Abstract:

Biomass gasification is gaining attention as a route for biomass energy production. One of the main technical barriers in the development of biomass gasification process is the problems due to the presence of heavy organic impurities, especially tarry compounds, in the flue gas. Tar creates various difficulties, for example, condensation or formation of tar aerosols, causing downstream problems in the process equipments.

In this study, the catalytic activity of nickel supported on Ce_0.75Zr_0.25-xMn_xO₂ (x = 0, 0.10, 0.20 and 0.25) mixed oxide catalysts prepared by urea hydrolysis and incipient wetness impregnation was investigated via steam reforming reaction of naphthalene as a model biomass tar compound. Experiments were carried out in a fixed bed reactor at 700 ^oC and atmospheric pressure using a molar ratio of steam/carbon (S/C) equal to 2. The catalysts were characterized by various techniques, such as surface area, temperature-programmed reduction (TPR), X-ray diffraction (XRD) and temperature-programmed oxidation (TPO).

The catalytic performance data showed that nickel supported on mixed oxide catalysts exhibit high activities and stabilities for the steam reforming of naphthalene and high resistance toward carbon deposition. The steam reforming products are mainly CO, CO₂ and H₂ for Ni/Ce_0.75Zr_0.25-xMn_xO₂ (x = 0.10, 0.20 and 0.25) and the H₂/CO ratio is about 2.40 ± 0.12. On the other hand, the main products of naphthalene steam reforming over Ni/Ce_0.75Zr_0.25O₂ are CO, CO₂, H₂ and trace of CH₄ and the H₂/CO ratio is about 1.84 ± 0.05, indicating carbon formation.

The results showed that, Mn-doped Ni/Ce_0.75Zr_0.25-xMn_xO₂ (x = 0.10, 0.20 and 0.25) catalysts yielded much lower amounts of carbon formed (1.1 ± 0.3 mg g_cat^-1) than Ni/Ce_0.75Zr_0.25O₂ catalyst (26.7 mg g_cat^-1). It is then, suggested that the incorporation of manganese ions into ceria lattice improved the oxygen storage capacity as well as the oxygen mobility on the surface of mixed oxides, facilitating gasification of surface carbon. The promotion effect of manganese on the nickel supported on Ce_0.75Zr_0.25-xMn_xO₂ catalyst for steam reforming of naphthalene is probably through a redox mechanism. The lower valence state manganese might adsorb water and dissociate it, the resulting species –O or –OH transferring to the nickel and reacting with surface carbon species to form carbon monoxide, carbon dioxide and hydrogen.

Keywords: Naphthalene steam reforming, Ceria-zirconia, Manganese.