This paper was first published in October 2017.
INTRODUCTION
Low rank coal is a major part of the total Resources of coal in Indonesia and will most likely be mined as steaming coal. For this reason, it is an essential part of the energy planning of Indonesia into the future.
Low rank coals in Indonesia commonly occur in thick seams often greater than 20m thick. Such thick coal seams near the surface allows large scale mining, at low strip ratio, by conventional open-cut methods and provides the opportunity for economies of scale. Low rank coal is a proven source of fuel for power generation and new technologies will give additional opportunities for improving efficiencies of exploitation and utilization in the future.
DEFINITION OF LOW RANK COAL
Coal is a rock made up of 4 basic things; moisture, ash, volatile matter and fixed carbon (see figure 1).
COAL COMPOSITION
Figure 1 Coal composition
Coal rank is a measure of the maturity or degree of alteration, in the process known as coalification, of these organic deposits. For this reason, coal forms a series of rocks ranging from peat to anthracite. The rank of coal indicates the thermal history (temperature) that these deposits have experienced. Table 1 shows a maturation range chart comparing coal rank with maturity of oil and gas source rocks and the relative temperatures (in degrees Celsius) they may have experienced over geologic time.
TABLE 1
COAL RANK STAGES
Low rank coal is a brownish black coal in which the alteration has progressed further than in peat but not so far as sub-bituminous coal. It is considered as the lowest rank of coal. Figure 2 shows the range of coal ranks and where low rank coal sits within the peat anthracite series of rocks.
Figure 2 peat anthracite series of rocks
The term lignite or brown coal are used indiscriminately in the coal industry. Although there have been attempts in Australia, Europe, USA and other places, to define standards of suitable classifications of these coals, the attempts have not been successful in gaining widespread use in the industry. This may be because this type of coal has a wide variation in properties and composition.
For this study of Indonesian low rank coal, we define these as coal deposits as having total moisture contents of more than 35%, volatile matter content greater than 40% and calorific value less than 4000kcal/kg(gar). The properties of low rank coal suggest shallow depth of burial which is most likely to occur in younger deposits. A comparison of the composition of Indonesian coals is shown in Figure 3.
Figure 3 Indonesian coal rank series comparison
DISTRIBUTION
In the geological record throughout the world there have been several periods where climate, latitude and tectonic conditions to have favoured the accumulation of photosynthesized energy, as against aerobic decay (Parkin, 1975). The Tertiary period is one of these periods and Indonesia accumulated massive deposits of coal throughout mainly Kalimantan, Sumatra and Papua. Prolific coal accumulation and preservation occurred in 2 different phases; Paleogene from around 65-25 million years ago and the Neogene from 25 million years ago until recent times. The location of these coal measures are shown in Figure 4.
Figure 4 coal and peat location map of Indonesia showing Paleogene coal measures in brown, Neogene in yellow and peat areas in green
Low rank and sub-bituminous coal deposits are found in the Neogene (yellow) areas. These areas are where the youngest sediments of coal measures less than 10,000,000 years old occur adjacent to active peat areas where coal is forming today. Indonesia is geologically young and for this reason many of the low rank coal deposits are still in the tectonic settings in which they were formed. This puts them relatively close to the present-day coastlines and also near to large active peat swamp areas where accumulations of organic material is continuing at this time.
Figure 5 many low rank coal deposits in Indonesia occur close to active peat environments
It is not difficult to imagine the environments in which most of Indonesia’s low rank coal formed. These environments often still persist until this day in relative close, proximity to where the low rank coal deposits occur.
Figure 6 shows peat swamps in Kalimantan which are active coal forming environments
It seems apparent that these stable conditions have continued for a long period of time (up to 10 million years in some cases). Tropical climate, high rainfall, slow subsidence and a water table in equilibrium with the rate of sedimentation has allowed very thick deposits of peat to accumulate and subsequently be preserved by active sedimentation
caused by erosion of uplifted mountains. This is why very thick low rank coal seams in Indonesia are relatively common.
LOW RANK COAL RESOURCES AND QUALITY
The distribution of the main low, grade coal deposits are summarized in Figure 7.
Figure 7 distribution of some of the main low rank coal deposits in Indonesia
Resources of low rank coals are undoubtedly large and probably not all deposits have yet been discovered or explored. Low rank coal deposit resources are summarized in Table 2.
TABLE 2
LOW RANK COAL RESOURCES OF INDONESIA (millions of tonnes)
These resource numbers published by the Geological Survey of Indoneisa in 2006, appear to be conservative, restricted to areas where exploration had already taken place in 2005. Current resources are expected to be significantly larger and should include Papua where recent exploration has indicated significant additional resources.
CONCLUSION
Low rank coal is a major part of the total Resources of coal in Indonesia and should not remain undeveloped. For this reason, low rank coal will remain an essential part of the energy planning of Indonesia into the future.
Low rank coals in Indonesia commonly occur in thick, laterally extensive seams (greater than 20m) near the surface which allows large scale mining, by conventional open-cut methods, and the opportunity for economies of scale to keep mining costs low.
Low rank coal is a proven source of fuel for power generation and new technologies in the future are likely to give additional opportunities for improving efficiencies of exploitation and utilization.
Resources of low rank coals in Indonesia are undoubtedly large and not all deposits have yet been discovered or fully explored. This gives enormous potential for the discovery of new deposits.
References
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Durie R.A. 1991, The science of Victorian brown coal: structure, properties and consequences for utilization. CSIRO, Division of Coal and Energy Technology, Butterworth Heinemann
Preston K.P. & Sanders R.H. Estimating the insitu Relative Density of coal. Australian Coal Geology Volume 9 14 July 2013