Coal Preparation in India – A Historical Perspective
The earliest record of systematic coal washing study in India was an attempt by Prof. William Galloway using a pre-Baum type Jig washer before World War I on tested Assam coals. Sometimes later Prof. Henry Louis of Newcastle-upon-Tyne conducted similar experiments on Jharia coals. However, systematic washability study was made, for the first time, by Rev. E.H Roberton, Prof. of Mining at the Bengal Engineering College, Shibpur, during World War I.ref 1
Again, in 1920 E. C. Evans, a chemist from London conducted experiments for washing of Jharia coals in a Draper Washer. All of them concluded that Indian coal cannot be economically washed, to the level of British coals.
It is interesting to note that K. Reinhardt (Germany) invented the present day Float and Sink method in 1926. Before that the washability characteristics used to be determined by pulsating crushed coal on a perforated pan, dipped in water, simulating jigging.
A. Farquhar of the Tatas carried out washability studies on Jharia coals from 1918 to 1924 and confirmed in 1938 that Jamadoba, Malkera and Bhowra Coals can be washed economically. After further studies during 1938 to 1940, Tatas decided for two washeries in 1946 and established West Bokaro Washery in 1951 and Jamadoba Washery in 1952. Dr C Forrester of the Indian School of Mines, Dhanbad along with J. N. Majumdar conducted systematic washability studies of Indian coals, under the auspicious of the Fuel Research Committee, which is the fore-bear of CFRI, commissioned in 1946 as the first Central laboratory at Digwadih, by the side of the Tatas, under CSIR in 1946.
In both West Bokaro and Jamadoba Washeries, upper seam U/G coals of JCF were crushed to 75mm and size fraction +25mm was washed by Chance Cone process. It was interesting to note that the Sinks were further crushed for liberation and their recycling facility was also incorporated.
After creation of Coal Board in 1952, synonymous with the erection of Steel plants in the public sector, the erstwhile Coal Washing Committee suggested installation of 4 Central Washeries at the Railway Marshalling Yards (viz Dugda, Patherdih, Bhojudih and Kargali) by the Second Five Year Plan. Lodna washery was installed for washing raw coal from U/G mines, working on Upper seams in the Eastern Sector of JCF, for IISCO. Raw coal was crushed to 13mm and washed in a Feldspar Jig. Subsequently, 4 Central washeries were created at the above mentioned railway yards during 3rd and 4th Five Year Plans, followed by Durgapur washery near the steel plant in West Bengal.
Dudga washery (1962) was the first Central Washery in JCF, located on the Western fringe near the Railway Marshalling Yard, serving as the junction point between the Eastern and the South Eastern Railways. More than 150 washability studies were conducted at CFRI and the individual small private mines (generally U/G) were classified into high and low yielding groups. Higher yielding coals were taken to the Washery I by railway wagons, unloaded by tipplers, crushed to 75mm, sampled and stored in large number of circular ground bunkers ( 2 steams x 22 pockets x 150 t, with individual Vibratory feeder) , with a view to simultaneous discharge to blend the feed. The circuit comprised of Deshaling Baum Jig and subsequently screening at 25and 6 mm, and HM Baths for both the size fractions (75-25 and 25-6mm) with individual media control measures, apparently with a view to maximize the yield of cleans. As mentioned above, the smalls (below 6mm) were dry screened and mixed with the cleans. The average feed ash was 24%, yield was 60 to 70% at 17% ash, and sinks were dispatched by conveyors to nearby Chandrapura Thermal Power Station of DVC.
Dugda II Central Washery was installed in 1968, having large over ground trough type concrete bunkers (2 nos x 8000 t each) for raw coals crushed to the same 75mm. Coal from individual mines was proposed to be stacked in large over ground slit bunkers in layers and cut vertical / inclined sections by plough reclaimers for proper blending. Each bunker was sufficient for stacking one day’s consumption and the blended feed was designed to be crushed to 13mm, sampled, washability studies done and sp.gr. of cut presumably determined beforehand. The feed was deslimmed at 0.5mm and washed in 600mm HM Cyclones. Fines were thickened, filtered and mixed with the cleans. The feed ash of Dugda II washery was considered to be 24 to 28% and yield of cleans was ~50% at the same 17% ash. The sinks were sent to the same power station. The dirty slurry coming out of the Cyclone sinks’ rinsing screen was fed to a series of Hydrocyclones and the underflow was dried by solid bowl centrifuge. The overflow used to go back to the thickener. Thus, Dugda II was an “All Cyclone Washery”, quite novel at that period of time.
