Distributed feeding for improved CH4 selectivity in Ni-Fe catalyzed CO2/biogas methanation

Carbon dioxide methanation is a promising route for renewable natural gas production, although its performance is often limited by kinetic constraints under severe operating conditions. In this work, distributed reactant feeding in a four-inlet polytropic fixed-bed reactor was investigated as a process intensification strategy for CO 2 methanation and biogas upgrading over a conventional methanation catalyst (Ni-Fe). The effects of feeding configuration, GHSV, temperature and feed composition on CO 2 conversion and CO selectivity were systematically assessed and compared with conventional co-feeding operation. Distributed CO 2 feeding consistently outperformed both conventional and hydrogen-distributed configurations, providing higher CO 2 conversions and lower CO selectivities. The beneficial effect was particularly evident at high space velocities and low conversions, where kinetic limitations and selectivity to the unwanted CO are more pronounced, reaching reduction of 50 % in this parameter in the more kinetically-hindered conditions. The same qualitative behaviour was observed when upgrading a synthetic sweetened biogas (70 vol% CH 4 and 30 vol% CO 2 ), demonstrating the applicability of the proposed strategy under feed compositions relevant in practice. The improved performance is attributed to the establishment of locally H 2 -rich environments along the catalytic bed, which promote methane formation and mitigate CO accumulation without modifying the global operating conditions. These results highlight the potential of distributed reactant feeding as an effective reactor engineering approach for enhancing methanation performance and intensifying renewable natural gas production processes.