Experimental And Mechanisms Study Of Direct Pyrolytic-liquefaction Of Sewage Sludge In Supercritical Water

Basing on the low temperature pyrolysis and direct thermo-chemicalliquefaction of sewage sludge, direct pyrolytic-liquefaction of sewage sludge insupercritical water(SCW) was put forward in this study. The raw sewage sludgefrom waste water treatment plant, without air drying, was used as the feedstockdirectly. During the reacting process, water containing in the sewage sludge washeated till its supercritical state, where some special properties of SCW appeared,sludge pyrolytic-liquefaction was thereafter carried out in it. Comparing with theexisting technologies, this new method avoided feedstock drying, therefore offeringan opportunity to reduce energy input and improving economic benefit.First, physico-chemical characteristics and fuel properties of sewage sludgewas analyzed. Then, in order to study the pyrolysis process, thermogravimetricanalysis(TGA) was carried out under different heating rates, so as to obtain thekinetic parameters and final pyrolysis kinetic models. According to the variation ofmass loss fraction and mass loss rate, process of sewage sludge pyrolysis can bedivided into three stages. With Fourier Transform Infrared Analysis (FTIR) afterTGA, type and features of pyrolysis reactions under different stages were inferred,and H2O、CO2、CO、CH4and C2H4were concluded as the gaseous main contentsfrom pyrolysis of sewage sludge.The self-design experimental system for sewage sludge supercritical conversionwas installed, and temperature(T), sodium carbonate catalyst content(Cc),residence time(tR) and feedstock filling ratio(g/v) were chosen as elements oforthogonal experimental design. Using SCW as reacting medium, sewage sludgewas pyrolyzed directly, and then tRand T were resulted as the most effective twofactors on sludge pyrolysis.The optimal operating conditions were thereafterdetermined by testing the above-mentioned parameters. In supercritical reactingarea, the operation condition, under lower temperature, namely375°C, and noresidence time(or very short retention time), can increase the amount of targetedproduct of pyrolyzed oil and therefore improve the economics of sludge pyrolysisin SCW.Using FTIR and GC/MS to analyze the compositions of pyrolyzed oil andcarbon residue can deduce the reaction route of sewage sludge direct pyrolytic-liquefaction process in SCW and meanwhile clarify its reactionmechanism. In the end, fuel properties of pyrolyzed oil were analyzed in detail.Compared with general fuel oil, its application value and market potential wereanalyzed.