Zealand Coals Research Articles

96/03510 Spontaneous heating in New Zealand coals

96/03510 Spontaneous heating in New Zealand coals

Proximate analysis of New Zealand and Australian coals by thermogravimetry

Proximate analysis of New Zealand and Australian coals by thermogravimetry

95/05015 Low temperature oxidation of New Zealand coals

95/05015 Low temperature oxidation of New Zealand coals

95/04437 Proximate analysis of New Zealand and Australian coals by thermogravimetry

95/04437 Proximate analysis of New Zealand and Australian coals by thermogravimetry

Floral assemblage of the “D” coal seam (Cretaceous): Implications for banding characteristics in New Zealand coal seams

Abstract Two complete vertical sections were studied from the uppermost Cretaceous “D” coal seam near Greymouth, New Zealand. The thickest and most concentrated vitrain bands occur in the paleomire centre and bands are thinner and less abundant at the paleomire margin. Botanical analysis of the vitrain bands indicates they formed entirely from the secondary xylem (wood) of gymnosperms. Palynomorphs indicate that there is no consistent correlation between conifer pollen abundance and the degree of vitrain banding. However, maximum preservation of vitrain bands coincides with an inferred transition from a rheotrophic mire (as indicated by Phyllocladidites mawsonii pollen) to an acidic and possibly ombrotrophic system (as indicated by the abundance of Gleicheniaceae spores). This suggests that the presence/absence of gymnosperm secondary xylem as vitrain bands is controlled at least in part by mire chemistry.

Coal type and rank variation in Tatu Mine, New Zealand

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.

Botanical control on banding character in two New Zealand coal beds

Two New Zealand coal beds were examined using a palaeobotanical approach to determine controls on banding characteristics. The Morley (Cretaceous) coal is thickly (−5 to −1 δ or 32 to 2 mm) and well (>25%) banded whereas the Brunner (Eocene) coal is essentially non-banded. Palynological analyses indicate that the Morley was formed from a gymnosperm-dominated vegetation with a secondary component of angiosperms. In contrast, the Brunner was dominated by angiosperm vegetation with only a minor component of gymnosperms. However, it was found that any (xylite/vitrain) bands >1 mm in either coal bed were comprised exclusively of secondary xylem of probable gymnosperm origin. The presence of (gymnosperm) wood (i.e. secondary xylem) forming the larger bands in both coals can be related to the relative resistance of this material to chemical and physical decay as compared to all other plant tissues. An absence of angiosperm wood in either the Morley or Brunner coal beds may be the result of the relative chemical degradability of angiosperm wood as compared to gymnosperm wood. Alternatively, angiosperms in either of the two palaeo-mires may have been shrubby and thus did not produce abundant woody tissue. In either case, the banded nature of the Morley coal bed is the result of the presence of gymnosperm wood and the non-banded nature of the Brunner is the result of being dominated by a flora (angiosperm) which is easily degraded.

Diagenetic quartz and amorphous silica in New Zealand coals

New Zealand coal seams (Late Cretaceous-Miocene, lignite-semi-anthracite) contain numerous and varied occurrences of diagenetic quartz and amorphous silica. Early diagenetic silicifications include: sub-horizontal and laterally extensive, quartz-mineralized bands; micro- and megascopic, euhedral quartz crystals, dispersed or concentrated in layers; and silicified in situ tree stumps. Late diagenetic forms include: sub-horizontal, quartz-mineralized bands, mostly < 1 m in length; and fracture-infillings. The origins of the silica remain uncertain but leached basement rocks and coal measure sediments, and siliceous phytoliths within the coal-forming plants, are possible sources. The silicification of coal seams may have significant implications for exploration, mining and geological studies.

COAL RANK AND TYPE VARIATION IN ROCK‐EVAL ASSESSMENT OF NEW ZEALAND COALS

Rock‐Eval and coal analytical data are compared for New Zealand coal samples ranging from peat to medium‐volatile bituminous coal. Most are humic coals within the New Zealand coal band, which is representative of the Type III maturation series defined for kerogens. However, four were chosen as particularly high‐hydrogen coal types. Relations between individual parameters — principally H/C, O/C, Tmax, HI and OI — are generally similar to those of Espitalié et al. (1985, 1986), but important differences include a broader range of HI values in the peats and lignites, and a differentiation between maturation and coal‐type effects in the relationship of HI to H/C. Correlations of the effects of maturation on HI, OI, Tmax and vitrinite reflectance are made by the use of the independent Rank(S) scale of Suggate (1959).Rank(S)/reflectance relations vary between well sequences. The Rank(S) scale is used to integrate biomarker data on hydrocarbon generation and expulsion. The oil zone (“oil window”) corresponds to the range of bituminous coals, beginning at Rank(S) 10 (Tmax425°C) and ending at Rank(S) 17 (Tmax estimated at 550°C). Release of oil begins at or soon after Rank(S) 12.5 (Tmax 435°C), and culminates in the range Rank(S) 14.5‐ 15.5 (Tmax 460–480°C). For the Taranaki Basin, New Zealand, the results imply that a substantially greater volume of coal measures has been subject to oil generation and migration processes than previously thought.

