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Longwall mining is a form of underground coal mining where a long wall of coal is mined in a single slice (typically 1-2 m thick). The longwall "panel" (the block of coal that is being mined) is typically 3-4 km long and 250-400 m wide. Longwall mining produced 176 million tons of coal in 2007, about 15 percent of total U.S. production. Longwall mines are located in Appalachia as well as and in Illinois, Utah, Colorado, and New Mexico.
Southwestern Pennsylvania has six of the United States’ top 25 longwall mines, underlying 138,743 acres of rural terrain, or 15 percent of the surface area. The remaining 19 U.S. mines are scattered among West Virginia, Ohio, Virginia, Kentucky, Indiana, and western states.
The basic idea of longwall mining was developed in England in the late 17th century. Miners would undercut the coal along the width of the coal face, removing coal as it fell, and using wooden props to control the fall of the roof behind the face. this was known as the Shropshire method of mining. While the technology has changed considerably, the basic idea remains the same, to remove essentially all of the coal from a broad coal face and allow the roof and overlying rock to collapse into the void behind, while maintaining a safe working space along the face for the miners.
Starting around 1900, mechanization was applied to this method. By 1940, some referred to longwall mining as "the conveyor method" of mining, after the most prominent piece of machinery involved. Unlike earlier longwall mining, the use of a conveyor belt parallel to the coal face forced the face to be developed along a straight line. The only other machinery used were an electric cutter to undercut the coal face and electric drills for blasting to drop the face. Once dropped, manual labor was used to load coal onto the conveyor parallel to the face and to place wooden roof props to control the fall of the roof.
Such low-technology longwall mines continued in operation into the 1970s. The best known example of this was the New Gladstone Mine near Centerville, Iowa. This longwall mine did not even use a conveyor belt, but relied on ponies to haul coal tubs from the face to the slope mining where a hoist hauled the tubs to the surface.
Longwall mining has been extensively used as the final stage in mining old room and pillar mines. Room and pillar is a mining system in which the mined material is extracted across a horizontal plane, while "pillars" of untouched material is left to support the overburden, leaving open areas or "rooms" underground. It is usually used for relatively flat-lying deposits, such as those that follow a particular stratum. When used in this context, longwall mining can be classified as a form of retreat mining, a term used to reference the final phase of an underground mining.
The gate road along one side of the block is called the maingate or headgate; the road on the other side is called the tailgate. Where the thickness of the coal allows, these gate roads have been previously developed by continuous miner units, as the longwall itself is not capable of the initial development. In thinner seams the advancing longwall mining method may be used. In this system the gate roads are formed as the coal face advances. Only the maingate road is formed in advance of the face. The tailgate road is formed behind the coal face by removing the stone above coal height to form a roadway that is high enough to travel in. The end of the block that includes the longwall equipment is called the face. The other end of the block is usually one of the main travel roads of the mine. The cavity behind the longwall is called the goaf, goff, or gob.
Fresh air travels up the main gate, across the face, and then down the tail gate. Once past the face the air is no longer fresh air, but return air carrying away coal dust and mine gases such as methane, carbon dioxide, depending on the geology of the coal. Return air is extracted by ventilation fans mounted on the surface. A series of seals are erected as mining progresses to maintain goaf gas levels. 
Typically to avoid coal in the goaf spontaneously combusting, goaf gases are allowed to build up so as to exclude oxygen from the goafed area. This means that there is an explosive goaf fringe between the face and the goaf at all times requiring constant monitoring, which can become dangerous for miners if coal companies neglect or ignore safety precautions.
The longwall equipment includes:
- A number of hydraulic jacks, called powered roof supports, chocks or shields, which are typically 1.75m wide and placed in a long line, side by side for up to 400 m in length in order to support the roof of the coalface. An individual chock can weigh 30-40 tonnes, extend to a maximum cutting height of up to 6 m and have yield rating of 1000-1250 tonnes each, and hydraulically advance itself 1m at a time.
- The coal is cut from the coalface by a machine called the shearer (power loader). This machine can weigh 75-120 tonnes typically and comprises a main body, housing the electrical functions, the tractive motive units to move the shearer along the coalface and pumping units (to power both hydraulic and water functions). At either end of the main body are fitted the ranging arms which can be ranged vertically up down by means of hydraulic rams, and onto which are mounted the shearer cutting drums which are fitted 40-60 cutting picks. Within the ranging arms are housed very powerful electric motors (typically up to 850 kW) which transfer their power through a series of lay gears within the body the arms to the drum mounting locations at the extreme ends of the ranging arms where the cutting drums are. The cutting drums are rotated at a speed of 20-50 revs/min to cut the mineral from coal seam.
