Underground coal gasification (UCG) - Potential to increase coal reserve worldwide:

Underground coal gasification (UCG) - Potential to increase coal reserve worldwide:

Introduction: Worldwide, coal reserves are quite vast - over 10 trillion tonnes. However, unless cleaner and cheaper ways can be found to convert coal to gas or liquid fuels, coal is unlikely to become an acceptable replacement for dwindling and uncertain supplies of oil and natural gas. Mining coal is dangerous work. Coal is dirty to burn and much of the coal in the ground is too deep or too low in quality to be mined economically. Today, less than one sixth of the world’s coal is economically accessible. However, there is a renewed interest world over to revive the old technology that offers promise to substantially increase usable coal reserves and make coal a clean and economic alternative fuel. Known as underground coal gasification (UCG), this technology converts coal to a combustible gas underground.

Underground Coal Gasification (UCG) is the process by which coal is converted in situ into a combustible gas that can be used as a fuel or chemical feedstock. It is a process to convert unminable underground coal/lignite into combustible gases (i.e., combustible syngas – a combination of hydrogen and carbon monoxide) by gasifying. 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 the coal takes place underground instead of at the surface.

Despite considerable research and testing, no commercially viable project has yet been demonstrated anywhere. Research has been conducted principally in Western Europe, USA, China, the former Soviet Union and Australia.

Benefits of UCG - As a method of exploiting coal, UCG represents an environmental improvement on the combination of coal mining and surface combustion of coal. It is also safer and intuitively more efficient.

Environmental benefits of UCG over underground coal mining for fuelling power generation include:

(i) Lower fugitive dust, noise and visual impact on the surface

(ii) Lower water consumption

(iii) Low risk of surface water pollution

(iv) Reduced methane emissions

(v) No dirt handling and disposal at mine sites

(vi) No coal washing and fines disposal at mine sites

(vii) No ash handling and disposal at power station sites

(viii) No coal stocking and transport

(ix) Smaller surface footprints at power stations

(x) No mine water recovery and significant surface hazard liabilities on abandonment.

Additional benefits include:

(i) Health and safety

(ii) Potentially lower overall capital and operating costs

(iii) Flexibility of access to mineral

(iv) Larger coal resource exploitable

Note: At present, natural gas offers attractions as a clean fuel that UCG may find difficult to compete.

Process of Underground Coal Gasification (UCG):

The basic UCG process involves drilling two wells into the coal, one for injection of the oxidants (water/air or water/oxygen mixtures) and another well some distance away to bring the product gas to the surface. Coal is gasified underground by creating a linkage through the coal seam between the injection and production wells and injecting air (or oxygen) and water (or steam) into the underground reaction zone. The injected gases react with coal to form a combustible gas which is brought to the surface in a production well, cleaned and used as a fuel or chemical feedstock. A cavity is formed as the coal burns and the roof collapses. This results in lateral growth and is allowed to continue until the product gas quality deteriorates. The greater the lateral growth, the longer the life of a gasifier and the more cost-effective the operation. When the quality of the product gas falls, fresh coal is ignited further along the injection well. Once the coal within the underground gasifier has been exhausted, new injection and production wells are drilled and the process is repeated.

Injecting oxygen rather than air reduces the nitrogen content and raises the heating value of the produced gas to the ‘medium-Btu’ gas range – of heating value roughly one-fourth of natural gas. If the goal is high-Btu gas (also called as substitute natural gas or SNG), the percentage of methane in the produced gases needs to be boosted. For methane formation in UCG, two additional steps are required. First, some of the carbon monoxide made in the gasification process is reacted with steam to form additional hydrogen. This step, called shift conversion, sets up the proper ratio of gases for the next step called methanation. The hot gas thus produced is allowed to pass through the coal seam to the exit boreholes and is carried to the surface where it is cleaned and upgraded for use. The whole aspect is elaborated in next paragraphs.

In fact, gasification differs from combustion which takes place when coal is burned in excess oxygen to produce carbon dioxide and water. Another important difference between coal combustion and coal gasification is in pollutant formation. The reducing atmosphere in gasification converts sulphur (S) from coal to hydrogen sulphide (H2S) and nitrogen (N) to ammonia (NH3), whereas combustion (oxidation) produces sulphur dioxide (SO2) and oxides of nitrogen (NOx).

