The flowsheet in this study illustrates the general practice in the United Kingdom. A large number of coal preparation plants have been established by the National Coal Board during the past 10 years. The primary consideration is to achieve a maximum recovery of low ash coal. The secondary consideration is to reclaim the plant water and avoid pollution of streams or rivers in the area. The flowsheet incorporates three stages of treatment for the various size ranges, namely, Dense Medium for the 8, +2 fraction, Baum Jigs for the 2, + mm. fraction, and Sub-A Flotation for the mm. fraction. In addition to the low ash coal fines recovered, flotation is an essential part of the flowsheet for preparing coal flotation tailings for media use in the dense medium separation and also to enhance the disposal of the fine refuse or tailing.
Dense Medium Section: The 8, 2 material is delivered to the Dense Medium Separators which are generally of the deep bath type. In the flowsheet the primary separator delivers the floats to the secondary unit which produces medium grade and high grade coals. The sink product from the primary separator is delivered to a Bradford Breaker fitted with 2 perforations which rejects the hard shale and reduces the middlings to 2 for retreatment in the Baum Jig Section. The products from the separators are passed over spraying screens (one shown) and the resultant effluent is treated by Sub A Flotation Machines for the removal of fine coal. Without this flotation step, the fine coal would upset the specific gravity range of the natural media and create a large circulating load of coal fines in the media.
Jig Section: The 2 fraction from both Bradford Breakers is treated by Baum Jigs and the clean coal screened into various size ranges for marketing. The mm. x 0 is partially dewatered on mm. wedgebar screens and then further dewatered by a centrifuge preparatory to marketing. The underflow from the mm. wedgebar screen is pumped to large diameter settling cones for treatment in the froth flotation section.
Flotation Section: The feed for the flotation section is siphoned from the settling cones at 10% solids and is fed to Conditioners. The reagents are added at this point. A mixture of cresylic acid- creosote, which acts as a collector and frother, is used in England.
The separation is made using No. 30 Sub-A Flotation Machines equipped with four-bladed froth paddles. At 10% solids each 100 cubic foot of cell capacity will handle 2-3 tons throughput per hour or roughly one ton per 40 cubic feet per hour.
The froth concentrates (cleaned fine coal) are filtered on a Disc Filter and the filter cake combined with either the , + mm. jig product or marketed separately. From an initial feed containing 20 25 % ash, the froth concentrate will average 8 12% ash and the tailings 80 85% ash. These figures, of course, will vary depending on the inherent ash content of the coal.
The tailings from flotation are fed to a thickener where a flocculant is usually added to promote settling. The thickener underflow is pumped to multi-framed filter presses for removal of water to a final moisture content of 18-20%. This waste material is trucked to a disposal area.
The standard practice on coal in England has been to use cell-to-cell Sub-A Flotation Machines of the double overflow type. Several hundred of the large No. 30 cells (100 cu. ft./cell) are in use. Super-charging is not generally used. Flotation is carried out at low solids, usually under 10% dilution.
In the U. S. on bituminous coal the trend is toward open type No. 30 Sub-A Cells, single overflow, and with supercharging. Flows of dilute pulp up to 1500 GPM are effectively treated with excellent results through a single 4-cell machine. Capacities up to 7 TPH of clean coal per 100 cubic ft. cell are not uncommon even on material as coarse as minus 14 mesh. Low ash froth concentrate and high ash refuse can usually be obtained in a simple one pass flotation system.
In 1958 the American Potash & Chemical Corporation, Trona, California, installed two 15-6 diameter x 28 high Agitators equipped with 120 diameter Turbine Type Propellers. These agitators are in parallel and circulate 91,000 gpm of 25% solids slurry. The new feed slurry to the agitators is of 29% KCl and NaCl crystal slurry, 95% +50 mesh.
Each unit consumes less than 20 HP in handling the coarse leach and crystalization service. This company is highly pleased with the operation of their big Agitators and reports no maintenance problems.
The feed to the mill is 4 and the discharge is 20 mesh. To reduce iron contamination, no rods or balls or other grinding media are used in the mill. Disintegration of the material is aided by large lifter bars inside the mill.
The feed end trunnion liner is equipped with a deep spiral to convey the large pieces of pot lining into the mill. The discharge trunnion liner has a reverse spiral to minimize the discharging of oversize material. The mill is equipped with a Spiral Screen on the discharge to scalp out tramp oversize.
A major phosphate producer in Florida purchased four 4-cell No. 30 (56 x 56) Sub-A Flotation Machines in 1954. These machines are on Phosphate Rougher Flotation service (fatty acid section). Feed to this section is 1250 long tons per hour of 35 +150 mesh feed. The machines are still equipped with original neoprene impellers and diffusers which are still in good condition.
This arrangement can be used to sample any number of streams simultaneously. Amount of each sample can be identical or proportional depending on feed volume and cutter opening. Samples can be combined or kept separate.
This unit is a standard Automatic Sampler Mechanism with special designed cutter arrangement using standard type B wet cutters carried on end track and wheels. Cutter openings are adjustable from to . Cutter travel can be 16, 21, 30 or more. Cutters move at 7, 12, 18 or 30 per second and make sample cuts at any interval from 2 to 55 minutes. Timer is adjustable in 1-minute increments. Samplers can be arranged for continuous sampling if desired.
In coal flotation, the most commonly used collectors are oily materials collectors such as diesel oil and kerosene. For higher rank coals, the reagent consumption in flotation is low because of the high natural hydrophobicity of the coal. However, for low-rank coals (containing a greater amount of oxygen), flotation performance with these oily collectors is poor and large amount of collectors are required in order to obtain acceptable coal recovery.
Polyethoxylated nonyl phenols constitute a major part of the general class of non-ionic surfactants, whose hydrophobic/hydrophilic characteristics can be controlled by varying the number of ethoxy groups on a hydrophobic nonyl phenol.
Two bituminous coals, namely Illinois No. 6 (from Peabody Coal Co., Marissa, IL) and Pittsburgh No. 8 (from R&F Coal Co., Warnock, OH) were used in this investigation. The proximate and sulfur analysis results of these two coals are given in Table 1.
Since the non-ionic surfactants contain a relatively small portion of polar groups in their structure, the flotation results for both coals indicates that these polar groups may play an important role in the interaction of the reagents with the surface of the coal. This interaction may be enhanced if the surface of the coal contains numerous oxygenated groups, as in the case of Illinois No. 6 coal or a coal that has been weathered. Because of this, the non-ionic surfactants can spread on a hydrophilic coal surface much more easily than the oily collector dodecane, thereby creating a hydrophobic surface.
The flotation results of Illinois No. 6 and Pittsburgh No. 8 coals show that non-ionic surfactants GH0, GH1.5, GH4 and THF are more effective collectors than the oily collector dodecane for both unoxidized and oxidized coals. Although ethoxylated (with an average of 4.5 ethoxy groups) dodecanoic acid has a similar balance of hydrophobicity and hydrophilicity, it was a poor collector.
Utilizing engineering ingenuity and todays developing computational fluid dynamics tools, a new classifier design is now available that significantly improves fineness from pulverizers without the heavy costs associated with dynamic classification or any downsides on pulverizer capacities, maintenance, and parasitic power. Instead, operational flexibility and improved emission control options are enhanced.
