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Characteristics in the Global Scene
GR Gillard and TA Moore
CRL Energy Research & Testing, Christchurch
…Introduction
…Coal Rank and Type
…Age of Coals
…Carboniferous
…Permian
…Cretaceous and Tertiary Coals
…New Zealand Coals
…New Zealand Coals Compared with Other Coals
…Lignite
…Subbituminous
…Bituminous
…Anthracite
…Conclusion
…References
Although New Zealand mines only a small amount of coal annually (approximately 3 million tonnes), it competes with other countries for global markets. To be internationally competitive coal mining in New Zealand has to be not only cost effective but also supply a superior product to existing and potential new markets, utilising the unique nature of New Zealand coals.
The properties unique to New Zealand coals can be highlighted by comparing them to other coal types from throughout the world. This allows a perspective upon which New Zealand coals can be ranked for specific users overseas, maximising the return for the producer.
For such a small landmass, New Zealand has a large variety of coal resources. Ranks of coal range from low rank lignite through to high rank anthracite. The geological age of these coals spans Late Cretaceous (75 million years ago) to Miocene (~30 million years ago). In this report, we will examine coal from three perspectives:
1. controls on coal rank and type,
2. characteristics of coal from different geological ages, and
3. specific attributes of coals of different ranks.
The rank of a coal is the degree or stage the coal has reached during its coalification; that is, the degree of metamorphism or geochemical maturity. Coalification represents the natural chemical and physical transformation of peat into coal, firstly by biochemical processes and subsequently by geochemical processes, which are mainly controlled by increases in temperature, pressure and time that accompany the progressive burial of coal by sediments within a subsiding basin. The coalification process can be accelerated by exposure of the coal seams to elevated temperatures, such as localised igneous intrusions. Although temperature is very important in the coalification process, most coals even of coking grade have been exposed to temperatures of less than 200°C. Both pressure and length of burial are important in coal maturation.
Rank and coal type are highly influenced by the age, original plant material and subsequent tectonics of a coal deposit. However, post-depositional igneous activity and/or higher geothermal gradients can have a great impact on coal development. For example Tertiary coals found in Indonesia have higher rank than would normally be expected as a result merely of the passing of geological time. Similarly, South African coals exhibit a wide range of rank across the Karoo basin, with those of highest rank having been effected by intrusive activity.
Coalification therefore only refers to the progressive transformation of peat through lignite, subbituminous, bituminous and anthracite coals. Anthracite, the highest rank coal rank, is typified by very low volatile and inherent moisture contents. Bituminous coals with higher volatile content often have a good caking propensity; that is, they have certain characteristics which make them suitable for making coke. High volatile bituminous coals, on the other hand, may or may not have caking properties and have higher inherent moisture. The term 'low rank' coal is usually used to describe the subbituminous coal and lignites. Low rank coals are typified by high inherent moisture contents, high volatile contents and low carbon contents.
Historically, coals of the world have been broadly subdivided into four distinct age categories: Carboniferous (northern hemisphere), Permian (generally southern hemisphere), Cretaceous and Tertiary. It should be noted that coal has been formed in every geological age since land plants evolved in the Devonian, but that the ages given above represent the optimal times for ancient peat accumulation. It is also recognised that coals from these different ages possess different attributes merely as a result of differing source material (Shearer et al 1995). However, as a generality, older coals tend to be higher in rank and younger coals are lower in rank. It does have to be stressed that secondary processes such as tectonics can override any generality.
The Carboniferous coals tend to be composed of variable quantities of vitrinite. The equatorial climate associated with this period promoted luxuriant growth of seed ferns and lycopods that produced thick walled spores and pollens, characteristic of exinite (liptinite) macerals. Carboniferous coals tend to have relatively high sulphur contents with low ash and high rank, although there are many individual exceptions. Most Carboniferous coals were deposited in foredeep basins which underwent rapid subsidence allowing the development of many thin seams, in stratigraphic sequence. These were often later subjected to tectonic disturbance in the form of folds, faults and igneous intrusions. Examples of Carboniferous coal deposits are the Appalachian coalfields in the eastern USA, East Penine coalfield in the United Kingdom and the Upper Silesian Basin in Czech Republic and Poland.
