Fly Ash Handling Challenges and Solutions

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Fly Ash Handling: Challenges and Solutions

By Jayant Khambekar, Ph.D. and Roger A. Barnum, Jenike & Johanson, Inc,USA

Fly ash is a general name used for the residual products of combustion that risewith flue gases. More than 100 million tons of fly ash is produced in the UnitedStates every year; most coming from the combustion of coal in power plants.Nearly half of this fly ash is reused for purposes such as producing cement.Chemically and physically, fly ash can have many forms depending upon the typeof fuel burned and handling methods. A typical fly ash contains a significantamount of silicon dioxide and calcium oxide, which make it frictional and abrasive.Usually, fly ash has a fine particle size distribution with most less than 100 microns.Given the fine particle size, frictional nature and high temperature, fly ash can be adifficult material to handle reliably.In this article, we will look at various types of flow problems that can occur in flyash handling and storage systems, including arching, ratholing, flow rate limitationsand flooding. We will describe a proven, scientific method that can be used tocharacterize flow properties of fly ash. Also, we will describe various options tohandle fly ash reliably in a deaerated mode as well as in a fluidized state. Thisdiscussion will apply to handling of fly ash in precipitator or baghouse hoppers aswell as in storage silos.ChallengesIn a typical fly ash handling system, the material that is generated as a result ofcombustion is captured by an electrostatic precipitator (ESP) or a baghouse beforethe flue gases reach the stack. These ESPs and baghouses generally havemultiple pyramidal hoppers at the bottom, in which the ash is collected by gravityand then is transferred to a storage silo. These storage silos generally haveprovisions for a truck load-out to carry the fly ash for disposal or reuse. As a resultof the frictional nature and fine particle size distribution, fly ash handling systemsoften experience problems if they are designed without following a prudent


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engineering approach. In the following, we first describe the common flowproblems that can occur when handling and storing fine dry fly ash [3, 4, 6].No-flow from hopper or silo outletThis 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 outlet as a result of the material's cohesive strength. When fly ash forms astable arch above the outlet, discharge is prevented and a no-flow conditionresults. Fig. 1 on page 46 shows an example of an arching problem.

Ratholing occurs when material empties out through a flow channel above anoutlet. As the level of fly ash in the flow channel drops, a resistance to further flowinto this channel occurs due to the material's cohesive strength. No further material

discharge occurs from the outlet, resulting in a no-flow condition. Fig. 2 on page 46shows an example of ratholing. The pyramidal shape of typical ESP or baghousehoppers makes potential arching and ratholing problems worse.


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When flow obstructions switch/interchange between arches and ratholes, erraticflow results. In a typical erratic flow problem, an arch formed over a hopper outletmay fail due to an external force, such as vibrations transmitted to the hopper, andthen material flow will resume until the flow channel has emptied out. This willresult in the formation of a rathole preventing material discharge. The rathole can

collapse due to a similar external force and the falling material often getscompacted over the hopper outlet and again forms an arch resulting in no-flow.Flow rate limitationThe permeability of fly ash is typically very low due to its fine particle sizedistribution. As a result, when deaerated, fly ash provides a considerableresistance to the flow of air or other gases (simply referred to as air in this paper).During discharge from a silo or hopper outlet, air counter-flow through the fly ashbed provides an opposing force to gravity. This air ingress occurs as a result of thenatural expansion of the ash bed within the hopper as it flows, or simply due toleakage from the conveying system below. As a result, fly ash hoppers and silosare limited in terms of the maximum discharge rates that they can provide bygravity alone. This behavior increases the time required to fill the trucks as well asto empty out the storage silos. This situation can cause further problems whensufficient storage capacity is not available for newly collected fly ash due to slowunloading from the storage silo.Flooding or uncontrolled flow

As a fine powder, fly ash can behave like a fluid when sufficient air is present.Flooding can result, particularly when the handling rate is too high to allowsufficient time for the entrained air to escape. In this case, the fly ash may becomefluidized and flush through the outlet unless the feeder can contain it. Flooding notonly creates a challenge in metering the discharge, but can also lead to seriousenvironmental, health and safety concerns.Fig. 3 shows an example of the impact of fly ash flooding. In this case, the materialbecame aerated when a rathole developed in the silo, and then collapsed, resultingin a rapid and uncontrolled discharge through a screw feeder, emptying the entirecontents of the silo through the building wall in a matter of minutes or seconds.

Figure 3 Jenike Shear Tester

Structural problems


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As a result of the collapse of ratholes and the formation of arches, sudden dynamicforces acting on the silo shell can result in structural damage. Also, thedevelopment of eccentric flow channels within the silo, particularly due to multipleor offset outlets, can result in non-uniform loading along the outer walls, that maycause wrinkling or buckling of the silo.