Pulverized coal has been the dominate fuel used for power generation since the early 1900s. Perhaps the best reason for its popularity was improved combustion flame stability and heat release rate by a fuel that burns substantially as a gaseous mixture when combusted. This approach to combusting coal has since been extended to all grades of coal, from anthracite to lignite, for steam and power production. Of course, the balance of the combustion system and furnace geometry must be carefully selected for a given rank and quality.
Thirty years ago, pulverized coal burners were generally very turbulent with combustion efficiency the primary design concern. During this period, general industry standards for the degree of coal fineness were developed that produced acceptable combustion efficiency. A sampling of these older standards is presented in Table 1.
Since that time, pulverizer capacity has increased to meet the corresponding growth in unit sizes with incremental improvements in grinding efficiencies. The pulverizer configuration usually found in most coal-fired power generation plants is the vertical air swept design, although there are a number of ball tube pulverizers still in service. Both of these styles of pulverizers were, for the most part, placed into service with static classifiers. Changes in combustion systems implemented in the 1980s with New Source Performance Standards and post-1990 through Title IV of the Clean Air Act (CAA) utilized controlled mixing between pulverized fuels and combustion air and the application of low NOx burners and staged combustion air or overfire air.
Also, these more stringent emission controls rules have necessitated the addition of selective catalytic reduction catalysts to further reduce NOx emissions, flue gas desulfurization systems, and high-efficiency particulate collection systems. Today, there is another tool available to incrementally improve combustion efficiency and therefore reduce flue gas emissions: an improved coal classification technology.
One of the challenges facing plants today is to achieve complete combustion during the short time fuel and air particles spend in the combustion zone where they must be exposed to sufficient oxygen and temperature. Since oxidation rates are also limited by the exposure of the char or carbon in the fuel to oxidizing conditions, this puts increased emphasis on the initial fuel particle size. Reducing the pulverized coal particle size (in particular, eliminating the coarse particles) increases the surface area to volume or mass ratio, effectively making the coal more reactive. Consequently, improved fuel fineness will improve plant operating economics as well as reduce air emissions (see sidebar).
Coal fineness is a relative measurement of particle size distribution typically applied to the product leaving a pulverizer. Standardized wire mesh screens are stacked and used to shake down samples. The standard meshes allow particles of a specific size or smaller to pass through, so the largest screen openings are oriented towards the top of a stack. The weights for each particle size range are then determined and converted to a percentage of total sample weight.
Figure 1 shows the correlation between US standard meshes and the nominal sieve opening. A common practice is to plot the results on a Rosin-Rammler chart. The Rosin-Rammler equation, shown below, was originally derived in the 1920s from theoretical considerations.
A sample set of results from a vertical air swept pulverizer with a static classifier might be 99.25% passing 50 mesh (297 m), 87.38% passing 100 mesh (149 m), 72.12% passing 140 mesh (105 m) and 55.76% passing 200 mesh (74 m). These are plotted in Figure 2.
If the pulverizer is functioning properly and the coal sample was properly collected, the Rosin-Rammler plot gives a good straight-line representation of the data. The x-axis uses a log scale with the particle sizes plotted at their square hole U.S. standard mesh equivalent. The y-axis is a probability distribution based on the Rosin-Rammler formula. The slope of this Rosin-Rammler line is n in the equation. The absolute size constant k represents the most common particle size in the coal particle distribution. The value of k is the size at which the line crosses R equal to 36.79 (or 63.21% passing).
As an example of the importance of effective fuel classification, a typical classifier for a vertical air swept pulverizer which have been limited to the upper portion of the pulverizer is illustrated in Figures 3 and 4. Heated (primary) air is supplied in the lower portion of the pulverizer to provide some coal drying and to transport the pulverized coal to the downstream combustion process.
The coal classification actually begins upstream of the static classifier in Zone A of Figure 3. In this zone, the drag forces created by the primary air are overcome by gravitational forces for the coarsest particles. The result is that essentially no coal particles greater than 700 m in size reach the classifier. The flow is then turned from vertical to horizontal and radially inward through the classifier vanes (Zone B) where a swirl component is introduced (Zone C). The intent of the swirl is to add radial inertia to the coal particles to encourage them to migrate outward in Zone D. Historically, the goal has been to minimize the number of coal particles greater than 300 m that fail to be classified and are returned to the pulverizer (through Zone E) for further size reduction. These coarser fractions are considered to be the largest contributors to unburned carbon residues in the fly ash. Coarse particles also contribute to in-furnace ash deposition and corrosion potentials.
Any or all of these adjustments can produce marginal fineness improvements, the magnitude of which is dependent upon the scale of the classifier used and the starting conditions. However, each also adds pressure drop to the system and the associated increase in draft power required. This approach can also lead to pulverizer capacity restrictions. Some changes in the shape and surfaces of the vertically oriented classifier vanes have also been attempted, although with marginal improvement in performance.
During CAA Title VI compliance upgrades there have been some limited applications of dynamic classifiers. Dynamic classifiers introduce a rotating component with vanes. Larger particles have more mass and thus higher momentums and inertia than finer particles. The higher momentum (and inertia) makes it more difficult to avoid components that create a change in direction. The result is that coarse particles are more likely to be impacted by the rotating components of the dynamic classifier and like a baseball bat impacting the baseball, redirecting their trajectories, ideally back to the grinding zone. The performance of such retrofit applications was rarely as good as promised, and the installations were relatively expensive and had high maintenance requirements in addition to increasing parasitic power. For external classifier applications applied to ball tube pulverizers, the performance of dynamic classifiers was significantly less effective, presumably due to the lack of proximity to the grinding zone and decreased effectiveness in rejecting coarse particles.
Many of the pulverizers in service today were designed and built prior to the ready availability of computational fluid dynamic (CFD) modeling as a design tool, let alone two-phase CFD modeling. Over the last couple of years, Reaction Engineering International has been working with LP Amina and SAVvy Engineering to further develop their ability to reasonably simulate particle classification using their proprietary in-house CFD tools. The objectives for this work were:
The CFD modeling effort focused on a typical MPS pulverizer classifier. The classifier inlet conditions were based upon actual coal fineness test results from a pulverizer with a plugged classifier reject section. Figures 5-9, provide a brief comparison of the modeling results for the baseline and new S-Type classifier designs.
9. Sample comparison of coarser particle trajectories for Baseline and S-Type classifier. Note that the baseline classifier was predicted to still be passing a number of 425 m particles while the S-Type classifier was rejecting essentially all particles at 330 m (and larger). Source: SAVvy Engineering LLC
The Rosin-Rammler plots in Figure 10 include the comparative fineness predicted through CFD at the classifier outlet for the Baseline and S-Type classifier designs. The baseline CFD results reasonably match field measurements for similar operating conditions, and the predicted changes between the two designs (as discussed further below) have essentially been duplicated through a field demonstration.
These plots indicate a substantially improvement in the overall coal fineness results (Figure 11). Each bar in the two charts presented in the Figure 11 represent the percentage of the particles of a given size range entering the classifier that are either passed through the classifier or rejected back to the pulverizer grinding zone. Clearly, the S-Type classifier significantly improves the percentage of fine coal particles allowed to pass to the combustion process while passing far less percentage of the two coarser ranges of particles. Also, the S-Type classifier more effectively rejects coarse coal particles back to the grinding zone with very small percentages of fines rejected, relative to the Baseline classifier design.
