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stone overburden. According to a paper presented by two authors from NLC at a meeting in the late 2000s, typical strata cutting resistance at Neyveli was 15-20 MPa (2,100-2,800 psi), compared to con- ditions of 7-10 MPa that were then found in Germany's lignite mines, and on which the machine designs had been based. Downtime for changing excavator bucket teeth was excessive. And not only were teeth wearing out in as little as 3.5 hours, but the shock loading from digging this material was causing extensive structural damage to other machine components. NLC was forced to undertake major modifications to the teeth, buckets, booms, slewing mechanisms and drives, among other things. As the authors noted in their paper, the modifications resulted in sub- stantial performance improvements, while "the experience gained helped in arriving at the present set of equipment in which most of the problems have been eliminated." Expansion From the 1980s With Mine I established, in 1978 the Indian government gave the go-ahead for NLC to develop Mine II, initially with a capacity of 4.7 million mtpy, but soon increased to 10.5 million mtpy. Although located only 5 km from Mine I, overbur- den removal for the new operation threw up a different set of challenges, as well as those involving the hard sandstone. Here, the surface alluvium overlying one area of Mine II consists of a sticky clay that expands when it gets wet, becoming soft and plastic. It stuck in the excavator buckets, clogging them and causing extensive soil spillage. Again, NCL found remedies through lining the buckets and material chutes of machines working in this material with HDPE teflon sheets, and using open- backed buckets with chains to knock the clay free. Mine II began producing lignite in early 1985. The next stage of expansion came in 1998, with the development of Mine IA at a capacity of 3 million mtpy. Output from this mine feeds a local private-sector company's power plant, and also helps balance NCL's feed to its own generating stations. Mine IA came into production in 2003, with a capacity increase to the cur- rent 15 million mtpy at Mine II coming on stream in 2010. As for the future, NCL is currently in the process of updating both its lignite- fueled power plants, with civils works for a new 2×500-MW station to replace the original Thermal Power Station (TPS) I — commissioned in the 1960s — having started last year. Meanwhile, expansion of the second existing station has involved the construction of two 250-MW circulating fluidized-bed units, which came on stream in March — six years behind schedule as a result of boiler tech- nology problems. Completion of these projects will increase its total generating capacity at Neyveli to 3,390 MW, to which the company is adding 10 MW through the construction of a solar power plant. In terms of fuel, the previously com- pleted Mine II expansion was designed to supply the additional requirements for TPS II, while a restructuring of operations in Mines I and IA will provide an addi- tional 1.5 million mtpy for the new TPS I. Capacity at Mine I is being cut to 8 mil- lion mtpy, while that of Mine IA is being increased to 7 million mtpy, at a cost of some $250 million. The Mining System According to a presentation made by NLC's chairman and managing director, Shri B. Surender Mohan, at a workshop on technology development and the mecha- nization of mines, held in New Delhi in January, the company currently operates 29 BWEs. Of these, 13 are fitted with 1,400- liter (50 ft 3 ) buckets, with the remainder at 700 liters. At the other end of the overbur- den-removal system, there are four 20,000-mtph (22,000 tph) spreaders, seven capable of handling 11,000 mtph and four rated at 5,000 mtph. Mohan described the working system used at Mine II, where five overburden stripping benches have a total capacity of 78 million m 3 py (102 million yd 3 ). The top four benches are each 25 m (82 ft) high, with the lowest overburden bench at 18 m (59 ft). In sequence from the top, the first three benches are each equipped with two 1,400-liter bridge-type BWEs jointly feeding a 20,000-mtph spreader. The bench below that has a single 1,400- liter machine that feeds an 11,000-mth spreader, while the bottom bench uses two 700-liter BWEs feeding another 11,000-mtph spreader. Lignite seam mining, using an 18 m- high bench, is carried out using four, 700- liter excavators. In his presentation, Mohan explained that a 1,400-liter BWE working 4,000 hours a year has the capac- ity to move 9 million m 3 of overburden — a volume that would require a fleet of 26, 10-12 m 3 mining shovels and 230, 100- mt-capacity haul trucks to accomplish. Not only does NLC face challenges with the hard, abrasive overburden, but also with materials handling and water control. The stripping ratio is high, at around 6 m 3 of waste per metric ton of lignite, while the presence of a pressur- ized artesian aquifer beneath the lignite seam means that the operation has to pump 8-10 m 3 (2,100-2,650 gallons) of water per ton of fuel mined. With the aquifer exerting an upward thrust of 5-8 kg/cm 2 (70-110 psi), depressurization through permanent pumping is the only way of preventing the seam from heaving once overburden has been stripped, Mohan told the meeting. i n d i a n l i g n i t e c o n t i n u e d July 2015 www.coalage.com 47 Neyveli Lignite operates 29 bucketwheel excavators in India.