Mixed zirconia–alumina supports for Ni/MgO based catalytic filters for biomass fuel gas cleaning

Abstract

Localized, small scale renewable energy generation by biomass gasification will only find widespread use if simple and efficient technology is available for removing undesired impurities such as particles and tar from the producer gas in order to avoid serious operating problems in the downstream equipment and coupled diesel engine. This work focuses on the further improvement of a novel one-step cleanup with a catalytic candle filter by using alternative ZrO 2 or ZrO 2–Al 2O 3 mixed supports in order to improve the activity of Ni-based catalysts at lower temperatures. Porous α-alumina filter materials were prepared with the urea method to deposit 2.5 wt.% Al 2O 3 or ZrO 2, or a combination of ZrO 2 and Al 2O 3 to increase the surface area, followed by 1 wt.% Ni and 0.5 wt.% MgO deposition. The surface area increases with the number of deposition cycles of ZrO 2 or ZrO 2–Al 2O 3, but was lower than for pure Al 2O 3 deposition. Uniform distributions of the zirconia secondary support and the active ingredients such as Ni and MgO throughout the sample could be obtained. Tar conversion achieved from the three types of supports: ZrO 2, Al 2O 3 and a mixed ZrO 2–Al 2O 3 are generally similar, except at high velocity and low temperature where the mixed support is best. As an example, for the mixed oxide deposit of 1.20 wt.% ZrO 2 + 1.28 wt.% Al 2O 3 followed by 0.46 wt.% MgO + 0.996 wt.% Ni at 900 °C, 2.5 cm/s, with 15 g/Nm 3 benzene or 5 g/Nm 3 naphthalene loading in presence of 100 ppm H 2S, conversions of benzene and naphthalene were 95% and 99.5% respectively, which is extremely promising, since the surface area of these new ZrO 2 based materials is lower than with pure alumina and leaves room for significant improvement. Extended time testing over 170 h under the condition of 15 g/Nm 3 benzene and 5 g/Nm 3 naphthalene fed together at 900 °C and in presence of 100 ppm H 2S, showed 99.2–99.6% benzene conversion and 100% naphthalene conversion, even at lower temperature (850 °C) a good activity was still achieved in comparison with our previously developed catalysts.