Biomethane Production in an Innovative Two-phase Pressurized Anaerobic Digestion System

Publication Type

Doctoral thesis

Authors

Yuling Chen

Year of publication

2015

Generation of biogas from biomass through anaerobic digestion is receiving increasing attention. Over the past decade, the biogas industry has been developing rapidly in Germany, as well as the rest of the world. In Germany, biogas is generally used in a heat and power plant (CHP) for electricity and heat production. However, most biogas plants are located in a rural area, where heating demands are quite low. Except for biogas plant thermal control, a huge amount of cogenerated heat is often wasted. In order to increase the overall energy utilization efficiency, biogas can be alternatively converted to biomethane of natural gas quality and injected into existing gas grids. By making use of the mature gas transportation and storage systems, biogas production and end utilization can be temporally and spatially separated. Therefore, it is regarded as an efficient and flexible solution to energy issues. Nevertheless, in terms of this application, raw biogas requires, above all, gas purification and upgrading. Carbon dioxide content, in particular, must be reduced from 40–50% in the raw biogas to approximately 4% in the purified gas. Conventional technologies are generally expensive in investment and/or operation. Therefore, an economical option is desired.
Within this research project, a two-phase pressurized anaerobic digestion system was developed. The innovative concept aimed to reduce the cost involved in biomethane conversion and injection into the natural gas grids by integration of biogas production, purification and compression in one system. It was expected that a great amount of carbon dioxide could be directly removed from the pressurized digester due to its high solubility. In addition, the methane-rich biogas could be produced at an elevated pressure which could meet the injection standard, and therefore could reduce or even avoid the expenses for further compression. In order to gain better understanding of two-phase pressurized anaerobic digestion, three major studies were conducted:
- The pressure effects on two-phase anaerobic digestion
- Effects of organic loading rate (OLR) on the performance of a pressurized anaerobic filter in two-phase anaerobic digestion
- Effects of liquid circulation on two-phase pressurized anaerobic digestion
By this means, the system performance could be examined and the technical feasibility and potential of the new concept could be explored. Moreover, an optimization of the process in a two-phase pressurized anaerobic digestion system could be realized. From both economic and ecological perspective, two-phase pressurized anaerobic digestion offers an interesting process option for biomethane production, making a great contribution to sustainable energy supply.