Abstract
Some of high rank coal occurrences in Kalimantan Basins – Indonesia have been identified and documented as coking coal deposits. The occurrence of the coking coal deposits is essentially controlled by the distribution of the bituminous coal. These coking coals generally containing low ash and low to high sulfur have a very high fluidity, although the content of inert macerals appears to be very low as represented by the Central Kalimantan coking coals. Petrographically, these coals in general are dominated by vitrinite macerals with minor inertinite and liptinite contents. The vitrinite reflectance (Rv) ranges from 0.7% to 1.1%. With these characteristics, in carbonization process, these coals produce high CRI and low CSR Cokes. Nevertheless, these clean and high fluidity coking coals might become a good blending material for coke industries.
INTRODUCTION
In Indonesian archipelago, coal deposits are mainly distributed in the islands of Sumatera and Kalimantan. Most of Kalimantan coals are deposited and associated within the Paleogene and Neogene coal measures in Tertiary sedimentary basins. The formation of Kalimantan coal quality was strongly influenced by the geological environments resulting in a wide variation of coal characteristics including the coking and caking properties. The variation of Kalimantan coal properties is commonly expressed by two main coal parameters, namely type and rank.
Coal type is determined by the type of original plant input, the availability of nutrients, climatic conditions, the level of water table, the pH and Eh conditions and other biochemical environments in the peat swamp. Coal rank refers to the degree of maturation as the result of coalification endured by the organic matterin particular geo-and physico- chemical conditions related to tectonics, burial history, and heat-flow environments.
The quality of Kalimantan coal varies quite markedly across the island especially from the coal rank is concerned. The coal rank starts from lignite, sub-bituminous, high to low volatile bituminous, semi-anthracite, to anthracite. In some places, the rank increases gradually from sub-bituminous coal to anthracite. Paleogene coals are commonly bituminous or higher in rank, whereas the rank of Neogene coals, in normal geological conditions, are relatively low, except for heat-effected coal deposits. Therefore, the occurrence of high rank coal in Kalimantan is mainly controlled by the distribution pattern of the Paleogene coal measures, and for some extent also affected by the occurrence of volcanic activities associated with Neogene coal measures.
Regarding the quality variation, Kalimantan has areas with the coal of Low, Medium, to High Volatile Bituminous coal. Theoretically, coals with this rank series should have coking and caking properties. Data collected from recent studies also indicate that some coal reserves contain bituminous coal with coking coal deposits. However, due to the coal type constrain, the physical properties of the Tertiary coking coal of Kalimantan would be different with Pre-Tertiary coking coal deposits in other countries.
In Kalimantan Island, coking coal deposits are known in West, Central, and East Kalimantan Provinces. In Central Kalimantan coking coal deposits are distributed within the Northern Barito and Western Upper Kutai Basins (Fig.1) such as coking coals from Barito (North, South, and East) and Murung Raya Regencies.
GEOLOGY OF CENTRAL KALIMANTAN COAL DEPOSITS
Central Kalimantan coal deposits are located in the Northern Barito and Western Upper Kutai Basins (Fig. 1), and included in the Regional Geological Map of Muara Teweh and Buntok Quadrangles (1:250,000) published by GRDC (1995).
Stratigraphically, this area comprises four units namely Pre-Tertiary Unit, Eocene Unit (Batu Ayau and Tanjung Formations), Oligocene Unit (Ujoh Bilang, Karamuan, Purukcahu, Berai, and Montalat Formations), and Miocene Unit (Warukin Formation). The Pre-Tertiary Unit consists of igneous, metamorphic, interbedded sedimentary and volcanic rocks, which are not differentiated in the geological map. The Eocene Formations are composed of sedimentary rocks deposited within a wide range of environments starting from a fluviatile system to transitional and marine environments.
The Oligocene Formations consist of wide variation of sedimentary rocks deposited within fluvial, transitional and marine environments Coal seams occur within the Batu Ayau and Tanjung Formations (Eocene Unit) and Montalat Formation (Oligocene Unit), and Warukin Formation (Miocene Unit). High rank coal, however, is mostly found in the Eocene Formations. Some igneous intrusions, particularly in the Eocene formations, were also found in this area possibly intruding the sedimentary rocks.
