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A Review of Current Coal Research in New Zealand
NA Moore and TA Moore
CRL Energy Research and Testing, Christchurch
…Introduction
…Geological Investigations
…Exploration
…Climate
…Trace Elements
…Demethanation
…Artificial Maturation
…VFR
…Mining Activities
…Rockbursts
…Trace Element Pathways
…Coal Gasification
…Spontaneous Combustion
…Coal Quality Monitoring
…Summary
…References
Coal is a widely researched aspect of New Zealand's geology, for both industrial and academic applications. Ongoing research is being carried out at universities, Crown Research Institutes (CRIs) and by private research organisations and individuals, on such diverse topics as plant evolution, mining methods, combustion technology and the geological history of New Zealand.
Much of the academic research into coal is focused on the fundamental mechanisms of coal formation, specifically how coal is deposited and buried, and its relationship to surrounding rock types. While such information might not seem immediately relevant to coal producers, it can provide valuable information on properties such as a coal's rank range, volatility, swelling ability, sulphur and ash contents, oil and gas generating potential, and the likely distribution of additional coal deposits.
Many New Zealand coals are remarkable for their extremely low ash contents. New Zealand coal is sold internationally, both as a source of coke for iron smelting and as an energy source (thermal coal). Most New Zealand exported coals are not used on their own, but are instead mixed with coal from other countries to provide tailored coal blends with specific properties for individual clients' needs. Finding new uses and markets for New Zealand coals is an important area of industrial research.
New Zealand's coal, oil and gas resources are of vital importance to the economy. Researchers and exploration geologists have been studying the evolution of many of New Zealand's sedimentary basins, the origin of coal-bearing deposits, and in some cases the formation, migration and entrapment of petroleum reserves. Known reserves of both coal and petroleum are being depleted, and ongoing research is necessary to ensure New Zealand's security of supply and global competitiveness.
The opening of the Tasman Sea approximately 80 million years ago caused the formation of a segmented system of fault-bounded rift basins, of which the Taranaki, Paparoa and Ohai basins were the largest. The faults are important because they can effectively keep dirt out of the paleomires (i.e. coal). Thus, coals that are thick, low ash and low sulphur can be formed, as seen in the Paparoa and Ohai basins. Thick coals were also deposited in the Taranaki Basin, much of which presently lies offshore. The coals are believed to be a major source of the regions oil and gas deposits.
Two areas where geological principles can be used to find good quality coal are in the Ohai and Greymouth regions. Both these areas have been mined for over a century (Barry et al 1994), but much of the in-ground resource remains. More recently, geological investigations have helped to define deposits and understand their origin.
For example, a recent study of the Morley Coal Measures of the Ohai Coalfield (Shearer 1992, 1995) demonstrated that the individual coal seams within the Ohai Basin resulted from episodic tectonism. Active periods caused uplift and erosion of basement, with resultant deposition of clastic sediments within the basin. Quiescent periods allowed the formation of peat mires relatively unaffected by clastic sedimentation.
The sedimentological history of the Paparoa Basin, examined by Ward (1997), is more complex than that of the Ohai Basin, due to faulting within the basin during deposition. Older sediments are therefore more structurally disturbed and complex than younger, overlying deposits.
The sediments of the Paparoa basin were derived from basement metasediments and granites, with the two sources contributing from opposite sides of the basin. As well as the sediment entry points, the river courses were fault-constrained, allowing mires to exist for long periods without sedimentary debris intruding (Ferm & Moore 1997; Moore in press, Sedimentology). The resulting coals tend to contain low-ash levels, and, as they tend to be overlain by lacustrine (i.e. lake) sediments rather than marine rocks, they do not contain elevated levels of sulphur.
Research has been undertaken to identify variations in the characteristics and petroleum source potential of coals resulting from various floral and climatic influences. Climate affects not only which plants grow in a peat-forming mire, but also how fast they grow and how well they will be preserved. This has an impact on the resultant coal and thus to the oil and mining industries.
