Published on EGEE 101: Energy and the Environment (https://www.e-education.psu.edu/egee101)

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Lesson 4: Coal: A Rock that Burns!

In Lesson 1 we covered electricity use in our daily lives, and in Lesson 2 we discussed the fossil fuel sources (natural gas and coal) of that electricity.  In this lesson,  we turn our focus to coal. Pennsylvania has provided more coal to the nation than any other State (and we live with the historic environmental challenges).

Coal the fossil fuel upon which 20th-century modern life and productivity were built.
Click Here for the transcript of Lesson 4 Introductory Video

[Dr. Mathews is standing next to a pillar filled with coal in a room with other coal displays.] Dr. Mathews: You should know by now that I got my doctorate pretty much in this material coal, so it is one of my favorite things to talk about. And I can talk about this for several weeks and obviously bore everyone to tears. But today’s lecture is about this material coal, the true black gold. This is a coal column that is 100 years old. Today we are going to talk about how we get this out of the ground and how we process it and move it around. And the origins of coal; where did it all come from? That and natural gas. [Video ends]

Credit: JPM
Picture of a miner.
Credit: Smithsonian

The main energy source powering our modern life comes from the remains of plants (coal and coalbed methane) or phytoplankton (oil and natural gas) that was buried millions of years ago. Coal was a key component in the industrial revolution, its extraction has long been associated with danger, poor work conditions, and death. It is commonly thought of as a dirty energy source. The truth is, however, that coal continues to be a critical piece of our energy puzzle, and will continue to be so for some time. To understand the role that coal plays in our energy system, we'll explore its origins, qualities, examine extraction, remediation, and historic environmental challenges. We'll also look into the vastly improved methods for extraction, transportation, and use — which will explain why coal continues to be among the leading sources of electricity production Worldwide. Its greenhouse gas emissions (specifically CO2) remain a challenge, however, and with cheap natural gas being available in the U.S., coal is being used less. It will, however, be used extensively in China, India, and other nations.

Lesson Objectives

Success in this lesson will be based on the following learning goals:

  • Describe the formation of coal and coalbed methane (an unconventional natural gas source)
  • Articulate coal mining approaches (underground and surface mining) along with mining remediation strategies
  • Discuss coal transportation
  • Recognize historic coal mining environmental challenges

Enjoy your trip underground in Lesson 4: Coal

Wake Up Your Brain

Question: Have you ever seen lumps of coal?

Yes
No

Click for answer.

ANSWER: Coal used to be used for home heating, fueling steam locomotives, etc. and so would be a common sight. Currently, coal is about 25% of our electricity supply but remains hidden away. If you are from a coal region you might well have seen the old mining waste piles, etc. At one point in time, there were >9,000 mines and >700,000 miners (1921) in the U.S. so many of you may well have some mining heritage. Now ~600 mines have 50,000 coal miners.

Origins of Coal (From the World Coal Institute)

Read

This is a brief overview of the Origins of Coal from the World Coal Institute. As you read, please note that coal can be very different depending on its maturation (extent of maturing). Also, note that we classify coal according to its properties (the classification being called "rank"). It's important that you understand that the term "rank" here does not mean a true "ranking" but rather a classification. All coals are used for combustion within electricity generation, select bituminous coals are more valuable and are used for coke making (used in the manufacturing of steel from iron ore).

Read up on coal here with the goal of understanding coal rank and how coal forms (it is fair game for the exam — see the learning objectives). 

 link to the Origins of Coal pdf [1]
Click on the image to open the pdf
Credit: World Coal Institute

Coal Formation

Coal Formation

Coal is one of the fossil fuels (along with crude oil, natural gas, oil shale, and tar sands). The name fossil fuel invokes the notion that at one point in time coal was alive. Well, almost, the coal precursors, mostly the plants, were alive growing in the sunshine. We know this because we can find fossil imprints of leaves and branches in coal, amber containing flies, and other organic material preserved, bark imprints in the coal, coalified trees, coalified roots, and biomarkers which are chemical compounds produced by living organisms, etc.

The Carbon Cycle - again

You've seen the carbon cycle once already in relation to Biomass back in Lesson 3. This time, focus on how the living matter (plants and animals) breaks down but the decay is limited, creating the organic basis for fossil fuel formation.

Photosynthesis & Decay Revisited

green leave with sun shining behind it.
Credit: Aelita [2]/ adobe.stock.com [3]

Recall that photosynthesis is the process by which plants absorb the sunlight (solar energy), store it, and convert it into energy to grow and survive. The plant takes in carbon dioxide (CO2) and water (H2O), stores and uses the glucose to grow and live, then releases oxygen (O2) back into the environment.

When plants die, this process simply works in reverse. Walking through almost any forest is the best way to witness the decaying process in action. The forest floor is generally strewn with dead and decaying leaves, branches, and sometimes entire trees.

Normally the process of growth and decay occurs but in the formation of the fossil fuels the normal decay path did not occur. Instead, the organic material is somewhat preserved. The key is the absence of oxygen, which is necessary for the normal decay process. Bogs are ideal for this to occur so in swamps (a particular type of bog) the plant material dies but is protected from the normal decay process by the stagnant water, which is low in oxygen. Decay still occurs but the bio-resistant material (the chemical structures which are resistant to the bugs dining in the swamp) is enriched. Over long time periods, considerable quantities of organic material might be buried in the swamp which might eventually form the material peat. As the peat is aged and buried deeper in the ground the slow coalification process (the maturation process for coal) continues and eventually transforms peat (a coal precursor) into low-rank lignite coal. This brown/black coal is typically a "young" coal (~60 million years old). With further maturation, long time periods (millions of years), warmer temperatures (within the earth), and higher pressure as the coal is buried deeper, other ranks of coal are produced: subbituminous, then bituminous coal. If there is uplifting, then anthracite coal can be formed because of the high temperatures and stresses involved. Magma can also bake some coals enhancing the rank in some locations. (refer back to the World Coal Organization PDF on the previous page for a refresher of coal rank: lignite, subbituminous, bituminous, and anthracite).

During the various stages, other materials such as minerals might be washed into the bogs, mud will form clay which will turn to shale over the years. The organic material will have contained inorganic material (it forms the ash left behind in a wood fire). Thus, coal is not pure organic material, and as the coalification process is over such long time periods we have different ranks of coal in the US coal of different qualities.

Coal is very old. The formation of coal spans the geologic ages and is still being formed today, just very slowly. Below, a coal slab shows the footprints of a giant armored salamander (the footprints were made during the peat stage but were preserved during the coalification process). 

dinosaur footprints in a slab of coal
Coal Footprints
Credit: JPM
see link in caption for a text description
Coal Formation Process
Credit: ND Geological Survey, Becky Barnes, artist.

