Coal-bed Methane (CBM) drainage from Underground Coal Mines – Safer and eco-friendly option of green energy:
A. Introduction – The primary energy source of natural gas is a substance called methane (CH4). Coal bed methane (CBM) is simply methane found in coal seams. CBM is generated either from a biological process as a result of microbial action or from a thermal process as a result of increasing heat with depth of the coal. Often a coal seam is saturated with water, with methane is held in the coal by water pressure. CBM as a source of clean natural gas has immense potential. Methane gas is found trapped in fissures in coal and extraction reduces explosion hazards in mines, thereby reducing safety risks for miners. Coal-bed methane (CBM) – deleterious to the environment when released to the atmosphere – is actually a remarkably clean fuel when burned. Methane gas when burnt as fuel gives zero emission, whereas when released into the atmosphere its global warming potential is 21 times that of carbon dioxide. Coal mining contributes an estimated 10 percent of total methane emissions from human activities.
Thus, coal mine methane, a byproduct of mining operations, can be recovered to provide various types of benefits to a mining company. The benefits include, (a) reduced ventilation costs, downtime costs, and production costs; and (b) the ability to use the recovered gas as an energy source, either at or near the mine site or by injecting it into a commercial gas pipeline system.
Coal-bed Methane (CBM) Drainage system is degasification system to extract much of the coal-bed methane from coal seams before or during mining. There are many variables that play a part in the decision to implement a coal mine methane drainage project. Mining companies can employ basic decision-making logic to determine the feasibility of draining and/or using methane at specific coal mines.
Over the past few decades, emissions of methane from coal mines have increased significantly because of higher productivity, greater comminution of the coal product, and the trend towards recovery from deeper coal seams. Under current coal mine regulations of many countries, methane must be controlled at the working faces and at other points in the mine layout. This has traditionally been performed using a well-designed ventilation system. However, this task is becoming more difficult to achieve economically in modern coal mines. In addition, scientists have established that methane released to the atmosphere is a major greenhouse gas, second only to carbon dioxide in its contribution to potential global warming. In order to improve mine safety and decrease downtime as a result of methane in the mine openings, many mines are now using a degasification system to extract much of the coal-bed methane from their seams before or during mining.
Methane drainage offers the added advantages of (a) reducing the ventilation costs, (b) reducing the development costs of the mine, (c) reducing the global warming threat, and (d) allowing a waste product to be productively utilized.
This byproduct can be gathered to produce three levels of benefits to a mining company, depending on the market potential of the methane. The benefit levels are as follows:
(1) The methane is gathered from the coal seam to reduce ventilation costs, downtime costs, production costs, and shaft development costs or to benefit from increased coal resources. All of these benefits are achieved internal to the mining operation and can be easily analyzed by the mining company.
(2) The coal-bed methane is extracted from the seams to be mined and is utilized as a local energy resource to heat buildings, dry coal output from the coal preparation facility, generate electrical power, power vehicles by compressing the gas, or other local uses.
(3) The extracted methane can be upgraded, if necessary, or immediately compressed and introduced into a commercial gas pipeline system. This may provide the highest possible benefit to the mining company providing that the methane is of high quality and the mine location is near a gas pipeline. With this option, the value of the methane as an energy resource may be very large and it can make a significant contribution to profits.
However, in the process of coal-bed methane extraction, water from the coal bed is first pumped out and since the water is usually rich in sodium, its salinity makes it unfit for irrigation or drinking purposes. Therefore, waste-water disposal from coal-bed methane system needs to take care in order not to adversely affect the local water table or stunt plant growth. Unless it is diluted sufficiently with normal water, saline content will be far too high to use as normal irrigation.
USA and Canada have been exploring methane obtained from coal beds (CBM) since the early 1970s and 1980s, respectively. Production of coal-bed methane for domestic energy needs has grown significantly in USA. Other coal-producing countries like Australia, China, Russia, Germany, Great Britain, Poland, etc. including India too have paid attention to the exploration of this new resource and have initiated several research programmes on different aspects of coal-bed methane.
B. Methane degasification methods – With the increasing coal production and depth of coal mines, traditional ventilation methods are not always the most economical methods of handling methane in the coal seam. Degasification systems have been developed that recover the gas before, during, or after mining. The degasification methods, coupled with mine ventilation, may be the most economical method of keeping methane concentrations low in many mines.
