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In Case Studies

Hog Fuel (Wood Waste) Tall Day Bin Retrofit

Hog Fuel (Wood Waste) Tall Day Bin Retrofit

 

This case study is an example of a retrofit of a 37-foot tall, 6,000 cu-ft silo used to store and feed hog fuel (wood waste) into a power boiler. The hog fuel at the mill is particularly challenging because it contains hickory bark and can be very stringy.

When Kamengo first met the mill, the plant engineers were exploring options to replace the entire hog fuel feeding system. However, upon learning of a retrofit to a similar size tall bin that Kamengo had completed for a pulp mill in Washington State, USA, the mill found that it could solve its feed issues by retrofitting its existing bin

 

Kamengo replaced the lower half of the bin with a rectangular plane-flow mass flow hopper and Kamengo Feeder. The Kamengo Feeder is ahead of a live bottom bin with screw feeders feeding biomass fuel into the boiler. Since the screw feeders cannot maintain a significant head of material above them, the Kamengo Feeder is used to reliably meter just enough material that the screw feeders can handle to ensure good, steady material discharge into the boiler.

 

This retrofit was completed in 2004.

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Why the Existing Storage and Feed System is Suffering from Chronic Bridging and Inconsistent Discharge

Prior to the retrofit, the storage silo converged with a 60-deg cone down to a vibrating hat. The mill could not put more than 6-feet of material in the storage bin before it would plug. This was problematic, because this storage silo was providing critical storage between the hog fuel pile and the power boiler’s metering bins.

The existing silo was suffering from chronic plugging for three reasons:

  1. Poor bin geometry. The sloping walls and discharge opening of the cone were insufficient to produce a reliable mass flow or first-in, first-out discharge pattern. First, the sloping walls were too shallow. As a result, material could not slide along the hopper walls, but would instead try to slip within itself in a funnel flow, or first-in, last-out discharge pattern. Second, the discharge opening was too small. The smaller the opening, the less strength a material requires to form a stable arch over the bin outlet that gravity cannot reliably break.
  2. The vibrating feeder withdrew material unevenly from the storage bin – pulling material primarily from around the silo walls, leaving behind a relative stagnant core of fuel. This selective withdrawal of material, in addition to the shallow sloping walls, induced a funnel flow, or first-in, last-out discharge pattern in the bin. In principle, there is nothing wrong with funnel flow as long as the effective opening of the bin exceeds the bulk solid’s piping dimension (or distance over which the bulk solid can form a stable rat-hole). Unfortunately, the piping dimension for wood waste is quite large – in fact, it is larger than the silo diameter. To handle hog fuel (wood waste) reliably without hang-ups, it must be discharged in a mass flow, or a first-in, first-out discharge pattern – where all the material in the storage bin descends as a single body and where all the material in the storage bin is in motion. To do achieve a mass flow discharge pattern, the Feeder must withdraw material evenly from the storage bin’s full discharge outlet.

Kamengo’s Solution

The solution to fixing the silo had two parts:

 

The first half of the solution is to modify the bin geometry of the silo such that it will promote a first-in, first-out discharge pattern. In this case, Kamengo cut off the bottom portion of the silo, including the cone and replaced it with a plane flow, mass flow hopper and Kamengo Feeder. A plane flow hopper is the most conservative hopper shape.

 

The new hopper converged to wide and long 6-foot wide by 18-foot long discharge opening. This wide and long opening is required to ensure gravity will always be sufficient to break the strength of the arch that hog fuel would create above the Feeder. In summary, by replacing the cone with the new plane flow hopper, Kamengo fixed the geometry of the silo such that if the Feeder were removed, the silo would self-empty with only the aid of gravity. The minimum geometry required for gravity discharge, including minimum slope angle of the plane flow hopper and minimum discharge opening were all selected based on the flow properties of hog fuel.

 

The second half of the solution was to pair the plane flow hopper with a fully-effective Feeder – in this case a Kamengo Feeder. A fully-effective feeder is one that withdraws material evenly from its entire opening, which by definition is necessary to actually achieve a mass flow discharge pattern in the hopper. By definition, to achieve mass flow, the bulk solid must descend the storage bin as a single body with all the stored material in motion, and the only way to achieve this is for the feeder to withdraw material evenly from its entire opening. If the Feeder withdraws material selectively from the bin discharge outlet, sections of material in the bin will be stagnant and funnel flow will ensue.

