By Jayant Khambekar and Roger A. Barnum, Jenike & Johanson
As a result of recent environmental regulations, many utilities in the United States have already switched or are thinking about switching to Powder River Basin (PRB) coal. One of the primary reasons for this is the low sulfur content of PRB coal. While low in sulfur (0.4 to 1.4 LB/MMBTU), PRB coal is also low in heating value (8,000 to 9,400 BTU/LB, on an as received basis, depending upon the mine or area it is coming from). Thus, typically, larger volumetric feed rates are required when using PRB coal. While changing fuel to PRB coal can help emission control, one also needs to be aware of the effects of changing fuel on the existing material handling system at the plant.
|Wet PRB coal|
PRB coal is extremely friable and will break down into smaller particles virtually independent of how the coal is transported or handled. Once it is exposed by mining, the attrition process of coal particles begins – the majority of the breakage can occur in a very short time, even as short as a few days. It is believed that the root cause of the degradation is loss of moisture that impacts the coal both mechanically and chemically through the generation of additional surface reaction area. PRB coal can represent the extremes of handling problems: dust is an issue when the coal is fine and dry; when the same fine coal is wet, plugging in bunkers and chutes is an issue.
The existing silos, bunkers, feeders and transfer chutes that were designed to handle bituminous and coarse coal may not be suitable to handle PRB coal and thus may require modification to ensure reliable handling without flow problems. Also, PRB coals generally have a much higher potential for spontaneous combustion. This issue introduces new requirements for material handling systems that the present fuel may not have needed.
What follows is an examination of the various types of flow problems that can occur in fuel handling systems when handling PRB coals. The discussion will include problems such as arching, ratholing, feeder hangups, dusting and plugging in chutes.
We will also describe how a proven, scientific method can be used to characterize flow behavior of the PRB coal you are handling and to design your handling system to achieve reliable flow.
MATERIAL HANDLING CHALLENGES
In a typical coal-fired power plant, coal is received from either railcars or barges. Then it is unloaded in a coal-yard and often stored in stockpiles. Material can be reclaimed from the stockpile using an automatic reclaimer or a gravity-reclaim arrangement consisting of reclaim tunnel hoppers and feeders. It is then conveyed to a crusher house for sizing and then transferred to fuel silos or bunkers that typically have belt feeders beneath them. For a pulverized coal boiler system, coal is then transferred to a mill using a mill-feed chute, from where it is conveyed to the boiler. If it is a fluidized bed boiler, the coal is directly transferred to boiler-feed chutes. In the following, we describe the common flow problems that can occur when handling and storing PRB coal through the above equipment.
The most common flow problem experienced when handling PRB coals is flow stoppage; that is, a no-flow condition. This condition can result either from arching (also known as bridging) or ratholing.
Arching occurs when an obstruction in the shape of an arch or a bridge forms over the silo, bunker or hopper outlet as a result of the material’s cohesive strength. When coal forms a stable arch above the outlet, discharge is prevented and a no-flow condition results. Fig. 1 shows an illustration of the arching problem.
Ratholing can occur when coal flow takes place in a channel located above the silo, bunker or hopper outlet. As the level of material in the flow channel drops, a resistance to further flow into this channel occurs due to the material’s cohesive strength. If coal has enough cohesive strength, the stagnant material outside the channel will not flow into it, forming a stable rathole. Once the flow channel has emptied, no further material discharge will occur from the outlet resulting in a no-flow condition. Fig. 2 shows an illustration of ratholing. In addition to causing a no-flow condition, ratholes significantly reduce the live capacity of a bunker, silo or stockpile. While there may be a large amount of material present, if that material remains stagnant and would not come out on its own, the volume occupied by that material is essentially a dead volume. As a result, the process or equipment may require frequent filling to keep up with the discharge rate.
When flow obstructions switch or interchange between arches and ratholes, erratic flow results. In a typical erratic flow problem, an arch formed over a hopper outlet may fail due to an external force, such as vibrations transmitted to the hopper, and then material flow will resume and continue until the flow channel formed above the outlet empties out. This will result in formation of a rathole and prevent further material discharge. This rathole can collapse due to a similar external force and the material falling down from the collapsed rathole may get compacted over the hopper outlet and again form an arch, resulting in a no-flow condition. Thus, in an erratic flow situation, material discharges intermittently from the hopper outlet.
While flow stoppages can be a big issue by themselves, when handling PRB coals any material remaining stagnant can pose a danger. This could occur if material flow takes place through a channel within the silo. Then the material outside of this channel may remain stagnant for a long time, depending on how often the silo or bunker is completely emptied. The same problem can occur within gravity-reclaim stockpiles, particularly if significant amount of material remains un-reclaimed. As PRB coals have a tendency for spontaneous combustion, such stagnant material present can pose risk of fires.
In addition to these problems associated with no-flow conditions, when arches and ratholes collapse, sudden dynamic forces act on surrounding equipment. These forces can result in structural damage to walls, floors and feeders. Also, the development of eccentric flow channels within a silo, particularly due to multiple or offset outlets, can result in non-uniform loading along the outer walls of the silo. This situation may cause wrinkling or buckling of the silo.
