Study On Characteristics Of Sewage Sludge Gasification And Heavy Metal Migration And Transformation

How to properly handle sewage sludge attracts more and more attention of our society with the rapid growth of urban sewage sludge production in China.Under this background,as a more environmental friendly sewage sludge disposal and recycling technology,sewage sludge gasification is becoming a hotspot in academic and industry circles.However,compared with traditional higher temperature gasification,the medium-low temperature gasification of sewage sludge has more prominent environmental benefits because of its low emission of heavy metals and other pollutants.Therefore,this paper focuses on the the characteristics of of medium-low temperature gasification and heavy metal migration and transformation.Firstly,characteristics of N₂ pyrolysis,CO₂ gasification and air combustion of sewage sludge were preliminarily studied by TG-FTIR,and the dynamic parameters of each process were calculated.The main functional groups in sewage sludge organic matters include carboxyl,carbonyl,amido,amino,methyl,methylene,aromatic group and so on.Thermogravimetric analysis showed that the pyrolysis,gasification and combustion process of sewage sludge can be divided into three stages all.The first stage of the three processes is all evaporation of water.The second stage of pyrolysis is the thermal decomposition of organic matters,the third stage is the thermal decomposition of residual organic matters and inorganic carbonate.The second stage of gasification is the thermal decomposition of organic matters and the gasification of fixed carbon,the third stage is the gasification reaction of the residual carbon and the decomposition of the inorganic carbonate.The second stage of combustion is the combustion reaction of volatile matters and residual organic matters,the third stage is the combustion reaction of residual organic matters and the thermal decomposition of inorganic carbonate.After the second stage,the conversion rates of pyrolysis,gasification and combustion reached 84%,90%and 94%respectively.This stage is the maximum weightlessness stage of each process,and the corresponding activation energy is the largest in each stage too.The gasification process of sewage sludge contains pyrolysis,combustion and gasification reaction.TG experiment of sludge preliminarily proved that the middle-low temperature gasification of sludge can achieve higher conversion rate.Secondly,CO₂ gasification and H₂O gasification experiment of sewage sludge were carried out by tube furnace,and compared with N₂ pyrolysis experiment.The characteristics of sewage sludge medium-low temperature gasification and heavy metal migration and transformation in tube furnace were systematically studied.It is found that the time sequence of each combustible gas getting peak releasing rates in the process of pyrolysis and gasification is CO peak,CH₄peak,CnHm peak and H₂ peak.The initial mass and particle size of the sample have a certain influence on the CO and H₂ summit.When the mass of the sewage sludge is larger and the particle size is bigger,the H₂ sub peak will be produced when in N₂ pyrolysis.The CO sub peak will be produced when it is in CO₂ gasification.The cold gas efficiency of CO₂ gasification,steam gasification and N₂ pyrolysis is close to each other within 450?and 550?,and steam gasification is slightly higher than CO₂ gasification after 550?,while the gap between N₂pyrolysis and the above two increases gradually.The weight loss rate and carbon content of the samples in CO₂ gasification,steam gasification and N₂ pyrolysis from 450?to 700?are all close to each other.After 700?,CO₂ gasification and steam gasification still keep close,and the gap between N₂ pyrolysis and the above two is increasing.The cold gas efficiency of CO₂gasification is 50%at 700?and 87%at 850?.Because of the high activity of steam at high temperature,the cold gas efficiency of steam gasification is 70%at 700?,and even 106%at850?.The residual rates of Cr,Ni,Cu,Zn,As and Pb in the CO₂ gasification residue were all slowly decreasing from 450?to 700?,but decreasing rapidly in the interval of 700?-850?.The residual rate of Cd decreased slowly from 450?to 550?,then decreased rapidly from550?to 700?,lastly decreased slowly from 700?to 850?.In particular,the residual rate of Cr,Ni,Cu,Zn and As in the gasification residue of middle temperature?about 700??is about 1.2 times that of higher temperature?about 850??,Pb is about 2.7 times and Cd is about7.5 times.The proportion of stable morphology of Cr,Ni,Zn,As and Cd in the gasification residue is significantly higher than the original sludge,and the proportion of Pb and Cu is similar to that of the original sludge.Especially,the proportion of stable form of each heavy metal in the medium temperature residue is close to that of higher temperature.Finally,the sludge air gasification experiment was carried out by a self-built fluidized bed thermal experimental device,and the characteristics of medium-low temperature gasification and heavy metal migration and transformation in fluidized bed were studied systematically.Among the main parameters affecting the sludge gasification characteristics in fluidized bed,the most important factor affecting the cold gas efficiency and carbon conversion rate is gasification temperature,the second is air equivalent ratio.The effects of air ratio and fluidization number on the gasification characteristics are weaker.Without the occurrence of slugging,bed height increases,the cold gas efficiency and carbon conversion rate increase too.Compared with higher temperature gasification,the output of tar in medium-low temperature gasification increased significantly.With the increase of secondary air and air ratio,the tar yield of medium-low temperature decreased monotonously,the scope of which is smaller than higher temperature.With the gasification temperature rises from 600?to 850?,the cold gas efficiency and carbon conversion rate are all increasing.In the process of the air equivalent ratio rising from 0.2 to 0.4,the cold gas efficiency increases firstly and then decreases,and reaches the maximum at 0.3,while the carbon conversion rate increases monotonously.In the process of gasification temperature rising from 600?to 850?,the residual rate of heavy metals in bottom slag decreased monotonously,but the mobility in fly ash,gas and tar increased monotonously.With the increase of equivalent ratio?from 0.1 to 0.6?,the oxidation of atmosphere in the gasifier increases,the mobility of Ni and Cu decreases,while Cr increases,and the other heavy metals change slightly.In summary,it is found that the gasification temperature can be chosen at about 700?in medium-low temperature sludge fluidized bed gasification industry.Comparing with higher temperature gasification,the cold gas efficiency of medium-low temperature gasification is lower than,but the proportion of heavy metals migrating to fly ash,gas production and tar is much smaller,and the proportion of heavy metals in the bottom slag did not decrease significantly.While ensuring a certain economic value,this technology highlighted its environmental friendly characteristic.