Case studies and analyses of bioenergy systems

Biogas production via anaerobic digestion is an extensively explored and reliable method to convert biomass to bioenergy. A 90% increase in world research in the biogas industry over the past 10 years, as reported in 2021, stresses the growing captivation among researchers. Although lab-scale analysis with well-defined conditions is widely available in the literature, full-scale studies on anaerobic digestion plants are limited in comparison. Three published case studies on full-scale working plants are analyzed, and some unique observations, differing from published literature at times, are observed. The first case study on a plant using a substrate with high substrate variability confirmed that increasing the organic loading rate of biomass produced an increase in the ratio of volatile fatty acids to alkalinity (VFA/TAC) that resulted in reduced operation stability. Results of the second case study, using a substrate containing a high concentration of inhibitory molecules, demonstrated that decreasing the biomass flow rate increases the biogas production, reduction in total volatile solids is more pronounced for purer substrates dictating an effective biomass conversion, and inhibitory molecules did not impact biomass conversion because of excellent process design coupled with high heat exchange efficiency and increased homogenization of the substrate. In the final case study, foaming and overacidification were analyzed. While the study did not manage to confirm early warning signs for foaming, VFA/Ca, VFA/Mg, VFA/P, and VFA/S were proposed as early warning indicators for overacidification. Finally, some published indicators for overacidification, such as EWI-PO4/Ca, EWI-PO4, propionic/acetic acid, TOC/TN, and, in cases of high cattle rumen, the occurrence of high potassium levels and low sodium levels, were not deemed applicable in this study.