Additionally, there is often a reduction of BTU content when switching to PRB coal, resulting in an increase in the required volumetric feed rate to compensate. This change can worsen effects of the above-mentioned problems.
Whenever material is transferred from one point to another, conveying systems and transfer chutes come into the picture. Unfortunately, many times transfer chutes get designed at the end of the development phase, requiring them to fit within or stretch to an already fixed layout.
This situation can often result in material flow problems through poorly performing chutes. When material discharged from a belt conveyor impacts a chute surface, its velocity decreases. The larger the impact angle, the bigger is the change in velocity. Sliding friction with chute surface can decrease the stream velocity even further, and the flowing material may halt on the chute surface, creating a plugging condition. Poor chute design or performance can also result in material spillage.
Dusting can occur when material has fines and air currents are present that can carry the fines away. In transfer chutes, if the material stream is not well-controlled, it can lead to hard impacts. The air entrained with the material (especially in fines) is then suddenly expelled, carrying these fine particles away as dust. Dust generation also occurs when local air currents have sufficient velocity to pick up particles from the surface of a stockpile. Dust by itself is a nuisance and, more importantly, it can result in safety concerns including the health effects of operator exposure and the potential for explosions. Hence, OSHA has a strong policy for controlling dust generation.
MATERIAL FLOW PATTERNS IN SILOS AND BUNKERS
The flow problems that can occur in silos and bunkers are related to how a bulk material flows within them. There are two primary flow patterns that can develop during material discharge: funnel flow and mass flow. Both patterns are shown in Figure 3.
In funnel flow, an active flow channel forms above the hopper outlet with stagnant material at the periphery. As the level of material in the silo decreases, material from stagnant regions may or may not slide into the flowing channel, depending on the material’s cohesive strength.
When the material has sufficient cohesive strength, the stagnant portion does not slide into the active flow channel, which results in the formation of a stable rathole. In addition to flow stoppages that occur as a consequence of ratholing, funnel flow can cause oxidation of material, results in a first-in-last-out flow sequence and increases the extent to which sifting segregation impacts the uniformity of the discharging material.
In mass flow, all of the material is in motion whenever any is withdrawn from the hopper. Material from the center as well as the periphery moves toward the outlet. Mass flow hoppers provide a first-in-first-out flow sequence, eliminate stagnant material, reduce sifting segregation and provide a steady discharge with a consistent bulk density and a flow that is uniform and well controlled. Requirements for achieving mass flow include sizing the outlet large enough to prevent arching, as well as ensuring the hopper walls have sufficiently low wall (material/surface boundary) friction and are steep enough to achieve flow along them.
A third type of flow pattern, called expanded flow, can develop when a mass flow hopper (or hoppers) is placed beneath a funnel flow hopper, as shown in Figure 4. The mass flow hopper is designed to activate a flow channel in the funnel flow hopper, which is sized to prevent the formation of a stable rathole.Particularly for large diameter silos, the major advantage of an expanded flow discharge pattern is the savings in headroom as compared to an all mass flow design. This approach not only reduces the capital cost, but also facilitates retrofitting existing silos by minimizing the additional headroom requirement. The mass flow hopper beneath the funnel flow hopper still has the benefit of discharging material reliably with a consistent bulk density. Note that segregation and material degradation problems are not necessarily minimized with an expanded flow pattern.
Which flow pattern develops in a silo or a bunker is a function of the flow properties of the material being handled within it.
FLOW PROPERTIES OF BULK MATERIALS
Several critical flow properties influence a material’s handling characteristics. These have been described in detail in the literature [1, 2, 3, 4] and are summarized here for completeness.
Cohesive strength of a material determines its potential for forming a stable arch or rathole. This property is measured as a function of consolidating pressure; moisture content, temperature and storage time at rest also need to be considered for testing. Information obtained from these tests is used to calculate minimum outlet dimensions for silos and bunkers to prevent arching and ratholing.
Wall friction between a material and a hopper wall surface has a critical influence on the resulting discharge flow pattern. Wall friction is measured as a function of consolidating pressure, and also should consider moisture content, temperature and storage time at rest. Information obtained from these tests is used to determine hopper angles required to achieve mass flow. Cohesive strength and wall friction tests are performed using a Jenike Shear Tester, in accordance with ASTM standard D 6128 .
Compressibility of a material measures the change in its bulk density as a function of consolidating pressure. It is used to determine the capacity of storage equipment such as silos and bunkers and to calculate material induced loads. It also provides information necessary to determine outlet sizes and mass flow hopper angles.
Chute angle tests are run to determine minimum (shallowest) required chute angles in order to maintain flow after impact of a material stream with a wall surface.
Permeability of material measures its resistance to the flow of air through a bed of particles. It is particularly important when material contains a significant portion of fines. The data obtained from this test is used to calculate critical, steady-state discharge rates of the material in a deaerated state as a function of outlet size and consolidating pressure.