Permian coals (Gondwana) were deposited on stable cratonic shelf areas, sometimes forming thick seams (5–15 m) that grade into carbonaceous shale. They originated from Glossopteris and Gangamopteris flora that grew in a cool temperate climate, where drier conditions often promoted biochemical oxidation of the peat material resulting in coals rich in inertinite macerals.
Coals of Permian age probably have more inertinite for two reasons: low accumulation rates for the peat mire (i.e. pre-coal) and higher levels of plant decay. Perhaps as a result of the type of tectonic environment (i.e. stable, cratonic) during the Permian, slow rates of coal accumulation allowed fire to create relatively more inertinite and be volumetrically more important. In addition, the slow accumulation rates may have allowed greater levels of degradation of the plant material by fungi and other aerobic micro-organisms that convert plant material to charcoal (Shearer and Moore 1996; Moore et al 1996).
The vitrinite content is variable and the coals also tend to have high mineral matter content (therefore high ash content) in the form of finely dispersed grains of clay and quartz. This form of mineral matter is thought to have been blown or washed in from surrounding glacially eroded landscapes. Other sources may have included volcanicity contemporaneous with swamp development, increased amounts of river sediments being deposited in the swamps, and enhanced porosity of the peats which provided more space for the precipitation of minerals from permeating water. However, Permian coals laid down in foredeep basins are similar to those of Carboniferous age. Many Permian coals were, however, deposited in geologically stable inland cratonic basins. These tend to have lower sulphur contents, higher ash and lower ranks. Seams tend to be thicker and more extensive where subsidence was slow but continued for a longer period. The rank and degree of disturbance within cratonic basins is not as high as that associated with fore deep basins as the coals were subjected to a lesser degree of subsidence and therefore lay at shallower depths. The basins were also at some distance from intense orogenic effects. Permian coals of this nature include the deposits of South Africa in the Karoo Basin, Australia in the Bowen and Sydney basins and India in the Ranigani/Jharia Basin.
Cretaceous and Tertiary Coals…
In Cretaceous to Tertiary times, a combination of cratonic and foreland basins also existed and exhibit the same characteristics as those of previous times. Plants were rapidly evolving, changing from plants bearing thick-walled pollen and spores to angiosperm floras having smaller, thinner-walled, more diverse types. Their ability to grow under a greater variety of conditions thus resulted in a greater range in ultimate coal type with more complicated chemical characteristics due to the varied plant composition and paleoenvironment. Examples of these deposits include the Powder River Basin in the western USA, the coal basins in Indonesia, and all New Zealand coal deposits (Walker 1993).
The coals found in New Zealand are widely spread from the Far North to Southland (Fig. 1). In geological age the deposits span the Late Cretaceous though to Miocene. The bulk of New Zealand's coal resources are the Southland and Central Otago lignite deposits (Figs 2 & 3).
Since New Zealand is situated in a tectonically active structural setting between the Australian and Pacific plates, it is not surprising that the structure of the coalfields and rank of the coals differs from that of many major coals in other parts of the world. Many of the large well documented Australian and South African coal deposits of Permian times were deposited and buried slowly over long periods of time before being uplifted, eroded and exposed for exploitation. This often resulted in coalfields with uniform rank characteristics over broad areas, and because of the stable cratonic structural setting, an absence of complicated folding or faulting structures. Thus, exploitation of these resources by coal mining companies has proved a reasonably straightforward task.
However, taking into account New Zealand's complex and active structural setting it is not surprising to find New Zealand coalfields vary from the Permian example. Faulting is a major complication for miners in virtually all New Zealand coalfields and folding and shearing is particularly prevalent in the West Coast deposits near to the Alpine fault. Coal rank is also variable due to differing rates of burial and uplift throughout New Zealand. For example the Late Cretaceous coals of New Zealand (Fig. 1) (Pakawau, Paparoa, Broken River, Papakaio and Morley Coal Measures) are all of similar age but vary in rank significantly. The Papakaio and Taratu Formation coals of coastal Otago are predominantly lignites, as are the Broken River Formation coals (with the exception of some anthracites resulting from igneous intrusives); the Morley Coal Measures of Ohai and the Pakawau Group of Collingwood are subbituminous; and the Paparoa Coal Measures of the West Coast range from bituminous to semi-anthracite.
