Study On Co-combustion Behavior And Interaction Mechanism Of Municipal Sewage Sludge And Coal Slime

At present,the output of municipal SS（Sewage Sludge）and CS（Coal Slime）in China continues to grow and has ranked first in the world.The traditional disposal of SS and CS has brought about double problems of environmental pollution and resource waste.With the increasing awareness of national environmental protection and the increasingly stringent requirements for sustainable development,the efficient and clean utilization of SS and CS will become the focus of the future environmental field.In view of the theoretical advantages of SS-CS co-combustion,it is a promising disposal method to use the direct co-combustion technology of coal-fired units to generate power from SS and CS,which is expected to achieve the stabilization,reduction,harmless and resource utilization of SS and CS rapidly and efficiently.The co-combustion effect of SS and CS in furnace is closely related to their co-combustion behavior,and the study of co-combustion characteristics is a necessary prerequisite for the application of co-combustion technology.In this thesis,SS and three kinds of CS（XCS,TCS and QCS）were taken as the research objects.Using TGA（Thermal Gravimetric Analyzer）,TG-MS（Thermogravimetry-Mass Spectrometry）,fixed bed combustion system,XPS（X-ray Photoelectron Spectroscopy）and ICP-MS（Inductively Coupled Plasma-Mass Spectrometry）,SS-CS co-combustion behavior was studied from three aspects of combustion characteristics,interaction and its mechanism,and pollutant migration/emission.（1）The combustion/co-combustion characteristics and kinetics characteristics of SS and CS were evaluated from three aspects of the basic combustion process,slagging characteristics and combustion kinetics by thermogravimetric analysis.The ignition temperature of SS is211.0℃,which is much lower than that of CS（476.4～492.3℃）.The ignition temperature and burnout temperature of CS are positively correlated with the ash content.During co-combustion,SS reduces the ignition temperature and burnout temperature of the fuel,while CS enhances the slow oxidation ability of the fuel.The lower ash in CS（such as XCS）helps to improve the comprehensive combustion performance of SS,while the higher ash in CS（such as TCS and QCS）reduces the comprehensive combustion performance of SS.Among the indicators for determining slagging tendency,R_s and R_z have good applicability for predicting the slagging tendency of CS.In the process of co-combustion,the three kinds of CS can improve the slagging characteristics of SS.When the ratio of TCS is greater than 40%and the combustion temperature is lower than 900℃,the slagging problem can be effectively avoided.During SS-TCS co-combustion,the addition of TCS helps to reduce the apparent activation energy of the fuel.And when the ratio of TCS is 50%,the apparent activation energy decreases by 20.0k J/mol compared with SS combustion alone.（2）The interaction between SS and CS in the co-combustion process was revealed by the deviation between the experimental and theoretical weight loss curves.Based on the influence of pretreatment methods on the interaction,the interaction mechanism was proposed.The inhibition and synergism coexist in SS and CS co-combustion process.In the co-combustion process of SS-XCS and SS-QCS,there is an inhibitory effect on the volatile weight loss stage due to the blockage of mass and heat transfer processes.The free radical induction and thermal synergy generated by SS volatiles during SS-TCS co-combustion promote the release of CS volatiles,leading to a synergistic effect in the heavy volatiles weight loss stage.In the fixed carbon combustion stage,SS ash catalyzes the combustion of fixed carbon in XCS and TCS and inhibits the combustion of fixed carbon in QCS.The minerals with catalytic combustion effect in SS ash are Ca O,Fe₂O₃ and Fe₃（PO₄）₂,among which Fe₂O₃ has the strongest catalytic activity.By the cascade chain catalytic mechanism,Fe₂O₃ and Ca O reduce the activation energy in fixed carbon combustion stage of TCS by 25.5 k J/mol.（3）Combined with TG-MS and XPS,the migration and transformation of N/S in gas and solid phases during SS-TCS combustion were studied,and the related interactions were revealed.The main N-containing gases produced during SS and TCS combustion are NH₃produced by N-A（amine-N）decomposition at 200～400℃and HCN produced by heterocyclic nitrogen（N-5 and N-6）decomposition at 400～600℃,respectively.During co-combustion,the interaction promotes the transformation of N functional groups to N-A and NH₃,and significantly inhibits the formation of NO.When the ratio of TCS is 50%,the amount of NO released is reduced by 36.9%compared to the theoretical value.The forms of S in SS are S2（aliphatic-S）,S3（thiophene-S）,S5（sulfone-S）and S6（sulfate-S）,and the forms of S in TCS are S2,S3,S4（sulfoxide-S）and S6.SO₂ is the main S-containing gas produced during the temperature-programmed process.During co-combustion,the interaction promotes the release of H₂S,and the sulfur fixation of minerals in SS is significantly enhanced,promoting the conversion of S5 and SO₂ to S6.（4）The characteristics of NO/SO₂ emission during SS and TCS combustion were studied using a fixed bed combustion system,and the contributions of different factors to the interaction of NO/SO₂ emission were revealed.NO conversions of SS and TCS are 1.8～2.3%and 8.0～15.0%at combustion temperatures of 700～1000°C,respectively,and the lowest NO conversions are obtained at 1000°C and 900°C,respectively.During co-combustion,the reducing gas released from SS has a reducing effect on NO released from TCS combustion,and the char formed from TCS has a reducing effect on NO released from SS combustion,both of which lead to an enhanced in situ reduction of NO.SS and TCS exhibit the lowest SO₂ conversion at 700℃and900℃respectively,with SO₂ conversion rates of 14.0～43.8%and 8.5～13.8%at different combustion temperatures.The enhancement of sulfur fixation of the minerals leads to the existence of self-desulfurization in the co-combustion process.When the ratio of TCS is 50%and the combustion temperature is 900℃,NO in-situ reduction and self-desulfurization are the most significant,and NO and SO₂ are reduced by 0.3mg/g and 1.4mg/g respectively compared with the theoretical values.（5）ICP-MS was used to study the migration characteristics of heavy metals during the combustion of SS and TCS,and the effects of typical minerals in SS on the migration of heavy metals were revealed through experiments and DFT（Density Functional Theory）calculations.The volatilization rate of heavy metals in TCS is As>Pb>Cu>Zn>Cr when the combustion temperature is 1000℃,and the volatilization rate of As and Pb is significantly higher than that of SS.The interaction during co-combustion significantly suppresses the volatilization of As,and the residual rate of As at 1000°C increases by 9.3～14.9%compared to the theoretical value.The influence of Ca O and Fe₂O₃,typical minerals in SS,on the migration of heavy metals in TCS is mainly derived from chemical reactions,in which Ca O fixes As in the form of Ca₃（As O₄）,while the influence of Fe₃（PO₄）₂ on heavy metals is reflected in the change of physical structure.Ca O has a strong chemisorption effect on As,and the optimal adsorption sites of As and As₂O₃ on Ca O（001）surface are both O-top site.When the surface O defect exists,the adsorption of As is inhibited on Ca O surface,the interaction between As₂O₃ and Ca O is enhanced,and the adsorption site changes from O-top to Ca-top.