The principal processes can be divided into two stages, namely (i) pyrolysis (also known as carbonisation, devolatilisation or thermal decomposition) and (ii) gasification. During pyrolysis coal is converted to a char releasing tars, oils, low molecular hydrocarbons and other gases. Gasification occurs when water, oxygen, carbon oxide and hydrogen react with the char.

The main gases produced are carbon dioxide, methane (CH4), hydrogen and carbon monoxide (CO) and oxygen. CH4 is essentially a product of pyrolysis, rather than gasification. Its formation is favoured by low temperature and high pressure.

In a theoretical appraisal of the gasification process, the Autothermal Chemical Equilibrium (ACE) condition exists. This is a condition at which the heat value of the product gas and the conversion efficiency of the gasified coal (chemical energy of product gas/chemical energy of gasified coal) is a maximum. At high temperatures and pressures (say 5MPa, 900°C), ACE conditions are approached rapidly but at lower temperatures and pressures the time to attain equilibrium greatly exceed the residence time of the gases in the gasifier and therefore ACE will not be attained.

The basic reactions can be generalised to a simple empirical form:

C + O2 → CO2 (+heat)

C + CO2 (+heat) → 2CO

C + H2O (+heat) → H2 + CO

C + 2H2 → CH4 (+heat)

During pyrolysis coal, subjected to high temperatures, yields higher heat value gases than ACE gasification products for a relatively small consumption of O2. Pressure increases the proportion of coal pyrolysed to form methane thus raising the heat value of the product gases. There is also some evidence to suggest that elevated pressures cause pyrolysis processes to penetrate in situ coal, further enhancing the gasifier yield.

Gasification circuit and Cavity behaviour: The gasification circuit requires a flow link to be achieved between an injection and a production well. Methods of achieving the link are:

* Accurate drilling assisted by a target device in the vertical well if necessary.

* Reverse combustion, involving ignition at the base of the production well.

Initially, channel created in coal seam using special drilling techniques. As reaction proceeds, channel grows, creating underground ‘cavity’. Volume of cavity increases progressively with progress of reaction.

Installation of well pairs (injection and production wells) is costly and therefore it is desirable to gasify the maximum volume of coal between a well pair. As gasification proceeds, a cavity is formed which will extend until the roof collapses. This roof collapse is important as it aids the lateral growth of the gasifier. Where the roof is strong and fails to break, or where the broken ground is blocky and poorly consolidated, some fluid reactants will by-pass the coal and the reactor efficiency could decline rapidly.

The most successful gasifier or reactor control process, developed in the USA, involves the use of a burner attached to coiled tubing. The device is used to burn through the borehole casing and ignite the coal. The ignition system can be moved to any desired location in the injection well. This ‘controlled retraction of ignition point’ (CRIP) technique enables a new reactor to be started at any chosen upstream location after a declining reactor has been abandoned.

Carbon capture and sequestration (CCS): If the CO2 is to be captured at the surface and sequestered, it must be separated from the syngas. At a UCG production site, a significant percentage of the CO2 would likely be sequestered in the void left by the burned coal seam. Ideally, remaining CO2 can be sequestered in deep geologic formations nearby.

If the CO2 is not sequestered in place, it can be piped to oil fields. Oil companies can then inject it underground to increase production from oil and natural gas wells, a process called enhanced oil recovery. This represents an opportunity to sequester carbon at a lower cost compared with storing it in geologic repositories.

Monitoring and Control: In order for the gasification process to be controlled, it is essential that continuous analytical measurement of the product gas stream is available.

Injection flow rate and composition, temperature and pressure were measured at various parts of the circuit to facilitate control of the gasifier and to ensure pressure and temperature design limits of system components were not exceeded. The manipulation of the following variables allowed the reaction rate and the gas quality to be adjusted within certain limits:

(i) Injected gas flow rate and composition

(ii) Reactor back pressure

(iii) Production well base temperature

(iv) Safety monitoring and alarm control

Technical requirements: Important technical requirements and considerations in designing a commercial gas production scheme:

(i) A cost-effective means of acquiring high-resolution coal seam geological data

(ii) Reproducible drilling accuracy

(iii) Multiple, independent gasifier units (with separate injection and production wells) to ensure systems failures do not totally halt gas production

(iv) Integrated surface plant using readily available off-the-shelf equipment wherever practicable.

The most critical element of deep UCG is arguably the directional drilling. Technologies exist which are capable of achieving the required precision but there is considerable uncertainty about the general drillability of coal seams in other than ideal conditions.

