The importance of clean coal technologies2009-12-09 19:10:00 By...

The importance of clean coal technologies
2009-12-09 19:10:00

By Prabhavati Akashi
With growing public concern about global warming and climate change, efforts are being made globally to keep the environment clean. Coal, which is primarily used for the generation of electricity, contributes heavily to carbon dioxide emissions. Hence there is a growing emphasis on clean coal technology aiming at reducing the environmental impact of coal energy generation. The Government has also laid special emphasis on development of clean coal technologies including Underground Coal Gasification (UCG) under National Action Plan on Climate Change.

What is UCG?
Underground Coal Gasification (UCG) or In-situ Coal Gasification is a method of converting un-worked coal/in-situ coal into a combustible gas which can be used for industrial heating, power generation or the manufacture of hydrogen, syn gas or diesel fuel. UCG technology allows countries that are endowed with coal to fully utilise their resource from otherwise unrecoverable coal deposits in an economically viable and environmentally safe way. This is one of the identified Clean Coal Technologies.

UCG uses a similar process to surface gasification. The main difference between both gasification processes is that in UCG the cavity itself becomes the reactor so that the gasification of coal takes place underground instead of at the surface.

UCG has the potential to exploit the coal resources regarded as either uneconomic to work by conventional underground coal extraction or in-accessible due to depth, geology or other mining and safety considerations.

Basic Process
The basic UCG process involves drilling tube wells into coal; one for injection of the oxidants (water/air or water/oxygen) and other well some distance away to bring the product gas to the surface. The product gas is a combustible Syngas containing hydrogen, carbon monoxide and methane that can be used as a fuel or chemical feedstock. A cavity is formed as the coal burns and the roof is allowed to collapse. Eventually a stage is reached when gasification efficiency declines and a new gasifier must be initiated.

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The principal UCG processes can be divided into two stages viz., pyrolysis and gasification. During pyrolysis coal is converted to char which releases tars, oils, low molecular hydro carbons and other gases. Gasification occurs when water, oxygen, carbon dioxide and hydrogen react with the char. The main gases produced are carbon dioxide, methane, hydrogen and carbon monoxide. Methane is a product of pyrolysis and its formation is favoured by low temperature and high pressure.

During the UCG process, much as in conventional gasification methods, an oxidant reacts with coal of the underground coal seams, and part of released sensible heat is used in coal drying, pyrolysis and the endothermic reactions that reduce the combustion products. The resulting mixture is UCG gas. The gas composition depends on the coal geology as well as the process parameters and type of oxidants, including air and oxygen-rich gaseous blends.

Potential Benefits
There are many potential benefits of UCG. In general most of the activities related to underground coal mining like coal stockpiling, coal washing and waste disposal and activities at user end like ash handling and disposal are removed. Other benefits relate to health and safety issues and potentially lower overall capital and operating costs. The environmental effects of the UCG are perceived to be fairly low as the main product of the process is gas and byproducts are either left in the ground, are removed by conventional processes or re injected back into the seam. However, some issues like its effect on surface and sub-surface environment need to be studied in detail.

International Developments
The feasibility of UCG has been examined in many countries and few have undertaken field trials. The Angren plant produces gas for burning in boilers in the old Angren power station, generating a power output varying between about 50 and 150 MW. The plant is one of the oldest in the world and is operating since 1963. Linc Energy, Australia conducted UCG operations at Chinchila continuously for two and half years starting from 1999 for trial power generation and is now planning for use of syn gas for CTL purposes. Eskom, South Africa has commenced trial UCG operations in January 2007 at Mazuba (3.5 m thick coal seam at 300 m depth) for power generation and now planning for up scaling the operations to meet the requirements of 350 MW IGCC power generation plant. Sasol initiated a new UCG trial project at Secunda as a potential feedstock for CTL Plant. Xinao, China has commenced pilot ignition in August 2007 for producing methanol.

Indian Initiative
In India, almost 40% of the available coal resources are deep seated i.e., beyond 300 meters depth and winning these deposits through conventional mining techniques is difficult as well as capital intensive. In such a situation in order to utilize the energy available from these sources, promotion of clean coal technologies like UCG is important.

Since UCG is relatively a new area in the country, Ministry of Coal has earlier constituted a committee in August 2005 under the chairmanship of the then CMD, CMPDIL with other members from ONGC, DGMS, DGH, MECL, NLC etc. to study various aspects of UCG and suggest a regulatory frame work for development of UCG in the country. The report of the committee was submitted in August 2006. After detailed examination of the same certain issues were raised for clarification. Subsequently, these were addressed in an International Workshop on UCG conducted by Ministry of Coal under the aegis of Indo-US Coal Working Group and Asia Pacific Partnership for Clean Development in November 2006 in Kolkata.

Further, under Indo-EU Coal &Clean coal technology Working Group another international Workshop was held in March 2009 in New Delhi. Based on the deliberations in these forums, Ministry of Coal issued a set of Guidelines in July 2009 for conducting UCG operations and allotment of coal blocks for UCG in the country. Five lignite blocks namely, Sindhari West, Chokla (North), Nimbalkot, Nagurda (all in Barmer district of Rajasthan), Dungra (in Surat district of Gujarat) and two coal blocks, namely Bandha in Singrauli coalfield and Yellandu dip side in SCCL command area have been identified for offer for UCG purposes and some companies have approached the Ministry seeking allotment of blocks for taking up UCG on their own. The proposed guidelines will facilitate allotment of blocks by the Ministry and conducting of UCG operations by the operators.

It has also been emphasized that since it is a high technology area we may need to depend on technology providers from outside the country. Capacity creation, technology development and R&D are critical for development of the subject. It is important for the public sector coal companies to have the technology issues studied in detail by undertaking visits to already operating projects outside India for adoption of technology in Indian condition.

Underground Coal Gasification, no doubt, is one of the identified clean coal technologies having good potential for development in India which may play a crucial role in making the environment clean and safe for the future generations. (Courtesy: Press Information Bureau