Multidisciplinary analysis of an ideal sample set with diverse floral origins has demonstrated that coals with different floral origins generate different amounts and types of oils and gases, and require different temperatures to commence generation. This research represents significant progress toward enhanced evaluation, more efficient utilisation and a more robust understanding of the factors determining petroleum source potential of the nation's largest fossil fuel reserve (Newman 1989, 1997).
Climatic influences in depositional basins have also been examined by researchers from the University of Canterbury (e.g. Ward et al 1995, Kennedy 1993). These studies have shown that the climate at the end of the Cretaceous Period, when the Paparoa Coal Measures of the West Coast were deposited, were cool and dry. It is believed that the Paparoa Coal Measures were deposited at approximately 75° south, resulting in several months of darkness in winter, followed by short, well-lit growing seasons. These conditions resulted in annual pauses in growth and peat deposition.
Although trace elements occur in coals in minute quantities, combustion can concentrate some trace elements in the ash residue, while releasing others into the atmosphere in gaseous form. Concentrations of vanadium and phosphorus in coking coal can lead to the contamination of smelted steel, causing brittleness. Vanadium is also known to be corrosive to steam turbines. The concentration of trace inorganics such as titanium can also allow their utilisation as a resource.
By world standards, New Zealand coals contain high levels of boron. It has been reported that up to 80% of boron in coal is discharged into the atmosphere in vapour form (Clemens et al 1997). The boron remaining in the ash is susceptible to leaching after burial, causing contamination of groundwater.
One method of preventing contamination of the environment or manufactured products is to mine selectively, identifying and then avoiding coals or in-seam horizons known to contain high levels of trace elements. A study by Shearer et al (1997), for example, used three-dimensional models to study the distributions of titanium, vanadium and fluorine within the Eocene Kupakupa seam of the Waikato Coalfield. Significant areas of enrichment were identified, and these were then compared with similar trends found in the Kopuatai peat dome, a nearby mire in a geographic and geological setting thought to be similar to that of the Kupakupa coal seam. The Kopuatai mire contains several tephra horizons containing high levels of trace elements. It was deduced that the enrichment of the Kupakupa seam was the result of two ancient ash-fall layers.
As the trace elements in the Kupakupa seam are associated with the mineral matter fraction, it may be possible to wash the coal to remove the tephra layers, or to mine the seam selectively and avoid the enriched horizons.
Research into demethanation is important because methane in coal seams is both a mining hazard and a potential energy source. Large volumes of methane are produced by peats during coalification. Some of this gas is released into surrounding sediments, but a large proportion is retained within the coal in micropores, either adsorbed onto the coal or dissolved in water. Due to their porosity, coals make excellent gas reservoirs, in some cases holding three times as much gas as surrounding sandstones.
Coal bed methane is commonly considered a mining hazard, requiring extensive ventilation, and often drilling ahead of the working face to release pockets of pressurised gas. However, in the last few years, the commercial extraction of methane from coal seams and coal measures has been under commercial investigation.
The southern sector of the Greymouth Coalfield has historically been recognised as a source of methane, with reports of both gaseous and liquid hydrocarbons from mines and drillholes. Several companies have investigated the possibilities of methane extraction from deep, unmineable seams in the southern sector of the coalfield.
After the Second World War, Superior Oil drilled to the south of Greymouth, to examine the down dip potential of the Brunner and Paparoa coal measures. Minor oil flows were recorded. Since then New Zealand Petroleum, Shell BP Todd, Offshore Mining Ltd, Petrocorp, Petroleum Resources Ltd, New Zealand Oil and Gas and Westgas Resources have carried out drilling programs and seismic and gravity surveys.
Recent research by Cave and Newman (1995) has examined the coal rank gradients across the southern Greymouth Coalfield. The coal measures have been subject to a high geothermal gradient, with the rank of the Paparoa coals increasing across the basin to the east with increasing depth of burial (Suggate 1998). This high geothermal gradient has resulted in a narrow oil generation window.
