| Coal is the safest,most economic and reliable energy source in China,of which the total consumption was 1,793,110,000 tons for power generating and heating in 2018.Half of it was consumed by coal-fired power stations,which contributed 63.9% of total power generated in China.To remove the pollutants mainly of SO2,NOx and dust,above 750,000,000 k W of coal-fired units totally were retrofitted targeting ultra-low emission in China,and limestone-gypsum wet flue gas desulfurization(Wet FGD)is widely applied(up to 90%).The daily production of FGD sludge,which is the end solid waste,exceeds 41,200 tons based on the total installed capacity of coal-fired units of 1,000,000,000 k W.The FGD sludge contents high sulfate with very fine particulates,and a variety of heavy metals enriched in FGD system,which hinders high-effective dewatering and simultaneous stabilization various heavy metals.Presently the disposal of FGD sludge is not drawing enough attention.Mostly they are piled outdoors or sent to slag yard for disposal after blending,which brings potential and severe hazard to soil and water environment.Therefore it is important to solidify the FGD sludge effectively and to dispose it in a safety way.This article intends to reveal the migration and transformation of heavy metal among gas-liquid-solid phases in the entire wet FGD process by means of monitoring the data of heavy metals in FGD waste-water and gypsum,and finds out the origin of heavy metals in FGD sludge.A special conditioning agent is developed for solid waste FGD sludge,optimizing the conditioning process and improving the dewatering feature of sludge,studying its micro-mechanism.Based on characteristics of sulfur-rich FGD sludge,building a self-stabilization mechanism excited by sulfate and alkali,mitigating the leachability of heavy metals and finally clarify the principle of it.The study and achievement is stated as below:1)Aiming at high volatile heavy metal,a corona plasma system is introduced to enhance oxidation so that gaseous heavy metals can be removed,realizing a higheffective capture of polluting heavy metal by FGD slurry.The volatile heavy metal Se is selected as a representative in this paper for test and research.The results show that O3 produced by corona plasma can enhance the oxidation of Se4+,which 2.4 times more efficient than that by auto-oxidation in air.It can increase absorption of Se O2 significantly and minimize heavy metal appeared in flue gas.This is an exploration in new field regarding to volatile heavy metal to be removed in gaseous phase.2)Investigating the distribution of heavy metals in flue gas,waste-water and sludge produced in several coal-fired power plants,the migration and transformation of heavy metals in the whole wet FGD process is revealed.The results show that the heavy metal content in flue gas after FGD is low.The content of As,Pb,Cr and Mn in flue gas satisfies the emission requirements specified in American EPA standards revised in 2016 for coal-fired power plant.Large amount of heavy metals are enriched in FGD waste-water and sludge,including gypsum which is the main discharge.The heavy metal of As,Pb,Cr occupies approximately 70-85% of total discharge,while heavy metal Mn of 30-40%.Analyzing the origin,As and Pb originates from limestone and inlet flue gas,which coming from limestone contributes 40% of its total amount.Cr and Mn mainly come from limestone,which contributes more than 85% of its origin.3)Using high-iron slag and fly-ash,a special modifier for FGD sludge is developed,optimizing the conditioning process based on response surface methodology(RSM),improving the dewatering of sludge.Combining the microstructure,mineral composition and thermal analyzing,the conditioning principle is manifested.The results show that 38% of special modifier,60 oC of water bath temperature will be the optimal conditioning process,where the sludge’s specific resistance is up to 3.75×1011m/kg and capillary suction time is 9.6s.The specific resistance is reduced by 89% while capillary suction time down to 67%.The treated sludge is pressed by plate-frame filtration,of which the water content is 71.51% down to 24.61%,showing the dewatering performance is improved remarkably and reduction of sludge.The conditioning principle of FGD sludge could be generalized as: the special modifier chemically reacts with water and consume part of it,in the mean time the hydration generates large amount of gel C-S-H to bind the gypsum crystal,forming a dense and systematic skeleton structure,good for release of bound water.4)Based on the sludge’s characteristics of rich-sulfur,a self-solidification treatment is applied.Assessing the long-term leachability of solidified sludge,it reveals that a double excitation of sulfate and alkali takes place.The results show that the compressive strength of solidified sludge is up to 6.36 MPa,with high anticorrosiveness against sulfate.The leaching of heavy metal(As,Cr,Pb,Mn)meets the requirements specified in Wastewater Quality Standards for Discharge to Municipal Sewers(CJ343-2010),and maintaining a long-term stabilization.The self-stabilization of double excitation of sulfate-alkali could be generalized as: at early stage of hydration,the targeted heavy metals are fixed at the surface of gypsum mainly by means of chemical absorption,which is a meta-stable state.With the process of hydration,the heavy metals will stay within the silicon-aluminate crystal constantly by means of absorption and crystal lattice immobilization,forming a precursor of zeolite structure existing as nesosilicate,stabilizing heavy metals and preventing them from release to the environment. |
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