Lengthy
straw/stalk of biomass may not be directly fed into grinders such as hammer mills and
disc refiners. Hence, biomass needs to be preprocessed using coarse grinders like a knife mill to allow for efficient feeding in refiner mills without bridging and
choking. Size reduction mechanical energy was directly measured for
switchgrass (
Panicum virgatum L.),
wheat straw (
Triticum aestivum L.), and corn
stover (
Zea mays L.) in an
instrumented knife mill. Direct power inputs were determined for different knife mill screen openings from 12.7 to 50.8 mm, rotor speeds between 250 and 500 rpm, and mass
feed rates from 1 to 11 kg/min. Overall
accuracy of power measurement was calculated to be +/-0.003 kW. Total specific energy (kWh/Mg) was defined as size reduction energy to operate mill with biomass. Effective specific energy was defined as the energy that can be assumed to reach the biomass. The difference is
parasitic or no-load energy of mill. Total specific energy for
switchgrass,
wheat straw, and corn
stover chopping increased with knife mill speed, whereas, effective specific energy decreased marginally for
switchgrass and increased for
wheat straw and corn
stover. Total and effective specific energy decreased with an increase in screen size for all the crops studied. Total specific energy decreased with increase in mass
feed rate, but effective specific energy increased for
switchgrass and
wheat straw, and decreased for corn
stover at increased
feed rate. For knife mill screen size of 25.4 mm and optimum speed of 250 rpm, optimum
feed rates were 7.6, 5.8, and 4.5 kg/min for
switchgrass,
wheat straw, and corn
stover, respectively, and the corresponding total specific energies were 7.57, 10.53, and 8.87 kWh/Mg and effective specific energies were 1.27, 1.50, and 0.24 kWh/Mg for
switchgrass,
wheat straw, and corn
stover, respectively. Energy
utilization ratios were calculated as 16.8%, 14.3%, and 2.8% for
switchgrass,
wheat straw, and corn
stover, respectively. These data will be useful for preparing the feed material for subsequent fine
grinding operations and designing new mills.
