Producing high CH4-content (>95%) biogas in high-pressure reactors with bio-hydrogen addition and with minimal CH4 loss

초록

Producing high-CH4 content biogas directly from anaerobic digesters can reduce burdens in the upgrading process, either by supplying external H-2 or by using high-pressure digestion. However, these processes face challenges, including high H-2 costs and significant CH4 losses to the digestate. Therefore, this study proposes a two-stage high-pressure digestion (2St-HPD) system comprising high-pressure dark fermentation (HPDF) followed by high-pressure anaerobic digestion (HPAD), in which H-2-rich gas from HPDF is fed to HPAD. Initially, a mass-balance and chemical-equilibrium-based calculation was developed to assess the feasibility of producing high-CH4 content biogas with minimal loss. The results showed that single-stage HPAD required 50 bar to reach 98% CH4 content (with > 86% CH4 loss), whereas 2St-HPD achieved the same at 5 bar with only 9% loss. The system was then tested experimentally using glucose as a model substrate. First-stage HPDF operated at 1 to 9 bar increased H-2 content from 55.0% to 89.1%, approaching the theoretically calculated value of 90.5%. The second-stage HPAD was initially operated on HPDF effluent only, with pressure increasing from 1 to 5 bar, resulting in a CH4 content in the biogas of up to 92.8%. Later, after injecting the H-2-rich gas, the CH4 content further increased to 95.7% at 5 bar, while the COD loss to dissolved CH4 was decreased (11.7% -> 9.0%). To our knowledge, this was the first report to produce high-CH4 content biogas directly from AD without external H-2 addition, with < 10% CH4 loss.

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