Slow pyrolysis and gasification reactivity of thermally treated spent coffee grounds pre-treated by wet oxidation

Abstract

Slow pyrolysis and gasification reactivities of waste biochar samples, produced after hydrothermal treatment of spent coffee grounds, were determined using a thermogravimetric analyser (TGA) at 900 °C under CO₂. Biochar samples were prepared by wet oxidation treatment in 30 vol.% hydrogen peroxide at room temperature for 2, 6, and 24 h and at 80 °C for 0.5, 1, and 2 h. NMR results showed that the samples treated at 80 °C showed a slight decrease in the amount of aromatic carbons and a significant reduction in the number of aliphatic carbons. The wet oxidation-treated biochar was thermally treated at 900 °C (10 °C min⁻¹) under N₂ for 2 h to produce a coke substitute. Results showed that the wet oxidation treatment of the samples at room temperature did not show much degradation, but samples treated at 80 °C showed higher mass losses. The temperature at which wet oxidation is performed has a significant effect during the process. The pyrolysis chars were investigated for their CO₂ or gasification reactivities. CO₂ isothermal gasification was used to determine the reactivities of the produced substitute coke char samples and industrial coke sample. The prepared substitute coke chars and the industrial coke samples were subjected to thermogravimetric analyses at an isothermal temperature of 900 °C in a CO₂ atmosphere. Characterization of the prepared substituted coke samples was carried out using nuclear magnetic resonance (NMR). The CO₂ gasification results showed that the average initial reactivities, the reactivities at 50% conversion, and the average final reactivities of the prepared samples were up to three times that of the industrial coke sample. The pyrolysis char of the coke substitute sample prepared for 2 h at 80 °C with a wet oxidation method has an average initial reactivity of 6.59 × 10^–3 min⁻¹, reactivity at 50% conversion of 15.73 × 10^–3 min⁻¹, and 9.6 × 10^–3 min⁻¹ for average final reactivity. The coke substitute samples produced from the waste coffee ground biochar subjected to a wet oxidation treatment method may be utilized as a coke substitute, with similar or higher gasification reactivity than industrial coke.