Biogas Upgrading by CO2 Methanation with Ni-, Ni–Fe-, and Ru-Based Catalysts

This piece of work dealt with the concept of ‘biogas upgrading’ or enrichment of the CH4 contained in a sweetened biogas to proportions and features comparable to those of synthetic natural gas (SNG). For this, the behavior of three lab made catalysts (Ni/Al2O3, Ru/Al2O3, and Ni–Fe/Al2O3) was tested in a CO2 methanation reaction (Sabatier reaction) under different feeding conditions (with and without methane). In the first set of experiments (without methane), the good catalytic behavior of the solids was validated. All three catalysts offered similar and increasing CO2 conversions with increasing temperature (range studied from 250 to 400 °C) at a constant WHSV of 30 × 103 STPmL·gcat−1·h−1. The CH4 selectivity remained close to one in all cases. Considering their total metallic load, the Ru (3.7 wt%)-based catalyst stood out remarkably, with TOF values that reached up to 5.1 min−1, this being six or three times higher, than those obtained with the Ni (10.3 wt%) and Ni–Fe (7.4–2.1 wt%) catalysts, respectively. In the second set (cofeeding methane), and also for the three catalysts, a high correspondence between the conversions (and selectivities) obtained with both types of feeds was observed. This indicated that the addition of CH4 to the system did not severely modify the reaction mechanism, resulting in the possibility of taking advantage of the ‘biogas upgrading’ process by using H2 produced off-peak by electrolysis. In order to maximize the CH4 yield, temperatures in the range from 350–375 °C and a H2:CO2 molar ratio of 6:1 were determined as the optimal reaction conditions.