Lignite Mine Report

Neyveli Lignite Corporation Mine Visit Report

Luke Rogers

Overview

            Neyveli Lignite Corporation (NLC) was formed in 1956 by the Indian Government for the purpose of finding lignite and harvesting it to help meet the growing power needs of the country. From an initial capacity of 3.5 million tons (MT) in 1956, NLC has now grown to lead the lignite industry with a total capacity of 30.6 MT in four separate mines as of April 2011.

            The newest NLC mine was opened in 2006 in Rajasthan. The first the three mines, though, were excavated in the state of Tamil Nadu, which, along with Pondicherry, contains 81 percent of India’s lignite. The nation’s annual production capacity of coal and lignite is 530 MT and 36MT, respectively.

            Fifty-five percent of India’s electricity is generated from coal or lignite. NLC’S total power generating capacity reaches 3240 MW, which constitutes almost two percent of the total 182,344 MW installed capacity nationwide. 37 percent of this power remains in Tamil Nadu, while the rest is distributed to other southern states like Andhra Pradesh, Kerala, and Karnataka.

Lignite

So what is this lignite material anyway? It is mined like coal, and burned like coal, but listed in a separate category by most energy professionals. It turns out that lignite is just a younger relative in the coal family, and even referred to sometimes as brown coal. The Neyveli lignite, despite its relatively young age, originates 25 million years ago in the Miocene age.

            Like other coal varieties, it is formed from the decay of organic plant material, which becomes peat, and is the metamorphosed by pressure within the earth as it is buried over time. Lignite is simply a less dense form of coal that has not experienced as much pressure. The Neyveli lignite is made of a wide variety of dead plants, primarily conifers.

            Lignite contains more volatile compounds and less carbon by mass than higher grades of coal. NLC lignite contains 65%-70% carbon, 20%-25% oxygen, and small amounts of sulfer and nitrogen. The heat content per kilogram is 28,470 kilojoules, which is lower than all other coal types. Unfortunately, this means that more lignite by mass must be burned to supply the same amount of power as higher grade coal, and it generally causes more carbon emissions per megawatt. The high content of volatile compounds, though, makes lignite readily combustible, and it leaves relatively little ash behind. Lignite also has a high moisture content, which increases its weight per kJ of potential energy. Its low ratio of energy to weight and volume means that lignite is rarely transported far from its source for use. Worldwide, most lignite mines have power plants beside them, and this is true of the NLC locations.

Another quality of lignite complicates industrial processing. Lignite has a low temperature of combustion, and when it is stored in large quantities aboveground, bacteria fermentation can generate enough heat to start it burning spontaneously. To avoid this danger, the lignite at NLC is never stored for more than 15 days before use in the power plant. The head geologist mentioned the stress caused by this prohibition of extensive stockpiling. The power plants must run constantly to provide reliable electricity, and even brief lapses in service jeopardize their contracts. The mines, therefore, must also work constantly to prevent serious problems for everyone in the lignite-to-energy production line. The mine operates 24/7 between the efforts of three shifts of workers, and equipment breakdown is the greatest concern of managing officials.     

           

Geology

Mine II, which we entered in our visit, was started in April 1981 with a capacity of 4.7 MT annually, and expanded to 10.5 MT/A in February 1983 before the first lignite was exposed in September of 1984. Its current annual capacity is 15MT. The three year delay is caused by the geology of mine II.

Before mining lignite, the workers must first remove up to 100m (over 300ft) of useless material called overburden. It starts with young alluvial clay on top, and all other layers down to the lignite date back to the upper Miocene era. These consist of sandstones, clays, and then semi-confined aquifer sand. The sandstone is about forty percent quartz and over fifty percent clay, with small amounts of feltsbar. A lignite seam about 14m thick is found below these layers, followed by a smaller one about 4m thick. The lignite is fairly devoid of geologic disturbances, and dips deeper into the earth towards the Southeast.

Water Issues

Confined aquifer pressure is the major geologic obstacle faced by NLC. The buildup of ground water below the lignite threatens to burst through and flood the mine back up to the water table. The water pushes upward with a pressure between 500 and 1200 kN per square meter. Water is pumped out from mine II at a rate of 130 cubic meters per minute from about forty strategically- placed wells to attenuate this hazard.

Rainwater collection in monsoon season necessitates additional pumping capacity. Each mm of rainfall creates about 10,000 cubic meters of runoff influx into the mine pit. Specially designed pumps on pontoons are used to transport this water to canals on the surface. It is tested monthly for particulates and other contaminants, and purified as necessary. This water provides the source for the large boilers in the lignite power plants.     

