Lignite coal of relatively recent origin. It is a non-agglomerating brownish black low-rank consolidated coal with abundant woody material. In coalification process it is intermediate between peat and bituminous coal. It is also known as Brown Coal. The lignite burns readily with long smoky flame. The heating value of this type of coal is less than 8,300 BTU/lb on a moist mineral matter free basis. The lignite is separated into two groups: Lignite-A and Lignite-B. Lignite-A has the heating value in the range of 6,300 BTU/lb to 8,300 BTU/lb and Lignite-B has a heating value of below 6,300 BTU/lb. The average composition of lignite is: moist 31.06-35.00%, ash 7.41-7.45%, volatile matter 45.32-56.71%, and fixed carbon 43.29-54.69%. Lignite beds occur in Mesozoic and Tertiary rocks in different parts of the world, but in the South Asian region it is deposited mainly in the Tertiary rocks.
In Bangladesh lignite is found at Lamakata and Bandarban of Bandarban district, Patiya of Chittagong district and Lubachhara of Sylhet district. Lignites of these places are present as lenses and or as beds. Besides these, lenses and small beds of lignite are also found in sandstones of Dupi Tila, Tipam, Bokabil, Bhuban, Kopili and Tura formations. Ligno-bituminous coals are present in the subsurface of Takerghat-Bhangerghat area of Sunamganj district. The reserve of the ligno-bituminous coal is 3 million tons in an area of 3.048 sq km. The lignite in Bangladesh has of little economic value.
Lignite shale is dark grey or black shale with significant amount of carbonaceous materials in the form of disseminated particles or flakes. In this shale the lignite material is generally less than 40%. The lignite shale is the characteristic feature of the lower part of the Oligocene Burdwan Formation (Barail) of West Bengal, India. The lignite shale is soft and found in alternation with the sandstone and shale. Decomposed vegetal matters and leaf impressions are present in the lignite shale. The shale is laminated and breaks into laminar planes. The vegetal matters are seen in the laminar plane. [Md Nehal Uddin]