Samples of coking coal were taken from Batu Ayau Formation in Muara Teweh and Tanjung Formation in Buntok.
DISTRIBUTION OF HIGH RANK COAL IN EASTERN KALIMANTAN
In general, the Paleogene coals appear to have a various trend of increasing in rank to the north from South Kalimantan to East and Central Kalimantan coalfields. Neogene coals also increase in rank to the north from Sarongga to Samarinda up to Sangatta coalfields; and also to the west from Samarinda to West Kutei coal fields.
In the Central part of Eastern Kalimantan, two trends of high rank coals of prospective for coking coal deposits have been reported by the recent study, namely Puron – Dalit Trend in Upper Kutei/Upper Barito Basin and Swalang – Bayan Trend in North Barito/SW Kutei Basin (Fig. 2). These trends are located almost along the structural lineament so called Adang Flexure, and the increasing rank is probably controlled by this structure along with the thermal effect of igneous rock activities in the area.
It has been known that the Silantek high rank coal in the Ketungau Basin – West Kalimantan indicates coking and caking properties (Nas, 2005). This deposit is also positioned in extending zone with the Eastern Kalimantan coking coal deposits, and is probably still controlled by the Adang Structure.
In Kalimantan, the occurrence of coking coal deposits has been recorded by Nas (2005) from several areas, namely Meruai and Juloi (Murung – Central Kalimantan), Marunda, Nantoy, Puron, and Basau (Muara Teweh - North Barito, Central Kalimantan), Ayuh and Swalang (Buntok – South Barito, Central Kalimantan), Bayan and Mahakam (Kutei – East Kalimantan), Long Ikis, Jatus, and Telakai (Pasir – East Kalimantan). Two coking coal deposits in Muara Teweh and Buntok have been well identified during the current coal study.
Coking coal in Puron – Nantoy area is located 50 km northwest of Muara Teweh, and it occurs in the lower part of Batu Ayau Formation (Fig. 3). In the South Barito Regency, coking coals are found along the Swalang Mountain approximately 100 km northeast of the capital city – Buntok. The coking coals occur in lower and middle parts of the Tanjung Formation (Fig. 4).
RANK AND TYPE
The variation of coal characteristics is commonly expressed by two main coal parameters, namely the type and rank. Coal type is determined by the type of original plant input, the availability of nutrients, climatic conditions, the level of the water table, the pH and Eh conditions and other biochemical environments in peat swamp. Practically, a coal type is determined by the composition and association of macerals in the coal. Most of Central Kalimantan coals are dominantly composed of vitrinite macerals, with a lesser amount of inertinite and liptinite macerals.
Coal rank refers to the degree of maturation as the result of coalification process endured
Figure 1. Locations of Central Kalimantan Coking Coal studied
Figure 2. Coking Potential High Rank Coals in Eastern Kalimantan (Nas, 2005)
Figure 4. Stratigraphy of coking coal deposits in Buntok Area (Nas, 2001)
Figure 3. Stratigraphy of coking coaldeposits in Muara Teweh Area (Nas, 2001)
by the organic matter of peat in particular geo- and physico-chemical conditions related to tectonics, burial history, and heat-flow environments. Rank building in coals is mainly determined by heat and time, while pressure only plays some roles in the early stage of coalification. Heat source is normally from heat flow, geothermal gradient (depth of burial), and magma activities with time or duration of heating is also important. The rank is defined to range from lignite, sub-bituminous, bituminous, semi-anthracite, and anthracite. It is estimated by measuring the moisture content, calorific value, and reflectance of vitrinite or volatile matter (these are known as rank parameters).
Coal rank is a main parameter used to distinct coking coal terminology. Although the coal type, especially maceral composition, is also important, for Central Kalimantan coal it is clear that most of the coal is dominantly composed of vitrinite, while liptinite and inertinite only occur as minor constituents. As a reactive maceral group, vitrinite is categorized into structured vitrinite (less reactive) and unstructured vitrinite (more reactive), therefore the ratio between the two should be considered. The grain size of structured vitrinite would also be useful to determine. The ratio and size of vitrinite maceral in coking coals, for some extent, would influence the strength of the coke produced.
