Pyrolysis Of Sewage Sludge And The Control Of Nitrogen/Sulfur/Carbon Containing Pollutants By Using Biochar

With the acceleration of urbanization,the scale of urban wastewater treatment significantly increases,and the resulted sewage sludge increases accordingly.The pyrolysis in thermochemical conversion has advantages of significant volume reduction and high resource utilization rate.However,the sludge may decompose to produce pollutants,such as NO_x,SO_x,CO₂during the thermochemical conversion and there are sparse studies on the evolution of nitrogen/sulfur/carbon present in sludge and corresponding pollutants regulation.Therefore,this work firstly investigated the evolution of nitrogen/sulfur/carbon in sludge by using thermogravimetric-mass spectrometry（TG-MS）and thermogravimetric-infrared（TG-FTIR）.Based on this,the biochar was prepared from paper mill sludge and characterized using X-ray diffraction（XRD）,scanning electron microscopy（SEM）,and X-ray photoelectron spectroscopy（XPS）.Then,the effect of biochar addition on pollutant control was investigated from the perspectives of pollutant release characteristics,nitrogen/sulfur/carbon fugacity pattern and retention rate in the pyrolysis residue before and after exogenous biochar addition,and response surface optimization was performed.Finally,the control mechanism of exogenous biochar addition on nitrogen/sulfur/carbon pollutants was investigated.The main research contents and conclusions are as follows:1.Paper mill sludge was used as the raw material for the preparation of biochar,and the effects of different pyrolysis end temperatures（550,600,650 and 700℃）on the physicochemical properties of biochar were investigated.The physicochemical properties of the biochar prepared at different pyrolysis temperatures also differed to some extent.The results of elemental analysis showed that the contents of nitrogen,sulfur and carbon decreased gradually with the increase of the pyrolysis temperature,and the biochar prepared at 700℃had the lowest contents of nitrogen,sulfur and carbon,while the results of BET and SEM showed that the specific surface area and the number of pores of biochar increased and then decreased with the preparation temperature,and the optimum pore structure parameters were found at 650℃.The chemical composition of the biochar was obtained by XRF and XRD analysis,and the inorganic material was mainly Si O₂.In addition,the relative content of alkali metal compounds increased and then decreased with the temperature,and also had the highest relative content at 650℃.2.Based on the evolution of nitrogen/sulfur/carbon during thermochemical conversion,the effect of biochar addition on the pollutants regulation was investigated.Firstly,the influence of the addition of various nitrogen,sulfur,and carbon containing gases to biochar on the release was analyzed,with NH₃,HCN,COS,SO₂,and CO₂as the main gases.When sludge was pyrolyzed individually,the release law of the above gases followed the principle that the gas release intensity first increased and then decreased as the temperature increased.At the peak release intensity,the corresponding temperature was close to the temperature corresponding to the maximum decomposition rate in thermogravimetric analysis,The addition of biochar still follows the law of gas release from the mixed sample,which decreases with increasing temperature.The addition of biochar at 650℃can achieve the minimum release of NH₃,HCN,COS,and SO₂.Compared to sludge pyrolysis alone,these four gases reduce by 54.42,40.90,46.61,and 65.56%,respectively.The reduction of the above typical gas release amount will also lead to an increase in the retention rate of nitrogen,sulfur,and carbon in the residue.When the sludge is pyrolysed alone,the retention rates of nitrogen,sulfur,and carbon are 14.27,44.38 and 39.56%,respectively.Adding biochar at 650℃showed the best nitrogen and sulfur retention effects,increasing to 37.29 and 80.62%,respectively.The addition of biochar at550℃showed the best carbon retention effect,and the carbon retention rate increased to 90.08%.Through XPS analysis of species before and after pyrolysis,it was found that biochar also had a certain impact on the morphology of nitrogen,sulfur,and carbon functional groups in the pyrolysis residue,and they were found to be fixed in a more stable form in the residue.3.Optimize the control parameters of pollutants through the Box-Behnken response surface method simulation.The model was constructed using the preparation temperature and addition amount of biochar,sludge pyrolysis rate,and final pyrolysis temperature as input parameters,and the nitrogen/sulfur/carbon retention rate as response values.The correlation coefficients R²between the predicted value and the actual value of the model are 0.9864,0.9561,and 0.9882,respectively,indicating that the accuracy of the regression model is high.The parameter sensitivity results show that the preparation temperature and addition amount of biochar have a significant impact on the response value.4.Based on the intrinsic characteristics of biochar,the release characteristics of pollutants before and after biochar addition,and the occurrence forms of nitrogen,sulfur,and carbon in pyrolysis residue,the control mechanism of nitrogen,sulfur,and carbon pollutants was discussed.During the pyrolysis conversion process,Main mineral components in exogenous biochar such as Mg O and Fe₂O₃with pollutants such as HCN,SO₂,H₂S,and CO₂,thereby retaining stable quaternary nitrogen,sulfate,and aromatic compounds in the residue.In addition,the porous structure of biochar is also conducive to the adsorption of pollutants.Through this research,it will greatly promote the development of biomass energy,the construction of"zero waste city"and achievement of the"dual carbon"goal in China.