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Hog Fuel (Wood Waste) Boiler Feed Metering Bins

Hog Fuel (Wood Waste) Boiler Feed Metering Bin

 

This case study highlights the retrofit of four live bottom metering bins that were suffering from chronic plugging at a pulp mill in Port Angeles, Washington State. Kamengo replaced the four metering bins with new bins, Kamengo Feeders, chute and expansion joint. Prior to the retrofit, the plant was experiencing lost steam production and swings in the header pressure, which made it particularly problematic to keep emissions within the plant’s permit. Post retrofit, the plant is producing more steam, and the boiler is able to meet the changing steam demands from the paper plant while maintaining an even header pressure. The retrofit was completed in 2015.

The Challenge

Prior to the retrofit, the pulp mill had four live bottom metering bins, each discharging into one of four spouts into the boiler.

 

Each metering bin consisted of two augers below a negative taper storage bin. Although the boiler was only two years old, the mill was keen to improve the reliability fuel feed into the boiler. The screw feeders had a propensity to compact material at the front of the bin walls as well as rat hole in the storage bin. The mill tried a number of solutions, including installing UHMW along the front of the bin walls. Nothing worked.

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Why the Conventional Screw Feeders Were Suffering from Chronic Plugging and Inconsistent Discharge

The screw augers were suffering from chronic plugging for two reasons:

 

First, the screw augers have a tendency to compact material against the front of the bin wall. Wood waste that is being carried or sheared from the storage bin, through friction or interlocking, carries fuel above it. And this fuel travelling above the screw feeder outlet is compacted against the bin wall. The problem is that wood waste gains significant strength when it is compacted. Once the fuel above the outlet gains sufficient strength it will bridge over the outlet resulting in inconsistent discharge and plugging.

 

Second, the screw augers withdraw material primarily from the rear of the bin. This limited withdrawal of material has several consequences. First, it means that gravity is acting on a smaller area than the total footprint of the screw auger, leaving a stagnant pocket of material at the front of the bin. This tagnant 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. Second, the uneven withdrawal of fuel by the screw auger promotes a funnel flow discharge pattern, which promotes rat-holing, which will lead to plugging through the center of the bin.

Kamengo’s Solution

In contrast to the screw feeders, the Kamengo Feeder withdraws material evenly from its full opening, while avoiding the compaction that promotes material bridging. 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.

 

To minimize disruption to structural steel and the existing slide gates between the distribution chain conveyor and the metering bins, Kamengo kept the upper half of the existing metering bins and replaced everything to the boiler distributor. For each metering, Kamengo supplied a new lower half storage bin, Kamengo Feeder, collection screw conveyor, screw conveyor discharge chute and expansion joint.

 

The screw conveyor is required because the Kamengo Feeder discharges material across its full length. There is no head of material above the screw auger – in fact the screw trough is never more than 1/3 full and only requires a 5HP drive. As such the screw is only acting as a conveyor and not as a feeder.  Although the screw conveyor introduces an additional mechanical item, its inclusion permits a layout that maximizes storage at the boiler.

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. 

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.