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

Coal seam gas (CSG) is natural gas contained in coal seams. CSG content is primarily methane (CH₄) and carbon dioxide (CO₂). Methane contents vary greatly in different areas and coal seams. Some CSG contains almost 100% methane and the Illawarra coal measures contain contents up to 95% methane with localised variations. Pure methane is colourless, odourless and tasteless but small amounts of impurities make CSG detectable by smell. 

Coal Seam Gas is contained within many coal seams in huge amounts and has been exploited over many years by means of drilling from the surface. Drill rigs as shown below are used to drill vertical and directionally controlled wells through overburden, coal seams and interburden in order to provide an escape route for the gas. Various methods of well completion are used but production rates of gas vary greatly between individual areas and individual wells and also depend on well completion techniques. The drill rig shown below is a typical example of a rig used for CSG production.

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.

Gas vs Coal

Undoubtedly, high efficiency natural gas-fired power stations can produce up to 70% lower greenhouse gas emissions than existing brown coal-fired generators, and less than half the greenhouse gas emissions of the latest technology black coal-fired power stations. Notice the distinction between black and brown coal, however, exactly how much less CO₂ also depends upon the type of gas-fired station.

There are three main types: the steam boiler, the gas turbine and the combined cycle. The most efficient natural gas turbines are the combined cycle plants where hot exhaust gases are used to raise steam in a waste heat boiler. A combined-cycle gas turbine power plant consists of one or more gas turbine generators equipped with heat recovery steam generators to capture heat from the gas turbine exhaust. Steam produced in the heat recovery steam generators powers a steam turbine generator to produce additional electric power. Use of the otherwise wasted heat in the turbine exhaust gas results in high thermal efficiency compared to other combustion based technologies. 

The CO2 emissions from Natural Gas Combined Cycle (NGCC) plants are reduced relative to those produced by burning coal given the same power output because of the higher heat content of natural gas, the lower carbon intensity of gas relative to coal, and the higher overall efficiency of the NGCC plant relative to a coal-fired plant.

Compared to the average air emissions from coal-fired generation, natural gas produces half as much carbon dioxide, less than a third as much nitrogen oxides, and one percent as much sulphur oxides at the power plant.

In the gas vs coal debate... gas is a viable option in the short-medium term, with renewable energy the solution in the long term.

(Gas vs Coal, 2010)

http://www.global-greenhouse-warming.com/gas-vs-coal.html

Gas Uses

Power generation

Natural gas is a major source of electricity generation. Particularly high efficiencies can be achieved through combining gas turbines with a steam turbine in combined cycle mode. Natural gas burns more cleanly than other Hydrocarbon fuels, such as oil and coal, and produces less carbon dioxide per unit of energy released. For an equivalent amount of heat, burning natural gas produces about 30% less carbon dioxide than burning petroleum and about 45% less than burning coal. Combined cycle power generation using natural gas is thus the cleanest source of power available using hydrocarbon fuels and this technology is widely used wherever gas can be obtained at a reasonable cost. 

Domestic use

Natural gas is supplied to homes where it is used for such purposes as cooking in natural gas-powered ranges and ovens, natural gas-heated clothes dryers, heating/cooling and central heating. Home or other building heating may include boilers, furnaces, and water heaters. Compressed natural gas (CNG) is used in rural homes without connections to piped-in public utility services, or with portable grills.

Transportation

CNG is a cleaner alternative to other automobile fuels such as petro) and diesel. As of 2008 there were 9.6 million natural gas vehicles worldwide, led by Pakistan (2.0 million), Argentina (1.7 million), Brazil (1.6 million), Iran (1.0 million), and India (650,000). The energy efficiency is generally equal to that of gasoline engines, but lower compared with modern diesel engines. Petrol vehicles converted to run on natural gas suffer because of the low compression ratio of their engines, resulting in a cropping of delivered power while running on natural gas (10%-15%). CNG-specific engines, however, use a higher compression ratio due to this fuel's higher octane number of 120–130.

Fertilizers

Natural gas is a major feedstock for the production of ammonia, via the Haber process, for use in fertilizer production.

Aviation

The advantages of liquid methane as a jet engine fuel are that it has more specific energy than the standard kerosene mixes do and that its low temperature can help cool the air which the engine compresses for greater volumetric efficiency, in effect replacing an intercooler. Alternatively, it can be used to lower the temperature of the exhaust.