Therefore, when faced with the combined complications of unfavourable structure and coal quality variability due to rank variation within a deposit, it is hardly surprising to note the absence of large scale opencast and longwall mining operations as is found in the Australian and South African coal industries. The complex structure renders large scale mechanised mining techniques not viable. Local variability in coal quality can also be related to proximity to major structures, as is the case in many West Coast mines. Here, a common problem, particularly with coking coals, is the impact that faults have on various coal quality parameters due to crushing and weathering. Such examples of in-situ quality variability calls for micro-management of production faces to monitor coal quality to meet market specifications on a daily basis. This kind of 'hands on' quality management is a feature of New Zealand mines which is not necessarily an essential requirement for many of the uniform quality, large scale mining operations overseas.
To see how New Zealand coals compare with those from around the globe, coal types will be looked at individually within rank categories of peat, lignite, subbituminous, bituminous and anthracite. We will look at coals in New Zealand and around the world and for each coal rank the geological age, structural setting, coal quality parameters and coal usage will be compared.
Lignites often, but not always, represent the youngest coals. Different terminology can be used in different countries and lignites are sometimes referred to as brown coals. There are no standard universally accepted definition of lignites and this can create confusion when comparing lignites from one country to another.
Lignites generally have a moisture content between 35-70%; a low heat value from about 6-16 MJ/kg and a high oxygen content compared with bituminous coals. Lignites also tend to have a higher volatile matter content and lower fixed carbon content than bituminous coals. The ash content can be highly variable from a few percent to as much as 50%. Substantial quantities of lignite occur near surface throughout the world and it provides a low cost fuel, predominantly from opencast ines supplying minemouth power stations. Little lignite is traded on international or domestic markets because of its low energy value and high transport cost per unit energy (Couch 1988).
Unlike hard black coals, large scale use of lignite is a twentieth century phenomenon. During the first half of the century production was concentrated in Europe, mainly in Germany, and in Australia. During the second half of the century European use has increased and the CIS, USA, China and India have become significant users.
There are substantial unworked lignite reserves in New Zealand, Portugal and Northern Ireland but the largest known deposits are in China, the CIS, USA, Germany and Australia. The annual world production of lignite was 894 Mt in 1996. This compares with New Zealand's production in the same year of 198 Kt (EIA 1998). Figure 4 plots the top 20 lignite producers by country in 1996.
One notable characteristic of lignites the world over is the marked variability in quality. Even within a single deposit variation of ash, moisture, volatile matter and sulphur is generally much greater than is normally observed in hard black coal deposits. Some of this variability can be attributed to the generally young age of the lignite deposits. As most lignites are tertiary in age they were formed during periods when plant life had diversified to a greater degree than when the Carboniferous and Permian coals were deposited. Thus, the initial peat bog, later coalified to lignite rank, was itself already a non-homogenous collection of a wide range of plant species. So how do New Zealand lignites compare with lignites world wide? To investigate this New Zealand lignites can be compared with those found in major deposits in Australia, USA, Germany and Thailand. Comparisons of depositional basin settings, geological age, and coal quality parameters are made as well as comments on the end use of lignites. Table 1 lists the various countries with details on the nature of the local lignites.
The New Zealand lignites are of similar age to the Australian, German and Thai lignites but the US lignites are a little older. Structurally, New Zealand lignites are 'middle of the road' being both gently dipping and forming multiple seams of no vast thickness. This compares with the Australian, USA and German lignites which exhibit phenomenal thickness and are flat-lying near-surface often as only one major seam.
