Migration Of Phosphorous And Production Of Bioavailable Phosphorous During Sewage Sludge Combustion

Since the beginning of the new century,the rapid advancement of urbanization in China had resulted in a large amount of urban sewage to be treated every day.The sewage treatment rate in China had reached nearly 100%.Sewage sludge(SS),as a companion product of sewage treatment,contained considerable organic matter and phosphorous resources.The organics provided valuable calorific value for the sludge,and the phosphorous resources made it possible to use the SS back to the field for composting.However,according to the current SS disposal methods in China,a large amount of SS was directly or indirectly disposed of by soil landfilling.Not only did landfilling SS cause the soil to be threatened by pathogens and bacteria and wasted the precious phosphorous resources in the sludge,but also would occupy China’s land resources Therefore,in order to effectively use and reduce the sludge volume,recycle the energy of organic matter and enrich the phosphorous resources in ash,SS incineration technology was attracting more and more attention.In this thesis,the drop-tube furnace experimental system was used to carry out combustion experiments on two different kinds of SS.Combined with XRF,sequential phosphorous extraction experiment and XRD methods,the migration and transformation of phosphorous during SS combustion were studied.Then,the SS was mixed with three different coals and co-combusted to investigate the effect of co-combustion on the migration and transformation of phosphorous.Finally,the effect of magnesian minerals on phosphorous migration and transformation was studied,and the bioavailable phosphorous content in the ash of SS was increased by potassium carbonate calcination method.Research shows that:(1)Increasing temperature effectively improved the phosphorous fixation rate in the ash of SS.However,if the content of active calcium oxide produced by SS is too low,increasing the temperature to 1000℃ would detrimental to the fixation of phosphorous.CaCO3 effectively captured phosphorous during combustion and pyrolysis,while CaSO3 reacted with phosphorous vapor only during pyrolysis.2%moisture facilitated to higher phosphorous enrichment in ash.More moisture usually reduced the amount of phosphorous that could be captured by identical CaO,and consequently reduced the fixation rate of phosphorous in SS due to the generation of hydroxyapatite.In addition,moisture also promoted the reaction between calcium and sulfur,thus inhibiting the capture of phosphorous by calcium.The addition of CaCO3,CaO and MgO significantly increased the fixation rate of phosphorous in SS.At 1000℃,moisture reduced the efficiency of CaCO3 capturing phosphorous.5%moisture exhibited obvious positive effect on CaO capturing phosphorous,while CaO captured less phosphorous with 10%moisture.Magnesium oxide was still effective in fixing phosphorous in an atmosphere containing 10%moisture(2)Due to the introduction of sulfur in Tianchi(TC)coal,calcium was occupied by SO2 and lost the ability to catch phosphorus.When the temperature rose to 900℃,the sulfur competition for calcium was weakened somewhat,and accordingly,the fixation rate of phosphorus in ash was improved.The abundant silicon and aluminum in Xiheishan(XHS)coal exhibited ability in phosphorus capture at 800℃ and increased the fixation of phosphorus in ash.But because of the weak thermostability of the generated aluminum phosphate at higher temperature,silica and aluminum could no longer effectively catch phosphorous at 900℃ and phosphorus fixation rate decreased.The high content of calcium in Xiaolongtan(XLT)significantly facilitated to phosphorous capture.During co-combustion of SS1 and coal,10%moisture inhibited the mineral capture of phosphorus.Both TC and XLT coals introduced more calcium-containing minerals to the reaction tube,from which the promoted fixation of phosphorous by 5%moisture resulted.The calcium and magnesium content in XHS coal was less than SS1,and 5%moisture inhibited the enrichment of phosphorous in ash.(3)After doping 7.2%MgO in the SS2,the complete fixation of phosphorous in SS2 was basically realized.The capacity of hydromagnesite in phosphorous capture was positively correlated with moisture content.The phosphorous recovery rate of SS2 was slightly increased with 0-2%moisture;its capacity to capture phosphorous was improved by 5%H2O;when doping 10%H2O into air,90.21%phosphorous was captured in the ash of SS2.Following SS combustion,the recovery rates of bioavailable phosphorous in SS ashes were all less than 25%,which were very limited.Ca3(PO4)2 was more suitable to be used as the initial phosphorous substance than Mg3(PO4)2 for the modification of K2CO3.The neutral ammonia citrate(NAC)solution well extracted bioavailable phosphorous from the modified ash,indicating that this modified ash had the potential as a calcium-magnesium phosphate fertilizer,and this fertilizer might be more suitable for application in acidic soil.