Surface Mining Techniques

In addition to the forms of underground mining previously described on this blog, there are also several forms of aboveground mining used in some areas because of the increased convenience.

Strip mining is a technique used when the seam of coal is very near the surface or when the "overburden," or earth above the seam, is too unstable to mine under. As mining progresses, the removed overburden fills in the coal cavity. Any increased safety afforded by strip mining is offset, however, by the damage done to the natural landscape. The land surrounding the coal seam must be destroyed, and recovery of the area is often not attempted.

Contour mining is functionally very similar to strip mining, with the same problems and advantages. The only difference is that it follows the contours of the mountains being mined, creating "terrace" levels on the mountainside.

Mountaintop removal is by far the most controversial form of surface mining, wherein the tops of hills are removed to access coal seams. The overburden from this is put into surrounding valleys. This technique is extremely damaging to forest ecosystems and to mountainous water sources, and it damages the landscape as well, because the original mountain shape is not restored. Though like all surface mining, mountaintop removal is financially effective for the companies and safer for the miners, many think the ecological destruction is not worth the coal gained.

Sources: http://en.wikipedia.org/wiki/Mountaintop_removal_mining

Underground Mining Methods

There are several common methods of underground coal mining. Among these are drift mining, shaft mining, room & pillar mining, continuous mining, and longwall mining. Some of these are used in combination with each other--for example, shaft mining refers to the way coal is transported, while room & pillar refers to the way it is mined, so they may both be used together in the same seam of coal.

The first among these, drift mining, refers to the way the seam of coal is entered. It is used when the coal seam intersects the earth's surface. The mining area follows the coal horizontally from the surface as it extends outward. This method is advantageous in situations where the coal seam is on the side of a hill and accessible by this method, but is obviously untenable in anything other than that specific situation.



Shaft mining is used with coal deposits which rest deep underground without intersecting the surface. Vertical shafts enter the seam of coal and elevators take miners down and coal up. These mines are commonly deeper than 1000 ft below the surface. One of their primary disadvantages is that the heavy coal must travel against gravity up the elevators to reach the surface; another is that the coal must be blown up underground to put it in pieces small enough to be transported, a potentially dangerous practice. However, it's commonly practiced as it is by far the easiest way to reach seams of coal which are far underground.

Room and pillar mining is a method combined with both drift and shaft mining, as it refers not to the way the mine is entered but to the way coal is extracted. In room and pillar mining, the roof of the mine area is supported by pillars of coal left in the mine. This is a wildly inefficient method, leaving nearly half the coal behind in the mine. It's also dangerous, because soft bituminous coal can compress under the weight of the roof, putting extra stress on other pillars and leading to roof falls. Even with bolting in the roof for support, it's a dangerous method. It is, however, the most convenient way to mine a seam of coal, being quick and easy if not very financially sound.

Continuous mining refers less to a technique of mining and more to the technology used with it: continuous mining machines are run and can mine up to 5 tons per minute. These machines have been in use various places since the 1940s and are generally used with conveyor systems to transport the coal. This is an extremely fast method of mining; however, the machines are expensive and difficult to upkeep.

The final form of mining discussed here today is longwall mining. It's extremely efficient: machines mine a seam while supporting its roof with hydraulics, and after the coal is extracted the seam is allowed to collapse behind the retreating machine. Because no support pillars are necessary, much more coal is extracted than in room and pillar mining. It also resolves the problem of filling in mines after they've been used up by causing a cave-in. However, it suffers from the same machine cost problem as continuous mining.




 Sources: http://en.wikipedia.org/wiki/Drift_mining http://library.thinkquest.org/05aug/00461/shaft.htm

The Formation of Coal

Coal is formed by a process of compression. Decaying plant matter in swamps is trapped without oxygen, slowing the decay process. This forms peat, a substance with relatively low carbon content which is burned in parts of Europe to heat homes. Peat is considered a "coal precursor," being the substance which eventually develops into coal.

Peat, when compressed, forms lignite, or brown coal. It's often found in Alaska and other Western states in the US. Its carbon content is higher than that of peat, but lower than other forms of coal, and its heat value corresponds to that carbon content as in all the substances discussed here today. It's often used not as a heat source but instead to generate electricity.

The next stage in coal's compression process is bituminous, or black, coal. This is the most common form of coal and almost the only form found in West Virginia. Its carbon content is highly variable, ranging from 45% to 85% concentration, and again, heat value changes accordingly. It is used as a heat source and is also processed into coke for the creation of steel.

A final type of naturally forming coal is anthracite, or hard coal. Unlike at all previous stages of compression, anthracite coal is not a sedimentary rock but a metamorphic one, produced from tremendous heat and pressure put on bituminous coal. It has the highest carbon content of all naturally occurring forms of coal and produces relatively little smoke, making it useful for heating homes.

Coke, as previously mentioned, is a coal product created from bituminous coal. It is heated without oxygen to remove water, tar, gas, and other non-carbon substances. This runoff is used for energy production, while the coke, now at 100% carbon content, is used to make steel.