Catalytic reforming of acetic acid as main primary tar compound from biomass updraft gasifiers: screening of suitable catalysts and operating conditions

Tar compounds have been defined as Achilles’ heel in biomass gasification. Catalytic reforming solves problems caused by tar by converting them into H₂ and CO. Most of the research has focused on secondary and tertiary tar reforming while some information on primary tar can be derived from bio-oil reforming. However, these studies use humidified N₂, Ar or He as gas carrier. Therefore, in this work, three catalysts are compared for reforming 40 g/Nm³ acetic acid as main primary tar compound from biomass updraft gasification using simulated biosyngas as gas carrier. The catalysts were tested over a 72 h period between 680 and 750 °C with a gas composition of 35.0 vol% H₂O, 2.3 vol% CO, 19.5 vol% CO₂, 3.6 vol% CH₄, 24.0 vol% H₂ and 15.6 vol% N₂. Olivine completely converted acetic acid, but a considerable amount of carbonaceous deposits was found on the catalyst and the catalytic activity decreased over time with 0.2 g/Nm³ hydroxyacetone measured in the last day of testing. Dolomite showed promising performances by completely converting acetic acid and accumulating carbonaceous deposits only in the low temperature part of the catalyst bed. The carbonaceous deposits could be suppressed increasing the steam content to 50.1 vol% and the temperature to 900 °C. However, the catalyst became excessively brittle. The metal-based catalyst out-performed the naturally-occurring catalysts by completely converting acetic acid with almost no carbonaceous deposits accumulation. These results are expected to help the further development of tar reformers, and the commercialisation of biomass updraft gasifiers based systems.