Degasification methods that have been used in the U.S. include vertical wells, gob wells, horizontal boreholes, and cross-measure boreholes.
(1) Vertical wells method - The term “vertical well” is generally applied to a well drilled through a coal seam or seams and cased to pre-drain the methane prior to mining. The wells are normally placed in operation 2 to 7 years ahead of mining and the coal seam is hydraulically fractured to remove much of the methane from the seam. The water in the coal seams must be removed to provide better flow of gas. This water is separated and must then be treated and/or disposed of in an environmentally acceptable manner. To enhance the flow of gas from a vertical well, either hydraulic fracturing or open-hole cavity completions are generally used.
Vertical wells recover high-quality gas from the coal seam and the surrounding strata. The gas quality is ensured in most cases because the methane will not be diluted by ventilation from the mine. The total amount of methane recovered depends on site-specific conditions such as the gas content of the coal seams and surrounding strata, permeability of the geologic materials, the drainage time, the amount of negative head applied, and other variables of the geologic and extractive systems. Vertical wells can recover 50% to 90% of the gas content of the coal and are normally placed in operation two to seven years before mining commences.
Vertical wells offer an advantage over other methods because they can be applied to multiple coal seams simultaneously. These wells produce greater gas yields that can make them commercially economic as well as further reduce the potential for gas influx into the operating mine.
(2) Gob Wells - The designation “gob well” refers to the type of coalbed methane (CBM) recovery well that extracts methane from the gob areas of a mine after the mining has caved the overlying strata. Gob wells differ from vertical wells in the sense that they are normally drilled to a point 10 to 50 feet above the target seam prior to mining, but are operated only after mining fractures the strata around the wellbore. The methane emitted from the fractured strata then flows into the well and up to the surface. The flow rates are mainly controlled by the natural head created by the low-density methane gas or can be stimulated by blowers on the surface. Gob wells can recover 30% to 70% of methane emissions depending on geologic conditions and the number of gob wells within the panel.
(3) Horizontal Boreholes - Horizontal holes are drilled into the coal seam from development entries in the mine. They drain methane from the unmined areas of the coal seam shortly before mining, reducing the flow of methane into the mining section. Because methane drainage occurs only from the mined coal seam and the period of drainage is relatively short, the recovery efficiency of this technique is low.
(4) Cross-Measure Boreholes - Cross-measure boreholes are drilled at an angle to the strata, normally from existing mine entries. The boreholes are strategically placed above areas to be mined with the goal of pre-draining the overlying strata and exhausting gas from the gob area. Like horizontal borehole systems, the individual holes must be connected to a main pipeline which ordinarily is coursed through a vertical borehole to the surface.
C. Economic benefits of coal-bed methane (CBM) drainage - There are many mining benefits that accrue from a methane drainage system. Coal-bed methane drainage systems can: (1) enhance coal productivity because of less frequent downtime or production slowdowns caused by gas; (2) decrease fan operating costs because of reduced air requirements for methane dilution; (3) reduce shaft sizes and number of entries required in the mains, (4) increase tonnage extracted from a fixed-size reserve as a result of shifts of tonnage from development sections to production sections; (5) decrease dust concentrations due to reduction of velocities at the working face; (6) improve mine safety resulting from lower methane contents in the face, returns, gobs and bleeders; (7) reduce problems with water; (8) improve worker comfort through reduction of velocities in the working faces; and (9) provide miscellaneous other benefits. Other benefits, such as reduced dust concentration, improved safety, or improved worker comfort, are difficult to estimate; while they constitute a real and significant benefit.
(1) Reduced Downtime – Enhanced coal productivity is probably the most significant benefit to be obtained from methane pre-drainage systems where coal-bed methane is encountered in significant quantities. The benefits come in the form of added production that occurs when downtimes or slowdowns resulting from high methane occurrences are avoided using methane drainage.
(2) Ventilation Power Cost Savings – The power costs associated with the mine ventilation system will ordinarily be the second most significant benefit associated with the addition of a methane drainage effort. In many mines, ventilation to ensure continuous production is quite expensive. Methane drainage would normally be used instead of increased ventilation because the overall costs associated with drainage will be lower than the costs associated with ventilation.
