外文翻译--选煤概述和煤的可选性

44 The outline of coal preparation and Economics of Coal Cleaning Abstract Coal preparation, simply put, is the conversion of run-of-mine (ROM) coal (or coal as it leaves the mine complete with impurities and prior to any processing) into a marketable product. Originally, coal preparation began as a line of equipment-crushers, feeders, screens, etc.-to control the size of the mined coal. Perhaps the easiest way to understand the evolution of coal cleaning and to understand the evolution of coal cleaning and to understand the variations found within the industry is to become familiar with the levels of coal preparation. Level 0 processing is the mining and shipping of ROM coal. The product of Level 1 processing is commonly termed raw coal. Level 2 processing involves the cleaning of the coarser sizes of raw coal (or coal which is larger than 1/2”).The coal finer than 1/2” would be added to the cleaned coal (the plus 1/2mm coal) or sent elsewhere. Level 3 processing extends the cleaning of the raw coal to the intermediate size raw coal-1/2” by 1/2mm.The minus 1/2mm material is added to the cleaned coal (the plus 1/2mm coal) or sent elsewhere. Level 4 processing extends the cleaning to include the minus 1/2mm raw coal. The feed to the coal preparation plant is then raw coal from Level 1 processing. Coals impurities are numerous, but by far the largest have specific weights greater than coal. The raw coal is thus characterized by partitioning the very heterogeneous coal into relatively homogeneous subpopulations on the basis of size and specific gravity. The separation unit operations normally process water/raw coal slurries, thus the term Coal Washing. Coal preparation is the quality control arm of the coal industry. It is an integral part of the coal business. 2. The Cumulative Float Curve-a plot of the cumulative float weight percent versus the cumulative float ash percent. The outline of coal preparation 45 Coal preparation, simply put, is the conversion of run-of-mine (ROM) coal (or coal as it leaves the mine complete with impurities and prior to any processing) into a marketable product. (A quality-controlled substance whose composition meets the ever-increasing specifications required for its use whether its combustion, liquefaction, gasification or carbonization.) The coal we mine today represents the deposition of phytogenic material 50 to 350 million years ago. The resulting horizontal strata, what we call coal seams, will vary in thickness from several inches to several hundred feet. They are usually separated by varying thicknesses of sedimentary rocks such as shales, clays, sandstones and, sometimes, even limestone, OR-when combined with coal-what are known as impurities in terms of preparation. Originally, coal preparation began as a line of equipment-crushers, feeders, screens, etc.-to control the size of the mined coal. Among the product line was the conveying picking table which was used to visually inspect the ROM coal so that obvious impurities could be removed manually. Thousands of men, women and children performed this unfulfilling work until mechanization replaced it withmore modern coal cleaning equipment. Generally speaking, this coal cleaning equipment was developed for British and European mines because their coal was of much greater value per ton than in the U.S. Its value reflected its cost of mining-which was high because the seams were more difficult to mine compared with American coal seams. However, although U.S. seams are among the easiest in the world to mine, preparation took on a new significance with the unionization of mines during the New Deal. A rapidly rising demand for machines to mine coal both underground and above ground was created； machines which were not and are not selective and which mine whole seams, including partings and some roof and floor mater ials. Mechanical mining meant mechanical cleaning. Perhaps the easiest way to understand the evolution of coal cleaning and to understand the evolution of coal cleaning and to understand the variations found within the industry is to become familiar with the levels of coal preparation. Each level is indicative of the intensity of the work