The CFD results have provided a better understanding of the actual operation of the historical designs and provided confidence in the new S-Type classifier design. Some of the new S-Type classifier design components are illustrated in Figure 12. The flow diverter (upper right) is intentionally radiused and integral to pitched classifier blades (lower right).
The new design was completed during the summer of 2010 and the first field demonstration was installed in March of 2011 at the Fengtai Power Station, a 600 MWnet unit in Anhui Province, China on one of six MPS-style pulverizers. The experience to date has closely duplicated the CFD modeling estimates developed by Reaction Engineering International. The significant operational improvements have included:
In addition, reduction in unburned carbon deposition on tube surfaces, reduced slagging potential, and improved combustion side NOx emission control is expected when all pulverizers on a unit use the new classifier. Similarly, low load pulverizer turn-down will improve with the increased fineness and effective reactivity, lowering minimum load without igniter support. The performance data collected during those tests is illustrated in Figures 1315.
14. Comparison of Average Fengtai Baseline versus with S-Type classifier pulverizer inlet pressures (1 kPa 4 inches w.c.). Also, note the expanded range of mill operation. Source: SAVvy Engineering LLC
As a result of this success, Fengtai has since issued an order for five more units to retrofit the balance of their pulverizers on the unit. Similar considerations also apply to the S-Type classifier application on other pulverized coal fired power plants worldwide. The first US S-Type classifier installation is delivered and scheduled to be installed on an MPS-89 pulverizer processing bituminous coals in the September-October 2011 timeframe.
Scott A. Vierstra, P.E. is the principal of SAVvy Engineering LLC, located in Canal Winchester, Ohio. SAVvy Engineering provides engineering support to the power industry, focusing heavily on combustion and related auxiliary equipment. Scott is the inventor of the S-Type classifier (patent pending). Marc Cremer, PhD is the manager of engineering analysis at Reaction Engineering International, located in Salt Lake City, UT. Reaction Engineering International (www.reaction-eng.com) provides engineering solutions to problems dealing with energy and the environment by leveraging its internal and external specialist talent and computational tools. David B. Piejak is the general manager for LP Aminas North American Operations, headquartered in Charlotte, NC. LP Amina (www.lpamina.com) is a multinational environmental engineering company with research and development activities in the US, Europe and Asia focused on delivering the most advanced environmental technology and solutions to support the sustainable use of coal resources for power, chemistry and other applications. LP Amina has the exclusive worldwide license to market the S-Type classifier.
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Economic and operating conditions make it important to provide a simple, low cost, efficient method for recovering fine coal from washery waste. Not only is the water pollution problem a serious one, but refuse storage and disposal in many areas is becoming limited and more difficult. Many breakers and washeries efficiently handle the coarser sizes, but waste the coal fines. This problem is assuming major importance due to the increase in the amount of coal fines being produced by the mechanization of coal mining.
Flotation offers a very satisfactory low-cost method for recovering a fine, low ash, clean coal product at a profit. Often this fine coal, when combined with the cleaned, coarser fractions, results in an over-all superior product, low in ash and sulphur, giving maximum profit returns per unit mined.
Generally a very simple flotation flowsheet, as illustrated above, will be suitable for recovering the lowash coal present in waste from coarse recovery washeries.Assuming the fines are approximately all minus 20 mesh and in a water slurry of about 20% to 25% solids, the first step is to condition with a reagent which will promote flotation of the fine coal particles. Kerosene, fuel oil, coal tars and similar hydrocarbons will accomplish this effectively when added to thecoal slurry in a (Patented) Super Agitator and Conditioner. A frothing agent such as pine oil, alcohol frother, or cresylic acid added to the slurry as it discharges from the conditioner is also used. The separation between low ash coal and high ash refuse is efficiently accomplished in a Sub-A Flotation Machine. As the amount of clean coal floated represents a high percentage of the initial feed, provision is made to remove the cleaned coal from both sides of the cell. Fine coal is dewatered with a Disc Filter, as the Flotation Machine can usually be regulated to produce a product low in ash and with proper density for direct filtration.
It is highly desirable to extend the range of coal flotation to include the coarser Sizes. Not only will this simplify general washery practice but will result in a superior product having desirable marketing characteristics for metallurgical and steam power plant uses. It is now possible to efficiently recover coal by flotation through the entire size range beginning at about 4 mesh down to fines, minus 200 mesh.
With the flowsheet as outlined for coarse coal recovery, the feed is first deslimed for removal of high ash slimes and excess water. The hydroclassifier underflow is conditioned at 40% to 45% solids with kerosene or fuel oil and diluted with water to 20%-25% solids prior to flotation. If pyrite and coarse high ash material are present, it is often helpful to pass the conditioned pulp over a Mineral Jig for removal of a portion of these impurities. Hindered settling in the jig against a rising pulsating water column classifies out the high gravity impurities and eliminates them from the flotation circuit. Water requirements are low and feed density to flotation can easily be maintained at the proper level.
The Sub-A (Lasseter Type) Flotation Machine has proved successful for treating coarse coal with the flowsheet as indicated. A frother of the alcohol type is generally added to the flotation feed after conditioning with kerosene. Floated coal will collect in a heavy dense matte at the cell surface and as raked off, will contain up to 60% solids. Mechanical dewatering is usually not necessary. Natural drainage, dewatering on porous bottom screw conveyors, and vibrating screen dewatering are all being used successfully in coarse coal recovery circuits.
Flotation, with the Sub-A gravity flow principle, provides the ideal way to treat coal fines even as coarse as 3/16 top size. According to reports from plants operating for the production of metallurgical coke, each percent ash in the coal carries a penalty of 2$ per ton of coal. Thus there is a considerable margin for operating costs in a fine flotation cleaning method that will materially lower the ash of the cleaned coal. Further convincing evidence that ash removal from coal is of major importance is found in the weekly magazine of metal working, Steel, January 29, 1951, reporting on a modern coal preparation plant. The report states that a 1% reduction in ash content of coal means a reduction of 30 cents in cost of pig iron. One large plant reduces the ash from 7% to 3.5% by cleaning, thus cutting the cost of producing pig iron a dollar or more per ton.
A coal flotation machine must not only be able to handle a coarse as well as a fine feed, but it must also be simple to operate. Gravity circulation permits the treatment of difficult unclassified feeds.
High cost of mining makes it very important from a profit standpoint to recover all of the low ash coal, both coarse and fines. With the present trend toward mechanization, more fines are produced in mining. In many operations it is no longer economical to discard these fines to waste even though ash contiminants render the fines unmarketable without additional cleaning.
Water conservation, stream pollution and refuse storage are also factors which must be taken into consideration along with marketing requirements for the clean coal product. Flotation offers an efficient and low-cost method for recovering coal fines at a profit. In many cases floated coal fines can be blended with the coarser fractions without affecting ash, moisture or size limitations. This is being done successfully in coking coal operations. Fine coal is also being used extensively in steam plants for electric power generation.
The above flowsheets are based on existing small coal flotation plants. They illustrate clearly the simplicity and feasibility of adding Sub-A Coal Flotation as an additional process to small washing plants.