There is an unconformity between the Cretaceous Paparoa Coal Measures and the overlying Tertiary Brunner Coal Measures. Cave and Newman have suggested that this has led to offset oil generation windows between the two units, complicating past petroleum exploration efforts. The stratigraphy and geothermal history of the two units require further examination to accurately determine the coalfield's future methane production potential.
Petroleum source potential has also been examined using artificial maturation techniques, whereby samples of coal types with varying oil-bearing characters are artificially matured under pressure and temperature conditions closely akin to those experienced in nature. A lignite, for example, would gradually increase in rank to a sub-bituminous or bituminous coal as it was buried more deeply. This process can be simulated in the laboratory.
Different burial temperatures control the yields of various liquid and gaseous hydrocarbons. It is hoped that the potential yields and forms of hydrocarbons generated by coal in specific geological settings can be accurately predicted.
Vitrinite Reflectance and Fluorescence (VRF™) is a technique developed by CRL researchers at the University of Canterbury. Although developed originally to characterise coals (Newman 1997), it has since proven equally valuable for analysing dispersed organic matter in sediment samples. The technique provides a more accurate evaluation of burial temperatures than conventional rank indicators. In the case of hydrocarbons originating in marine source rocks, dispersed plant fragments can provide information on burial history, the major control in the process of petroleum generation.
VRF™ is more reliable than traditional reflectance analysis, which can give unreliable maturity data due to loss of hydrogen-rich volatile matter.
Mining has historically been conducted with less regard for environmental safety impacts than efficiency of extraction. While modern mining is required to produce few environmental contaminants and for open-casted areas to be rehabilitated, some historic mine sites require monitoring. The University of Otago has been researching arsenic and mercury leachate production and dispersal rates from tailings dams and piles in Otago, Coromandel and Northland, and acidity and toxic metal discharge from coal mine tailings at Kaitangata and Reefton. This research will benefit the industry in New Zealand by allowing accurate pre-mining predictions of possible leachate minerals and elements, based on the geochemistry of the surrounding rocks.
Coalification of peats commonly produces gaseous hydrocarbons such as methane. These gases pose dangers for underground mining operations. When sufficiently concentrated, methane is explosive, and good ventilation is essential in underground mines to prevent flammable buildups. In some instances, pockets of gas under high pressure may be released violently, as outbursts.
In an attempt to predict where and when outbursts may occur, Beamish et al (1996, 1998) studied the methane adsorption mechanisms and capacities of coal seams in the Greymouth Coalfield. It was found that the methane sorption capacity decreased as rank increased, with a minimum occurring in medium volatile bituminous coals. Sorption capacity was found to decrease with increasing moisture content, as water and methane compete for sorption sites on the internal surfaces of coal pores. Coals with discrete vitrain bands had higher sorption capacities than non-banded coals.
New Zealand coals were found to have lower sorption capacities than similarly ranked Australian coals, due to higher amounts of volatile compounds in New Zealand coals and a resultant lower microporosity. It is hoped that these trends will prove useful in determining possible hazards associated with mining specific coal deposits.
Utilisation
To minimise the loss of trace elements to the atmosphere during combustion, research has been undertaken to understand the complex ash chemistry reactions occurring in the firebox and elsewhere within the combustor (Clemens et al 1997, 1998). These reactions influence the partitioning behaviour of volatile trace elements and sulphur gases.
The researchers concluded that the presence of calcites in Waikato sub-bituminous coals led to the formation of glassy calcium silicates in the ash, which retained significant quantities of most trace elements which would otherwise have been discharged into the atmosphere in the vapour phase. This was taken to indicate that the release of most toxic elements might be significantly less than expected on the basis of their trace element contents. Mercury was an exception to this, with most still being discharged as vapour, and requiring a separate scrubbing system.
One of the analytical tools used by Clemens et al 1997, proton induced x-ray emission (PIXE) is gradually being accepted internationally into the arsenal of methodologies available for trace element determination in coal. In the above study it was found to deliver reliable, useful concentration data on up to 40 trace elements found within the coal and peat samples.