performed on run-of-mine coal and each is an extension of the previous level. Level 0 processing is the mining and shipping of ROM coal. Level 1 processing combines top-size control by crushing, with some removal of undesirable constituents such as tramp iron, timber and perhaps strong rocks. The product of Level 1 processing is commonly termed raw coal. 46 Level 2 processing involves the cleaning of the coarser sizes of raw coal (or coal which is larger than 1/2”).The coal finer than 1/2” would be added to the cleaned coal (the plus 1/2mm coal) or sent elsewhere. Level 3 processing extends the cleaning of the raw coal to the intermediate size raw coal-1/2” by 1/2mm.The minus 1/2mm material is added to the cleaned coal (the plus 1/2mm coal) or sent elsewhere. Level 4 processing extends the cleaning to include the minus 1/2mm raw coal. Developing the appropriate circuitry for processing raw coals at Levels 2,3 and 4 involves four areas-characterization, liberation, separation and disposition. Characterization is the systematic examination of the ROM coal in order to determine the make up of the feed to the coal preparation plant. A coal processing engineer will develop a flowsheet of the unit operations required to achieve the desired preparation level. Liberation is the creation of individual particles whose composition are predominantly coal or refuse. This is achieved by size reduction or the crushing of the justmined coal to a particular top size as determined by the characterization study. The feed to the coal preparation plant is then raw coal from Level 1 processing. Unfortunately, particles containing both coal and refuse known as middlings-are also created Separation is, simply, the dividing of the particles into their appropriate groups-coal, refuse and middlings. Coals impurities are numerous, but by far the largest have specific weights greater than coal. The dominant method for separating the liberated coal is by gravity concentration which relies on two physical property differences-size and specific gravity. The raw coal is thus characterized by partitioning the very heterogeneous coal into relatively homogeneous subpopulations on the basis of size and specific gravity. Disposition is the cleaning up of the various streams. The separation unit operations normally process water/raw coal slurries, thus the term Coal Washing. The predominant disposition operation is the dewatering (separating the liquid and the solis ) of the various atre ams after the separations have been made. The second most important disposition operation is refuse disposal, followed by other environmental control operations. Coal preparation is the quality control arm of the coal industry. It is an integral part of the coal business. 47 Washability Washability studies are conducted primarily to determine how much coal can be produced at a given specific gravity and at what separation difficulty and size. The importance of the size analysis is perhaps more clear if you think of the cleaning process as removing impurities form individual pieces of coal, rather than in terms of tons of coal. As the individual pieces get smaller they become harderand more costlyto clean. Generally , the testing procedures of a washability study begin by obtaining a representative sample of the material already reduced to a designated top size, Next, the sample is sized at several different screen apertures, with each fraction held separately for further evaluation. A typical size analysis for a feed material is shown in Table 1. The table presents the percent of total weight, as well as an analysis of ash, sulfur content and Btu of each fraction, both individually and cumulatively. Then the material of each size fraction undergoes a float-sink test in liquids of pre-selected, carefully controlled specific gravities, beginning with the lowest. The float material from each specific gravity bath is then weighed and sink material is tested in the next heavier bath. The procedure is repeated until the desired number of float-sink result for the fraction in Table1 is given in Table 2. Since wider ranges are treated commercially, composite results are usually made by properly combining the