Study On Sub-micron Particle Formation And Agglomeration Mechanism From Coal Combustion

Plenty of sub-micron particles emitted from coal-fired boiler are enriched with many trace elements, such as Hg, As, Se, Pb, Cd, and Cr, which would do great harm to environment and mankind health。The serious pollutant problem brings the maximum attention of many science researchers. However little information about the formation mechanism of sub-micron particles and its controlling during the combustion methods was reported because of the complexity of combustion course, microcosmic of sub-micron particles formation and limitation of removing devices. Then the paper carried out systemic research about the formation and controlling of the sub-micron particles enriched with trace elements during the coal combustion, which is helpful for perfecting fossil fuel combustion mechanism and pollution controlling theory and has a far-reaching impact on continuing development of energy sources and environment. Firstly, this thesis discusses the serious harm of sub-micron particle emission on human and inhabitation environment, which means it important and necessary to study sub-micron particle formation and controlling method. According to the research work on the formation fundamental theories and control ways of sub-micron particles enriched with trace elements up to date in detail, their advantage and its disadvantages are systematically analyzed about the latest developments in numerical simulation and experiment study on formation theories and control ways (including electricity, lightening, sound, magnetism, heating and so on ) of sub-micron particles. A new way about adopting chemical agglomeration controlling sub-micron from combustion processes is proposed. Based on this idea, the relevant research word follows. Based on aerosol theory, a mathematical model about sub-micron particle formation during combustion is developed successfully and the research is lack in our country. The model describes the formation of two sub-micron particles, including free molecule and chain molecule, analyzes the effect rule of nucleation, condensation and coagulation. Through the contrast of model calculation and particle formation experimental result, the model feasibility has been validated and can rationally reflect the formation rule of sub-micron particles. So the model can direct the formation of sub-micron particles during combustion. In this thesis, synthetic study about the enrichment of trace elements (including As, Cr and Pb) and main elements of fly ash , K and so on) on different diameter of sub-micron particles are carried out. The results through drop coal combustion experiment show that particles mass of different diameter is distributing by two-apex, that As content distribution is independent of coal kinds and is relating with As volatility and content in coal. Cr distribution is main relating with existing form. So the model about the connection of particle diameter and trace element is developed. Through the calculation and validate, calculation is consistent with the test result under the condition of 0 < Kn< ∞, though the connection is not consistent with the weakening volatility of trace element. The study is of meaningful direction for describing the transferring and translation of sub-micron particle enriching with trace element. In order to study the adsorption rule of fly ash composition on As, Al2O3,CaO,MgO and SiO2 are selected and tested about As volatilization and their adsorption to As. At the same time, base on adsorption mechanism and quality diffuse theorya, the model about the adsorption of CaO to As has been developed according to absorb theory and mass pervasion theory. Test result show As vapor can easily react with Cao, MgO and make arsenic-compound. The model can discovery that As gasification rate, translation rate, temperatur, CaO transform rate and CaO particle surface area are important for absorbing As. Both CaO transform rate and CaO particle surface area are first great and after lessening. The model can well direct the study of As adsorption mechanism. A new way about adopting chemical agglomeration controlling sub-micron from combustion processes is proposed to decrease sub-micron particle emission during coal combustion. Three models are developed to study the agglomeration mechanism between sub-micron particles and atomized particles of agglomerator, including particle self-agglomerating physical model without adding agglomerators, agglomerating between ash particles and agglomerator particles, also agglomerating effect forces between ash particles and agglomerator particles, which is proved to be a very useful tool in the process. The first self-agglomerating model results show that particle number, diameter, flow rate and mass are changing with the length of agglomerating tube and the agglomeration is not obvious. The second model results are consistent with the test results and better reflect the effect rules of agglomerator. The effect force model results show that resistance and viscosity force of agglomerator particles have main effect on agglomeration processes, gravitation and elasticity force are in the next place. Finally, In allusion to the issue of efficiently controlling sub-micron particle emissions through agglomeration way in coal combustion, a minitype agglomeration of fly ash stovethat simulated gas flow is made up in order to do experiment researches. The test results show that the agglomerator adding can greatly decrease gas concentration at the exit and that gas flux,gas concentration,agglomerator flux, PH and concentration are the important factors impacting on the agglomeration of sub-micron particle, which proves the way validity. The agglomeration way can not change running operation of boiler and removing device and is of low investment, simple technique and easy operation.