Coal-Fired Power Generation Handbook. James G. Speight

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СКАЧАТЬ technique in which rooms of coal were extracted with pillars left to support the roofs. Both of these techniques, however, left a considerable amount of usable coal behind.

      Deep shaft mining started to develop in England in the late 18^th century, and rapid expansion occurred throughout the 19^th century and early 20^th century. The English counties of Durham and Northumberland were the leading coal producers and they were the sites of the first deep pits. Before 1800, a great deal of coal was left in places as support pillars and, as a result in the deep pits (300 to 1,000 ft. deep) of these two northern counties (i.e., Durham and Northumberland) only approximately 40% w/w of the coal could be extracted. The use of wood props to support the roof was an innovation first introduced in 1800. The critical factor was circulation of air and control of explosive gases. In the current context, coal mining depends on the following criteria (i) the thickness of the seam, (ii) the thickness of the overburden thickness, (iii) the ease of removal of the overburden by surface mining, (iv) the ease with which a shaft can be sunk to reach the coal seam for underground mining, (v) the amount of coal extracted relative to the amount that cannot be removed, and (vi) the market demand for the coal.

It is important to recognize that coal quality control begins at the mine. The mining engineer is responsible for developing the mining plan, monitoring production, and managing operations. One objective of any mining plan is to maximize recovery of the deposit of suitable quality coal. This is an economic issue – it is cost effective to retrieve as much of a given resource that is economically possible. Mine development has sunk costs that should be spread over as much coal as possible. There are economic cut-off parameters that impact the mine plan. For open cast mines, the issues include strip ratios, how much overburden or interburden must be removed to expose a given quantity contained in a coal seam. For underground mines, it can be parameters such as (i) the seam height, (ii) the pitch of the seam, (iii) the depth of the seam, and (iv) the roof stability.

      Mine plans recognize the spatial attributes of coal quality; some seams will be better than others. As a result, the plan will typically manage mining areas to balance coal quality. Mining only the highest quality seams at the outset will truncate the life of the mine. Coal quality for any given mine can also change over time, as lower seams are used or new areas exposed. Long-term relationships with a mine should recognize how quality can change and continue to be vigilant, rather than complacent.

      Exploration for coal is emerging as a future potential source for thermal coal. The process involves discovering new regions and extracting coal economically from earth. Various coal mining techniques include underground coal mining, surface coal mining and mountain top removal method (Speight, 2013).

      The most economical method of coal extraction from coal seams depends on the depth and quality of the seams, the geology of the deposit, and environmental factors. Coal mining processes are differentiated by whether they operate on the surface or underground. Coal-mining operations can be described under three main headings (i) underground or deep mining, in which the coal is extracted from a seam without removal of the overlying strata, (ii) surface mining, in which the strata – the overburden – overlying the coal seam are first removed after which the coal is extracted from the exposed seam or partially covered seam.

      Each mining technique has its own individual merits and the method eventually employed to extract the coal and the technical and economic feasibility of coal recovery are based on (i) the regional geologic conditions, (ii) the overburden characteristics, (iii) the continuity of the coal seam, (iv) the thickness of the coal seam, (v) the structure of the coal seam, (vi) the quality of the coal seam, (vii) the depth of the coal seam, (viii) the strength of the strata above and below the coal seam for roof and floor conditions, (ix) the topography – especially altitude and slope, (x) the climate, (xi) the ownership of the land as it affects the availability of land for access and subsequent mining, (xii) the surface drainage patterns, (xiii) the groundwater conditions, (xiv) the availability of labor and materials, (xv) the requirements of the coal purchaser in terms of tonnage, quality, and destination, and (xvi) the capital investment requirements (Cassidy, 1973; Lindberg and Provorse, 1977; Martin, 1978).

      There are two predominant types of mining methods that are employed for coal recovery: (i) surface mining methods, in which the strata (overburden) overlying the coal seam are first removed, after which the coal is extracted from the exposed seam, and (ii) underground mining methods which currently account for recovery of approximately 60% w/w of the available coal for use.

      3.2.1 Surface Mining

Surface mining is the application of coal removal methods to reserves that are too shallow to be developed by other mining methods (i) the open pit method and (ii) the drift mine method. Relatively shallow coal deposits are generally extracted by surface mining, and deeper deposits are extracted by underground mining. There are also situations in which a seam is mined by surface methods first, and then if adequate reserves are still available, the mine is developed for underground extraction. Where remaining reserves are limited, other methods of mining—such as auger mining or highwall mining—may be used.

      Surface mining developed as a natural extension of the early mining techniques by which man recovered coal from a seam. In the early stages, the recovered coal would come from exposed ledges or outcrops. In time, this supply would be exhausted and the earth would be scraped away to lay bare more of the seams that led to these outcroppings, thereby exposing even larger amounts of coal. However, as the overburden to be removed became too much for the primitive equipment then in use, the workings would be abandoned and fresh outcrops sought.

      Typically, seams relatively close to the surface, at depths less than approximately 200 feet, are surface mined. Coal that occurs at depths of 200 to 300 feet is usually deep mined but, in some cases, surface mining techniques can be used. For example, some coal in the western United States (such as coal in the Powder River basin of Wyoming) that occur at depths in excess of 200 feet are mined by open pit methods – the thickness of the seam (60 to 90 feet) renders the method economically feasible.

      In surface mining, or strip mining, earth-moving equipment is used to remove the rocky overburden and then huge mechanical shovels scoop coal up from the underlying deposit (Speight, 2013). The modern coal industry has developed some of the largest industrial equipment ever made, including shovels (part of a piece of equipment known as a dragline) capable of holding in excess of 300 tons of coal.

      To reach the coal, bulldozers clear the vegetation and soil. Depending on the hardness and depth of the exposed sedimentary rocks, these rocky layers may be shattered with explosives. To do this, workers drill blast holes into the overlying sedimentary rock, fill these holes with explosives, and then blast the overburden to fracture the rock. Once the broken rock is removed, coal is shoveled from the underlying deposit into giant earth-moving trucks for transport.

      The characteristic that distinguishes open pit mining is the thickness of the coal seam insofar as it is virtually impossible to backfill the immediate mined out area with the original overburden when extremely thick seams of coal are involved. Thus, the coal is removed either by taking the entire seam down to the seam basement (i.e., floor of the mine) or by benching (the staged mining of the coal seam). Frequent use is made of a drift mine in which a horizontal seam of coal outcrops to the surface in the side of a hill or mountain, and the opening into the mine can be made directly into the coal seam. This type of mine is generally the easiest and most economical to open because excavation through rock is not necessary.

      Another surface mine is a slope mine in which an inclined opening is used to trap the coal seam (or seams). A slope mine may follow the coal seam if the seam is inclined and outcrops, or the slope may be driven through rock strata overlying the coal to reach a seam that is below drainage. Coal transportation from a slope mine can be by conveyor or by track haulage (using a trolley locomotive if the grade is not severe) or by pulling mine cars up the slope using an СКАЧАТЬ