- The shearer moves along the length of the face sat upon the armoured face conveyor, driving through a chainless haulage system (which resembles a ruggedised rack and pinion system especially developed for mining),Previous to chainless haulage systems, a heavy duty chain was run the length of the coal face for the shearer to pull itself along the face. The shearer moves at a speed of 10-30m/min depending on cutting conditions. The AFC on which the shearer sits, is placed in front of the powered roof supports, and the shearing action of the rotating drums cutting into the coal seam, disintegrates the coal this being loaded onto the AFC. The coal is removed from the coal face by a scraper chain conveyor called the AFC to the main gate. Here it is loaded onto a conveyor belt and transported to the surface, usually via a network of conveyor belts.
- At the main gate the coal is usually reduced in size in a crusher, and loaded onto the first conveyor belt by the beam stage loader (BSL).
As the shearer removes the coal, the AFC is snaked over behind the shearer and the powered roof supports move forward into the newly created cavity. As mining progresses and the entire longwall progresses through the seam, the goaf increases. This goaf collapses under the weight of the overlying strata. The strata approximately 2.5 times the thickness of the coal seam removed collapses and the beds above settle onto the collapsed goaf. This collapsing can lower surface height considerably, causing serious problems like changing the course of rivers and severely damage building foundations.
The advantages of longwall mining include:
- better resource recovery (about 80% compared with about 60 percent for room and pillar method).
- safety of the miners enhanced by being under the hydraulic roof supports when they are extracting coal.
The disadvantages of longwall mining include:
- surface subsidence, which may considerably alter the landscape above the mine which can damage natural or man-made structures or features.
- deterioration in groundwater quality, as it can become more brackish with increased sulfate levels.
- disruption of ecosystems and loss of stream and wetland functions.
- shock waves on the surface.
According to the Center for Public Integrity's study of southwestern Pennsylvania, the following damages have occurred:
- Due to subsidence, homes suffer shattered foundations, crooked roofs, and cracked plaster. As of September, 2008, 1,819 property owners had reported longwall damages since the state of Pennsylvania began documenting such complaints.
- Longwall is permanently lowering the area’s water table and draining its aquifers; state regulators have reported damages to 23 stretches over 97 miles of mined streams.
There is still the possibility of coal mining accidents, as roofs can collapse, and the process of longwall mining stirs up harmful coal dust and explosive methane, thought to be the cause of the explosion at Massey Energy's Upper Big Branch Mine in April 2010, which killed 29 miners.
How much land is disturbed by longwall mining?
Estimate for 2009: 13,235 acres
Background and calculations:
Longwall mining, a form of underground mining designed to completely remove underground coal seams, results in land subsidence over large areas. As documented by reports describing subsidence impacts in Pennsylvania and elsewhere, longwall mining produces serious impacts to buildings, surface water supplies, aquifers. In 2009, longwall mining accounted for 166.4 million tons of coal, or 50% of U.S. underground coal production. This included 101.2 million tons in Appalachia, 12.2 million tons in the Illinois basin, and 53.1 million tons in the West. According to the Energy Information Administration, longwall mine seam heights average 71 inches in Appalachia, 86 inches in the Illinois Basin, and 127 inches in the West. Assuming 1800 tons of recoverable coal per acre foot, the amount of surface area affected by longwall mining in 2009 was 13,235 acres.
TABLE 4: Longwall coal production, area disturbed, and land intensity by region (2009)
|Region||Production (million tons)||Average seam (inches)||Acres annually disturbed||Acres per million tons|
Sources: "Underground production by state and mining method, 2009," Energy Information Administration; "Longwall Mining," Table 4: Average Mining Height of Longwall Units, by State and Region, Energy Information Administration, DOE/EIA-TR-0588, March 1995. Note: Appalachia includes Alabama, Eastern Kentucky, Maryland, Ohio, Pennsylvania, Virginia, and West Virginia. Illinois Basin includes Illinois and Wesetrn Kentucky. West includes Colorado, New Mexico, Utah, and Wyoming.
Global longwall production
World coal production in 2009 is estimated at 5,990 million metric tons (5,348 million short tons) for hard coal and 913 million metric tons (815 million short tons) for brown or lignite coal. Worldwide, approximately half of coal production is produced via longwall mining, mainly due to the high proportion of Chinese mining performed using longwall methods. In China, underground mining accounted for 85.9% of total production in 2008. There are more than 20 underground coal mining methods in China, almost exclusively longwall. Applied to estimated 2009 production of 3,210 million short tons, the total amount of coal produced via longwall mining in China is 2,757 million short tons. In 2008, China's 268 national strategic mines operated 1,545 longwall faces and 4,808 development faces. Of these, 88.3% are mechanized.
