| The problem of heavy metal pollution in soil is becoming more and more serious.As a green and low-cost method,phytoremediation technology has been widely concerned.However,with the application of phytoremediation technology,how to safely deal with the heavy meatal contaminated plants is the key to limit the extension of phytoremediation technology.The incineration method can not only recover heat,but also reduce its mass and volume to the maximum extent,which is convenient for subsequent treatment.However,how to control the release of volatile heavy metals in the combustion process and avoid secondary pollution is an urgent problem to be solved.At present,many scholars have studied the formation mechanism and emission reduction methods of heavy metals in the combustion process of coal,garbage and sludge,but there is little research on the heavy metal emission in the combustion process of mining plant for phytoremediation.In view of this,four new types of phytoremediating plants in mining area,including Sapium sebiferum,Robinia pseudoacacia,poplar and tufted bamboo,were selected as research objects to study their basic combustion characteristics and emission characteristics of semi-volatile heavy metals Pb,Zn and Cd in the combustion process.Meanwhile,in order to explore the emission reduction characteristics of Pb,Zn and Cd,thermodynamic software was used to simulate the reaction process of heavy metal chloride,oxide and different kinds of addition,to explore the law of interaction between heavy metals and additives.Firstly,the combustion characteristics and kinetic mechanism of four heavy metal contaminated biomass（Robinia pseudoacacia,Sapium sebiferum,poplar and Tufted bamboo）were studied by thermogravimetry.It is found that the combustion weightlessness of four new phytoremediating plants（Robinia pseudoacacia,Sapium sebiferum,poplar and tufted bamboo）mainly occurred at 180℃-400℃and 400℃-500℃,corresponding to the emission and combustion of volatile matter（weight loss rate～45%）and the combustion of fixed carbon（weight loss rate～35%）.The ignition index,burnout index and flammability index of Sapium sebiferum were higher than those of other three plants,which indicated that Sapium sebiferum was easier to ignate and burn out.According to the comprehensive burning index,the flammability of selacted plants was as follows:Sapium sebiferum>tufted bamboo>poplar>Robinia pseudoacacia.The combustion dynamics of four kinds of biomass in the mining area were analyzed by using Coats-Redfern method.The combustion of four kinds of plants were in accordance with the first-order chemical reaction series model except the first stage of combustion of poplar and tufted bamboo.The results showed that the combustion process of four kinds of biomass could be divided into two stages,the first stage activation energy was 53.39k J/mol-83.48k J/mol.The second stage activation energy was 40.62k J/mol-116.73k J/mol,and the activation energy of Robinia pseudoacacia is the lowest 40.62k J/mol.Then,the relationship between the volatilization rate of Pb,Zn and Cd and temperature was studied in the small horizontal furnace.The results showed that the release rate of Pb,Zn and Cd increased with the raise of temperature.The comparison of volatilization rate of heavy metals at different temperatures is basically Cd>Pb>Zn,and the stability of Zn is the highest.The residual rate of Zn is still greater than 80%at 1000℃.the volatilization rate of Cd is the highest,and the volatilization is basically completed at 1000℃.After combustion,the bottom ash of Robinia pseudoacacia or poplar was mainly composed of alkali and alkaline earth metal oxides and sulfates,such as CaO,MgO and K2SO4;the bottom ash of Sapium sebiferum is mainly composed of alkali and alkaline earth metal silicates and sulfates;the tufted bamboo melts at 900℃,and only SiO2 crystal is detected in the bottom ash.With the increase of temperature,the crystal phase of poplar ash has no obvious change and the heavy metal phase has not been detected in ash.The result show that the content of heavy meatal is low after combustion.Combustion can be used as treatment of selected biomass,and the subsequent treatment of bottom ash is not difficult relatively.Finally,in order to screen an effective additive to control heavy metal emission,the law of chemical reaction of chloride and oxide of Pb,Zn and Cd with Ca based additives,Si-Al based additives and P based additives were studied by using the thermodynamic software HSC.The simulation shows that Ca based additives can transform heavy metal chlorides into heavy metal oxide which is difficult to volatilize and reduce the yield of gaseous heavy metal chloride.The selected calcium based additives can reduce the yield of gas phase PbCl2,ZnCl2 and CdCl2 mostly by 10%,76%and 60%respectively at 1000℃.P salt can transform PbO and ZnO into Pb3（PO4）2and Zn3（PO4）2,and keep stable at 25-1000℃.More than 60%of PbO,ZnO and CdO can be transformed into more stable heavy metal silicate or aluminate at1000℃.This study can provide some theoretical basis for the clean combustion utilization and heavy metal emission control in the combustion process of plants for phytoremediation. |
