Modeling and Simulation of Microbial Fuel Cells

Microbial fuel cells are one of the newly emerged technologies in the field of energy recovery and treating wastewaters simultaneously which have attracted interest in recent years. In this study, MFC model based on the direct conduction of electrons in biofilm has been presented and validated by available experimental data from litrature. The presented model is able to predict the measured data with an acceptable accuracy. Furthermore, the investigation of anode potential variations demonstrated that the produced current density shifts from zero to a saturated value in a narrow range around half-saturation voltage. The substrate concentration showed similar effects but in a wider range. According to these observations, it is determined that the maximum attainable current density from a pure-culture biofilm composed of Geobacter sulfurreducens is 2.42 (A/m2) with acetate as substrate under the specified conditions. If the anodic chamber would perform in batch mode under the defined conditions, the current pattern will be repeated with each fresh substrate injection with the existence of a mature biofilm. The maximum current will reach 3.12 (mA) in this case. The performance of anodic compartment was examined in continuous flow for the first time. The results obtained for continuous flow of substrate in a cylindrical anodic chamber with the biofilm developed over its inner surface showed that the overall process was controlled by external mass transfer resistance. In this situation reducing the diameter and increasing the length of the tube are generally favorable in viewpoint of both elevated current density and treatment efficiency. The inlet substrate concentration showed no influence on treatment efficiency while resulted in raising the current density. Treatment efficiency decreased with flow rate; this effect saturated finally for high flow rates. With defining a novel boundary condition for potential equation, it would be possible for the first time to investigate the effects of external electrical load using polarization curves by conduction-based model. The obtained results demonstrated that for a biofilm made up of G.sulfurreducens and with the presence of acetate the major overpotentials were associated with the mass transfer resistances. It would be possible to determine quantitatively the effects of biofilm conductivity and concentration film thickness on MFC performance. The contributions of ohmic and mass transfer polarization increased with decrease in the biofilm conductivity and increase in the thickness of the concentration film adjacent to biofilm, respectively. These polarizations decreased the maximum output power density. Finally by using the verified model, the design parameters for an annular MFC with spiral anode were estimated employing genetic algorithm and then validated, for the first time. The optimized parameters implied that the effective anode surface area increased by 64.76% for a screen with a mesh of 300. It is not possible to measure surface area experimentally. The polarization behavior of this system clarified that the ohmic overpotential was responsible for the major voltage drop due to the low biofilm conductivity obtained for the present biofilm. The behavior of the MFC was examined with the constant concentration of 1 (mgCOD/cm3) and external load of 50 (Ω). The potential gradient inside the biofilm was remarkable compared to the biofilm made of G.sulfurreducens. The biofilm thickness increased unlimitedly due to not considering the detachment of the biofilm causing the internal mass transfer resistance to increase continuously.

Keywords:

Microbial fuel cell, modeling and simulation, direct conduction, parameter estimation, genetic algorithm