Because of its limited output, treatment must be very simple and operating costs kept to a minimum. At the washery, illustrated by flowsheet A, the entire mine output is sold for coking coal. Mining the relatively narrow seam produces a product with 15 to 20% ash, although the coal when cleaned will carry only 3 to 3% ash. This low ash coal brings a premiumprice, so it is an economic necessity to remove the impurities.
The mine run coal is crushed to a size for coking coal requirements. The entire production is treated over a coal jig which removes as waste primarily the coarse refuse. The coarse clean coal passes over the jig along with the fines and is elevated to a wedge bar stationary screen with 1 millimeter openings for dewatering. The coarse clean coal passing over the screen discharges by gravity into a storage bin. The fine coal, along with clay and its high ash fractions and water averaging 15 to 18% solids, discharges by gravity into a (Patented) Super Agitator and Conditioner. Kerosene and pine oil are added and the conditioned slurry or pulp then is introduced into the Sub-A Coal Flotation Machine.
The low ash coal product removed from the Sub-A Coal Flotation Cells contains 35 to 40% solids and is transferred to the coarse coal storage bin through a Vertical Concentrate pump. The flotation coal mixes with the coarse product which allows for adequate drainage and minimum loss of fines.
In the operation as illustrated by flowsheet B, approximately 15 tons per hour of coal flotation concentrate are produced. This installation requires more control to meet specifications and consequently a more elaborate system is necessary.
Screen undersize and water containing fines from the gravity separator are thickened in a centrifugal or cyclone separator to give the proper water-to-solids ratio for subsequent treatment. The effluent from the cyclone contains collodial slimes and high ash fines in addition to the bulk of the water from screening and gravity systems. Thickened coal fines from the cyclone pass over a Mineral Jig which removes a high ash refuse and free pyrite down as fine as 150 to 200 mesh.
The coal fines passing over the Jig are conditioned with reagents in the (Patented) Super Agitator and Conditioner and subjected to flotation treatment in a 6-cell Sub-A Coal Flotation Machine at approximately 20-25% solids. Double overflow of froth is used due to the low ratio of concentration and the high weight percentage of floatable coal recovered by flotation.
The coal flotation product at 35% solids is dewatered by a Disc Filter. Coarse coal from the gravity section and fine coal from the flotation section are blended and transferred by rail to the coke plant.
In some cases the coarse and fine coal are dewatered by Dillon Vibrating Screens. The coarser fractions of coal are first added to the screen to form a bed and flotation fines are added on top of this bed for dewatering. Where operating conditions are favorable, this system is preferred to other means of dewatering as it assures a well blended product low in moisture and uniform in ash content.
Effluent from the cyclone, high ash jig refuse and flotation tailing refuse are thickened in a Thickener to conserve and re-use water. Thickener refuse is disposed of without contaminating local streams.
Sub-A Coal Flotation with its gravity flow principle and selective action makes it possible to recover low ash coal from 1/8 down to minus 200 mesh. If an appreciable amount of recoverable coal is plus 20 mesh in size, the Sub-A Lasseter Type Coal Flotation Machine should be used. It is no longer necessary to use a complex system for fine coal recovery. Flotation will effectively handle the entire fine size range at low cost and produce a low ash marketable product.
In the washing of coal the problem exists in having to clean the fines in an economical and efficient manner without an excessively complex flowsheet. Mechanized mining creates fines not considered as problems in older methods of selective mining and underground loading. In many cases the minus 1/8 inch fines require cleaning to lower the ash content and frequently it is also necessary to reclaim all of the water for re-use in the washing system. Most plants use a closed water system to conserve water and comply with anti-stream pollution regulations.
Flotation offers a means for handling the entire size range minus 1/8 inch x 0. Efficient recovery of the fines at a low ashcontent is accomplished in a relatively simple flowsheet. Thesubstantial amount of coarser sizes in the concentrates aids in subsequent dewatering either by vacuum filters or dewatering screens.
In the flowsheet shown mine run coal after proper size reduction treatment is passed over heavy duty screening equipment to removethe minus 1/8 inch fines. Wet screening down to 10 or 12 mesh offers no particular problem. Water sprays are generally employed to thoroughly wash the fines from the coarse coal and prepare it for treatment. A surge tank or a thickener ahead of the conditioning and flotation section may be necessary to provide a uniform feed rate both as to solids content and density.
The coarse coal is washed and up graded in a conventional manner through heavy media or coal jigs to produce a clean coal and a coarse refuse. Any fines due to degradation through the coarse cleaning system is collected, partially dewatered and combined with the fines from the screening section.
Minus 1/8 inch x 0 coal fines are conditioned with the required amount of fuel oil or kerosene (approximately 1 to 3 lbs/ton) to thoroughly activate the low ash coal particles and render them floatable. Density in the conditioner should be as high as possible; however, for the open circuit system as shown it very likely will be maintained between 20 to 25% solids. A Super Agitator and Conditioner is preferred for this service since any froth accumulation on the surface is drawn down the standpipe and thoroughly dispersed throughout the pulp. This also aids in the most effective use of reagents.
The discharge from the conditioner at 20% solids is floated in a Sub-A Flotation Machine of the free flow type for handling coarse solids. Some dilution water may be necessary to maintain the feed density at 20% solids. A frother such as pine oil, cresylic acid, or one of the higher alcohols is added to the head of the flotation circuit at the rate of about 0.5-1.5 lbs/ton.
In the primary flotation section a high recovery of the coal fines minus 28 mesh is secured. In addition some of the more readily floatable coarse coal, low in ash, is also recovered. However, ability of the machine to handle all 1/8 inch feed permits recovery of coal over wide range of mesh sizes, thus improving filtering and handling characteristics. This coal, if not clean enough, is refloated in cleaner cells and middlings are recycled back to the feed. Clean coal will contain about 35% solids which is ideal for vacuum filtration. A Agitator Type Disc Filter is used as solids are effectively kept in suspension giving uniform distribution of cake for greater dewatering.
Generally the refuse from the primary flotation cells will contain a very high ash content in the -28 or-35 mesh size fraction. By screening the refuse the excess water and undersize high ash fines are eliminated while screen oversize is re-treated by flotation. This screening need not be highly efficient since only a partial sizing is satisfactory. Handling the coal in this manner reduces size degradation to a minimum.
The coarse coal from the foregoing dewatering and screening step is repulped to about 40% solids and conditioned with reagents. The conditioned pulp after dilution to 25 to 28% solids is floated in a second bank of flotation cells. The coarse coal in the absence of fines will form a dense, heavy matte at the surface of the cells. For this type of flotation, slow moving rakes are provided to remove the coal as final concentrate. This clean coal will generally contain over 50% solids, thus making it ideal for dewatering over vibrating screens or on a horizontal or top feed vacuum filter. In some plants where moisture is not too critical a screw conveyor with wedge bar bottom sections is used for the dewatering step.
The refuse from the coarse coal flotation cells may still contain some coal notresponsive to flotation recovery but low enough in ash to be saved. In such cases the refuse can be screened and the oversize fraction jigged or tabled. The tonnage at this point is usually only a very small percentage of the initial fines so the equipment requirements for this gravity section are moderate.
All refuse in the 1/8 inch x 0 coal recovery section is collected in a thickener for water reclamation. The thickened refuse or sludge underflow may be pumped to waste ponds, or if water is in short supply, filtration of this refuse may be necessary.