Over the years the upper seams were exhausted and the raw coal from the middle seams contained higher ash and were of poorer washability characteristics. The fines contained ash more than acceptable as cleans and Flotation Plants were incorporated in mid 80’s. Jigs were also installed to sweeten the middlings. At the same time, another Jig was installed in the same dovetailed building to rewash the dry screened smalls (-13mm) of Dugda I washery.
When Dugda I outlived its life, then for utilizing the receiving and loading out sections and the above mentioned Jigs, it was converted to a non-coking coal washery by 1998.
The Patherdih Central Washery (1964) was subsequently installed at the Eastern fringe near the Railway Marshalling Yard. Raw coal was received by railway wagons, tippled, crushed to 75mm and stored by layers in large over ground concrete bunkers (2 nos x each 7000t) and reclaimed by ploughs. Though jigs were installed to deshale the coal, the back stroke jigs were always found to be over loaded (>25 tpm2/h) and Star gates were difficult to operate due to blocking with oversize brick shaped lumps. The deshaled coal was screened into two fractions at 13mm and HM Bath and Cyclones were used with individual media circuits. The fines were thickened, filtered and mixed with the cleans. Over the years, the Eastern JCF mines were exhausted of upper and middle seams and surface fires at Lodna and adjoining areas proved to be hindrance to receiving of raw coals by railways. It is worth mentioning that a 10tph Oil Agglomeration Demonstration Plant was installed, having Ball Mill grinding facilities, to study the liberation and the recovery potential from the slurry . The Patherdih washery had a miniature CHP under an adjoining shed to systematically process the manually sampled coal, presumably from the blended feed to the washery, more as a post mortem analysis than for day to day operational control. Subsequently, a 5tph Jig and a 2tph cyclone and a high speed screen were given full scale trials in that pilot plant before industrial acceptance. Flotation cells, salvaged from closed Barora washery, were reinstalled at the main Patherdih washery, more as a stop gap arrangement.
Though, it was originally planned to fill up one bunker at a time by layering individual receipt of raw coals from small (pvt) mines, and sample before the next day’s operation, it failed due to frequent shortage of raw coal supply and difficulties in producing F&S results overnight. Similar to the above washeries, Patherdih was designed to get water from the Damodar and discharged effluents into lagoons, due to inefficient operation of the vacuum filters.
By far Bhojudih Central Washery (1962) located at the South Eastern fringe of JCF, by the side of the Damodar / Goai river, has proved to be most successful. Firstly, the Southern mines (SER) produced best coals, with highest yield and easy washability characteristics, and secondly the design and construction made by the Belgium Company incorporated HM Bath for 75-25mm coarse coal and Jigs for smalls, a standard European circuit for good coals. Moreover, sufficient live spare capacity was also proved to be beneficial in the long run, like three thickeners, multiple vacuum pumps, etc. However, the Cyclone circuit provided for sweetening the sinks of HMB, after crushing to 13mm, did not prove to be successful. However, the middlings were largely acceptable by the neighbouring Santaldih Power Plant (WBSEB).
Over the years, due to deterioration of raw coal characteristics, the popular circuit of Batac Jigs and Flotation plants were installed in late 80’s to early 90’s.
Sudamdih (1981)and Moonidih (1983) Washeries were installed after the Nationalization of Coal and formation of BCCL (1972), as the first large Govt pit head plants in JCF. The projects were to win coal from modern U/G mines with Polish collaboration, expected to produce 2Mtpy. As the areas were disturbed by tectonic movement and the seams were highly inclined, horizon mining with long wall faces and sand stowing facilities were tried for the first time in India. The U/G mechanized mining, though the only alternative so far to exploit deep seated coals, has proved to have taken more time to stabilize, though all modern infrastructures like Training Centers, Welfare amenities, well laid quarters, water treatment, swimming pool, clubs, schools, markets, etc. were provided. It is appreciated that at least the miners learnt to use helmets, iron toe shoes and modern safety gadgets.
