Optimum mixing rate in biogas reactors: Energy balance calculations and computational fluid dynamics simulation

Limited renewable energy resources, population overgrowth, development of industries and environmental pollution have made countries tend towards the use of renewable energy resources, e.g. solar, wind and biogas energies. In this respect, this study attempts to determine the best rate of mechanical stirrers for biogas reactors through energy balance calculations and fluid dynamics simulations. The conducted experiments utilized a semi-industrial scale bioreactor (1.2 m3 volume) containing a 45° six-bladed turbine stirrer while the simulations evaluated the effect of 0, 40, 80 and 120 rounds per minute (rpm) stirrer rates. The stirrer rate of 80 rpm provided appropriate mixing in the center and walls of the reactor, reduced dead spaces and enhanced mass and heat transfer in the reactor. In other words, the experiments on biogas generation from cow manure at different stirrer rates identified 80 rpm as the rate that can present the optimum performance. Also, energy balance and evaluation of the energy measures distinguished the stirrer rates of 40 and 80 rpm for achieving the highest energy efficiency and net energy gain. However, these energy related values were found to decrease at 120 rpm rate, noticeably. The highest obtained energy efficiency and net energy gain values, i.e. 1.182 MJ and 43.382 MJ respectively, were obtained at 80 rpm. Therefore, this stirrer rate is suggested to be applied under similar conditions.