An integrated two-step process of reforming and adsorption using biochar for enhanced tar removal in syngas cleaning

Abstract

Tar reforming processes conducted at typically high-temperatures for syngas cleaning using biochar in biomass gasification plants, fail to remove the thermally stable smaller aromatic compounds (such as naphthalene, indene, anthracene) thereby retaining the risk of clogging downstream equipment. In this context, integration of an additional step of adsorption at relatively lower temperature, targeting the lighter tar compounds is proposed. An integrated approach for removal of tar generated from mallee wood bio-oil, involving reforming at 750 °C and adsorption at 220 °C was systematicaly evaluated against conventional reforming process. The comparative assessment included tar analysis estimating tar yield and examining tar constituents through UV-fluorescence spectroscopy, biochar structural analysis based on Raman spectroscopy supported by reactivity studies through TGA, and biochar morphological analysis by FESEM imaging and BET analysis. Tar analysis reveals a drastic drop in the two-step process by ∼ 80%: with ∼ 60% tar reduction in reforming and ∼ 40 % of residual tar in adsorption. The removal of lower aromatics was confirmed through UV-fluorescence of residual tars. The Raman spectroscopy shows a growing predominance of higher and lower aromatics in catalyst and adsorbent respectively. These trends were explained based on cross-linking of aromatic compounds through a polymeric condensation in reforming and the penetration of light tars into the micropores in adsorption. The effects of aromaticity are reaffirmed in reactivity studies by TGA. The BET results show a manifold decrease in porosity, implying coverage of the pore surface by the adsorbing species, which is also evident in FESEM images.