Poultry Litter Boiler Feed Metering Bin
This case study is an example of boiler feed metering bins that are metering poultry litter into a power boiler. This installation was delivered in 2007 and operated until 2019 when the plant was decommissioned.
This fuel is particularly challenging because it consists of wood waste mixed with poultry litter. The result is a fuel that is not only fibrous and compactible, but also sticky and corrosive.
This installation was a for a new power plant. The owners, who had built several poultry litter power plants in the UK prior to this plant in Benson, Minnesota, had experienced a lot of difficulty handling poultry fuel, in particular in delivering an even feed into the boiler at their existing plants. For their first planned plant in the USA, the owners wanted a solution that was capable of reliably metering poultry fuel and ultimately selected the Kamengo Feeder.
Why Conventional Live-Bottom Feed Systems Suffer from Chronic Bridging and Inconsistent Discharge
The owners had previously used live bottom bins and individual metering bins with screw augers to meter fuel into the boiler of previous plants. These bins suffered from inconsistent discharge and plugging for several reasons. First, the screw augers have a tendency to compact material against the front of the bin wall. As poultry litter fuel is sheared from the storage bin, due to interlocking and friction, the fuel being carried out carries fuel above it. And this fuel travelling above the outlet is compacted against the bin wall. The problem is that poultry litter 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, leaving a stagnant pocket of material at the front. This limited withdrawal of material has several consequences. First, it 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 poultry litter compacts, the stronger it gets. Eventually the material will be strong enough to form a stable bridge. Second, uneven withdrawal promotes a funnel flow discharge pattern, which promotes rat-holing.
In contrast to the live bottom bin, 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 installation, Kamengo supplied eight individual metering bins, each with its own Kamengo Feeder. Because the Kamengo 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 poultry litter 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 is necessary to maximize steam delivered to the turbine.
There are eight inlets into the boiler, with a Kamengo supplied 430 cu-ft bin, Feeder, screw conveyor and chute with expansion joint for each inlet. The Kamengo equipment receives fuel from a distribution chain conveyor. Fuel is continuously fed into the first seven bins. Level sensors in the seventh and eighth bin indicate to the plant PLC when to start and stop the distribution conveyor. The even feed from the Kamengo bins permitted the plant to forgo a return conveyor.
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 subject to failure and wear, its inclusion permits a layout that maximizes storage at the boiler. Kamengo was chosen as the vendor for this project after extensive testing, including feeding the material through our pilot scale Kamengo feeder and bin located in Vancouver, Canada.
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.