Environmental Impact and its Control: The main environmental issues concerning UCG are:

(i) Atmospheric emissions;

(ii) Surface water;

(iii) Drinking water pollutants;

(iv) Noise;

(v) Site operations;

(vi) Groundwater;

(vii) Subsidence.

Conclusion: Today, high prices of oil and gas and uncertainties about political stability in most of oil producing countries, have renewed interest in all kinds of fuel. A renewed interest in coal gasification is therefore not surprising. Further-more, hydrogen is now a welcome by-product because of the current interest in alternatively fuelled vehicles. UCG is potentially the most important clean coal technology of the future with worldwide application. Ultimately, it could be a substitute for deep mining coal for power generation use.

Applying improved UCG technology to gasify deep, thin, and low grade coal seams could vastly increase the amount of exploitable reserves. The coal could be converted to gas for a variety of uses and emissions of sulphur, nitrous oxides and mercury could be dramatically reduced. UCG could increase recoverable coal reserves by as much as 300 to 400 percent. Another benefit of UCG is that hydrogen accounts for nearly half the total gas product which can be separated and actively used as automotive fuel or as feed-stock for the Chemical Industry.

Countries are turning to UCG to fully utilize their coal resources in an economically viable and environmentally acceptable manner. Using UCG technology even without a carbon-capture-and-sequestration plan could also be eligible for carbon credits.

Summary: Underground coal gasification (UCG) involves injecting steam and air or oxygen (O2) into a coal seam from a surface well. The injected gases react with coal to form a combustible gas which is brought to the surface in a production well, cleaned and used as a fuel or chemical feedstock. A cavity is formed as the coal burns and the roof is allowed to collapse. This process results in lateral growth of the gasifier in the seam and is allowed to continue until the quality of the product gas declines. When this occurs the seam is re-ignited at a new location further along the gasifier. Once the coal within the underground gasifier has been exhausted, new injection and production wells are drilled alongside the exhausted gasifier and the process is repeated.

UCG has the potential to exploit coal resources which are either uneconomic to work by conventional underground coal extraction, or inaccessible due to depth, geology or other mining and safety considerations. The successful development of UCG will not only depend on advances in the use of technology but also on demonstrating that a clean energy can be produced without detriment to the environment. As a method of exploiting coal, UCG represents a substantial environmental improvement on the combination of coal mining and surface combustion of coal.

Energy Mix Strategies for oil importing country – Important in the scenario of energy security and global warming:

Energy Mix Strategies for oil importing country – Important in the scenario of energy security and global warming:

A. We are aware of the problems of environmental pollution and the adverse consequences of global warming causing due to CO2 emission. To restrict environmental pollution, to mitigate the CO2 emission and rate of increase of CO₂ concentration in the atmosphere, responsive long term energy mix strategies exploiting the maximum potential of non-greenhouse gas emitting energy sources need to be developed and implemented as rapidly as possible. The future energy mix will not only depend on environmental issues, but also will depend on technological, economic, supply, logistics and political factors. It is generally accepted that for many decades fossil fuels will continue to be the major energy source world over. Natural gas being the lowest fossil fuel greenhouse gas emitter will increase the share in energy scenario world over. Countries having or exporting fossil fuels cannot easily turn away from their use and likewise the industrially & economically dynamic countries of Asia such as China, Japan and India cannot radically shift from fossil fuels towards uncertain and currently costly renewable for their growing power needs.

B. National and regional factors are the most important in guiding country's energy mix. Percentage share of energy differ considerably today and they will in the future. For example, today China is more than 90% dependent on various forms of fossil fuels. On the other hand, France and Sweden have reduced their dependence on fossil fuels to less than 50% and 35% respectively by using nuclear and hydro-power to a great extent. Moreover, out of all the fossil fuels coal is the workhorse of global electric power sector and is used to generate more than half of the electricity world consumes. Coal is also world’s most abundant fossil fuel, with supplies projected to last almost 250 years or more. As coal-fired power plants generally produce the lowest-cost electricity and coal is abundant, most of the country’s economic and energy security depend on the continued use of the fuel.

C. Therefore, on the global level, it is difficult to make a policy decisions to foster a reduced reliance on fossil fuel. Decision makers are confused on how to proceed for country’s energy mix for the future as there is general support for cost effective energy efficiency techniques and on the supply side an endorsement of an increased use of renewable and sustainable energy sources. In fact, both the efforts are necessary at present; but ‘renewable and sustainable energy sources’ have limited potential over the near term. However, in the developed and industrialized countries, the significant energy efficiency gains and use of renewable energy sources have been seen over the past two decades, that changed the dependence on fossil fuels and energy scenario to a great extent on their industrial and residential front.

