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
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
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.
