Hydrogenation of CO/CO2 mixtures under unsteady-state conditions: Effect of the carbon oxides on the dynamic methanation process

Abstract

The Power-to-Gas (PtG) process offers the opportunity to store fluctuating renewable energy in form of chemical energy by hydrogenating carbon oxides into methane. In addition, potential carbon point sources often consist of CO/CO2 (COx) mixtures. Hence, reactor design requires kinetic models valid for unsteady-state operation and a broad spectrum of feed gas compositions. In order to provide the required experimental data basis for derivation of kinetic expressions valid under transient conditions, the dynamic response of a continuously operated fixed-bed methanation reactor is studied by applying periodic step-changes in the feed composition. The obtained results are evaluated based on a simple reactor model, providing the molar flow rate exchanged between the gas bulk and the solid surface for CO, CO2, CH4, and H2O. The results further reveal that the transient kinetic processes at the catalyst surface strongly affect the reactor response under reaction conditions of technical relevance.

The Impact of Support Material of Cobalt‐Based Catalysts Prepared by Double Flame Spray Pyrolysis on CO2 Methanation Dynamics

“…This is also the reason for the CH 4 response exceeding the expected values in a later stage, where the H 2 saturation of the catalyst surface is still high and the CO 2 availability reaches an optimal value. This situation leads to the observation that CH 4 responses faster to the step change than expected from Ar, which is expressed by an overshoot in CH 4 frequently reported for similar experiments [26,27,31,39] . The slower decrease in CH 4 response than expected by the RTD during the back‐transient phase indicates a CH 4 source at the catalyst surface even though the feed was carbon free.…”

“…After reaching the limit cycle, the periods do not differ from one another, except for the noise [35] . This limit cycle is then statistically analyzed and averaged, resulting in a mean step response for each individual component with a narrow confidence interval [31,35] . With the mean step response from each component, as well as the theoretical response from the RTD, important catalyst and support properties for dynamic operation can be investigated, such as adsorption and desorption behavior of the involved reactants and products.…”

Frequency Response Analysis of the Unsteady-State CO/CO2 Methanation Reaction: An Experimental Study

“…In principle, this consideration agrees with our experimental results obtained for a fixed cycle period duration of 240 s, where a delay of the H2O response is reported compared to CH4. 39 Furthermore, a high sorption capacity of the active and support material is known for H2O 40 , which is responsible for the delayed desorption.…”

“…From Figure 3b it is apparent that the hysteresis branch in the COx-rich half-period exhibits only a local maximum (red diamond) for a cycle period duration of 240 s. This maximum corresponds to an overshoot in CH4 formation rate, which is most likely caused by high H2 coverage prior switching to the COx-rich phase as reported previously. 39 Reduction of the cycle period duration leads to shorter COx-lean phases and therefore only partial conversion and subsequent desorption of the reaction intermediates adsorbed at the catalyst surface. Consequently, a certain amount of those intermediates is still adsorbed after switching to the COx-rich phase occupying sorption sites and thereby reducing the number of active sites available for H2 to adsorb.…”

“…For 0 < 𝑧 < 1 the amount of Cα scales with 𝑧, which affects the extent of the observed overshoot. 39 Consequently, based on the form of the hysteresis the participation of CO and CO2 during COx methanation can be differentiated.…”