A model-based comparison of Ru and Ni catalysts for the Sabatier reaction
The differences between Ru- and Ni-based catalysts for the Sabatier reactor are assessed on the basis of appropriate kinetic models and reactor designs. The origin of the higher performance of the Ru-based catalyst is analysed in detail. Ru activates the reactor and initiates the thermal runaway at about 100 °C lower than commercial Ni/Mg/Al2O3 catalysts and 10–20 °C lower than Ni/Al2O3 catalysts. In addition, the higher catalytic activity at low temperature of Ru-based catalysts allows the thermodynamic curve to be followed-up to the region of high CO2 conversion. Over steam reforming Ni catalysts, the highest attainable conversion is instead limited to 90%, while tailored Ni catalysts for CO2 methanation can reach 96% conversion in a single pass reactor. In the intermediate conversion areas, the two catalysts show comparable activity, due to the reaction limitations related to the required cooling and to the diffusional limitations that are more pronounced in the highly active Ru. We developed a reactor design routine to define the amount of catalyst required to reach grid-compatible CO2 conversion, and found that 99.5% conversion is attainable in a single step with a 0.5% Ru/Al2O3 catalyst or with a high load of Ni/Al2O3 by introducing an intermediate water condensation step. As a consequence, we calculated that the cost of the catalyst is approximately two to three times higher for the Ru-based reactor than the Ni-based. However, this difference in catalyst cost cannot compensate for the cost of a more complex system, including several different units, when developing energy storage solutions at a small-scale. For this reason, the Ru-based system is, at the current price and technological state, the most economical solution for small-scale applications, while efficient Ni-based catalysts can be the ideal choice for large scale applications.