Thermal Modeling of a Pilot-scale Liquid Recirculation Anaerobic Digester

Abstract

A multitude of parameters affect the speed, maximal yield, quality of obtained biogas and several other important factors in the complex process of anaerobic digestion. One of these parameters is the temperature distribution inside the digester. It was noticed that when no form of stirring takes place inside the digester, a temperature gradient forms due to the natural convection phenomenon. The presence of this gradient is undesirable in most anaerobic digestion applications, since it would cause the anaerobic bacteria to suffer from upsetting temperature changes, triggering a noticeable slowdown and even a failure of the process. As stirring would fix this temperature gradient issue, an alternative low-cost solution is investigated in this study. This potential solution manifests in changing the placement of the solid waste layer inside the liquid recirculation digester to allow it to always be at an optimal temperature. Experiments and simulations have been conducted to better understand the thermal behavior of the digester, then a model was developed and validated to numerically simulate the observed phenomena. It was concluded that the occurrence of this gradient is independent of the contents of the digester, while in the same time it was established that the optimal location for the solid waste layer is between the heating elements.