Satisfactory catalyst stability in SNG production using real biogas despite sulfur poisoning evidences at different reactor zones

Abstract

The performance of a nickel-ceria micro-catalyst in biogas methanation was evaluated in a complete pilot plant during 1,000 h. The core of the exothermic methanation process consisted in two micro-reactors using a decreasing temperature profile, intermediate water removal and moderate pressure. The obtained gas quality and the reactors temperature profile remained constant during operation, indicating no signs of catalyst deactivation. After the experimental campaign, catalyst samples from different reactors sections were withdrawn, collected and independently characterized. It has been demonstrated that the different reaction conditions, in which the catalyst operated, played a significant role on the different level of degradation of the catalyst samples. On one hand, various characterization techniques agreed that sintering of nickel and ceria nanoparticles (+10-30%) and loss of surface area (−20%) was restricted to the initial reactor zones, which is attributed to the higher operation temperatures. On the other hand, despite the cautions undertaken for biogas cleaning and gas monitoring, sulfur was detected along the entire reactor longitudinal profile (0.25-0.91%). Accordingly, a progressive diffuse flow poisoning mechanism is expected from very long operation times. In particular, higher amount of sulfur was detected in the latest reactor zones, which operated at lower temperatures and under more oxidizing conditions. Beneficially, sulfur was predominantly detected as Ce2O2S phase, confirming thereby the crucial sacrificial role of CeO2 that allows for maintaining the catalytic activity of nickel active sites. The overall outcome of this work is very promising and reveals a sufficient catalyst lifespan for industrial application.