individual size fraction results. A typical composite result of the material (Level 3 processing)in Table 1 is shown in Table3. This type of data is then used to develop washability curves-curves as unique to the coal as fingerprints to a hand-which describe the various characteristics of the coal. For example, Figure 1 shows three curves, generated from the data in Table 3, which are generally employed: 1. The Yield Curve-a plot of the cumulative float weight percent versus specific gravity； 2. The Cumulative Float Curve-a plot of the cumulative float weight percent versus the cumulative float ash percent； 48 3. The Cumulative Sink Curve-a plot of the cumulative sink weight percent versus the cumulative sink ash percent. The theoretical cleaning capability can be determined from the curves. For example, if it is desired to produce a 28m product of 10% ash, the theoretical product quantity will be 75.8% of the feed. The separation must be made at a specific gravity of 1.665 and the rejects should analyze 82.1% ash. 49 Economics of Coal Cleaning Abstract A second stage of evaluation is then based on user costs deriving from coal properties. Losses of yield in cleaning represent the biggest item contributing to total cleaning costs because the size, and hence the capital cost of a cleaning plant, is based on the throughput capacity for raw coal .It is generally accepted that capital costs reduce with increases in plant size, although not all the items comprising a cleaning plant are directly units .For example ,the sizes of raw coal storage and handling units ,and the sizes of cleaned coal bunkers and loading facilities ,are often determined by the needs for strategic stockpiles or the requirements of the transportation system . Economic analysis is also greatly influenced by the relationship of the coal producer to the coal user .At least three different considerations may arise:(1) cleaned coal is produced for sale under comparatively short term contracts (1 to 3 years) in a competitive market, (2) coal is produced on long-term (7years and more) supply contracts, and (3) coal production and cleaning forms part of a totally integrated operation in which coal is mined and used by a single industrial undertaking. The elements of the cost of coal cleaning comprise fixed costs arising from capital charges, and fixed or variable costs arising from plant operation. For a given annual production, capital costs are highly dependent upon raw coal quality as determined by ash content and size consist. The former determines the yield of clean coal, and since plant capacities are dictated by raw coal throughputs, this factor has the major influence on capital requirements. The average yield of current American coal cleaning plants is 71%. Economics of Coal Cleaning In recent times modern wash plants in the United States has been standing idle because the premium on price necessary to cover cleaning costs could not be recovered in a slack coal market tend to evaluate their coal purchases by methods that indicate minimum costs in energy terms (i.e., delivered cost per million Btu). A second stage of evaluation is then based on user costs deriving from coal properties. These can be complex and include such items as ash content, ash composition and fusion temperatures, fixed carbon (coke making), sulfur, and crushing and pulverizing characteristics. Considerations of these factors may modify the primary evaluation ,but as a general rule will justify a premium for cleaning only if substantial cost savings in utilization will result . However, calorific value is not a primary control function in coal cleaning .It has an approximately linear relationship to ash and moisture contents. But after gross rock dilution in a raw coal has been removed, the yield in terms of thermal efficiency of recovery with ash content becomes nonlinear and an increasing penalty in thermal recovery for unit decreasing in ash content becomes the rule. Losses of yield in cleaning represent the biggest item contributing to total cleaning costs because the size, and hence the capital cost of a cleaning plant, is based on the throughput capacity for raw coal .It is generally accepted that capital costs