WearThe presence of a significant portion of silicon dioxide makes fly ash very abrasiveand frictional. As a result of material sliding and impacting within the handlingequipment, wall surfaces undergo tremendous wear. This often results in the needfor frequent patching and replacement.Dust generationDust can be encountered when air currents have sufficient velocity to capture andmove fine particles. Dusting can particularly occur at transfer points where the airentrained in the powder is suddenly expelled, carrying these finer particles with it.Dust generation also occurs when local air currents have sufficient velocity to pickup particles from the surface of a pile. Dust by itself is a nuisance and, moreimportantly, it can result in safety concerns including the health effects of operatorexposure and the potential for explosions. Hence, OSHA has a strong policy forcontrolling dust generation.Other problems

Agglomerated lumps of fly ash and foreign materials can create flow problems,especially when handling fly ash with airslides or aerated bin bottoms. Theselumps are usually too large and heavy to remain in suspension, and settle on themembranes. This can cause the fluidizing air to short circuit and channel throughfly ash, thus allowing the surrounding material to deaerate. Such conditions oftenlead to flow rate limitations or incomplete discharge.In addition, the pneumatic conveying lines carrying fly ash from the ESP orbaghouse hoppers to storage silos also experience plugging, conveying ratelimitations, as well as pipeline wear issues.Fly ash can contain excess unburned carbon. When stored in an aerated bin, theinjected air can provide sufficient oxygen for combustion to take place, resulting inan unsafe condition.The material handling challenges and flow problems described in this section arerelated to how fly ash flows through a hopper or silo. Hence, before looking at thesolutions for these problems, it is important to understand the fundamentals of theflow of bulk materials.Bulk Material FlowMany flow problems are related to how a bulk material flows within a hopper or asilo. As shown in Fig. 4, there are two primary flow patterns that can developduring material discharge: funnel flow and mass flow.


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In funnel flow, during discharge, only a portion of the material is in motion while theremainder is stationary. Thus, an active flow channel forms above the hopperoutlet, with stagnant material at the periphery. If the bulk solid has sufficientcohesive strength, the stagnant material will not slide into the flow channel,resulting in the formation of a stable rathole. In addition to reducing the live storagecapacity, funnel flow can result in caking and exacerbate particle size segregation.Often times, shallow hoppers result in funnel flow discharge. Pyramidal hoppers,which have shallow valley angles, commonly discharge in funnel flow. Thesevalleys can also serve as hang-up points due to rough welds and the high surface

area, promoting material buildup.In mass flow, all of the material is in motion whenever any is withdrawn from thehopper. Material from the center as well as the periphery moves towards the outlet.Mass flow hoppers provide a first-in-first-out flow sequence, eliminate stagnantmaterial, and provide a steady discharge with a consistent bulk density and a flowthat is uniform and well-controlled. Requirements for achieving mass flow includesizing the hopper outlet large enough to prevent arching and ensuring the hopperwalls are sufficiently smooth and steep enough to promote flow along them.

A third type of flow pattern, called expanded flow, can develop when a mass flowhopper (or hoppers) is placed beneath a funnel flow hopper. The lower mass flowhopper is designed to activate a flow channel in the upper funnel flow hopper,

which is sized to prevent the formation of a stable rathole. The major advantage ofan expanded flow discharge pattern is the savings in headroom. Particularly forlarge structures, a configuration consisting of a funnel flow hopper above a massflow hopper results in a significantly lower overall height, compared to a mass flowhopper only. This approach not only reduces capital cost, but also facilitatesretrofitting existing hoppers and silos by minimizing the additional headroomrequirement. The mass flow hopper beneath the funnel flow hopper still has thebenefits of discharging material with a consistent bulk density.


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wear (a risk if it is moved too quickly). Pneumatic conveying tests can be used togather this data, recognizing the influences of line pressure and size.

All these tests must be run at conditions representing the actual handlingenvironment, particularly material moisture content, storage time at rest andtemperature.

Designing a Reliable SystemWhen properly designed, a handling system can provide reliable flow forchallenging materials, whereas poorly designed equipment may not be able tohandle even the most free-flowing materials. The key to reliable systemperformance is to ensure that the design takes into account flow and other relevantproperties of the materials involved, in this case fly ash.Selecting the appropriate flow pattern is critical for a reliable storage system.Funnel flow is suitable only for coarse, free-flowing, non-degrading bulk materialswhere segregation is not important. Furthermore, funnel flow discharge is prone toratholing, and it exacerbates arching tendency of materials. For materials like flyash, mass flow is recommended due to their cohesiveness and ability to cake withtime. Expanded-flow designs are used for large storage volumes or when limitedheadroom prevents the use of a mass flow design.When an existing handling system is not performing well, it can be modified toimprove reliability, even in the presence of limitations due to space, time andbudget.In order to ensure mass flow, the hopper walls must be steep and smooth enough,as per the results provided by wall friction tests, to ensure that fly ash flows alongthe sloping walls of the hoppers and silos. In addition to this requirement, the outletmust be sized large enough to prevent cohesive arching as well as to achieve thedesired steady-state discharge rate. Rathole formation is not possible in mass flow,which is a significant advantage.Many fly ashes can be handled very well in an aerated state. Fluidization testresults can be used to set superficial air velocity and the expected pressure dropfor the air supply system. These tests can also be used to gage the ability to re-aerate the material after stoppage periods or in the presence of large storageheads. When aerated, the internal friction and wall friction of the material reduceconsiderably, thus avoiding cohesive arching and ratholing. This behavior canallow the use of shallow converging sections in the area of the aerating membraneand smaller outlet sizes for achieving reliable flow. Aerated fly ash can alsoachieve very high discharge rates. In large aerated storage silos, only the materialclose to the discharger's membrane becomes fluidized, whereas in smaller bins,such as blow tanks, the entire contents can reach this state.In addition to hopper and silo design, feeder design is also critical for ensuringreliable flow. The type of feeder most suited for a given application depends uponthe flow characteristics of the fly ash, flow pattern selected, and site-specificrequirements such as material handling conditions, available space, and flow ratecontrol. Of primary concern is the containment of fly ash when it becomes aerated.Rotary valves are suitable for handling aerated fly ash and when a pressure seal isrequired between the storage and conveying systems. For a rotary valve towithdraw fly ash uniformly from the entire outlet, there must be a sufficiently tall