D. The supply potential from renewable energy sources, at present, is difficult to assess since they are only emerging technologies and currently not suitable for meeting large energy demand of a country. With differing relevance for the various renewable energy sources, technological improvements are needed and basic challenges exist in reducing costs, improving efficiency and reliability, solving energy storage problems and integrating the technologies into existing energy systems. In most of the developed countries many decision makers in the opinion that, non-hydroelectric renewable energy sources, such as solar and wind will not be economically competitive for large scale production in the foreseeable future and that they will play no more than a limited role in the decades to come. They opine that, even with adequate support and subsidies the share of such renewable energy sources could reach only 5-8% (including about 3% non-commercial energy share) of primary energy supply by 2020.

E. Fortunately, hydroelectric has already been extensively developed and in use in Europe and North America (some 50% of the estimated maximum economic potential). Its greatest potential lies primarily in Asia, South America and Africa, where the trend will likely be towards small capacity units as concerns grow about the damaging environmental and social impacts of large dams.

F. Energy security and implementing proper energy mix strategies for oil importing countries are very much crucial especially in the scenario of rapid industrialization. For those countries, in my opinion, renewable energy must be developed in parallel with nuclear power and a clean-up of coal-fired power station technology, if these nations are to meet increasing demand without relying on enormous and potentially debilitating natural gas imports. For a nation the provision of sufficient, affordable and secure energy is crucial for any modern economy. Many countries are facing the challenge of bridging the widening gap between energy supply and demand. At the same time, across the globe, those same economies are facing challenges such as climate change, limited resources and rising costs. Therefore, for oil importing countries, energy mix should shift more towards, nuclear power and clean coal technology.

G. There are very large amounts of remaining oil, gas and coal left in the world and in the absence of concerted government initiatives, it may take many years before alternative energy sources such as wind and solar become a significant part of the world’s energy mix. It is true that some renewable sources such as bio-fuels and wind have attained ten-fold production increases throughout the past ten years. However, global energy demand is increasing at such a rate that, if we ignore hydro-electricity, renewable energy - as a proportion of total energy supply - may well remain at less than 2 per cent of the total market for many years to come. It may be noted here that, global climate change may be best addressed in the short term by energy conservation, by increasing fuel efficiency, and by subsurface storage of the carbon dioxide that results from burning fossil fuels. At the same time, the greatest advantages of nuclear power is that it avoids the wide variety of environmental problems arising from burning fossil fuels, apart from economically generating a high amount of electrical energy in one single plant using small amount of fuel.

H. Therefore, concerted efforts by such economies, in order to have secured energy for their sustainable developments, should involve large scale nuclear expansion, the development of clean coal-fired power stations, implementation of hydro-electric power to maximum potential and a increase in renewable energy sources such as solar and wind.

Green coal for power - To take care of post-Kyoto issues from energy security point-of-view:

