Lignite coal is a natural resource which is readily available. It is often referred to as brown coal. It has some special characteristics which make it different from other coals.
Lignite coal is a soft, brown, combustible, sedimentary rock formed from naturally compressed peat. It is considered to be the lowest rank of coal due to its relatively low heat content. It has lowest carbon (C) content amongst all types of coals. It is mined all around the world and is mainly used as a fuel for steam and electric power generation. Since it is not economical to transport lignite coal, it is not traded extensively on the world market when compared with higher grades of coal.
Large reserves of lignite coal are available in limited areas of the world. Australia, USA and China have the major reserves of lignite coal. Germany has the largest number of power plants based on the lignite coal. In USA, most of the reserves are located in the North Dakota province while in India, the lignite coal reserves are in Neyveli in Tamil Nadu and in Rajasthan. Around 17 % of the world’s coal reserves are lignite coal. As the world’s oil and gas reserves decline, other sources have become attractive. That is why there is a sustained interest in the use of lignite coal.
Coals are classified by rank according to their progressive alteration in the natural metamorphosis from lignite to sub bituminous coal to bituminous coal and to anthracite. Coal rank depends on the volatile matter, fixed carbon, inherent moisture, and oxygen, although no one parameter defines rank. Typically coal rank increases as the amount of fixed carbon increases and the amount of volatile matter decreases.
Coal is a complex combination of organic matter and inorganic ash formed over eons from successive layers of fallen vegetation. Coals are classified by rank according to their progressive alteration in the natural metamorphosis from lignite to anthracite. The process of coal formation from organic compounds includes two distinct stages namely (i) biochemical, and (ii) geochemical. In the biochemical stage the plant material had first converted into peat and then to lignite while in the geochemical stage the lignite had first converted into the bituminous coal and then to anthracite.
The process of conversion of organic substances into coal involves three phases consisting of microbiology, chemistry and physics. During the microbiology phase plant remains sink in a body of stagnant water. Mud and sand are deposited on top. The flow of oxygen is cut off. Anaerobic bacteria convert lignin and cellulose into humus. During the phase of chemistry, humic acid develops. Polymerization takes place. The pH level drops. The resulting brown substances are high in acid. Conversion into peat begins. During the phase of physics the peat is compressed and dehydrated under the weight of sands and glaciers. The conversion to lignite coal is reached at a water content of less than 75 %. Increasing depth, pressure and temperature determine the carburization over the course of time.
Lignite coal geology and mining
As per the history of coal formation the age of the peat ranges from present to 2 million years, the age of lignite coal from 2 million years to before 70 million years, the age of sub-bituminous coal is before 100 million years and the age of bituminous coal is between 200 million years to 300 million years. Lignite is the first product of the coalification process and has properties intermediate to those of bituminous coal and peat.
Lignite coal begins as an accumulation of partially decayed plant material, or peat. Burial by other sediments results in increasing temperature, depending on the local geothermal gradient and tectonic setting, and increasing pressure. This causes compaction of the material and loss of some of the water and volatile matter (primarily methane and carbon dioxide). This process, called coalification, concentrates the carbon content, and thus the heat content, of the material. Deeper burial as well as the passage of time result in further expulsion of moisture and volatile matter, eventually transforming of the material into higher rank coals such as bituminous coal and anthracite. The geological processes for formation of lignite and coal are shown in Fig 1. Lignite deposits are typically younger than higher-ranked coals, with the majority of them having formed during the Tertiary period.
Fig 1 Geological processes for formation of lignite and coal
Lignite is normally mined by surface mining. In surface mining, the material lying on the mineral deposits are removed, and the underlying mineral deposits are exposed for extraction. The stripping of the overlying material is done using draglines and motorized scrapers. Once lignite is exposed, it is extracted with a hydraulic backhoe and loaded into trucks before being transported.
Since lignite coal veins are located relatively near the surface, it means that underground excavation in tunnels is not necessary and there is no risk of methane or carbon monoxide buildup, a primary safety concern in underground mining.
Characteristics of lignite coal
Lignite coal is generally yellow to dark brown combustible material. It has a low calorific value, which means that transporting it over any significant distance is uneconomical in comparison to other types of coal. It has some special characteristics. These characteristics vary from mine to mine. These characteristics make burning of the lignite coal different from burning of the bituminous coals.
Lignite coal usually has carbon content in the range of around 60 % to 70 %. The ash content of lignite coal generally ranges from 6 % to 20 %. The volatile matter (VM) content of lignite coal is high. On an ash and moisture free basis, the volatile matter percentage is in the range of 45 % to 55 %. The high content of volatile matter makes it easier to convert it into gas and liquid petroleum products in comparison to higher ranking coals.
