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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. Coal’s 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
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 it’s 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.
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. Coal’s 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.
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 harder—and more costly—to 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;
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.
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 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 noise—have 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.
The elements of the cost of coal cleaning comprise fixed costs arising from 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.
選煤概述和煤的可選性
摘要
煤炭加工選煤概述簡(jiǎn)單說(shuō)來(lái)，選煤就是把原煤（即開(kāi)采后未經(jīng)加工含有各種雜質(zhì)的煤）。商品煤是具有一定質(zhì)量規(guī)格的產(chǎn)品，它能滿足燃燒，液化氣化等方面所不斷提高的技術(shù)要求。現(xiàn)在開(kāi)采的煤是五千萬(wàn)到三億五千萬(wàn)年前的植物沉積而成，所形成的水平層狀物稱之為煤層，厚度不一，從數(shù)英寸到數(shù)百英尺。煤層中經(jīng)常夾雜著厚度不等的頁(yè)巖，粘土砂巖，有時(shí)還夾雜石灰?guī)r等沉積巖。從選煤的角度來(lái)說(shuō)，這些和煤結(jié)合在一起的夾雜物稱之為雜質(zhì)。三級(jí)加工——選別中等粒度（1/2英寸×1/2毫米）的原料煤，小于1/2毫米粒級(jí)的則歸入精煤（大于1/2毫米）或送往他處。
原煤可選性的研究主要是為了決定在某一比重下可能獲得的產(chǎn)品數(shù)量和洗選的難以程度，并確定入洗煤的粒度。如果把選煤看成是從一塊塊的煤中除區(qū)雜質(zhì)，而不是根據(jù)成噸的煤去考慮問(wèn)題，就能比較清楚的理解粒度分析的重要性。粒度越小，選煤難度越大，成本越高。在可選性研究的試驗(yàn)程序開(kāi)始之前，通常是先取出經(jīng)破碎達(dá)到規(guī)定粒度上限的煤樣，然后用各種篩子進(jìn)行篩分。各粒級(jí)產(chǎn)物要分別存放，以便進(jìn)行可選性評(píng)定。表1所示為入料粒度的典型分析。表中列出了各粒級(jí)產(chǎn)物的重量百分?jǐn)?shù)灰分硫分和發(fā)熱量，分本級(jí)和累計(jì)兩項(xiàng)。先配好重液，準(zhǔn)確調(diào)節(jié)其比重，然后對(duì)各粒級(jí)產(chǎn)物進(jìn)行浮沉試驗(yàn)，從比重最小的重液開(kāi)始。每一級(jí)重液中的浮起物要稱記重量，下沉物移入較高比重的重液，依次進(jìn)行，直到獲得個(gè)級(jí)比重物為止。表2為表1中產(chǎn)物的浮沉實(shí)驗(yàn)結(jié)果。由于工業(yè)上處理的粒級(jí)范圍較廣，經(jīng)常把某些粒級(jí)浮沉試驗(yàn)結(jié)果加以適當(dāng)組合，形成綜合結(jié)果。
近年來(lái)，由于支付選煤成本所需的額外費(fèi)用不能在蕭條的煤炭市場(chǎng)上回收，美國(guó)的現(xiàn)代化選煤廠一直處于停產(chǎn)狀態(tài)。在競(jìng)爭(zhēng)性的市場(chǎng)上，用戶往往首先根據(jù)按能量計(jì)算最低價(jià)格（即每百萬(wàn)英熱單位包括交貨費(fèi)用在內(nèi)的價(jià)格）的方法判斷他們是否要購(gòu)買。其次是判斷按煤的性質(zhì)決定的使用價(jià)值。煤的性質(zhì)比較復(fù)雜，它包括灰分，灰的組成及熔點(diǎn)，固定碳，硫分，破碎和磨碎特性等各項(xiàng)因素?？紤]了這些因素以后，還會(huì)改變最初的判斷，但按一般規(guī)律來(lái)說(shuō)，只有在使用中如能明顯地節(jié)約費(fèi)用，才能認(rèn)為選煤這項(xiàng)額外費(fèi)用是合算的。
然而，熱值并不是選煤中應(yīng)加以控制的主要方面。它與灰分和水分呈近似的線性關(guān)系，但是在原煤中大塊巖石被揀除后，產(chǎn)率和灰分之間變成非線性關(guān)系，一般的規(guī)律是降低灰分就要增加熱值回收率的損失。選煤中的產(chǎn)率損失是影響選煤總成本的最大項(xiàng)。由于選煤廠的規(guī)模乃至基建投資是以處理原煤的能力為基礎(chǔ)的，因此產(chǎn)率損失對(duì)投資費(fèi)產(chǎn)生最直接的影響。雖然構(gòu)成選煤廠的各個(gè)項(xiàng)目并非全部都直接涉及選煤設(shè)備的處理能力，但通常人們認(rèn)為隨著工廠規(guī)模增大，投資費(fèi)反而減少。按慣例，還本期很長(zhǎng)，一個(gè)燃煤的廠的還本期為30-40年。
關(guān)鍵詞：煤炭加工 選煤 原煤可選性
簡(jiǎn)單說(shuō)來(lái)，選煤就是把原煤（即開(kāi)采后未經(jīng)加工含有各種雜質(zhì)的煤）。商品煤是具有一定質(zhì)量規(guī)格的產(chǎn)品，它能滿足燃燒，液化氣化等方面所不斷提高的技術(shù)要求。
現(xiàn)在開(kāi)采的煤是五千萬(wàn)到三億五千萬(wàn)年前的植物沉積而成，所形成的水平層狀物稱之為煤層，厚度不一，從數(shù)英寸到數(shù)百英尺。煤層中經(jīng)常夾雜著厚度不等的頁(yè)巖，粘土砂巖，有時(shí)還夾雜石灰?guī)r等沉積巖。從選煤的角度來(lái)說(shuō)，這些和煤結(jié)合在一起的夾雜物稱之為