Assessing tar removal in biomass gasification by steam reforming over a commercial automotive catalyst

Biomass gasification is a primary thermochemical conversion technology for transforming woody biomass feedstocks into a range of renewable fuels and chemicals. However in practice during biomass gasification, tar formation is practically unavoidable and tar removal during downstream synthesis gas cleaning is crucial to achieving high-quality synthesis gas at commercial scale. A category of catalytic tar reforming (tar cracking) catalysts typically utilizes precious metals including rhodium, which shows high reactivity toward gasification tar, resists coking and is more tolerant to sulphur compared to nickel. As such, they are similar to automotive exhaust after-treatment catalysts. In this study we evaluated how a commercial automotive catalyst performed as a gasification tar reforming catalyst. We tested the catalyst at bench scale for methanol reforming and tar reforming at 700 °C using 80/20 mixtures of methanol/water, and 79.5/20/0.5 mixtures of methanol/water/tar and methanol/water/guaiacol in flowing nitrogen. Methanol was 95% converted to synthesis gas at 700 °C and the catalyst did not deactivate during 48 h on stream. Methanol/water mixtures containing 4925 ppm gasification tar also reacted readily over the catalyst to produce syngas, but catalyst deactivation occurred over tens of hours of continuous operation, indicated by decreased conversion of the methanol/tar feed. The catalyst was regenerated by calcining in air at 500 °C, which allowed catalyst to operate for 120 h. Methanol/water/guaiacol mixtures also reacted readily to produce syngas, but as with tar, the catalyst deactivated over tens of hours continuous operation with methanol/guaiacol feed. SEM data confirmed that coking of the catalyst was the likely cause of deactivation. At relatively high reaction temperature and contact times of seconds used in this study, guaiacol was completely deoxygenated, but a fraction of the guaiacol was methylated over the catalyst to form methyl-substituted benzenes, toluenes and xylenes (BTX).