Development of Biogas-Based Power-to-Methane Technology

The world’s current environmental state requires a solution. The increase of greenhouse gas(GHG) emissions has negatively impacted the world, by trapping the heat and thereby increasing the temperature globally. One of the major contributions to the increase of GHG emissions has been the increase in usage of fossil fuels such as oil, coal, and natural gas, whichhave caused an increase in carbon dioxide (CO2) emissions and CO2 concentration in the atmosphere. Besides this, the war between Russia and Ukraine in 2022 have severely impacted the energy stability, as gas supplies have been decreased to the EU. Energy solutions are needed which will utilize the CO2 to produce fuels that can substitute the fossils fuels and create more energy stability.This PhD thesis was conducted within the EUDP funded project efuel [Project ID 64018-0559]. The project was a collaboration between the University of Southern Denmark, the Technical University of Denmark, Nature Energy A/S, and Biogas clean A/S. The focus of this project was the development of a biological technology which can be implemented in large scale to convert CO2 to methane (CH4). A goal was to have a technology utilize CO2 directly from the source such as raw biogas. The focus was biological methanation (biomethanation), where hydrogenotrophic methanogens utilize CO2 and hydrogen gas (H2) to produce CH4. For utilization of hydrogenotrophic methanogens a trickling filter was used. During the thesis, tests were performed in laboratory scale with biological trickling filters (BTFs) of 0.3 and 8 L active volume and in pilot scale of 1000L per BTF. During laboratory tests the enrichment method was investigated, to significantly decrease the time needed for enrichment of the methanogens. This was accomplished by using a direct enrichment, compared to previous externally pre-enriched inoculum, where the inoculum was enriched directly in the BTF. With this direct inoculation of a BTF >90% CH4 content in the outlet gas was reached within 14 days, whereas studies with pre-enrichment used 1-2 months from start of enrichment to same gas quality from their BTF. The laboratory reactor operated in this laboratory research had an operational period of 110 days, with a production capacity(PC) at the end of 2.54 ± 0.16 mCH43 mr -3 d-1 and a product gas of 95.4 ± 4.4% CH4. DNA sequencing was performed and confirmed successful enrichment with an increase in Methanotermobacter, which is a group of hydrogenotrophic methanogens.
Another laboratory tests focused on the flexibility of biomethanation in a BTF. Having anintermittent energy system is a reality and this causes times where there will be no H2 production. To test the flexibility of biomethanation, periods of 6, 24, and 72 hours were tested where the supply of H2 was removed. For this, a laboratory setup of four 8 L BTFs was used.The BTFs had an operational period of 350 days, whereas 120 of these were prior to the testing of the flexibility. In this period the BTFs obtained a PC of 5.1 mCH43 mr -3 d-1 with a product gas quality of 96.9 ± 1.6% CH4. The study showed the potential for having biomethanation implemented into an intermittent energy system, as the results indicated a return to initial performance within 60 min after 24 hours of no H2 feed, with 95.6% CH4 in the outlet. For longer down times of 72h it would take more than 180 min to reach 90% CH4 in the outlet. This was achieved with optimized parameters of temperature and maintenance flow during thedowntimes to ensure optimal downtime conditions for the hydrogenotrophic methanogens.Pilot scale operation was used to validate the laboratory results, showing the potential for thetechnology, and moving it into the next phase. The pilot-scale plant was designed with twoBTFs of 1000 L each, built in configuration for both parallel and serial operation. The pilotplant had been running for more than 500 days at the time of writing this thesis and was set torun for at least 6 months more. During the parallel period an average production capacity of9.44 ± 2.37 mCH43 mr - 3 d-1 was reached during a 78-day period, where the product gas reached99% CH4 at multiple occasions. The pilot scale was also tested in serial operation, and here theavg. production capacity was 10% higher than in parallel operation. Besides this, the firstreactor in the series reached a PC of 16.8 ± 2.6 mCH43 mr -3 d-1, which was almost 78% higherthan the PC reached in parallel operation. This shows the potential of the technology, ifoperated under the right conditions. The pilot scale also validated the flexibility and duringseveral occasions the BTF regained performance within a few hours.Based on all results found in this thesis, it has been confirmed that biomethantion is a possibilityin larger scale operation. This thesis and the results show that the technology is no longer atechnology readiness level (TRL) of 4-5 (laboratory scale) or 6 (pilot scale), but it is ready togo into TRL 7+ (large scale). This thesis has paved the way for future energy systems andbiological power-to-x technologies. This thesis has proven that large scale biological powerto-x is possible today and can help reach the climate goals for 2030 and 2050.