The Migration Principles Of Pb And Hg During Solid-Waste Co-processing In Different Boilers

Under the background of energy-saving,emission reduction and environmental protection,the co-processing in power boilers is an important way for resources utilization of solid waste which means burning traditional fossil fuels blended with solid waste.However,attention needs to be paid to environmental risk of heavy metal pollutants in co-process.Therefore,this paper has studied the migration and transformation characteristics of Hg,Pb and other heavy metals under the background of solid-waste co-processing in power plants.Comparison and discussion were carried out on the migration characteristics of heavy metals in different boilers.The gas phase migration and transformation of heavy metals Hg and Pb in co-processing were calculated with the gas-phase reaction kinetics in CHEMKIN.The results showed that a small amount of gas-phase Hg0 would be chloridized below 500℃,and it will be gradually oxidized to HgCl2 by reacting with Cl,Cl2,HCl and other radicals.Maintaining an oxidizing atmosphere and ensuring a sufficient amount of oxidizing radicals are the prerequisites for the chlorination and oxidation of Hg0.Increasing the concentration of Cl radicals could significantly increase the low-temperature chlorination rate of Hg0 and greatly increase the final oxidation proportion.The reaction between Pb0 and HCl occurred in gas phase above 1000℃ and undergoed chlorination.When the temperature was lowered to 700℃,the reaction tended to stagnate,and PbCl2 became the main presence of Pb.Increase of Cl content and maintaining a strong oxidizing atmosphere could speed up the Pb chlorination rate above 800℃ and slightly increase the final oxidation proportion.The different working conditions of the pulverized coal(PC)boiler and the circulating fluidized bed(CFB)boiler have little effects on the migration trend of heavy metals,and mainly affect the reaction and migration rate.Gas-phase migration and solid phase Pb presence of heavy metals Hg and Pb under thermodynalic equilibrium were calculated.The results showed that gas phase Hg0 remained stable at high temperature under oxidizing atmosphere,and migrated to HgCl2 below 800℃ while HgCl2 was the main presence below 500℃.Hg0 failed to undergo chlorination when the excess air ratio is lower than 0.94.The high-temperature gas-phase Pb was mainly present in PbO,which was remarkably converted into PbCl2 in the range of 1200℃-800℃.When the temperature was lower than 800℃,the gas-phase PbCl2 gradually solidified into the solid-phase PbSO4.Pb0 was directly sulfided into gas-phase PbS at middle-high temperature range under reducing atmosphere,which hindered the Pb chlorination reaction at middle-temperature range,and PbS was directly solidified when the temperature was lower than 600℃.The reaction tendency of the solid-phase Pb at all temperature range was weak,which means it was difficult to react with SiO2,Al2O3 in fly ash.PbO presented in inert liquid slag above 900℃.At 900℃-400℃ Pb was mainly present in magnetoplumbite,litharge and other lead-iron binary oxide crystals.The competition mechanism between Cl and S in gas-phase for the migration of heavy metals is revealed via thermodynamic equilibrium calculation.Cl has a strong affinity and reaction tendency for Hg.The chlorination reaction of Hg0 at medium-low temperature is significantly delayed and slowed down when Cl is traced,and part of mercury solidifies to HgSO4 below 500℃.S has a stronger affinity for Pb.The low-temperature sulfation ability of PbCl2 decreases when S is traced,and partial PbCl2 is further oxidized to PbCl4.However,when Cl is traced,the middle-temperature chlorination reaction of lead is not competitive,and the sulfation of Pb is advanced.Solid-waste co-processing engineering tests were carried out in a 300MW pulverized coal boiler and an 130t/h circulating fluidized bed boiler.The migration and distribution characteristics of heavy metals Pb,Hg,Ni,Co in PC boiler and CFB boiler were compared and analyzed.The results showed that Pb was hardly retained in bottom slag,and Pb released into flue gas solidified when it was cooled,mainly adsorbed on the fine particles of fly ash.Only a trace amount of Pb entered the desulfurization system.Hg was mainly discharged as gas phase in the form of Hg0.HgP in the gas phase was mainly captured by the dust removal equipment,and Hg2+ mainly entered the desulfurization slurry and gypsum.Ni and Co are involatile metals.The proportion of Ni/Co volatilization in PC boiler was relatively high,and they were considerably retained in bottom slag.Strong adsorption characteristics of volatilized Ni and Co were not observed,and the rest not captured by the dust removal equipment entered the desulfurization slurry and gypsum.The higher temperature of the PC boiler is particularly conducive to the volatilization of semi-volatile(Pb-like)and involatile heavy metals(Ni,Co-like).And it can reduce retention proportion in bottom slag and increase the content in fly ash and desulfurization system.Temperature is the prerequisite for volatilization of heavy metals in the fuel.The collision and abrasion of the material particles in CFB boiler produces debris with larger specific surface area.The long mixing time and flue gas circulation are conducive to the deposition and adsorption of semi-volatile and involatile heavy metals on fine particles in the flue gas.Also,it is beneficial to the enrichment of Hgp.In the actual solid-waste co-processing process,the dust removal equipment and the desulfurization system and other pollutant control equipment can remove heavy metals effectively.