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

Rautenbach, Rudelle; Strydom, Christien A.; Bunt, John R.; Matjie, R.H.; Campbell, Q.P.

doi:10.1080/19392699.2018.1533551

Cited by 13 publications

(34 citation statements)

References 36 publications

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“…It can be seen from Table 1 that the coals used in this research are of lower quality, especially the discard coal (C2), which was found to have a very high ash content (41.95%) compared to the coal used in the South African coal fired plants. The ash content of a typical coal used in the South African coal power plants is within the range of 25-35.9%, this was reported in a recent study conducted by Rautenbach et al (2018) [26]. The ROM coal (C1) used in this study, with an ash content of 29.42% is similar to the quality used in the present South African coal power plants, but this coal may need to be washed in order to upgrade its volatile matter content to above 30%.…”

Section: Physicochemical Properties Of the Rdf And Coal Samplessupporting

confidence: 58%

The co-combustion performance and reaction kinetics of refuse derived fuels with South African high ash coal

Isaac

Bada

2020

Heliyon

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This research focuses on the co-firing of low-quality coal with refuse derived fuel (RDF) as a means to reduce the volume of waste dumped in landfill sites. The co-combustion behaviour and kinetics of various RDF/coal blends at different weight ratios, along with their physicochemical characteristics were investigated. The physicochemical analysis revealed that the run-of-mine and discard coal have relatively low calorific values of 21.7 MJ/kg and 16.7 MJ/kg, respectively. The RDF samples, plastic blend (31.2 MJ/kg) and paper blend (22.4 MJ/kg), were found to have higher energy contents. The thermogravimetric analysis was performed in an atmosphere of air, over a temperature range of 25-850 C, and the results showed that the RDF samples had lower ignition, devolatilisation, and burnout temperatures compared to the coals. The ignition temperatures for the blended fuel occurs in the lower temperature region when RDF is added to the blend, likewise the peak temperatures and burnout temperature shifted to a lower temperature zone. The activation energies (E a ) were determined using the Coats-Redfern method. The E a for the run-of-mine (ROM) coal of 104.4 kJ/mol, was found to reduce to 31.4 kJ/mol for the 75% PB þ 25% ROM coal blend and 35 kJ/mol for the 75% PL þ 25% ROM coal blend, respectively. The discard coal which had an E a of 109.9 kJ/mol was reduced to 30.9 kJ/mol for the 75% PB þ 25% discard blend, and 33.5 kJ/mol for the 75% PL þ 25% discard coal blend. It was determined that the most favourable blend for co-combustion was 70% discard coal þ 30% PL RDF due to the similarity of the combustion profile to that of 100% coal and the simultaneous reduction in apparent activation energy.

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Section: Physicochemical Properties Of the Rdf And Coal Samplessupporting

confidence: 58%

The co-combustion performance and reaction kinetics of refuse derived fuels with South African high ash coal

Isaac

Bada

2020

Heliyon

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“…Minor proportions (<0.5%) of carboxylic acid salts may contribute to catalyzing the pyrolysis reactions in South African coals. , The minor proportions of carboxylic acid salts, inherent minerals, and significant proportions of extraneous minerals contained in these coals do not contribute to the pyrolysis reactions at 450–550 °C during the South African coals pyrolysis under an inert atmosphere. However, significant fractions of carboxylic acid salts contained in the overseas lignites may catalyze pyrolysis and gasification reactions …”

Section: Resultsmentioning

confidence: 99%

Pyrolysis of Tetralin Liquefaction Derived Residues from Lighter Density Fractions of Waste Coals Taken from Waste Coal Disposal Sites in South Africa

Uwaoma¹,

Strydom²,

Matjie³

et al. 2019

Energy Fuels

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The lighter density (<1.5 g/cm3) fractions produced from two waste coals sampled from the waste coal disposal sites at thermochemical plants situated in South Africa were used as feed materials for liquefaction with tetralin. The liquefaction residues from the lighter density and untreated lighter density fractions were used in pyrolysis experiments. Pyrolysis of the lighter density fractions was carried out in a Fischer Assay oven at 750 and 920 °C under an argon atmosphere. Advanced analytical techniques (gas chromatography–mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy) were employed to characterize the pyrolysis products. Also, the lighter density fractions, liquefaction residues, and their chars were examined using conventional and advanced analytical techniques. The pyrolysis char yields of the liquefaction residues ranged between 74% and 76%, and those of the coal float fractions ranged between 67.0 and 71.5%. Gas pyrolysis yields ranged between 16.0% and 20.0% for the residues and between 14.5% and 18.4% for the lighter density fractions, while the pyrolytic water and the tar products of the lighter density fractions were slightly higher than those of the liquefaction-derived residues. The proton NMR analysis of the tars from the residues shows marginally higher amounts of aromatic protons than those of the lighter density fractions. Chars which were generated after pyrolysis of the liquefaction-derived residues show higher porosity values than those from the pyrolysis of the lighter density fractions. The differences in the porosities are attributed to the opening of pores and extraction of some lower molecular mass aliphatic species from the coal matrix during liquefaction. The pyrolysis products distribution and characterization of the products showed that the residues (waste material) generated after tetralin liquefaction of the float fractions from the float–sink experiments of waste coals may be utilized for thermochemical processes (pyrolysis).

