(402e) Syngas Production By Biogas Steam and Oxy Steam Reforming Processes on Rh/CeO2 Catalyst Coated on Ceramics Monolith and Open Foams

Today there is a worldwide interest in the use of biofuel as an environmental friendly renewable energy source [1]. Biogas can be considered as one of the most widespread renewable fuel obtained from biomass produced from a variety of organic raw materials in various sectors ranging from zootechnical to agro-industrial [2-3]. Biogas, basically, it is constituted by 50-75% CH₄, 25-45% CO₂, 2-7% H₂O (at 20-40 °C), 2% N₂, less than 1% H₂/H₂S and traces of O₂, NH₃, halides and siloxanes [4]. The utilization of biogas to produce syngas/hydrogen by reforming processes [5] to feed fuel cells represents an alternative route to the traditional utilization of this renewable fuel in less efficient and polluting engines to produce energy and heat. Among the possible reforming reactions, steam reforming (SR) is widely used for hydrogen production. It is the largest and generally most economical route especially for industrial applications [6]. However, SR is strongly endothermic, and large amount of heat supply is required. Another option can be represented by the oxy steam reforming (OSR) process. The simultaneous occurrence of the reactions due to the presence of O₂ and H₂O can help to overcome critical factors as the deactivation by coke deposition reducing at the same time the overall endothermicity of the process [7]. One of the central issue of reforming processes is the development of efficient, stable and low cost catalysts. The most used catalysts in reforming processes are based on transition metals supported on oxides. Noble metals (Pt, Rh, Ru, Pd, Ir) are normally, more active and tolerant to coke formation [8] with respect to other transition metals as Ni. However, an excessive use of these elements increases the total cost of the catalyst. Another important challenge is the development of more compact and lightweight fuel processors, especially to achieve the targets for small-scale applications such as for distributed hydrogen production [9]. In this respect different catalyst configurations (foams, honeycombs, gauze, microchannels) were proposed as alternative to traditional packed-bed reactors for the realization of compact reactors [10]. The advantages of structured catalysts are related especially to the large surface-to-volume ratio that leads to good heat and mass transfer properties and low pressure drop. In this work, the performances of structured catalysts, based on Rh (1.5 wt.%), supported on CeO₂, coated on cordierite monoliths (400 cpsi, diameter 1 cm, length 1.5 cm) and alumina foams (20, 30, 40 ppi) were investigated and compared. The structured support was lined by combining the solution combustion synthesis with the impregnation technique [11-13]. The deposition process was repeated several times until reaching the desired amount of catalytic layer over the supports (6.5 mg cm^–2 referred as weight of active metal plus oxide carrier on the bare support geometric surface). In addition to the preparation of the coated monoliths, the corresponding catalyst at powder level was prepared and used for the chemical-physical characterization of the supported catalyst (XRD, TEM, SEM and chemisorption analyses). The pressure drop and the porosity across the bare ceramics supports and the structured catalysts were determined at different superficial velocities and compared. In addition the adherence between the catalytic layer and the support walls was evaluated via sonication treatment [12,13]. Biogas OSR and SR experiments were conducted in a fixed-bed quartz reactor with inner diameter of 1 cm at atmospheric pressure. The structured catalysts were placed between two quartz wool plugs in the center of the quartz tube, inserted into a furnace heated at the reaction temperature T_SET and controlled through a PID temperature controller. Instead, the influence of temperature and of the WSV was assessed on all the prepared structured catalysts varying the temperature (T_SET) between 700–900 °C and the WSV between 34,000 and 18,000,000 Nml g_cat^–1 h^–1 at fixed S/C =1 for the SR reaction, and the temperature (T_SET) between 700–900 °C and the WSV between 36,000 and 24,0000,00 Nml g_cat^–1 h^–1 at fixed S/C =1 and O/C = 0.2 for the OSR reaction. Model biogas containing 60% CH₄ and 40% CO₂, was prepared from high purity CH₄, O₂ and N₂ gas cylinders. Steam was added to the feed by using an isocratic pump and a specially designed evaporator. The feed and reaction products were analyzed using an Agilent 6890 Plus gas chromatograph equipped with thermal conductivity (TCD) and flame ionization (FID) detectors. Before testing the catalytic performance of the prepared monoliths, each coated support was reduced under hydrogen-rich atmosphere directly in the quartz reactor of the test rig. All the structured catalysts showed a uniform thin coating with thickness between 20–30 µm, high mechanical strength and low pressure drop. In detail the monolith based catalyst has shown a negligible weight loss (0.4%) of the catalytic layers after two sanction baths (1 h overall) in a mixture of water and acetone (50–50 wt.%), instead the foam based samples have shown a higher loss of catalytic layer. This different behaviour is probably due to the different surface morphology of bare supports, the bare monolith is characterized by the presence of a more intense macro-porosity over the walls respect the surface of the foams supports, this can help to obtain a more stable layer of CeO₂ during the first step of coating procedure. Also, regarding the pressure drop measurement there are differences between the foam and monoliths. The monolith based catalysts show a lower pressure drop respect that of the foam catalysts, in addition the values increase increasing the ppi. Both monolith an foam catalysts have shown good performances during SR and OSR processes at all the WSV investigated in terms of high CH₄ conversion (99.9–98% for SR and 99.9–96% for OSR), high H₂ concentration (64-62% for SR and ≈58% for OSR, in dry basis and nitrogen free), the H₂/CO ratio changed from about 2.8 for the SR to 1.9 for the OSR. The utilization of CO₂ contained in the biogas in both reforming processes was also registered, the results have shown that the CO₂ conversion can be regulated principally tuning the S/C and the O/C molar ratio. In SR reforming condition, increasing the WSV, the CO₂ conversion decreased from 10 to 3%, instead in OSR condition it decreased from 49 to 42%. The presence of O₂ and lower amount of steam in the OSR process probably leads to convert more CO₂ trough the dry reforming reaction that is disadvantaged in SR condition by the large presence of steam.

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Acknowledgements

This work was funded by the Italian Ministry of Education, University and Research (MIUR, PRIN 2010–2011) within the project IFOAMS (“Intensification of catalytic processes for clean energy, low-emission transport and sustainable chemistry using open-cell FOAMS as novel advanced structured materials”, protocol no.2010XFT2BB).