Coal flotation concentrates produced in this primary section are filtered direct and the filtrate is re-cycled back to the flotation cells for re-use. This filtrate is high in reagent content and is particularly useful as dilution water. Generally the density of the coal from the primary cells will contain about 35% solids and thus does not require thickening ahead of filtration.
Coal from the coarse flotation and the gravity section, if employed, can be readily dewatered over screens or horizontal or top feed filters. In some cases it may be possible to divert part or all of this coal to the filter handling the fines provided it is equipped with proper agitation equipment and a high displacement vacuum system. Some of the new synthetic filter bag fabrics such as Saran and nylon materially aid in securing high filter rates and low final moisture content.
Sub-A Coal Flotation Systems have been successful for recovery of both coarse and fine coal. It is important, however, to employ a two-stage circuit for maximum efficiency in saving the plus 28 mesh fraction which is normally the most difficult to float. The development of the free flow and Type M flotation cells offers a means for efficiently handling coarse coal in a size range heretofore reserved for other more complex systems.
Ash and sulphur content is desired to be as low as, or lower than, for regular lump coal. Generally, for anthracite, not over 13 per cent ash is desired. Bituminous coal operations usually limit ash to not more than 8 per cent in the fines.
Flotation or gravity concentration are generally applied only to washery fines that otherwise would not be saleable and which generally have to be impounded to prevent stream pollution. Because of the low price secured, the expense of treatment must be held to a minimum. Pyrite and coarser ash-forming content may need to be removed by gravity treatment.
Kerosene or fuel oil with pine oil, or alcohol, frother are the more common reagents used. Cresylic acid frother may sometimes be advantageous. Fine pyrite, if free, may be rejected with the high-ash refuse by addition of lime to the flotation feed.
Under proper conditions, coal as coarse as 10 mesh maybe effectively floated with kerosene and pine oil. For this coarse flotation it is generally necessary to classify out the high-ash 200 mesh slimes ahead of flotation.
Referring to solvents used in pre-combustion capture (IGCC), Table 2 is presenting utility consumptions for one chemical solvent (MDEA) and two physical solvents (Selexol and Rectisol). It can be noticed that Selexol process is involving lower utility consumptions, being considered further in the evaluations presented in the paper.
Based on simulation results, main techno-economic al and environmental indicators were calculated. Table 3 presents the overall techno-economical and environmental plant performance indicators. For IGCC, Selexol process was used.
Evaluated case studies were integrated in term of heat and power aspects for maximising plant energy efficiency . As illustrative example, Figure 3 and 4 are presenting the composite curves for an IGCC power plant with CCS (Case 3b).
Introduction of CCS imply a significant energy penalty due to CO2 capture, compressing and drying processes. For PC power plants, carbon capture penalty is about 8 9 % and for IGCC plant is about 7 % (net energy percentage points). Lower energy penalty for IGCC can be expected by advantages of CO2 capturing in a pre-combustion arrangement (CO2 partial pressure is about 10 12 bar) compared with post-combustion capture from nearly atmospheric flue gases (CO2 partial pressure is about 0.1 0.15 bar). All CCS designs capture more than 90 % of the feedstock carbon.
As can be seen from Table 3, there are minor variations of capital costs between the two PF-type power plants without CCS (about 5 % in favor of the super-critical case due to superior construction materials). Introduction of CCS implies a significant increase of capital cost (almost double) compared with designs without CCS. This is due to the influence of post-combustion capture (CO2 capture and conditioning are about 32 % of total plant costs). Effect of overall plant efficiency can be seen more clearly on specific capital cost per kW net power when comparing sub-critical and super-critical cases. Super-critical case has lower specific capital cost investment compared with sub-critical case with about 5 % (without CCS) and 9.5 % (with CCS).
Regarding IGCC capital costs, introduction of carbon capture step implies an increase of capital cost with about 20 24 % compared with designs without CCS. What is really important to notice is that the introduction of CCS in case of IGCC power plants is involving a significant lower increase of capital costs compared with combustion (PC) power plants [23,5]. Specific capital cost investment per kW net is showing comparative figures with a slight advantage of super-critical PC plants.
A coal-fired thermal power station is a power plant in which the prime mover is steam driven. Water is heated, turns into steam, and spins a steam turbine, which drives an electrical generator, as schematically shown in Fig. 1.9. After it passes through the turbine (one to three stage), the steam is condensed in a condenser and recycled to where it was heated; this is known as a Rankine cycle.
The efficiency of the Rankine cycle can be increased by introducing reheating of the steam after passage of the high-pressure turbine. Reheating not only improves the plant efficiency but also reduces the excessive moisture problem in low-pressure turbines. Reheating is commonly used in modern steam power plants, and for the USC plants or the A-USC plants, double reheating, where an additional reheating after the second turbine occurs, is adding another benefit to the plant efficiency.
The ideal reheat Rankine cycle differs from the simple, ideal Rankine cycle in that the expansion process take place in two stages. In first stage (the high-pressure turbine), steam is expanded isentropically to an intermediate pressure and sent back to the boiler where it is reheated at constant pressure, usually to the inlet temperature of the first turbine stage. Steam then expands isentropically in the second stage (low-pressure turbine) to the condenser pressure.
The energy efficiency of a conventional thermal power station, considered salable energy produced as a percent of the heating value of the fuel consumed, is typically 33% for the subcritical up to 48% for the most recent ultra-supercritical plants.
The greatest variation in the design of thermal power stations is due to the different fossil fuel resources generally used to heat the water. Some prefer to use the term energy center because such facilities convert forms of heat energy into electrical energy.
A pulverized coal-fired boiler is an industrial or utility boiler that generates thermal energy by burning pulverized coal (also known as powdered coal or coal dust since it is as fine as face powder in cosmetic makeup) that is blown into the firebox.
The basic idea of a firing system using pulverized fuel is to use the whole volume of the furnace for the combustion of solid fuels. Coal is ground to the size of a fine grain, mixed with air, and burned in the flue gas flow.
Biomass and other materials, like waste, can also be added to the mixture. Coal contains mineral matter that is converted to ash during combustion. The ash is removed as bottom ash and fly ash. The bottom ash is removed at the furnace bottom.
The feeding rate of coal according to the boiler demand and the amount of air available for drying and transporting the pulverized coal fuel is controlled by computers. Pieces of coal are crushed between balls or cylindrical rollers that move between two tracks or races. The raw coal is then fed into the pulverizer along with air heated to about 330C (650F) from the boiler.
As the coal gets crushed by the rolling action, the hot air dries it and blows the usable fine coal powder out to be used as fuel. The powdered coal from the pulverizer is directly blown to a burner in the boiler.
The burner mixes the powdered coal in the air suspension with additional preheated combustion air and forces it out of a nozzle, similar in action to fuel being atomized by a fuel injector in modern cars. Under operating conditions, there is enough heat in the combustion zone to ignite all the incoming fuel.
Denmark was first in Europe to build ultra-supercritical (USC) plants, but Germany followed very soon after and later on also Italy. The USC plant with steam temperatures around 600C is now considered state of the art in Europe, Japan, and China, whereas the United States has still not implemented this concept. It was the development of the steel grades 91 and later 92 for heavy section tubular components and derivatives for the cast and forged components that made this possible .
