Experimental And Mechanism Study On Mercury Emission And Control During Coal Combustion

As the largest source of atmospheric mercury emission, mercury emitted from coal-fired power plants has been identified as a hazardous to both human health and environment. And the relative research concerned with the speciation and control of mercury in coal-fired power plants is currently an active topic.Based on the overview of researches related to characteristics of mercury distribution and occurrence in coal and the mercury transformation and emission control, experimental and simulation studies were conducted to investigate the mercury emission and speciation characteristics and the mercury oxidation and adsorption mechanisms of several typical Chinese coals. Investigations interpreted the influences of factors including coal type, Cl, S, CaO and temperature on mercury speciation mechanisms and the homogenous oxidation mechanism between mercury and the flue gas constituents such as Cl, S and so on. A model describing mercury adsorption by CaO was established and the rate constant of adsorption reaction was calculated. The overall research provided scientific knowledges for the development of mercury emission controlling technologies.The interactions of mercury with chlorine, sulfur and calcium oxide additive were investigated with F*A*C*T. Results showed that in the oxidative flue gases atmosphere of coal combustion, sulfur can inhibit Hgo(g) formation. For the"low ratio"of S/Cl condition, sulfur couldn't affect the mercury speciation, while chlorine did with HgCl2(g) as the main form of mercury at low temperature. High S/Cl ratio will inhibit the elemental mercury oxidation and the HgCl2(g) formation, and Hg0(g) would exist as the main form of mercury in a wider range of temperature. Analysis also forecasted that CaO(s) addtition couldn't notably affect mercury speciation in flue. The influence of CaO(s) on mercury speciation in flue gases was due to the decrease of particulate surface area and/or the change of ash particle morphology & mineralogy.With the Ontario Hydro method recommended by EPA, experimental studies were conducted on vertical alundum furnace and one-dimensional combustion test facility, respectively, to determine the mercury speciation and emission during coal combustion. Experimental data on vertical alundum furnace system showed that the Hg(G) and Hg(P) percentages were 63-90% and 5-37%, respectively. And the Hg2+(g) percent is nearly the same for all four coals. The main form of mercury in flue gases was Hg0(g), accounting for 59%-75% of gaseous mercury, while the remained 25%-41% was Hg2+(g). On one dimensional furnace, much mercury in flue presented as particulate mercury. The Hg(P) andHg(G) percentages were 52-89% and 11-48%, respectively. It was possibly due to the adsorption of fly ash. With a percent of 52%-83%, Hg0(g) was the primary form in gaseous mercury of gaseous mercury, while the other 17%-48% was Hg2+(g).Mercury capture by CaO addition was studied using high-temperature electric resistance furnace and vertical alundum furnace. On grate firing condition, the CaO addition into XLT lignite, LPS bituminous and HS anthracite coals could greatly inhibit the gaseous mercury emission. The Ca/S ratio could greatly influence the mercury capture efficiency and the optimal adsorption temperature. Experiments on vertical alundum furnace showed that with more CaO addition, there was a clear increase in particle-bound mercury fraction. Meanwhile, CaO(s) could effectively capture HgC12(g) with a adsorption efficiency nearly 50%. However, the Hg0(g) capture was not obvious.Based on the homogeneous mercury chemistry in coal combustion flue gas, CHEMKIN software package was adopted to analyze the influences of different gas components on mercury oxidation. Cl(g) and Cl2(g) were the most important active species for mercury oxidation. The rapid mercury oxidation could occur only for high concentration of"Cl-atom pool". NO(g) can strongly affected the mercury oxidation in flue gas, showing promoting effect at lower concentration and inhibiting effect at much higher concentration. In the concentration range of typical coal combustion flue gas, O2(g) weakly promoted the homogenous Hg oxidation and SO2(g) showed the inhibition effect, whereas moisture was a stronger inhibitor. A mathematic model about mercury capture by CaO addition was developed on the basis of the Freundlich adsorption isotherm equation. Model analysis indicated that the rate constant of the reaction between CaO and HgCl2(g) was 2.56×107×exp(635.9/RT).The reactions between CaO additive and other trace elements in coal, for example, As, Se, were also investigated through thermodynamical equilibrium analysis and experiment study. CaO could not only desulfur and control mercury emission, but can also inhibit the vaporization of As, Se in coal, depositing As, Se as arsenate in the solid phase and be removed with fly ash.