The organic matter that falls or is washed into the swamp will be protected from the usual decay process because of the low oxygen concentration in the water. Decay will occur in the less bio-resistant material, leaving behind the bio-resistant organic material. Eventually, this material will form peat. With burial and the "cooking" of the earth (as you get deeper, it gets warmer) for geological time periods, low-rank coals will form such as the lignite shown here for North Dakota. The process of coal formation might repeat itself and so layers of coal and rock are present in many locations within the US. There the deeper you go the higher the coal rank. In Pennsylvania, we have bituminous and anthracite coal, the mountain formation transformed the bituminous coal into anthracite and it can occur at the surface.

Where is Coal Located?

Coal Fields in Pennsylvania

There are a few mining operations (historic) in northern Centre County (where the University Park Campus is located) but coal occupies much of the western portion of the state (bituminous rank) with a smaller but well-defined anthracite region in the north-east.

Map of PA showing the bituminous coal field occupying nearly all of the western portion of the state, and the much smaller (area) for the anthracite fields in the north east.
Map of the Pennsylvania distribution of Coal.
Credit: USGS

US Coal Fields

Coal is well dispersed throughout the continental United States. We have about 25% of the World's reserve of coal, and all the various coal ranks. The yellow is lignite, pale green is subbituminous coal, the other colors are various bituminous ranks, except the orange in NE Pennsylvania which is anthracite.

Map of the U.S., showing the age of its coal deposits.
Map of the US showing Geologic Age of Coal.
Credit: USGS

Here is a global view of the deposits. The major coal regions (and users) are China, the U.S., India, Europe, and Australia (major exporter).

View of the global deposits of coal
Map of global coal deposits
Credit The Open University

The take-away message from this now dated graphic below is that Asia (specifically China) is the highest user for most of the Worlds Coal (still true in 2021). The growth was related to the transformation to an economic powerhouse and increased electricfication.

Enter image and alt text here. No sizes!
World coal consumption by region, 1980 - 2010
Credit: U.S. Energy Information Administration [4]

We also have lots of coal, far more than the energy in oil (shown in red) and natural gas (shown in brown) in the U.S., Asia (China and India), Europe, etc.

Location of the worls's main fossil fuel reserves (Mtoe) See link in caption for text description
Coal is much more plentiful in North America, Europe and Eurasia, India, China, Asia Pacific, and Australia than oil or natural gas.
Credit: BP Statistical Review of World Energy 2018 (WCA Analysis)

There is also a robust international trade in coal (see the coal transport page later in this lesson).

Coal Quality

Coal Quality Issues

We have learned that coal is very old. Much of the Pennsylvania coal is about 300 million years old, so, much older than the dinosaurs. To put it in perspective, the Jurassic era was about 80 million years ago! But, coal is still being formed today so we have a range of coal with highly variant properties (and quality), from a brown coal, which is relatively young, to a bituminous or anthracite coal, which is relatively old. The material is distinguished by a rank system that is very informative regarding how we expect that coal will behave during the combustion process or other coal applications.

Coal Rank

There are 2 officially accepted methods of determining the rank of a coal in the U.S. (but numerous other approaches). Which approach you use depends on the calorific value of the coal. There are 4 general rank classifications, shown from lowest to highest rank;

Table lists different coal ranks
Coal Rank
Lignite low-rank
Subbituminous low-rank
Bituminous high-rank  (soft coal)
Anthracite high-rank  (hard coal)

There are lots of sub-classifications but we only need to consider the groups. The rank of coal is an important feature of the coal, it is used for taxation purposes, contracts, etc. Coals that have a calorific value below 14,000 Btu's are put into a rank classification based on the calorific value. Coals with calorific values above 14,000 Btu's use the proximate analysis to determine rank (specifically volatile matter or fixed carbon values).

Calorific Value

When coal is burned, the exothermic reaction produces heat;

C + O2 → CO Carbon and Oxygen yields Carbon Monoxide

CO + O2 → CO2 Carbon Monoxide and Oxygen yields Carbon Dioxide

2 H + O2 → H2O Hydrogen and Oxygen yields Water (Steam)

This is, after all, why we burn coal! But how much calorific energy we obtain is dependent on the chemical composition of the coal, which tends to change with maturation (age), so bituminous coal will provide more energy than biomass, lignite or a subbituminous coal. This is very important information when buying coal or, as we will see later, very important information when determining pollutant emissions.

Proximate Analysis

This has 4 components, which are useful in determining price and behavior of the coal during combustion, as well as some other quality issues.

1. Moisture

Moisture adds mass to the coal which impacts the transportation costs as well as reducing the useful energy obtained from the combustion of the coal. The steam produced from within the coal (from moisture) is not used to turn turbines but rather goes out of the stack. In low ranks coal, such as lignite, water might be 60% of the total mass! Moisture levels are rank-dependent and bituminous coals may only contain 1-2% moisture by weight (subbituminous coals will have more moisture ~30% unless the coal has been dried). High moisture levels and high oxygen content is why the calorific value is low for lignite and other low-rank coals.

2. Volatile Matter

Most coal is burned, how easily it burns depends on the quantity (and the quality) gasses that are released when the coal is heated. Under prescribed heating rates, under nitrogen (so no combustion losses), and times, the weight loss is determined to be volatile matter. This value is used in rank determination for coals above 14,000 Btu's/lb. Coals with a higher volatile matter yield are easier to burn.

3. Ash Yield

Coal contains inorganic material too, which we call mineral matter. This diluent impacts transportation costs, and after the combustion process, the coal will leave behind the now chemically altered mineral matter (high temperatures, as well as oxygen, will change the composition of the mineral matter) into ash, which will have to be removed influencing the combustor design and operation. Thus, the coal is combusted, leaving behind the ash which is weighed to roughly determine the contribution of mineral matter to the coal mass. Remember coal is purchased on a per ton basis so ash values are important indicators of quality.

4. Fixed Carbon

After the volatile matter determination, the char contains fixed carbon and ash, if we know the ash values the subtraction of the moisture, volatile matter, and ash produces the fixed carbon. Fixed carbon -= 100 - moisture - volatile matter - ash values

Ultimate Analysis

This analysis determines the relative abundance of the organic elements that are contained in the coal. It is influenced by the rank of the coal.