The main advantage between the subcritical, the supercritical, and the USC/A-USC units and the reason for a higher pressure operation is the increase in the thermodynamic efficiency of the Rankine cycle, as simply shown in Fig. 1.12. From subcritical to current state of art of USC, the efficiency increase is about 7%, targeting about 16% with the most optimistic A-USC project.
The single most important factor that determines the use of higher and higher pressure and temperatures is the availability of materials to withstand these conditions. Increases in operating pressure and temperatures have to go hand in hand with developments in metallurgy .
The last steel-based USC power plant is targeting a steam temperature in the range of 650C. For the A-USC the use of nickel-based alloys is mandatory to reach a steam temperature over 700C, both for creep and corrosion/oxidation properties. Fig. 1.13 summarizes the operating temperature and pressure of the different available material classes, and Fig. 1.14 shows the schematic view of the modified boiler for an A-USC power plant proposed by Babcock and Wilcox.
The other chapters of this book will give detailed information about the materials currently available for the construction of the USC plants and the materials R&D activities around the world for the development and manufacture of components able to be used in the next generation of A-USC plants.
Pulverized coal firing ensures complete combustion of coal, thus ensuring higher efficiency of steam generators. It is predominantly adopted in large coal-fired utility boilers. The finer the grinding of coal, the more efficient its combustion. The total time required from entry of a coal particle to a furnace to combustion of the particle is very short. This time, however, is dependent on various factors. The heart of a pulverized coal-fired boiler is the pulverizer, also known as the mill. Depending on speed pulverizers are classified as low-speed, medium-speed, and high-speed mills. Pulverized coal burners may be located on the front or opposed walls or in the corners of the furnace. There are two types of fuel-firing systems: bin system and direct-firing system. Coal feeders are either the volumetric or gravimetric type.
Pulverized coal is used in many utility boilers. Due to the small particle size, pulverized coal can be transported pneumatically. Coal particles contain four major components: moisture, volatiles, char, and ash. When heated, the moisture is dried and the volatiles are devolatized. The volatile vapor mixes with oxygen and the mixture burns. The char left on the particles is burned with oxygen when contacted. The drying and devolatilization processes are controlled by the heat transfer from the gas to the particles. Similar to the liquid vaporization, the heat transfer is convective. Empirical correlations are used to determine the interfacial heat transfer rate. Similar Lagrangian and Eulerian approaches can be applied for particles.
Pulverized coal can be used as a reducing agent by injecting it through the tuyeres. Coke has been replaced up to 250kg/THM with pulverized coal . Coals for PCI are usually cheaper noncoking thermal coals. Important properties of the coals are:
Coal is dried and ground to <0.1mm grain size. Pulverized coal is carried pneumatically to a bin and further to weighing and distributing system to be injected in the blast furnace, as illustrated in Figure 1.1.15 .
Waste materials, e.g., plastics and fluff, can be recycled by injecting in the blast furnace if they are ground to suitable grain size to be transported pneumatically and if they contain enough of carbon that can be oxidized. These materials must not contain too much of undesirable impurities such as Cl, Zn, P, S, etc.
Injection of oil was widely used before oil crisis in the 1970s. Since then PCI has been dominant. Equipment for oil injection is quite simple. Oil has been injected up to 150kg/THM. The replacement ratio decreases from about 1.3kg coke/1kg oil with small oil rates down to 1kg coke/1kg oil with large oil rates. Light oil fractions are easier to inject and they have a better replacement ratio. The heaviest fractions, e.g., bottom oil must be preheated up to 220C before pumping and injection. The main disadvantages of oil are high sulfur content, strong soot formation with high injection rate, and the price. A typical composition of heavy bottom oil is C=86.6%, H=10.0%, S=1.8%, N=0.8%, and O=0.8%.
Surplus tar from coke plant can also be injected like oil in the blast furnace. Tar is more abrasive for the pumps than oil. A typical composition of coal tar is C=91.5%, H=5.5%, S=0.6%, N=0.9%, and O=1.5%. Tar contains also some 25% water.
Natural gas is widely used where it is available at reasonable price. It consists mainly of methane (over 80% in volume) with a few percent of ethane and other alkanes. The main impurity is nitrogen and the rest is small amount of carbon dioxide. Table 1.1.1 lists the constituents of the natural gases from different countries.
Natural gas is easy to inject and it is cleanpractically without sulfur. It has a strong reducing effect on the adiabatic flame temperature. The lowest limit is claimed to be 2073K (1800C). The price of natural gas is usually high.
COG is a strong fuel but it is less useful compared to other injected fuels in the lower part of the blast furnace. COG contains H2=60%, CH4=24%, CO=6%, CO2=2%, and N2=6%, but only carbon in CH4 is oxidized in the raceway to CO releasing heat. CO2 reacts with coke carbon to CO consuming energy, whereas H2, N2, and CO are only heated up in the raceway. The advantage of H2 and CO as reducing gases comes in use in the upper part of the furnace. COG lowers the adiabatic flame temperature more than other injected materials.
Pulverized coal-fired supercritical steam boilers (e.g. 250 atm, 2853 K), have been in use since the 1930s, but improvements in materials and increasing demand for higher efficiency are making this system the choice of new coal-fired utility plant worldwide. Their increased efficiency is due to the higher mean temperature of heat addition in the supercritical steam cycle. Because of a strong increase in the moisture content of high-pressure steam as it expands through the last stages of the steam turbine, the steam has to be reheated by taking it back from the turbine to the boiler. Reheating, once or multiple times, will raise also the thermodynamic efficiency of the power cycle because it further increases the mean temperature of heat addition. Advanced supercritical steam power plants with steam parameters of 300 atm pressure, temperatures of 866 K, and cycle efficiencies up to 43% are expected to play a major role in new power generation in the near future. As the steam temperature approaches 973 K, the efficiency may reach 47%, but new advanced designs and materials for the boiler, the steam turbine, and the associated piping will be required; development will likely yield commercial applications after 2010.
The carbon of the coke and of the auxiliary reducing agents supplies the major part, approximately 80%, of the heat required for the process (Babich et al, 2008). Heat is required for the endothermic reactions, preheating and melting of the charge and heating of liquid products.
Carbon and oxygen react to carbon monoxide either directly (2C+O2=2CO) or at high temperatures (above 900-1000C) by means of the Boudouard reaction (C+O2=CO2 and then CO2+C=2CO). The carbon monoxide (and also the hydrogen) acts as reducing media. Below 9001000C iron oxides are reduced indirectly: FenOm+mCO=nFe+mCO2. This process is slightly exothermic. At temperatures above 900-1000C direct reduction starts: FenOm+mC=nFe+mCO. Direct reduction is an endothermic process and consumes heat.
Coke also maintains burden permeability. First liquid phases appear in the cohesive zone at temperatures between 900C and 1350C (Gudenau et al., 1998). Reduced iron and slag drop through the supporting checker-work of glowing, solid coke. Coke keeps its solid form until the raceway level.
Dust in the form of char and soot, which might be generated in the hearth while injecting the high rate of auxiliary reducing agents, is transported upwards by the gas stream. They decrease gas permeability and increase apparent viscosity of liquid phases. These negative phenomena are diminished when char and soot cover coke pieces and react later (Gudenau et al, 1998). Figure12.7 illustrates the effect of coke quality on the BF operation.
