Wednesday, August 26, 2009

Coal deposits, mining perspective in northwest Bangladesh

How could the Phulbari basin extend up to 60 sq km? Is it true? The present study reveals that if the in situ coal reserve was around 500 million tonnes, the deposited area in the Phulbari basin could never exceed 6 km2. Therefore, the scientific and general community of the country should rethink the possible fraud by some people, writes Md Rafiqul Islam*

NewAge, August 26, 2009

NATURAL gas is the major indigenous non-renewable energy resource in Bangladesh. Gas production has increased sharply over the last decade so that natural gas resources are likely to be exhausted in 12 to 15 years. Considering the critical situation in power sectors, the government of Bangladesh wishes to produce coal from the Gondwana coal deposits because of the gradual increase in demands of electricity and to safeguard the energy crises in 21st century. Highly volatile B bituminous rank coal has been discovered in varying amounts in 13 specific places of the northwestern districts of the country like Bogra, Jaipurhat, Rangpur and Dinajpur. Seven coalfields, among them, like Barapukuria, Phulbari, Dighipara, Nawabjonj, Shimnagar, Dangapara, and Burirdoba basins are found in Dinajpur district. Three coal basins — Khalaspir, Baradgonj and Osmanpur — have been found in Rangpur district. Other two coal basins are the Kuchma basin, found to the eastern Bogra shelf, and the Shingra basin, found to the western frontier of Bogra district. The Shingra basin is the northern part of the greater Atrai basin (Chalan Beel). The Jamalganj basin, which is the largest coal basin of the country, is found in Joypurhat, close to the Jamalganj town. The Kuchma and Jamalganj basins were discovered in 1959 and 1962 respectively. It is reasonable to state that after the liberation war in 1971, there was no satisfactory advancement in discovering the coalfields in Bangladesh until 1985. The Geological Survey of Bangladesh discovered the first coal basin in Barapukuria in 1985-88. The surface drilling confirmed the existence of a sequence of Gondwana coal-bearing sediments. The GSB has discovered the Khalaspir coal basin in 1987 and the Dighipara basin in 1995. The coal basins like, Phulbari, Nawabgonj, Shimnagar, Dangapara and Burirdoba have been recognised based on the negative gravity anomaly, a survey that was carried out by the Geological Survey of Bangladesh in 1992. Based on the gravity-anomaly survey report of the GSB, an Australian company, BHP (Broken Hill Proprietary), completed the surface drilling and specified the existence of coal in the Phulbari basin in 1997. Other basins as mentioned above have not been considered for exploration, reservoir estimation, and feasibility study yet. The five discovered coal basins which have the most estimated coal deposits are Jamajganj (1053 million tonnes), Barapukuria (377 million tonnes), Khalaspir (828 million tonnes), Phulbari (about 500 million tonnes) and Dighipara (about 500 million tonnes).

Recently, a vast conflict has been raised between the basinal extent and estimated reserve of the Phulbari coal basin compared with the extent and reserve of Barapukuria as well as the Jamalganj basin. For example, the Barapukuria basin has six coal seams with an average thickness of about 44.8 m that extends up to 5.16 square km with an in situ total reserve of 377 million tonnes. The Jamalganj basin has seven coal seams with an average seam thickness of about 64 m and it covers about 11.7 sq km. In contrast, BHP boreholes reveal the Phulbari basin has only two coal seams with an average total thickness of about 38 m. This author has calculated the preliminary estimated reserve of about 483-500 million tonnes, while the deposited area is 6 square km. If the area is ten times of 6 km2, then the estimated reserve should be ten times, i.e. 4830-5000 million tonnes. Therefore, the author wonders and his critical query is here. How could the Phulbari basin extend up to 60 sq km? Is it true? The present study reveals that if the in situ coal reserve was around 500 million tonnes, the deposited area in the Phulbari basin could never exceed 6 km2. Therefore, the scientific and general community of the country should rethink about the fraud of some people. Why some particular people are shouting that the extent of the Phulbari coal basin is about 60 km2, which is ten times the in situ coal reserve area of the basin. Is there any hidden agenda behind this publicity?

Some people are trying to motivate the government for open-pit mining in Phulbari, and Barapukuria. Worldwide mining reveals that a larger-scale area is needed for open-pit mining than that needed for underground mining. If the angle of internal friction of the overburden rocks strata of an open mine is minor, in that case, a huge amount of area is wanted to safeguard the slope stability for optimum production of coal. The depth of coal seams of the Phulbari basin ranges from 152 to 255 m. The angle of internal friction of the overburden of the Barapukuria basin ranges from 10 to 32 degree. Belonging to the same geological belt, the friction angle of rock strata of the Phulbari basin would be identical to that of Barapukuria. Therefore, if the mining company considers the above-mentioned range of friction angle for safe operation, it would get a huge amount of land. In addition, the mining company should think of shifting the Phulbari town along with many densely populated villages elsewhere. Perhaps these are the reasons for shouting 60 km2. More clearly, excluding 6 km2 area of the coal deposit, another 54 km2 land would be required for supplementary purposes rather than for mining.

The problem for open-pit mining in northwest Bangladesh lies with the depth of coal seams and overburden unconfined water-bearing formation. The mining methods are usually selected based on the depth of overburden of ore body, geographic and topographic locations, and geology of the basin. Most of the open-pit coalmines in Australia, for example, Surat Basin, Galilee Basin, Washpool open-pit coal project, the Red Hill open-pit coal project, the Burton, Ellensfield and Wallanbah open-pits, etc, are found in and around the hilly regions. However, open-pit mines in Australia are not located within the arable and densely populated land. The depths of these open-pit mines in that area are shallower (maximum of 120m). For example, in the Washpool open-pit coal project, the coal seams occur at a shallow depth up to roughly 60m. In the Red Hill open-pit coal project, the shallower (100m) seam has been considered for small open-pit, while the deeper seam (depth ranges from 150m–300m) has been considered for underground mining. The depths to the top of coal seams in the Burton, Ellensfield and Wallanbah open-pits are almost 100m, 120m and 90m respectively. In Coppabella mine, a major open-cut has been considered to be around 150m depth, after which an underground mining has been proposed. In India, the eventual pit depth of the Chandmari Coppermine of Rajasthan is 148m. The Lajkura opencast coal mine is located in the Orissa state of India. The depth of overburden rock is 22m, immediately above the main coal seam, having a thickness of 18m. The Jharia coalfield in Bihar state of India is about 40km long and about 12km in average width, having 50 coal seams and an in situ reserve of 17077 million tonnes. Opencast mining in Jharia has a maximum depth of 70m. The mine has been exploited with a combination of opencast and underground mining. Therefore, the study implies the depth of coal seams and unconfined aquifer in the Phulbari and northern part of the Barapukuria basin does not allow open-pit mining. Other alternatives for coal utilisation like underground coal gasification, coal bed methane and coal to liquid technologies should be considered in this connection. For underground mining, Longwall Top Coal Caving mining method associated with descending order of extraction and backfill technology should be preferred.

*Dr Md Rafiqul Islam is a professional mine geologist. At present he is a research fellow of the Department of Earth Sciences, University of the Ryukyus, Japan