Christien A. Strydom

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.

Effect of some nitrogen donor ligands on the optical and structural properties of CdS nanoparticles

The cadmium complex of N-ethyl-N-phenyl dithiocarbamate, 1, and its 2,2′-bipyridine and 1,10-phenanthroline adducts, 2, and 3, respectively, have been used as single source precursors for the synthesis of CdS nanoparticles. The formation of CdS nanoparticles was achieved by thermal decomposition of the complexes (pyrolysis) and thermolysis in the presence of hexadecylamine – HDA (solvothermal). Scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and high resolution transmission electron microscopy (HRTEM) analyses were carried out to study the structural properties of the nanoparticles. Complex 1 afforded rod-shaped nanoparticles while star-shaped nanoparticles were obtained from complexes 2 and 3. The optical property of the CdS nanoparticles was studied by UV–visible and fluorescence spectroscopy. The spectral features for the nanoparticles prepared via the solvothermal route were significantly sharper and blue shifted to a greater extent relative to the corresponding bulk semiconductor, compared to the observed shift in the nanoparticles prepared via pyrolysis. All the as-synthesized material exhibited band edge luminescence.

Laser induced and controlled chemical reaction of carbon monoxide and hydrogen

Bimolecular chemical reaction control of gaseous CO and H(2) at room temperature and atmospheric pressure, without any catalyst, using shaped femtosecond laser pulses is presented. High intensity laser radiation applied to a reaction cell facilitates non-resonant bond breakage and the formation of a range of ions, which can then react to form new products. Stable reaction products are measured after irradiation of a reaction cell, using time of flight mass spectroscopy. Bond formation of C-O, C-C, and C-H bonds is demonstrated as CO(2)(+), C(2)H(2)(+), CH(+), and CH(3)(+) were observed in the time of flight mass spectrum of the product gas, analyzed after irradiation. The formation of CO(2) is shown to be dependent on laser intensity, irradiation time, and on the presence of H(2) in the reaction cell. Using negatively chirped laser pulses more C-O bond formation takes place as compared to more C-C bond formation for unchirped pulses.

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.

Comparative Study of the Dissociative Ionization of 1,1,1-Trichloroethane Using Nanosecond and Femtosecond Laser Pulses

Changes in the laser induced molecular dissociation of 1,1,1-trichloroethane (TCE) were studied using a range of intensities and standard laser wavelengths with nanosecond and femtosecond pulse durations. TCE contains C-H, C-C and C-Cl bonds and selective bond breakage of one or more of these bonds is of scientific interest. Using laser ionization time of flight mass spectrometry, it was found that considerable variation of fragment ion peak heights as well as changes in relative peak ratios is possible by varying the laser intensity (by attenuation), wavelength and pulse duration using standard laser sources. The nanosecond laser dissociation seems to occur via C-Cl bond breakage, with significant fragmentation and only a few large mass ion peaks observed. In contrast, femtosecond laser dissociative ionization results in many large mass ion peaks. Evidence is found for various competing dissociation and ionization pathways. Variation of the nanosecond laser intensity does not change the fragmentation pattern, while at high femtosecond intensities large changes are observed in relative ion peak sizes. The total ionization yield and fragmentation ratios are presented for a range of wavelengths and intensities, and compared to the changes observed due to a linear chirp variation.

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.

(2-Ethyl-2-oxazoline-κN)bis(N-ethyl-N-phenyl­dithio­carbamato-κ2S,S′)cadmium

In the title compound, [Cd(C9H10NS2)2(C5H9NO)], the CdII atom is five-coordinated in a distorted square-pyramidal geometry by four S atoms from two chelating N-ethyl-N-phenyl dithiocarbamate ligands and one N atom from a 2-ethyl-2-oxazoline ligand. Intermolecular C—H⋯π interactions are observed in the crystal structure.

Validation of Using a Modified BET Model to Predict the Moisture Adsorption Behavior of Bituminous Coal

A good correlation exists between the amount of moisture adsorbed and the oxygen content of medium-rank B vitrinite-rich and medium-rank C inertinite-rich bituminous coal. Total mass moisture adsorbed per gram of coal determined from the modified BET model provides a better approximation than the BET model. The relationship between the modified BET moisture monolayer coverage and the area of surface oxygen groups suggests that moisture is adsorbed at these sites. Oxygen content is a good indicator of the amount of moisture adsorbed on bituminous coal. Open hysteresis loops indicate that moisture is retained on medium-rank C inertinite-rich bituminous coals.

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.