Which of these method is the best for mining?
¿son seguras esas minas?
Today we have instruments that can capture carbon sing today is technology like pre combustion method not forgetting post combustion method.
What is Oxy fuel method?
What is carbon sequestration?
Once the geological data gathered during the exploration phase has been evaluated, geologists will estimate the quality and quantity of coal present. Coal reserves (in tonnes) are calculated from volume × density. The volume of coal is controlled by seam area and seam thickness.
Hence: tonnage = seam area × seam thickness × coal density
Seam area is not the same as surface land area, as the coal seam may dip. However, the surface land area will define the land area required for mining. Other considerations are the purity and rank of the coal, the likelihood of encountering any geological problems, and whether the seam has been worked in the past.
The first decision to be made is whether to open a surface mine, or an underground mine. In surface mines, exposed and/or shallow coal seams are accessed by removal of waste rock or overburden from the surface. They are therefore rarely deeper than 100 m below ground level.
The ratio of the amount of overburden to the total amount of workable coal is therefore of critical importance. Within the coal industry, this is known as the stripping ratio. Stripping ratios can be calculated either in tonnes or in thickness ratios (commonly used in coal operations). The maximum economic stripping ratio for surface mining has steadily increased over the years to around 20:1, helped by improvements in the productivity and life of the plant and equipment.
Mines working high-rank coal, which is more valuable, can operate higher stripping ratios than those working lower-rank coal - the surface anthracite workings in south Wales for example, operate a stripping ratio of 35:1. Note also that because coal is a moderately high place-value resource, it is economic for some present-day surface mines in the UK to extract coal at greater depths than underground mines in other parts of the world that are remote from points of sale.
Once it has been decided whether to use either a surface or underground mine, engineers will begin planning the optimal mine layout and the processes that will be used to extract and remove the coal from the coalface. Computers are often used to model the continually changing layout of the mine with time.
In surface mining (sometimes called ‘opencast’ mining in the UK), the coal seam is accessed by removing the rock overburden, a process that benefits from economies of scale by using some of the world's largest machines.
Topsoil is removed and stored or used immediately for land restoration elsewhere. Shallow or soft overburden is removed by draglines, hydraulic shovels and dump trucks, but at deeper levels with harder layers of rock explosives may have to be used.
When the first coal is reached, seams are usually worked by bench mining methods. The top surface of coal exposed on each bench is carefully cleaned to remove any adhering waste rock. This careful exclusion of non-coal material enables high-quality coal to be produced consistently by surface mining.
The clean coal is then broken up by diggers or with the aid of explosives, and loaded into trucks using mechanical shovels. Bench methods allow every seam to be worked, not just the thickest. Seams as thin as 0.1 m can be worked where they form part of the overburden to a lower, profitable seam.
The quantity of coal extracted per worker-day in surface mining can be many times that in underground mines - a factor that is reflected in the generally lower cost of surface mined coal (helped also by lower capital and operating costs). Other advantages of surface mining are that geological problems are more easily resolved and the working environment is safer for mining personnel.
One disadvantage for mine operators is that some major coal-producing countries, including the UK and US, have legislation requiring rehabilitation of the mined area, or laws prohibiting surface mining on land where rehabilitation would not be possible. In such cases, and where the coal is deeper than the economic stripping ratio, underground mining is the only option.
Coal extraction is of course less straightforward using underground mining techniques. The associated costs are higher, and these begin with the sinking of two shafts, an ‘upcast’ and a ‘downcast’ shaft for ventilation. Sinking these to a depth of a kilometre may take a few years and during this time, no coal is extracted.
However, not all underground mines involve deep, vertical shafts. Coal seams less than 350 m deep may be reached by inclined tunnels, or drifts. In hillsides, horizontal adits may be opened up directly into the coal seam, producing an earlier return on investment.
The coal seams exploited in most European mines are typically about 1.5 m thick, but can vary from 0.5 m to 3 m. Modern underground mines use longwall extraction methods, relying on highly mechanized extraction techniques.
A cutting machine works its way along the start of the planned extraction to develop a roughly 250 m long coal face. Temporary hydraulic jacks support the roof at the face. Cut coal falls onto a conveyor belt laid out parallel to the coal face, and is carried away.
The diagram illustrates the two common types of underground mine layout used in the UK. At an advance face, the coalface is advanced into a block of coal. The access gates at either end of the face are also advanced to keep pace with the face.
At a retreat face, the two access gates are first driven to the far boundary of the block of coal before the coalface is opened at their extremities. The face is then worked back towards the main roadways and the roof collapses in the same direction.