Читать книгу Coal-Fired Power Generation Handbook - James Speight G., James G. Speight - Страница 49

Recovery of coal (coal mining, coal recovery) is the act of removing coal from the ground and has been practiced throughout history in various parts of the world and continues to be an important economic activity (NRC, 2007; Speight, 2013, 2020).

A modern coal mine is a highly mechanized industrial plant that has to meet strict standards of engineering design and operation. The size, power, strength, monitoring and control features, and automation of mining equipment dwarf those of even a few decades ago. The overall coal mining process consists of several sequential stages: (i) exploration of a potentially economic coal seam to assess minable reserves, environmental issues, marketable reserves, potential markets, and permitting risks, (ii) analysis and selection of a mining plan, (iii) securing the markets, (iv) developing the mine, (v) extracting the coal, (vi) processing the coal if necessary, and (vii) decommissioning the mine and releasing the property for post-mining use.

The uncertainties concerning resource and reserve estimates also apply to the grade or quality of the coal that will be mined in the future. At present, it is difficult to project spatial variations of many important coal quality parameters beyond the immediate areas of sampling (mostly drill samples). Almost certainly, coals mined in the future will be lower quality because current mining practices result in higher-quality coal being mined first, leaving behind lower-quality material (such as coal with a higher mineral content leading to higher yields of combustion ash yield, higher sulfur emissions, and/or higher concentrations of potentially harmful elements. The consequences of relying on poorer-quality coal for the future include (i) higher mining costs, such as the need for increased tonnage to generate an equivalent amount of energy, greater abrasion of mining equipment, (ii) transportation challenges, such as the need to transport increased tonnage for an equivalent amount of energy, (iii) beneficiation challenges, such as the need to reduce ash yield to acceptable levels, the creation of more waste, (iv) pollution control challenges, such as capturing higher concentrations of particulates, sulfur, and trace elements; dealing with increased waste disposal, and (v) environmental and health challenges. Improving the ability to forecast coal quality will assist with mitigating the economic, technological, environmental, and health impacts that may result from the lower quality of the coal that is anticipated to be mined in the future (NRC, 2007).

Furthermore, coal seams are located in a variety of geologic settings and their characteristics, including variability in thickness and continuity, can differ markedly from basin to basin. Therefore, any definition of geological reliability (measured, indicated, and inferred) that is intended for the entire country or for any specific region is not as precise as a system that takes into account the geological differences between regions and between coals of different geological ages. Thus, the two essential requirements that must be fulfilled before a prospective coal mine can enter the development stage are confirmation that there are (i) sufficient minable reserves of adequate quality with no unacceptable environmental or permitting risks, and (ii) confirmation of an assured or contracted market for a substantial fraction of the coal that will be mined.

Early coal mining (i.e., the extraction of coal from the seam) was small-scale, the coal lying either on the surface, or close to the surface. Typical methods for extraction included drift mining and bell pits. In Britain, some of the earliest drift mines (in the Forest of Dean) date from the medieval period. As well as drift mines, small-scale shaft mining was used. This took the form of (i) a bell pit mining technique in which the extraction of the coal worked outward from a central shaft, or (ii) the room and pillar mining 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.