(3) Reduced Development Costs- Another important issue in assessing the costs and benefits associated with mining is the possibility that a reduction in the ventilation requirements will result in a reduced requirement for development openings. This can result in two types of cost benefits. The first benefit is the reduction in the size and number of shafts and other development openings connecting the coal seam to the surface. This can at times result in a significant level of economic savings. The second benefit results if the coal from the development entries of a mine is more costly on a cost/ton basis than that in the production sections. In a longwall mining operation, the coal produced from development openings will be much more costly than that produced in a longwall panel.
(4) Increased Reserve – The benefit of an increased reserve is also provided in a mining operation when a gas drainage system allows for a reduced number of entries in the development of mains, submains, headgates, and tailgates of mining layouts. This results in an increased number of tons of coal that can be extracted from a fixed-size coal block. The extra tonnage is derived from the fact that only about 50% of the coal in development sections is extracted while production sections may extract 85% to 95% of the coal under good conditions. The extra coal extracted when this occurs is an economic benefit of significant value under many conditions. Thus, it should be evaluated as a potential benefit in every operation where degasification is considered.
(5) Mine Safety - The effect of a methane drainage system on the safety of a mining system will certainly result in positive benefits. Any high-methane operation will incur a higher level of hazardous operating conditions than an equivalent mine with a methane drainage system in place.
(6) Reduced Dust Problems – The relationship between gas drainage activities and the costs of providing proper dust control in a mining section is another possible source of cost benefits from gas drainage.
(7) Reduced Water Problems - The presence of water in coal mine roof strata can be a costly source of delays in some underground mining operations. The most sizeable delays will ordinarily be encountered in the development sections of the mine and will be quite variable depending upon the geologic parameters of the roof strata. The water in the roof, when occurring in conjunction with high methane contents, can be mitigated by a methane drainage system.
(8) Worker Comfort - The level of comfort of work in a mining environment deteriorates if high air velocities are required to keep methane contents below the regulatory limits. The difficulty of working in an air velocity above 600 ft/min is that ordinary tasks become more difficult and the high velocities will generate more dust.
Thus, extraction of coal-bed methane provide lot of economical benefits in running coal mines, apart from providing cleaner environment by preventing release of major greenhouse gas, methane, in the atmosphere and recovering extra energy source as well.
D. Potential uses of coal mines methane obtained from Coal bed methane (CBM) drainage – One of the major decisions facing a mine owner, when considering the implementation of a CBM drainage program is the potential use for the gas. The gas is a clean energy resource. However, the location of the mine and the ability to convert the gas into a marketable product may severely test the mine planners’ perseverance in finding an economic way of using the gas and producing the accompanying reduction in greenhouse gases. Here we would try to outline some possibilities for the gas whether it is a high-Btu, medium-Btu, or low-Btu product.
(1) High-Btu Gas (> 950 Btu/scf) – High-Btu gas is generally defined as having enough heat content to be used in a natural gas pipeline. Several potential uses exist for high-Btu gas. If the drainage system provides primarily CH4 and little in the way of inert gas, the product may be gathered, compressed, and marketed to a pipeline company. This is one of the most desirable options if natural gas pipelines are located near the mine. Thus, marketing of coal mines methane to a pipeline company would be a very desirable goal.
In case, pipelines are not readily available or the pipeline companies are not ready to buy coal mines methane, several other options are available for high-Btu gas. The first of these would be to use the gas as a feedstock to produce ammonia, methanol, or acetic acid. Currently, these chemicals are produced from natural gas, but coal-bed methane would be equally useful if it is available in sufficient quantities and if the chemical plants were in a favorable location. Another potential method of using CBM would be to compress or liquefy it for use in buses, trucks, and automobiles. This implementation has been successfully used in many of the CIS countries like Ukraine, Czech Republic etc.
(2) Medium-Btu Gas (300 to 950 Btu/scf) – There are many possible uses for medium-Btu gas. If the gas is at the high end of the heat content scale, enrichment by blending with a higher-quality gas or ‘spiking’ of the gas to produce a gas of pipeline quality is possible. Enrichment is the removal of gases like nitrogen, oxygen, and carbon dioxide to improve the heat content of the gas. ‘Spiking’ is the process of combining another fuel gas (like propane) with the methane to increase the heat content. Spiking will normally be economic only if the supplement gas is available cheaply in the area. A major and growing use of medium-Btu gas is as a substitute for other fuels in space heating and other applications where natural gas, fuel oil, or coal is normally used. For example, CBM can be used for heating mine facilities, heating mine intake air, heating greenhouses and institutional facilities, as a heat source in a thermal dryer and as a heat source for treating brine water.