Coal quality is plotted for each lignite region in Figure 5. New Zealand lignite compares favourably with the world contenders. Ash levels in New Zealand lignites are reasonably low by world standards and also show only minor variation. In volatile matter and fixed carbon New Zealand coals are fairly similar to most of the world's lignites, however, the fixed carbon levels show a smaller range of variability than the volatile matter. Calorific values are reasonably standard and do not vary much. In sulphur levels, New Zealand lignites are very low, on a par with the best of the Victorian lignites, the Fort Union lignites and the Rhenish area lignites.
In summary, New Zealand lignites exhibit favourably low ash and sulphur in comparison with world lignites.
For New Zealand's energy future, the Southland and Central Otago lignites are a great energy source waiting in the wings. Potential exists for development of these resources for power generation, briquette making for domestic burning, and gasification to create Synthetic Natural Gas (SNG).
Subbituminous coals represent the next stage in the coalification process up from lignites. The main feature of subbituminous coals compared with lignites is the significantly lower levels of moisture (10-35%). Subbituminous coals are generally black as opposed to brown in colour though some subbituminous C rank coals do exhibit some brown colouration. Production of subbituminous coals is not reported consistently in all countries, lower rank subbituminous coals often get lumped in with lignites and higher rank subbituminous coals are frequently reported with bituminous production figures. Therefore, no coal production figures are presented here.
World wide, the main use for subbituminous coals is for thermal power and steam generation. In New Zealand, the Waikato coals are also used as a feedstock for a direct reduction steel making plant. Because of the lower moisture content the heat value per unit weight of subbituminous coal is significantly higher than for lignite, therefore, in many cases it is economically viable to transport subbituminous coals to customers remote from the minesite. Handling of subbituminous coals can be a problem, however, as subbituminous coals, like lignites, are prone to spontaneous combustion, whereas bituminous coals are generally more stable for transport.
Most subbituminous coals deposits are relatively young being of Cretaceous to Tertiary age. Deposits in New Zealand, Indonesia and the USA all exhibit development of thick coal seams, compared with the Carboniferous deposits, some of which are amenable to large scale open pit mine development. Table 2 lists some coal quality parameters from selected subbituminous coal deposits.
From the New Zealand perspective, the quality of local subbituminous coals compares favourably with the foreign examples (Fig. 6). Ash levels in New Zealand subbituminous coals are low compared with the wide range of values from Australia, the Powder River Basin (USA) and Indonesia. Volatile matter and fixed carbon levels of New Zealand subbituminous coals are within the common range, though the Ohai Morley coals is quite high in fixed carbon. Calorific values of Waikato and Ohai subbituminous coals are reasonably high or on a par with the Powder River and Indonesian coals. However, sulphur levels of New Zealand's subbituminous coals are consistently low, even compared with those of Indonesia. So overall, New Zealand subbituminous coals are low in ash and sulphur compared with their overseas counterpart.
Bituminous coals occur as a later stage of development in the coalification process. Bituminous coals range from high volatile bituminous C rank through to low volatile, and the coal properties change significantly with the increase in rank. The issue of bituminous coal quality is in fact such a diverse subject that a short paper such as this could not do justice. So, from the New Zealand perspective we shall not compare all coal quality parameters in detail, but will briefly focus on what makes New Zealand's bituminous coals special in world markets.
The two main features which prove New Zealand coals to be unique in the world scene are extremely low ash levels, and the dominance of vitrinite macerals over inertinite and liptinite in the coal organic matrix.
Low ash is a characteristic of bituminous coals in the Buller and Greymouth coalfields on the West Coast of New Zealand. The Paparoa (Late Cretaceous) and Brunner (Eocene) coal measures both host coals which are structurally complex due to faulting, occur in thick seams up to 20 m thick and have extraordinarily low ash levels. For example, some sections of the Brunner coal measures coal are less than 1% ash for large parts of the seam. These low ash levels have enabled New Zealand coals to be marketed as unique premium products for thermal coal markets, or if coking properties are present to international coking coal markets. In both thermal and coking coal markets the low ash levels have led New Zealand coals to be used as high quality 'sweeteners' to blend out problems with other, cheaper, overseas products. A good example of the higher ash bituminous coals would be the Permian coals found in South Africa, India and Australia. In fact, one of the prime markets for New Zealand coal is India, where very high ash coal is blended with low ash Brunner coal to achieve a workable blend at the power station or steel mill. As shown in Figure 7, New Zealand's production of bituminous coal is small and will never challenge the major players, however, at current production rates New Zealand bituminous coals should still retain their niche in world markets.
