Energetic use of CO₂ to increase the methane yield during anaerobic digestion of renewable energy crops

Background/Summary

In recent years, the demand for renewable energy and the reduction of greenhouse gases such as CO₂ has become increasingly important within the framework of energy transition.

The aim of the project is an energetic use of CO₂ to increase the methane yield during biogas production from renewable energy crops such as maize silage. The utilisability of CO₂ is based on the bioconversion process first described almost 30 years ago. Sato and Ochi showed in 1994 that CO₂ is converted into energetically usable methane (CH₄) without the addition of hydrogen (H₂). However, this effect has so far only been demonstrated using sewage sludge or food waste as substrates.

The main task of this project is to evaluate the bioconversion of CO₂ to CH₄ during the anaerobic digestion of different NawaRo substrates (maize, grass, whole plant silages). The CO₂ uptake potential and the additional CH₄ production will be quantified under various operational conditions (organic loading rate, CO₂ injection). Accompanying isotope analyses and microbiological investigations will reveal the underlying mechanisms and will provide a better understanding of the process.

In a next step, the results from the laboratory experiments will be transferred to a pilot plant. Possible applications in practice are the use of highly concentrated external CO₂ waste streams (e.g., from industrial processes) or the recirculation of CO₂ that accumulates as a waste product during biogas upgrading.

Thus, CO₂ emissions are avoided or reduced and converted into energetically usable CH₄.  

Schematic of the lab-scale digester system, including anaerobic digesters (control R₀ and test R_CO2), recirculation loops and bubble column reactor (scheme by Meriam Muntau, TU München)