Coal is the world’s most affordable and abundant fossil fuel, contributing to about 41% of electrical power and 26% of primary energy needs of the world. At current production levels, proven coal reserves are estimated to last around 147 years. In contrast, proven oil and gas reserves are equivalent to around 41 and 63 years respectively (World Coal Institute, 2007). While over 68% of oil and 67% of gas reserves are concentrated in the Middle East and Russia, coal reserves are available in almost every country, with recoverable reserves in around 70 countries, making it most secure energy source than oil and gas. Despite the concerns over global warming for burning fossil fuels, especially coal, its role as dominant fuel is increasing throughout the world. According to World Energy Outlook 2008, world demand for coal advances by 2% a year on average, its share in global energy demand climbing from 26% in 2006 to 29% in 2030. Some 85% of the increase in global coal consumption comes from the power sector in China and India to support their impressive economic growth.
Good news for Bangladesh comparing the primary energy sources of its neighbors. The country has good quality bituminous coal with an estimated reserve of 2-3 billion tones in the five discovered coal fields. But the further potential of coal is still to be explored, though the country made the first discovery in 1962 at Jamalganj. Bangladesh has entered into the era of coal mining through the development of Barapukuria underground mine with projected output of 1 million tones per annum for 25-30 years. Detailed geological, geotechnical, social and environmental studies following international standards have been completed for the Phulbari coal field. This two well assessed coal fields jointly share an extractable reserve of about 1 billion tones. Jamalganj coal field with significant resource (>1 billion tones) is so far not considered technically and economically feasible for mining due to the depth (>600m). Other two coal fields, Khalaspir and Dighipara are not systematically assessed and significant efforts will be required to define the mine-able reserve and the method of mining and associated issues.
The Phulbari coal is mostly embedded in two major seams within the Gondwana basin accounting for almost 90% of the deposit with few other thin seams at different depths. Open pit mining will allow considering all the seams in mine design and thus maximize the recover to around 90-95% with other valuable co-products like kaolin, glass sand, gravels etc from the overburden materials. Experts suggest that the underground mining is not feasible considering the seam thickness, geotechnical conditions, mining conditions and economic returns (as already experienced with the Barapukuria mining). Thickness of the coal seams in underground mining significantly influences the resource recovery and mine safety, specially in a geotechnical conditions like Barapukuria and Phulbari. Barapukuria underground mining is being carried out using longwall mining method and coal is being extracted from seam-VI in slices (each slice around 2.5m). This multi slicing extraction method doesn’t allow recovery of major amount of coal and increases the risk of safety hazards like land subsidence, roof collapse, inundation of water, spontaneous combustion of coal etc as the mining advances. Land subsidence is inevitable in underground mining and Barapukuria mine has been facing the experience of such incidents in its very early stage. The subsidence will be more prominent over the large area following the progress of mining with permanent loss of land and other assets. The highly faulted and fractured nature of the basin might have some links to the land subsidence of Barapukuria and may further play bigger role as mining advances.
The coal resource is overlain by a major regional aquifer, the Upper Dupi Tila sequence. This poses significant technological challenge and environmental risks for mining. The presence of impervious ‘Lower Dupi Tila’ clay layer in the lithologic sequence immediately below the aquifer acts as a barrier to prevent water entering the lower sequences and its thicker presence is very vital for underground mining. This clay layer is relatively thinner and at times absent in the shallower section of the Barapukuria and Phulbari basin and gradually thicker towards deeper section. Therefore, underground mining in the shallower basin increases the risk of water inrushes from the overlying aquifer. The numerous faults and fractures of the basin might create some passage with the overlying aquifer which may open up further with coal extraction and increases the risk of mine flooding. Barapukuria coal mine already had such experience in 1998 that flooded the mine and forced to change the design. There should be a thorough study of the origin of huge amount of mine water which is being pumped out every day to keep the mine dry. If the source is overlying aquifer then there is a significant cause of concern for the mine safety and environment in future.
On the other hand, open pit mining requires to managing this regional aquifer efficiently to ensure safe working environment and stabilize the pit slope. The environmental and social implications of groundwater extraction need to be managed through ensuring supply of water for irrigation, community and other environmental uses. Fortunately the world mining industry has made significant technological advancement in handling large open pit mining operation at greater depth in complex geological and hydrogeological conditions. Major coal producing countries like India, Australia, USA and Indonesia produce most of their coal from open pit mines. RWE’s Hambach, Inden and Garzweiler mines in Cologne, Germany are the classic example of successful large open pit mining operations. Hambach alone produces 40 million tones of lignite annually at greater depth (>300m) managing huge aquifers (water extraction rate 16,000-17,000 litre/sec) and its social and environmental impacts.
Bangladesh coal is of good quality but considering other major coal producing countries the reserve is pretty limited- almost equivalent to the annual production of China. Therefore, mining method has to be appropriate to maximize the recovery taking into consideration of geological, hydrogeological reality of the coal fields, and economic viability and energy needs of the country. Underground mining experience at Barapukuria has already demonstrated that coal extraction with this method is not a sustainable solution, and will not meet the required demand for the country. It is uneconomic and will lead to loss of valuable resources of the country. With the present mine design at Barapukuria mine, the resource recovery will be less than 10%, leaving most of coal underground after spending millions of dollars for mine development. The mine which had all the potentiality to become a major energy source for the country is now running uneconomically and struggling to feed only 250MW power plant. Considering the limited reserve and fast growing primary energy market, open pit mining is the most logical option for coal development. An open pit coal mine with an annual output of 10-15 million tones can easily support generation of few thousand megawatts of electricity (approx. 3 million tones/annum of coal may ensure generating 1000 MW). Moreover, current import of 5-6 million tones of low quality, high sulphur coal can be substituted. Coal based briquette industry can be developed to supplement fire wood for domestic cooking and other light industries.
Open pit and underground, both are well established and well practiced mining methods for extraction of coal. But the choice of mining method depends on basin geology, and economic viability of extraction of the deposit. Social and environmental impacts of mining are also important consideration. Generally open pit mining method is preferred for appropriate geotechnical conditions of a coal deposit as it maximizes the resource recovery and offers better economics, and surplus revenues for environmental and social care related to mine development. Bangladesh already has the experience of underground coal mining. The option for open pit mining should be evaluated at least for one coal field with introduction of state of art technologies and best practice mitigation measures of mining world. Country can’t effort to go for another uneconomic venture like Barapukuria. This will neither ensure the energy security nor protect the interest of the country.
Zubayer Zaman, Geologist geozaman@gmail.com
