Pyrolysis of Mixed Plastic Wastes for the Recovery of Benzene, Toluene, and Xylene (BTX) Aromatics in a Fluidized Bed and Chlorine Removal by Applying Various Additives

Abstract

Mixed plastic wastes are very difficult to mechanically recycle into new products because of their nonhomogeneity. Therefore, pyrolysis and gasification seem to be very effective in the extraction of energy from mixed plastic wastes in the form of oil and gas. In this study, a fraction of mixed plastic wastes was pyrolyzed in a bench-scale plant equipped with a fluidized-bed reactor and a char removal system. This study has two aims. The first is to find out the optimum reaction temperature for a high yield of benzene, toluene, and xylene (BTX) aromatics. The second is to find out the best additive for chlorine removal. To find out the optimum reaction temperature for the maximum BTX content, experiments at a temperature range of 660−780 °C were carried out. The pyrolysis oils that were obtained from the experiments were composed of aliphatics and mono- and polyaromatic compounds. The reaction temperature had a positive effect on the BTX aromatics yield. The maximum BTX aromatics yield was obtained at 719 °C, with a value of 18 wt % of the organic product. The pyrolysis oils had almost no metal content. The metals that were present in the feed material were concentrated in the solid product, the char. It seems that the char removal system, which was composed of a cyclone and a hot filter, almost perfectly removed the char particles. Besides the pyrolysis oil, a combustible gas was also obtained. The main components of the gas were CH4, C2H4, C3H6, C4H6, and C4H10. The higher heating value of the product gas was about 50 MJ/kg. To absorb the hydrogen chloride that was formed from the degradation of the polyvinyl chloride (PVC) in the mixed plastic wastes, additives (calcium oxide, calcium hydroxide, crushed oyster shells, and rice straw) were added to a fraction of the mixed plastic wastes. In the experiments without any additive, the chlorine contents of the pyrolysis oils were 350−500 ppm. When the additives, especially calcium oxide, calcium hydroxide, and crushed oyster shells, were applied, the chlorine content of the pyrolysis oils significantly decreased to 50 ppm. The results of this study suggest that the use of oyster shells as a HCl absorbent can stimulate the recycling of waste oyster shells, which are abundant in Korea.