imposed external force and condition, or even the design and optimization of high-efficiency capture strategy. On this condition, the integrated framework of the stochastic model of population balance modeling is constructed for the numerical simulation of dynamic evolution of PM from coal combustion, including particle collision, coagulation, breakage, condensation/evaporation, nucleation and deposition. Besides, the stochastic model of population balance modeling is used to simulate numerically the dust-removal process of electrostatic precipitator and wet scrubber, the dry deposition and wet scavenging of particles in the natural environment. Those removal mechanisms in those industrial or natural processes are analyzed quantificationally. Based on this, two techniques with high PM-removal efficiency, i.e., the gravitational wet scrubbers with electrostatic enhancement and the electrostatic-bag hybrid precipitator, are proposed. Therefore, feasibility analysis and operation optimization are taken against the two techniques, which will provide theoretical basis and technical guide for their industrial application. The main points are as following:(1) Stochastic algorithm for zero-dimensional population balance modelingThe concept of"weighted fictitious particle"is instructed and a new multi-Monte Carlo (MMC) method is developed to realize numerically zero-dimensional population balance modeling. The MMC method has characteristics of time-driven Monte Carlo, constant-number and constant-volume method. It has the receivable computational cost in engineering because the fictitious particle population is tracked, whose number is greatly less than that of real particle population; it exhibits the stable and high computation precision because of the constant number of fictitious particles; it also shows the friend expansibility coupling perfectly with two-phase turbulent models due to its constant-volume characteristic and time-driven frame. The MMC method is used to simulate some special cases including initial monodisperse or polydisperse particle population, independent or simultaneous dynamic events, in which complete or partial analytical solutions exist. The good agreement between MMC solutions and the corresponding analytical solutions proves that MMC method has high and stable computational precision and efficiency.For the first time, the accuracy of MC methods has been quantified on basis of standard deviations in calculation of properties of particle size distribution. This approach is be used for accuracy analysis of time-driven direct simulation Monte Carlo, stepwise constant volume, constant number and MMC method. And then the source of numerical error and the contributor of numerical cost are analyzed qualitatively on the basis of quantitative comparison. With respect to the so-called"constant number error"of MMC method, it is improved by some measures including the establishment of the procedure of"differently weighted fictitious particle population"and the corresponding coagulation rule between"differently weighted fictitious particles"; the introduction of the acceptance-rejection method to realize numerically the standard Markov process in the time-driven MC; the development of"constant number scheme"and"stepwise constant number scheme"to restore the number of fictitious particles. The quantitative comparison among some kinds of MCs shows that the improved MMC method has been one of the most high-precision and high-efficiency methods among time-driven MC techniques.With the understanding that event-driven MC exhibits the congenital immunity to"uncoupling error"and high precision and efficiency, a new event-driven constant volume (EDCV) method for zero-dimensional population balance modeling is developed. The method still introduces the concept of"weighted fictitious particle"and conserves the characteristic of constant volume. Furthermore, the following different techniques, the procedure of"equally or differently weighted fictitious particle population", the cumulative probabilities or acceptance-rejection methods,"constant number scheme"or"stepwise constant number scheme"or"resetting scheme", are unified within the one framework. By the comparison of several popular MCs, it's concluded that the EDCV method has come into the MC family of the highest precision and efficiency.(2) Stochastic algorithm for multi-dimensional population balance modelingThe multi-dimensional MMC method for multi-dimensional population balance modeling is founded by means of the perfect coupling of the improved MMC method for the particle dynamic evolution and the Euler-Lagrangian model for two-phase turbulent flows. The method introduces the spatially grid-plotting technique and sets the right time-step. Furthermore, the stochastic process is utilized to judge the occurrence of the dynamic events such as collision/coagulation, to search the collision/coagulation partner, and to establish the collision dynamics. Two standard cases, fine particle flows and heavy particle flows, are chosen to validate the multi-dimensional MMC method for the description of particle collision and coagulation. The simulation results of MMC method are in good agreement with those of direct numerical simulation (DNS), which indicates the proposed multi-dimensional MMC method has constructed a high-efficiency and high-precision numerical platform for four-ways coupling two-phase turbulent models and multi-dimensional population balance modeling. In order to investigate the influence of particle collision on the two-phase flows field, the proposed MC method is used to simulate a swirling gas-particle flows which tack place in an axial symmetry and suddenly expanded pipe. The result shows, the velocity and Reynold stress of particle phase are redistributed and is inclined to isotropy; and turbulence kinetic energy of particle phase and fluctuation velocity correction of particle-gas phase are attenuated.(3) The research on the PM collection mechanisms in traditional dust control units and natural environmentIn order to understand the dynamic mechanism of PM when imposed by external force and condition, the event-driven constant volume method is adopted to simulate the dynamic process of fly ashes in typical electrostatic precipitator and wet scrubber. The simulation results indicate that, the competition between the inertia force and the electric force of fly ash in electrostatic precipitator results in the low collection efficiency of fine particles with size 0.1~1μm; and fine particles with size 0.1~1μm in wet scrubber are scavenged by drops with low efficiency because not only Brownian diffusion mechanism but also inertial impaction mechanism have a weak effect on those intermediate particles. Moreover, low pressure impactor is used to sample locally the fly ashes of the inlet and outlet of electrostatic precipitator in a 300MW coal-fired boiler, and then the collection process is decribed by population balance modeling. The simulation results agree with the experimental results well.In order to describe quantitatively the sedimentation process of PM from the outlet of dust separator, and to learn the self-purification mechanism of nature, the dry deposition and wet removal process of particles are simulated by the multi-Monte Carlo method. The numerical results show that, it's difficult for nature to settle the intermediate particles of about 0.3μm by its self-purification mechanism; the increase of rainfall intensity will benefit the wet scavenging of particles with any size; the smaller geometric mean diameter or geometric standard deviation of raindrops can help scavenge small and intermediate particles better, though large particles are prevented from being collected in some ways.(4) The feasibility analysis and operation optimization of two techniques with high PM-removal efficiencyOwe to the understanding of the dynamic laws of PM in the natural and imposed condition, the hybrid external conditions or forces are adopted to enhance the collection efficiency of PM. Population balance modeling and then feasibility analysis are taken against two PM-removal techniques with high efficiency, saying, the gravitational wet scrubbers with electrostatic enhancement and the electrostatic-bag hybrid precipitator. The simulation results indicate that overall number and mass efficiency reach to 99% in the two techniques. Operation optimization of the gravitational wet scrubbers with electrostatic enhancement shows that, the faster gas velocity or the slower droplet velocity, and the smaller geometric mean diameter or geometric standard deviation of droplets (that is, the finer or evener droplets), and the bigger liquid-to-gas flow ratio or charge-to-mass ratio of droplets, can help remove PM.