Power Engineering

Waste Coal: How to Avoid Flow Stoppages During Storage and Handling

By Roderick J. Hossfeld and Roger A. Barnum, Jenike & Johanson Inc.

There is no such thing as coal, there are coals.1 This phrase is often quoted by operating and engineering companies when trying to assess their handling equipment needs. The primary bulk properties governing the handling of coal are particle size, moisture content and bulk density. Coal handling properties can vary significantly based on geographical region, as one might expect, but coal properties can even vary among mines within a specific location. This makes it difficult to standardize a handling system design that will account for all variables.

In most solids processing plants, more attention is placed on designing the process and controls than on the bulk solids handling equipment. Yet, bulk solids handling problems are often the major cause of costly downtime for many plants, especially during startup.

The situation is no different in a solid fuel-fired power plant. The design time spent on a boiler and controls is often justified by the subsequent predictable operation; and, after all, power plants have been handling solid fuels, such as coal, for years. So what is so special about designing equipment for storing and handling waste coal? After all, waste coal is still coal - isn’t it?

Waste coal is just as its name implies: unusable or unwanted coal that was discarded years ago, primarily due to its low energy value, but also due to its handling challenges. Typically, waste coals are reclaimed from uncovered piles or ponds and then transported to a plant either to sit in another exposed pile or in a large storage silo before being conveyed into fuel bunkers to feed the plant boilers. Belt conveyors are typically used to transfer the waste coal from location to location within the plant. Belt-to-belt transfers involve transfer chutes to direct the material stream from one belt onto another belt. With some fuels, spontaneous combustion of stagnant material is a particularly dangerous problem related to flow stoppages. Many have worked on addressing these concerns through the development of safety standards; for example, a solid fuel bunker designed to provide mass flow.4

Designing the handling system components based on the measured flow properties of the intended solid fuel can avoid these types of problems. This article focuses on waste coals, specifically, gob (bituminous and sub-bituminous) and culm (anthracite). However, the principles discussed here apply to all bulk solid materials handled at power plants.

Bunkers, Silos and Bins

Two of the most common flow problems experienced in an improperly designed bunker, silo or bin (hereafter referred to as silo) are no-flow and erratic flow. No-flow (Figure 1) from a silo can be due to either arching or ratholing.

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Arching occurs when an obstruction in the shape of an arch or a bridge forms above the outlet of a hopper and prevents further discharge. It can be an interlocking arch, where the particles mechanically lock to form the obstruction, or a cohesive arch. Common with waste coals, a cohesive arch occurs when particle-to-particle bonds form, allowing the material to pack together to form an obstruction.

Ratholing occurs in a silo when flow takes place in a channel located above the outlet. If the waste coal has sufficient cohesive strength, the stagnant material outside of this channel will not flow into it. Once the flow channel has emptied, all flow from the silo stops.

Erratic flow often results from alternating between an arch and a rathole. A rathole may fail due to an external force, such as vibrations created by a plant pulverizer (mill), a passing train or a flow aid device such as an air cannon, vibrator and so on. While some waste coal discharges as the rathole collapses, falling material often gets compacted over the outlet and forms an arch. This arch may break due to a similar external force. Material flow resumes until the flow channel is emptied and a rathole again forms.

Delayed startup time caused by problems related to fuel handling can add significantly to a plant’s cost. In addition, any stagnant region in a silo can be dangerous, especially when handling coals that are prone to spontaneous combustion. If flow takes place through a channel within the silo, the material outside of this channel may remain stagnant for a long time, increasing the likelihood of fires. Stagnant coal, typically caused by this type of flow pattern (funnel flow) (Figure 2), is one of the main causes for spontaneous combustion. Coal residence time should be limited in the silo, which can only be achieved by emptying the silo frequently or by using a first-in, first-out flow pattern (mass flow) (Figure 3), where all the material is in motion whenever any is discharged.

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Most waste coals tend to have large amounts of fines. Because up to 50 percent of the composition can be ash (and for many waste coals a large portion of this ash is clay) waste coals tend to be cohesive. This problem is further compounded by high moisture contents associated with outdoor storage in piles and ponds. Spontaneous combustion can be a concern with some of the lower rank waste coals, especially as new wet waste coal is added to a pile of older waste coal that is drier by comparison. For these reasons, flow-related problems are very common in funnel flow silos; therefore, mass flow silos should be used whenever possible.