SOME QUALITY PARAMETERS OF COKING COAL
Crucible Swelling Number (CSN) CSN is an index to measure swell ability of coal when heated in the absence of air. To determine the CSN by the ASTM method, one gram of powdered coal is heated and the result is compared to a series of standard sections. The CSN of the coal sample is the number according to the standard ranging from 0 to 9++. Tests of CSN are commonly used as an initial indication of coking properties of coal. The value of CSN is very sensitive to coal oxidation, thus the variation is probably related to the freshness of the samples. Since CSN is non-additive the values would be reduced as samples are not fresh.
Dilatation
Dilatation is a measure of change in the length when a coal pencil is slowly heated in the absence of air in a confined tube using “Audiebert Arnu Dilatometer”. The change caused by the increase of temperature is continuously recorded. At the early stage of the heating, the pencil firstly shortens and then starts to lengthen as the temperature is slowly increased. The data obtained from this dilatation test including:
1) the softening temperature;
2) the temperature of maximum contraction;
3) percentage of maximum contraction;
4) the temperature of maximum dilatation; and
5) percentage of maximum dilatation.
The most important value from the data would be the total maximum dilatation. The maximum dilatation can be regarded as a more precise measure of swelling and should be used together with the values of CSN.
Fluidity
Fluidity is a measure of the degree of plasticity or ductility of coal when heated in the absence of air using Gieseler Plastometer. In the Plastometer, a constant torque is applied to a mechanical stirrer within the coal sample. The coal will show initial softening at a particular temperature and then is fused when the stirrer starts to move five dial division per minute (5ddpm). As the heating temperature is increased and the sample reaches its maximum fluidity, the stirrer will rotate in maximum ddpm, and finally slows gradually when the coal starts the process of resolidification. The data obtained from this fluidity test including:
1) the temperature of initial softening;
2) the fusion temperature;
3) the temperature at the maximum fluidity;
4) the ddpm value at the maximum fluidity;
5) the temperature of resolidification.
QUALITY OF CENTRAL KALIMANTAN COKING COALS
Central Kalimantan bituminous coals are very rich in vitrinite macerals, while inertinite and liptinite only occur as subordinate constituents. Vitrinite macerals generally perform as semi-reactive and reactive materials during carbonization process, while inertinite macerals are commonly inert (non reactive) and semi reactive. Most of liptinite macerals become volatile matter during carbonization process.
In this study, data collected from proximate - calorific value analyses and Crucible Swelling Number (CSN) have been used as initial parameters indicating coking coal properties. Dry Mineral Matter Sulfur Free (dmmSf) basis of Volatile Matter (dmmSf VM) and Calorific Value (dmmSf CV) is plotted into the Suggate Curve to identify the plot position within the coking coal zone (Fig. 5).
Data collected from CSN tests have been used to confirm the coking properties of the coal. As CSN is very sensitive to coal oxidation, if it is known from the analytical results that CSN values vary from the bottom to top within seams, this variation is probably related to the freshness of the samples taken.
Therefore, it is fair to consider the highest CSN value as the most representative coal. In this case, CSN test on the composites (plies composite) is not recommended as CSN is non-additive, so that the CSNvalues would be reduced as samples are not fresh. Proximate analysis on the composite would be a good check on how representatively the sample was mixed (plies composited on RD and ply thickness).
During this study, complete coking coal analyses were undertaken for two coalfields namely Ayuh – Swalang in Buntok (South Barito Regency) and Puron – Nantoy in Muara Teweh (North Barito Regency). As a result, the properties of the Buntok and Muara Teweh coking coals might represent the general characteristics of Central Kalimantan coking coals.
