In our previous work [1], a novel catalytic candle filter was developed to integrate the high temperature removal of particles and tars from biomass gasification gas in a one-step gas-cleaning process in order to reduce the equipment inventory and meet the specifications demanded by end-use devices and environmental constraints. It consists of a ceramic á-alumina porous filter substrate in which a suitable nickel-based catalyst is deposited onto the pore walls to destroy the tars. In the laboratory scale, a small filter disk with a diameter of 29mm to 30mm and a thickness of 10mm is used to represent the full-scale candle filter. After a large amount of development and optimization work, an optimal catalyst composition had already been found, which approximately consists of 2.5 wt% secondary supports (Al2O3, ZrO2 or a mixture of ZrO2-Al2O3), 1.0 wt% Ni and 0.5 wt% MgO in the porous, preformed alumina filter material with low specific surface. In typical conditions with a face velocity of 2.5 cm/s, the presence of 100 ppm H2S and at 900 °C, the conversion of naphthalene is 100%. At 850 °C and the same conditions mentioned above, the naphthalene conversion is about 99.0%. However, when reducing the temperature to 800 °C, only 77% reduction in tars is achieved. This means that this catalytic gas cleaning process needs further improvements to reduce the required reaction temperatures, significantly reducing the equipment cost.
Recently, some groups [2] introduced air or oxygen into the biomass gasification gas to avoid carbon deposits or increase temperature by partial combustion, which resulted in better tar removal and stable operation. High conversions of tars (e.g. toluene) were obtained in the presence of 1% to 3% oxygen below 600°C with the zirconia-containing catalysts, whereas the tar conversions are negligible without oxygen in the feed [3]. To identify the role of thermal oxidation in removing the tars, experiments were carried out using a blank alumina disk with the 1vol% O2 in the gas mixtures: in typical conditions (900 °C, 100ppm H2S, 2.5 cm/s) with 1 g/Nm3 naphthalene as tar model compound, the conversions of naphthalene are 54.0% with 1% O2 and 9.9% without O2. Additionally, at 800 °C, the conversions of naphthalene are 47.3% with 1% O2 and 2.0% without O2. At high temperature, O2 reacts with naphthalene and result a high tar conversion in comparison to the decomposition in absence of O2, where the naphthalene conversion was negligible.
However, with the optimal catalyst compositions, the conversions are much higher and show that the addition of a small amount of oxygen in the biomass gasification gas increases the tar conversion significantly. For example, even as low as 800 °C, the naphthalene conversions are 98.2% with 1% O2 versus 79.5% without O2 in the feed. Conversion with only the secondary support is only 70% at 800°C, higher than for the blank material, as ZrO2 is known to be a catalyst for this conversion in the presence of O2 [3]. These improvements are clearly linked to the catalytic conversion and cannot be explained on the basis of the thermal conversion only. The addition of O2 also profits the reforming of CH4 to make synthesis gas and enhances the ratio of H2/CO. Long term experiments were performed at 800°C over 250 hours with 1g/Nm3 naphthalene and 1% O2. The conversion of naphthalene was ranging from 98.2% to 99.4%, while the pressure drop remained stable over 250 h. Adding a small amount of oxygen to the produced fuel gas allows almost complete removal of tars at low temperature without significantly modifying the gas composition and resulting in significantly simpler and less expensive equipment.
We also believe to have elucidated the mechanism of improvement with the help of a large number of experiments with different catalyst and gas compositions and thermodynamic calculations.
REFERENCES
1. Ma, L., H. Verelst and G.V. Baron, Catalysis Today, 105, 3-4 (2005).
2. Corella, J., J. M. Toledo and R. Padilla, Ind. Eng. Chem. Res., 43, 10 (2004).
3. Juutilainen, S. J., Pekka A. Simell and A. Outi I. Krause, Applied Catalysis B: Environmental, 62, 1-2, (2006).