(338b) Electrified Steam Reforming on Rh/Al2O3 Washcoated Open-Cell Foams: Experimental and Modeling Study

1. Introduction

In several endothermic catalytic processes, a significant amount of CO₂ is produced by fuel combustion to provide heat to sustain the desired reaction. Recently, electrification was proposed as an effective solution to decarbonize these processes.[1] Renewable electricity can be converted into heat and thermally drive chemical reactors. In this context, electrified methane steam reforming (eMSR) is a promising concept for low-carbon H₂ production since it offers potential to utilize excess energy to produce either building blocks for chemical processes or can be used as clean fuel. At the small scale, methane steam reforming is strongly limited by heat transfer. Proper reactor design for e-MSR enables the possibility to bring the heat source closer to the catalytic sites, allowing higher volumetric heat transfer rates and to operate the system at higher space velocities. Recently, Wismann et al.[2] proposed an innovative reactor concept for direct Joule heating of a FeCrAl-alloy tube washcoated with Ni-catalyst for MSR. Methane conversion of 87% was reported with outlet temperatures up to 900 °C. The performance of such a reformer configuration is controlled by external mass transfer limitations.[3]

Open-cell foams have been proposed as enhanced catalyst substrates thanks to their remarkable heat and mass transfer coefficients and large surface areas for catalyst deposition. Moreover, open-cell foams feature a continuous solid matrix that ensures electrical continuity and provides distributed heating inside the reactor tube. For these reasons, the direct electrification of open-cell foam structures, washcoated with a thin layer of Rh/Al₂O₃ catalyst is investigated both numerically and experimentally for the strongly endothermic methane steam reforming reaction, in view of low-carbon H₂ production.[4,5]

2. Materials and Methods

Commercial cylindrical SiSiC foams (d = 3.2 cm, L = 9.9 cm, Erbicol, CH) were adopted in the present work. 1% Rh/Al₂O₃ catalyst was prepared via an incipient-wet impregnation method by using Al₂O₃ powder (Sasol, PURALOX) and rhodium precursor (Rhodium (III) nitrate solution, Alfa Aesar). The washcoating of the foam was performed by dipping, spinning and flash drying processes until a final loading achieved. Catalytic tests were performed at different GHSV up to 200000 cm³/h/g_cat with a non-diluted gas feed of H₂O and CH₄ (S/C = 4.1/1). Downstream of the reactor, water was removed from the products by a condenser and the dry gas mixture was analyzed using an online micro-GC (Agilent, 900 Micro GC).

Figure 1(a) shows the schematic representation of the electrified methane steam reforming reactor layout proposed in the present work. The washcoated SiSiC foam was placed in a tubular stainless-steel reactor (OD = 5 cm) for eMSR reaction. A ceramic tube was inserted between the foam and the stainless-steel tube to avoid electric contact. To connect the foam with the power generator, home-made electric contactors were adopted. A thin layer of copper foam was placed between the SiSiC foam and the electrical plate to ensure a good electrical contact. The contactors are connected to a power generator, which applies a DC current to the system. K-type thermocouples, electrically insulated by ceramic thermocouple wells, are placed inside the electric contactors to measure the temperatures at the upper side and at the bottom of the foam.

A mathematical model was built to describe eMSR reactors based on washcoated foams. The model is built in Matlab and describes mass, energy, momentum balance and electric field in the system. In a previous work, the kinetics of Rh/Al₂O₃ catalysts in concentrated MSR conditions have been derived.[6] Heterogeneous energy and mass balances are considered to describe possible external mass transport limitations. Heat losses of the system are evaluated by considering the dissipation between the system and the environment considering the thermal insulation layer. The system was discretized with finite differences in axial difference and orthogonal collocations in radial directions.

3. Results

We tested foams with different catalyst loadings, at different space velocities and different operative pressures. During the test, the voltage of the power generator was varied to reach a target outlet temperature, measured at the end of the catalytic bed. Our tests demonstrated the possibility to reach conversions close to equilibrium at space velocities in excess of 150.000 Ncc/h/g_cat. In these conditions the system was operated with a power density of 8 MW/m³ , almost twice the power density of industrial reformers. The thermal efficiency of the system increases at the increase of the space velocity and of the catalyst inventory, since heat losses are a sole function of the internal reactor temperature, reaching unprecedented values of 80%. The comparison of experimental and modelling results in terms of outlet methane conversion and thermal efficiency is shown in Figure 1 b). The model is able to predict with a reasonable accuracy for both methane conversion and thermal efficiency.

On this basis, we used the model to design a preliminary scaled-up unit. The system is designed such that the total ΔV across the system is equal to 380 V, the catalyst inventory is equal to 100 g/lit. The foam diameter was set equal to 12 cm, considering manufacturing limits of SiC foams. This unit is able to produce 200 Nm³/h of hydrogen.

4. Conclusions

In this contribution we showed the feasibility of eMSR based on electrified washcoated SiC foams. Thanks to the proper choice of the foam (material and geometry) and active catalyst, it is possible to intensify clean H₂ production with this novel reactor concept. The very good agreement between the model and experiments provides the basis for model-based design of optimized reactor geometries.

Acknowledgement: This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (GA No. 694910 -'INTENT'), and the project "PLUG-IN" funded by the MUR Progetti di Ricerca di Rilevante Interesse Nazionale (PRIN) Bando 2020.

References

[1] KM. Van Geem et al., Science, 2019; 364(6442):734-735.

[2] ST. Wismann et al., Science, 2019; 364(6442):756-759.

[3] ST. Wismann et al., Ind. Eng. Chem. Res., 2019; 58(51):23380-23388.

[4] M. Ambrosetti et al., Front. Chem. React. Eng., 2021; 3:747636-747652.

[5] L. Zheng et al., AIChE J., 2022; doi.org/10.1002/aic.17620.

[6] M. Ambrosetti et al., Catal. Today, 2022; 387: 107-118.