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    Two-stage high pressure anaerobic digestion for biomethane production
    (2017) Merkle, Wolfgang; Jungbluth, Thomas
    The use of natural gas for power and heat generation in the EU has become particularly prominent since the 1990s. As a result, the whole natural gas infrastructure has been continuously expanded and today has a total length of 2.15 million km and a storage capacity of about 108.3 billion m³. The production of biomethane in the EU and its distribution by natural gas network offers an interesting alternative for the reconfiguration of EU’s energy supply system. Up to now, biomethane is obtained by purifying and upgrading raw biogas in a complex process. In this study, a novel two-stage high pressure anaerobic digestion system was developed. This innovative concept aims to integrate biogas production, purification and pressure boosting within one system. The process is based on the enhanced water solubility of carbon dioxide compared to methane. By operating the methane reactor for biogas production at increased pressures, high amounts of dissolved carbon dioxide can be removed with the liquid effluent from the reactor, resulting in a high-calorific biogas. In batch experiments at pressures up to 30 bar, a significant influence of pressure on the pH-value in the reactor was observed, due to the augmented formation of carbon hydroxide. The study on the effect of a rapid pressure increase up to 100 bar showed no inhibition of the microorganisms in the batch-rigs too, although the microorganisms were not adapted to these environmental conditions. Furthermore, a continuously operated methane reactor was run at pressures up to 50 bar for the first time. The experiments showed that a stable anaerobic digestion process could be run at these pressures nearly without any problems and methane contents above 90% could be achieved. The promising results showed that this technology has great potential in producing on-site high calorific gas also in smaller units. In addition, the costs of post-production gas purification can be significantly reduced, due to the fact that the size of a subsequent gas purification unit can be decreased. Furthermore, the produced gas can be injected into the transnational gas grids without post pressurization or can be used in the transportation sector.