Further studies of Gieseler fluidity development in New Zealand coals

The hydrogen donor ability of fresh samples of a Webb coal with high Gieseler fluidity is shown to be greater than that of samples of the same coal after oxidation has caused a significant decline in fluidity. Model compounds containing hydrogen donor sites similar to those in coal are found to donate hydrogen when heated through a Gieseler temperature profile and to undergo oxidation at these sites under the conditions used to destroy fluidity in coals. The addition of sulphided metal catalysts is found to destroy Gieseler fluidity. The results support the theory that a major factor in fluidity development is a supply of donor hydrogens involved in the generation of low molecular weight ‘solvating’ species.

Low temperature oxidation studies of dried New Zealand coals

The chemical and thermal responses of six dried coals on exposure to a flow of oxygen or air were studied using isothermal differential thermal analysis (DTA) and temperature-programmed in situ diffuse reflectance infrared Fourier Transform spectroscopy (DRIFTS). A range of samples varying from lignites to bituminous coals and reaction temperatures from 30 to 180 °C were used. The major DTA response was an immediate sharp exotherm which for each coal increased with preset oxidation temperature and was always smallest for the coal least prone to spontaneous combustion. These exotherms may prove useful as indicators of spontaneous combustion propensity. The first product signal detected by DRIFTS was assignable to the carbonyl-stretch frequency of a carboxylic acid/aldehyde functionality. At higher temperatures the rate of increase of this signal correlated closely with the thermal response profiles suggesting that, regardless of coal rank, the reactions responsible for coal heating across the 30–180 °C range involve the formation and breakdown of the peroxy-precursors of these carbonyl-containing products and that susceptibility to spontaneous combustion is strongly influenced by the uncontrollable factors that govern such reactions. Oxidations were also carried out under a static blanket of O 2/argon at each temperature and the gas mixture analysed after five hours. At the lower temperatures no changes were seen. At 90 °C carbon oxides were first detected and their yields increased with temperature. An oxidation mechanism is proposed.

Comparison of aromaticity and phenolic content as parameters for characterization of coal by 13C n.m.r. spectroscopy

Cross-polarization and magic-angle spinning techniques were used to obtain 13C n.m.r. spectra of 27 New Zealand coals sampled from drill cuttings at depths between 1.4 and 4.0 km in the Tara-1 oil exploration well, offshore south of New Zealand. Aromaticity and phenolic content were estimated from each spectrum. Of these two parameters, the phenolic content showed a stronger correlation with depth. The correlation was improved further by expressing the phenolic content as a fraction of the aromatic content. It was concluded that a combination of these parameters provides a new method for obtaining information about differences both in depositional environments and in the extent of subsequent coalification for suites of coal samples.

Prediction of ash fusion temperature from ash composition for some New Zealand coals

Three improved techniques are presented for predicting the fusibility temperatures of coal ash from ash composition for particular coalfields. The first uses ternary equilibrium phase diagrams where the vertices are percent base, percent fluxing acidic oxides and percent non-fluxing acidic oxides. For reducing conditions FeO is a base, but for oxidizing conditions Fe 2O 3 is a non-fluxing acid. Plots of ash fusibility results on these diagrams show the range of behaviour of a particular coal and where low temperatures occur. They can be used to predict behaviour of fresh samples. The second method uses multiple regression equations above and below the minimum in the ash fusion temperature-percent base plot (usually 38% base), using as predictors, percent base ( B), B 2, and acid flux and base flux factors F A , F B (or their logarithms). The third technique, the most accurate, uses stepwise regression, again on split observations above and below the minimum in the ash fusion temperature-percent base plot. With 40 parameters it is possible to identify the significant ash fusion parameters present for each ash fusion measurement in each coalfield. Two sets of observations which included measurements of borate (B 2O 3) showed that it caused a significant but negligible reduction in ash fusion temperature.