Green coal for power - To take care of post-Kyoto issues from energy security point-of-view: a. Coal is the world’s most abundant and important source of primary energy. Turning a potential pollutant into a clean, green fuel for economical power production has become a matter for concern on a global scale. Coal continues to dominate the energy industries as the single most important and widely-used fuel. Delivering around 27 per cent of the world’s consumption of primary energy, almost half of which is used for electricity generation; reserves of coal are spread worldwide throughout some 100 developed and developing countries, sufficient to meet global needs for the next 250 years. b. Although a combination of economic and environmental pressures has forced the closure of older, inefficient, fossil fuelled thermal stations, the massive growth in power demand on a world scale will continue to be met predominantly by coal-fired plant for the foreseeable future. In many of the rapidly developing and industrializing regions of the world the rate of consumption of coal as a primary fuel for electricity generation is actually increasing. In energy-hungry India alone, coal-burn for power generation is forecast to more than double in the next few years to 350 million tonnes per year. Annual coal production in China, the world’s largest producer, has rocketed to over 1,500 million tonnes. Nevertheless, post-Kyoto issues have heightened environmental awareness, forcing the pace of technological change in the use of this abundant but potentially polluting fuel for power generation. The environmental threat posed by the release of even more millions of tonnes of toxic pollutants, acidic and greenhouse gases from both new and existing coal-burning power stations is widely accepted. Currently, signatories to the Kyoto Protocol are focusing on solutions to the problem of global warming, including the reduction of CO2 and other ‘greenhouse’ gases. In many other non- signatory countries, major programmes have been implemented by utilities and power producers to reduce SOx, NOx and CO2 emissions. Additional environmental concerns have also emerged, including the potential health impacts of trace emissions of mercury and the effects of particulate matter on people with respiratory problems. c. In contrast with both natural gas and LPG, hard coal can contain a wide range of compounds including sulfur in addition to useful hydrocarbons. The percentage of sulfur can vary widely, with relatively low concentrations in the highest quality anthracite and very high amounts in lignite, generating large volumes of SOx. As well as the need to treat the fuel prior to firing and control closely the combustion process itself to limit the production of nitrogen oxides, coal-fired stations based on conventional pulverized coal technology can only reduce SOx emissions through the use of post-combustion treatments. Further problems still remain through the safe disposal of fly ash which can contain high levels of toxic compounds including heavy metals. d. Enormous environmental problems faced by operators of older, coal-fired generating plants all over the world, plants were forced to take drastic action after various public protests about the deadly effects of SOx emissions and other emissions. Emissions from coal and lignite-fired units at various power generating stations caused widespread damage, killing livestock and crops downwind of the plant and causing respiratory illness in the population in many countries. The plants were forced to cut output. This tends to place an unacceptably high strain on the commercial viability of an existing power station in many of the developing nations and represents a completely uneconomic option for the majority of obsolescent installations. Power producers in industrialized developed countries are therefore adopting a variety of leading-edge clean-coal technologies for electricity generation. e. New clean coal technologies are providing an attractive and economically viable option to post-combustion systems. Applying the latest combustion, steam and process technologies in new power plant or upgrading existing coal-fired generating facilities provides significant improvements in thermal efficiency, reducing environmental impact and energy costs to the consumer. At the same time, higher thermal efficiencies result directly in reduced fuel costs, improving profitability and market position for the independent power producer. (i) For new and smaller coal-fuelled generating plant, boilers using well-proven circulating fluidized-bed CFB technology provide a cost-effective and efficient system capable of meeting current and future environmental standards. They are now being widely used and successfully operated in coal-fired generating units, burning a very wide range of coal and other fuels with widely differing heat values and mineral content. These can typically include anthracite, semi-anthracite, bituminous and sub-bituminous coal, lignite and even ‘gob’ – a form of high-ash bituminous coal waste. (ii) As an alternative to direct combustion based systems, coal gasification is becoming increasingly attractive, with Integrated Gasification Combined Cycle (IGCC) technology offering one of the best ‘clean’ options for effective power production. Gasification systems use steam and controlled amounts of air or oxygen under high temperatures and pressures to react with coal to form clean synthetic gas or ‘syngas’. Current systems provide efficient clean-up of the gas-stream to produce a mixture of carbon monoxide and hydrogen which can be used subsequently as a ‘clean’ fuel or a basic feedstock for liquefaction. f. Used as a fuel for power generation in a typical IGCC generating plant, a syngas-fired gas turbine drives a generator, with exhaust heat from the gas turbine recovered to produce steam to power a steam turbine in conventional ‘combined cycle’. IGCC power generating systems are presently being developed and operated in Europe and the US, with commercial systems capable of operating at thermal efficiencies approaching 50 per cent. NOx and Sox emissions levels are minimized with the potential for carbon-capture and sequestration of the CO in the sysngas stream being actively researched as design strategies for near-term and future coal-based IGCC plants. Elemental sulfur is removed from the syngas before combustion and is a highly saleable commercial byproduct. If the gasifier is fed with oxygen rather than air, the flue gas contains highly concentrated CO2 which can readily be captured, at about half the cost of that from conventional plants. g. As an alternative to the direct use of syngas as a fuel for electricity generation, it can also be processed using modern gas-to-liquids (GTL) technologies to produce a wide range of liquid hydrocarbon fuels such as gasoline and diesel oil. Coal-to-oil is a long-established technology in coal-rich South Africa. h. Nevertheless, clean coal technology is moving very rapidly in the direction of coal gasification, with a second stage designed to produce a concentrated and pressurized carbon dioxide stream followed by separation and geological storage. This has the potential to provide extremely low emissions of conventional coal pollutants, and as low-as-engineered carbon dioxide emissions – a vital step in the fight to prevent irreversible climate change.