A secondary method for hot syngas cleaning in fluidized bed gasifiers: tar removal by activated carbons

The removal of tar compounds from the syngas produced by biomass or waste fluidized bed gasification is the greatest technical challenge for a wider application of the technology. These heavy polynuclear hydrocarbons are characterized by high dew point temperatures (generally above 300°C), and then can condense along pipes, heat exchangers and particulate filters, leading to fouling, plugging or choking, with strong limitation of plant availability and increased operating costs. An innovative method for tar removal based on the combination of adsorption and thermal/catalytic cracking on new-generation activated carbons, which may operate at temperatures significantly higher than the dew points of the main tar compounds and up to 900°C, is presented in this work. To this aim, an experimental apparatus has been specifically designed and adopted for adsorption tests of naphthalene, considered the reference tar compound, with three selected activated carbons, under different conditions of temperature, residence time and initial tar concentration. Each carbon has been fully characterized in terms of particle and bulk densities, porosimetry as well as scanning electronic microscopy, X-ray (EDAX), and other chemical and thermo-gravimetrical analyses. The results allow: i) to validate the ability of the proposed experimental rig to carry out reliable tar removal tests; ii) to individuate the activated carbons that provide the highest tar removal efficiency, and the operating conditions under which they have to be utilized; iii) to propose possible links between the performances of selected activated carbons and their physical and chemical characteristics.