Another use for medium-Btu methane is in electric power production. Using methane in coal-fired utility and industrial boilers and as a supplement to natural gas in blast furnaces is common where methane is extracted from coal mines.
(3) Low-Btu Gas (< 300 Btu/scf) – In most cases, these mines, where low-Btu gas is available, handle methane using ventilation alone and the gas is released into the atmosphere with the exhaust air of the mine. The concentration of methane is below 1%, making it impossible for use as a primary energy source. However, the option of using this waste energy is favorable under the right conditions and should be considered where the mine and a production facility can be located close to each other.
E. Environmental issues related to CBM drainage – The core environmental issues related to CBM drainage are listed below:
(1) Groundwater table draw down due to pumping large quantities of groundwater;
(2) Disposal of large volumes of produced water;
(3) Methane contamination of shallow groundwater;
(4) Noise pollution from compressors and other sources;
(5) Air pollution from compressor exhaust gases, methane leakage, and dust; and
(6) Surface disturbance from construction of roads, pipelines, and other facilities.
In CBM production, water is produced in large volumes and must be disposed of. Because waters produced from coal beds are often fresh and subsurface disposal is expensive, disposal to surface drainages, wherever possible, carries a strong economic incentive. Such disposal may erode soils and sediments, change microclimate, create unsustainable aquatic habitats, or salinize soils. Additionally, the organic and inorganic chemistry of coal waters has not been studied comprehensively; dissolved contaminants in coal waters, such as phenols or arsenic, may damage the environment.
F. Summary and Conclusion - CBM is a potentially important energy resource in many of the major coal mining countries of the world. Significant volumes of CBM are exploited worldwide with most of the gas originating from operational deep coal mines, and lesser quantities recovered from abandoned mine workings. Many coal-producing countries are now looking at the potential for wider application of CBM technologies to maximise the exploitation of gas from coal seams. CBM is a clean fuel with similar properties to natural gas when not diluted by air or other non-combustible mine gases. CBM can be recovered from coal seams by:
• Drainage in working coal mines (CMM),
• Extraction from abandoned coal mines (AMM),
• Production from unmined (virgin) coal using surface boreholes (VCBM).
The characteristics of each of these CBM sources are different in terms of reservoir characteristics, production technology and gas composition. The principal constituent of CBM is methane (typically 80-95%), sometimes with lower proportions of ethane, propane, nitrogen and carbon dioxide (CO2). The gas produced from coal mines consists of mixtures of methane and higher alkanes, water vapour, air, nitrogen (deoxygenated air) and CO2.
CBM technologies are being developed in Australia and North America with emphasis on VCBM production and CMM utilisation options. Technologies for using methane in ventilation air are receiving attention in the USA and developments are being pursued in Australia, Canada and Sweden.
Summery of specific options for utilization of Coal-bed methane from mines:
a. Power Generation – CBM can be ideal fuel for co-generation Power plants to bring in higher efficiency and is preferred fuel for new thermal power plant on count of lower capital investment and higher operational efficiency.
b. Auto Fuel in form of Compressed Natural Gas (CNG) – CNG is already an established clean and environment friendly fuel. Depending upon the availability of CBM, this could be a good end use. Utilization of recovered CBM as fuel in form of CNG for mine dump truck is a good option.
c. Feed stock for Fertilizer – Many of the fertilizer plants in the vicinity of coal mines where coal-bed methane is drained, have started utilizing fuel oil as feedstock for its cracker complex.
d. Use of CBM at Steel Plants – Blast furnace operations use metallurgical coke to produce most of the energy required to melt the iron ore to iron. Since coke is becoming increasingly expensive, in the countries where CBM is available, the steel industry is seeking low-capital options that reduce coke consumption, increase productivity and reduce operating costs.
e. Fuel for Industrial Use – It may provide an economical fuel for a number of industries like cement plant, refractory, steel rolling mills etc.
f. CBM use in Methanol production – Methanol is a key component of many products. Methanol and gasoline blends are common in many countries for use in road vehicles. Formaldehyde resins and acetic acid are the major raw material in the chemical industry, manufactured from methanol.
g. Other uses – Besides above, option for linkages of coal-bed methane produced by coal mines, through cross country pipe lines may be considered.
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