reduce with increases in plant size, although not all the items comprising a cleaning 50 plant are directly units .For example ,the sizes of raw coal storage and handling units ,and the sizes of cleaned coal bunkers and loading facilities ,are often determined by the needs for strategic stockpiles or the requirements of the transportation system . Economic analysis is also greatly influenced by the relationship of the coal producer to the coal user .At least three different considerations may arise:(1) cleaned coal is produced for sale under comparatively short term contracts (1 to 3 years) in a competitive market, (2) coal is produced on long-term (7years and more) supply contracts, and (3) coal production and cleaning forms part of a totally integrated operation in which coal is mined and used by a single industrial undertaking. Types (1) and (2) are most representative of past and present practices for metallurgical and thermal coals. Type (3) may apply following the current reorganization and restructuring of the coal industry and the growth of large coal conversion plants and mine-mouth generating stations. Additional complexity arises from tougher environmental regulation in which coal cleaning is only one aspect of control technology available for limited emissions of sulfur oxides and particulates, including hazardous trace elements. It has already been noted that environmental regulation relating to the operation of cleaning plants liquid effluents, fugitive dust, and noisehave resulted in significant increases in capital and operating costs. The future growth of coal cleaning in the United States will be largely determined by the attitude of the electric utilities industry and the extent to which utilities companies enter into full or joint ownership of the means of coal production. This may result in decisive changes in the way in which economic evaluation of new coal projects, including cleaning, are made .The financial yardsticks applied, until very recent times, for determining the economic worth of coal cleaning were relatively simple measures .The quality, tonnage, and expected market price of raw coal were compared with similar parameters for cleaning were then estimated and added to the basic production costs of ROM coal .The difference between anticipated gross productions costs and forecast selling prices because the basic for applying various accounting devices accounting devices to determine the balance of economic advantage .Usually , a discounted cash flow (DCF ) analysis enabled calculation of return on investment (ROI ) over periods of about 10 to 15 years for the different options. This exercise was principally of concern to coal-producing companies because they carried the fiscal responsibility for the decision to clean or not to clean. As a general rule of thumb, a decision to proceed with cleaning depended on a DCF-ROI of at least 15%per year, and more commonly, 20%. If the case for cleaning was a foregone necessity, because of the markets requirements, the analysis was used to calculate a selling price that would produce the necessary ROI. These rules had worked well in an industry that, although still important and substantial, had been declining. The utilities employ different accounting principles from the DCF-ROI type of evaluation and they employ different funding methods, particularly as regards debt/equity ratios. Traditionally, their payback periods are substantially longer, 3o to 40 years for fossil-fired plant. These differences can result in substantial changes in the fixed capital largest element in coal cleaning costs, being greater than 50% at all levels of preparation. Although a number of cost studies are in progress, the full impact of these changes and their wider importance for coal cleaning cannot be assessed at the present time. 51 The elements of the cost of coal cleaning comprise fixed costs arising fro m capital charges, and fixed or variable costs arising from plant operation. Capital charges are determined by the depreciation of costs incurred in land acquisition and preparation； design procurement and construction； provision of utilities and transportation and communications facilities； taxes； working