Coal gasification may be undertaken in two different ways, by burning coal deposits in situ and then tapping the released gases, or in sealed, pressurised reaction vessels. The chemical processes of the two techniques are similar, in that coal is burned in a reducing atmosphere, and coal gas (a mixture of CO and H2) is generated. The coal gas is then burned in a combustor.
Underground coal gasification may be used where a coal seam is unmineable, due to depth or structural complications. Old underground workings using room and pillar extraction often leave the majority of a seam in place, and the gasification technique can be used here to exploit the remaining coal.
Plant coal gasification is receiving new attention worldwide in conjunction with new advanced clean coal technologies. New technologies have the potential to produce less CO2 emissions per unit of power produced. Coal gasification involves three stages:
1. pyrolysis of finely ground coal to produce coal gas and char;
2. combustion of the coal gas, while the char undergoes partial combustion with oxygen; and
3. gasification of the remaining char with CO2 and steam.
The coal gas produced by gasification is sent to a gas turbine, where it is burnt with compressed air, the products of which drive a turbine coupled to an electric generator. The research highlights areas where the conventional understanding of chemical factors underpinning gasification is incomplete. This implies that some New Zealand coals, which conventional wisdom has suggested would not be sufficiently reactive to undergo gasification, are in fact well suited (Clemens et al 1997).
A study comparing combustion, gasification and pulverised coal injection (PCI) chars by Bailey et al (1997) examined the combustion efficiency of the different techniques with different coal ranks. It was found that coals with highest gasification volatile yields were obtained from coals with a high vitrinite:inertinite ratio, which yielded a porous char. Volatile matter alone did not prove a reliable indicator, as some high-volatile coals with high-reflectance inertinite formed dense, non-porous chars which inhibited the release of the coal gas. The authors concluded that given the variety of conditions coal seams were deposited under, it was unlikely that any one indicator could give an accurate indication of a coal's behaviour under gasification.
Spontaneous combustion is responsible for fires in stockpiles and, in the worst case, fires and explosions in mines. It is a result of chemical weathering, and occurs when air is introduced to large quantities of coal, allowing it to oxidise exothermically.
When heat produced exceeds the rate of heat loss from the deposit or stockpile, the coal temperature increases. The oxidation reaction rate doubles with every 10°C increase (Speight 1983), and the resultant runaway heating often leads to combustion.
The oxidation of pyrite to iron sulphate and sulphuric acid is highly exothermic, producing two-thirds as much heat as the combustion of carbon in coal.
There are several factors influencing a coal's susceptibility to spontaneous combustion. A small grain size means that oxygen has access to a greater surface area, thus allowing more rapid oxidation. Fractured coal seams, as found in many New Zealand coalfields, allow more oxygen into the seam than a massive, uncleated deposit. In addition, high vitrinite coals tend to be more susceptible to oxidation than low vitrinite coals (Stach 1982). An important influence on spontaneous combustion in New Zealand coals is the presence of finely disseminated pyrite.
Coal quality varies throughout a coal seam, and the properties of coal from any one mine may change as different plies of sections of the mine are worked. To ensure consistent coal quality, frequent run-of-mine sampling is required, with different grades of coal being put in separate stockpiles.
While such sampling is necessary, the turnaround time on samples is at best several hours. Technology to produce an advanced dual-beam radiation scanner to determine bulk chemical compositional analyses of coal is now being developed within New Zealand by the Institute of Geological and Nuclear Sciences. Advantages of such methods include real-time coal quality analyses as coal is being carried on a conveyor, either out of the mine or while being loaded aboard ships.
The future of New Zealand's coal industry appears positive. While New Zealand's deposits are smaller than many overseas, the coal's quality ensures that markets will exist for both thermal and coking coals. Likewise, continuing research into the deposits, properties and uses of New Zealand coals aims to aid the discovery, extraction and utilisation of coal deposits. Research that focuses on the unique attributes of New Zealand's coals will aid its saleability into distinct, niche markets worldwide.
