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 security – ‘Clean Coal’ has potential to change the world from pariah to paragon of virtue in high oil price regime:

Energy security – ‘Clean Coal’ has potential to change the world from pariah to paragon of virtue in high oil price regime:

A. The thirst of populous emerging economies for energy and the industrial countries’ sustained need for energy will ensure a further rise in demand. However, it looks as if the supply of oil, and later also natural gas, will not keep pace with this demand. Only by leveraging every possible means will it be possible to compensate the imbalances emerging on the horizon. However, during the transition to the renewable sources of energy such as wind and solar age, an energy gap will have to be filled.

B. With oil currently trading at around USD 140/bbl, coal-to-liquid technology is already an interesting alternative from a purely commercial point of view. Coal offers great potential as a substitute for oil and natural gas in the medium term, but so far its versatility has been underestimated. Going forward, coal could attract more attention in all three major energy sectors – power generation, the heating market and transport – provided that the right technologies delivering higher efficiency and lower environmental burdens take root. Environmental risks emphasize need for “clean coal”. Global warming is one of the biggest dangers facing human existence on earth, and combating this danger is therefore one of the greatest challenges. Since coal causes 40% of global CO2 emissions, only advanced technology can pave the way to a better future. The required quantum leaps in technology could, however, open the doors to the global mass markets. The need for investment is very high not only in emerging economies like China and India but also in US and Europe. CO2-free coal-fired power plants could become a milestone on the way to a better energy future in spite of their additional fuel consumption.

C. Worldwide prospects for energy is actually quite good, but only if all possible levers are used. These include steps – apart from urgently needed conservation and efficiency-enhancing strategies – to diversify the range of energy carriers with an even greater drive to mobilize renewable energies and to continue developing potential alternative technologies. In public debate about our energy options after the petroleum age virtually no consideration is given to coal or else it gets very bad reviews. In the developed countries, coal is usually considered synonymous with a dangerous climate killer; in the developing countries, for inhuman labor conditions in the mining industry, the talk is of ‘blood coal’. At best, coal is given credit for its valuable contribution to energy security during the industrialization era.

D. Today, coal is used in the industrial countries above all as a source of fuel for generating electricity, for the heating market and for metal production. In some of the emerging economies, coal is still used in some places to fire steam engines. Going forward, coal could attract much more attention in all three major energy sectors – power generation, heating and transport – provided that the right technologies with higher efficiency levels and a low environmental impact take root. In this sense, the versatility of coal has been underestimated. As a substitute for the hydrocarbon fuels oil and natural gas, which will become increasingly scarce in the relatively near future, coal offers considerable potential for improving our energy structures in future. However, only advanced technologies and innovations will be able to pave the way for coal into a better future.

E. As mentioned above, environmental risk emphasizes need for “clean coal”. Global warming is one of the biggest dangers facing human existence on earth, and combating this danger is therefore one of the greatest challenges facing mankind. Fossil fuels, especially those that pose the greatest threat to the earth’s climate, will only have a future if they can be reinvented from an ecological standpoint. Coal accounts for 40% of global output of carbon dioxide (CO2). The ‘bridge to the future’ must therefore lead to ‘clean coal’, which if possible has to be climate neutral and thus acceptable to the public at large. If ‘King Coal’, the mythical figure of the coalmining saga, stops wearing a black robe in future and instead dons an environmentally-friendly white robe, his days will not be numbered and he may go on to prosper the second time round. Until renewable sources of energy are finally mature and established enough to shoulder the burden of the world energy supply largely on their own, the purified ‘clean coal’ may develop into one of the biggest sources of hope for a more secure energy supply.

F. One advantage of coal is that it offers the greatest range of global reserves among the fossil fuels. Plenty of coal reserve, up to about more than 200 years worth, is readily available, almost all over the world. By contrast, the ranges for oil (42 years) and natural gas (63 years) are much smaller.

G. The search for alternatives to conventional fuels did not seem to be an urgent task in late 1998 when a barrel of oil cost less than USD 10. Not quite 10 years later, with oil going for an annual average price of USD 65 in 2006. The most prominent participants in the contest for the fuels of the future are:

(i) First-generation bio-fuels are increasing in popularity in the most diverse countries of the world, such as Brazil, the US and Germany. In Brazil, they have long since become commercially competitive. Research on the second generation, the synthetic bio-fuels (biomass-to-liquids, or BTL), is continuing briskly.

(ii) Natural gas has been a common fuel in some countries for years. More appears to be possible if the catalytic conversion of natural gas proves able to secure the availability of a synthetic fuel, so called GTL (gas-to-liquids), on an industrial scale. GTL and BTL will mean fewer emissions and higher efficiency.