Element Normal Contribution range (%) in Coal
Carbon 60-90% ( Higher Rank - Higher % )
Hydrogen 2-6% (Higher Rank - Lower %)
Oxygen 1-30% (Higher Rank - Lower %)
Nitrogen 1-2% (No real change with Rank)
Sulfur 0.5-5% (no significant change with rank)

We will find out later in the course that nitrogen and sulfur contribute to environmental problems when their oxides are released into the atmosphere following combustion. For those who are impatient, you can find out more information concerning the NOx and SO2 [5](and sulfate aerosols) emissions here. Nitrogen values for coals are typically around 2%, so nitrogen content does not influence the coal quality when comparing two similar-rank coals. Sulfur, however, can vary dramatically and is certainly an element that impacts coal quality. High S content negatively influencing the price of the coal, making it cheaper as more expense will be incurred in cleaning the coal or reducing the SO2 emissions (acid deposition-Lesson 10). The S content along with the calorific value is useful because you can use it to predict the SO2 emissions per million Btu's of thermal input (which is the measure used in the clean air act of 1990 to ensure pollution controls).

Thus, several measures impact coal quality– how much organic material is actually in the coal (minus ash and moisture values) as this impacts transportation, the quantity of useful energy that will be produced (calorific value and moisture content), removal of the ash from the boiler (ash), and emissions from the combustion process (S content). Of course, the quality of the coal will impact the value of the coal so remember the $ is an important influence on the cost of the electricity, and where the coal is purchased from (transportation).

Coking Coals

Select bituminous coals can be used in coke manufacturing for use in steel production. They need to have low S, low ash yield, and other properties, but they are more valuable. The Pittsburgh coal seam is a classic example and one of the reasons that it is Steel City with the Steelers (not Stealers) football team, and Iron Brew beer.

Left: Pile of shiny coal pieces. Right: Pile of gray dull coal pieces
Left: A few coal lumps from a bituminous coking coal. After being in the coke oven (heated) for almost a day (~18 hours) a silver-grey coke block is produced.
Right: A pile of coke lumps that will be used in a blast furnace (steel production).
Credit: Left: Stef [6] / adobe.stock.com [7], Right: Renate Promitzer [8] / adobe.stock.com [9]

Coalbed Methane

As coal ages, it undergoes chemical and physical structural changes. You will see below some of the proposed structural changes that occur. I love this stuff! This is very much why I am Dr. Mathews: 6 years of study and the generation of a few structures like these. The structures here are simplistic representations of some of the structural features found in coals of those ranks. We still do not know the structure of coal, because it is highly variable and complex. As the structure changes one of the gases it produces is Coalbed methane, which supplies on the order of 8% of the domestic natural gas production and so is an important unconventional resource (shale gas is also an unconventional resource and a much larger contribution to the domestic natural gas supply).

Look at each of these three models. They are very different in appearance. Look at the hydrogen and oxygen atoms -there are fewer as the coalification proceeds. The anthracite coal is the most carbon-rich. These changes influence calorific value.

The make-up of coal
Diagram of atoms for low-rank coal, bituminous Vitrinite, and Anthracite coal
Credit: (in order): Leigh Clemo, Australia's CRC for Clean Power from Lignite; JPM, PSU Energy Institute; Peter Pappano, PSU Energy Institute

Coalification Changes

A coal lamp
The Police sang about
a "Canary in a Coal
Mine", but if you don't
have one handy the
coal miners' lamp is
a pretty useful tool
for detecting methane.
Any guesses as to
why it does not
cause an explosion?
Credit: JPM

Okay, a few structural changes are evident as coalification proceeds from low-rank coal to anthracite:

  1. Lots of moisture is associated with low-rank coals (water is actually part of the structure, up to 60% of the mass of some coals).
  2. Less oxygen (which is why the calorific value increases over much of the rank range).
  3. Less hydrogen, particularly in the anthracite structure (this is why on reaching anthracite rank the calorific value decreases slightly).
  4. Larger aromatic structures; when transitioning between bituminous coal and anthracite, the number of aromatic rings in a layer (or the size of the aromatic portion of the molecule) increases from a few rings (around 3) to perhaps 100's. These chicken-wire like aromatic structures are becoming more like graphite and is less reactive during combustion.
  5. The structure is becoming more carbon-rich.

The only way for the structure to become more carbon-rich is to either 1) have carbon added (not likely 100's of feet down), or 2) lose other material. The latter is the case here. As coal matures, the oxygen is lost probably as water and hydrogen as methane (as most of the oxygen formed water there is not enough oxygen left and so the hydrogen generated from the coalification process is lost as methane). The methane will either migrate to the surface or into other structural traps, with a considerable quantity being retained in the coal.

For over a century, this coalbed methane has been problematic. In the right mixture with air, it is explosive. Coal mine methane explosions are among the worst mining accidents. Methane in the mine causes ventilation problems trying to prevent the methane content from building up. If the levels are too high in modern mines (in the US) work has to shut down for hours before the miners can return, causing huge losses in productivity.

In the (4:13) video below, Dr. Mathews describes the perils of being a miner and the cues miners used to avoid serious danger (and explains why the lamp doesn't explode!)

Perils of Being a Miner
Click Here for Transcript of Perils of Being a Miner Video

[Video opens with Dr. Mathews standing next to a pillar of coal.] Dr. Mathews: One important thing about mine safety is the gases. Now, there are several things that can go wrong in the mine. The roof can fall in on you, you could have an explosion, or you could get trapped or crushed by any of the machinery or the explosive devices you are using. One of the key things, however, is the gases. And this is a very important miner's safety piece of equipment. [Dr. Mathews hold up a lamp-like object that is in his hands.] Dr. Mathews: This is a Davey's Miner's Lamp. Probably about eighty years old by the looks of it. What I would have is some fuel source, probably kerosene back at about 1920. Prior to that, we may have been using Sperm Whale oil. And there is a naked flame. One thing you don't want to have in a coal mine is a naked flame. And so it is protected by this wire mesh. When I was in the mine, if I was to lift it up, and I would see a bluish tinge, and if the light would get brittle, I would know that there was methane in the coal mine. Now methane now-a-days is a good thing. It is a very valuable resource. But if you are in a coal mine, one spark, which is very easily done, is going to start an explosion, if it is in the right mixture. And that could kill everybody. So certainly there have been coal mine explosions where more than 100 people have died. It is one of the main reasons coal miners today will still wear a very thick leather belt with their name stamped on it so you can find their remains. But of course, there are other gasses in the mines as well. If you lowered your miner’s lamp and the light was to go dim, when you put it toward the bottom of the mine, it means there is probably not a lot of oxygen there. There is probably either carbon monoxide or carbon dioxide. Carbon monoxide will kill you. Carbon dioxide means there is a lot less oxygen in the air and you might want to leave. It is one of the good ways of staying alive. The other methods of using are of course high tech equipment which we have now, but in the good old days, you would use a canary. A canary has very small lungs and breathes very fast. [A picture of a miner holding a canary in a small cage.] Dr. Mathews: If your canary stops singing, and is lying in the bottom of his cage, it is probably a good time to leave. If you are in a drift mine, that is a mine that goes into the slope of a mountain, then there is probably going to be rats there as well. If they start leaving it means either the roof is going to come down, or it is the presence of dangerous gasses. Thank goodness for rats and canaries. [Video ends]