Achieving Mass Flow

To achieve mass flow, two conditions must be met: the sloping hopper walls must be steep enough and sufficiently low in friction for the particles to slide along them and the hopper outlet must be large enough to prevent arching.

How steep and how smooth must a hopper surface be? The answer depends on the friction that develops between the particles and the hopper surface. This friction can be measured in a laboratory using an ASTM test method8 developed by Dr. Andrew Jenike9. These angles are used as design criteria for achieving mass flow in new hopper and silo installations and are invaluable when considering retrofit options using inserts, liners, coatings and polished surfaces with existing designs.10

In general, a number of factors can affect wall friction for a given waste coal, such as:

  • Wall material. Generally, smoother wall surfaces result in lower wall friction (there are exceptions), thus allowing shallower hopper angles for mass flow to take place.
  • Bulk solid condition. Moisture content, variations in material composition and particle size can affect wall friction.
  • Time at rest. Some waste coals adhere to a wall surface if left at rest in a hopper. Wall friction tests can be performed to measure the increase in wall friction (if any) due to storage at rest. If adhesion takes place, steeper hopper angles are required to ensure mass flow.
  • Corrosion. Wall materials that corrode with time generally become more frictional.
  • Abrasive wear. Often, abrasive wear results in smoother wall surfaces; therefore, designs based on an unpolished surface are usually conservative. However, abrasive wear can occasionally result in a more frictional surface, which can disrupt mass flow. When handling abrasive materials, wear tests can be performed to determine the effect on wall friction, as well as calculate the amount of wear expected. The second requirement for mass flow is that the outlet must be large enough to prevent arching caused by particle interlocking or the cohesive strength of the bulk solid.

Interlocking arches can be overcome by ensuring that the outlet diameter is at least six to eight times the largest particle size in a circular opening, or the width is at least three to four times the largest particle size in a slotted opening. (Slotted outlets must be at least three times as long as they are wide for such conditions to apply.)

The cohesive arch can be analyzed by determining the cohesive strength of the material.

A number of factors affect the minimum outlet sizes required, including:

  • Particle size. Generally as particle size decreases, cohesive strength increases, requiring larger outlets to prevent arching.
  • Moisture. Increased moisture content generally results in an increase in cohesive strength, with the maximum typically occurring between 70 percent and 90 percent of saturation moisture. At moistures higher than these, many bulk solids (including waste coals) tend to become slurry-like and their cohesive strength decreases.
  • Time at rest. Similar to wall friction, some waste coals exhibit an increase in their cohesive strength if left at rest for some period of time. Cohesive strength can be measured using a direct shear tester simulating storage time at rest.

Waste fuels, such as bituminous gob and anthracite culm, are inherently difficult to handle because they are high in all the factors that negatively influence material flow: high fines, high ash (much of which is clay), high moisture (due to open stockpiles and ponds), and storage time at rest. All of the variables listed above must be accounted for in the test program. A robust design requires testing samples from multiple sources over a range of moisture contents.

Feeders

Assuming that the silo is an independent piece of equipment should be avoided; it must be designed to operate in conjunction with the feeder below the hopper to ensure smooth, trouble-free operation. In addition to ensuring that reliable flow takes place in the hopper above, the outlet’s entire cross-sectional area also must be active. A restricted outlet, possibly caused by a partially open slide gate, will result in funnel flow with a small active flow channel regardless of the hopper design. It is, therefore, imperative that a feeder be capable of continuously withdrawing material from the entire hopper outlet.12 This feature allows mass flow to take place in the hopper above, if it is so designed. It also reduces the potential for ratholing in funnel flow by keeping the active flow channel as large as possible.

An essential aspect of using a slotted outlet is to ensure the feeder capacity increases in the direction of flow. As an example, when using a belt feeder, this increase in capacity is achieved by using a tapered interface (Figure 4). The increasing capacity along the length is achieved by the increase in height and width of the interface above the belt. Poor feeder design is a common cause of flow problems in silos.

