Properties and reactivity of two oxidized and unoxidized South African Highveld fine coal rejects and their density-separated fractions

ABSTRACT

Fine coal reject samples (FCRs) derived from the South African (SA) feed-coal preparation are unsuitable for gasification/combustion. In this novel investigation, two SA FCRs were taken from a thermal-export plant (TEP) and steam-station (STEM) for the coal-property comparison study and pyrolysis/gasification reactivity. H₂S/SO₂/CO₂/Cadmium/Arsenic/volatile organic compounds (VOCs) emissions/dust/particulate-matter pollution linked to pyrite-spontaneous combustion and the interaction of organic sulfur and calcium in FCRs are serious threats to the global human health and atmospheric environment. Numerous investigations primarily focused on SA unoxidized FCR (STEM sample) rather than oxidized FCR (TEP sample) and beneficiated fractions. Furthermore, no Raman/X-ray diffraction (XRD)/density-separation/thermogravimetric results for the TEP (40–400 µm; 83% recovery) and STEM (111–500 µm; 20% recovery) samples have been previously reported. FCRs/their beneficiated fractions were characterized by the different coal analyses. Results indicate that the FCRs have similar properties compared to gasification/combustion feed coals with the exception of higher mineral matter (39% versus 29%), higher vitrinites (43% against 24%) and higher alkali index (8 versus 5) in the TEP sample. Thermogravimetric gasification experiments illustrate higher reaction rates/higher carbon conversion for the TEP sample than the STEM sample. Raman/XRD detected aromatic/aliphatic hydrocarbons/minerals, which positively contribute to the coal’s reactivity. Results provide a baseline toward FCRs utilization in pyrolysis/gasification applications on the basis of their reactivity, and a minimization of higher disposal-costs/health-hazards/air-pollution/fines volume.

KEYWORDS:

• Gasification
• char reactivity
• fine coal rejects
• mode of occurrence
• mineral matter

Acknowledgements

The information presented in this paper is based on the research financially supported by the South African Research Chairs Initiative (SARChI) of the Department of Science and Technology and National Research Foundation of South Africa (Coal Research Chair Grant No.86880 and NRF free standing bursary MND200715544211/UID: 136078). Any opinion, finding or conclusion or recommendation expressed in this material is that of the author(s), and the NRF does not accept any liability in this regard. Prof N. Wagner of the University of Johannesburg, Geology Department, Dr Liberty Kapesi of the University of Johannesburg, Department of Physics and Morne Van Zyl of SGS, Secunda, are thanked for their assistance with different aspects of the analytical techniques.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was supported by the Department of Science and Innovation, South Africa [86880]; National Research Foundation [136078]