Study On The Co-liquefaction Of Sewage Sludge With Rice Straw/sawdust To Produce Bio-oil And Biochar

With the increasing number of wastewater treatment plants and the increasing awareness of environmental problems,sludge treatment and disposal has become one of the most important challenges in the daily management of wastewater treatment plants.Sludge is the residue produced in the sewage treatment process of sewage plant,which is usually regarded as a typical pollution source.However,because of the certain calorific value of the sludge itself,people are eager to recycle it as a kind of energy through thermochemical conversion technology such as gasification,combustion,pyrolysis and liquefaction.Compared with other technologies,liquefaction technology does not need deep dewatering and drying treatment of sludge,so it has wider application prospects.The basic principle of sludge liquefaction is cracking reaction of organic matter in sludge under a certain temperature and pressure.Research has shown that sewage sludge is a good raw material for producing bio oil and biological carbon by liquefaction technology,but if the products are not properly pretreated after liquefaction,the direct utilization of the sludge will still cause some pollution to the environment.The preliminary study shows that the introduction of agroforestry in the process of sludge gasification and pyrolysis is one of the effective methods to improve the reaction process,improve the yield of the target products and promote the clean production.However,the introduction of common agricultural or forestry waste biomass in the process of sludge liquefaction can improve the yield of bio-oil or improve the characteristics of bio-oil.The further reduction of the activity and environmental risk of heavy metals in biological carbon remains to be further explored.To solve this problem,the following three tasks have been carried out.First,the effect of the reaction parameters?reaction temperature,reaction time,solid to liquid ratio,sludge-rice straw/sawdust ratio and type of catalyst?on the yield of liquefied products was preliminarily discussed.The results show that the reaction temperature is the most important parameter for the distribution of liquefaction products.The optimum reaction temperature of sludge and sawdust/rice straw is 280/300?,and the optimum residence time is 10/30min,respectively.Under this condition,increasing or decreasing reaction temperature or time will decrease or almost no effect on bio-oil yield.Generally speaking,prolonged reaction temperature and time will lead to lower biochar yield and higher gas and water rate.The higher the solid-liquid ratio,the lower the yield of bio-oil,but the higher the yield of biochar,that is,the lower the conversion rate.No positive synergistic effect was found between sludge andrice straw and sawdust on the yield of bio-oil and biochar.Alkali metal catalyst?Na/K-OH and Na₂/K₂-CO₃?is an effective catalyst to improve the yield of bio-oil.Second,we discussed the basic properties of bio-oil and biochar products in order to characterize their potential applications.With the addition of rice straw/sawdust,the content of nitrogen and sulfur in the bio-oil decreased obviously,but the content of nitrogen and oxygen was still higher than that of oil,and the carbon and hydrogen were still lower than that of oil.The content of ester compounds in bio-oil decreased,and the content of other organic compounds increased.The main coke-oil in bio-oil after sludge liquefying is kerosene,diesel and lubricating oil.When rice straw is added,the percentage of bio-oil in diesel and lubricating oil is reduced.When wood chips are added,the percentage of bio-oil in almost every tar type is reduced,resulting in very high residual rate.The introduction of sawdust/rice straw reduced the thermal stability,surface area,pore volume,carbonization degree and aromaticity of biochar,and the gap also began to become strong.The content of organic matter and total nutrient?TN+TP+TK?in biochar increased,that is,the effect of biochar on agricultural soil improvement.Third,we studied the migration and transformation of heavy metals during co-liquefaction.Heavy metals remain in biological carbon after liquefaction.After adding rice straw or sawdust,more heavy metals were transferred to bio-oil.The total content of heavy metals Ni and Cd exceeded the relevant standards.In addition,the content of heavy metals in the relative active state increased and the relative stable state decreased,and the leaching amount of heavy metals showed an increasing trend,in which the leaching amount of Ni and Zn exceeded the related standards.According to the comparison of GCF and GRI evaluation models,the pollution level and ecological risk of sawdust and rice straw increased more than that of sludge liquefaction alone.Generally speaking,after the introduction of sawdust/rice straw in the process of sludge liquefaction,it has little effect on the yield of bio-oil of the target product,but it can reduce the content of nitrogen and sulfur in bio-oil,improve the quality of bio-oil and improve the value of biochar to land use.On the other hand,the stable heavy metals of liquefied products will transform to active state,resulting in the increase of their pollution and ecological risk.This is also an urgent problem to solve.In addition,the mechanism of adding rice straw and sawdust to other pollutants produced by liquefaction of sludge should be further studied.