Investigation On Sub-/Supercritical Liquefaction Behaviors Of Municipal Sewage Sludge And Camellia Oleifera Cake

Municipal sewage sludge is a typical biomass with high concentration of water and large amount of organic matters which has been named as“bio-solid”.In recent years,supercritical fluid liquefaction technology has been widely applied in treating sewage sludge for liquid energy recovery.Besides,solvent property,reaction temperature and catalyst are deemed to be main factors influencing the liquefaction process.Based on the status and shortcomings of supercritical liquefaction of sewage sludge,this work carried out research following several aspects.Firstly,based on the shortcomings of investigation on carbon-based catalyst in sub-/supercritical liquefaction of sewage sludge,effects of sewage sludge-based activated carbon on liquefaction of sewage sludge in sub-/supercritical water were investigated.Synergetic effect of sewage sludge-based activated carbon and sub-critical water was found on increasing the yield and higher heating value of bio-oil to 29.25%?daf.?and 39.06 MJ/kg,respectively.Besides,after the treatment of sewage sludge in sub-critical water with sewage sludge-based activated carbon,Igeovalues of Cu,Zn,and Pb were below zero,namely in a clean level;Igeovalue of Cd>5,in extremely contaminated level.However,after the treatment of sewage sludge in supercritical water with sewage sludge-based activated carbon,I_geo values of Cu,Zn,and Cd were below zero,namely in a clean level;I_geoeo value of Pb was between4 and 5,in a heavily to extremely contaminated level.Results of RAC also demonstrated that sub-critical water could present better immobilization effect on heavy metals.However,the bio-oil originated from liquefaction of sewage sludge in sub-/supercritical water mainly composed of pyridines,indoles,ni triles,esters,phenol and its derivatives which resulted in low quality.Considering the low quality of bio-oil originated from liquefaction of sewage sludge in sub-/supercritical water,liquefaction of sewage sludge in supercritical organic solvents was investigated with ethanol and acetone in the second part.Compared to liquefaction in water,sewage sludge liquefaction in organic solvents was found to obtain higher bio-oil yield of 62.43%?180.11%.Besides,the greater the polarity of solvent,the higher the conversion rate of sewage sludge.When acetone used as the solvent,ketones,pyridines,esters,phenol and its derivatives were main components in bio-oil;while esters?C10?43.59%?66.59%?,pyridines,phenol and its derivatives were main components in bio-oil when ethanol as solvent,which was in line with the composition of biodiesel.Moreover,bio-oil generated from liquefaction in ethanol?32.97?34.78 MJ/kg?presented higher heating value while lower amount of heavy metals than that from liquefaction of acetone.For high temperature acetone could corrosion the reactor wall.Considering the yield,composition and heavy metals in bio-oil,liquefaction of sewage sludge in supercritical ethanol was preferred.It has been confirmed that liquefaction of sewage sludge in ethanol could obtain bio-oil with higher quality than in water and acetone.Therefore,existing forms and environmental risk of heavy metals in sewage sludge before and after supercritical ethanol treatment?300 and 350 ^oC?was investigated in comparison with sub-critical water?300 ^oC?treatment.Sub-critical water favored the immobilization of Zn,Pb,Cr,Cu,while supercritical ethanol favored the immobilization of Zn,Pb,Cr,Cu,and Cd.Moreover,the higher the reaction temperature,the stronger the stabilization of Pb,Cd,and Cr.Evaluation results of I_geo,RI,and RAC suggested that liquefaction of sewage sludge in ethanol at 350 ^oC could obtain the lowest environmental risk of sample,the result of toxicity characteristic leaching procedure also confirmed it.Considering the high concentrations?Zn,7.68?20.40;Pb,1.20?1.84;Cd,0.04?0.16;Ni,3.20?5.72;Mn,1.48?7.40;Cr,2.80?5.40;Cu,22.92?94.44 mg/kg?of heavy metals in the bio-oil originated from sewage sludge,co-liquefaction of sewage sludge with other biomasses was proposed to decrease the contents of heavy metals while remaining the bio-oil yield unchanged,and Camellia oleifera cake was chosen to be the one.Thus,feasibility of liquefaction of Camellia oleifera cake in supercritical organic solvents were studied with methanol,ethanol,and acetone as solvents.In comparison with supercritical organic solvent liquefaction of sewage sludge,liquefaction of Camellia oleifera cake obtained higher yield of bio-oil.Besides,concentrations?Zn,3.32?9.20;Pb,0.48?1.64;Cd,0?0.08;Ni,2.60?124.48;Mn,0.52?8.48;Cr,1.72?4.00;Cu,37.32?198.44 mg/kg?of heavy metals were lower.Moreover,bio-oil from liquefaction of Camellia oleifera cake in methanol and ethanol mainly composed of esters?>80%?,which similar to biodiesel.Therefore,liquefaction of Camellia oleifera cake in alcohols could be an effect way for generating bio-oil.Based on the studies and results obtained,co-liquefaction of sewage sludge and Camellia oleifera cake was proposed.Compared to liquefaction of sewage sludge only,co-liquefaction could generating higher bio-oil while liquefied in ethanol at 350^oC?76.62%?and in acetone at 300 ^oC?89.38%?.However,results of GC-MS showed that esters?69.95%?77.76%?were main composition in bio-oil generated from liquefaction in ethanol which comparable to that of biodiesel.Moreover,concentrations of heavy metals?Zn,4.16?11.56;Pb,0?1.92;Cd,0?0.12;Cr,1.20?3.36;Cu,8.60?153.44 mg/kg?in bio-oil originated from co-liquefaction were lower than that from liquefaction of sewage sludge alone,and most of lowest concentrations obtained in ethanol at 300 ^oC.Further,ethanol presented better immobilization effect on heavy metals in solid residue than acetone did.The leaching rate and leaching concentration of heavy metals were quite lower than that in sewage sludge and consistent with the standards requirement.Therefore,co-liquefaction of sewage sludge and Camellia oleifera cake might be preferred for the reduction,reuse,and recycle of both solid wastes.