Research On The Synergistic Control Of Multiple Pollutants In Flue Gas From Municipal Solid Waste Incineration By Cement Clinker

Municipal solidwaste incineration(MSWI)has been widely paid attention,and has also been rapidly developed and applied in the world due to its advantages of significant reduction,complete harmlessness and recovery of waste heat resources.However,the pollutants involving acidic gas(such as HCl,SO₂),dioxins and heavy metals emitted during the incineration process will directly endanger the environment and human health,which leads to a greater potential for environmental pollution.Cement kiln co-disposal of municipal solid waste(MSW)technology has been developed rapidly in China in recent years because of the advantages of its huge consumption capacity of MSW,complete decomposition of dioxins and inhibition of their resynthesis,and no secondary pollution due to fly ash emission.At present,the traditional co-disposal process is to apply an independent waste incinerator to incinerate MSW separately and then directly pass the high temperature MSWI flue gas into the precalciner to heat the raw materials.However,this process still has the problem of enrichment of elements of sulfur and chlorine in the cement kiln system.In this paper,based on the background of cement kiln co-disposal and the new process of passing MSWI flue gas directly into the cement kiln grate cooler,experimental studies on the characteristics of desulfurization and dechlorination,adsorption and fixation of heavy metals bound in MSWI fly ash,as well as dioxin thermal decomposition over cement clinker were carried out in a fixed-bed reactor to obtain the optimal design and operation parameters of the new process.The main work and conclusions of this paper are shown as follows.The effects of reaction temperature,initial concentrations of SO₂ and HCl and clinker type on the desulfurization and dechlorination characteristics over cement clinker were investigated in a fixed-bed reactor,and the adsorption mechanismsof SO₂ and HCl were discussed based on the characterization of raw and spent clinker.The results showed that:when SO₂ or HCl is presented alone in the simulated flue gas,the desulfurization efficiency increases rapidly and dechlorination efficiency decreases continuously as reaction temperature increases from 600°C to 1000°C.The maximum unit adsorption capacities of clinker for SO₂or HCl is 4.90 or 10.29 mg·g^-1,and the corresponding maximum calcium utilization ratio is33%or 50%,respectively.At the same reaction temperature,the unit adsorption capacity of SO₂ shows a tendency of increase first and then stabilization with increasing its initial concentration,while the unit adsorption capacity of HCl shows a tendency of first increase and then decrease.When SO₂ and HCl are simultaneously present in the simulated flue gas,the dechlorination efficiency is slightly lower than the dechlorination efficiency at the same reaction temperature,however the desulfurization efficiency is enhanced significantly.The migration of chloride ions of HCl and Ca Cl₂ in the clinker is the main reason for the enhancement of desulfurization efficiency.The effects of reaction temperature,holding time and blending incineration on the immobilization characteristics and leaching toxicity of heavy metals(As,Pb,Cd,Cr,Cu,Ni,Zn,Ba,Mn)in clinker and MSWI fly ash were investigated in a fixed-bed reactor.The results indicated that the contents of targeted nine heavy metals in MSWI fly ash and its leaching toxicity are significantly larger than that in clinker,and the corresponding magnification between MSWI fly ash and clinker is ranged from 3.9-102.4 and 2.2-35,respectively.Under the condition of MSWI fly ash individually incinerated,the fixation fractions of Zn,Pb,Cu,Cd and As and the leaching toxicity of heavy metals(except Ba)decrease continuously with increasing reaction temperature.Under the condition of blending incineration of MSWI fly ash and clinker,the fixation fraction of Cd and Zn decrease,Cr and Mn increase,and the other heavy metals have a little change compared with the calculation value during individual incineration of MSWI fly ash and clinker.The leaching toxicities of Ba,Cd,Cr and Mn also have a little change,ranging from 0.01-0.08 mg/L,and the leaching toxicities of Ni,Pb,As,Zn and Cu fluctuate in the range of 0.10-0.23 mg/L.With the incineration time increased from5 min to 10 min,the contents of Zn,Pb,Cd and Cu in the blended incineration bottom ash decrease,furthermore,the contents of targeted heavy metals have a little change when the incineration time increase to 60 min.This indicates that the targeted heavy metals have been well fixed when the incineration time is over 10 min.The leaching toxicities of the targeted heavy metals decrease slightly with the increase of incineration time,but the overall change is not significant,in which,the leaching toxicity of Ni is the largest,reaching 0.19 mg/L.The thermal decomposition characteristics of dioxins in MSWI fly ash and its mixture with clinker were investigated in a fixed-bed thermal decomposition apparatus.The results showed that dioxins in the bottom ash of incinerated fly ash decrease and the corresponding dioxin degradation ratio increases with increasing reaction temperature.Dioxin content in the bottom ash incinerated fly ash is about 94%and the corresponding dioxin degradation ratio is only 0.6%at 200°C,however,the corresponding dioxin degradation ratio reaches 64.0%and99.2%while the temperature increases to 600 and 800°C,respectively.As the incineration time increases from 5 min to 60 min,the dioxin distribution and its degradation ratio have a little change at 200°C,the gas phase dioxins increases at 400°C,and dioxinsis completely degraded at 800°C.Blending incineration has little effect on the distribution and thermal decomposition of dioxins at 200 and 800°C,however,it will inhibit the migration of dioxins from bottom ash to gas phase at 400°C.This indicates that an appropriate reaction temperature can achieve complete degradation of dioxins whether MSWI fly ash is incinerated alone or mixed with clinker.For the cement kiln co-disposal process of MSWI flue gas directly into the grate cooler,the synergistic control of multiple pollutants could be achieved if the MSWI flue gas containing acid gases,heavy metals and dioxins is fed into the high temperature section of the grate cooler(about 700-800°C),which has little influence on the quality of cement clinker.