Other countries with major amounts of longwall mining include:
- United States: The United States operates 49 longwalls producing over 175 million metric tons (156 million short tons) per year.
- Australia: China has 29 operating longwall faces producing 47.5 million tonnes (42.4 million short tons)
Longwall mining in Illinois
Mining companies are buying up land in Illinois from struggling farmers for longwall mining. In Litchfield, IL, Cline Mining Corp. began buying land, prepping the ground for mining but leaving it unusable. Removing the slabs after mining can lead to sinking on the surface, with some land sinking down 15 feet, causing damage to roads and formations of creeks in what was once profitable farmland. Yet as of 2010 Cline Mining continues to buy land from struggling farmers and its mining efforts are expanding, with new mines starting closer to Hillsboro. Many townspeople begrudgingly support the business. In a time when jobs are increasingly hard to find, the development of longwall mining in the area promises to bring jobs to many people in the area, despite the cost for others. While the more traditional practice of room-and-pillar mining could be used in the area instead, the practice takes considerably longer to set up and execute, and cannot extract as much coal.
Longwall mining in Pennsylvania
2010 Citizens Coal Council report on damage to PA water
In July 2010, the Citizens Coal Council (CCC) - a national alliance of social and environmental justice grassroots groups - released the results of a nine-month long investigation into the regulatory files of three major longwall mining operations in southwestern Pennsylvania: Bailey Mine (Consol Energy), Enlow Fork Mine (Consol), and Emerald Mine (Foundation Coal). Analyzing over 75,000 pages of Pennsylvania Department of Environmental Protection (PADEP) mining files, CCC found "an internal administrative quagmire of illogical permit monitoring, baseless decisions, and lax oversight of coal operators by the PADEP." The report finds that despite improved data collection requirements and strong state constitutional and regulatory safeguards, the PADEP has failed to adequately evaluate and protect against threats to Pennsylvania streams and watersheds adversely affected by the longwall “full extraction” method of coal mining. The report can be found here.
CCC said that while the report only focuses on the past several years of mining records, subsidence damage to streams and watersheds has been allowed by PADEP since Act 54 of 1994, which paved the way for widespread use of longwall mining in Pennsylvania. Communities impacted by longwall-related water losses and property damage have been raising their concerns with legislators and PADEP for the past 16 years. CCC’s website Longwall Mining states: "Like a slow-moving earthquake, longwall mining damages the complex and interrelated system of groundwater and surface water in streams and watersheds. A typical longwall mining panel is more than 1,000 feet wide and several thousand feet long and is usually between 200 to 800 feet below the surface. As the longwall mining shearer moves forward the roof supports move with it and the ceiling behind the supports collapses, generally 4 to 6 feet, causing ground movement and strata displacement and disrupting groundwater flow by creating cracks and fractures as the overlying rock drops into the void left after the coal is removed."
Among the investigation’s key findings:
- Lax enforcement of state and federal coal mining laws that protect hydrologic balance, including the Cumulative Hydrologic Impact Assessment (CHIA), resulting in stream dewatering and widely-damaged aquatic systems.
- State water quality standards are inadequately enforced, and permit discharge limits are routinely exceeded; for example, the anti-degradation requirements which can protect high quality water resources most at risk from longwall mining damage.
- Longwall mining permits are routinely issued based on inadequate assessments of likely impacts to streams, wetlands, and the hydrologic balance.
- Oversight and enforcement of pre-mining mitigation and post-mining restoration and coal mine reclamation are piecemeal and inadequate, and may in fact be illegal under the Clean Water Act and the PA Clean Streams Law.
2011 Citizens Coal Council report on Act 54
In Pennsylvania, the passage of Act 54 in 1994 altered the state’s 1966 mining regulations from the “prevention of damage from mine subsidence” to “restoration of damage from mine subsidence.” According to a 2011 report by the national Citizens Coal Council - "The Increasing Damage from Underground Coal Mining in Pennsylvania: A Review and Analysis of PADEP’s Third Act 54 Report" - analysis of PA DEP’s five-year review shows longwall mining has become highly destructive and is not compatible with environmental protection, landowner protection, or taxpayer protection. The study examined 50 underground mines across the state and concluded that Act 54 is fundamentally flawed because:
- it allows certain impacts to occur without any compensation or restoration;
- where it requires compensation or restoration, those remedies are incomplete and take long times to be realized, with “resolution” of impacts often partial or incomplete;
- as presently administered by PADEP, the Act appears to be in direct conflict with Article 1, Section 27 of the Pennsylvania Constitution, as there exists practical alternatives to longwall mining that make its impacts unnecessary.