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Standpipes are an important piece of equipment, which minimize the amount of gas leakage into the silo from a pressurized CFB and minimize the upward (positive) gas pressure gradient that can actually exacerbate the waste coal’s arching potential. The finer the coal, the more adverse this latter effect will be.

Chutes must be designed to prevent plugging while minimizing surface wear, particle attrition and dust generation. Dust is not an issue with higher moisture waste coals, although moisture introduces other problems. In particular, the inherent waste coal fines when combined with high moisture and ash (clay) contents make chute pluggages more likely. Proper chute design based on the properties of the material can avoid or solve these problems.13

Typical Remedies

The key to reliably handling waste coals is to design the equipment based on the measured flow properties of the materials being handled. Given waste coal’s variability, it is imperative to test samples from multiple sources over the expected range of moisture contents.

However, if the plant is already built, three methods exist to address the types of problems mentioned here (these methods can also apply to new plant design):

  • Change the bulk solid (minimize moisture by covering storage pile, mechanical drying or blending wet and dry materials; increase particle size).
  • Change the operating procedures (limit storage time at rest to reduce its arching tendency, lower silo fuel operating level frequently to minimize stagnant material, use flow aids sparingly, use all outlets simultaneously in multiple outlet silos).
  • Change/modify the equipment (line existing hopper or chute with a less frictional liner, enlarge outlet, steepen the angle of the lower hopper section or chute, install a flow correcting insert, change feeder and/or modify feeder interface).

The key to reliable waste coals handling is to design the handling system equipment based on the measured flow properties of the materials to be handled. Preventing stagnant regions of waste coal in a silo is an essential part of preventing spontaneous combustion.

REFERENCES

[1] Luckie, P.T., Penn Sate University, University Park, 1988.

[2] Merrow, E.W.: Estimating Startup Times for Solids-Processing Plants, Chemical Engineering, October 1988, pp. 89-92.

[3] Merrow, E.W.: Problems and Progress in Particle Processing, Chemical Innovation, January 2000, pp. 35-41.

[4] NFPA 85 “Boiler and Combustion Systems Hazards Code”, 2007 ed. (section 9.4.6.5, Bunker and Hopper Designs).

[5] Purutyan, H., Bengston, K.E. and Carson, J.W.: Identifying and Controlling Silo Vibration Mechanisms: Part I, Powder and Bulk Engineering, 1994, pp. 58-65.

[6] Carson, J.W. and T. Holmes: Why Silos Fail, Powder and Bulk Engineering, November 2001, pp. 31-43.

[7] Carson, J.W. and Jenkyn, R.T.: Load Development and Structural Considerations in Silo Design, Presented at Reliable Flow of Particulate Solids II, Oslo, Norway, 1993.

[8] “Standard Test Method for Shear Testing of Bulk Solids Using the Jenike Shear Cell,” American Society for Testing and Materials (ASTM), D 6128-00, 2000.

[9] Jenike, A.W.: Storage and Flow of Solids, University of Utah Engineering Experiment Station, Bulletin No. 123, 1964.

[10] Purutyan, H., Pittenger, B.H. and Carson, J.W.: Solve Solids Handling Problems by Retrofitting, Chemical Engineering Progress, Vol. 94, No. 4, 1998, pp. 27-39.

[11] Johanson, J.R. and Royal, T.A.: Measuring and Use of Wear Properties for Predicting Life of Bulk Materials Handling Equipment, Bulk Solids Handling, Vol. 2, No. 3, 1982, pp. 517-523.

[12] Carson, J.W. and Petro, G.J.: Feeder Selection Guidelines, Chemical Processing, Powder Solids Annual, 1997, pp. 40-43.

[13] 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.

Authors:

Roderick J. Hossfeld is a Senior Consultant for Jenike & Johanson Inc., an international specialized engineering firm focusing on providing reliable bulk solids flow. He has published several technical papers, participated in various conferences, and presented numerous short courses in the field of bulk solids flow. He holds both a B.S. and M.S. in Mechanical Engineering from the University of Massachusetts, Amherst, Mass.

