Mechanism Study On Pyrolysis/Hydrothermal Liquefaction Of Cattle Manure

The large-scale and intensive development of the livestock industry has led to the dramatic increasing of livestock manure discharge in recent years.However,abundant livestock manure without being treated effectively would cause serious problems on both environment and human health.Treating livestock manure by thermochemical conversion method can not only mitigate environmental pollution but get high value-added products such as bio-oil.Hence,it would realize the energy and resource utilization of livestock manure.In this paper,cattle manure and its model compounds representing its main components（cellulose and protein）were selected as the research objects.The reaction mechanisms of cattle manure during the pyrolysis and hydrothermal liquefaction process were studied by combining the product distribution analysis and reaction kinetics analysis.The main research contents and results are as follows:（1）The pyrolysis process of cattle manure could be divided into five stages:dehydration stage,equilibrium stage,lignocellulose decomposition stage,protein decomposition stage,and inorganic components decomposition stage.The Coats-Redfern method was used to calculate the average activation energy values of the last three stages,which were 68.9k J/mol,2.6 k J/mol,and 55.9 k J/mol,respectively.Kinetic parameters calculated by DAEM,FWO and VM method were similar,and the activation energy values during the process varied from 119.1 to 454.4 k J/mol.The power law（P3）model was better fitted to the TG curve of cattle manure by the main plot method.（2）The pyrolysis products of cattle manure mainly included ketones,aldehydes,phenols,acids,hydrocarbons,and nitrogen-heterocyclic compounds.With the temperature increasing,the contents of ketones and nitrogen-heterocyclic compounds showed increasing trends,while the contents of aldehydes and hydrocarbons decreased significantly.Yet the content of phenols reached the maximum at 400℃.The initial decomposition temperature of cattle manure was around 180℃.The cellulose and protein mainly generate furfuryl alcohol,acetic acid,cyclopentenone,and nitrogen heterocyclic compounds through the Maillard reaction.Lignin decomposed into phenols through C-C bond breaking,and the methoxy group and hydroxyl group on the benzene ring in phenols were removed at high temperatures,thus generating phenol,cresol and aromatic hydrocarbon compounds.（3）The bio-oil yield of cattle manure from hydrothermal liquefaction reached the maximum 19.3%at 360℃.With the increase of reaction temperature,the O/C and N/C in the bio-oil samples decreased but the H/C_eff increased gradually,indicating that high temperature is beneficial to improve the quality of bio-oil.Compared with the pyrolysis reaction,the Maillard reaction was enhanced during hydrothermal liquefication,which greatly increased the yields of nitrogen heterocyclic compounds such as 3-pyridinol and inhibited the formation of acetic acid.In addition,hydrothermal liquefaction also promoted the decomposition of lignin in cattle manure and generated benzenediols through substitution reaction of methoxy groups on the benzene rings.With the temperature increasing,the contents of ketones and nitrogen heterocyclic compounds in bio-oil increased,while the contents of nitrogen heterocyclic compounds in water-soluble organic compounds decreased gradually,and the contents of phenols and benzenediols decreased after increasing first.（4）Co-liquefaction of cattle manure and corn cob could improve the yields of bio-oil and water-soluble organic compounds but inhibit the formation of solid products.After co-liquefaction,the ratios of N/C and O/C in bio-oil were significantly lower than expected,which indicated that co-liquefaction is conducive to the deamination and deoxygenation of bio-oil products.In addition,the yields of phenolic compounds,polyhydroxy alcohols,and nitrogen heterocyclic compounds also increased,indicating that the co-liquefaction was beneficial to the Maillard reaction and lignin decomposition reaction.Protein is the main component that promotes the synergistic effect,for its decomposition products can not only react with other components but also influence their reaction path by changing the p H value of the liquefaction environment.（5）The activation energies of glucose,glycine,and their mixture in the continuous hydrothermal liquefaction process were 83.0 k J/mol,52.3 k J/mol and 11.9 k J/mol,respectively,indicating that co-liquefaction could promote the decomposition reaction.Dehydration of glucose occurred mainly at low temperatures,but when the reaction temperature exceeded 340℃,the reaction rate of C-C bond breaking increased rapidly and became the main reaction path of glucose.The dehydration and decarboxylation were the main reactions of glycine at low temperatures.However,after 340℃,the deamination reactions were enhanced and became the major pathway.While Maillard reaction and deamination reaction were the main reactions in co-liquefaction,and their reaction rates increase with increasing temperature.Some glycine could also form products like pyrazines by dehydration or decarboxylation,but they would be inhibited at high temperatures.In conclusion,the Maillard reaction plays an important role in the decomposition process of cattle manure,and it is significantly enhanced during hydrothermal liquefaction.There are obvious synergistic effects among biomass components in the liquefaction process,which are mainly manifested in reducing the apparent activation energy of the reaction,reacting with other components,and changing the selectivity of pathways.