A novel concept for syngas biomethanation by two-stage process: Focusing on the selective conversion of syngas to acetate.

Thermal gasification of nonrenewable and renewable sources produces syngas, containing CO, H2, CO2 and N2. Anaerobic conversion of syngas to CH4 is a promising way to replace natural gas. However, the high N2 content (>50%) in syngas would result in the low CH4 content in the biogas and CO in syngas also had serious inhibition on methanogens. The present study proposed a two-stage anaerobic process for syngas biomethanation, and syngas was first anaerobically converted to acetate by mixed culture, which could be further converted to methane easily without the negative effects of N2 and CO. The results showed that mesophilic condition was more suitable for the conversion of syngas to acetate compared to thermophilic and ambient conditions at pH 5.5 considering the higher acetate yield and syngas conversion rate. Although CO was efficiently converted at thermophilic condition, it was mostly converted to H2, which was then converted to acetate. CO was much easier to be converted compared to H2. Further study showed that pH 6.5 and 7.5 were optimal for selective conversion of syngas to acetate. The other products including butyrate and ethanol were also detected in relatively higher amounts at pH 4.5 and 9.5. Although pH 5.5 and 8.5 had relatively lower syngas conversion rates compared to pH 6.5 and 7.5, they might inhibit methanogens naturally without adding methane inhibitors. Finally, batch experiments showed that the acetate concentration had obvious inhibition on syngas conversion when the acetate concentration was higher than 2 g/L.