Research On Mechanism Of Coal-Fired Fly Ash And Flue Gas Mercury

In this thesis, the flue gas mercury emissions from coal combustion, mercury control technology and the hazards of mercury on environment and human health, the composition and principle of the experimental system was expounded in detail.In this thesis, fly ash samples from three coal-fired power plant were sized by physical methods, and the different distribution size of fly ash particles was attained. Some analysis apparatuses were used to describe the characterization of these fly ash samples including morphology, chemical composition, unburned carbon surface area and so on.Firstly, experimental evaluation of the affecting factors on coal fly ash adsorption of mercury in flue gas was carried out at the self-developed fly ash sorbent adsorbing experiment system. The result showed that removal efficiency of mercury was affected by different sources of fly ash, temperature, residence time, mercury's entrance concentration, fly ash size distribution, specific surface area, modification of fly ash and ect.It was found that mercury removal efficiency in the flue gas by the modified coal-combustion derived fly ash obviously increased, up to74.34%. The mercury speciation transformation was affected by different particle size distribution, the mercury removal efficiency in the flue gas of D1AS-1 fly ash sample is 47.17%. It has also been found that large particles of coal fly ash is of high mercury removal efficiency, while small fly ash particles are not necessarily with low mercury removal efficiency. The mercury capture ability of different fly ash sources in flue gas is not identical; the mercury removal efficiency of fly ash from power plant 1 is 34.6%.Fly ash had different mercury removal efficiency under different reaction temperature conditions, the mercury is more likely to be adsorbed by the coal combustion derived fly ash when the temperature is high. The transformation of the mercury speciation in the flue gas is promoted when the inlet mercury concentration is higher. The mercury removal efficiency in the flue gas by fly ash was 34.6% when the inlet concentration is 10.39 ug/m3. The adsorption of mercury was higher when fly ash was with longer residence time in the multiphase flow reactor. The relationship between specific surface area and the mecury removal efficiency is proportional. The elemental mercury in the flue gas became less when the fly ash particles were larger.It was showed that the residual carbon content of coal fly is decreased with particle size decreasing, and the decrease of the removal efficiency of gas mercury was no direct connection with the particle size. However, the mercury removal efficiency was decreased with particle size decreasing when coal fly ash particles was larger than 50μm, the gas mercury removal efficiency and particle size was inversely proportional when the fly ash particles was less than 50μm. Overall, the residual carbon content for coal fly ash samples and the corresponding mercury removal efficiency was not directly mathematical relationship. Inorganic mineral composition of coal fly ash for the adsorption of flue gas mercury may be related to calcium oxide (CaO), iron oxide (Fe2O3), alumina (Al2O3) and other active substances.Experimental results showed that SO₂, NO₂, NO and HCl played an important role in promoting mercury removal efficiency in the flue gas by coal fly ash under the conditions of the basic gas atmosphere (compressed air as equilibrium gas, 12% content of CO₂, 6% of oxygen levels) with HCl or SO₂ or CO₂ or O₂ or NO or NO₂ or H2O (water vapor) while O₂ and H2O promoting effect was less and CO₂ had certain inhibition effect. The transformation of mercury speciation in the flue gas was inhibited by SO₂ under the condition of basic climate conditions with SO₂ and HCl. HCl and NO played a greater role in promoting mercury speciation transformation than that inhibited by SO₂ under the conditions of basic atmosphere with SO₂ , HCl and NO.The mercury removal efficiency in the flue gas was affected by fly ash in the state of accumulation and deposition, up to 67.34%. The mercury removal effiency in flue gas was less by coal fly ash in flight, only for 21.46%.In this thesis, simple mathematical models of the fly ash in the flight adsorbing mercury were developed based on literatures and the results of this research and the process of the adsorption equilibrium and kinetics about mercury which was absorbed by the fly ash particles was studied. The results showed that the adsorption model of the mercury in coal-fired flue gas absorbed by fly ash was more consistent with two adsorption kinetic model and the flow field, temperature field, pressure field and particle trajectories was simulated inside the multiphase flow reactor. The homogeneous and heterogeneous reaction of the flue gas, fly ash and mercury was simulated using CHEMKIN software and the important reaction mechanism of the role of fly ash and flue gas mercury was obtained.