 

Again, discharging in mass flow is often necessary when handling a difficult flowing bulk solid. What makes the Kamengo Feeder unique is that it can be made as wide as needed and as long as desired. This feature is particularly valuable given that the chosen minimum discharge opening of the silo is 6-foot by 18-foot. To achieve mass flow, the Feeder inlet must match this outlet. This is very difficult to do with conventional technologies, but very easy to achieve with a Kamengo Feeder.

Learn More

To learn more about the physics of storage bin and feeder design as well as the root causes of bin plugging, please download our white paper entitled: The Design of Reliable Storage Bins and Feeders for the Biomass Industry. 

In Case Studies

Hog Fuel (Wood Waste) Boiler Feed Metering Bins

Hog Fuel (Wood Waste) Boiler Feed Metering Bins

This case study is an example of a retrofit of wood waste or hog fuel metering bins that are metering fuel into two power boilers. The retrofit was completed in 1994.

 

In 2021, Kamengo replaced its previously supplied metering bins with new wider and longer Feeders capable of handling more difficult flowing fuels including roadside slash and long stranded cedar.

Existing System Prior to the Retrofit

Prior to the retrofit, the pulp mill had twelve star feeders, each feeding a separate spout into one of two boilers. In 1993, the pulp mill retrofitted one star feeder with a small Kamengo Feeder. While the star feeders continued to struggle, the Kamengo Feeder proved that it could deliver consistent discharge without hang-ups. In 1994, the pulp mill retrofitted the remaining eleven metering bins with new Kamengo Feeders, hoppers and chutes.

 

By 2021, the originally supplied Kamengo equipment reached its end-of-life. However, instead of replacing the Feeders as they were, Kamengo opted to replace the metering bins to include larger, wider Feeders. These new Feeders are capable of handling much worse fuels including roadside slash and long, stringy cedar.

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Why the Existing System Suffered From Chronic Plugging

Prior to the retrofit, the pulp mill struggled with the star feeders. They suffered from chronic plugging which impacted both emissions and steam-output. Star feeders suffer from two problems. The first problem, is that like most rotary feeders, they withdraw material from a relatively small footprint. Remember, gravity is acting on the area in which material is discharging. When the area through which material is flowing is small, the opening may not be sufficient for gravity to break the strength of the material and keep it flowing. The second problem is that star feeders have a propensity to withdraw material from the back of its opening, leaving a stagnant pocket of material at the front. This limited withdrawal of material means that not only is gravity acting on a smaller area, but it also means that the stagnant material is allowed to compress and compact over time under its own weight. The more wood waste compacts, the stronger it gets. Eventually the material will be strong enough to form a stable bridge.

Kamengo’s Solution

In contrast to the star feeders, the Kamengo Feeder withdraws material evenly from its full opening. Further, the opening of a Kamengo Feeder can be made as wide as needed and as long as desired. For this retrofit, Kamengo is using Feeders with a 3-foot by 7-foot opening, which is several times the footprint of the star feeder. And because the Feeder withdraws material evenly from its entire opening, all of the material above the feeder is in motion during discharge, resulting in a mass flow, or first-in, first-out discharge. Handling a low bulk density, easily compactible bulk solid like wood waste in mass flow is important to ensuring a reliable system.

 

Finally, the Kamengo Feeder delivers a very steady, predicable discharge of fuel into the boiler. This is necessary for maintaining a predictable pyrolytic reaction in the boiler. The steady discharge of fuel from the Kamengo Feeders improved boiler efficiency and resulted in higher steam-output with lower emissions.

Learn More

To learn more about the physics of storage bin and feeder design as well as the root causes of bin plugging, please download our white paper entitled: The Design of Reliable Storage Bins and Feeders for the Biomass Industry. 

In Solutions To Common Problems

Retrofit of Wood Chip Chute Suffering From Chronic Plugging

Retrofit of Wood Chip Chute Suffering From Chronic Plugging  

 

This is an example of when a chute becomes a hopper, and when a conveyor becomes a feeder and plugging ensues.

