Processing Switchgrass

Switchgrass bale destringer (Click to Enlarge)

Loading bales on conveyor (Click to Enlarge)

The biomass processing system at Ottumwa Generating Station (OGS) has been designed to operate at an average feed rate of 12.5 tons of switchgrass per hour. General operation of the system is as follows. Switchgrass bales are loaded onto a bale feed conveyor using a front-end loader. While on the conveyor, the bales will be scanned for moisture content using non-contact microwave sensors, then will be weighed by load cells that are built into the conveyor. “Wet” bales (moisture content greater than 25%) are rejected. An automatic “D-Stringer” cuts the twine from the bales, deposits it in a waste bin for disposal, and maintains a running total at the operator control panel of the number of bales processed. A feed rate indicator on the bale feed conveyor was tied into the OGS data acquisition system (DAS) to allow automated tracking of switchgrass feed rates. Bales are then fed into the “debaler” hammer-mill which processes the 3’ x 4’ x 8’ bales to a size that will pass through a 2-inch screen. The debaled switchgrass drops onto a “Take-away” conveyor which transports the material across a magnetic belt to remove tramp metal, then dumps it into the infeed chute of an attrition mill (the “Eliminator”). The processed material is discharged from the mill and sucked into a cyclone which separates dust from the larger particles. The cyclone drops the larger particles into a surge bin. The dust continues to a cyclone/baghouse which filters the dust from the transport air and drops the dust onto a fully-enclosed conveyor. This conveyor transports the dust to the surge bin. The processed switchgrass is fed by two screw conveyors in the bottom of the surge bin into two rotary airlocks. The airlocks feed the material into steel blow lines which will transport it about 1000 feet, through two burners, and into the power plant furnace at OGS.

To view other steps in the Chariton Valley Biomass Project’s “cofiring cycle,” click on one of the links below:

Establishing and Growing Switchgrass
Transportation and Storage
Power Conversion
Green Power Sales