Study On Low-temperature Catalytic Pyrolysis Of Municipal Sewage Sludge For Bio-oil Production

The amount of sewage sludge from wastewater treatment plant in Chinapresently has increased rapidly, causing a important issue of constraining urbandevelopment and polluting urban environment. Therefore, utilization of sludge hasbrought about great interets of the national environmental scientists. Compared withthe disadvantages and shortages of the traditional treatment methods of sewage sludge,to find an innovative and effective new method of sludge recycling is emergentlyneeded. The low-temperature catalytic pyrolysis of sludge is a new developingtechnology with the dual characteristics of sludge treatment and energy recovery. Ithas been used in this study to treat residual sludge produced by an urban wastewatertreatment plant. Pyrolysis conditions, i.e., residence time and final pyrolysistemperature and catalyst, were optimized. The reaction mechanism of pyrolysisprocess was also discussed.The pyrolysis process of sewage sludge and dynamics was studied bythermogravimetry. The results showed that the pyrolysis process of sewage sludgeconsisted of three stages, i.e.,161-350℃,350-450℃and645-750℃. Each largearea of mass loss corresponded to a exothermic peak. The former two were the phasesof volatile separating out, and the third one was the phase of fixed carbon burning.Therefore, the sludge pyrolysis temperature range could be determined among thetemperature range of200and500℃. By means of Coats-Redfern method, the kineticequation and reaction parameters were calculated. The theoretical TG curves fittedexperimental TG curves very well.The catalytic pyrolysis conditions of sewage sludge were also optimized. Theresults showed that using catalyst could decrease the char fraction yield and increasethe oil fraction yield. Additionally, the quality of oil also improved. Compared withthe non-catalyst method, the final pyrolysis temperature needed for maximising thebio-oil yield decreased from450℃to400℃, the maximum oil yields increasedfrom34.53％to38.71％with composite catalyst. Using residue solid obtained fromlow-temperature pyrolysis of sewage sludge, the final pyrolysis temperature neededfor maximising the bio-oil yield decreased from450℃to410℃, the maximum oil yields increased from34.53％to37.79％.The chemical compositions of pyrolysis oil were characterized by GC/MS.Results indicated that the pyrolysis oil contained large amounts of straight-chaincompounds(C11-C19), and that aliphatic compounds were the main source of derivedoil. A large amount of nitrogen compounds (amide and cyanide class) of derived oilshowed that protein of sludge participated in pyrolysis reaction. Because of lessbenzene series of pyrolysis oil, it was the characteristics of pyrolysis products ofcellulose, hence the pyrolysis of cellulose was not a thermo-chemical conversion ofthe main reactions. With the help of thermal gravimetric analysis (TGA) for sewagesludge and GC/MS analysis for derived oil, the main reaction mechanism could bepresumed the evaporation of aliphatic compound and the conversion of protein from200℃to450℃.