Credit: JPM

Methane is also a greenhouse gas (more on that in unit 3). So now that natural gas has value, so does Coal Bed Methane. Coal Bed Methane is a danger to the miner, as methane is an explosive gas (when it is present with air at the correct concentration levels). It is there because of the maturation of the coal and the changes in the molecular construction of the coal. With age, the coal becomes more carbon-rich by losing oxygen and hydrogen in the form of methane. Now methane has more value, the methane in coal has value and is now being extracted. This can extend the lifetime of our natural gas supply. Locations with bituminous coal are extracting the methane from abandoned coal mines and from deep unminable coal seams. They also look to extract the methane from coal seams that haven't been mined yet, which have the added benefits of creating mining operations later that are safer and less expensive, because the air flow into the mine can now be minimized, saving time, money, and overall productivity. We will see in unit 3 that we might be able to store CO2 (sequestration) within deep coal seams as a climate change mitigation option.

Underground Mining

 Dr. Mathews in a miners outfit

The picture to the left is a dirty, smelly Dr. Mathews as happy as a pig in swill! It was taken three decades ago when I was just starting my Ph.D. studies here at Penn State. Thinner and braver in those days, I would venture into mines looking for coalified trees. Six years later I had a doctorate in Fuel Science - "Following the changes in the constitution of rapidly heated bituminous vitrinites." Ahhh, the good old days!

Mining has always been hard work, but without the availability of modern machinery, lots of manpower was necessary.

For an in-depth and very interesting look into the jobs of miners, and the methods they use to do their jobs, check out this United Mine Workers Association [10] link.

2 drawings. One with a vertical entrance from the ground into the earth and through the side of a mountain.
For underground coal mining in some cases they are accessing from above via a shaft, in other cases, you can enter via the side of a mountain to chase the seam(s). Once underground, there are two underground mining approaches: room and pillar mining, and longwall mining.
Credit: left: blueringmedia [11]/ adobe.stock.com [12], right: borodatch  [13]/ adobe.stock.com [14]

Room and Pillar Mining

The following video from Catapillar Mining provides a good overview of room and pillar mining.

Click here for a transcript of the Principles of Room & Pillar Mining video.

In room and pillar mining, the coal seam is mined in a checkerboard style, leaving pillars of coal to support the roof, which allows for instant coal access with a relatively low invest compared to longwall mining, however, only utilizes the coal reserves between 50% and 75%. It is a mining method of its own right, as well as a supporting technology to develop roadways in order to prepare the coal face for a long wall operation.

In room and pillar mining, continuity is the key to profit, from the continuous miner to the continuous flow of material. The coal is cut by a continuous miner, which delivers the product to haulers. They bring the product to feeder breaker units that prepare and deliver it under the belt system.

While feeder breakers are moved only occasionally, the other equipment is in constant motion, so maneuverability, cableless operation, and maximum load capacities are vital. With a full range of battery or diesel powered vehicles, Caterpillar has an answer to every challenge in room and pillar mining. It all starts at the coalface with the right cutting technology.

Today's continuous miners are designed to cut at highest efficiency while keeping dust levels to a minimum with water sprays and dust collectors. They are available for operations from as low as 70 centimeters up to a maximum of 5 meters. From production to the delivery point, it's just a question of volume and velocity.

That's easy physics for our range of utility vehicles. Being equipped with rubber tires and an industry leading capacity, they keep the circulation of product and material at a healthy and profitable level. The roof bolter follows production on its tail to create a safe mining environment. Driving the bolts into the roof in a safe and efficient way is most important in this real hands-on job.

Therefore, ergonomic controls and easy material access are one of the most important features in our roof bolters. The Scoops Multipurpose Contoured Bucket will carry equipment, serve as a multi-tool, or clean roadways and feeder sections. Extended battery life and a dual motor option make it versatile yet powerful. The scoop has often been called the miners Swiss army knife. As a matter of fact, it is a real workhorse too.

Credit: Caterpillar Mining [15]

The image below provides a birds-eye view. The pillars are a bit large but you get a general idea. The continuous miner (the machine with the big drum and all of the teeth) extracts the coal, it is loaded directly or picked up by the coal hauler that takes the coal to the conveyor system for transport out of the mine. The roof bolter adds metal rods to the mine ceiling so the rocks don't fall (the roof bolts are needed for safety). Obviously, the pillars are coal and so we leave behind 30% or more of the coal.

diagram of a room pillar mine. It has square 'pillars' of coal with 'road' around them where the mining equipment works.
Room and pillar mining
Credit: Advanced Mining Solutions
coal on a conveyor belt headed out of the mine
Conveyor system carrying the coal out of the mine.
Credit: Sunshine Seeds [16] / adobe.stock.com [17]

Mining anthracite is quite different from bituminous coal (where the seams are mostly horizontal). Anthracite seams can be at a 60° pitch so slightly different techniques are used either mining the seam from above or below. 

Longwall Mining

A longwall miner. Described  in text below.
A longwall miner
Credit: Consol

The productivity in underground coal mining has dramatically increased due to mechanization. Watch this video of a longwall miner that can run the length of a football (American) field or longer (3:25 minutes).

Click here for a transcript of the Principles of Longwall Mining video.

Whenever mining coal underground, longwall mining is a highly productive, efficient, and safe way of doing it. The coal seam is mined cut by cut with a plow or a shear until a complete panel is mined out. Such a longwall panel would typically be 3 to 4 kilometers long and 250 to 400 meters wide.

Four seam heights of up to 1.8 meters or 71 inches, the plow is the cutting tool of choice. It travels fast, with speeds of up to 3.6 meters per second along the coal face pulled by a plow chain that transmits a force of up to 2 times 800 kilowatts. The plow is cutting coal at predefined depth up to 25 centimeters or 10 inches.

In seams from 2.3 meters to 6 meters and above, the shear has proven to be the most efficient cutting technology. It travels typically with a speed of 16 meters per minute, while normally cutting 100 centimeters with its powerful cutting drums, generating a production capacity of 5,000 tons per hour. Constant loading of the face conveyor, which transports the coal to the crusher and the belt, is the goal.

