Power Engineering

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 rise with flue gases. More than 100 million tons of fly ash is produced in the United States 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 type of fuel burned and handling methods. A typical fly ash contains a significant amount 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 a difficult material to handle reliably.

In this article, we will look at various types of flow problems that can occur in fly ash handling and storage systems, including arching, ratholing, flow rate limitations and flooding. We will describe a proven, scientific method that can be used to characterize flow properties of fly ash. Also, we will describe various options to handle fly ash reliably in a deaerated mode as well as in a fluidized state. This discussion will apply to handling of fly ash in precipitator or baghouse hoppers as well as in storage silos.

Challenges

In a typical fly ash handling system, the material that is generated as a result of combustion is captured by an electrostatic precipitator (ESP) or a baghouse before the flue gases reach the stack. These ESPs and baghouses generally have multiple pyramidal hoppers at the bottom, in which the ash is collected by gravity and then is transferred to a storage silo. These storage silos generally have provisions for a truck load-out to carry the fly ash for disposal or reuse. As a result of the frictional nature and fine particle size distribution, fly ash handling systems often experience problems if they are designed without following a prudent engineering approach. In the following, we first describe the common flow problems that can occur when handling and storing fine dry fly ash [3, 4, 6].

No-flow from hopper or silo outlet

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 outlet as a result of the material's cohesive strength. When fly ash forms a stable arch above the outlet, discharge is prevented and a no-flow condition results. Fig. 1 on page 46 shows an example of an arching problem.

Ratholing occurs when material empties out through a flow channel above an outlet. As the level of fly ash in the flow channel drops, a resistance to further flow into 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 46 shows an example of ratholing. The pyramidal shape of typical ESP or baghouse hoppers makes potential arching and ratholing problems worse.

When flow obstructions switch/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 until the flow channel has emptied out. This will result in the formation of a rathole preventing material discharge. The rathole can collapse due to a similar external force and the falling material often gets compacted over the hopper outlet and again forms an arch resulting in no-flow.

Flow rate limitation

The permeability of fly ash is typically very low due to its fine particle size distribution. As a result, when deaerated, fly ash provides a considerable resistance 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 ash bed provides an opposing force to gravity. This air ingress occurs as a result of the natural expansion of the ash bed within the hopper as it flows, or simply due to leakage from the conveying system below. As a result, fly ash hoppers and silos are limited in terms of the maximum discharge rates that they can provide by gravity alone. This behavior increases the time required to fill the trucks as well as to empty out the storage silos. This situation can cause further problems when sufficient storage capacity is not available for newly collected fly ash due to slow unloading 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 allow sufficient time for the entrained air to escape. In this case, the fly ash may become fluidized and flush through the outlet unless the feeder can contain it. Flooding not only creates a challenge in metering the discharge, but can also lead to serious environmental, health and safety concerns.

Fig. 3 shows an example of the impact of fly ash flooding. In this case, the material became aerated when a rathole developed in the silo, and then collapsed, resulting in a rapid and uncontrolled discharge through a screw feeder, emptying the entire contents of the silo through the building wall in a matter of minutes or seconds.

Figure 3 Jenike Shear Tester

Structural problems

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

Wear

The presence of a significant portion of silicon dioxide makes fly ash very abrasive and frictional. As a result of material sliding and impacting within the handling equipment, wall surfaces undergo tremendous wear. This often results in the need for frequent patching and replacement.

Dust generation

Dust can be encountered when air currents have sufficient velocity to capture and move fine particles. Dusting can particularly occur at transfer points where the air entrained in the powder is suddenly expelled, carrying these finer particles with it. Dust generation also occurs when local air currents have sufficient velocity to pick up particles from the surface of a pile. 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.

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. These lumps are usually too large and heavy to remain in suspension, and settle on the membranes. This can cause the fluidizing air to short circuit and channel through fly ash, thus allowing the surrounding material to deaerate. Such conditions often lead to flow rate limitations or incomplete discharge.

In addition, the pneumatic conveying lines carrying fly ash from the ESP or baghouse hoppers to storage silos also experience plugging, conveying rate limitations, as well as pipeline wear issues.

Fly ash can contain excess unburned carbon. When stored in an aerated bin, the injected air can provide sufficient oxygen for combustion to take place, resulting in an unsafe condition.

The material handling challenges and flow problems described in this section are related to how fly ash flows through a hopper or silo. Hence, before looking at the solutions for these problems, it is important to understand the fundamentals of the flow of bulk materials.

Bulk Material Flow

Many flow problems are related to how a bulk material flows within a hopper or a silo. As shown in Fig. 4, there are two primary flow patterns that can develop during material discharge: funnel flow and mass flow.

In funnel flow, during discharge, only a portion of the material is in motion while the remainder is stationary. Thus, an active flow channel forms above the hopper outlet, with stagnant material at the periphery. If the bulk solid has sufficient cohesive 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 storage capacity, 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. These valleys 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 the hopper. 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 stagnant material, 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 hopper outlet large enough to prevent arching and ensuring the hopper walls 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 flow hopper (or hoppers) is placed beneath a funnel flow hopper. The lower mass flow hopper 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 of an expanded flow discharge pattern is the savings in headroom. Particularly for large structures, a configuration consisting of a funnel flow hopper above a mass flow hopper results in a significantly lower overall height, compared to a mass flow hopper only. This approach not only reduces capital cost, but also facilitates retrofitting existing hoppers and silos by minimizing the additional headroom requirement. The mass flow hopper beneath the funnel flow hopper still has the benefits of discharging material with a consistent bulk density.