The circuit was considered to be revolutionary at that point of time. Coal was crushed to 37mm and after deslimming at 0.5mm, was washed in gravity fed cyclones of 600 mm dia. The second stage Cyclones at higher gravities proved to be techno-economically less viable. Flotation circuit at Sudamdih washery had short comings due to less retention time in cells and required multiplication. Moonidih washery had Autogenous Cyclones and filters. As the filters were difficult to maintain for optimum performance, they were strengthened by high speed dewatering screens. The concrete slurry ponds, provided in the original layout, proved to be insufficient and had to be dovetailed to low lying impoundments.
Unfortunately, neither Sudamdih nor Moonidih mines reached target production from U/G, which had to be supplemented with production from neighbouring mines and quarries of lower inferior seams, and sometimes of patch deposits of middle seams. Thus, the dream of achieving >85% yield of cleans with less than 17% ash did not materialize.
Under the instance of KSR Chari, the only mining engineer who could become the Secy (Coal) and the pillar of the Nationalization of Coal, a few Unit Washeries were proposed to be installed at JCF. Barora (1985) and Mahuda (1986) were installed as pit head washeries @ 100 to 150 tph. Raw coal was crushed to 75mm and stockpiled in small 750t circular ground storage, deshaled by a Jig and crushed to 15mm for HM Cyclone treatment. Slurry was tried to be directly treated by Flotation in dilute phase and the tailings were thickened for recovery of process water. As quite a substantial amount of coal was locked under the washery area, it was dismantled in 90’s.
However, the similar unit washery at Mahuda was successful, possibly due to easy washable coals of Ranigunge series, where the coal was deshaled at 75mm, deshaled coal crushed to 25mm and as usually deslimmed, before cyclone washing and flotation.
BCCL installed Madhuban Washery in 1998, where raw coal from Block II OCP (IX/X seams) i.e the Western fringe of JCF was crushed to 50mm, de-dusted at 3mm and finally washed in a 3-P modern Jig. The middlings rewashing in HM Cyclones, to recover the misplaced cleans in them, was proved to be difficult due to very high NGM (>90%), particularly when both cleans and rejects were removed from the raw coal. The long haul of washery conveyors (around 4 km) for receipt of raw coal and loading out of products proved to be difficult to maintain. Jigs were found to be inefficient when further lower seams (IV/V/VI/VII) started coming due non availability of middle seam coals when Block II OCP dwindled production. Ultimately it was temporarily transformed to a non-coking coal washery dealing mainly with lower seams coals available nearby. Recently it has again been reverted back to washing coking coal.
The Kargali washery (1959) was installed with Japanese collaboration. It is interesting to note that the 20 tph Pilot Plant at CFRI was constructed as leading so as to fine tune the proposed circuit and study possible alternatives. The circuit was originally simple, feed size 80mm, screening at 12mm, HM Drum (two stages) for coarse coal and Jig for the smalls. It is interesting to note that the sinks of the first Jig was rewashed in a smaller capacity second Jig for reducing the overall costs.
The circuit was first by utilizing the Jigs in parallel circuits (-16mm size fraction) and secondary crushing of the Drum sinks to -16mm to recover further cleans by the incorporation of Cyclone washers.
And it was renovated Batac Jig (1984) for -16mm fraction, once again by incorporation of deshaling Jigs (1997), deduster before feeding to the Wemco Drum (two stages) for coarser coal (80-16mm. The deslimmed slurry was dewatered by thickening Cyclones and the slurry of better quality from the cleans dewatering screens was thickened in a thickener.
Kathara washery was installed by NCDC under Russian collaboration and commissioned in the year 1970. As expected it was equipped with one large Gyratory Crusher, having opening mouth radius sufficient to let go a medium size dumper. However, it has so much of power and excess capacity that with minimum maintenance and hardly utilizing the original spares that came along with the first supply, it worked dauntingly since commissioning. Raw coal from neighbouring Kathara OCP (containing large boulders up to 1200 mm size) was crushed to 200mm and further crushed to 75mm before storage in ground bunkers.
