Development of a biomass industry will depend on development of ways to economically densify biomass. Potential biomass crops include corn stover, switchgrass, miscanthus, woody materials and other dedicated energy crops. Pelletizing biomass is one way to increase the density. It also creates a form that can be stored, conveyed and transported using existing machinery and trucks. Bulk density is a measure of the weight of a material for a given volume. Loose biomass has a density of about 3 to 5 pounds per cubic foot.
Biomass can be harvested and removed from the field in a variety of forms, from chopping and ensiling to baling round or square bales. Baling typically increases the density by a factor of at least three, which is helpful for the transportation of biomass from the field to a processing facility, but it is not enough to alleviate further handling, processing and transportation hurdles which increase costs to the end user.
An industry standard in handling of large volumes of biomass has been established by grains including shelled corn which has a density of 45 pounds per cubic foot. This density is in some ways a standard that the biomass industry strives to match. There are different levels of densification that can be achieved by available equipment. A briquette (sometimes called cubes) requires less processing and less expense, but does not produce a form that is comparable to corn. A pellet requires additional processing, more expense and energy. The pellet form is comparable to the density of corn and allows semis to carry a full load and meet weight targets. With low density materials, the freight cost per ton is higher due to light loads that fill the trailer, but does not reach the weight capacity of the trailer. If you can increase product density, your shipping costs will be reduced significantly.
Bulk density for a given material
|Form||Density (lbs/cubic foot)|
|Large round bale||12-15|
|Large square bale||13-16|
Adapted from Sokhansanj et al. 2006.
Michigan State University Extension has developed a portable biomass pellet system that can be pulled from farm-to-farm to make pellets. In this system, biomass is fed into a hammer mill and ground to pass through a quarter-inch sieve. It is then blown into a holding tank called a surge bin. An auger at the bottom of the surge bin pulls ground material out of the bin where it drops into the pellet mill. Pellets come out of the machine and drop into the conveyor. The conveyor is hooked up to the blower to pull air in, which cools the pellets. It also picks up any fines and blows them back into the surge bin. There is a large cyclone mounted on top of the surge bin that recycles dust from throughout the system.
The pellet mill has a ring die. This means the die spins and rollers are stationary, forcing the material through the die. Different types of biomass require different thickness of dies. For switchgrass, you want a 10:1 ratio. For a quarter-inch diameter pellet, you will want a die that is 2.5 inches thick. The MSU Extension portable biomass pellet system is equipped with a die sized for switchgrass and for wood shavings.