CFD simulation and performance evaluation of gas mixing during high solids anaerobic digestion of food waste

To analyze the flow characteristics of the gas-mixed high solids anaerobic digestion (HS-AD) system and optimize the mixing performance, the mixing performance of gas and non-Newtonian fluid in the HS-AD system was investigated using the CFD simulation. The mixing energy magnitude, input pattern and nozzle inlet angle were optimized by evaluating available mixing indices. The performed experiments showed that the non-Newtonian fluid properties and consistency index of the food waste increased as the total solid increased. Moreover, it was found that the higher the reactor diameter, the greater the input energy, the smaller the dead zone, and the higher the global velocity gradient. Under specific input energy conditions, more mixing was achieved with the gas dispersed inlet compared with the centralized inlet. Symmetrical nozzles were arranged in the digester with a spacing of 180°, and mixing was the most effective when each nozzle formed an angle of 45° with the horizontal tangent of the reactor wall. As for the available mixing indices, both local velocity gradient and UI were the key ones and more meaningful to reflect mixing homogeneity of the system. The obtained results demonstrate that CFD is a powerful tool to optimize the mixing performance in high solids anaerobic digestion systems.