The washing circuit of Kathara is having an international flavor. The deshaling of the coarse coal (75 – 13mm) is done by a French Drewboy bath, the crushed deshaled coal of size 13-0.5mm is washed in Polish supplied H.M.Cyclones and the slurry is beneficiated in Russian made Flotation cells. In one word it has every thing best in the then world, but the synchronization seemed to be poor. The French buiding has French windows, the Polish building has floor heights standardized for Poland and the flotation plant was heavily constructed with 2 systems of flotation circuits (one for the cleans and the other presumably for the middlings circuit). It is a fact that one system remained mostly unutilized , but for cannibalisation . Each system has 4 streams x 6 cells (individual trough type with adjustable wooden partitions) with dedicated conditioners, dosing of reagents and vacuum disk filters. The flotation banks have a fixed volume large size mug like containers to measure the time taken for its fulfilling for determination of the quantitative flow of the slurry. This shows the Russian dexterity of the flotation subject so as to tackle slow floating medium coking coals like that of Central Coalfields.
Sawang Washery (1970) was another pit top plant where the feed is primary crushed to 80mm, prewashed in Jigs and floats crushed to 20mm for deslimming and feeding to HM Cyclones. The slurry (-0.5mm) was treated by Hydro-cyclones, thickened and filtered by vacuum filters. The underflow of the hydro-cyclones was dewatered by Dirty Slurry Screens and centrifuged along with the sinks of the HM Cyclones for production of middlings.
Gidi Washery (1973) was installed, under Polish collaboration, to wash non-coking coals for the Railways. Raw coals from U/G mines were crushed to 150mm, screened at 25mm and coarse fraction was treated in HM DISA Bath. As a standard circuit, the smalls were washed in Baum Jig, after deslimming at 0.5mm.
As the consumption of coal reduced for the Railways, Gidi Washery was converted to coking coal and flotation of fines was added, along with the replacement of the Baum Jig by FCB Jig.
Rajrappa Washery (1987)has a more chequered history. Located by the side of the Rajrappa River and the holy temple of Kshin Mastika, it is the most popular visiting place of Bureaucrats and Political hobnobs. Unfortunately, soon after commissioning, the river water became blackish and fine coal particles settled on the river bed. As the coal was non-coking and less matured than coking coal being washed in most other washeries, the problem of effluent was more serious. It was subsequently tackled by creating shallow unlined lagoons, from which the fines bearing good market value could be manually recovered.
The washery has one novel equipment. Raw coal crushed to 1200/200/80 mm was screened at 10mm. both the coarse and the smalls (after passing through a deduster and deslimmed at 0.5mm) were washed in separate 4-P Batac Jigs, perhaps an infrequent equipment t in the world. The idea was to recover the cleans misplaced in the middlings II, due to higher Probable Error of the Jig. The middlings II of the coarse coal Jig was also crushed to -10mm, mixed with the middlings II of the smaller coal jig and were proposed to be washed in HM Cyclones. Unfortunately, the NGM value of this product was beyond 80%, which made it formidable for further recovering cleans.
Rajrappa washery also has flotation banks and vacuum filters for recovery of fines and a vortex thickener for tackling the tailings.
Kedla washery was the latest coking coal plant for CCL. OCP coal was crushed to 200/75 mm and screened at 13mm. The coarser coal was sent to Batac Jigs and the smalls were passed through Deduster and deslimming screens before putting into separate Jigs. The middlings of the coarser Jig was crushed to -13mm mixed with the middlings of the small coal Jig, deslimmed at 0.5mm and washed in HM Cyclone. Both dewatering screens and Centrifuges are used to dry the coal.
The slurry is froth floated, cleans filtered and the tailings thickened in Vortex thickener.
Pipawar (1997) washery is the most modern designed “new gen” plant to wash more than 6.5 Mtpy. it was constructed within a few years only, with the help of total digital drawings, prefabricated tubular structure and strict disciplines in checks and measurements. The sides are open and non-cladded; computerized control room is in a separate RC column building (to make it free from vibration, with an isolation gap of some 50mm from the main plant). From the control room the whole material handling sections are visible, unlike most of the earlier designs having the control room ”window less” and with “wall to wall mimic panels, in an artificially illuminated room, feeling all the vibrations of the screens and crushers.