The measurement of a material’s angle of repose can aid in estimating storage capacity, along with use of compressibility (bulk density) test data. However, angle of repose should not be used for characterizing material flow and cannot determine hopper angles to achieve mass flow.
All these tests must be run at conditions representing the actual handling environment, particularly material moisture content, storage time at rest and temperature.
DESIGNING A RELIABLE HANDLING SYSTEM FOR PRB COAL
|PRB coal dust during unloading|
When properly designed, a handling system can provide reliable flow for challenging materials, whereas poorly designed equipment may not be able to handle even the most free-flowing materials. The key to reliable system performance is to ensure that the design takes into account flow and other relevant properties of the materials involved, in this case PRB coal.
Selecting the appropriate flow pattern is critical for a reliable storage system. Funnel flow is suitable only for coarse, free-flowing, non-degrading bulk materials where segregation is not important. Specifically, funnel flow discharge is prone to ratholing and it exacerbates arching tendencies. Any amount of PRB coal that will remain stagnant in the silo under funnel flow conditions will be prone to spontaneous combustion. Thus, for materials like PRB coal, mass flow is recommended due not only to their cohesiveness but also their ability to spontaneously combust. In fact, the National Fire Protection Association (NFPA) has stated in its code 85, Section 22.214.171.124.1, that new coal silos and bunkers should be designed for mass flow as a preventative measure . This Section also mentions the importance of achieving uninterrupted flow and avoiding arching and ratholing conditions. Although not covered by the NFPA code, stockpile designs should also minimize stagnant material. If smoldering occurs in the stagnant regions and if the coal gets sent to handling systems and bunkers downstream, fires can occur.
These philosophies apply not only to new handling system designs but also to existing systems. When an existing handling system is not performing well, it can often be modified to improve reliability, even in the presence of limitations due to space, time and budget
In order to provide mass flow, the hopper walls must be steep and smooth enough, as per the results provided by wall friction tests, to ensure that material flows along the sloping portions. In addition to this requirement, the outlet must be sized large enough to prevent cohesive arching as well as to achieve the desired steady-state discharge rate. Rathole formation is not possible in mass flow, which is a significant advantage.
In addition to ensuring that reliable flow takes place in the silo and bunker through the selection of hopper geometries and angles, it is also necessary for the entire cross-sectional area of the hopper outlet to be active. A restricted outlet, such as due to a partially open slide gate, will result in funnel flow with a smaller active flow channel regardless of the hopper design. It is therefore imperative that a feeder be capable of continuously withdrawing material from the entire outlet of the hopper. This feature will allow mass flow to take place in the hopper above, if it is so designed.
As an example, when using a slotted outlet, it is required that the feeder capacity increase in the direction of flow. When using a belt feeder, this increase in capacity is achieved by using a tapered interface. The increasing capacity along the length is achieved by the increase in height and width of the interface above the belt.
By far the most common problem with chutes is plugging at impact points, such as after a free fall or where the material stream changes direction . Chutes must be sufficiently steep and smooth to permit sliding and clean-off of the most frictional material that they will handle. However, chutes should be no steeper than required in order to minimize velocities, thereby reducing wear and dusting.
In order to reduce chute wear, free fall heights and changes in the direction of material flow should be minimized. This approach not only reduces impact forces, but also attrition, dusting and fluidization of fine materials. Dusting can also be kept to a minimum by keeping the material in contact with the chute surface, concentrating the material stream, centering the stream and keeping it in the direction of flow, and keeping the velocity through the chute to as near constant as possible.
Discrete Element Modeling (DEM) provides an excellent way to understand material stream flow behavior in a given chute configuration. This knowledge can then be used to ensure that material stream trajectory is optimum such that the above-mentioned chute design criteria are satisfied in a modified or a new chute configuration.
Following the approaches outlined in this paper can improve the performance of existing systems and ensure new systems are designed in a manner that results in reliable flow of PRB coals.
 Jenike, A.W.: Storage and Flow of Solids, University of Utah Engineering Experiment Station, Bulletin No. 123, 1964.
 Barnum, R.A., Hossfeld, R.J., Khambekar, J., Geisel, K., (2009). “Improving Plant Performance by Retrofitting Coal Bunkers at Mt. Storm, presented at 2009 Power-Gen International Conference in Las Vegas, NV, USA.
 Khambekar, J., Rulff, M., Cabrejos, F., (2009). “Improving Storage and Handling of Ores in Mining and Processing Applications”, Mining Engineering, October 2009, Vol. 61, No. 10.
 Purutyan, H. and R. Barnum: Fuel-handling considerations when switching to PRB coals, Power, November/December 2001, Vol. 145, No. 6, pp. 53-64
 American Society for Testing and Materials (ASTM), Standard D-6128.
 “Standard for Pulverized Feed Systems 1997 Edition,” National Fire Protection Association (NFPA), 8503, 1997.
 Stuart-Dick, D. and Royal, T.A.: Design Principles for Chutes to Handle Bulk Solids, Bulk Solids Handling, Vol. 12, No. 3, 1992, pp. 447-450.
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