Hydrogen

Natural gas can be used to produce hydrogen, with one common method being the hydrogen reformer. Hydrogen has many applications: it is a primary feedstock for the chemical industry, a hydrogenating agent, an important commodity for oil refineries, and a fuel source in hydrogen vehicles.

Other

Natural gas is also used in the manufacture of fabrics, glass, steel, plastics, paint, and other products.

Storage and Transport

Because of its low density, it is not easy to store natural gas or transport by vehicle. Natural gas pipelines are impractical across oceans.

LNG carriers transport liquefied natural gas (LNG) across oceans, while tank trucks can carry liquefied or compressed natural gas (CNG) over shorter distances. Sea transport using CNG carrier ships that are now under development may be competitive with LNG transport in specific conditions.

Gas is turned into liquid at a liquefaction plant, and is returned to gas form at regasification plant at the terminal. Shipborne regasification equipment is also used. LNG is the preferred form for long distance, high volume transportation of natural gas, whereas pipeline is preferred for transport for distances up to 4,000 km over land and approximately half that distance offshore.

CNG is transported at high pressure, typically above 200 bars. Compressors and decompression equipment are less capital intensive and may be economical in smaller unit sizes than liquefaction/regasification plants. Natural gas trucks and carriers may transport natural gas directly to end-users, or to distribution points such as pipelines.

In the past, the natural gas which was recovered in the course of recovering petroleum could not be profitably sold, and was simply burned at the oil field in a process known as flaring. Flaring is now illegal in many countries. Additionally, companies now recognize that gas may be sold to consumers in the form of LNG or CNG, or through other transportation methods.

Natural gas is used to generate electricity and heat for desalination. Similarly, some landfills that also discharge methane gases have been set up to capture the methane and generate electricity.

Natural gas is often stored underground inside depleted gas reservoirs from previous gas wells, salt domes, or in tanks as liquefied natural gas. The gas is injected in a time of low demand and extracted when demand picks up. Storage nearby end users helps to meet volatile demands, but such storage may not always be practicable.

Natural gas is a gas consisting primarily of methane, It is found associated with other hydrocarbon fuel, in coal beds, as methane, and is an important fuel source and a major feedstock for fertilizers.

Most natural gas is created by two mechanisms: biogenic and thermogenic. Biogenic gas is created by methanogenic organisms. Thermogenic gas is created from buried organic material.

Before natural gas can be used as a fuel, it must undergo processing to remove almost all materials other than methane. The exception to this is where systems are designed to utilise raw gas. Natural gas is often informally referred to as simply gas, especially when compared to other energy sources such as oil or coal.

CO2 emissions

Natural gas is often described as the cleanest fuel, producing less carbon dioxide per joule delivered than either coal or oil and far fewer pollutants than other hydrocarbon fuels. Natural gas itself is a greenhouse gas more potent than carbon dioxide when released into the atmosphere, although natural gas is released in much smaller quantities. However, methane is oxidized in the atmosphere, and hence natural gas has a residence lifetime in the atmosphere for approximately 12 years, compared to CO2, which is already oxidized, and has an effect for 100 to 500 years. Natural gas is mainly composed of methane, which has a radiative forcing twenty times greater than carbon dioxide. Based on such composition, a ton of methane in the atmosphere traps in as much radiation as 20 tons of carbon dioxide, but remains in the atmosphere for a 8-40 times shorter time. Carbon dioxide still receives the lion's share of attention over greenhouse gases because it is released in much larger amounts. Still, it is inevitable when natural gas is used on a large scale that some of it will leak into the atmosphere. (Coal methane not captured by coal seam gas extraction techniques is simply lost into the atmosphere; however, most methane in the atmosphere is currently from animals and bacteria, not from human's leaks.).  

Other pollutants

Natural gas produces far lower amounts of sulphur dioxide and nitrous oxides than any other hydrocarbon fuel. Carbon dioxide produced is 117,000 ppm vs 208,000 for burning coal. Carbon monoxide produced is 40 ppm vs 208 for burning coal. Nitrogen oxides produced is 92 ppm vs 457 for burning coal. Sulfur dioxide is 1 ppm vs 2,591 for burning coal. Mercury is 0 vs .016 for burning coal. Particulates are also a major contribution to global warming. Natural gas has 7ppm vs coal's 2,744ppm