Simulation of Mixing Effect on Methane Production in Anaerobic Digestion

Anaerobic digestion of materials is one of the accepted technologies in the field of industry. In this technology, providing the necessary conditions for optimal mixing of materials is of great importance. In this research, the mixing process was simulated using Computational Fluid Dynamics (CFD), and the possibility of predicting the kinetic process of biogas production from animal waste and determining the optimum speed of the stirrer during the process was investigated. In the initial stages of the work, data related to an anaerobic digester with an agitator, and mixing speeds of 0, 100, and 150 rpm were recorded for one month, and the measured characteristics were converted into the inputs of the ADM1 model. Then, the initial values that were reported during the start-up stage of the digester were estimated. The reactor was optimized to a volume of 400 liters. The stirring system is mechanical and works for five minutes every 6 hours at speeds of zero, 100, and 150 rpm. According to the graphs and the results, the produced gas has almost a constant trend from the 15th day onwards and reaches 75% of methane, and the highest average percentage of methane produced is 64%, which happened at a stirring speed of 100 rpm. During the experiments, the pH was in the range of 5.7 to 7.3.
Introduction
As the population increases and energy resources are limited, all countries will face energy problems. The crises that threaten countries and societies are the lack of energy resources (fossil fuels) and the increase in environmental pollution caused by excessive consumption of fossil fuels, which shows the necessity and importance of using alternative energy sources. The close connection between economic and environmental issues has created new approaches in the field of international environmental law, one of the most prominent of which is the green economy, and since one of the main goals of the green economy is to reduce greenhouse gas emissions, the use of renewable energy sources is a fast way to Achieving a green economy. Mixing is an important process in AD that has the following advantages: (1) It promotes direct contact among enzymes, bacteria, and substrates; (2) It avoids foam formation and sedimentation; (3) It enhances heat and mass transfer; (4) It facilitates the release of biogas; (5) It disperses any toxic materials in the influent to avoid inhibitions. Some researchers also compared different intermittently mixed anaerobic digesters.
Materials and Methods
In this research, a digester with a diameter of 60 cm was used and the volume of the digester was 0.4 cubic meters. The standard volume of the maximum substrate that can be loaded is 0.325 cubic meters. Digester stirring is done by a mechanical stirrer connected to an electric motor in the central part of the top of the cap. The whole digester has a capacity of three layers of materials, each layer has its own sensors. Acidity and temperature sensors collect the relevant variable status and store and transfer it to virtual memory through the electronic control system. The anaerobic digester system in the bioenergy and recycling laboratory unit has been repaired, and a sample of cow manure was prepared from the animal husbandry unit around Tabriz and transferred to the laboratory as a substrate for conducting research. The experiment was done in three repetitions, and in each repetition, 150 kg of fresh animal waste was poured into the digester tank with 150 liters of water. Then, to add methanogenic microorganisms to the substrate, 10% of the total weight of the tank (substrate), i.e. 30 kg of animal rumen, was prepared and added. Each repetition of this process continued for 30 days, and the temperature inside the tank was kept at the same temperature as the outside environment (30 degrees Celsius) in the first repetition, and at 35 degrees Celsius in the second and third repetitions. Mixing was done automatically for 5 minutes only in the second and third repetitions and every 6 hours, and the mixing speed was set to 100 and 150 rpm, respectively. After the system started working, gas was discharged twice a day (every 12 hours) according to the production rate and pressure. The total amount of methane produced until that day was measured on the meter and the percentage of methane gas produced daily was measured by the methanometer. Also, in this research, using computational fluid dynamics (CFD), the prediction of the kinetic process of biogas production from animal waste and the provision of the appropriate stirring cycle during the anaerobic digestion process was investigated. In the initial stages of the work, data related to an anaerobic digester with an agitator, and mixing speeds of 0, 100, and 150 rpm were recorded for one month, and the measured characteristics were converted into the inputs of the ADM1 model. Then, the initial values that were reported during the start-up stage of the digester were estimated.
Results and Discussion
According to the graphs and the results, the produced gas has almost a constant trend from the 10th day onwards and reaches 75% of methane, and the highest average percentage of methane produced is 64%, which happened at a stirring speed of 100 rpm. During the experiments, the pH was in the range of 5.7 to 7.3. Gas production is approximately fixed from the 15th day onwards and reaches 80% of methane. As can be seen in the figure, biogas production usually decreases on days with increased loading; In every change of the input organic load, the largest amount of freshly undigested feed enters the system, so the digestion steps begin with cell destruction and hydrolysis. These steps are often time-consuming and, in addition, the products of the hydrolysis step are acidic. Therefore, it is expected that the activity of biogas production microorganisms will decrease with the acidification of the environment.
Conclusion
In this work, the effects of mixing time on AD performance were studied experimentally and numerically. The results showed that when the mixing time in intermittent mixing mode was long enough to homogenize the digester, then increasing the mixing time would not increase biogas yield but increase energy input. However, if the digester cannot reach homogeneity within the mixing time in intermittent mixing mode, then the digester cannot operate in its optimum condition. These results indicated that simulated mixing time can be used as a reference to determine the minimal experimental mixing time to increase the AD efficiency.