Roger A. Barnum is also a Senior Consultant at Jenike & Johanson Inc. He is heavily involved in consulting with projects ranging from portable bin design to solving solid dosage form content uniformity problems. Examples of other projects include large silos for coal feeding of power plants, ceramic powder processing facility design, feed systems for contaminated soil, gravy mix storage and conveying, improving cement plant operation, and lime kiln storage and feeding. He received a B.S. in Mechanical Engineering from Rensselaer Polytechnic Institute, Troy, N.Y.

Materials Handling for Unconventional Fuels

State governments increasingly set goals that target a minimum portion of the state’s electrical supply as coming from renewable sources. At least 19 states have such targets ranging from 2 percent to more than 20 percent. In addition, seven northeastern states have jointed the Regional Greenhouse Gas Initiative which implements a cap-and-trade system for CO2, which will also drive renewable generation.

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While solar and wind tend to get most of the coverage, generating electricity from biomass has the potential to be the largest contributor to providing renewable-based generation.

Long popular in Europe, biomass accounts for nearly half of the generation from renewable energy sources when hydro is excluded. More than 24 percent of municipal waste in Europe is burned for energy. Now, biomass-based generation is finding a place on the U.S. electric grid.

Biomass generation may come from dedicated plants or co-fired in circulating fluidized bed, Stoker or pulverized coal boilers. New fuel sources include wood waste, fuel derived from animal or human waste, agriculture residue, tires or just about anything that will burn.

Experience in Europe has shown that the greatest challenge related to biomass may not be the combustion so much as material handling issues.

Consider poultry litter. The current estimated volume of poultry litter available as fuel in the United States is in excess of 30 million tons annually. With an average energy content of 4,500 Btu/lb, the fuel source may be a viable alternative for 25 MW to 60 MW power plants.

Because animal wastes have a low energy density compared to conventional fuels such as coal, much greater volumes of the material are required. Even relatively small power plants need to burn 100 tons of fuel per hour – or more.

In some cases, fuel may come from mixed waste materials from multiple farms. That means the litter may contain poultry waste along with straw, sawdust, wood chips and other “tramp” material. Seasonal and dietary variations can cause differences in the fuel that also must be accommodated. Local weather conditions, the unloading environment and even the configuration of the truck hauling the fuel can complicate and influence material handling system design.

Experiences at plants near the center of the UK’s turkey industry show that material handling must be kept simple. Conveyors that run well on tight particle size distributions and moisture contents are far less adaptable to these types of biomass and alternate fuels. A common front-end practice is to avoid using conveyors that intimately mix the material with the conveying media. That means pneumatic systems and flighted conveyors, such as drag chains and screw conveyors, are likely to make the conveying more difficult than necessary.

Storage-to-conveying transfer points can be automated. In many cases the live bottom receiving bin can directly feed the conveying system using the trailer as a storage bin. Some degree of volumetric feed control almost always is needed.

Mechanisms are also needed to keep the material static and unaffected either by material placed in or on them. Pipe conveyors can be used along with pocket belts and apron conveyors. Using belt scales to provide inventory control favors conventional belts for overland distances.

In some cases dust control, odor abatement or bacterial control must be considered and isolated from the broader environment. Galleried conveyors running between fuel and boiler buildings need sufficient air and ventilation. Large fuel halls or feed conveyors can present fire control challenges. Fire suppression must be factored into material handling system design.

Due to the material’s highly variable nature, it is critical to consider material bed depth and conveyor flight design to avoid operating and feed problems. Unique flight shapes, brushes or knife edge flight profiles have all been used effectively to avoid material build-up or adhesion to the conveyor components and walls.

Bio-fuel materials also demand hopper and flow conditions that are far different from coal. In many cases, avoiding static holding altogether is the only solution. For example, modern poultry litter plants have no day or holding bins until the delivery path nears the small feed screw hopper at the boiler itself.

Biomass is expected to produce 200 billion kWh before 2025. States including New York, California, Maine, Nevada and Minnesota have committed to producing 20 percent of their energy with renewables. Biomass can help reach these targets and fuel feed systems will be a critical part of making biomass generation successful.

After all, in the new world of biomass, the simple conveyor isn’t quite so simple any more. - By Ralph Harris and Simon Shipp, Stock Equipment Co.

Sponsored by FLSmidth

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