Direct Liquefaction of South African Vitrinite- and Inertinite-Rich Coal Fines.

In this investigation, coal fines enriched with inertinite were used for direct liquefaction experiments. For comparison, a vitrinite-rich coal typically utilized in coal-to-liquid processes was also employed. To assess the impact of mineral matter content, demineralization was used to remove most of the inorganic constituents. The findings revealed that the inertinite-rich coal exhibited lower liquefaction conversions due to a reduced proportion of reactive macerals and elevated levels of inorganic mineral matter. These conversion values exhibited a strong correlation with the quantity of reactive macerals present in the parent coals. For the inertinite-rich coal, the presence of inorganic mineral matter impeded the liquefaction process but facilitated the CO2 gasification reactions of the derived chars. To evaluate their potential in gasification processes, CO2 gasification experiments were conducted and the reactivities and apparent gasification activation energies of both coal chars, liquefaction residue chars, and preasphaltene and asphaltene (PAA) chars were calculated. These calculations were carried out using the random pore model (RPM) and volumetric reaction model (VRM). The chemistry, reactivity, and kinetics of residue gasification conversion are not thoroughly understood, yet they hold significant importance in optimizing syngas production within gasification processes. The findings from this work highlight significant differences in liquefaction conversion values, product distribution, and composition. These differences are influenced by factors such as maceral composition, inorganic mineral matter content, hydrogen-donor capabilities of the solvent, and liquefaction reaction temperatures. Additionally, these variables affect the CO2 gasification reactivity of liquefaction solid residue chars.

Slow Pyrolysis Products Derived from Extrudates Produced from Discard Coal Fines and Recycled Plastics as Binders.

The pyrolysis products derived from both coal and plastics have been extensively evaluated in literature; however, their copyrolysis product yields together with the characterization of the generated products have received less attention. Most studies use high heating rates with small particle sizes of the mechanically mixed blends. This study aims to improve the understanding of the slow copyrolysis behavior of extrudates produced from coal fines from the South African Highveld coalfield combined with recycled waste low-density polyethylene (LDPE) and polypropylene (PP). The fraction of plastic was varied between 10 and 100 wt % during extrusion. The extrudates (10 mm diameter) underwent slow pyrolysis in a modified Fischer assay setup by increasing the temperature (5 °C/min) under a nitrogen atmosphere to 520, 720, and 920 °C. The pyrolysis products (char, condensable products, water, and gas) were collected and characterized. The coal fines produced up to 83% char, whereas the plastics produced over 90% condensable products. The slow heating rate and the small reactor volume favored the production of condensable products, which increased with plastic concentration. The extrudates produced char and condensable product yields that fit the additive model of the raw materials. Statistical equations were developed to predict the pyrolysis yields and characteristics as functions of coal and plastic composition. The equations accurately predict the char yield, condensable product yield, fixed carbon content, ash content, sulfur content of chars, and carbon and hydrogen contents of chars and condensable products. Gas chromatography-mass spectrometry results indicate that the extrudates' condensable products consist mainly of alkanes and alkenes, similar to the plastics. Furthermore, the condensable products derived from PP and coal extrudates have fewer components with lower boiling points compared with the raw materials.

Chemical and physical characterization of spent coffee ground biochar treated by a wet oxidation method for the production of a coke substitute.

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%). The biochar was treated for different time durations (0.5, 1, 2, 6 and 24 h) and at different temperatures (room temperature and 80 °C). Thereafter, the various samples were characterized and pyrolysed to obtain a coke substitute. Characterization 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. The investigated characteristics of the produced coke substitute obtained from the pyrolysed biochar treated for 24 h with 30 vol% H2O2 at room temperature, showed the most promising results when compared to blast furnace coke.

Dataset on the carbon dioxide, methane and nitrogen high-pressure sorption properties of South African bituminous coals.

The dataset presented in this article supplements the result and information published in the report "The carbon dioxide, methane and nitrogen high-pressure sorption properties of South African bituminous coals" (Okolo t al., 2019). Four run of mine coal samples from selected underground coal mines from the Highveld, Witbank, and Tshipise-Pafuri coalfields of South Africa were used for the study. The CO2, CH4, and N2 sorption data were acquired from an in-house built high-pressure gravimetric sorption system (HPGSS) at the CSIRO Energy, North Ryde, Australia; at an isothermal temperature of 55 °C, in the pressure range: 0.1-16 MPa. The resulting excess sorption isotherm data were fitted to the modified Dubinin-Radushkevich isotherm model (M-DR) and a new Dubinin-Radushkevich - Henry law hybrid isotherm model (DR-HH). The dataset provided in this article, apart from being informative will be useful for comparison with available and future data and for testing other sorption isotherm models developed by other investigators in the area of CO2 storage in geological media, especially coal seams.