Which flow pattern develops in a silo 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. 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 as conditions for testing. Information obtained from these tests is used to calculate minimum outlet dimensions for hoppers and silos 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, shown in Fig. 3, in accordance with ASTM standard D 6128 [7].

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 hoppers, and to calculate material flow-induced loads. It also provides information necessary to determine outlet sizes and mass flow hopper angles.

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. Discharge rates attempted above these critical values can result in two-phase (air:solid) flow problems, such as limitations and flooding described earlier in this paper [8].

Aeration can be used to overcome discharge rate limitations. To design a storage system with an aerated discharger, the superficial airflow rates and supply pressure requirements need to be determined. These are established by running a fluidization test. In this test, a fluidization column is filled with the bulk material and air is permeated through a membrane and into the column. Different membrane types can be used, depending on the application, ranging from relatively inexpensive polyester to high-temperature duty sintered metals. Plotting air flow versus pressure drop allows the designer to determine the minimum requirements for the air supply system. The state of aeration must be considered as part of the design basis, as to whether the entire hopper contents will be fluidized or whether just a portion near a silo outlet will be in this state. Discharge controlling devices such as feeders and gates must perform in a manner that supports the desired state.

Measurement of a material's angle of repose can aid in estimating storage capacity, along with use of compressibility (bulk density) test data. However, this test has little value in characterizing material flow, even though it is often used for this purpose.

If a material is pneumatically conveyed, its minimum conveying velocity should be understood to prevent plugging (a risk if it is moved too slowly) and excess pipe wear (a risk if it is moved too quickly). Pneumatic conveying tests can be used to gather this data, recognizing the influences of line pressure and size.

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 System

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 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 materials where segregation is not important. Furthermore, funnel flow discharge is prone to ratholing, and it exacerbates arching tendency of materials. For materials like fly ash, mass flow is recommended due to their cohesiveness and ability to cake with time. Expanded-flow designs are used for large storage volumes or when limited headroom prevents the use of a mass flow design.

When an existing handling system is not performing well, it can be modified to improve reliability, even in the presence of limitations due to space, time and budget.

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 along the sloping walls of the hoppers and silos. 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.

Many fly ashes can be handled very well in an aerated state. Fluidization test results can be used to set superficial air velocity and the expected pressure drop for 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 storage heads. When aerated, the internal friction and wall friction of the material reduce considerably, thus avoiding cohesive arching and ratholing. This behavior can allow the use of shallow converging sections in the area of the aerating membrane and smaller outlet sizes for achieving reliable flow. Aerated fly ash can also achieve very high discharge rates. In large aerated storage silos, only the material close 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 ensuring reliable flow. The type of feeder most suited for a given application depends upon the flow characteristics of the fly ash, flow pattern selected, and site-specific requirements such as material handling conditions, available space, and flow rate control. 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 is required between the storage and conveying systems. For a rotary valve to withdraw fly ash uniformly from the entire outlet, there must be a sufficiently tall vertical spool piece between the rotary valve and the hopper. Also, it is important to provide venting when feeding into a positive pressure environment.

Double dump valves are also commonly used to handle fly ash, and can handle high air pressure differences. In the actuating sequence for the two valves, first, the top 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 or evacuated as needed. Next, the bottom valve is opened, discharging the fly ash to the downstream process. Using a double dump valve arrangement results in a batch or pulsing flow.

Moderate flow rate control and hopper area recovery can be achieved when handling aerated fly ash by using airslides. Screw feeders can be used for handling deaerated fly ash; however, caution must be exercised in the design and operation of 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 collection hoppers to the storage silos. While the equipment used for pneumatic conveying has advanced significantly over the years, it is still not uncommon to encounter problems with insufficient conveying capacity, plugging, erosive wear in elbows and 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 system must be designed based on required minimum conveying velocities to avoid pluggages, while providing the needed air pressure and flow rate to move the material through the line [5]. In these situations, the pipeline diameter can be used as a design variable, with step increases made over the line length to minimize conveying velocities to reduce line wear while increasing the line's capacity.

Following the approaches outlined in this paper can improve the performance of existing systems, as well as ensure new systems are designed in a manner that results in reliable fly ash flow.

Footnotes

1 Moisture must be avoided in a dry handling system, since many fly ashes are hygroscopic and will react with water. If moisture is inadvertently added, caking, agglomeration and build-up can occur. Fly ashes are well known for their cementitious properties and are often used in low strength concrete mixes.

2 A valley forms at the intersection of two adjacent hopper walls. The valley angle is 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 Engineering Experiment Station, Bulletin No. 123", November 1964.

[2] Carson, J. W.: Toward a Better Understanding of the Storage and Flow of Bulk Materials. Proceedings of Bulk 2000, London, Oct. 29-31, 1991.

[3] 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.

[4] 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.

[5] Khambekar, J., Maynard, E. P., (2011). "How to Reliably Feed Material into Your Pneumatic Conveying System", Powder and Bulk Engineering, July 2011, Volume 25, No.7.

[6] Khambekar, J., Barnum, R. A., Geisel, K., (2009). "Dominion Addresses Generating 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 Particle Size Distribution Upon Adverse Two-Phase Flow, presented at m3: An International Conference on the Role of Materials Science and Engineering in Drug Development, Reykjavik, Iceland, May 20-23rd, 2007

Authors:

Dr. Jayant Khambekar (jkhambekar12@jenike.com) is Power Industry Specialist and Mr. Roger Barnum (rbarnum12@jenike.com) is a Senior Consultant at Jenike & Johanson. Jenike & Johanson is a specialized engineering firm focusing on providing reliable bulk solids flow.

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