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.
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.
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.
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.