Fate of sulfur compounds in coal during oxidative dissolution in perchloric acid

The sulfur forms in coal (sulfate, pyrite and organic) have been clearly delineated by oxidation with perchloric acid. Perchloric acid was chosen as an oxidizing agent because its oxidizing power can be varied by changing its concentration and subsequent boiling point. Using this property, the stepwise oxidation of an Illinois No. 6 coal, a coal-derived pyrite, and a New Zealand coal (in which almost all of the sufur was organic) was effected. At low temperatures, sulfate was removed without oxidizing the pyrite or organic material. At a moderate temperature (155°C), the pyrite was oxidized to sulfate and a sulfur-containing gas. Little or no organic sulfur was oxidized at 155°C. Boiling concentrated perchloric acid (203°C) converted all the sulfur to sulfate and a sulfur-containing gas. The sulfur-containing gas was produced primarily from the pyrite; organic sulfur produced very small amounts of a sulfur-containing gas as an oxidation product.

The apparent relative density of New Zealand coals

Abstract Results of measurements of apparent relative density are presented for two sets of samples of New Zealand coals. The first set, from recent borecores, enables a relationship between density and ash to be established for 12 coalfields. The second set are from most currently producing New Zealand mines. Calculations of moist and dry coal density and of moist and dry mineral density are made from these figures. Dry coal density displays a linear relationship with coal rank (as measured by moist mineral free specific energy) over the range of coals studied. The results provide a means of more accurate calculation of coal quantities and a guide to bene-ficiation by gravity methods.

Comparison of Australasian tertiary coals based on resolution-enhanced solid-state 13C n.m.r. spectra

13C solid-state nuclear magnetic resonance spectroscopy was used to characterize 32 low-rank coals from New Zealand and Austrlia. A combination of high magnetic field (4.7 T) and resolution ennhancement was used to extract spectral details beyond those seen in published spectra of coals of similar rank. Signal heights were used to characterize organic functional distributions. The spectra showed close similarities between Australian brown coals and low-rank New Zealand subbituminous coals, particularly those mined in the North Island. The spectra of New Zealand lignites all showed stronger signals from cellulose, methoxyl groups and phenols. Almost all of the New Zealand coals showed a relatively strong signal from polymethylene chains, compared with the Australian brown coals. This led to a prediction of higher alkene yields from pyrolysis of the New Zealand coals. Variations in phenolic substitution patterns were attributed to variations in the relative proportions of tannins and lignins in the depositional environments.

Assessment of quality of New Zealand coals from borecore analyses

Methods are needed for predicting the properties of as-mined coals from analyses carried out on borecores. Statistical techniques are described for isolating the mean moisture, coal and mineral properties from sets of laboratory analyses carried out on borecores. Comparisons between mine and borecore samples from the same area can be used to derive general methods for predicting properties of mined coal. This Paper deals with the derivation of moisture, ash, voltaile matter, specific energy and elementary composition of coal and minerals for mined coal from borecore analysis results.

Capillary viscometry of a New Zealand coal at high pressures and shear stresses

The practicality of using a capillary viscometer for measuring the flow properties of coal is established. Sources of error, particularly viscous heating effects and thermal lags, are discussed and precautions are outlined. Foaming is thought to have been prevented over much of the capillary. Some results obtained with a high vitrinite coal are presented for temperatures 679–683 K, and pressures up to 77 MPa. The coal viscosity is highly temperature dependent, and also apparently dependent on shear rate.

Radiometric assessment of abrasiveness of coal in pulverizers

A laboratory method is described for the measurement under water, of the abrasiveness of a coal as it affects wear in pulverizers. The coal is ground by a rotating blade containing radioactive material. The determination of the level of radioactivity on the ground material provides an indication of the amount of material abraded from the blade. A coal which caused excessive wear in commercial grinding in a pin-disc mill gave an extremely high ‘abrasive index’ when subjected to this laboratory test. Abrasive indices have been determined for a series of New Zealand coals. High values correlate with increased ash level, with a high silica-to-alumina ratio in the ash and especially with a high free-silica content of the coal.

Coal as a source of sulphur for perennial ryegrass

Abstract Three New Zealand coals, ground to pass a 150 µm sieve, were evaluated as a source of -ulphur for ryegrass in a factorial pot experiment on two sulphur-responsive soils. All coals examined supplied useful amounts of sulphur to ryegrass. Sulphur uptake over a 90-day period. by ryegrass supplied with Webb coal, was similar to uptake from gypsum at 40 ppm S, and higher than uptake by ryegrass supplied with Pirongia or Waimumu coals. The Pirongia coal, in common with other Waikato coals, contains relatively high levels of boron which, in This study, caused boron toxicity in ryegrass.