Kinetically consistent detailed surface reaction mechanism for steam reforming of methane over nickel catalyst

Abstract

A one-dimensional model, LOGEcat is used to develop a detailed surface reaction mechanism for modeling the steam reforming of methane over a nickel-based catalyst. The focus of the paper is to develop a kinetically consistent surface reaction mechanism. The two terms, kinetically and thermodynamically consistent mechanisms, will be used frequently in this article. Note that when the mechanism is thermodynamically consistent then the thermodynamic data or the thermochemistry of the species is not used and all the reactions in the mechanism are forward reactions (no backward reactions included) which need the Arrhenius parameters (pre-exponential factor, $$A_{r}$$ A r ; activation energy, $$E_{r}$$ E r ; temperature exponent, $$\beta _r$$ β r ) explicitly for each reaction. The kinetically consistent mechanisms need Arrhenius parameters only for the forward reactions and does not need these parameters for backward reactions making the backward reactions independent of the kinetics. The rate for the backward reactions is calculated with the help of thermochemistry of the intermediate species involved in the reaction mechanism. Since the thermochemistry of the intermediate species are not easily available, this makes such studies much more complex. The original multi-step reaction mechanism consists of 42 reactions which are thermodynamically consistent. In this study we have modified this reaction mechanism from literature and used only 21 (reversible) direct reactions. This aspect is important because by using only 21 reactions, we use the thermochemistry of the species to achieve thermodynamic equilibrium instead of providing the Arrhenius parameters for forward and backward reactions. We use the same $$A_{r}$$ A r , $$E_{r}$$ E r and $$\beta _r$$ β r values as used in the reference paper for the considered 21 reactions which are in equilibrium. However, the value of $$A_{r}$$ A r , $$E_{r}$$ E r and $$\beta _r$$ β r for the inverse/backward reactions are not given in the mechanism and the reverse rates are calculated by using the equilibrium and forward rate. Therefore, a sensitivity analysis on thermochemistry of the species is performed for a better agreement with the literature data. The method is presented for ensuring kinetic consistency and bringing thermochemistry of species in play while developing a surface reaction mechanism. The applicability of the mechanism is tested for two different simulation set-ups available in literature considering various reactor conditions in terms of parameters given as temperature and steam-to-carbon (S/C) ratio. Several chemical reaction terms, such as, selectivity, conversion and $$\mathrm {H_2}/{\text{CO}}$$ H 2 / CO ratio are shown for different parameters in comparison with the available reference data. The detailed mechanism developed in this study is able to accurately express the steam reforming of methane over the nickel catalyst for complete ranges of temperature for both the set-ups and within acceptable limit for S/C ratio considered for the analysis.

A detailed surface reaction mechanism to investigate oxidation of methane over nickel catalyst

“…In this work, Ni-catalyzed conversion of methane under oxidative conditions is investigated using a one-dimensional model known as LOGEcat [19]. The model is successfully utilized in previous studies discussed in [20,21]. The numerical model is based on a detailed surface reaction mechanism containing 26 multi-step reversible reactions with 6 gas-phase and 14 surface species.…”

“…A sensitivity analysis on the thermochemistry of the species was performed to modify the thermodynamic data for CO(s) which we do not aim to cover in this paper. This sensitivity analysis is presented elsewhere [21].…”

“…The conservation equations discussed above are solved in each PSR for each time step along with the 1D Navier-Stokes equations for flow velocity by an operator splitting method over all cells. For more details related to the model and derivations for the equations, the reader is referred to our previous work [20,21].…”

“…Note that the thermodynamically consistent mechanism needs Arrhenius parameters for the forward as well as backward reactions explicitly, whereas for the kinetically consistent mechanism, the Arrhenius parameters are required only for the forward reactions and the thermochemistry of the species is used to calculate the reverse parameters without explicitly giving the kinetics. We have discussed about the details for calculation of forward and backward rate constants in our earlier work presented in [21] for the two approaches, i.e., the thermodynamically as well as kinetically consistent surface reaction mechanism.…”

Investigation dry reforming of methane over nickel using a one‐dimensional model

“…The conservation equations are solved in each PSR for each time step in addition to the 1D Navier-Stokes equations for flow velocity solved by an operator splitting method over all cells. For more details, we refer the reader to our previous work [23,26]. Dry reforming process is investigated by simulating a Ni-coated monolithic catalyst.…”