To fulfill this task, the armored face conveyor can be equipped with three drive systems, each holding a power of up to 1,800 kilowatts. This power package enables them to handle extreme peak loads. The AFC provides rail on which the cutting device travels. But it's not only for this, but also for the capability to handle all the strains, especially on the chain and the constant wear and abrasion, that the AFC is called the backbone of the longwall.

Roof supports are vital, not only for a constant production and advance of the longwall but for the safety of the miners. With a bearing force of up to 1,750 tons each, they can handle even severe roof and floor conditions.

They are available for a seam heights from 0.8 meters to 7.5 meters.

Our roof supports have been tested to advance 60 kilometers and more underground before a complete overhaul is necessary.

Integrated automation has become unrenouceable in longwall mining. Constant optimization of the entire longwall system, control of thousands of kilowatts of power in motors and drives, as well as a few 100,000 tons of combined yielding capacity of the roof supports surmounts human abilities.

A network of intelligent control units collects and shares data, thereby optimizing the entire longwall system, achieving maximum productivity and availability. Automation systems also keep the miners away from hazardous areas, increasing safety standards even further.

Credit: Caterpillar Mining [15]

What is the role of the shearer, roof supports, face conveyor, and gob? The machinery behind the operator holds up the roof and then the whole device walks forward leaving the roof to fall safely behind the work area. Thus, the extraction is much higher and more of the coal is removed by this technique. Safety has improved in the mines but one of the reasons for lower loss of life is the reduction in the number of miners because of productivity enhancements and fewer miners.

Mine Safety

Roof falls are a major danger in coal mines. The roof is bolted together with meter-long bolts but the roof has to be checked anyway. How the roof "sounds" is a good indication of the stability. If you happen to have rats handy, and they leave the mine, it is a good idea to follow, as their hearing can often pick up the sound of the roof straining when it's not audible to humans.

Black Lung

The small coal dust particles (and silica particles) enter the lungs and stay there. Long-term exposure produces a debilitating disease and often premature death. In the early years of coal mining, a miner was an old man at age 44 and the life span was not expected to be much longer.

Photograph of a healthy lung in comparison to one showing black lung (discolored, and shrunken) from the National Institute of Occupational Health and Safety
Here is the comparison of a healthy lung and a black lung from a coal miner. 
Credit: National Institute of Occupational Health and Safety
coal dust on a white background
Coal dust
Credit: dule964 [18] / adobe.stock.com [19]

One of the major dangers to coal miners had nothing to do with physical injury (although that is highly possible, especially in the "good old days"), but rather with lung damage. The medical community calls the infliction pneumoconiosis but the miners know it as black lung. All coal mining methods will produce some dust but underground mining methods produce more and the dust (small coal particles - smaller than 100 microns) in an enclosed space. 

Many miners still do not wear masks to filter out some of the particles but the technology is much better at controlling dust, water sprays help to reduce the problem as well as air handling systems. Fewer miners in the mine also help to reduce exposure, as does the remote control machinery that allows the miner to operate the continuous miner of a roof bolter from several meters away. Historically, this was a massive problem, resulting in the premature death of perhaps 100,000 miners. Modern miners can still "get" black lung disease and it is increasing in some locations.

More information if you're interested in learning more about these hazards is available from the United Mine Workers Association's Black Lung webpage [20].

Open Cast (Strip) Mining

 An enormous coal truck with Dr. Mathews standing next to the wheel. He comes up the middle of the wheel.
There are some very large "Tonka toys" around
surface mines. This truck cost @ $1,000,000 -
no wonder they run it about 22 hours of the day,
about 340 days of the year. 
Credit: ES

In some locations, the coal lies deep in the ground, but in others, only a few feet from the surface. If the material above the coal (overburden) can be removed, then there is easy access to the coal. Sometimes the tops of mountains can be removed to expose the coal, other times vast acreage of mines is created. Either way, this technique requires the ability to move vast quantities of rock and coal. Surface mining is an expensive operation, with environmental challenges, and a great deal of expense. It can, however, extract lots of coal with which to provide the engines of industry (and the computer chips, etc.) the energy they require.

The removal of both the coal and the overburden is performed with some of the largest mining vehicles in existence. First, the overburden is drilled into with power drills. The intent is to use explosives to fragment the various rock layers into manageable chunks that can be removed. Then, the holes are filled with an explosive mixture of fuel oil and fertilizer. The fragmented rock is then scooped with very large buckets via draglines. The process then repeats but this time the coal is fragmented with explosives. This is done by truck, or by miles of coal conveyor belts. After collection, the coal is transported to the breaker. There the coal is crushed and a cursory cleaning is performed to remove the large pieces of rock.

 Picture of a surface mine with holes drilled in the side of a hill with an inset image showing one of the holes going down 40 feet. See caption
A field of holes 40 feet deep awaits the explosive mixture and detonation fuses that set off the explosions one after the other separated by a very small fraction of a second to fragment the rock.
Credit:JPM
 Picture of a small dragline
The draglines are very heavy and slow when moving along on the caterpillar tracks but when in motion, moving the bucket is a swift beast of burden, moving tons of coal with ease. This is a small dragline. Realize that the bucket on a dragline can be the size of a house!
Credit: JPM
 Picture of an area which was surface mined.
This image does not capture the impressive scale of surface anthracite mining at this site. The drop is about 200 feet with the water being another 100 feet deep. This operation had shut down to replace the pumps that keep the workings dry otherwise the water would not be as high.
Credit: JPM
A large quarry with many horizons and ledges.
Mountain Top Removal Mining
Credit: nordroden [21]/ adobe.stock.com [22]

A special form of surface mining is Mountain Top Removal. This occurs in locations such as West Virginia and eastern Kentucky. This is a highly contentious form of mining that provides jobs and tax benefits to the region, but damages streams and removes mountains. Dam bursts of coal dust retention ponds have also been a problem.

a valley.  The walls of the valley are covered in gravel. [23] The walls of the valley are covered with grass and wild flowers [24] a surface mine. [25]
Click on the images to learn more about what you are seeing.

The scale of these mines can be very impressive, check out a drone view of a surface coal mine.