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“…The proximate and ultimate analyses results for the caking feed coal and its density-separated fractions (without potassium compounds addition) are presented and discussed in the literature. ,, In addition, the proximate and ultimate data of their chars, which were produced at different temperatures (450, 550, and 600 °C) and pressures (0.87, 15, and 30 bar) during the pyrolysis experiments, are given and fully discussed in these different studies.…”

Section: Resultsmentioning

confidence: 99%

“…Approximately 124 kg of the representative coarse ROM caking coal particles (>0.5 mm <75 mm) were sampled from the Waterberg Matimba station coal stockpiles for the pyrolysis experiments to study the chemical reactions preventing caking propensity using International Organisation for Standardization (ISO) 18283 and ISO 13909-4 . The preparation methods of coal and its sampling procedures that were employed in this investigation were reported earlier by Tsemane et al The physical method of cleaning coal using dense media was followed to produce different density-separated fractions needed in this study as mentioned in Tsemane et al The proximate, ultimate and petrographic analytical methods and results of the Waterberg ROM coal particles and their density-separated fractions are presented and discussed elsewhere. ,, The selected density-separated fraction particles (<1.5 g/cm 3 float) were ground to produce >75 μm <1000 μm particles to study the interactions of mineral matter, organic matter, and potassium compounds, which may inhibit caking propensity of coal particles during pyrolysis. Also, the similar fine coal particles (>75 μm <1000 μm) used in this study were produced in the South African commercial gasifiers during the coal gasification. , …”

Section: Methodsmentioning

confidence: 99%

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Interactions between Kaolinite, Organic Matter, and Potassium Compounds at Elevated Temperatures during Pyrolysis of Caking Coal and Its Density-Separated Fractions

et al. 2021

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Large coal agglomerates (cakes), which form in slow heating fixed-bed commercial gasifiers, result in low carbon efficiency and carbon loss in the ash assemblage. In an earlier study, laboratory-scale pyrolysis (600, 720, or 920 °C for 15 min) of a bituminous coal and its beneficiated fractions (>75 μm <1000 μm) was conducted to determine chemical reactions, which may inhibit caking propensity. It was found that the <1.5 g /cm 3 float achieved 70% caking compared to feed coal (18%) and its >1.9 g/ cm 3 sink (0%). In this study, blends of the same <1.5 g/cm 3 float and either 2.7 M (Molarity) KOH or 2 M KCl or 0.9 M K 2 SO 4 or 1.5 M KNO 3 or 1.1 M K 2 CO 3 or 1.5 M KCH 3 CO 2 were pyrolyzed to gain valuable insights into the reactions inhibiting caking propensity. Kaolinite, organic matter, and potassium compound interaction effects in the blends, KOH concentrations, and temperature effects on the caking propensity were therefore investigated. X-ray diffraction (XRD) results indicated that the potassium compounds added to coal resulted in the formation of noncaking oxygen-containing potassium minerals (illite, sanidine, nepheline, microcline, and muscovite) at elevated temperatures due to the reactions of Al 2 O 3 •2SiO 2 and K + in the chars. The addition of high proportions of potassium compounds to coal may result in gasifier blockage/corrosion/erosion and carbon loss due to clinker/slag formation at elevated temperatures and should be considered with caution. Nuclear magnetic resonance results further show that polycyclic aromatic hydrocarbons and carboxylic acids present in coal and KOH blends reacted with KOH to form oxygenates (23%) in the noncaking chars. Large cakes derived from the <1.5 g/cm 3 float contained 11% oxygenates. Also, Fourier transform infrared (FTIR) results for the 2.7 M KOH blend char confirmed higher intensities of peaks of oxygen-containing compounds, which are associated with caking propensity inhibition during pyrolysis. These chemical reactions in the presence of organic matter, kaolinite, and KOH to form oxygenates and potassium feldspars in the chars significantly reduced the highest coal particles caking (70%) to <2 or 0%. Based on these findings, the <0.5 M KOH blend strategy should be considered for use in commercial gasifiers to inhibit severe coal particle caking.

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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

Cited by 13 publications

References 36 publications

The co-combustion performance and reaction kinetics of refuse derived fuels with South African high ash coal

The co-combustion performance and reaction kinetics of refuse derived fuels with South African high ash coal

Pyrolysis of Tetralin Liquefaction Derived Residues from Lighter Density Fractions of Waste Coals Taken from Waste Coal Disposal Sites in South Africa

Interactions between Kaolinite, Organic Matter, and Potassium Compounds at Elevated Temperatures during Pyrolysis of Caking Coal and Its Density-Separated Fractions

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