Non-coking coal constitutes about 85% of Indian coal reserves which are of high moisture, high ash, high volatile and sub-bituminous types (with Ro% ranging between 0.4 to 0.65). Washing of non-coking coal generated much interest in view of the Ministry of Environment and Forests’ stipulation of movement and consumption of thermal coal beyond certain ash percentage i.e. drive towards washing non-coking coal is essentially re-active from the point of view of both producer and consumer of coal. Transporting of coal of ash not exceeding 34% beyond 500 km is posing problems to different coal suppliers for the dispatch of its coal to the power plants. The solution to this problem will call for setting up of washeries to reduce the ash content.
At the behest of erstwhile Planning Commission a working group committee set up to look into consider the two important scenarios namely, (1) 2 × 500 MW power plant at the pit head using ROM coal and (2) integrated 2 × 500 MW power plant along with a coal washery at pit head and CFBC or AFBC based power plants utilizing washery rejects. These two points are elaborately discussed and different physical processes are adopted in cleaning raw coal from mine with or without any chemical reagents, to get a product which could be sold in the market or can be used for various purposes as coke making or for power generation. The distinguished scientists and experts in the field of coal preparation from the CSIR-Central Mining & Fuel Research (CIMFR), Dhanbad have contributed a set of research papers in the subject area which are of immense value for the industry and have suggested some useful and innovative methods, new approaches among them, are online coal washability.
Hopefully, this special issue of the Journal of Mines, Metals & Fuels should prove to be of some interest and would provide insight into the “black art” of coal washing.Read more
Indian coal industry is the world’s third largest in terms of production and fourth largest in terms of proven coal resources. Coal deposits are of drift origin, high in ash content but low in sulphur. There are 20 major coalfields located in east and south eastern quadrant of the country. The coking coal reserves are less compared to the non coking coals and the good quality coking coals has been exhausted leaving behind poor quality feed stock for the metallurgical sector. For production of Iron & Steel through blast furnace route, coking coal is an important raw material. The good quality coking coals of the upper seams are fast depleting leaving behind the inferior quality lower seam coal. The lower seam coals presently being mined are mostly low volatile coking coal (LVC). They constitute about 50% of the total coking coal reserves in India. These coals are characterized by high raw coal ash content and poor washability characteristics. Beneficiation of the lower seam coals in the existing washery circuits (2 or 3 product) does not yield requisite quality demanded by the steel sector of the country and as a result it is termed as Non-Linked Washery (NLW) coals, and the entire production of inferior coals is diverted to the thermal power sector, thus, wasting the scarce coking coal resources. In this paper, an attempt has been made to focus the cost-effective and eco-friendly approach for utilization of the LVC/NLW coking coals illustrating with a case study.Read more
The non-coking coal constitutes about 85% of the Indian coal reserves and the depositions remained mostly as low rank sub-bituminous and non-coking type. The vast reserves of the non-coking coal are usually high moisture, high ash, high volatile and sub-bituminous types (Ro% ranging between 0.4 and 0.65). The stipulation laid by the Ministry of Environment and Forest (MOEF), Government of India to transport coal of ash not exceeding 34% beyond 500 km is posing problems to different coal suppliers for the dispatch of its coal to the power plants. The only solution to this problem appears to be the setting up of washeries to reduce the ash content. With a 66% share of installed power generation capacity, the coal industry has a major role to play in the nation’s development. The coal washing capacity from all the 52 washing plants is 131Mtpa, indicating that around 30% of the coal is being washed before it is used in power stations. The paper highlights the washability studies carried out on high ash coals, the beneficiation methods required for coal washing in India and various technologies being adopted thereof for washing of Indian coals.Read more
Most of the Indian coking coal washeries were installed during the period 1960-1980 where the design of the washery was based on relatively easy or moderately difficult washing coking coals, mined in the eastern and western part of Jharia coalfields. The coal fines below 0.5 mm were not processed and though enriched in vitrinite content, nowadays the fines cannot be mixed directly with clean coal due to their high ash content (more than 25%) and of high percentage of silica content.
Due to an increase in opencast mechanized mining of coals, quality of the coal fines deteriorated and they need to be beneficiated before blending with the coarse coals. Some of the washeries installed fine coal treatment plants, but due to varied reasons these circuits were not operated as a result, huge quantity of unwashed coal fines is left as such and discharged to nearby ponds or lagoons, and in the process they had been accumulated in adjoining settling ponds causing environmental pollution and are being sold to domestic market for use in non-metallurgical purposes.