The circuit is most simple. OCP coals are crushed to 200/100 mm, screened at 20mm and the coarse coal is washed in a Jig. The slurry from cleans dewatering screen is thickened in Hydro Cyclones and its underflow after dewatering in high speed screen is mixed with the cleans. The overflow of the thickening cyclone goes to settling cone and slime ponds to re-circulate the wash water.
The cleans ash is controlled depending up on the ash content of the smalls (-20mm), so as to guarantee the overall product quality. However, no online sampling system for these smalls was provided, which could help in making quality control easy and dependable.
As the first non-coking coal washery, Bina (1999) was constructed by Humboldt Wedag at Singrauli coalfield. The novelty of the plant is to wash “rom coal” size -200mm by a rocking pan Jig, which separates cleans and rejects into two streams while the coal is winnowed on an inclined screen, dipped in water. The most interesting part is the low water consumption (8m3/h) and almost no slurry is thrown out of the plant. The plant supplies cleans to Dadri TPP, near Delhi. Though the plant construction was completed in time, it remained idle for a few years, because to NTPC could not decide on price of washed coal. But, the plant broke the ice subsequently and has been satisfactorily supplying washed coal to Dadri.
Almost at the same time, with USAID collaboration, CLI installed a cyclone washery at Dipika, where raw coal is crushed to 50mm and after screening at 15/13/10 mm is washed in 1000 mm dia Cyclone washers. It may be noted that as the sp.gr.of media is normally maintained lower than 1.8 or so, the ash content of coarse washed coal from the Cyclones is likely to be 25 to 28%,which exactly matched with the unwashed screen fraction which could have ash content slightly more than 35 t 36%.the washery had for sometime problems with disposal of sinks, however, ir paved the way for installation of Cyclone Washers for washing of no-coking coal in India.
The only washery in WCL is at Nandan project (1984) constructed by Voist Alpine , Austria and ICB. In fact this is a pocket of coking coal in midst of all around non-coking coal in WCL. The washery has simple circuit: crushing raw coals, mostly from U/G mines, to 75mm, screening at 10mm, washing both the coarse and small coals in 3-P Batac Jig, with usual deslimming at 0.5mm.The slurry is thickened in thickeners, floated and vacuum filtered. The tailings pass through High Speed Screens.
What is unique in Nandan Washery is the automatic sampling, sample preparation – even up to bottling in size -3mm, by a series of crushers, dividers, etc, untouched by human hand.
There are two more washeries in the captive coking coal sectors – Chasnalla at IISCO and DSP washery at Durgapur. Chasnalla was constructed in 1969. Raw coal comes generally from U/G mine and is supplemented by OCP. Coal from Jitpur U/G mine comes by Ropeways. Raw coal is crushed to 75mm and used to be dry screened at 6mm and like most of the old washeries of JCF, the fines had acceptable ash content for mixing with the cleans. The dry screen was difficult to be kept unclogged, even with heating arrangement. The coarser fraction was washed in HM Bath.
Subsequently, the ash content of fines increased beyond acceptable limits and the whole washery was re-constructed by incorporating Batac Jigs for -20mm coal and froth flotation of slurry. This is the first flotation plant where a U-tube (Coriolis effect) type Mass Flow meter was installed to monitor the flow and solid content of the slurry, so as to proportion the reagents dose systematically. Chasnalla washery was also equipped with an automatic mechanical sampler, sample processing crusher/ divider / bucket elevators and Nucleonic Ash Monitors. A secondary HM Bath operating at high gr. rewashes the primary HM Bath’s sinks to recover sweetened middlings. Thus, the real rejects have very high ash making it suitable for U/G sand stowing. Subsequently, spirals were also installed to reduce the load on flotation and Jigs, by feeding the coarser fines (3mm-0.4mm) to spirals.
The cleans and middlings are partly dispatched through Rope Ways, rest by road.
Durgapur washery (1968) meant for washing Central JCF coals (linked by ER) supplies cleans to the Durgapur Steel Plant. Raw coal passes through Rotary Breakers to eliminate the hard stones and stored in Silos (6 nos x 1200t). After primary screening at 13mm, the coarser fraction passes through primary and secondary HM Baths. Smalls (13-0.5mm) is washed in Feldspar Jig and -0.5mm is processed in Classifying Cyclones and dried in Vacuum Filtration.
