Methane production from co-electrolysis products applying a microchannel reactor

Abstract

This PhD thesis comprises the development of a polytropic microstructured methanation unit for conversion of CO/CO$_2$ mixtures. As part of the MINERVE Power-to-Gas project, conducted from 2012-2015, the developed reactor contributes to transformation of electrical surplus energy into chemical energy by using methane. The theory focuses on the examination of thermodynamic equilibria, methane formation mechanisms, catalyst degradation and kinetics. From a technological point of view, microreactors were identified as a promising tool due to the possibility of precise temperature control, which is key in the highly exothermic methanation reaction. The experimental procedure encompasses the catalyst behavior under variation of H$_2$/C-ratio, temperature and concentrations of CO and CO$_2$. Under CO methanation conditions, strong deactivation of the commercial Ni catalyst was observed, while almost no catalyst degradation was encountered under pure CO$_2$ methanation conditions. The assumption of deactivation by coke was supported with surface carbon and BET-surface decrease. Preferential methantion of CO was observed in CO/CO$_2$-mixtures with deactivation of catalyst being similar to that under CO conditions. Two microstructured packed bed reactors were developed showing few novelties in respect to temperature control and pressurized operation of both cooling and reaction zones. It could be shown, that the methanation of CO/CO$_2$-mixture is possible in one step by controlling the temperature of the reactor to a certain degree while evaporating the cooling water. Due to strong exothermicity of the methanation reaction, a partial overheating of the catalyst took place in both reactors, however, was clearly below adiabatic temperature rise. The hot spot occurrence pointed to heat transfer resistances either in the packed bed or the metal housing. The Prototype 2 showed superior performance compared to Prototype 1 due to additional cooling zone. The idea is filed as an international patent and a scaled-up version is successfully utilized in industrial application for methane generation. An extensive CFD study of Prototype 1 revealed the proper fluid distribution in the cooling zone and the positive effect of fins on heat transfer rate. Besides, valuable information could be extracted by using parameter variation in the packed bed and in the metal housing to determine the heat transfer bottleneck of the system, which could be narrowed to the separating metal wall. In the final chapter, a few literature models in respect to porosity, flow and heat conductivity distribution in packed beds were discussed, which served as a benchmark to successfully validate the presented meshing strategy using CFD. Ideal plug-flow behavior, as in case of microreactors, was found for packed beds, even at low d$_t$ /d$_P$-ratios, regarding flow and mass transport in the laminar and transition regions. Pronounced heat transfer issues could occur in the wall areas for Re<100.