The application of gasification to thermally treat biomass as carbon neutral resources has been constrained by the technical challenges associated with tar formations, which cause operational problems in downstream equipment for syngas processing. Catalysts, such as transition metals, calcined rocks and char, can be used to catalyse tar reforming. Biochars, which are naturally produced during biomass gasification, are particularly attractive as an alternative catalyst due to their catalytic functions, low cost and long endurance. Despite these promising characteristics, adequate knowledge on the relationship between the porous structure of biochar and its deactivation by coking during the steam reforming of tars is not available. In this work, the influence of the porous structure of biochar on its performance across time for reforming tar was investigated in a fixed-bed reactor, over a temperature range from 650 to 850 °C. Regular biochar and physically activated biochar from the same precursor biomass were employed as bed material. The tar samples were the composed mixture of benzene, toluene and naphthalene. Both fresh and spent catalysts were analysed with Brunauer-Emmet-Teller, tplot, Fourier Transform Infrared and Scanning Electron Microscopy/Energy Dispersive Spectroscopy. Results showed that, while at moderate temperatures of 650 and 750 °C, the activated biochar offered a higher tar conversion but more severe deactivation than that of the regular biochar. At the high temperature of 850 °C, the difference in the catalytic performance between the two chars was negligible, and over 90% of the initial tar species were removed throughout the 3-hour long experiments. At 850 °C, the coke deposited in the meso-and macro-pores of both chars was gasified, leading to a stable catalytic performance of both chars. The results indicated that meso-and macro-porous biochars are resilient and active enough to become a viable option for tar steam reforming.

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The presence of tars in syngas is a major technological constraint for upscaling biomass gasification to produce heat, power, and other value-added chemicals such as biofuels. At the same time, the solid remains from biomass gasification i.e. char and ashes, have capabilities to catalyse the reforming of gasification tars. This work presents a comprehensive analysis of the relevance of gasification chars and ashes as catalysts for tar reforming. A description of the solid products from biomass gasification, their formation, chemical characteristics and potential applications is given. Additionally, a review of the state of the art of the uses of regular char, activated carbon and ashes as a catalyst for tar reforming is presented. Further, kinetics reported in literature, and the homogeneous and heterogeneous mechanisms for tar reforming over char are discussed and explained. From reviewing literature it was found that activated chars exhibit the best reforming capabilities, followed by regular char and ashes. Knowing the role of the interactions between the char and the tars is a key factor for optimization of char catalysts. Ultimately, this work provides guidance for understanding the uses of biomass solids as catalysts for tar reforming, and aid in future research to increase the economic feasibility of biomass gasification.

Adequate tar removal is a recurrent challenge for biomass gasification. Materials such as char and activated char are promising catalysts for tar reforming because of their activity, inexpensiveness and constant production during gasification. Although the behaviour of char and activated char as catalyst has been previously studied, an evaluation of the thermodynamic efficiencies of the tar reforming process using char as a catalyst still lacking. This work analyses the performance of a two-stage system, where gasification is followed by tar reforming using char catalysts. For the study, a model based in a combination of equilibrium thermodynamics and chemical kinetics was developed. The first stage, where gasification occurs, was simulated with a thermodynamic equilibrium model. Gasification equilibrium models available in literature only predict the fractions of H 2 , CO, CO 2 and CH 4 ; the model developed for this work also predicts the formation of three model tar with different characteristics (benzene, toluene and naphthalene), providing information on the stability of formed tar. The second stage, simulated using kinetics from literature, consists on reforming the tar with catalysts made of residual char. The effects of the reactor temperature, equivalence ratio, and residence time were assessed via the gas quality, based on the gas lower heating value and tar concentration, and process efficiency, based on the energy and exergy efficiencies. Results showed that using char or activated char catalysts increases the heating value of the gas while reducing its tar concentration. Moreover, the process benefits thermodynamically (i.e. less exergy is destroyed) from low gasification temperatures and high reforming temperatures. Simulations indicate that a tarless gas with a lower heating value of more than 8 MJ/Nm 3 can be produced from gasification at 1023K with an equivalence ratio of 0.15 and subsequent reforming at 1123K with a residence time in the catalyst bed of 1 second.

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