After more than 50 years of practicing washing of Indian coals, which used to be considered all through previous 50 years as “un-washable” (like un-touchables..?), it was observed that the challenge of utilizing indigenous coking coals for the steel plants, as a self-sufficient measure, was successful till 1984. Since then, when the global perspective changed to self-reliance, coking coal started to be imported.
The real breakthrough in utilizing high ash difficult-to-wash but sufficiently matured (Ro = 1.15 to 1.25 or even 1.3 and low Sulphur) Gondwana coals was the concept of limiting Characteristic ash at the point of cut to about 25%, beyond which the individual coal particles lose their intrinsic coking property. This is a universally proven fact, even in Europe or America, the limiting Characteristic ash content at the point of cut for Steel Plant use is at the same level. That means that all the particles having Ch. Ash <25%, included in the cleans, contribute to the coking propensity, even if the overall ash content is 17% in India or 8 to 9% in Europe or USA.
Thus, there should be no laxity at producing washed coking coal from matured seams of JCF and CCL areas, for the sake of energy security and cost control. The remaining requirement for Steel Plants, for further sweetening to 15% ash or so, may be imported, considering matching carbonization properties.
One more aspect becomes visible. There are three schools of coal washing practice in India for washing coking coals:-
Dhanbad School (BCCL/CCWO/CFRI/ISM, etc.): HM washing, either all Cyclone or Bath / followed by small coal Jig or Cyclone and flotation of fines (which is also internationally practiced, by Australia and South Africa, the other two countries dealing with Gondwana coals) ,
Ranchi School (NCDC/CCL/CMPDIL, etc): Modern Jigs for both Coarse and small coals with Deduster where ever possible, and flotation (Generally practiced in Germany)
Tata School: Crushing coal to 20mm, HM Cyclone and flotation.
Before discussing non-coking coal washing, it may be mentioned that several Mini Flotation Plants are also working successfully in and around JCF, beneficiating the slurry recovered as effluent from the washeries mentioned above. The first plant (5 tph) was installed in 1994. Since then plants of 10 and 15 tph have also come up. The plants are low profile, all equipment are positioned / supported from the ground floor, flotation cells are individual sub-aeration type, vacuum pumps are used in place of vacuum legs for taking out the filtrate, etc. The water circuit is “closed”, as these are located by the side of G T Road, near the coke ovens. These plants (about 20 in number) have a cumulative capacity build up of 150 tph =0.6 Mtpy. The flotation concentrate (ash 14 to 15%) is dried by Belt Discharge Vacuum Filters with belt washing facilities. M/s IISCO made a trial of using the 1000 tpm of these flotation concentrate to substitute imported coals by the same quantity.
Clean Coal for Power Plants – Where does India Stand?
About 500 Mtpy is the production of non-coking coal at present. Thanks to MOEF ruling of compulsion in reducing the average Ash content of Coal to 34% for Power houses situated at a distance of more than 1000km from the dispatch end, and also for Thermal PP located in strategic areas, like cities and environmentally sensitive areas, the setting up of non-coking coal washeries has created no less than a “Clean Coal Revolution” in India.
Cumulative Annual Growth Rate (CAGR) for most of the non-coking coal reach coalfields in the installation of Coal Washeries has crossed all time record by a significant extent. We know that if CAGR for any industry is consistently more than 12%, the growth becomes Auto Catalytic. That means, the industry becomes self sufficient in creating further investment without external assistance.
I am honoured to mention that Singrauli and Singereni Coalfields have shown a CAGR of 25.7 to 29.4 % in last one year, where as Pench Kahnan & Wardha and Korba & Sohagpur registered CAGR of 33.4 and 43.4% respectively during last 8 to 10 years. Even Ranigunge Coalfield, which has not been industrially fast developing in other fields, showed CAGR of 35.7% during last 3 years. Talcher and Mahanadi Coalfields, with all round infrastructural developed industrial scenarios, recorded CAGR of more than 54.8% during last 9 years. Overall Indian CAGR for Power Coal Washery capacity build up is more than 34.3% for the last decade.
This is remarkable and we hope to further strengthen by installation of a series of 16 washeries (capacity 85 Mtpy of raw non-coking coal) envisaged by Coal India Ltd under BOMO contract.
