Study On Rheological Properties Of Mixed Materials In Anaerobic Fermentation

Anaerobic fermentation technology is an important way to deal with agricultural waste,solve the problem of environmental pollution,produce clean energy and recycle biomass resources.The rheological characteristics of fermentation materials in the process of anaerobic fermentation have an important impact on the calculation of heat and mass transfer,the selection of key equipment such as pumps and stirring devices and the selection of process parameters in the process of material treatment,disposal and transportation.However,there is no research on measuring the rheological properties of mixed fermentation raw materials in the process of anaerobic dry fermentation with on-line viscometer at home and abroad.Therefore,in this paper,cow dung and corn straw,which are produced in a large amount in agricultural waste,are used as anaerobic fermentation materials.The solid content of the fermentation system is 8%,15%,20%and 22%.The fermentation temperature is the medium temperature（37±1）℃and high temperature（52±1）℃.Cow dung fermentation alone and cow dung+straw mixed fermentation are carried out respectively.The physical and chemical indexes such as apparent viscosity,methane yield,volatile fatty acids,chemical oxygen demand and ammonia nitrogen concentration in the process of cow dung single fermentation and cow dung+straw mixed fermentation were analyzed,and the fitting of the kinetic model of each experiment was studied.It is expected to provide theoretical reference for the selection of anaerobic digestion equipment,the stable operation of the system,the adjustment of operation strategy and the reduction of energy consumption in the actual biogas project.The main results of this paper are as follows:（1）For rheological properties,with the progress of anaerobic digestion,the apparent viscosity of each experimental group showed a downward trend,and the decline rate was the fastest in the initial stage of the reaction.Under the condition of the same material and temperature,the greater the initial total solid mass of the material,the greater the apparent viscosity.Especially for anaerobic dry fermentation,the apparent viscosity is more obvious with the increase of total solid concentration.At high temperatures,the apparent viscosity of the material is significantly lower than that at medium temperatures.Under the same conditions,the apparent viscosity of mixed materials is significantly higher than that of cow dung.In addition,compared with cow dung fermentation alone,mixed fermentation can effectively shorten the delay period of the fermentation system,improve biogas production,and increase the buffer capacity of the fermentation system to avoid rancidity.At the same time,due to the participation of straw,the density of mixed materials did not change significantly compared with cow dung in the fermentation process.（2）For the gas production characteristics,the mixed materials have a significant effect on the gas production of the anaerobic fermentation system.The number of days required for the volume fraction of methane to reach more than 50%is significantly lower than that in the experimental group of cow dung fermentation alone.Under the condition of high temperature,the volume fraction of methane in the mixed material experimental group exceeded 50%on the 2nd,2nd,3rd and 4th day respectively,and reached the maximum values of 66.7%,68.9%,63.4%and 62.2%on the 3rd,3rd,3rd and 4th day respectively.From the perspective of cumulative methane production,the time for the mixed material experimental group to reach 80%of the total methane production is also significantly shorter than that for single cow dung fermentation.However,compared with the mixed material,the methane content in the biogas produced by cow dung in the fermentation process is lower.In the later stage of anaerobic fermentation,the methane volume fraction of the cow dung experimental group is stable between 50%-70%,while the methane volume fraction of the mixed material experimental group is between 50-60%.（3）For the results of kinetic analysis,in the medium temperature cow dung anaerobic fermentation system,the experimental group with TS of 8%had the best fermentation effect,with the shortest lag period,the maximum VS methanogenic rate and the maximum hydrolysis rate,which were 4.12d,13.18m L·（g·d）^-1and 0.03523respectively.In the anaerobic fermentation system of cow dung under high temperature,the experimental group with TS of 22%had the best fermentation effect,with the shortest lag period,the maximum methane yield and the maximum hydrolysis rate,which were 2.55d,222.55m L·g^-1and 0.04677,respectively.In the mixed anaerobic fermentation system of cow dung and straw at medium temperature,the experimental group with TS of 8%had the best fermentation effect,with the maximum cumulative VS methanogenic rate,the maximum VS methanogenic rate and the maximum hydrolysis rate,which were 245.515m L·g^-1,18.68m L·（g·d）^-1and0.06894 respectively.In the mixed anaerobic fermentation system of cow dung and straw under high temperature,the experimental group with the TS of 8%had the best fermentation effect,with the maximum cumulative VS methane production rate,maximum methane yield and maximum hydrolysis rate,which were 269.47 m L·g^-1,25.1 m L·（g·d）^-1and 0.1104 respectively.（4）For the change of stability parameters,in the whole fermentation process,the total solid mass fraction and volatile solid mass fraction of materials decreased rapidly and then tended to be flat with the progress of anaerobic fermentation.The concentration of ammonia nitrogen in each experimental group increased with the increase of initial total solid mass content,and the concentration of ammonia nitrogen in each experimental group was lower than the inhibition concentration,and there was no phenomenon of ammonia inhibition.Compared with single cow dung,the change of ammonia nitrogen content of mixed materials in the process of anaerobic fermentation is more stable.When cow dung is fermented separately,the pH value of the material has an obvious downward trend in the initial stage of fermentation,while the mixture does not,indicating that the mixed material can effectively reduce the impact of acidification.At the same time,increasing the initial total solid mass content can improve the pH value in the system and help to improve the buffer capacity of the system.