In Buntok area, coal is generally high to medium volatile bituminous in rank and very high in vitrinite content, and has coking properties. The coking properties of individual seams change gradually from South Swalang in the north to the South Ayuh in the south along 30km strike line, due probably to a decrease in the rank from medium to high volatile bituminous coal. This feature is shown by the value of Crucible Swelling Number (CSN), vitrinite reflectance (Romax), and dried ash free volatile matter (dafVM). CSN values are typically very high (9++) in the South Swalang area gradually decreasing to 8-9 in the North Ayuh area, and then to 5-6 in the South Ayuh.
Same trend is shown by the Romax, from 1.07% to 0.85% in the north to 0.75% in the south. Changes are also apparent in the values of dafVM from 30% in the north to 35% in the middle, and 40% in the south. The coal has high to very high fluidity varying from 1000ddpm to 25000ddpm, very low phosphorus (P) content (0.001% - 0.015%), and very low base-acid ratio in the ash. With these coking properties, coal in the Buntok area may be categorized as medium-high volatile coking coal in the north and semi-soft coking coal in the south.
Regarding individual seams, ash contents vary from 1.3% to 10.26%. The higher contents are due to the presence of partings in the seams. For the target seam (C Seam), the ash contents vary from 2.9 to 10.2%, that is commonly higher at South Swalang (4.3-10.2%) and lower at North Ayuh (2.9-7.6%).
The ash contents of this seam also change from top to bottom; generally very low ash (2.2-4.3%) at 1.6m of upper section and very high ash (22.4-42.9%) at 0.4m of lower section. In the South Ayuh area, ash contents are consistently low in the target seam (B seam) throughout the area.
Sulphur contents vary from low (<1%) to very high (>2%). In the South Swalang area sulphur contents range between 0.8% and 1.5%, whereas in the North Ayuh sulphur contents are consistently very high (2.5% to 4%). Sulphur contents in the South Ayuh coal vary from 1% to 2.64%. Fortunately the target seam (B Seam) typically contains less sulphur ranging between 0.57% and 1.4%.
Some quality parameters also change according to the stratigraphic positions. In the Lower Coal Unit, significant increase in rank as indicated by a decrease of the dafVM values, is noticeable particularly in the North and South Ayuh areas. Ash content is also much higher in the Lower Coal Unit seams, whereas the sulphur content is less. Coal seams in the Upper Coal Unit show variations in the ash and sulphur contents according to the stratigraphic positions.
Figure 5. Coking coal identification using Suggate Curve
Orientation washability tests indicate good washability characteristics of some samples from the lower high ash section of the Swalang seam collected from the South Swalang area. A good washability performance is also shown by the test of few samples from the main seam of the Lower Coal Seam collected from the Ayuh area.
Summaries of important quality parameters for A,B,C,D, and E seams at the South Swalang and North Ayuh areas, and for UA, A, and B seams at the South Ayuh area are listed in Tables 1, 2, and 3.
In the Muara Teweh coalfield, Nantoy coal is typically high-volatile bituminous in rank, and has coking properties (Table 4). Regarding the two parts of the coalfield, the coking properties change from Nantoy in the SE to Puron in the NW. In terms of the quality, coal in Nantoy has the best coking properties with very high value of CSN (9-9++), medium to high dafVM (32.79-37.26%), moderate Rvmax (0.83-0.93%), very high fluidity (23000-31000 ddpm). This coal also has low moisture (typically <1.5%), low ash (typically <5%), and low sulfur contents(typically <1%), low phosphorus content in air dried coal (0.003-0.008%), and low base-acid ratio in the ash. With these properties, this coal may be categorized as high quality coking coal.
Puron coal in Murangon is low-volatile bituminous to semi-anthracite in rank as indicated by very high vitrinite reflectance value (Rvmax) ranging from 1.28% to 2.50%, very low dafVM, high dafCV, and low moisture content. This very high rank caused the coking properties diminished as shown by no swelling and fluidity. It has probably been affected by the occurrence of an andesitic intrusion within the coal-bearing strata. It is apparent in coking coal that there is also relation between the amount and properties of coal ash and the quality of the resultant coke (Miroshnichenco, 2008).
This relationship is related to the basicity of ash composition and the temperature (oC) corresponding to a liquid state of the coal ash. Ash composition of Central Kalimantan coking coal indicates very low basic-acid ratio as shown in Table 5.
