kinetic modelling of autothermal biogas reforming over sic foam catalysts
Résumé
Biogas is a complex gas mixture primarily composed of methane and carbon dioxide, which is produced by anaerobic digestion of biomass. Biogas can be reformed into hydrogen-rich syngas for fuel-cell applications. In this study, autothermal reforming (ATR) of model biogas was performed. Previous screening of catalysts for this reaction led to the selection of a Ni-Rh/MgAl2O4 catalyst, which showed a stable performance over more than 300 hours [1]. SiC foams are promising catalyst supports for small size ATR units. They have excellent thermal characteristics and low pressure drops, permitting a very compact reformer. For a better reformer design a kinetic model of the biogas ATR reaction is highly desirable. Several kinetic models for methane reforming are available in the literature but only a few models exist for methane ATR [2], [3]. In this study, a kinetic model was developed for the ATR of model biogas over SiC foams coated with a 15-0.05wt.% Ni-Rh/MgAl2O4 catalyst.
The tested foam samples had a diameter of 2.5 cm and two different lengths were used: 2.5 and 1.4 cm (Figure 1). The carrier gas was a mixture composed of steam, CH4, CO2, O2 in a balance of inert argon. The operating variables were temperature, gas hourly space velocity as well as steam/CH4, CO2/CH4 and O2/CH4 ratios. Overall 65 experiments were performed.
Figure 1: SiC foam coated with 15 wt.% Ni-0.05 wt.% Rh/MgAl2O4 catalyst
Fig. 2 shows the effect of the inlet O2/CH4 and H2O/CH4 ratios on the catalyst performances at 650 and 700°C. The relatively low temperatures and high GHSV were chosen to ensure incomplete conversions of methane, necessary for kinetic studies. High H2/CO ratios and high methane conversions were observed even at severe GHSV conditions. At 650°C, the conversion of methane increased with the O2/CH4 ratio, since more oxygen translates into more methane burnt in CO2, and also more heat supplied to the endothermal reforming reactions. At 700°C the methane conversion was not modified with the O2/CH4 ratio but the H2/CO ratio increased with the O2/CH4 ratio. When the H2O/CH4 ratio increased, higher conversions and higher H2/CO ratios were reached. Indeed, the water gas shift reaction is greatly influenced by the inlet partial pressure of H2O
Figure 2: Observed methane conversion at 700°C ( ) and 650°C ( ) as well as H2/CO ratio at 700°C ( ) and 650°C ( ) as a function of (a) the O2/CH4 ratio (H2O/CH4 fixed at 3) and (b) the H2O/CH4 ratio (O2/CH4 fixed at 0.5) during the ATR reforming of biogas (GHSV = 15000 h-1).
The performance of the model was tested by using three objectives functions: the methane conversion, the temperature at the center of the catalyst foam and the H2/CO ratio at the reactor exit. Two different models were tested consisting of the same model for methane combustion combined with two different methane reforming models, one by Hou and Hughes [4] and one by Xu and Froment [5]. The latter one was also used by Halabi et al. to simulate an ATR reactor [2]. Initial simulations showed that the rate of methane combustion was too slow, giving incomplete oxygen conversion. The corresponding rate coefficient was therefore increased until a reasonable comparison with the experimental data was reached. Axial temperature gradient could not be avoided and had to be taken into account in the model. Therefore a one-dimensional pseudo-homogeneous plug-flow reactor with axial temperature gradient was applied. Initial simulations showed that the model by Xu and Froment [5] outperformed the model by Hou and Hughes [4] and further optimization was done using the Xu and Froment model with the model adequately described the measured data (Figure 3).
Figure 3: Parity plot
A kinetic ATR model reported in the literature has thus been successfully adapted to describe adequately the experimental ATR data over the foam catalyst. The main change concerned the higher rate of methane combustion, probably due to the presence of rhodium in our sample, which is a better combustion catalyst. Studies are ongoing on powder for comparison purposes.
Acknowledgements
The research leading to these results has received funding from the European Community's Seventh Framework Programme ([FP7/2007-2013] under grant agreement n° 325383 (BioRobur). The authors thank Alexander Khinsky and Sandro Gianelli for providing the monolith catalysts.
[1] M. Luneau, Y. Schuurman, F.C. Meunier, C. Mirodatos, N. Guilhaume, Catalysis Science and Technology, 5 (2015)
[2] M.H. Halabi, M.H.J.M. de Croon, J. van der Schaaf, P.D. Cobden, J.C. Schouten, Chemical Engineering Journal, 137 (2008)
[3] D.L. Hoang and S.H. Chan, Applied Catalysis A: General, 268 (2004)
[4] K. Hou and R. Hughes, Chemical Engineering Journal, 82 (2001)
[5] J. Xu, G.F. Froment, AIChE Journal, 35 (1989)