capital； and interest charges. They are a fixed-cost burden unaffected by actual plant throughputs. Operating expenses include salaries, wages, power costs, water costs, and productive supplies, including fuels, refuse, and waste disposal. This category may include fixed costs independent of plant throughput (e. g., salaries) or variable costs tied directly to throughput (e. g., productive supplies). For a given annual production, capital costs are highly dependent upon raw coal quality as determined by ash content and size consist. The former determines the yield of clean coal, and since plant capacities are dictated by raw coal throughputs, this factor has the major influence on capital requirements. The average yield of current American coal cleaning plants is 71%. The most expensive items of capital equipment to provide and operate are required for handling fine and ultrafine coal sizes (i.e., froth flotation vacuum filters, centrifuges, water clarification, and thermal drying). Cleaning costs are therefore highly sensitive to the quantity of sizes smaller than 1/4 in. in the feed. Capital and operating costs are also sensitive to plant utilization and the system of working. Plant practices vary from single-shift, 5-day-week operation to continuous-shift, 7-day operation. The latter normally allows one to three shifts per week downtime planned maintenance. It is clear that, for a given annual production, plant sizes and hence capital costs are related to the working practice adopted. 52 选煤概述和煤的可选性 摘要 煤炭加工选煤概述简单说来，选煤就是把原煤（即开采后未经加工含有各种杂质的煤）。商品煤是具有一定质量规格的产品，它能满足燃烧，液化气化等方面所不断提高的技术要求。现在开采的煤是五千万到三亿五千万年前的植物沉积而成，所形成的水平层状物称之为煤层，厚度不一，从数英寸到数百英尺。煤层中经常夹杂着厚度不等的页岩，粘土砂岩，有时还夹杂石灰岩等沉积岩。从选煤的角度来说，这些和煤结合在一起的夹杂物称之为杂质。三级加 工 选别中等粒度（ 1/2 英寸 1/2 毫米）的原料煤，小于 1/2 毫米粒级的则归入精煤（大于 1/2 毫米）或送往他处。 原煤可选性的研究主要是为了决定在某一比重下可能获得的产品数量和洗选的难以程度，并确定入洗煤的粒度。如果把选煤看成是从一块块的煤中除区杂质，而不是根据成吨的煤去考虑问题，就能比较清楚的理解粒度分析的重要性。粒度越小，选煤难度越大，成本越高。在可选性研究的试验程序开始之前，通常是先取出经破碎达到规定粒度上限的煤样，然后用各种筛子进行筛分。各粒级产物要分别存放，以便进行可选性评定。表 1 所示为入料粒度 的典型分析。表中列出了各粒级产物的重量百分数灰分硫分和发热量，分本级和累计两项。先配好重液，准确调节其比重，然后对各粒级产物进行浮沉试验，从比重最小的重液开始。每一级重液中的浮起物要称记重量，下沉物移入较高比重的重液，依次进行，直到获得个级比重物为止。表 2 为表 1 中产物的浮沉实验结果。由于工业上处理的粒级范围较广，经常把某些粒级浮沉试验结果加以适当组合，形成综合结果。 近年来，由于支付选煤成本所需的额外费用不能在萧条的煤炭市场上回收，美国的现代化选煤厂一直处于停产状态。在竞争性的市场上，用户往往首先根据按能量 计算最低价格（即每百万英热单位包括交货费用在内的价格）的方法判断他们是否要购买。其次是判断按煤的性质决定的使用价值。煤的性质比较复杂，它包括灰分，灰的组成及熔点，固定碳，硫分，破碎和磨碎特性等各项因素。考虑了这些因素以后，还会改变最初的判断，但按一般规律来说，只有在使用中如能明显地节约费用，才能认为选煤这项额外费用是合算的。 然而，热值并不是选煤中应加以控制的主要方面。它与灰分和水分呈近似的线性关 53 系，但是在原煤中大块岩石被拣除后，产率和灰分之间变成非线性关系，一般的规律是降低灰分就要增加热值回收率的损失。选 煤中的产率损失是影响选煤总成本的最大项。由于选煤厂的规模乃至基建投资是以处理原煤的能力为基础的，因此产率损失对投资费产生最直接的影响。虽然构成选煤厂的各个项目并非全部都直接涉及选煤设备的处理能力，但通常人们认为随着工厂规模增大，投资费反而减少。按惯例，还本期很长，一个燃煤的厂的还本期为 30-40 年。 关键词 ： 煤炭加工 选煤 原煤可选性 简单说来，选煤就是把原煤（即开采后未经加工含有各种杂质的煤）。商品煤是具有一定质量规格的产品，它能满足燃烧，液化气化等方面所不断提高的技术要求。 现在开采的煤是五千 万到三亿五千万年前的植物沉积而成，所形成的水平层状物称之为煤层，厚度不一，从数英寸到数百英尺。煤层中经常夹杂着厚度不等的页岩，粘土砂岩，有时还夹杂石灰岩等沉积岩。从选煤的角度来说，这些和煤结合在一起的夹杂物称之为杂质。 最初，选煤是用一系列设备如破碎机给料机筛分机等来控制煤的粒度。其中手选皮带是靠视力检查原煤，利用人工拣除那些明显的矸石。在手选带被较现代化的洗选设备取代之前，拣矸工作是靠成千上万的成人和儿童来完成。 一般说来，比较现代化的选煤设备是为英国等欧洲国家的煤矿研制的，因为这些国家每吨煤的价格大大地 高于美国。煤价反映了采煤成本。在英国和欧洲，煤层的难采度大于美国的，成本较高，因而煤价较高。 虽然美国的煤层属于世界上最容易开采的煤层之列，在新政策（译注：这是美国在二十世纪三十年代为缓解经济危机指定的一系列政策）时期，由于煤矿成立了工会，为了符合工会会章，选煤有了新的意义。人们对于无论是井下，露天用的或至今还没有被淘汰的，以及开采全煤层（包括夹层，一些顶板和底板在内的煤层）用的各种采煤机的需求都有迅速增长。 机械化开采意味着机械化选煤。下面熟悉一下选煤等级，这也许是了解选煤发展和变化的一条捷径。 54 每一等级 都表示出毛煤加工的程度，同时又是前一级的延续。 零级加工 原煤的开采和运输。 一级加工 用破碎方法控制粒度上限，并除去某些杂物，如混入的铁块，坑木和硬岩石等。该级产品常称为入选原煤。 二级加工 选别粗粒级（大于 1/2 英寸）的入选原煤，而小于 1/2 英寸粒级的则归入粗精煤或送往他处。 三级加工 选别中等粒度（ 1/2 英寸 1/2 毫米）的原料煤，小于 1/2 毫米粒级的则归入精煤（大于 1/2 毫米）或送往他处。 四级加工 选别包括小于 1/2 毫米在内的入选原煤。 适合于二，三，四级加工的选煤流程的制定必须包括煤质 特性鉴定，解离，分选和选后处理等四个方面的内容。 煤质鉴定是有系统地测定毛煤，以便确定入厂原煤的组成。选煤工程师制定机组操作所需流程图，以求出所需要的分选等级。 解离主要是使煤和矸石单体分离。其方法是减小粒度，即把刚采出的煤破碎到煤质研究所确定的粒度上限。入厂原煤是一级加工的产品。在解离过程中还有中煤即含煤和矸石的颗粒产生。 分选，简而言之，是把入洗原煤分组，分成精煤，矸石和中煤。煤中的杂质很多，但绝大多数杂质的比重大于煤。选别单体解离的煤的主要方法是重力选，该方法以粒度和比重两种物理特性的差异为基础，把各 部分极不匀质的入选原煤分成相对匀质的产品。 选后处理是指清除各种水分。分选作业过程通常在水或原生煤泥水中进行，因而叫做洗煤。选后处理作业最主要是选后产品的脱水（即把液体和固体分离），其次是废渣处理和其它的环境保护措施。 选煤是煤炭工业中控制质量的一个环节，是煤炭事业不可分割的一部分工作。 原煤可选性的研究主要是为了决定在某一比重下可能获得的产品数量和洗选的难以程度，并确定入洗煤的粒度。 如果把选煤看成是从一块块的煤中除区杂质，而不是根据成吨的煤去考虑问题，就 55 能比较清楚的理解粒度分析的重要性。粒度越小，选煤 难度越大，成本越高。 在可选性研究的试验程序开始之前，通常是先取出经破碎达到规定粒度上限的煤样，然后用各种筛子进行筛分。各粒级产物要分别存放，以便进行可选性评定。表 1 所示为入料粒度的典型分析。表中列出了各粒级产物的重量百分数灰分硫分和发热量，分本级和累计两项。 先配好重液，准确调