Bailey, J., Benfell, K., de Wit, M. 1997. Comparison of combustion, gasification and pulverised coal injection chars. Proceedings of the 7th New Zealand Coal Conference, 1997, 90-101.
Barry, J.M., Duff, S.W., MacFarlan, D.A.B. 1994. Coal resources of New Zealand. Ministry of Commerce Resource Information Report 16.
Beamish, B.B., Crosdale, P.J., Moore, T.A. 1996. Methane sorption capacity of Upper Cretaceous coals from the Greymouth Coalfield, New Zealand. Proceedings of the Mesozoic Geology of the Eastern Australia Plate Conference, 23-26 September 1996, Brisbane Australia, Extended Abstracts No.43, 45-51.
Beamish, B.B., Laxminarayana, C., Crosdale, P.J. 1998. Contrasts in methane sorption properties between New Zealand and Australian coals. Proceedings of the Coal 98 Conference, Wollongong, 561-565.
Berkowitz, N. 1979. An introduction to coal technology. Academic, New York.
Cave, M., Newman, J. 1995. Vitrinite reflectance variations between Brunner and Paparoa coal measure sequences in the Greymouth Coalfield and the implications for petroleum source rock demethanation; some initial results. Proceedings of the 6th New Zealand Coal Conference.
Clemens, A.H., Damiano, L.F., Matheson, T.W. 1997. Behaviour of New Zealand coals under gasification and combustion conditions relating to existing and new energy technologies. Proceedings of the 7th New Zealand Coal Conference.
Clemens, A.H., Deely, J.M., Gong, D., Moore, T.A., Shearer, J.C. 1998. Trace elements in coal: How they get in (during deposition) determines how they come out (during combustion). AIE 8th Australian Coal Science Conference, Sydney, pp.327-332.
Ferm, J.C., Moore, T.A. 1997. Guide to cored rocks in the Greymouth Coalfield (trial version). CRL report 97-11189.
Kennedy, E.M. 1993. Palaeoenvironment of an Haumurian plant fossil locality within the Pakawau group, Northwest Nelson, New Zealand. MSc thesis, University of Canterbury.
Newman, J. 1989. Why are some high rank Tertiary coals more peculiar than others? Some thoughts on climate and floral assemble. 3rd New Zealand Coal Research Association Conference. Coal Research Association of New Zealand, Wellington, New Zealand, pp.182-196.
Newman, J. 1997. VRF™: Combined vitrinite reflectance and fluorescence. Proceedings of the 7th New Zealand Coal Conference.
Shearer, J.C. 1992. Sedimentology, coal chemistry and petrography of the Cretaceous Morley coal measures and the Eocene Beaumont coal measures, Ohai Coalfield, South Island, New Zealand. PhD thesis, University of Canterbury.
Shearer, J.C. 1995. Tectonic controls on styles of sediment accumulation in the Late Cretaceous Morley Coal Measures of Ohai Coalfield, New Zealand. Cretaceous research 16, 367-384.
Shearer, J.C., Moore, T.A., Vickridge, I.C., Deely, J.M. 1997. Tephra as a control on trace element distribution in Waikato coals: Proceedings of the 7th New Zealand Coal Conference, v2, 505-520.
Speight, J.G. 1983. The chemistry and technology of coal. Marcel Dekker Inc.
Stach, E. 1982. Stach's textbook of coal petrology, Gebrüder Borntraeger, Berlin.
Suggate, R.P. 1998. Relations between depth of burial, vitrinite reflectance and geothermal gradient. Journal of Petroleum Geology 21(1), January 1998, 5-32.
Ward, S.D., Moore, T.A., Newman, J. 1995. Floral assemblage of the "D" coal seam (Cretaceous): Implications for banding characteristics in New Zealand coal seams. New Zealand Journal of Geology and Geophysics 38: 283-297.
Ward, S.D. 1997. Lithostratigraphy, palynostratigraphy and basin analysis of the late Cretaceous to early Tertiary Paparoa Group, Greymouth Coalfield, New Zealand. PhD thesis, University of Canterbury.
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