| Pyrolysis is regarded as a sustainable increasingly mature thermochemical process for converting sewage sludge(SS)into fuels.In this study,pyrolysis of sludge was carried out in a two-stage quartz tube furnace,and the factors including the yield,chemical composition and high calorific value(HHV)of the bio-oil were evaluated in detail to investigate the influences of the hydrothermal pretreatment(HTP)on the biooil produced by SS.In addition,in-situ catalysis of molecular sieve MCM-41 and exsitu catalysis of MCM-41 were used to improve bio-oil quality.In the combined treatment experiment of HTP and pyrolysis,the influence of different HTP temperatures on the characteristics of hydrochars was determined.The kinetic calculations results showed that the thermal stability of hydrochars is better.The content of N and O elements in the hydrochars after hydrothermal pretreatment were partially removed,the removal efficiency of N and O in the SS is between 44%～68%and 38.6%～79.4%,respectively.GC-MS analysis results showed that the quality of bio-oil obtained from hydrochars pyrolysis has been improved.The content of oxygen-containing compounds and nitrogen-containing compounds were reduced by 14.77%～17.75% and8.45%～17.93%,respectively,while the content of hydrocarbons increased by6.48%～26.22%.The analysis of the N form in the liquid and compound distribution of water-soluble oil(WSO)indicated that most of the organic nitrogen has transferred into the liquid and a part of N has converted into WSO in the form of cyclic compounds.The results of elemental analysis showed that HTP could improve the fuel performance of bio-oil.The H/C atomic ratio increased,while the O/C atomic ratio decreased.Moreover,the HHVs of bio-oil increased from 32.972 MJ/kg to 36.353～37.007 MJ/kg.Based on the influence of HTP on the properties of bio-oil,the analysis of mass and energy balance of the combined HTP and pyrolysis treatment process was carried out.It was found that with the increase of HTP temperature,the yield of hydrochars and bio-oi gradually decreased.However,after HTP process,the organic components in the hydrochars are prone to convert into bio-oil rather than gaseous products.The highest energy recovery rate(44.23%)was obtained under 190℃.In the catalytic pyrolysis experiment of sludge,it was found that in the low and medium temperature range(500℃～700℃),thermal catalysis has little effect on the components distribution of bio-oil.In contrast,cyclization reactions are easily occurred under high temperature.The content of aromatic hydrocarbons increased from2.82% to 37.04% at 800℃.Molecular sieve catalyst MCM-41 can promote the removal of O element content in bio-oil,and the deoxygenation efficiency gradually increases as the proportion of catalyst increases.After catalytic pyrolysis,the HHVs of bio-oil increased.The highest HHV(36.834 MJ/kg)was reached when the proportion of MCM-41 is 20%.In this paper,the production of bio-oil by combined hydrothermal pretreatment and pyrolysis of sewage sludge was investigated in depth,and the mass and energy balance were discussed to evaluate the feasibility of the combined process.In addition,the effect of thermal catalysis and MCM-41 molecular sieve catalyst on the properties of bio-oil was studied.This work could expand the possibilities of preparing bio-oil by SS. |
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