Sorption enhanced Methanation of Carbon Dioxide

Abstract

Searching for better ways to store the excess renewable energy at large scale, the topic regarding sorption enhanced methanation of CO2 is researched, pushing the Sabatier reaction over the theoretical equilibrium by means of different water-absorbing zeolites. Till now zeolites and catalyst were mixed separately in the reactor for sorption enhancement. In this thesis, the focus is on impregnating the catalyst on the zeolite as the support, so as to shorten the diffusion path length of water vapour. This material will be called ‘the bifunctional material’. This report covers 2 subtopics: Firstly, do bigger pore sized zeolites improve the sorption enhanced CO2 methanation? An appropriate zeolite is searched, resulting in just one zeolite found from the 237, namely the zeolite L. Thereafter, the zeolite L, 13X and 5A zeolites, were impregnated with nickel citrate or nickel nitrate. These bifunctional materials are characterized with the XRD, SEM/EDX and H2-TPR. In addition, two distinct tests in a fixed bed Sabatier setup were executed: The activity and sorption enhanced methanation tests. Overall, the results show that the nickel impregnated zeolite L activity is between impregnated 13X and the 5A activity (13X < L < 5A). The sorption enhanced test reveals that impregnated 13X and 5A did have sorption enhancement but not for impregnated zeolite L. Secondly, how do cleaned bifunctional materials affect the activity compared to regular bifunctional material from evaporation impregnation? Here, the influence of washing on the nickel loading is examined with SEM/EDX. Thereafter, the activity test shows the effect of cleaning on the activity. Finally, the activity of various bifunctional samples (zeolite supports) are compared with reference samples (impregnated Al2O3 supports), based on comparable support’s pore structure. The results show the following: Increasing the amount of washing did not have a noticeable effect on the nickel loading. Just by changing the impregnation method from evaporative impregnation method to filtered impregnation method without additional washing (1,2,3,4 or 5x) is sufficient to prepare clean bifunctional material, containing only little precipitated salt precursor material. Furthermore, by comparing regular and cleaned bifunctional material per zeolite, reveals that the 5A is impregnated primarily on the outside surface of the zeolite followed by L and 13X (nickel on the outside surface: 13X = 51.3%, L =58.1% & 5A = 84.9%). The activity test displays that the effective activity (conversion per gram nickel catalyst) of cleaned impregnated 13X and 5A are the highest in H2 conversion. This makes it interesting for further research since separation enhancement of the Sabatier reaction reduces byproduct formation (CO) to a very large extent. The activity test comparing bifunctional samples with reference samples result that, supports with similar-looking pore structures do not lead to comparable activity.