Hog Fuel (Wood Waste) Boiler Feed Metering Bins
This case study highlights the retrofit of eight small screw feeders experiencing chronic plugging at a mill in Quesnel, BC. The screw feeders where replaced by four surge bins and feeders. Each Kamengo Feeder feeds a split chute, which in-turn directs the hog fuel into two boiler spouts. Prior to the retrofit, the plant was experiencing lost steam production and swings in the header pressure. 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.
Prior to the retrofit, the pulp mill had eight small screw feeders that were experience chronic plugging. Each screw feeder consisted of a single auger with a very small and narrow storage bin above it. Although the mill limited the head of material above the screw augers, the metering bins were very unreliable.
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.
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 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.