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vertical spool piece between the rotary valve and the hopper. Also, it is important toprovide venting when feeding into a positive pressure environment.Double dump valves are also commonly used to handle fly ash, and can handlehigh air pressure differences. In the actuating sequence for the two valves, first, thetop valve opens, allowing the chamber between the two valves to fill with fly ash,

after which the top valve is closed. This space can then be pressurized orevacuated as needed. Next, the bottom valve is opened, discharging the fly ash tothe downstream process. Using a double dump valve arrangement results in abatch or pulsing flow.Moderate flow rate control and hopper area recovery can be achieved whenhandling aerated fly ash by using airslides. Screw feeders can be used for handlingdeaerated fly ash; however, caution must be exercised in the design and operationof the system, since a screw feeder will not hold back material that is aerated.

A conveying system for fly ash must also be designed properly. Often times,pneumatic conveying systems are used to transfer fly ash from the collectionhoppers to the storage silos. While the equipment used for pneumatic conveyinghas advanced significantly over the years, it is still not uncommon to encounterproblems with insufficient conveying capacity, plugging, erosive wear in elbowsand buildup in the line, particularly when emissions control systems are changed(such as after the introduction of an acid or mercury capturing sorbent system,which will increase the quantity of ash generated). A pneumatic conveying systemmust be designed based on required minimum conveying velocities to avoidpluggages, while providing the needed air pressure and flow rate to move thematerial through the line [5]. In these situations, the pipeline diameter can be usedas a design variable, with step increases made over the line length to minimizeconveying velocities to reduce line wear while increasing the line's capacity.Following the approaches outlined in this paper can improve the performance ofexisting systems, as well as ensure new systems are designed in a manner thatresults in reliable fly ash flow.Footnotes1 Moisture must be avoided in a dry handling system, since many fly ashes arehygroscopic and will react with water. If moisture is inadvertently added, caking,agglomeration and build-up can occur. Fly ashes are well known for theircementitious properties and are often used in low strength concrete mixes.2 A valley forms at the intersection of two adjacent hopper walls. The valley angleis always shallower than the angles of the wall surfaces that surround it. References[1] Jenike, A.W.: "Storage and Flow of Solids, University of Utah EngineeringExperiment Station, Bulletin No. 123", November 1964.[2] Carson, J. W.: Toward a Better Understanding of the Storage and Flow of BulkMaterials. Proceedings of Bulk 2000, London, Oct. 29-31, 1991.[3] Barnum, R.A., Hossfeld, R.J., Khambekar, J., Geisel, K., (2009). "ImprovingPlant Performance by Retrofitting Coal Bunkers at Mt. Storm, presented at 2009Power-Gen International Conference in Las Vegas, NV, USA.[4] Khambekar, J., Rulff, M., Cabrejos, F., (2009). "Improving Storage and Handlingof Ores in Mining and Processing Applications", Mining Engineering, October 2009,Vol. 61, No. 10.


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[5] Khambekar, J., Maynard, E. P., (2011). "How to Reliably Feed Material intoYour Pneumatic Conveying System", Powder and Bulk Engineering, July 2011,Volume 25, No.7.[6] Khambekar, J., Barnum, R. A., Geisel, K., (2009). "Dominion AddressesGenerating Problems due to Fuel Handling at Mt. Storm", Coal Power Online

Journal, March/April 2009.[7] American Society for Testing and Materials (ASTM), Standard D-6128.[8] Baxter, Thomas, Prescott, James K. and Barnum Roger "The Effect of ParticleSize Distribution Upon Adverse Two-Phase Flow, presented at m3: An InternationalConference on the Role of Materials Science and Engineering in DrugDevelopment, Reykjavik, Iceland, May 20-23rd, 2007Authors:Dr. Jayant Khambekar([email protected])is Power Industry Specialistand Mr. Roger Barnum([email protected])is a Senior Consultant at Jenike &Johanson. Jenike & Johanson is a specialized engineering firm focusing on

providing reliable bulk solids flow.
mailto:([email protected]:([email protected]:([email protected]:([email protected]:([email protected]:([email protected]:([email protected]


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Fly Ash Handling Challenges and Solutions