Avoiding tar formation in biocoke production from waste biomass

This paper focuses in avoiding tar formation and in optimizing pyrolysis gas (maximizing H₂ and CO) in the production of biocoke from waste lignocellulosic biomass. In order to obtain metallurgical grade biochar (biocoke) slow heating rate and high temperature are required. Under such conditions useless pyrolysis liquids, mainly composed of water together with some heavy-sticky tars, are obtained. In order to make biocoke a cost-effective process it is necessary to optimize pyrolysis vapors avoiding tar formation and maximizing the amount and quality of both coke and gases. With this objective, in this work different heating rates (3-20°Cmin^-1) and catalysts (zeolite, Ni/CeO₂-Al₂O₃) have been tested in a two step pyrolysis process. Olive tree cuttings have been pyrolyzed in a 3.5L batch reactor at 750°C and the vapors generated have been thermally and catalytically treated at 900°C in a second tubular reactor. About 25wt.% biocoke useful as reducing agent in certain metallurgical processes, ≈57wt.% gases with near 50 vol.% H₂, and no tar production has been achieved when a heating rate of 3°Cmin^-1 and the homemade Ni/CeO₂-Al₂O₃ catalyst were used.