John R. Bunt

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.

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.

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.

Influence of additives on the devolatilization product yield of typical South African coals, and effect on tar composition

Witbank area of South Africa were specifically chosen with a range of maceral content. Previous studies investigating the effect of inorganic additives on devolatilization have found that several additives affect the amount of volatiles derived from the studied coal (Liu et al., 2004; Ahmad et al., 2009; Wu, Sugimoto, and Kawashima, 2003). Studies investigating the effect of operational conditions (temperature, pressure, and heating rate) on the devolatilization product composition have found that changing these conditions will also influence the product yield and composition (Juntgen and van Heeck, 1977; Rennhack, 1964; Peters and Bertling, 1965). The importance of studying coals from South Africa becomes apparent when comparing coals from this region to coals that are mined in other regions. Coals from North America are spread between anthracite, bituminous, sub-bituminous, and lignite ranks (vitrinite-rich); this wide variety of rank is also the case with many of the world’s other top producers of coal, as reported by the World Coal Institute (2008). As a result, most previous studies (especially those done outside of South Africa) are not applicable to the highinertinite, high-ash bituminous coals from this region. Previous studies performed in South Africa were not focused on the devolatilization products, but investigated the process of devolatilization with regard to South African coals (Beukman, 2009). Due to the potential for the production of high-value chemicals from coal, it is deemed appropriate to investigate the effect of extraneous additives on the quantity and composition of the tars that can be derived from South African coal when pyrolised at a slow heating rate in the presence of an alkali metal catalyst. Should the tar yield be reduced due to additive addition, and lighter products be formed, then this could be seen as a positive step toward the production of valuable chemicals from coal.

Mineralogical, chemical, and petrographic properties of selected South African power stations’ feed coals and their corresponding density separated fractions using float-sink and reflux classification methods

ABSTRACT Three South African feed coal samples for the combustion process were beneficiated to produce carbon-rich and mineral-rich fractions. The mineralogical, petrographical, and chemical properties of these feed coals and their density separated fractions were investigated using XRD, XRF, QEMSCAN, Electron microprobe, and petrography analyses. This work was conducted with the goal of better understanding the processes and operational problems which could possibly occur during coal utilization, with particular focus on the included and excluded mineral matter transformational behavior at elevated temperatures. The conventional float-sink and reflux classification methods used were shown to successfully eliminate liberated minerals and produced maceral-rich float fractions (98%) macerals. The main differences between the feed coals were related to the mode of occurrence of mineral matter. An integration of these different analytical techniques allowed for better determination of the concentrations of mineral matter responsible for industrial ash-related problems. In this paper, we propose that blends of the different density fractions will reduce or minimize clinker and slag formation as well as the abrasive nature of the clinkers or slags. Possible blends to minimise clinker and slag formation include the float and sink fractions of the feed coals in varying proportions based on the specific mineralogical, petrographical and chemical data.

The influence of the roof and floor geological structures on the ash composition produced from coal at UCG temperatures

ABSTRACT The mineralogy of the ash and slag formed at typical UCG temperatures were investigated using a bituminous coal from the Theunissen UCG site in the Free State province of South Africa. The ash and slag samples were produced from the coal, and the surrounding roof and floor geological structures at 1000, 1100, 1200 and 1300°C. XRD results show an increase in crystalline phases, with a decrease in the amorphous content as the temperature increases, with mullite and quartz found to be the dominant minerals in the crystalline phase. FTIR spectroscopy results reveal the appearance of peaks related to the crystalline phase of mullite with increasing temperature. SEM results show the formation of spherical particles, with the appearance of cenospheres, as the temperature increases. The samples produced at 1000°C had significantly lower surface area and porosity values than the blended coal sample, but these values remain similar for the ash samples produced between 1000°C and 1300°C, where only slight decreases are observed with an increase in temperature. Results indicate that the higher the temperature in a UCG cavity, the less leaching of inorganic species should occur.

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.

Coal reactivity and selection for solid-based pre-reduction of sponge iron

ABSTRACT Solid-based direct reduction of iron ore requires the reductant coal to have a suitable CO2 reactivity to achieve optimum pre-reduction within a rotary kiln. The CO2 reactivity is affected by numerous factors including coal properties and operating conditions, and is traditionally determined using pulverized samples. The CO2 reactivity of nine South African coals, using 20 mm coarse coal particles, was measured. For this, an in-house constructed large particle thermogravimetric analyzer was used, at typical pre-reduction conditions: 1050°C and 25 vol.% CO2. The initial specific reaction rate was used to quantify the CO2 reactivity and a statistical analysis was performed to determine correlations between coal and char properties and coarse coal reactivity. For the coals, the chemical and petrographic characteristics, and for the char, the chemical and structural properties, showed the most significant relations. Multiple linear regression was applied to derive empirical equations from which the initial specific reaction rate could be determined as a function of coal or char properties. For the coal, the reactivity was a function of the fuel ratio and reactive maceral index, while for the char the initial specific reaction rate was a function of the nitrogen content and the carbon-based micropore surface area.

Evaluating the performance and emission reductions of a coal-derived low-smoke fuel in a conventional household stove

A low-smoke fuel (LSF) produced via the devolatilisation of coal lumps is proposed as an option to combat the health-damaging effects of domestic coal combustion. This study investigates the effect of the LSF particle size on its performance in a conventional domestic coal stove. Three size groups, 15mm, 40mm, and a mixed size LSF were tested against their coal counterpart for reduction in CO and PM emissions; two practical parameters, ignition time and combustion efficiency, were also considered. On a practical level, smaller particles, despite having difficulty igniting, showed the most promise. These particles achieved higher temperatures, provided heating for longer and, had higher combustion efficiency than their larger counterparts. Larger PM reductions were observed for smaller particles when comparing LSFs to coals.