The Low Temperature Thermal Behavior Of Dried Municipal Sewage Sludge And Its Effects

Large-scale storage and treatment of municipal sewage sludge in coal and sludge co-firing power plants have the thermal safety risk of self-heating and spontaneous combustion caused by low-temperature oxidation of dried sludge.Because of the diversities of the sludge source,composition,physical and chemical properties and treatment processes,the mechanisms of self-heating heat sources are complex.Studying the process and characteristics of the low-temperature oxidation from self-heating near room temperature to spontaneous combustion of dried sludge can provide a theoretical basis for predicting and preventing the self-heating and spontaneous combustion in large-scale sludge storage.Therefore,the present work took dried municipal sewage sludge as the research object,and used isothermal calorimetry to investigate the low-temperature oxidation characteristics,heat generation mechanisms and their contributions at near room temperatures.Meanwhile,thermogravimetric analysis-differential scanning calorimetry(TGA-DSC)was used to explore the effects of self-heating mechanisms on the combustion characteristics of stored sludge.Moreover,non-isothermal TGA/DSC simultaneous thermal analysis method was employed to investigate the characteristics and kinetics of the low-temperature chemical oxidation before spontaneous combustion,and evaluated the low-temperature oxidation capacity and spontaneous combustion risk of the dried sludge.The results of isothermal calorimetry analysis show that the low temperature oxidation heat generation during self-heating has three major mechanisms,including microbial biological oxidation,inorganic matter(Fe/S/O system)oxidation,and organic matter chemical oxidation.The exothermic characteristics of the low temperature oxidation of the sludge are the result of their combined performance.The moisture content and temperature affect the contribution of each heat source in the process of low-temperature oxidation of the sludge and lead to the transformation of the main heat source.Increasing the temperature and moisture content enhance the heat generation from the sludge,that is to say,the self-heating potential becomes stronger.By comparing the self-heating exothermic characteristics of the three sludges,it is found that Nanjing sludge and Yancheng sludge have strong self-heating potential,while the power plant dried sludge has weak self-heating tendency.Chemical oxidation of organic matter has little effect on the subsequent oxidation reactivity of sludge fuel before ignition,while microbial biochemical oxidation can not only change the oxidation reactivity of sludge fuel before ignition,but also affect the combustion characteristics after ignition.Nevertheless,regardless of the self-heating mechanisms,the stronger the reaction and the more oxidation heat the sludge,the worse the oxidation reactivity after the self-heating,which also indicates that the self-heating process will cause the consumption of easily degradable substances and consequently affect the combustion characteristics of the resulting fuel.The exothermic process of chemical oxidation of the sludge under air atmosphere is generally composed of three stages: low-temperature oxidation stage before ignition;pyrolysis of easily decomposable components and rapid release and combustion of volatile matter;and pyrolytic combustion of difficultly decomposable components and char combustion stage.The characteristics of the three exothermic stages are related to the composition and treatment methods of the sludge.While the endothermic pyrolysis process of sludge organic matter at low temperature obviously affects the exothermic characteristics of the air-oxidation reaction,especially the exothermic characteristics of the low-temperature oxidation stage before ignition,and the direct oxidation plays an important role in the heat release of the sludge in the low-temperature oxidation stage.Combining the low-temperature oxidation characteristics and spontaneous combustion risk assessment of the three sludges show that Nanjing sludge and Yancheng sludge have high spontaneous combustion tendencies,while the spontaneous combustion risk of power plant dried sludge is relatively lower.Therefore,power plant drying can reduce the risk of spontaneous combustion of the sludge.