Steam Reforming of Tar Impurities from Biomass Gasification with Ni-Co/Mg(Al)O Catalysts—Operating Parameter Effects

Abstract

The elimination of tar impurities from biomass gasification by catalytic steam reforming can provide clean syngas for downstream biofuel synthesis (Fischer–Tropsch). The effects of key operating parameters in CH4/tar steam reforming were investigated. Ni-Co/Mg(Al)O catalyst performance was tested at model conditions (10/35/25/25/5 wt% CH4/H2/CO/CO2/N2), changing the temperature (650–800 °C), steam-to-carbon ratio (2–5), tar loading (10–30 g/Nm3), and tar composition (toluene, 1-methylenaphthalene, and phenol). Complete tar elimination was achieved under all conditions, at the expense of catalyst deactivation by coke formation. Post-operation coke characterization was obtained with TPO-MS, Raman spectroscopy, and STEM analysis, providing vital insight into coke morphology and location. Critical low-temperature and high-tar loading limits were identified, where rapid deactivation was accompanied by increasing amounts of hard coke species. A coke classification scheme is proposed, including strongly adsorbed surface carbon species (soft coke A), initial scattered carbon filaments (hard coke B1.1), filament clusters and fused filaments (B2), and strongly deactivating bulk encapsulating coke (B3), formed through progressive filament cluster graphitization. High-molecular-weight tar was found to enhance the formation of strongly deactivating metal-particle-encapsulating coke (B1.2). The results contribute to the understanding of coke formation in the presence of biomass gasification tar impurities.