Model-Based Computation of Critical Operation Points in Biogas Producing Plants

Abstract

Reducing the use of fossil fuels for energy production is one of the main objectives in 21st century. In order to achieve this target renewable energy resources (like agricultural waste) in biogas plants can be used. An anaerobic bacterial fermentation process digests the substrates into methane and carbon dioxide. The process itself has strong fluctuations in terms of net methane yield due to different amounts and composition of agricultural influents. For increasing the space time yield two main difficulties are encountered. The first one is system-specific and includes stirrer design and reactor geometry. The second affects the biotechnological fermentation process. The following work is focusing on the fermentation process. The determination of critical parameters for the optimization of the anaerobic microbial digestion is investigated. An economic approach for solving these problems is only feasible by using mathematical models and simulation. Consequently two fermentation models are compared by regarding parameter sensitivity and critical operational points. The first one is based on simple Monod-kinetics while the second one is extended with two steps of fermentation and therefore two different microbial consortia and additive inhibition effects. The complex model is able to describe different phenomena in more detail. But its estimability and therefore its validation is difficult without further investigation of the model structure and the reduction of the model complexity. One important result of the investigation is that stable process conditions with simultaneous high yields are depending on a careful adjustment of the loading rate and therefore requiring precise model parameters.