| Sewage sludge(SS)is the product of sewage treatment.The sludge contains many bacteria,which can ferment to release malodorous gas,pollute the environment and affect human health.At the same time,there were a lot of heavy metals in the sewage sludge.Without properly treatment,it will cause water pollution and land pollution,and then pollute aquatic products or crops.Aiming at the demand for reduction,harmlessness,and resource-based sludge disposal,this paper takes the low operating cost and high promotion value as the guide,takes advantage of the existing municipal solid waste incineration power plant,and proposes the technical idea of using sludge as a low-quality fuel to combuste and dispose in a waste incinerator for power generation.Based on this,this paper studied the characteristics of co-incineration of SS and municipal solid waste(MSW),and the characteristics of pollutant emission,aiming at the main furnace type of waste incineration plant in China,grate incinerator.The volatilization characteristics of heavy metals in the co-combustion process of MSW and SS as well as the chemical forms and ecological risks of heavy metals in bottom slag and flue ash were also studied.Ca and Mg-based additives were prepare to realize the synergistic removal of HCl and heavy metals in the furnace.Taking a waste incineration plant as the research object,economic analysis of co-incineration of SS and MSW was studied.The main research contents of this paper are as follows:(1)Thermogravimetric analysis and infrared spectroscopy were used to study the characteristics of co-combustion of MSW and SS,as well as the pollutant emission characteristics.The effects of calcium-based adsorbents on the characteristics of co-combustion and pollutant emission were also studied.The results showed that positive interaction was found in the co-combustion,which promoted the combustion.Combustion characteristic index increased from 1.71×10-6 min-2?-3 to 1.73×10-6 min-2?-3 when the mass fraction of SS increased from 0%to 10%.Activation energy decreased as the proportion of sludge increased.Activation energy of the main reaction stage of MSW and SS were 321.12 KJ/mol and 63.15 KJ/mol,respectively.The emission of HCl and NO decreased during co-combustion.Ca CO3,Ca O,Ca(OH)2 or natural limestone decreased the emission of HCl and NO,and decreased the activation energy in the first stage but increased it in the second stage.(2)Heavy metal volatilization characteristics of co-combusted MSW and SS from different mass fraction of MSW and SS,different temperature and different O2 concentration atmosphere was investigated.Performance of calcium-based sorbents was also studied.Fitting formula was set to predict the heavy metals volatilization,and risk evaluation of gaseous heavy metals was exhibited.The results showed that volatilization efficiency of heavy metals increased as mass fraction of MSW in a blend increased and as temperature increased.Volatilization efficiency of all heavy metals examined decreased as O2 concentration increased at high temperature and that of Cu,Pb and Zn increased as O2 concentration increased at low temperature.Ca O,Ca(OH)2 and Ca CO3 declined the volatilization of As,Cr,Cu,Ni,and Zn,while enhanced that of Pb.With calcium-based sorbents,volatilization efficiency of As,Cr,Cu,Ni,and Zn decreased from 70.06%,39.91%,75.52%,44.08%and 40.10%to 54.24%,33.73%,39.98%,20.56%and 32.06%,while that of Pb increased from 47.23%to 100%.This was because Pb SO4(s)can react with Ca Cl2 to form a gaseous product Pb Cl2 in the presence of a calcium-based additive.(3)Chemical speciation and environmental risk of heavy metals of bottom slag during co-combustion of MSW and SS were evaluated.Fitting formulas between environmental index of bottom slag and combustion condition were giving.The results showed that when co-combusted with SS,the proportion of mild acid-souble fraction of Cr,Cu and Zn in the bottom slag decreased,the proportion of reducible fraction and residual fraction increased.As for As,with the increase of sludge blending ratio,the proportion of mild acid-souble fraction increased,and the proportion of oxidizable fraction decreased rapidly.The degree of soil contamination by As and the ecological risk of As of MSW combustion bottom slag increased when co-combusted with SS,while that of Cr,Cu,and Zn decreased.(4)Horizontal flue ash in waste incinerator was collected before and after the sludge was co-combusted.XRF,SEM-DES,and ICP were used to characterize the effect of sludge on the distribution of heavy metals on different heated surfaces.The fitting relation between the heavy metals content of each speciation and the gas temperature and the sludge mixing amount was given.The results showed that potential ecological risk index of all horizontal flue ash was higher than 600,indicating the high ecological risk.After adding sludge for co-combustion,heavy metals content in all ash sample increased.As for chemical speciation of heavy metals,the addition of sludge increased the content of mild acid-soluble fraction,reducible fraction and oxidizable fraction of heavy metals,while decreased that of the residual fraction.Environmental pollution and potential ecological risks of heavy metals were increased when sludge was co-combusted.(5)Three additives Ca2SiO4,Mg2SiO4,MgCa SiO4 were prepared by calcination method,and the effects of these three additives and the conventional additives Ca O and MgO on the adsorption of heavy metals and chlorine were studied.The results showed that the adsorption effect of Ca-based adsorbents on heavy metals and chlorine was better than that of Mg-based adsorbents,and the adsorption effect of prepared silicate additives on heavy metals and chlorine was better than Ca O and MgO.The adsorption of additives included physical adsorption and chemical adsorption.Temperature was one of the main factors affecting the adsorption effect.Integrating the effects of physical adsorption and chemical adsorption,there was an optimal temperature for the adsorption of heavy metals by additives,and the optimal temperature was500? for most additives.Additives were selective in the adsorption of heavy metals.Different additives can be selected according to the target removal heavy metal.For the comprehensive removal of Cd,Cu,Pb,it should choose Ca2SiO4;for the removal of Cu or Pb,it can choose Ca2SiO4 or MgCa SiO4;for removal of Cd,it should choose Ca2SiO4.Considering the adsorption effect of heavy metals and HCl,Ca2SiO4 was the adsorbent with the best synergistic removal effect among the five adsorbents studied.The optimal adsorption temperature was recommended to be about 500?.(6)Thermal calculation of sewage sludge co-combustion in waste incineration power plantwas done and economic analysis of four common sludge drying schemes was compared:flue gas drying,steam drying,electric heating drying in the waste incineration plant and in situ electric heating drying in the sewage treatment plant.Sensitivity analysis was also performed.Operating parameters optimization of co-combustion of sludge was done and ways to improve economic efficiency was discussed.The results showed that when the water content was30%?40%and the absolute-dry sludge ratio was 2%?3%,the boiler efficiency was reduced by0.56%?1.12%compared with the case without mixing sludge,and the power generation decreased by 0.2568?0.3767 MW in steam drying scheme and by 0.0037?0.0094 MW in other schemes.Net present value(NPV)of flue gas drying was more than 20?million,while the scheme of electric heating drying in the waste incineration plant can't recoup the initial investment.Sensitivity coefficient of flue gas drying was smallest among different schemes,showing that the risk of this program was the smallest.Absolute-dry sludge ratio and unit subsidy for sludge treatment were sensitive factors for NPV in flue gas scheme,which had a sensitivity coefficient of 1.123?1.171 and 1.232,respectively.Allover,this study provides the law of combustion characteristics for the application of co-incineration of sewage sludge and municipal solid waste and provides preparation method of additives for secondary pollutant in furnace with synergistic control. |
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