            Groundwater monitoring and recharge efforts receive much attention at NLC. The pumping to relieve aquifer pressure causes many concerns of groundwater depletion and salinization through encroachment from the nearby Bay of Bengal. A 400 square meter recharge area in the villages around the mines is monitored by the geology and exploration department at NLC. They practice conservation by minimizing power use in the mines, minimizing pumping from the aquifer, and promoting water recycling in the villages.

            The recharge efforts are an elaborate project that enlists the collaboration of IIT Chennai to build a system of dams, ponds and wells. The dams and ponds provide locations for surface water to collect and reenter the Neyveli aquifer. These collection structures often incorporate wells at their bottoms to aid the percolation of water downwards. Pressurized pumping is also used to move water into the ground from the surface.

             

Scale and Direction

Mine II’s fifteen million ton annual production capacity is achieved by a fleet of monstrous earth-moving machines. They gouge the earth with a tiered approach that creates five different “benches,” or shelves leading down to the lignite layer. Each bench is equipped with at least two bucket wheel excavators, which look something like a waterwheel spinning in reverse to claw into a vertical rock face. The buckets come in either 1400 or 700 liter capacities, depending on the model of the excavator. The machines, purchased from a German manufacturer, are shut down daily for a maintenance and inspection period. Their heavy loads are deposited onto a conveyor belt system that stores them outside of the pit until it is ready to be refilled.

The pit itself covers only about 5.5 square km at any given time, but the total planned scale of mine II is over 40 square kilometers. This is because the pit is constantly moving in a wide circle around the mining area, with the excavators advancing the leading edge as the trailing one is filled in. The cycle is expected to continue beyond 2030 before all of the planned area is mined.  

           

Power Generation

The lignite recovered from mine II is used exclusively to fuel Thermal Power Station II. The station consists of seven units of 210 MW each for a total capacity of 1470 MW. The complex was constructed in two distinct stages, with each of the seven units coming online individually between March 1986 and June 1993. The power plant boasts state of the art management software and technology for maximum efficiency. For many years it has stood as the largest lignite fired thermal station in Asia.

An operation the size of plant II generates lots of potential hazardous waste products. For one, the lignite combustion can generate pollutants in the exhaust that escapes the plant. NLC continuously operates six different air quality monitoring stations to address this risk. As a government-controlled institution, it adheres strictly to the standards set by the Central Pollution Control Board.

Large quantities of lignite ash are generated by combustion in the power plant. This ash can be used as a building material by mixing it into cement, and some of it is sold to outside parties for this purpose. NLC also manufactures lignite ash bricks and blocks on site, and some of these can replace wood products, which prevents large amounts of timber felling. 

Land Reclamation

Despite alternative uses of lignite ash, large quantities have been stored in pond structures. These tend to leak contaminants into the environment due to imperfect containment techniques. So far, 11.5 acres of ash pond have been reclaimed and restored by the planting of 3500 trees.

Additionally, since the mine pit is constantly moving and being refilled, NLC is already hard at work restoring tracts of the ecosystem it has destroyed. After mining is finished, overburden is used to refill the pit back to slightly higher than ground level. One of the geologists explained to us that after several years this expanded rock and soil will compact back down to its normal level. When that stage is reached, organic, inorganic, and biofertilizers can be applied to restore the land to agricultural productivity. Various crops are now being produced on about 250 hectares of already mined land throughout the NLC properties.

NLC directs afforestation efforts in the mined areas as well, using scientific assessments to select the most advantageous trees and shrubs to reestablish nutritional cycles in the soil. According to the head geologist, it takes 15-20 years for a mined location to fully recover from an open pit to a functional ecosystem. Reclamation efforts are currently mandatory for open cast lignite mines. In fact, anyone applying for a mining permit must also submit reclamation plans and budgets – long before any mining even takes place. NLC seems to have surpassed the minimum requirements, extending its reforestation efforts to areas surrounding its employee township outside the mine.

Reflection

            Primarily, I didn’t realize the extent of engineering that is required to safely operate a mine of this type. The open -cast technique looks just like a mindless task of digging a big hole, especially compared to shaft mining, oil drilling, or even blasting through hard rocks. I realize now that an intimate understanding of earth systems is required to run such an operation effectively. I was also impressed with the environmental consciousness of NLC’s presentations and practices. I wonder if this is case where government ownership delivers better results than liberalization would achieve.