Vitrinite levels are very high (generally >90%) in all of the Tertiary age coals in New Zealand. The reasons for this are unclear, but are believed to be the result of the original plant type. It has been noted elsewhere (Shearer et al., 1995) that Tertiary age coals world wide have abundant vitrinite as compared to other ages of coal. The predominantly angiosperm flora type (i.e. flowering plants) of the Tertiary, as opposed to previous geological eras, contributed much more vitrinite pre-cursors (Shearer and Moore 1994). The lack of inertinite, which often results from fires is also curious. There probably were no fewer fires in the Tertiary than other times, but because of the plant type and the amount and rate of plant material accumulating in Tertiary bogs, this resulted in volumetrically less inertinite.
The Tertiary coals in New Zealand also tend to be perhydrous (Newman and Newman 1982). Essentially this is the result of the coal generally being rich in hydrogen – especially the vitrinite component. This attribute, although not unique to New Zealand, lends the coal interesting properties upon utilisation giving the coal high fluidity whilst having virtually no coke strength.
Anthracite represents the highest rank of coal found in nature. The main characteristics of anthracite are the very low levels of moisture and volatile matter as a result of intense coalification. Thus anthracites have fixed carbon levels often above 90%. No deposits of anthracite occur in New Zealand in economic proportions. Small anthracites deposits do occur in Canterbury where Broken River Formation coals (Late Cretaceous) have been 'cooked' by local intrusives. Figure 8 presents world anthracite production figures for 1996.
New Zealand coals are young by world standards, all being younger than the Late Cretaceous. The main features of New Zealand coals that make them stand-out in the world scene are the very low levels of ash, particularly in the bituminous coals, very low sulphur levels in the subbituminous range, and low ash and sulphur in the lignites. The high vitrinite contents of the Tertiary age coals also lend the coking grade coals unique characteristics. However, the behaviour of vitrinite-rich coals is not well understood and requires more study.
Barry, J.M., Duff, S.W., MacFarlan, D.A.B. 1994. Coal resources of New Zealand. Resource information report 16, Energy & Resources Division, Ministry of Commerce, New Zealand. ISSN 0114-3719.
Couch, G.R. 1988. Lignite resources and characteristics. IEACR/13, Dec., 1988. IEA Coal Research, London. ISBN 92-9029-163-X.
EIA, 1998. http://www.eia.doe.gov/cabs/
Moore, T.A., Shearer, J.C., Miller, S.L. 1996. Fungal origin of oxidised plant material in the Palangkaraya peat deposit, Kalimantan Tengah, Indonesia: Implications for "inertinite" formation in coal. International Journal of Coal Geology, 30(1-2): 1-23.
Newman, J., Newman, N.A. 1982. Reflectance anomalies in Pike River coals: evidence of variability in vitrinite type, with implications for maturation studies and "Suggate rank". New Zealand Journal of Geology and Geophysics, 25: 233-243.
Shearer, J.C., Moore, T.A., Demchuk, T.D. 1995. Delineation of the distinctive nature of Tertiary coal beds. In: T.D. Demchuk, J.C. Shearer and T.A. Moore (Editors), Controls on the character of Tertiary coal beds. International Journal of Coal Geology 28, pp.71-98.
Shearer, J.C., Moore, T.A. 1994. Botanical control on banding character in two New Zealand coal beds. Palaeogeography, Palaeoclimatology and Palaeoecology, 110(1): 11-28.
Shearer, J.C., Moore, T.A. 1996. Effects of experimental coalification on texture, composition and compaction in Indonesian peat and wood. Organic Geochemistry, 24(2): 127-140.
Walker, S. 1993. Major coalfields of the world. IEACR/51, Jan., 1993. IEA Coal Research, London. ISBN 92-9029-208-3.
Used with permission Crown Minerals, Ministry of Commerce www.crownminerals.govt.nz
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