Study On The Co-pyrolysis Characteristics And Char Formation Mechanism Of Dried Paper Sludge And Biomass

Paper sludge（PS）is the main by-product of wastewater treatment in the pulp and paper process.Pyrolysis technology can achieve PS reduction,harmlessness,stabilisation and resource utilisation,which is a very promising way of sludge disposal.However,the ash content of PS is high and its bio-oil suffers from low yield and low calorific value,making it difficult to use directly as a fuel.Compared to PS,biomass has a lower ash content and a higher volatile content.The addition of biomass to the PS pyrolysis process can make up for the shortcomings of PS pyrolysis alone,increase the efficiency of pyrolysis and improve the quality of the product.Therefore,this paper takes PS and corn straw（CS）as the research object,and investigates the influence of factors such as pyrolysis temperature and feedstock ratio on the co-pyrolysis characteristics,product yield,composition and physicochemical properties as well as the interaction pattern,analyses the char formation mechanism during the co-pyrolysis of PS and CS,and explores the adsorption characteristics of PS and CS co-pyrolysis char（sludge biochar）on methylene blue.This study can provide a theoretical basis for the efficient thermochemical conversion and high value utilization of PS and biomass.Firstly,a study on the co-pyrolysis characteristics and kinetics of PS and CS was carried out using thermogravimetric analysis to investigate the effects of PS and CS blending ratios（8:2,6:4,4:6,2:8）and heating rates(10,15,20 and 25℃·min^-1)on the co-pyrolysis weight loss characteristics of the raw materials.As the proportion of CS increased from 20%to 80%and the starting decomposition temperature increased from259.9℃to 281.8℃,the maximum weight loss rate increased from 5.29%·min^-1to15.36%·min^-1,the solid residue at 800℃decreased from 56.25%to 34.09%and the activation energy E increased from 56.66 k J·mol^-1to 71.27 k J·mol^-1.The temperature increase rate increased from 10℃·min^-1to 25℃·min^-1,the onset of pyrolysis of mixture6CS4PS increased from 272.1℃to 279.7℃,the maximum weight loss rate increased from 5.58%·min^-1to 14.13%·min^-1,the solid residue at 800℃increased from 40.15%to41.39%and the activation energy E increases from 64.09 k J·mol^-1to 66.32 k J·mol^-1.The comparison between the theoretical and experimental values shows that there is an interaction between PS and CS co-pyrolysis,inhibiting the co-pyrolysis process in the low temperature region and promoting it in the high temperature region,with the interaction being particularly pronounced when CS is added at 60%.Secondly,the experimental study of PS and CS co-pyrolysis was carried out using a small fixed bed reactor to explore the effect of pyrolysis temperature（400,500,600 and700℃）and feedstock ratio on the yield,chemical composition and distribution of the three-phase products.The maximum yield of bio-oil was 31.00%when the ratio of PS to CS was 2:8 and the pyrolysis temperature was 500℃.When the PS to CS ratio was 4:6and the pyrolysis temperature was 600℃,the phenolic and acid content of the bio-oil reached the maximum of 27.24%and 43.62%respectively,and the aromatic hydrocarbon content reached the maximum of 45.03%at 700℃.When the temperature is certain,the content of CO and H₂gradually increases and the content of CO₂gradually decreases as the ratio of CS increases.When the ratio of CS to PS is 4:6,the content of C₁～C₃small molecules is the largest.The interaction analysis showed that the higher temperature and CS addition promoted the gas yield and inhibited the liquid yield,and the co-pyrolysis of PS and CS promoted the production of acids,inhibited the production of furans and aromatics.The addition of CS inhibited the production of CO₂and H₂and promoted the production of CO.Elemental analysis,industrial analysis,BET,FTIR,SEM and XRD were used to systematically characterize the elemental and ash composition,specific surface area and pore capacity,surface functional groups,morphology and microcrystalline structure of the sludge biochar.The results showed that sludge biochar is mainly characterised by the presence of functional groups with aromatic ring structures on its surface.The H/C value of sludge biochar decreased from 0.48 to 0.36 with the increase of CS addition from 20%to 80%at the pyrolysis temperature of 700℃.When PS:CS=6:4 and the pyrolysis temperature was 700℃,the specific surface area and microporous specific surface area of sludge biochar were the largest,64.12 m²·g^-1and 45.38 m²·g^-1respectively,which was the result of the combined effect of volatilization analysis out and ash in PS.The equilibrium capacity of sludge biochar for methylene blue adsorption decreased from 43.47 mg·g^-1to36.56 mg·g^-1as the CS addition increased from 20%to 80%at the pyrolysis temperature of 700℃.The maximum capacity of sludge biochar for methylene blue adsorption was44.66 mg·g^-1at a PS to CS ratio of 6:4 and a pyrolysis temperature of 400℃.The p H and ambient temperature had an effect on the adsorption performance of the sludge biochar.The adsorption capacity of the sludge biochar increased with increasing p H and then decreased,reaching 95.65 mg·g^-1when the p H was 7.0 and the temperature was 55℃.