Why the Chute is Suffering from Chronic Plugging and Bridging

In contrast to a bin or hopper, a chute is where material is continuously flowing and there is no head of material at the outlet. When a chute is flooded such that a head of material builds at the outlet, the chute becomes a hopper and the conveyor on which it discharges onto becomes a feeder. This is problematic because suddenly the chute is subject to minimum geometry constraints needed to avoid bridging and the conveyor needs assume the job of shearing material from the chute.

 

This is an example where the chute is receiving a flood of material from processes upstream causing a head of material to build in the chute. And because the geometry of the chute is insufficient to act as a hopper, the chute is subject to chronic plugging and is a major headache for the plant.

 

In summary, because it is carrying a head of material, the chute is not actually a chute, but a hopper and as such, its geometry, including outlet and sloping walls must be sufficiently large and steep to ensure reliable gravity discharge. However, with its four sloping walls and small discharge opening, the chute’s (turned hopper’s) geometry is insufficient, and hence it is subject to chronic plugging.

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Kamengo’s Solution

The easy solutions to fixing these chutes turned hoppers is to either decrease the discharge of material upstream or increase the speed of the conveyor to ensure no head of material builds up in the chute. If the easy solutions are not viable, as in the case of this case study, the plant needs to install a hopper and feeder designed to handle the bulk solid as an intermediary between the chutes and the conveyor.

 

Choosing a suitable storage and feed system has two components. The first is to choose a bin shape with a wide and long opening, where if the feeder were removed, the hopper would self-empty with only the aid of gravity. Kamengo typically suggests a plane flow hopper (which is a bin shape where the hopper walls only converge in one plane at a time). The plane flow hopper is the most conservative hopper shape.

 

The second half of the solution is to pair the plane flow hopper with a fully effective feeder – that is a feeder that withdraws material evenly from its entire opening – which is necessary for reliable discharge when metering a difficult flowing material from a small hopper. The Kamengo Feeder is a very good choice because not only is it a fully effective feeder, but it can also be made as wide as needed and as long as desired, which opens the range of solutions to fixing a problem storage and feeder system.

Learn More

To learn more about the physics of storage bin and feeder design as well as the root causes of bin plugging, please visit KamengoU.

In Solutions To Common Problems

Corn Lignin Filter Cake Metering Bin

Corn Lignin Filter Cake Metering Bin

This is an example of a solution to fix a corn lignin filter cake metering bin that is suffering from chronic bridging.

Why the Current Storage and Feed Arrangement is Suffering from Chronic Plugging and Inconsistent Discharge

The existing storage and feed arrangement consists of a long pyramid hopper with shallow walls that is discharged by a long chain conveyor. Filter cake falls through breaker bars into the hoppers, from where it is sheared from the bin by the chain conveyor.

 

The chain conveyor has a tendency to pull material from a short portion of the rear of the bin while compacting material at the front of the bin. The compaction is particularly problematic as it is leading to high loads being transferred to conveyor causing the conveyor to trip. The stagnant material at the front of the bin is permitted to gain strength both as a result of compaction from the conveyor as well as compaction under gravity. The result is that the material at the front of the hopper is permitted to gain significant strength, making it at risk of bridging over the conveyor.

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Kamengo’s Solution

The solution involves replacing the existing lower hopper with a new plane flow hopper that is constructed with sufficiently steel sloping walls and lined with a low friction liner. Below the new hopper is a long and wide Kamengo Feeder that will feed onto the existing chain conveyor.

 

The advantage of the Kamengo Feeder is that, unlike the chain conveyor, it meters material evenly from the hopper’s full discharge outlet. In combination with correct hopper geometry, the Kamengo Feeder will produce a first-in, first-out discharge in the storage hopper. Of critical importance, the full weight of the stored lignin sits on the Kamengo Feeder with no vertical load placed on the chain conveyor, which will reduce wear on the conveyor as well as power needed to drive the conveyor. Further, the Kamengo Feeder will do all the metering and as such the chain conveyor will simply be used as a conveyor. A second advantage of the Kamengo Feeder is that it discharges evenly across its full discharge length and as such over the full length of the chain conveyor infeed, avoiding problematic surge loading, which could place a strain on the conveyor and cause it to trip.

Learn More

To learn more about the physics of storage bin and feeder design as well as the root causes of bin plugging, please download our white paper entitled: The Design of Reliable Storage Bins and Feeders for the Biomass Industry.