Study On Mercury Removal Technology Based On Thermal Power Dust Control

Mercury pollution caused by coal fired plants, following the PM, SO2, NOx, is one of the pollutants which should be strictly controlled. Founded by the National Environmental Protection Public Welfare Industry Targeted Research Fund and the Key Technology Projects of Guodian, this paper has researched on the adsorption and oxidation mechanism of fly ash in thermal power plants, the controlling technologies of fly ash for removing mercury in the flue gas and the related factors of mercury removal from flue gas. The research on the mercury control technologies based on the dust control technologies was carried out, which will accelerate the development of coal-fired mercury control technology and help to improve the mercury measurement technologies in coal-fired fuel gas.The concentration of mercury in coal, fly ash, slag and outlet of dust collector which were collected from15coal-fired power plants was tested and mercury distribution and emission characteristics in each part of the coal-fired power plant products were analyzed. The results of the tested plants show that the average mercury content in coal is0.171mg/kg, the average mercury content in slag is0.0538mg/kg and the average mercury content in fly ash is0.223mg/kg. The mass balance of mercury is between47.3%and106%, and the results is in line with generally acceptable range of70%to130%. Mercury content in slag compared with mercury content in any other emission link is lower and it can be ignored. The mercury content in fly ash is accounted for about13%in all emission link. The Hgp content in flue gas of the outlet of the dust collectors is very low, and it can be ignored. The mercury content in flue gas of the outlet of the dust collectors is still accounts for a large proportion in all mercury emission link.Based on the fixed bed adsorption experimental system, the interaction mechanism of fly ash and mercury in power plants was researched, and the mercury adsorption process on the fly ash was analyzed from the point of view of adsorption dynamics, and the influence of the composition O2and HCl in flue gas to the fly ash to oxidized the mercury in flue gas was studied. The results show that:1)the rate of adsorption of Hg0by fly ash at the initial stage was faster, and the rate of adsorption is related to the surface adsorption in this stage. As the process of adsorption, the activity on surface was gradually occupied, and Hg0was needed to spread to the internal pore to be adsorbed. The rate of adsorption fall sharply until the mercury adsorption equilibrium is reached;2) Studies of adsorption kinetics show that accurate second-order adsorption kinetics model can predict the process of Hg0to be adsorbed on the fly ash better than other adsorption kinetics models. Chemical adsorption is the control steps in the Hg0to be adsorbed on fly ash;3)In N2atmosphere the oxidation of fly ash to mercury is weak, and it can be related to a large number of oxidizing functional groups on the surface of fly ash. O2play a weak role in oxidizing the mercury by fly ash and HCl plays an obvious one.Based on the experimental data in coal-fired power plant, the coordinated control of the dust removal facilities to mercury was researched, and the influence was studied which about the combustion mode(pulverized coal furnace and circulating fluidized bed boiler), the type of dust collector(electrostatic precipitator and bag filter), the residual carbon in fly ash and the effect of selective catalytic reduction(SCR) device for dust removal facilities to take off the mercury emission. The results show that:1) The average efficiency of electrostatic precipitator synergy to take off the mercury was37.4%, and the relation with the Hgp content in flue gas is obvious. The efficiency of CFB and ESP unit is higher than that of PC and ESP unit and the average efficiency of bag filter to take off the mercury was50%, which is better than the efficiency of electrostatic precipitator to take off the mercury;2) The carbon residue in fly ash plays an obvious role in content of fly ash and the relationship between chlorine and sulfur content in coal with the synergy efficiency of filter to take off the mercury was not obvious;3) The content of fly ash mercury in flue gas was increased after the SCR reactor. The SCR can improve the ability of fly ash to adsorb mercury through oxidizing Hg0to Hg2+, and the synergy efficiency of filter to take off the mercury was increased.