Aerial drone view of a huge opencast coal mine cut into a rural hilly area
Aerial drone view of a huge opencast coal mine cut into a rural hilly area
Credit: whitcomberd [26]/ adobe.stock.com [27]

Reclamation

Mining reclamation is now required for all active mines, but that was not always the case. There are many sites where the work finished and the miners and the owners just walked (or ran) away. Not only is mining reclamation the norm these days, but it is also required. Mined land is required to be returned to its original contours. A tax on every ton of coal (15 cents for surface coal) helped to fund "Superfund", a large-scale government environmental program for cleaning up the abandoned mine sites. The fund, which started in 1978, is now in the many millions (fines, interest, and late payments are also included). The ash from fluidized beds is currently used in the anthracite region to fill in the open pits from long ago abandoned strip mines. This reclamation and others help to remediate acid mine drainage, prevent landslides, and aids in recovering land for other useful purposes, such as land development to the West of Pittsburgh International Airport.

So after the coal is removed the overburden is moved back, leveled, the topsoil is returned, and vegetation planted.

Enter image and alt text here. No sizes!
Diagram of the Mining and Reclamation Process
Credit: Feng et al. Earth-Science Reviews, Volume 191, April 2019, Pages 12-25

Mining & The Environment

Fires

Mine fires are a familiar and common hazard, and also an environmental concern. The movie here shows a particularly unique occurrence, which is a fire that simply won't go out. The people of Centralia have had the unique problem of dealing with this fire for some 40 years now.

Here is an interesting YouTube video on the Centralia Coal Mine fire. The highway there has now been covered with overburden and it is dangerous, so don't go. Watch up to the 3:20 mark.

Centralia Burning Ghost Town - Pennsylvania USA
Click here for a transcript of the video.

There's a small town in America by the name of Centralia in Pennsylvania that looks like it has been hit by the apocalypse. The town was left abandoned after a coal mine fire began to burn more than 56 years ago.

Underground mine fires are common across the globe. There are thousands that have been burning uncontrollably for many years. Australia's Burning Mountain is believed to have been burning for 6,000 years.

Centralia's fire started in 1962, when residents turned an old strip mine into a dump and set the rubbish alight. The fire spread through an unsealed opening to the underground coal mines, igniting a seam of coal. And the fire has been burning to this day.

The fire stretches 12 kilometers and burns underneath an area of 15 square kilometers, 300 feet below ground. Authorities say the fire could burn for another 250 years.

The fire continued to rage unchecked into the 1980s. Giant plumes of smoke and deadly carbon monoxide gases billowed from fissures in the ground.

The local highway cracked and collapsed. Trees were bleached white and petrified. And people complained about breathing problems.

After estimating the cost of extinguishing the fire at over half a billion dollars, the government opted to raze the town and relocate its residents. Centralia used to have a thousand people living in the town. About five residents still live there today despite there being nothing there.

All real estate in the town was claimed under eminent domain in 1992 and condemned by the Commonwealth of Pennsylvania. The remaining residents were being forced to move. But in 1993, they started to fight for the right to stay.

After a lengthy legal battle, state and local officials reached an agreement with the seven remaining residents in 2013, allowing them to live out their lives, after which the rights of their houses will be taken through eminent domain. There is very little left in the town of Centralia, except for roads that lead nowhere and a few scattered buildings for the remaining residents.

Pennsylvania Route 61 used to stretch through Centralia. But it was destroyed by the underground fire. And cracks tearing through the tar would make you think a severe earthquake struck the area.

The town now mostly attracts tourists who visit an abandoned highway where many profanities and obscene pictures are sprayed onto it. Over time, the highway has earned the nickname Graffiti Highway, It sort of reminds me of the Cadillac Ranch, where there are 10 Cadillac cars facedown in the dirt. And people visit the cars to spray paint onto them.

When you see before and after images of the town when there was a thousand people who lived in it and now 5, it is very similar to the before and after images of Hiroshima. One picture had a whole city full of buildings. And the next is just an expansive parking lot. You'd be forgiven for thinking the town was nuked and wiped off the face of the Earth.

I asked a local YouTuber by the name of Joey Underground who let me use his footage for this video, does the ground still smoke, as I did not see any smoke in recent videos. He replied, it's smoking in certain parts of the woods. But the streets are no longer smoking. And you can only see the smoke on freezing cold days. I was there in March 2018 and couldn't see any smoke anywhere.

I'm not sure if it's love for the town and the house they live in or stubbornness, but when you think of what the remaining residents have to live with-- dangerous gases, cracks forming in the earth and roads, a raging fire below the ground they live and walk on, and an ever-present threat of sinkholes forming under their feet-- you have to ask yourself, would you stay? Anyway, that's the end of this video. Thanks for watching. And we'll see you next time. Bye, bye.

Credit: Wonder World [28]

Acid Mine Drainage

Acid mine drainage is an issue in both the Bituminous and Anthracite regions of Pennsylvania. Pyrite in the exposed coal (underground or on the reject (Culm/Gob) piles) acidify the water.

More on this in Lesson 10: Acid Deposition [29].

Acid Mine Drainage
Click Here for a transcript of Acid Mine Drainage Video

[Video opens panning down a discolored stream.] Dr. Mathews: This is a beautiful area of the anthracite region. This is one of the many sulfur creeks. It gets that name from its yellow or orange color in nature. This is due to acid mine drainage. What happens is the iron discolors the bed, the stream bed, with this yellow coloration, it is called a yellow boy, and it is actually iron hydroxide. Unfortunately, it means that the stream is in very poor health and it is not a good spot to go fishing. It is very sad to see the beautiful areas of this anthracite region devastated in this manner. [Video ends panning the stream.]

Credit: JPM

There is a correlation between coal mining locations and the occurrence of acid mine drainage.

Map showing streams impacted by mine drainage in Pennsylvania. [30] Map of streams and also shows where the coal beds are located. [31]
Click on the images to see a larger version.

We know that coal contributes a great deal to our production of electricity. The challenge is how to do it cheaply and in an environmentally responsible manner. Carbon dioxide emissions are a new challenge that we now have to face. More on this later in the course.

Here is how I think the lesson materials tie together the coal combustion materials in lesson 02 and the environmental challenge of acid mine drainage that we cover in lesson 10. This coverage map does not have interactive text so give some thought to what are the important concepts.

Graph the shows the diiferent polutants that come from coal.
Pollution from Coal Coverage Map
Credit: JPM

Abandoned Coal Mines

There are several other challenges that our historic (abandoned) coal mines contribute to.

High walls

You have to be careful when walking in the coal mining regions. The photograph below shows a high wall left when the mine was abandoned. This was illegal after 1977 but many abandoned coal mines still remain. High walls such as this are a danger for wildlife, off-road drivers, bikers, and hikers.

