Kinetics of model tar decomposition over activated biochar from a heavy metal-contaminated area

The utilization of waste materials, e.g. biomass contaminated with heavy metals (HM), has gained attention, as it combines phytoremediation with production of value-added products. This study investigates activated biochar from contaminated biomass as a catalyst for tar decomposition. Biochars were prepared from birch harvested around Miasteczko Śląskie, Poland. The samples were collected from two areas – a site polluted by a zinc smelter and an HM-free terrain. The contaminated and uncontaminated birch pellets were gasified with 50/50 vol.% CO2/H2O. Toluene was used for tar conversion tests to represent the most refractory tar compounds. Condensable reaction products were collected every 20 min to obtain toluene conversion as a function of time. The initial conversion at different temperatures (650 – 800 ℃) was used to calculate kinetic parameters of toluene decomposition, assuming first order reaction model. The decrease in the toluene conversion with time was used to fit the activity function that describes the deactivation of biochar due to coke deposition. Physicochemical properties of biochars were analysed with SEM, XRD, gas adsorption, and thermogravimetric method. The fate of HM was investigated with ICP-OES. Despite high retention of HM in the contaminated biochar, no significant differences between the structural properties of the contaminated and uncontaminated biochar were observed. Both had well-developed, although highly microporous, structure and high thermal stability. Toluene conversion was not affected by the presence of HM. The initial toluene conversion and the rate of biochar deactivation was strongly temperature dependent. Similarities in toluene conversion over both biochars suggest that the process was affected mainly by their structural properties and the presence of HM did not affect tar decomposition. The main reaction product was toluene-derived coke, which deactivated material with time. The second reaction pathway, demethylation, produced benzene. It became more favourable as the biochar deactivated. Interestingly, the relative benzene yield levelled off at the last 40 min of the conversion at temperatures ≤700 ℃, while in kept increasing at higher temperatures. The contaminated biochar showed good catalytic performance in tar removal. Such intersection of environmental sustainability and biofuel production could offer potential solutions to increasing pollution and energy demand.