Coal and Gas Formation

Coal beds were formed tens of millions of years ago through the progressive deposition and subsequent compression, decomposition and heating of the organic material. This process causes the generation of gas with varying compositions. Methane and other flammable gases are produced in varying amounts and is contained at the molecular level and adsorbed onto the coal surfaces within the coals structure. This gas is known as Thermogenic Gas. Methane is also progressively created by microbiological action and this gas is known as Biogenic gas. Coal seams can also contain large quantities per cubic metre of other gases such as Carbon Dioxide and Hydrogen Sulphide. The composition of the gas contained within coal is highly variable not only within a seam but between seams and can be affected by geological activity such as faults and igneous intrusions. The proximity of a coal seam to the surface can also cause variations in coal seam gas composition with factors such as gas relative density and molecular weight being causing variations. Therefore coal seam gas can be contained in quantities in excess of 30 cubic metres per tonne in wide variations of compositions from almost 100% Methane to almost 100% Carbon Dioxide. Gas exploited by means of methods not involving coal mining is known as Coal Seam Gas (CSG). Gas exploited or liberated by means of mining activity is known as Coal Mine Gas (CMG).

Coal Seam Gas

A combination of gas transmission boreholes and coal seam water extraction and handling facilities are utilised in order to effectively extract the gas. Multiple coal seams can be accessed with wide drilling patterns designed for optimum gas contact.

Coal Mine Gas

Aside from environmental considerations, methane emissions from coal mining are a very serious safety hazard. When methane of approximately 5% to 15% is mixed with air, the mixture is explosive and generally responsible for the all too familiar explosions in underground coal mines all over the world.

Coal mining companies employ different methods to reduce the possibilities of these mine explosions. The principal method is to force very large quantities of ventilation air into the underground workings to keep the methane content at the coal mining face below the lower explosive limit of 5% methane, actually in practice to much lower levels depending on relevant legislation. Air is caused to flow around a mine by means of a ventilation system. Pressure differential is the driver and this differential is created by means of ventilation fans situated on the surface and sometimes assisted by underground fan installations. The air picks up gas as it travels around the mine. This gas is known as Ventilation Air Methane (VAM). The VAM can be released directly into the atmosphere on the surface without treatment, burned through a flare or utilised for power generation. VAM has the lowest concentration levels of all forms of recoverable methane from coal seams because of its high exposure to air; often displaying levels of 0.05-0.8%.

The picture below shows an installation of surface ventilation fans.

Such wells can be drilled from the surface but are more commonly drilled by underground drill rigs and crews of skilled drillers. The picture below shows an underground gas well being drilled.

Gas produced from these wells may be of a sufficiently high quality that it can be sold to natural gas pipeline companies. This not only reduces methane emissions to the atmosphere but converts the methane into a useful resource.

When the mining operations reach the area of these gas extraction wells, the mine roof collapses forming goafs or gobs. Gas continues to be produced in these goaf/gob areas, but is typically mixed with air from the mine ventilation system. Gas extracted from these goaf/gob areas, even in coal seams high in methane content areas may contain only 30% to 95% methane. A combination of ventilation, surface extraction wells and wells drilled from underground can be used to reduce goaf/gob gas.

The diagram below shows how such a combination of methods can be used to capture this gas.

Destruction & Utilisation

CSG is exploited in order that the gas produced can be utilised. Utilisation includes the use of the produced gas as fuel for power generation facilities or the production of Liquid Natural Gas (LNG). CSG companies will typically burn produced gas through a flare during testing phases in order to establish the commerciality of a gas field. CMM can be also used for power generations, flared or simply vented untreated into the atmosphere. This last option is becoming unacceptable and coal mining companies are increasingly looking to treat any mine gas produced in a manner that reduces environmental impact and provides some financial benefit from gas capture.

(1) CSG & CMM Power Generation

Gas extracted from coal seams and coal mines has long since been utilised in various degrees of complexity and size throughout the world. Systems for the heating of the mines water systems to full power generation facilities have been used. Power generation facilities can be designed to use reciprocating engines burning the gas to turn generators but modern and efficient combined cycle gas turbines are becoming more popular. Localised power generation facilities of 1 - 4 MW to huge complex systems in excess of 500 MW are currently being used and the use of CSG and CMG is becoming more popular as nations try to reduce Green House Gas (GHG) emissions and also harness a previously wasted energy source. The picture below shows a large 400MW modern combine cycle gas turbine power station.

The picture below shows a reciprocating engine facility set up at head of a coal mine to generate circa 50MW of power.

(2) Ventilation Air Methane (VAM) Power Generation

If projects are seeking to take advantage of the benefits that CMM can provide as an energy source, there are alternatives to simply destroying the gas through flaring systems. Although both VAM and goaf/gob gas provide much lower methane concentrations than methane recovered from unmined coal seams, there are power generation technologies available today that can harness the energy production potential of these resources. VAM can not only be used for combustion dilution and cooling purposes in standard gas turbines, but it can also be used as a primary fuel in a number of ‘lean-burn' gas turbine systems. These systems can utilise VAM with methane concentrations as low as 1% (hence the term lean-burn). Therefore these systems can harness the energy potential of high percentages of the VAM recovered from working mines.

VAM's potential as an energy source can also be harnessed by a number of oxidation systems available on the market today. Methane can be converted to CO2 by the process of oxidation, thus reducing its global warming potential. This process also creates energy which can be used to generate heat or power. Oxidation systems can utilise VAM with methane concentration levels of less than 1%. These systems are often deployed on-site to provide auxiliary heat and power to the mine.

The artist's impression below shows a Hybrid Coal & Gas Turbine (HCGT) facility harvesting VAM at the top of a mine upcast shaft (www.eestechinc.com)

(3) Flaring

Options exist for destroying gas that would otherwise be released directly into the atmosphere. Flaring is an important technology for disposing of the methane safely and efficiently and can help to significantly reduce a major source of Green House Gas (GHG) emissions. The flared methane is converted to CO2, heat and water. Although flaring still leads to GHG emissions in the form of CO2, methane's global warming potential is 23 times greater than that of CO2. Therefore flaring actually reduces the overall greenhouse effect. However, the resulting CO2 emissions still clearly present a huge challenge in terms of combating global warming and flaring is therefore not regarded as the most efficient or environmentally friendly of end use options.

Flaring can be performed in either open or enclosed systems, and the technique is similar to that deployed in the oil and gas industries. This method of methane disposal is relatively cheap when compared to the extra costs incurred in developing power generation infrastructure or incorporating recovered methane into a region's natural gas pipeline network. The picture below shows a typical closed flare system.