Activation of solid residues from batch pyrolysis of waste tires

Abstract

The paper deals with a two-stage process of thermal treatment of waste tires in order to obtain a carbonaceous adsorbent. A fraction of 0.4 ‒ 0.8 mm of the ground material was involved in the experiments. In the first stage, pyrolysis took place in a retort apparatus, which provided about 43 % of solid residues, 41 % of condensates and 16 % of gas at temperatures of 600 and 800 ° C. The mass balance was confirmed by the TGA method independently of the retort apparatus. N-alkanes, mono- to trialkylated benzenes, cycloalkenes, higher alcohols, ethers, acetates and aliphatic and aromatic thiols were identified in the liquid product by GC-MS. The main components of the condensates were: 50% n-al-kanes and more than 25 % alkylated benzenes. According to GC-TCD-FID, the separated pyrolysis gases contained nine main components with a volume fraction >1 %. Of these majority compounds, methane with volume fractions approaching 30 % was the most represented. Other important compounds were in descending order: hydrogen (20.2 and 20.8 %), carbon dioxide (8.9 %), ethane (8.5 %) and ethene (7.0 %). Due to the upper calorific value of 44 ‒ 45 MJ m‒3, the gas can be advantageously used energetically, but the high sulfur content must be considered anyway. The solid residues were subjected to steam activation in a separate apparatus. The activation apparatus operated with a batch reactor of similar design as the pyrolysis retort. By activation, the specific surface area of the pyrolysis residues was increased from a very small initial value <59 m2 g‒1 to a maximum of 337 m2 g‒1. However, this result, in contrast to the reference sample prepared from hardwood, required aggressive conditions, namely 900 °C combined with a steam exposure time of 60 min. The combination of the lower of the selected pyrolysis temperatures and the higher activation temperature led to better results than the opposite setting. Both the crude pyrolysis residues and the obtained activated products were significantly mesoporous and pores with a diameter of 20 ‒ 80 nm predominated in their structure. Prior to activation, the pyrolysis residues always had a pore content of at least 60 %, while the activation further increased their proportion to 81 %. Such a significant proportion of mesopores 20 - 80 nm suggests that the material could be suitable for the next modification/improving step in the form of wet impregnation.