Following Power Point slides nos 1 to 9 present the detailed information viz CAGR for non-coking Coal Production and Washing Capacity Build up of major coalfields of India.
Technical Problems Envisaged for washing Non-coking coals.
Non coking coals available in India are of high ash (40 to 50% or more), less matured (Ro = 0.8% or less). Thus, they are less hydrophobic in particulate surface character than that of coking coals. Though, it is generally preferred to wash only the coarse size of coals and mix up the smalls (less than 20/15/10 mm) along with the washed coals, with a view to keep the moisture content of the supply within acceptable limits (less than 10 to 12%), the production of effluent due to degradation of coal particles by attrition with others and mechanical impacts in channels and launders, is apprehended to be significant.
It has been found that 1 to 1.5% of the feed to the washery may be produced as ultra fine particles as effluent (size less than 5 micron) whose quantity may be as large as 30 tph for a 10 Mtpy washery. Over the year it can produce as much as 1.5 lakh tonne. As the water loving functional group on the surface of these ultrafine particles is high, they are likely to remain in Brownian movement in the effluent, and as the surface charge of these particles (Zeta potential) is negative (-), it will be difficult to coagulate or floc them without high dose of costly flocculants.
It is estimated that the existing washeries (120 Mtpy capacity build up) might have already impounded at least 1.8 Mt effluent in different lagoons and low lying areas in and around coalfields, if they have not been manually recovered. With the envisaged washing capacity of about 200 Mtpy by next 5 years or so, the effluent production may cross 3 Mtpy. If not tackled at the washery sites, they can spoil the surface water bodies like the Damodar, Mahanadi, Son and even tributaries of the Ganges and Yamuna rivers.
Disposal of Washery Wastes
The main objective of the washeries is to separate the stones and high ash shales from the raw coals. The stones are generally coarse grain sandstones and shales, degrading to argillaceous clay. As some of the stone layers form partings of variable thickness in between coal seams, it is always possible to lose some coaly matters along with the wastes. The carbon content of these rejects can be as high as 10 to 15%, which imparts them a blackish color not favorable as building materials.
The generation of rejects from washeries in CIL in 2004-05 was 2.44 Mt . Accumulated stock of washery rejects up to March’05 was 18.15Mt . Presently, with the washing capacity of more than 80 non-coking coal washeries, having a total input capacity of more than 120 Mtpy, expected production of washery rejects can be as high as 24 Mtpy. Disposal of this huge quantity of rejects in an environment friendly manner poses a real problem.
Power Generation from Washery Rejects
As the washery rejects may have 1350 to 2000 kCal/kg, about 80 to 90 MW power can be generated from every Mtpy of rejects.(ref 3)
Possible Location & Capacity of FBC/CFBC Power Plants:
North Karanpura, (Jharkhand)Setting up of about 110 MW unit utilizing about 1.3 Mtpy of rejects of Piparwar washery(6.5 Mty) and an another 60 MW unit utilizing rejects of about 0.7 Mty from proposed washery(3.5 Mty) at KD Hesalong.
Talcher, (Orissa)2 X 110 MW FBC/CFBC plants may be installed Kalinga and Ananta-Bharatpurin Orissa state.Dipka/ Korba, (Chattisgarh)Installation of 2 X 110 MW FBC/CFBC power plants for utilization 3.2 Mty rejects from existing washeries at Dipka(5.0 Mty), Gevra(6.0 Mty) and Korba(5.0 Mty) in Chattisgarh State. Modalities for installation of these power plants can be worked out for US investment. (ref).
Quality Requirement Of rejects for FBC/CFBC Power Plants
GCV of rejects required for efficient burning in FBC/CFBC plants should be in the range of 1360 –2000 kcal/kg. Washery rejects from Orissa& Jharkhand have been tested & found suitable for such plants Likely Cost Of Generation (less than Rs 2 per kwh)
CIL has set up 7nos of 10 MW each FBC based power plants using washery rejects at various locations. Cost of generation from these plants varies from Rs.2.5 to Rs.3.5 per kWh due to operation of these plants in isolation mode at low PLF.The generation cost will reduce and will be comparable with other thermal power plant with bigger unit of FBC based plant as proposed above and operation of plant in parallel with the grid.