The coking coal fines constitute vitrinite content, which is a vital ingredient for metallurgical purposes. Hence, if these coal fines are beneficiated, they may add to the overall quality of the clean coal. In this paper attempt has been made to beneficiate the accumulated coal fines through laboratory flotation tests vis-a-vis pilot plant studies to generate maximum amount of cleans from settling pond coal fines suitable as blend for metallurgical coke making.Read more
The coal seams in India are mostly inter-banded, containing stone and hard shales, since such bands cannot be separated during the mining operations; these are mixed with the coal, thereby increasing the ash per cent of the run-of-mine (ROM) coal. In majority of the coalfields, it was found that the nature/ character of the coal and stone is such that the stone is harder than the coal. Hence, it may be possible to remove the hard stone by incorporating a rotary breaker at the pithead or coal handling plant. However, before installing a rotary breaker, it is very much essential to understand the breakage characteristics of the coal and stone.
The present investigation includes selective drop breakage tests of coals and stones from variable heights. The differential breakage patterns of this material have been quantified in terms of Selective Crushing Index and its dependence on number of drops and heights. Attempts have been made to find a breakage constant with the assumption of drop breakage as a first-order process dominated by volume breakage. Results of the selective drop breakage tests on coal and stone from non-coking coalfields showed that there is a good potential of installing a rotary breaker at the pithead.Read more
Heavy medium coal processing technology has been widely used in coal preparation industries in the world since it is the most accurate and efficient coal preparation process to obtain clean coal with a highest possible yield. Separation of coal and impurities using this technology is based on density differences. Tests were carried out on a 100mm dia heavy medium cyclone test rig treating coal in the size range -6 + 0.5mm. A total of twenty seven experiments were carried out by varying the parameters like vortex finder diameter, spigot diameter and inlet feed pressure for each cyclone dia. The results indicate that for 100mm dia H. M. cyclone it is possible to achieve about 24% clean coal at an ash content of 17.3% from feed ash of 35% at a feed pressure of 0.35 kg/cm2, vortex diameter of 42.5mm and spigot diameter of 28mm.Read more
The basic objective of coal washability is to establish the set of maximum possible separation performance for a given coal feed and to access the cut gravity for achieving clean coal of desired quality. The conventional float-sink (F-S) analysis is time taking, hazardous and at the same time costly. Indian coals are of drift origin and vary widely in the washability characteristics and there is also wide variation from seam to seam at the same mine. The washery is to be fed with coals from multiple sources/seams, which are having wide washability characteristics. The standard practice of washability will not help the washery to improve its performance as a result an alternative online analyzer especially for a commercial coal washing plants which maximize the reliability of actual coal washability in conjunction with particle size distribution, ash, etc is the need of the hour.
Researchers worldwide, are trying to establish the washability characteristics of coal through different routes X-ray/gamma, basically imaged based/processing to solve the problem. An automated coal washability analyser is being under developmental stage which will give near realtime F-S data that would maximize the yield at desired ash level and impact positively on the profitability of the plant operation. The results are very promising with the actual standard float and sink data to the coal analysed with the X-ray analyser, which are being highlighted in the present paper.
Keywords: Auto-oxidation, spontaneous heating, differential thermal analysis, intrinsic properties, coal
Clean coal technology is a very important area of research and development as it has enormous potential to reduce harmful emissions, improves fuel efficiency which meets the present and future energy requirements. Indian coals are known for drift origin, the associated mineral matter is responsible for formation of ash in the coal, which are intimately mixed with coaly matter during the time of formation. The mineral matter present in the coal has detrimental impact on different end uses. Gradual increase in ash percentage and deterioration of good quality Indian coals have attracted the researchers to look inside the coal for its judicious utilization. Coal is extremely complex and the most heterogeneous fuel, composed of a number of distinct organic entities called macerals and inorganic entities called minerals. The term, “mineral matter”, usually applies to all inorganic, non-carbonaceous material, includes those inorganic elements which may occur in organic combination. The inorganic mineral matter divided into two groups-inherent which is grown with plant life and seldom exceeds 2-3 per cent of whole mass while extraneous mineral matters generally consists of large bits and pieces of inorganic material typical of the surrounding geology. These extraneous mineral matters so finely divided and uniformly dispersed within the coal and enter into the coal matrices and behave like inherent mineral matter which cannot be removed by any means of physical washing. The coal at below -500 micron is density separated using centrifuge and studied with FE-SEM and EDS. Knowledge of the morphological relationship between the inorganic phases and organic matter is helpful to devise suitable schemes for cleaning coals containing such particles. The surface morphology, presence of pore and their sizes were carried out using scanning electron microscopy; while EDS analysis helps to determine the elemental composition of the mass of a particular area, advantage is simplicity with accuracy and speed.Read more