The carbon dioxide gasification characteristics of biomass char samples and their effect on coal gasification reactivity during co-gasification.

The carbon dioxide gasification characteristics of three biomass char samples and bituminous coal char were investigated in a thermogravimetric analyser in the temperature range of 850-950 °C. Char SB exhibited higher reactivities (Ri, Rs, Rf) than chars SW and HW. Coal char gasification reactivities were observed to be lower than those of the three biomass chars. Correlations between the char reactivities and char characteristics were highlighted. The addition of 10% biomass had no significant impact on the coal char gasification reactivity. However, 20 and 30% biomass additions resulted in increased coal char gasification rate. During co-gasification, chars HW and SW caused increased coal char gasification reactivity at lower conversions, while char SB resulted in increased gasification rates throughout the entire conversion range. Experimental data from biomass char gasification and biomass-coal char co-gasification were well described by the MRPM, while coal char gasification was better described by the RPM.

In Situ Capturing and Absorption of Sulfur Gases Formed during Thermal Treatment of South African Coals.

The objective of this study, the first of its kind on these specific South African low-sulfur coals, was to capture H2S and SO2 produced under inert and oxidizing conditions from sulfur compounds present in the coals. The capturing agents were calcium and magnesium oxides formed during the transformation of calcite and dolomite. The effectiveness of two different scrubbing solutions (0.15 M cadmium acetate and 1.1 M potassium hydroxide) for absorption of volatilized H2S and SO2 was also investigated. The bituminous coal (coal A) contained dolomite, calcite, pyrite, and organic sulfur. Lignite (coal B) had a high organic sulfur content and contained gypsum, no or low dolomite and pyrite contents, and no calcite. A third sample (coal C) was prepared by adding 5 wt % potassium carbonate to coal A. Under oxidizing conditions and at elevated temperatures, FeS2 produced Fe2O3, FeO, and SO2. It transformed to FeS and released H2S under inert conditions. Organic sulfur interacted with organically bound calcium and magnesium at 400 °C in an inert atmosphere to form calcium sulfate and oldhamite ((Ca,Mg)S). CaO, produced from calcite or dolomite, reacted with SO2 and O2 at 950 °C to form calcium sulfate. Treatment of lignite at 400-950 °C resulted in 96-98% evolution of sulfur as gases. Hydrogen sulfide formation increased with the increasing thermal treatment temperature under inert conditions for the three coals. Under oxidizing conditions, sulfur dioxide formation decreased with the increasing temperature when heating coals B and C. The lowest ratio (0.01) of H2S to SO2 was achieved during thermal treatment of the blend of coal and potassium carbonate (coal C), implying that almost all of sulfur was retained in the coal C ash/char samples. In situ capturing of sulfur gases by CaO and MgO and by the added K2CO3 in coal C to form calcium/magnesium/potassium sulfates and potassium/calcium/magnesium aluminosilicate glasses during utilization of these and similar coals could reduce the percentage of sulfur volatilized from the coals by 54-100%, thereby potentially decreasing their impact on the environment.

Chemical and structural characterization of char development during lignocellulosic biomass pyrolysis.

The chemical and structural changes of three lignocellulosic biomass samples during pyrolysis were investigated using both conventional and advanced characterization techniques. The use of ATR-FTIR as a characterization tool is extended by the proposal of a method to determine aromaticity, the calculation of both CH2/CH3 ratio and the degree of aromatic ring condensation ((R/C)u). With increasing temperature, the H/C and O/C ratios, XA and CH2/CH3 ratio decreased, while (R/C)u and aromaticity increased. The micropore network developed with increasing temperature, until the coalescence of pores at 1100°C, which can be linked to increasing carbon densification, extent of aromatization and/or graphitization of the biomass chars. WAXRD-CFA measurements indicated the gradual formation of nearly parallel basic structural units with increasing carbonization temperature. The char development can be considered to occur in two steps: elimination of aliphatic compounds at low temperatures, and hydrogen abstraction and aromatic ring condensation at high temperatures.

Structural and chemical modifications of typical South African biomasses during torrefaction.

Torrefaction experiments were carried out for three typical South African biomass samples (softwood chips, hardwood chips and sweet sorghum bagasse) to a weight loss of 30 wt.%. During torrefaction, moisture, non-structural carbohydrates and hemicelluloses were reduced, resulting in a structurally modified torrefaction product. There was a reduction in the average crystalline diameter (La) (XRD), an increase in the aromatic fraction and a reduction in aliphatics (substituted and unsubstituted) (CPMAS (13)C NMR). The decrease in the aliphatic components of the lignocellulosic material under the torrefaction conditions also resulted in a slight ordering of the carbon lattice. The degradation of hemicelluloses and non-structural carbohydrates increased the inclusive surface area of sweet sorghum bagasse, while it did not change significantly for the woody biomasses.