Fundamental Study On Supercritical Liquefaction Of Sewage Sludge To Produce Bio-oil With In Situ Hydrogenation

The hydrothermal liquefaction and incineration technologies were used to achieve the recovery of energy and resource during the process of sewage sludge(SS)treatment.The organic components in SS were firstly converted to bio-oil via sub-and supercritical liquefaction,and then phosphorus in SS liquefaction residue was recovered in the form of phosphate by incineration.Protein-carbohydrate and magnesium oxide-aluminum phosphate/ferric phosphate were chosen as model compounds to illuminate potential liquefaction reaction mechanisms of SS and the transformation mechanism of non-apatite phosphorus(NAIP)to apatite phosphorus(AP)during SS incineration processing which were using elemental analysis,FT-IR,GC-MS,TG-FTIR-MS and XRD.The experimental results showed that the yield and water content of bio-oil obtained in water were 16.70%and 15.51%,the value of O/C was high.The calorific value(HHV)of bio-oil was 31.46 MJ/kg.The yield and quality of bio-oils obtained by organic solvents via in-situ hydrogen were improved significantly.The H/C and O/C ratios in bio-oils decreased compared with those of SS,indicating that dehydration,dehydrogenation,decarboxylation and esterification reactions occurred during SS liquefaction with different organic solvents.The highest bio-oils yields were 42.3%in ethanol at 360?,43.1%in acetone at 360? and 26.3%in methanol at 280?.Esters were the main components of bio-oils in ethanol and methanol,which comprised 43.55%ethyl esters and 61.05%methyl esters,respectively.DSC-TG-DTG results illuminated that the bio-oil obtained from methanol was more stable than the others.Compared with bio-oil obtained in water,The highest bio-oil yield increased16?17%which was obtained in ethanol-water mixed solvents with Ni-Mo-S/Al₂O₃ at340? and 30 min.Catalytic liquefaction of SS in situ hydrogenation with the assisted of catalysts improved hydrocarbon contents due to heterocyclic compounds was destroyed and removal of O-containing function groups in phenols,esters,aldehydes and ethers by Ni-Mo-S/Al₂O₃.The weight losses of bio-oils was 89.45?92.01%and the main gas products were H₂,H₂O,CO,CO₂,CH₄,NH₃ and HCN.The results of model compounds liquefaction experiments indicated that esters,hydrocarbons,ketones,amines and N-compounds were produced underwent hydrolysis,decarboxylation,ammonolysis,esterification,cyclization and Maillard reactions.The synthesis pollution toxicity index model of heavy metals was established based on the study of distribution and speciations of heavy metls during SS liquefaction process.Most of heavy metals were concentrated in bio-chars.The potential ecological risk index results of heavy metals in bio-solids revealed the pollution levels were considerable risk and the monomial potential ecological factor indicated the effect of single heavy metal on heavy metals ecological risk was ranked in the order of Cu>Zn>As>Ni=Cr.The comprehensive toxicity order of heavy metals in different liquefaction residues was MSs>ASs>ESs.The contents of total phosphorus and AP increased with temperature increasing.High temperatures and additives were benefical for the transformation of NAIP to AP.Calcium oxide and magnesium oxide promoted the transformation of NAIP to Mg-P or Ca-P.The AP content increased from 27.57 mg/g to 44.15 mg/g at 950? with 13.5%Mg O.Thermogravimetric differential scanning calorimetry analysis was combined with X-ray diffraction measurements to simulate the process of SS liquefaction residue incineration of the crystal phase transition between Mg O and aluminum phosphate/ferric phosphate,forming stable AP such as magnesium pyrophosphate and magnesium phosphate in the temperature range of 625 to 950?.