Lignite coal has high content inherent moisture which is sometimes as high as 70 %. Moisture content of coal causes many difficulties during processing, storage, transport, grinding, and combustion. The high moisture content considerably reduces the CV and combustion efficiency of the coal. It also results into higher heat loss in the exhaust gas.
In the combustion of lignite coals, the important part of the energy is consumed to evaporate the moisture inside the coal. The combustion of the high moisture content coal creates several problems such as the additional energy consumption for the moisture evaporation, the insufficient combustion and the additional exhaust discharge etc.
Moisture content of the lignite coals can be classified into the following three types.
- Surface moisture – It is also known as external moisture. The moisture adheres to the surface of coal particulates or in the bigger capillary cavities. It is the moisture, which can be removed by the coal drying in air at ambient temperature (around 25 deg C). It depends on water conditions in deposit.
- Inherent moisture – It is a naturally combined part of the coal deposit. It is also called hygroscopic moisture which is the moisture in the air-dry state. It is chemically bound water, so called constitutional and intermolecular water. The hygroscopic water content decreases with the increase of the rank.
- Crystallized moisture – It is the chemically combined with the mineral matters in coal. It is also called decomposition moisture and is the water which is formed during thermal decomposition of the coal.
The drying or dewatering of the lignite coals decreases the problems caused by the high moisture content. This reduces the burden on the coal handling system, conveyers and crushers. Also, since dried coal is easier to convey, this reduces maintenance costs and increases availability of the coal handling system.
The calorific value (CV) of lignite coal is low. It generally varies in the range of 2000 kcals/kg to 4600 kcals/kg. This value of the calorific value is on a moist, mineral-matter-free basis.
Lignite coals are usually shallow buried facilitating its easy open mining. These coals besides high moisture content also have high volatile content and low calorific value (CV) with easy spontaneous ignition. High moisture content is the main restraint for the application of lignite coals.
The sulphur content of lignite coal usually low. It depends on the nature of the deposit and in some deposits it can be up to 1 %. However it creates issues if it is in the form of ferrous sulphide since it lowers the ash fusion temperature to around 900 deg C. This gives the coal high potential for slagging.
Lignite coal is softer than bituminous coal, with a Hardgrove grindability index (HGI) in the range of 100 making it easier to pulverize.
As like any other coal, lignite coal is also an organic rock (as opposed to most other rocks in the earth’s crust, such as clays and sandstone, which are inorganic). It contains mostly carbon (C), but it also has hydrogen (H), oxygen (O), sulphur (S) and nitrogen (N), as well as some inorganic constituents (minerals) and water (H2O).
Lignite coals are normally of two types. The first type is ‘xyloid’ lignite or ‘fossil wood’ and the second type is the compact lignite or perfect lignite. While xyloid lignite may sometimes have the tenacity and the appearance of ordinary wood, it can be seen that the combustible woody tissue has experienced a great modification. It is reducible to a fine powder by trituration, and if submitted to the action of a weak solution of potash, it yields a considerable quantity of humic acid.Leonordite is an oxidized form of lignite, which also contains high levels of humic acid. Jet is a hardened, gem-like form of lignite used in various types of jewelry.
Uses of lignite
The high moisture content and susceptibility to spontaneous combustion as well as relatively low energy density of lignite coal can cause problems in transportation and storage. Hence, the coal is generally used close to the mines (known as mine mouth operations). This is done to get the most output from the coal, without spending too much energy on transporting it. In these power plants the coal is burned and used in industrial boilers.
An environmentally beneficial use of lignite can be found in its use in cultivation and distribution of biological control microbes which suppress plant disease causing microbes. The carbon enriches the organic matter in the soil while the biological control microbes provide an alternative to chemical pesticides.
Reaction with quaternary amine forms a product called amine-treated lignite (ATL), which is used in drilling mud to reduce fluid loss during drilling.
79 % of all lignite coal is used in these boilers to generate electricity, and 13.5 % is used to generate synthetic natural gas. A small 7.5 % is used to produce various fertilizer products. Negligible amount of lignite coal is used as home heating fuel, as standalone fertilizer and as oil well drilling mud.
Issues related to environment
Lignite coal mining has several environmental issues related to its mining. These issues are described below.
- Since the calorific value of lignite coal is low, the quantity of coal needed is high.
- Lignite coal’s high moisture content and lower carbon content results in more carbon dioxide emissions than harder black coals.
- There is higher overburden to coal ratio when compared with other types of coal. This results in destruction of agricultural land and displacement of people.
- Lignite mining involves the removal of huge quantities of water. Depending on the quality of the water, it could be a boon for agriculture or an environmental issue.
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