Coal bed methane (CBM) drainage - Potential uses of coal mines methane:

Coal bed methane (CBM) drainage - Potential uses of coal mines methane:

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 (<>

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.

Coal-bed Methane (CBM) Drainage from Underground Coal Mines:

Coal-bed Methane (CBM) Drainage from Underground Coal Mines:

Coal mine methane, a byproduct of mining operations, can be recovered to provide various types of benefits to a mining company. These benefits include, but are not limited to, reduced ventilation costs, downtime costs, and production costs; and 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. 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 coalbed methane from their seams before or during mining. Methane drainage offers the added advantages of reducing the ventilation costs, reducing the development costs of the mine, reducing the global warming threat, and 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 coalbed 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.

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.

Use of methane as a fuel and for other purposes:

Use of methane as a fuel and for other purposes:

Methane (CH4) is the simplest alkane, used as a fuel for various use. Apart from methane being principal constituent of natural gas, it is obtained from coal seams (as coal bed methane, CBM) and also it is obtained from bio-mass. At room temperature, methane is a gas less dense than air. Methane's relative abundance and clean burning process makes it a very attractive fuel. Burning one molecule of methane in the presence of O2 (oxygen) releases one molecule of CO2 (carbon dioxide) and two molecules of H2O (water). Methane being gas in ordinary temperature, its transportation and storage is difficult. Methane is a powerful greenhouse gas. Methane is over 20 times more effective in trapping heat in the atmosphere than carbon dioxide (CO₂)

Some methane is manufactured synthetically by the distillation of coal. Coal also contains hydrogen and oxygen, with small concentrations of nitrogen, chlorine, sulfur, and several metals. Coals are classified by the amount of volatile material they contain. Volatile substances released from coal when it is distilled, in addition to methane, include water, carbon dioxide, ammonia, benzene, toluene, naphthalene, and anthracene. In addition, the distillation also yields oils, tars, and sulfur-containing products. The non-volatile component of coal, which remains after distillation, is coke.

At high temperatures (700 to 1100 degree Celsius) in the presence of nickel catalyst, steam reacts with methane to yield CO (carbon monoxide) and H2 (hydrogen). This hydrogen is used for manufacturing of ammonia (NH3). In near future, one of the greatest uses of hydrogen would be for running vehicle by using environment-friendly hydrogen cell technology.

Methane is important for electrical generation by burning it as a fuel in a gas turbine or steam boiler. Compared to other hydrocarbon fuels, burning methane produces less carbon dioxide for each unit of heat released. In many cities, methane is piped into homes for domestic heating and cooking purposes. Methane in the form of compressed natural gas (CNG) is used as a fuel for vehicles, and is claimed to be more environmentally friendly than alternatives such as gasoline/petrol and diesel.

Note: NASA is developing LOX/methane engines as an option for the future rocket engine, as methane is abundant in the outer solar system.