The Preparation of a Carbonaceous Adsorbent via Batch Pyrolysis of Waste Hemp Shives

Abstract

The study focuses on the two-stage process of the thermal treatment of waste hemp shives aimed at obtaining a carbonaceous adsorbent. In the first stage, pyrolysis was carried out in a retort apparatus, which, at temperatures in the range of 450–850 °C, provided approximately 22.6–29.9% of the solid residue, 31.6–34.0% of the condensate, and 38.5–43.4% of gas with respect to the charge. The organic part of the condensates showed the net calorific value of 30.8–31.3 MJ kg−1, which makes it well suited for energy purposes. In the separated pyrolysis gases, GC-TCD-FID determined only five compounds, namely carbon dioxide, carbon monoxide, hydrogen, methane, and ethane, which together accounted for 98.5–99.8% of the gas volume. The remainder was composed mainly of ethene, propane, and propene. The highest net calorific value (14.7–14.8 MJ m−3) was exhibited by the gas collected in the range of 450–650 °C, which can be advantageously used for energy purposes. Non-activated solid pyrolysis residues had relatively low specific surface areas (BET), with the highest value determined being 294 m2 g−1. Therefore, they were activated by steam in a separate apparatus in the temperature range of 740–900 °C. The activation apparatus operated with a batch reactor of similar design to the pyrolysis retort. The activation increased the specific surface area of the pyrolysis residues up to the maximum of 915 m2 g−1. This study discusses the significant trends in specific surface area and total pore volume caused by a combination of different pyrolysis and activation temperatures. Better results were generally obtained for residues pyrolyzed at lower temperatures and then activated at high temperatures. The structure of the activated products mostly contained pores with a diameter of < 6 nm. Their development was mainly facilitated by the activation temperature of 850 °C, resulting in the relative presence of these pores in the range of 57.9–59.1%. All activated products were also characterized by an increased proportion of 20–80 nm pores, which formed, however, approximately one-third in comparison with the pores < 6 nm. Last but not least, the study also discusses the effect of activation conditions on the process loss of the product.