Improvement of Environment by application of Non-coking coal washeries
One US Study projected for Year 2010 that, the potential benefits of coal preparation include reduction of the followingref 3:-
Ash disposal at power plants (78 Mtpy)
Greenhouse gases (30 Mtpy)
Particulate emissions (220,000 t/y)
SO emissions (440,000 t/y)
Land requirements for ash disposal (62,000 ha)
Coal Preparation – Quo Vadis?
It is an ambitious task for CIL to install 19 washeries in next 5 or 6 years. The choice of circuit will depend upon the washability of coals. As most of the coals from major sources are difficult-to-wash, as revealed by CFRI studies, and selection criteria like highly efficient performance, set by CIL are stringent, it is most likely that either efficient modern jigs or Large Diameter Cyclones or HM Baths may get initial preferences, though there could have been several other choices like Barrel washer, Hydro separator, Larcodem, etc.
As an example, it is highlighted that every 10 Mtpy washing capacity build up requires either:-
Thus, it is stressed that manufacturing / import of so many robust and efficient washers within such a short time is an involved task.
From the historical perspective it is observed that the Coal Preparation Engineering Institute was created at Ranchi in 1980s with a view to handle this enormous, gigantic and huge task. It is most unfortunate that more than two decades were lost in deciding who would take the onus of washing raw coal. With the passing of Pollution control bill in 19 and restricting the ash content of distant Power station feed to 34%, the good sense has prevailed that both the producer and the consumers are equally responsible for creation of the intermediate link of coal washing in the mine to power plant chain.
However, no organization can single handedly tackle such a behemoth task. CIMFR in Dhanbad is equipped with world’s biggest and best Pilot plant (funded by CCDA/MOC) to conduct crushing / liberation matrix and washability studies required for tackling most-difficult-to-wash coals. It is particularly required because green field washeries are to be installed in many places and the process data required for designing the large washeries are not readily available. ISM, IITs (KGP and Kanpur), BESUS and other engineering colleges are good in Mathematical modeling and theoretical guidance required for practical optimum designs and conducting laboratory physico-chemical studies to solve “new” pollution problems (could be funded by SSRC/ CIL R&D fund).
CIL has also envisioned resurrecting MAMC, Durgapur which was the store house of design and production of coal preparation equipment. Already turned around HEC, Ranchi can help in manufacturing large crushers including gyratory crushers of capacity 1500 to 2000 tph (mouth 1500 mm), and private large corporate houses like McNally Bharat / Wedag/Sayaji, TRF etc have reputation in construction of CP equipment and accessories.
What is most required is a synchronized and synergistic activity (like that of a Government Regulatory Commission) with the missionary zeal of cleaning coal to save the world from Green House Carbon activity. The penalty for not reducing the CO2 production, by improved combustion or at a later stage by sequestration, will be so high ( as already in Europe), that the cost and affliction of proper Coal Preparation will seem to be trifling.
Acknowledgements:
The author gratefully acknowledges the help and suggestions received from the Scientists of CIMFR, namely Dr Asoke Singh, Shri Asim Chowdhuri and Subhashis Biswas , Dr N.S.Das (Retd) and Shri Sachin Chattopadhyaya (Retd. CGM ,Engg & Washeries, CIL) and Shri V.K.Sahay, GM(Washery),CCL and Shri Chandan Das, CSIR-SRF, Mining Dept, BESUS.
Bibliography
1.Ghosh A.B: The Economics of Coal Washing in India, New Age Publishers Private Ltd, Calcutta- 12:New Delhi,1964.pp 35 & 36 .
2. Staats Gary, Rao Nagaraja and Gollakota Sai : The Status and Future of Coal Preparation in India: Coal Liquefaction & Solid Fuels Contractor’s Review Conference, Pittsburgh, Pennsylvania, 1997 (www.netl.doe.gov)
3. Coal Washing & Power Generation from Washery Rejects: 2nd Indo-US Coal Working Group Meeting, Washington, November, 2005.
4. Coal Statistics, Coal Controller’s Organization, GoI, Ministry of Coal, Kolkata
5. Coal Production Data: Directorate (Technical),Coal India Ltd.