Carbonisation of coking coal will produce coherent cellular or vesicular agglomerated mass comprised of very high carbon content so called “coke”. The quality of the resultant coke is stated using several parameters such as:
1) heat value;
2) porosity/permeability;
3) coke strength and reactivity;
4) percentage of inorganic content; and
5) occurrence of some particular chemical elements such as P and S.
The quality of coke is related to and derived from the quality of the parent coking coals. Coke samples produced from high volatile coking coals were tested for CRI and CSR (Reactivity of Coke to Carbon Dioxide, AS 1038 - Part 13). Two flow-rate of hot CO2 adjusted in the test are 2 liters per minute and 5 liters per minute. The first resulted in CRI ranging from 26.4 to 27.3% and CSR ranging from 51.6 to 53.6%, and the second showed an increase in the CRI (40.0 - 40.9%) and consequently reduced the CSR (40.2 - 40.8%).
DISCUSSIONS
Coking coal as a raw material in coke production needs to have appropriate properties to produce good and strong cokes. The coke strength indicated by CSR(Coke Strength after Reaction) and the reactivity indicated by CRI (Coke Reactivity Index) are mainly influencedby the rank, maceral composition and ash content and ash composition of the raw coal. An optimal combination of reactive and inert macerals in the raw coal will result in an optimal proportion of binding, matrix, and aggregate materials in the resultant coke produced, and this proportion will also in turn control the CSR and CRI of the coke produced.
The composition of most Central Kalimantan coking coals composed of abundant of vitrinite macerals as reactive and semi reactive materials, while inertinite as inert materials is only minor. Regarding predicted CSR and CRI, comparing the two Central Kalimantan coking coals with Australian prime and New Zealand coking coals as shown in Table 6, Australia would be the best followed by Indonesian coking coal and NZ coking coals. Indonesian coking coal has more structured vitrinite (less reactive), whereas NZ has more unstructured vitrinite (more reactive).
Their very high fluidity and low ash contents are considered as significant advantages for Indonesian and New Zealand coking coals. In coke making, therefore, these coals are approved to be good blending materials combined with Gondwana coking coals (Australia, South Africa, and India).
Furthermore, low contents of mineral and sulfur together with very high content of net-carbon, Indonesian coking coals will be beneficial to the reduction process in the steel making. Washed coking coals with less than 1.5% ash will also be appropriated to use in silicon metal industry.
For any rank of Central Kalimantan coking coals (Romax 0.7-1.1%), very low content of inert macerals (<5%) will produce relatively low CSR cokes as indicated by Pearson (1989; Fig. 6). Very high vitrinite content in the parent coking coal is also responsible for highly-moderately fractured and low to moderate CSR resulting cokes (Cook, 2008). This isproved by the CSR and CRI data indicating less than 50% CSR and more than 40% CRI.
Moreover, although the low basic-acid ratio of coal ash composition should positively affect the value of CSR (Ryan and Price, 1993), very low content of inorganic materials in the coking coal will not influence CSR significantly.
CONCLUDING REMARKS
- In Central Kalimantan, the occurrence of coking coal deposits is essentially controlled by the distribution of high rank coal (bituminous coal);
- The vitrinite reflectance (Rvmax) of coking coals from the study area range from 0.7% to 1.1%;
- Central Kalimantan coking coals generally containinglow ash, low to high sulfur, and low phosphorus have a very high fluidity, although the content of inert maceralsappears to be very low;
- Petrographically, these coals in general are dominated by vitrinite macerals with minor inertinite and liptinite contents;
- Cokes produced from these coking coals have slightly different properties with relatively high Coke Reactivity Index (CRI) and low Coke Strength after Reaction (CSR);
- These clean and high fluidity coking coals might become a good blending material in coke making industries.
Acknowledgments
This paper is written mainly based on researches by the main author for Kalimantan coking coals in PT. Austindo Nusantara Energi. The author thanks Mr.George Tahija, President Director of PT. Austindo Nusantara Jaya for his support and permission to write this paper.