An abandoned coal mine in Virginia showing the dangerous high wall (fall risk)
An abandoned coal mine in Virginia.
Credit: Adam Wells

Acidic Pools

Does anyone fancy a dip? On a hot day, the old pits that have become flooded are a tempting swimming hole. However, they are sites with high drowning fatalities both due to the very cold deep water and the fact that the pools can be very acidic and lead to acid-shock deaths. Stay away.

large mine filled with water
A dangerous swimming hole!
Credit: shootdiem [32]/ adobe.stock.com [33]

Subsidence

Worried you may live in a mining area? See the Pennsylvania Mine Map Atlas [34], and perhaps consider subsidence insurance. Subsidence is the gradual caving in or sinking of an area of land. Mining subsidence occurs when the hallowed out earth from the mines begin to cause the ground to shift or sink. If you live in a subsidence-prone area you can expect the cracking of walls etc. as the houses settle. In some cases, the house will become uninhabitable and will need to be demolished.

Graphic showing houses above an old coal mine tunnel. If the tunnel collapses there will be subsidence.
In this graphic, the houses were constructed above an unknown room and pillar mining location. The center has collapsed and the ground above has subsided.
Credit: Pennsylvania Department of Environmental Protection
Large crack in a concrete wall of a basement. The crack extends into the yard.
A large crack in the ground and the wall of a house due to subsidence, related to coal mining activity.
Credit: Eric Schmadel

Coal Production

Graph showing the rise of coal production from 1950 to 2008 (peak) and the current decline
With the growth in industry and electrification, there was a dramatic increase in coal production up until 2008. Climate change concerns, cheap natural gas, and increasing renewables, however, have caused a decline in production (see later).
Credit: EIA

The production of coal is directly related to the energy demand that is related to economic production. Another reason for the increase in coal production is an increase in the population. My two daughters are certainly using lots of electricity. Cold winters and hot summers are also important components in the overall coal demand. However, climate concerns and changing economics of cheap natural gas and increasingly cheaper renewable energy (initially wind and in the future solar) are having greater contributions and many older coal power plants (coal-fired utilities) are closing thus reducing the U.S. demand for coal. Some of the coal-plants generating electricity have also switched over to natural gas.

Plot of the declining coal production for the U.S. coal regions from its peak production in 2010, plot is from the Energy Information Agency
Coal Production, 2000- 2050
You should now be able to explain why this production (mining) of coal looks this way. The coal production typically rises due to economic growth, perhaps hot summers or cold winters. The dips are slow economic growth, mild summers/winters, with the overall decline being related to fuel switching to cheap natural gas for electricity generation and increasing renewables.
Credit: EIA

I expect most of you are familiar with coal, as most will have Pennsylvania ties. You are probably well aware that the number of mines and the number of coal miners has been dramatically reduced in the last 20 years. There are many good reasons for closing a mine: the coal might have been mined out, flooded, safety issues, switching to cleaner coal (lower sulfur coal) due to environmental regulations, and of course or cost issues. Cheap natural gas is the latest challenge along with increasingly stringent environmental regulations for coal mining and coal use.

We also tend to mine more of the lower Sulfur coal much of which is in Wyoming in the Powder River deposit, which has a significant impact on this region (we have higher sulfur coals) and where our coal comes from. Thus, due to environmental pressure (sulfur emissions), and changing economics (cheap shale gas) many coal mines have closed in Appalachia with significant losses of employment. 

Graph shows the change in coal employment from 2002. Much of the employment loss has been from the Appalachian coal basin (PA to AL).
Figure showing steady to decreasing coal production in the Appalachian basin
Credit: EIA
Worldwide however coal production has not declined due to the contributions of China. This plot shows the increase in Chinese coal production.
Coal has certainly declined in many nations due to carbon-neutral stances (abundant cheap shale gas is mostly an American phenomenon) and a deliberate shift to renewables for electricity generation.
Credit: 2018 BP Statistical Review of World Energy Data

Coal Transportation

How coal is transported is important, as often it will travel great distances (low S coal from the West to about 30 different States), across the State and International boundaries, and across the oceans. The United States is the "Saudi Arabia" of coal, so we transport coal across the nation. The final leg of the coal journey is normally to a utility site via rail. Lignite, however, is not shipped very far (much of the mass is water) so the coal-fired utility site (electricity generation site) sits next to the surface mine.

Conveyers Out of the Mine To The Next Transportations and Ultimately into the Pulverisors at the Utility Site

The conveyor is commonly used to move coal out of the mine and it can cover multiple miles carrying the material to the loading site for rail or barge transport. The coal may have been partially crushed in the mine to reduce the size of the lumps being transported. When the coal arrives at the utility site the conveyors are used again to put the coal in storage and to take the coal from the pile to the pulverizers for immediate use in the pulverized coal boiler.

coal on a conveyor belt
The conveyors can more coal efficiently over multiple miles, from the coal mine to the next transportation stage.
Credit: WN8540284 [35] / adobe.stock.com [36]

By Truck

Surface mines can also use very-large trucks to carry coal (or overburden) from the seam to the next location for transport. What is not evident in these pictures is the scale of these trucks, they are the size of some houses! Some mines are now using autonomous vehicles to transport coal (they are expensive vehicles and companies need to maximize their use).

Dump trucks full of coal
Massive coal trucks move the overburden, then the coal.
Credit: timofeev  [37]/ adobe.stock.com [38]

By Rail

Coal is commonly moved by rail. It was the movement of coal and goods during the era of "King Coal" that was responsible for an emerging infrastructure of railway lines that crisscross the country. There used to be a rail depot close to where the bus depot now stands in State College (there is a "Coal Lane" hidden there). You can still ride the rails from Bellefonte to Lemont. How important was the rail system? We currently ship coal all over the country from Wyoming because the coal has a low-sulfur content as the Clean Air Act restricts sulfur emissions from coal combustion. We also move over coal from the mining site to where it is needed (electricity generation, industrial use for steam generation, or movement to the cokers).

many trains in a train yard. All of the train cars are filled with coal.
Train yard
Credit: agnormark [39] / adobe.stock.com [40]

By River

It is a common site in Pittsburgh to see coal barges moving coal up and down the river. It is an efficient method of moving heavy materials. Here it is often coking coal being moved to the coker sites, or steam coal (for use in electricity generation) connecting to the rail system. Please listen to the audio, Did you know this [41]?

Click here for a transcript of "Did you know this?"