(iii) By means of liquefaction (coal-to-liquids, CTL), coal may directly replace oil even as a fuel. Thanks to higher reserve and resource ranges, coal as a substitute would clearly have an advantage over fuels based on natural gas.

(iv) Nuclear energy will be one of the major contributors to the world energy sources. Although, there is furious opposition against nuclear energy in some part of the world, the advantage of its potential of delivery of clean energy is the major plus point makes it better option.

H. A total of USD 10 trillion is expected to be invested in power generating plants around the globe up to 2030, with over USD 2 trillion being invested in China alone. The need for investment is very high all over the world. For investments, not only the direct costs but also the implications for the world climate will increasingly gain importance. This holds all the more so as over the past 30 years the share of CO2 emissions from coal has risen from 35% to 40% – with total emissions rising by 70% globally. One much more revolutionary project is a plan to develop emission-free coal-fired generating plants. Upstream and downstream CO2 sequestration, for which there are several different methods, aims for climate conservation. Thus, new power generation technology for fewer emissions will become the backbone of industrialization.

‘Clean Coal Technology (CCT)’ – methods to remove pollutants from coal.

‘Clean Coal Technology (CCT)’ – methods to remove pollutants from coal.

Carbon dioxide from burning coal is the main focus of attention today, since it is implicated in global warming, and the Kyoto Protocol requires that emissions decline, notwithstanding increasing energy demand.

A. Capture & separation of Carbon dioxide - A number of means exist to capture carbon dioxide from gas streams, but they have not yet been optimised for the scale required in coal-burning power plants. The focus has often been on obtaining pure CO₂ for industrial purposes rather than reducing CO₂ levels in power plant emissions. Capture of carbon dioxide from flue gas streams following combustion in air is expensive as the carbon dioxide concentration is only about 14% at best. This treats carbon dioxide like any other pollutant and as flue gases are passed through an amine solution the CO₂ is absorbed. It can later be released by heating the solution. This amine scrubbing process is also used for taking CO₂ out of natural gas. There is an energy cost involved. Captured carbon dioxide gas can be put to good use, even on a commercial basis, for enhanced oil recovery. Injecting carbon dioxide into deep, unmineable coal seams where it is adsorbed to displace methane (effectively: natural gas) is another potential use or disposal strategy.

B. Coal arriving at a power plant contains mineral content that needs to be removed, in order to make it clean, before it is burnt. A number of processes are available to remove unwanted matter and make the coal burn more efficiently.

(a) Coal cleaning by washing - Coal washing involves grinding the coal into smaller pieces and passing it through a process called gravity separation. One technique involves feeding the coal into barrels containing a fluid that has a density which causes the coal to float, while unwanted material sinks and is removed from the fuel mix. The coal is then pulverised and prepared for burning.

(b) Gasification of coal – The Integrated Gasification Combined Cycle (IGCC) plant is a means of using coal and steam to produce hydrogen and carbon monoxide (CO) which are then burned in a gas turbine with secondary steam turbine (ie combined cycle) to produce electricity.

Coal gasification plants are favoured by some because they are flexible and have high levels of efficiency. The gas can be used to power electricity generators, or it can be used elsewhere, i.e. in transportation or the chemical industry. In Integrated Gasification Combined Cycle (IGCC) systems, coal is not combusted directly but reacts with oxygen and steam to form a "syngas" (primarily hydrogen). After being cleaned, it is burned in a gas turbine to generate electricity and to produce steam to power a steam turbine. Coal gasification plants are seen as a primary component of a zero-emissions system. However, the technology remains unproven on a widespread commercial scale.

(c) Removing pollutants from coal - Burning coal produces a range of pollutants that harm the environment: Sulphur dioxide (acid rain); nitrogen oxides (ground-level ozone) and particulates (affects people's respiratory systems). There are a number of options to reduce these emissions:

(i) Sulphur dioxide (SO2) - Flue gas desulphursation (FGD) systems are used to remove sulphur dioxide. "Wet scrubbers" are the most widespread method and can be up to 99% effective. A mixture of limestone and water is sprayed over the flue gas and this mixture reacts with the SO2 to form gypsum (a calcium sulphate), which is removed and used in the construction industry.

(ii) Nitrogen oxides (NOx) - NOx reduction methods include the use of "low NOx burners". These specially designed burners restrict the amount of oxygen available in the hottest part of the combustion chamber where the coal is burned. This minimises the formation of the gas and requires less post-combustion treatment.

(iii) Particulates emissions - Electrostatic precipitators can remove more than 99% of particulates from the flue gas. The system works by creating an electrical field to create a charge on particles which are then attracted by collection plates. Other removal methods include fabric filters and wet particulate scrubbers.