Study On The Adsorption Of Mercury By Modified Low Calorific Value Coal-Fired Fly Ash

The mercury emissions from coal-fired power plants are the main source of anthropogenic mercury emissions.At present,activated carbon injection mercury removal technology（ACI）is a mature and effective mercury emission control method for coal-fired power plants.However,the high cost of activated carbon and its impact on the comprehensive utilization of fly ash have limited its large-scale commercial application.Fly ash,as a byproduct of coal-fired power plants,has a wide range of sources and low prices.Compared with activated carbon,it has certain advantages and is considered a potential mercury removal adsorbent.However,the mercury removal effect of raw fly ash from coal combustion is not ideal,and it is necessary to modify it to improve its adsorption performance.Therefore,this paper chemically impregnates fly ash from three typical low calorific value coal-fired power plants in Shanxi Province,prepares two types of mercury removal adsorbents,and studies their adsorption/oxidation mechanisms and influencing factors,providing theoretical and experimental support for the application of fly ash in mercury removal in coal-fired power plants.Fly ash was modified with NH₄Br and CuCl₂ solution impregnation,and its mercury removal performance was tested on a fixed bed adsorption device.The mercury removal mechanism of fly ash was analyzed through characterization methods such as specific surface area and porosity analysis（BET）,X-ray diffraction（XRD）,scanning electron microscopy（SEM）,and temperature programmed pyrolysis elemental mercury detection system（TPD-AFS）.The results showed that after modification with bromine,increase in average pore size and specific surface area of fly ash,the mercury penetration time of QX,YH,and YW fly ash was extended to 6 times,15 times,and 6 times,respectively;The main occurrence form of mercury in modified fly ash is Hg Br₂;The addition of Br will increase the number of active site on the fly ash surface and enhance the adsorption and oxidation capacity of fly ash to Hg⁰;The reaction process follows the Langmuir Hinshelwood mechanism and the Mars Masessen mechanism.The results of CuCl₂ modified fly ash show that with an increase in modified concentration,the mercury adsorption performance of QX modified fly ash is improved.The penetration time at 0.5%,1.0%,and 1.5%modified concentrations is increased to 7 times,11 times,and 12 times that of the original fly ash,respectively;Increasing the adsorption temperature can improve the mercury removal performance of modified fly ash.Excessive adsorption temperature can lead to the decomposition and desorption of Hg⁰ adsorbed on modified fly ash,weaken the mercury adsorption capacity of fly ash,and an increase in inlet mercury concentration can also improve the mercury adsorption efficiency of modified fly ash.The quasi second-order model can better describe the mercury adsorption process of CuCl₂modified fly ash.The Cl containing active groups generated by fly ash and the loaded Cu²⁺have a certain degree of oxidation to elemental mercury,promoting the oxidation and adsorption of mercury by modified fly ash.The presence of sulfur in fly ash also enhances the mercury removal ability of modified fly ash.A simulated flue gas pipeline spraying mercury removal device was designed and evaluated for mercury removal.The results indicate that within a certain range,increasing the amount of fly ash injection can increase the efficiency of mercury removal from flue gas,and the mercury removal activity of the adsorbent slightly increases.The maximum adsorption rate of mercury on fly ash can reach 28%.With the extension of adsorption time,the mercury removal efficiency of modified fly ash decreases,but there is still a certain removal effect within1 hour.The initial mercury removal efficiency under fixed bed conditions is higher than that under flowing conditions.