Coal type and rank variation in Tatu Mine, New Zealand

Abstract

Tatu State Mine in Taranaki, North Island, New Zealand, worked a Miocene subbituminous coal seam about 2 m thick over an area of nearly 3 km 2. Detailed face sampling of the seam from 1940 to 1961 provided 46 proximate and 15 proximate and ultimate analyses. They show great variation, particularly in moisture, volatile matter, calorific value and sulphur content. The relations between the various analytical parameters exemplify and amplify those found generally within coals from New Zealand and many other countries, providing basic elements for geological study of coal properties. The detailed inferences are: 1. (1) The relation between bed moisture and moisture in coal air dried under standard conditions of 20°C and 70% relative humidity (R.H.) is M B = ∼ 1.1 × M ADS(af). 2. (2) Sulphur content decreases from roof to floor of the seam, with a minor increase close to the floor. Although the sulphur contents of the top ply samples range from >6% to <2% (daf), the decrease from top to bottom plies is in all cases 1.5–2.0%. 3. (3) On the basis of the full seam thicknesses, sulphur content shows a clear areal pattern of variation, ranging from >6% to <1.5% (daf). 4. (4) At one sampling site, sampled in 9 plies, the following relations show the effects of coal type variations: Volatile matter and calorific value (dmmsf): +1% VM = +0.2 MJ/kg Volatile matter (dmmsf) and bed moisture (af): + 1% VM =c.−0.4% M 5. (5) Almost all coals lie within the New Zealand Coal Band. A few are lower in volatile content; rather more are higher. 6. (6) On the basis of the full seam thickness, volatile matter, ranging from >51% to <45%, shows a clear areal pattern of variation. 7. (7) The patterns of variation of volatile matter and sulphur are similar, the relation between them being: + 1% S (daf) = +1.5% VM (dmmsf). Although sulphur, introduced from sea water when overlying marine beds were deposited, appears to have affected coal type, as judged by volatile matter, petrographic data are needed to study this further. 8. (8) Rank of dry coal, in terms of Rank (S) (Suggate, 1959), increases westwards from rank ∼7.5 to >8.5. 9. (9) Moisture content shows no significant rank-related variation. 10. (10) Taken individually, almost all samples would be ASTM rank subbituminous A, but particularly high-volatile samples would be High-volatile C and B bituminous. Production from the mine has always been subbituminous coal.