Production of a metallurgical coke substitute from biochar depolymerised by wet oxidation

Abstract

Coke production relies on the availability, cost and quality of coking coal. Depleting coking coal resources and environmental pressure force the metallurgical industry to search for alternative methods to produce coke. Waste spent coffee grounds (biomass) treated via hydrothermal liquefaction (HTL) is an energy-efficient method to produce biochar. In this study the use of HTL biochar as feedstock for the production of a coke substitute was investigated. Wet oxidation treatment of the prepared biochar samples was done with different wet oxidant hydrogen peroxide concentrations (5, 15, 30 vol.%). The biochar was treated for different time durations (0.5, 1, 2, 6 and 24 hours) and at different temperatures (room temperature and 80°C). Characteristics for the prepared biochar samples were compared to characteristics for coking coal and values found in literature to evaluate whether a coke pre-cursor was produced. The various samples were characterised and pyrolysed at 1100°C (10°C/min) under N₂ for 2 hours to obtain a coke substitute. The prepared coke substitute samples were also subjected to gasification reactions in a CO₂ atmosphere to determine the reactivity of the produced coke substitute samples. The results from the prepared samples were compared to that of a coke sample obtained from industry. Characterisation of the various samples before and after thermal treatment was done using Fourier-transform infrared spectroscopy (FTIR), free swelling index, ultimate and proximate analysis, gross calorific value and compressive strength determination. Gasification reactivity of waste biochar samples was determined by a Thermogravimetric Analyzer (TGA) at 900°C under CO₂. The investigated characteristics of the produced coke substitute obtained from the pyrolysed biochar treated for 24 hours with 30 vol.% H₂O₂ at room temperature, showed the most promising results when compared to blast furnace coke as a coke pre-cursor. The 24-hour room temperature pellet had a significantly higher compressive strength value of 3.22 MPa (per unit area) compared to a coke sample obtained from industry that had a value of 1.02 MPa (per unit area). The results concluded that the average initial reactivity, reactivity at 50% conversion and average final reactivity of the prepared samples were mostly equal to the coke sample obtained from industry. The reactivity of the coke sample at 50% conversion was 4.7 x 10³ min⁻¹ with the closest reactivity calculated for the 2-hour room temperature wet oxidation treated sample with a value of 4.6 x 10³ min⁻¹. Thus, the coke substitute produced from the waste ground coffee biochar subjected to a wet oxidation treatment method using hydrogen peroxide solutions may be utilised as a coke substitute.