A 2011 Pennsylvania Department of Environmental Protection report, based on research done by the University of Pittsburgh, found that 50 underground coal mines were active in the state from 2003 through 2008, but eight of the state's operating longwall mines caused 94 percent of damage done to 456 structures, as well as 89 percent of the land impacts to 108 properties surveyed. During the five years covered by the new report, 683 wells, springs or ponds were damaged by mining subsidence, and 234 of those cases remained unresolved.
The average time to resolve a case involving those water sources was 321 days, almost a year, and considerably longer than the average time for resolution of structure and land damage claims. Mining operations impacted 55 streams, either reducing or stopping the flow of water or changing flow patterns. Flows had been restored on 20 of the streams, but the work took an average of 688 days, almost two years. Other stream damage has not been fixed.
A 2011 NY Times article reported that:
- Schmid & Co., a Wayne, Pa.-based environmental consulting firm hired by the Citizens Coal Council, also found agreements between a mining company and property owners often do not solve problems on water supply issues.
- Ben Stout, a biology professor at Wheeling Jesuit University of Wheeling, W.Va., said his studies of streams in the Pennsylvania longwall mining area show waterways suffering lasting and sometimes irreparable damage. Streams that have been longwalled, he said, have fewer or different organisms living in them, while some ponds never recovered from mining underneath.
- Residents of Greene and Washington counties, near the West Virginia line, say longwall mining caused the rupture of a dam at Greene County's Ryerson Station State Park. Whether Consol operations damaged the park will go to state court.
Citizen groups working on longwall
- ↑ 1.0 1.1 1.2 The Hidden Costs of Clean Coal: The Environmental and Human Disaster of Longwall Mining, Center for Public Integrity website, accessed 1/09
- ↑ "Longwall Mining", Office of Coal, Nuclear, Electric and Alternate Fuels, U.S. Department of Energy, March 1995, pages 9-10.
- ↑ A. Paxton, J. A. Biggs, "Ten Minutes in a Coal Mine", 1940, Pages 16-24
- ↑ Greg A. Brick, "Iowa Underground", Big Earth Publishing, 2004, pages 119-120
- ↑ The Last Pony Mine, a documentary film, Les Benedict, director, Steve Knudston, producer, 1972. Available on YouTube in 3 parts part 1part 2part 3
- ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 "Longwall Mining" EIA Report, March 1995
- ↑ "Underground Mining" Illinois Coal Association, accessed November 2009
- ↑ 8.0 8.1 "Positive and negative impacts of longwall mine subsidence on a sandstone aquifer" Environmental Geology Volume 34, Numbers 2-3, May, 1998: p. 223
- ↑ "The Dangers of Longwall Mining to Our Water" Sierra Club Hearing Alert, August 2009
- ↑ Bob Drogin and Nicole Santa Cruz,"25 miners killed in West Virginia explosion" LA Times, April 6, 2010.
- ↑ See Longwall mining and Citizens Coal Council for further information on impacts.
- ↑ "Underground production by state and mining method, 2009," Energy Information Administration
- ↑ "Longwall Mining," Table 4: Average Mining Height of Longwall Units, by State and Region, Energy Information Administration, DOE/EIA-TR-0588, March 1995
- ↑ "Coal statistics," World Coal Insitute, accessed October 7, 2010
- ↑ "International Energy Statistics," U.S. Energy Information Administration, accessed October 7, 2010
- ↑ Syd S. Peng, "Understanding the Chinese Coal Industry," Coal Age, August 26, 2010
- ↑ 17.0 17.1 S.D. Floor and J.J. Leeming, "Recent developments in longwall mining entry development, and room and pillar systems," Gospodarka Surowcami Mineralnymi, 2008
- ↑ Jackson Adams, "Longwall mining damages land near Hillsboro" Jackson Times, August 5, 2010.
- ↑ Jeff Biggers, "Big Coal’s Dirty Secret: Breakthrough New Study on Longwall Mining Regulatory Failure and Ruin in Pennsylvania" AlterNet, April 18, 2011.
- ↑ Don Hopey, "State report details longwall mining damage" Pittsburgh Post-Gazette, January 05, 2011.
- ↑ Manuel Quinones, "Reports of Property Damage Mount as Longwall Mining for Coal Proliferates in Pa." NY Times, September 29, 2011.
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- The Hidden Costs of Clean Coal: The Environmental and Human Disaster of Longwall Mining, Center for Public Integrity website
- "Protection of Water Resources From Longwall Mining Is Needed in Southwestern Pennsylvania," Schmid & Company Inc., Citizens Coal Council, July 2010
- "Longwall Mining," Energy Information Administration, DOE/EIA-TR-0588, March 1995