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The chemical properties and composition of Mokau Coal

, : The chemical properties and composition of Mokau Coal. New Zealand Journal of Geology and Geophysics 29(4): 447-461

Chemical and physical analyses carried out on Mokau coal as part of the recently completed New Zealand Government prospecting programme are treated statistically to define mean properties in-ground for each seam of each sector and to show geographical trends within and between seams. Properties studied include bed moisture, air-dried moisture, specific energy, volatile matter, sulphur, forms of sulphur, coal composition, mineral composition, ultimate analysis, ash constituents, trace elements, ash distribution, boiler fouling properties, ash fusibility, Hardgrove Grindability Index, and relative density. The median in-ground properties range from 15.9% moisture, 7.2% ash, 35.4% volatile matter, 41.5% fixed carbon, 23.14 MJ/kg specific energy, 1.6% sulphur (Mangatoi seam 1) to 14.4% moisture, 9.1% ash, 40.9% volatile matter, 35.6% fixed carbon, 23.76 MJ/kg specific energy, 4.7% sulphur (Manga-Awakino seam 5). The mean coal composition (dry, mineral free basis) ranges from 69.6% carbon, 4.85% hydrogen, 1.05% nitrogen, 1.25% sulphur, 21.0% oxygen, 1.14% calcium, 0.19% magnesium, 0.2% aluminium, 0.49% iron, 0.06% sodium, 0.043% boron (5.0% ash) for Mangatoi seam 2 to 70.2% carbon, 5.27% hydrogen, 1.0% nitrogen, 3.08% sulphur, 18.6% oxygen, 1.09% calcium, 0.21% magnesium, 0.11% aluminium, 0.03% iron, 0.30% sodium, 0.047% boron (5.2% ash) for Manga-Awakino seam 4. The coal minerals predominantly consist of silica and alumina with significant pyrite (or marcasite) and minor amounts of calcium, magnesium, potassium, and titanium. 33% of the sulphur, on average, was inorganic. Of the trace elements, boron, barium, and strontium are prominent. Ash fusion temperatures and Hardgrove Grindability are both comparable to those found in Waikato coals. Sulphur increases from the southeast to the northwest of the field. ASTM rank increases with the seam progression downwards. The top seam is predominantly subbituminous B and the lower seams predominantly subbituminous A with some patches of high volatile bituminous C. Volatile matter displays marked variability in each seam, associated with the liptinite content of the coal. Sodium is higher at the southeastern corner of the field. Mokau coal is comparable in quality to the best Waikato coals but with higher sulphur (particularly in the northeastern part of the field) and higher ash.


DOI: 10.1080/00288306.1986.10422166

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