Quality requirements for coke can be derived from the functions in the BF. The requirements of coke characteristics increase considerably with the growth of the volume (especially height) of blast furnaces and with the drop in the coke rate. In Table12.2, requirements of coke properties in Europe are given.
Decreasing the specific energy consumption rate has been a priority throughout the entire history of the blast furnace. Operating improvements have been remarkable over the years. Figure12.9 demonstrates an example of the development of the structure and rate of reducing agents in German blast furnaces. Injection of auxiliary reducing agents as oil, natural gas and mainly pulverised coal contributed in the past three decades largely to the drop in coke rate.
The BF technology with pulverised coal injection (PCI) is nowadays widely spread around the globe. PCI rate of 200kg/tHM and more is realised while the coke rate is less than 300kg/tHM. Table12.4 demonstrates achieved reducing agent rate at some best performing BFs in Europe.
About 60% of BFs in EU-15 operate with PCI (the average rate in 2008 was 124kg/t HM (Peters and Luengen, 2009), in 2009103kg/tHM) (Luengen et al., 2011a). In Japan all BFs operate using PCI; the majority of BFs in China, many BFs in the USA and in some further regions use this technology as well (the average injection rate in China was 147kg/tHM in 2009(Sha and Cao, 2011).
When pulverised coal (PC) is injected via the tuyeres, carbon reacts with oxygen of the blast and generates CO2, which reacts with burning hot coke and transforms to carbon monoxide. A crucial problem is related mainly to PC conversion (particularly at high injection rates) because its residence time within the tuyere and the raceway makes up only hundredths of a second. Unburnt coal particles may affect negatively gas permeability in the furnace shaft, slag viscosity, coke characteristics, and finally, coke consumption and furnace productivity. A number of measures for intensifying the PC conversion in the raceway have been developed and summarised by Babich et al. (2008); here they are only listed:
Furthermore, a part of injected PC which is not gasified by the oxygen of the blast can be utilised by reactions of secondary gasification like reactions with oxides in slag or with carbon dioxide in the shaft. In this context, char generation and its behaviour in the blast furnace are of great importance (Kruse et al., 2003; Sahajwalla and Gupta, 2005).
In the course of environmental challenges, injection of charcoal as a renewable carbon containing substance has been studied recently under BF simulated conditions alone and in the mixture with PC (Babich et al., 2010; Machado et al., 2010). Table12.6 gives the composition of used coals and charcoals. The results obtained allow for the following conclusions:
Tests under BF shaft simulated conditions showed that solution loss reaction for charcoal goes on faster than for PC. The reaction velocity rises exponentially with increase of temperature in the range of 9001300C (temperatures in the cohesive zone). Difference in reaction rates of charcoal and mineral coals lowers with rising temperature (Fig.12.11).
Waste plastics can also be used in the steel industry in different ways to recycle industrial and municipal wastes and to replace or supplement coal use. There is industrial experience of plastics injection into the BF via tuyres in Germany, Japan and Austria (Buergler et al., 2007); systematic study on reaction kinetics of waste plastic materials is being performed (Knepper et al., 2011).
70% < 75 m is required for bituminous coals, 80% for anthracite. Coal dried with air at 250350C for milling in low-, medium- or high-speed mills. Main milling costs are power and replacement of worn parts. Wear depends on fineness of grinding, and on hardness and abrasiveness of coal.
Coal and air distribution to burners must be uniform to avoid local reducing zones which encourage slagging. Burners for wall-fired boilers are concentric-jet, turbulent flow type, giving short, intense flame. Those for corner-fired boilers are parallel jet type, with primary and secondary ports arranged in a vertical line. For down-fired boilers, primary ports are in furnace roof and secondary ports in side-wall.
Combustion rate generally is controlled by eddy-mixing and burnout rate by diffusion of oxygen to particle surface, but that for anthracite is chemically controlled. Ash-softening temperatures are reached in flames, and gases must be cooled to 10501100C to avoid slag deposition on superheater tubes.
Pulverized coal (PC) systems are the most commonly employed methods of fuel combustion for power generation. Pulverized coal combustion includes wall-fired, tangentially fired, arch-fired, and roof-fired systems. Wall-fired and tangentially fired systems are the most common type of boilers. PC firing uses pulverizers to grind the fuel to particle sizes of typically >70% passing through 200 mesh or 74 m . PC firing typically occurs at temperatures of 2500F to 2900F (13701590C).
Optimal fuel chemistry can be designed for PC firing through fuel blending, as is discussed in detail in subsequent chapters of this book. Properties such as high calorific value, low moisture, low sulfur, and low fuel nitrogen are important parameters. Fuel blending can achieve desired characteristics that can be effectively and successfully fired in PC boilers. Furthermore, the use of lower-quality fuels such as PRB coals or opportunity fuels such as petroleum coke or biomass fuels has been successfully demonstrated. In PC firing, attention must be given to particle size and its consequence on the combustion process. Therefore, using oversized fuel particles, such as certain biomass fuels (e.g., wood chips and TDF), has a significant negative influence on the overall system. Comingling of the fuels when blending will typically result in decreased performance of the pulverizers, resulting in larger particle sizes.
Cyclone firing is fundamentally slagging combustion, occurring at 3300F to 3550F (18151950C). It uses centrifugal force to combust the crushed fuel particles. As the spiraled fuel particles are thrown outward, they build up against the outer wall of the cyclone barrels. Because the fuel particles are only crushed and not pulverized, the particle sizes typically are larger: usually -inch by 0-inch. The technology originally was developed to combust fuels with high slagging propensities, removing approximately 70% of the inorganic material as tapped slag or bottom ash [33, 34]. Cyclones are fuel-flexible boilers that typically burn a variety of fuel and fuel blendshigh slagging bituminous coal, petroleum coke, biomass, subbituminous coals, lignites, and others. High-ash, high-sulfur, and high-chlorine fuels are commonly encountered. Fuels that are high in iron and calcium are also very effectively combusted in cyclone boilers (Figure 1.7).
FIGURE 1.7. A cyclone burner at Paradise Fossil Plant Unit #3, the largest one that has been built. This unit has 23 cyclone burners, all fed by radial feeders, and generates 1150 MWe. The plant has been the location of numerous fuel-blending experiments, including petroleum coke/Interior Province slagging coal blend tests.
Despite the fact that the fuel is not pulverized, oversized fuel particles must still be managed in cyclone firing. Large fuel particles will not stick to the slag layer and will experience minimal combustion in the cyclone barrel. Particle sizing is a significant parameter. Due to high combustion temperatures and the nature of the fuels typically combusted in cyclone firing, NOx and SO2 emissions can be extremely high compared to other firing mechanisms. The need for postcombustion capture systems exists.
In stoker firing, the fuel is burned on a grate where the range of fuel types can be from coal to biomass. In this type of firing, as shown earlier in Table 1.1, combustion can be more easily achieved compared to other conventional firing systems. Depending on the manner in which the fuel is loaded onto the grate, there are two main types of stoker systems: chain grate stokers and spreader stokers [33, 34].
