Underground coal gasification: A new clean coal utilization technique for India
Introduction
Coal is the major fossil fuel in India and continues to play a pivotal role in the energy sector. Coal meets about 60% of the commercial energy needs and about 70% of the electricity produced in India comes from coal [1]. Hence, there is a need for technologies for utilization of coal efficiently and cleanly. Depleting oil and gas reserves can be substituted with abundantly available coal thus prolonging the reserves of all the fossil fuels for use by the future generations. Due to the availability of low price crude oil and natural gas, the coal consumption and process development was slow in the last few decades. But as the oil and natural gas reserves deplete, coal will again emerge as the best option for energy production.
Coal usage has been affected by the pollution caused by its transport, storage, and combustion [2]. To deal with these problems, “clean coal technologies” have been adopted worldwide [3] such as integrated gasification and combined cycle (IGCC), the pressurized bed combustor (PBC) combined cycle, British coal topping cycle in UK [4], low emission boiler system (LEBS) and high performance power system by the US Department of Energy [2].
Underground coal gasification is a promising technology as it is a combination of mining, exploitation and gasification. The main motivation for moving toward UCG as the future coal utilizing technique is the environmental and other advantages over the conventional mining process. Some of these benefits include increased worker safety, no surface disposal of ash and coal tailings, low dust and noise pollution, low water consumption, larger coal resource exploitation and low methane emission to atmosphere [5], [6], [7], [8], [9]. UCG is particularly advantageous for deep coal deposits and steeply dipping coal seams since at these conditions less gas leakages to the surroundings and high pressures favor methane formation. But UCG involves some environmental impacts such as land subsidence and ground water reserve pollution, which serve as disadvantages. Thus before the UCG site is selected there is a need for a thorough environmental impact assessment and complete risk analysis.
UCG is relatively well developed in countries like the USA, Russia, France, Spain and China [10]. They have performed a number of field trials and are ready to commercialize UCG technology. With a vast proven reserve of coal, India has the potential to use UCG technology to utilize coal effectively. The possibility of initiating UCG projects in West Bengal and Rajasthan have been indicated by companies such as the Oil and Natural Gas Corporation Ltd. (ONGC) and the Gas Authority of Indian Ltd. (GAIL), on a pilot basis [11], [12]. ONGC have signed a Memorandum of Understanding (MoU) with the Skochinsky Institute of Mining (SIM) of Russia and Coal India Limited (CIL) for an UCG pilot study [11]. These pilot projects are being carried out as per the recommendations of the consultant from the SIM of Russia [12]. The UCG site will be selected based on the suitability on various considerations including coal quality, area and environmental aspects. ONGC, Gujarat Mineral Development Corporation Ltd. (GMDC) [13], Gujarat Industries Power Company Ltd. (GIPCL) and Neyveli Lignite Corporation Ltd. (NLC) have also entered into an MoU for studies in UCG [13].
GAIL (India) is planning to use lignite, which cannot be mined commercially, to produce synthetic gas by employing underground coal gasification technology in Rajasthan. GAIL plans to use the gas so produced to generate 70–80 MW of power. It may tie up with Ergo Exergy Technologies Inc., Canada, for sourcing “in situ lignite gasification” technology for its proposed project [14]. Reliance is also interested to set up a pilot UCG plant. Essar want to use the product gas for their proposed steel plant in Orissa [15].
The objective of this article is to analyze the feasibility of UCG for application to various Indian coal mines based on quantitative information available in open literature.
Section snippets
Indian coal reserves
The study of reserves and availability of Indian coal have indicated that a major chunk of the reserve consists of weakly to non-coking variety of bituminous, sub-bituminous and lignite coal which are distributed all over India, located at different depths. Coal which when heated in the absence of air forms coherent beads, free from volatiles, with a strong and porous mass called coke, is called coking coal. Coals which do not have coking properties, are non-coking coals. Indian coal is mostly
Underground coal gasification
UCG typically consists of two adjacent bore holes drilled into a coal seam and pressurized oxidant such as air or oxygen/steam are used for ignition of coal seam [6]. The oxidant and the gasifying agent are fed through the injection borehole and the combustion and gasification products are recovered from the production bore hole. Injecting oxygen and steam instead of air produces the most useful product gas, since the dilution effect of nitrogen is avoided. The main constituents of the product
Feasibility study of UCG in India
In this section, Indian coal and lignite seam properties like depth and thickness are discussed. The previous UCG studies in India are reported along with the current selection of coal blocks for UCG by various agencies. Indian coals are compared with other coals, which are used for the worldwide UCG trials, based on seam depth, thickness, coal properties and quantity of coal available. The government policy for UCG and public issues are mentioned in brief.
Summary
India is the third largest producer of coal in the world. India has 253 billion tons of coal reserves and a significant portion is deep underground. Indian coal is of bituminous, sub-bituminous and lignite type. The high ash content and poor quality of these coals leads to operational problems in industries. Hence, the consumption of coal is reduced. To utilize the vast coal reserves underground coal gasification is a promising technology. UCG can utilize low-grade coal in India economically.
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