Dr. Mathews: During the search for the Titanic it was actually the coal debris field which helped located it. These submersibles went down and when they found a very large debris field of coal, they followed that and when it narrowed down they found the Titanic. Of course when the titanic flipped upside down and the boilers burst through the shell of the hull of the vessel and all the coal, which was Welsh Anthracite, poured through the hulled. Now when I was in Orlando in 2002, there was actually a Titanic exhibition and they had actual artifacts from the Titanic there. You could actually go and buy a piece of this Welch Anthracite coal which had been picked up from the sea floor. Now I wasn’t about to spend 15 or 20 dollars on a small piece of Welch Anthracite coal, but it is pretty interesting. There is also another story which is of relevance to the Titanic with coal. And that is its actual manufacturer. It was built in Ireland, cheap labor I guess. The problem with building things in Ireland is that if they are making steel, essentially you needed coal for making the coke. Unfortunately, they used cheap English coal to do this and it had a high sulfur content, so it was a relatively poor quality. The problem with a high-sulfur content in coal is that you get a high sulfur content in your coke. If you have sulfur in your coke it ends up in your steel, because it goes into the iron and the iron into the steel. This is very undesirable because it makes the steel brittle and this probably contributed to the sinking of the vessel. So the moral of the story - don't buy vessels built in Ireland unless they are made out of wood.)

Barge full of coal headed down a river
Pittsburgh has many bridges and if you look down you will see coal barges move up and down the river.
Credit: Tupungato  [42]/ adobe.stock.com [43]

By Ocean

We move coal in a similar manner to the movement of oil in the equivalent of tankers. These sea-going ships travel the Great Lakes, and across the oceans delivering coal.

ocean barge pulled up to port filled with coal
Coal loading dock and coal transport ship.
Credit: Parilov [44] / adobe.stock.com [45]

Movement of coal from Wyoming across the nation

Wyoming has low-sulfur coal and it is in high demand. It is shipped mostly by rail to ~30 States.

map of us showing movement of coal
Map showing where, and how much, coal is delivered from the Powder River Basis (mostly by rail) in millions of tons. Wyoming produces more coal currently than the next 5 states combined
Credit: Wyoming Mining Association

Coal Storage

We don't tend to store coal at the mine (instead we mine as needed) but we can store coal at the coal-fired utility. This allows for storage in case of supply disruptions (in winter) and permits the ability to pick up coal on the spot market (there are long-term contracts and coal that is available for more immediate sale). These piles are aligned into the wind to reduce dust problems. Supplies of 30 to 90 days are common.

piles of coal with a smoke stack behind it and a river in front of it.
The coal storage pile at the power plant — where it is stored for later use.
Credit: Marko Hannula [46] / adobe.stock.com [47]

There is also a robust international trade in coal by ship and rail with Indonesia, Australia, the former Soviet Union, and the U.S. being significant exporters. The coal goes to China, Europe, Japan, etc.

Global map showing coal trade
Coal is moved all around the world by various methods. Barges carry coal along the natural transportation routes (Rivers) and across the oceans. Indonesia and Australia Indonesia are the leading nations for exporting coal. 
Credit: CarbonBrief

Coal Coverage Map

As with previous coverage maps, this map represents a summary of the lesson, providing you with a way to quickly refresh yourself on the big ideas and connections in this lesson. This is interactive so move your mouse over the topics.

Accessible Version (word document) [48]

Deliverable

After looking at this map, please take the Lesson 4 quiz.


Source URL: https://www.e-education.psu.edu/egee101/node/517

Links
[1] https://www.e-education.psu.edu/egee101/sites/www.e-education.psu.edu.egee101/files/Lesson03/origins_of_coal.pdf
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[10] http://umwa.org/about/who-we-represent/mining-industry/
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[15] https://www.youtube.com/channel/UCjYrT--TpOrA86Iw_PkoVJw
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[18] https://stock.adobe.com/contributor/201635100/dule964?load_type=author&prev_url=detail
[19] https://stock.adobe.com/images/black-powder-coal-dust-isolated-on-white-background/141399101
[20] http://umwa.org/news-media/journal/black-lung-resurgence/
[21] https://stock.adobe.com/contributor/203116606/nordroden?load_type=author&prev_url=detail
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[23] https://www.e-education.psu.edu/egee101/sites/www.e-education.psu.edu.egee101/files/Lesson03/abandoned_coal_mine_APP.gif
[24] https://www.e-education.psu.edu/egee101/sites/www.e-education.psu.edu.egee101/files/Lesson03/L03_reclaimed_land_OSME.gif
[25] https://www.e-education.psu.edu/egee101/sites/www.e-education.psu.edu.egee101/files/Lesson03/surface_coal_mine1.gif
[26] https://stock.adobe.com/contributor/205336102/whitcomberd?load_type=author&prev_url=detail
[27] https://stock.adobe.com/images/aerial-drone-view-of-a-huge-opencast-coal-mine-cut-into-a-rural-hilly-area-dowlais-merthyr-tydfil-wales/215007863
[28] https://www.youtube.com/channel/UC0k173Oca1nPZurW2ITHlYw
[29] https://www.e-education.psu.edu/egee101/node/672
[30] https://www.e-education.psu.edu/egee101/sites/www.e-education.psu.edu.egee101/files/Lesson03/amd1_USGS.gif
[31] https://www.e-education.psu.edu/egee101/sites/www.e-education.psu.edu.egee101/files/Lesson03/amd2_USGS.gif
[32] https://stock.adobe.com/contributor/205198230/shootdiem?load_type=author&prev_url=detail
[33] https://stock.adobe.com/images/fantastic-view-of-open-pit-mining-on-cloudy-sky/299510614
[34] https://www.minemaps.psu.edu
[35] https://stock.adobe.com/contributor/205439036/wn8540284?load_type=author&prev_url=detail
[36] https://stock.adobe.com/images/opencast-mine-belt-conveyor-coal-stones-transport/339842530
[37] https://stock.adobe.com/contributor/203029387/timofeev?load_type=author&prev_url=detail
[38] https://stock.adobe.com/images/large-quarry-dump-truck-loading-the-rock-in-dumper-loading-coal-into-body-truck-production-useful-minerals-mining-truck-mining-machinery-to-transport-coal-from-open-pit-as-the-coal-production/251271397
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[43] https://stock.adobe.com/images/coal-transportation/210846570
[44] https://stock.adobe.com/contributor/206833199/parilov?load_type=author&prev_url=detail
[45] https://stock.adobe.com/images/loading-coal-mining-in-port-on-cargo-tanker-ship-with-crane-bucket-of-train-aerial-top-view/277788506
[46] https://stock.adobe.com/contributor/207577263/marko-hannula?load_type=author&prev_url=detail
[47] https://stock.adobe.com/images/a-large-pile-of-coal-stored-near-a-power-plant-located-next-to-the-waterfront/382687470
[48] https://www.e-education.psu.edu/egee101/sites/www.e-education.psu.edu.egee101/files/Lesson04/Lesson%204%20Coverage%20map.docx