Stoker grate designs can be traveling pinhole grates, chain grates, hydrogrates, sloping grates, and roller grates. The design depends on the material being burned. Fuel feeding is accomplished through several mechanisms. For chain and sloping grate systems, fuel is fed into a feed chute and dragged onto the grate. For units burning wood, a typical design uses a windswept spout, where the combustion air injects the fuel above the bed and the fuel falls onto the bed. For units burning coal, a paddle wheel type of feeder puts the fuel onto the grate.
Another type is the spreader style, where feeders spread the fuel onto the grates [33, 34]. From a fuel blending standpoint, stoker boilers are very flexible. Stokers have high combustion capacities and are capable of burning a large variety of fuels, including wood wastes, tire chips, municipal wastes, and many others. Mixing of fuels is easily achieved. Their limitation is total capacity and the consequent steam conditions that are appropriate. Since stokers are limited to a maximum of about 70 to 75 MWe, there is a practical limitation of about 1800 psig/1000F for steam conditions. Reheat cycles are not practical. Further, the quantity of fines in the fuel should be minimized.
Fluidized bed boilers offer an alternative to conventional firing systems, as discussed in Chapter 2 and detailed in Miller and Miller . The distinguishing feature of all types of fluidized bed boilers is the air velocity traveling through the boiler. Bubbling beds will have lower fluidization velocities, thus preventing solids elutriation from the bed into the convection pass. They are operated at gas velocities that are several times greater than the minimum fluidizing velocity, occupying 20% to 50% of the bed volume. The bed material experiences intense agitation and mixing, while maintaining relatively close contact with a well-defined upper surface .
Circulating fluidized bed boilers utilize higher velocities in order to promote solids elutriation. Bed particles are entrained and are removed from the combustor. The entrained solids are separated from the gas stream by utilizing a cyclone and are recycled to the bed. Bed inventory is maintained by the recirculation of solids separated by the off-gas. Effective combustion of the fuel and maximized sulfur capture with the sorbent can be achieved . Typically, the feed to fluidized bed systems consists of the fuel (or fuels), an inert material such as sand or ash, and usually limestone suspended by combustion air introduced below the combustor floor.
Inert materials have several key functions. These include dispersing the fuel particles throughout the bed, heating the fuel particles, acting as a thermal flywheel for the combustion process, and providing residence time for combustion to take place. The increased turbulence of fluidized bed boilers is an attractive feature; increased turbulence permits the generation of heat at a lower and more uniformly distributed temperature. High thermal energy allows for a variety of fuels to be combusted; blending within fluidized bed boilers is a common practice.
The chemistry of fluidized bed boilers is fundamentally different from those of PC boilers . Fluidized bed boilers can combust low-calorific-value fuels with higher-moisture and higher-sulfur contents. Operating temperatures for fluidized bed boilers are lower than those of PC boilers, so slagging is insignificant and agglomeration potential still exists. Fouling and corrosion are still of concern, particularly when fuels or blend of fuels with high concentrations of alkali metals and chlorine are combusted. Blending in fluidized bed boilers is commonly practiced and is successful.
Fluidized bed boilers operate normally in a temperature range of 1450F to 1650F (780900C). At this temperature range, most inorganic components will not melt and form slag. Thermally induced NOx is also not a major concern at the operating temperatures. The reactions of sulfur dioxide with sorbents (commonly limestone) are thermodynamically and kinetically balanced; sulfur capture decreases outside of the temperature range. Fluidized bed boilers enable the use of a wide variety of fuels. Fuels that are lower in calorific value and have higher concentrations of ash and moisture are commonly burned.
Blending of fuels is easily achieved and maintained in these boilers. Fuels can be added through the front wall or return leg(s), enabling multiple locations where fuel can be introduced. By dropping the fuel into the return leg(s), increased residence time in the furnace can be obtained. The practice of blending in fluidized bed boilers is used throughout the industry. Even though slagging is not a concern with fluidized bed boilers, fouling remains an issue. High concentrations of alkali metals such as sodium or potassium will cause fouling deposits.
Corrosion is also another area of concern; again, it is contributed by alkali metals. In the presence of chlorine, corrosion potential is exasperated. Agglomeration in the loopseal has been experienced where particle-to-particle bonding occurs when in close contact. With increasing regulations on emissions, polishing scrubbers and selective noncatalytic reduction (SNCR) systems are required on fluidized bed boilers in order to reduce SO2 and NOx. The advantage of traditional fluidized bed boilers compared to PC and cyclone firingnot requiring postcombustion systemshas decreased to some extent in certain applications and installations.
Pulverized coal consists mostly of particles capable of passing through a 200-mesh screen, which corresponds to diameters of 74 microns and lower. For coals of high volatility and low ash content, the combustion characteristics are comparable to those of liquid fuel sprays discussed on page 320. With decreasing volatility and increasing ash content the lower limit of inflammability shifts toward the fuel-rich side, indicating that the effective radius of a particle for flame transmission decreases. Particles of anthracite or coke containing little or no volatile matter do not transmit flame at ordinary ambient temperatures, because they loose heat by radiation more rapidly than can be supplied by the oxygen molecules that impinge and react on their surfaces, and thus do not maintain the high temperatures necessary for rapid reaction. In order to burn pulverized anthracite or coke it is necessary to surround the flame with a radiation shield; i.e., a heated surface which decreases the radiation loss sufficiently to permit heat evolution by chemical reaction to overbalance heat loss by radiation. In technical installations this is readily accomplished by a refractory mantle which is kept hot by the flame itself.4 In experiments with pulverized low-volatile char from bituminous coal, the critical shield temperature for flame propagation was found to be at approximately 1400F. as compared with flame temperatures of 1800F. and higher, depending on the fuel-air ratio.5
The functioning of the modern society depends increasingly heavily on large-scale, cheap, reliable and clean electricity. Coal has been the king in global electric power production and will continue to serve in this role in the foreseeable future.
Tween 80 was first used in flotation separation of magnesite and dolomite using NaOL as a collector.The effect of Tween 80 on the flotation separation of magnesite and dolomite was evaluated.The adsorption mechanism of Tween 80 with NaOL on the surface of magnesite and dolomite was studied.
Dolomite is difficult to be separated from magnesite by sodium oleate (NaOL) because of their same crystal structure, similar chemical composition, and high solubility. The work investigated the effect of Tween 80 as the accessory reagent on the flotation of magnesite and dolomite using NaOL as the collector by flotation tests, solution chemical calculations, surface tension measurements, Zeta potential measurements, contact angle measurements, FTIR spectra measurements, and XPS measurements. Flotation experiments and contact angle analysis showed that the addition of Tween 80 strengthened the floatability of magnesite and weakened that of dolomite when NaOL was used as a collector. Surface tension analysis, Zeta potential analysis, and FTIR spectra analysis revealed that Tween 80 could strengthen the adsorption of NaOL on mineral surface by promoting the dissolution and dispersion of NaOL solution. Solution chemical calculations, Zeta potential analysis, and FTIR spectra analysis also indicated that Ca2+ and CaOH+ were the active components of dolomite in flotation, and Tween 80 had a specific inhibitory effect on dolomite. XPS and Ca2+ dissolution tests further indicated that Tween 80 inhibited dolomite by binding Ca2+ with its EO groups. This action reduced the adsorption of Ca2+ with NaOL and improved the separation of magnesite and dolomite.