Biomass Feasibility Project Final Report - Xcel Energy

(Washington State University, n.d.)
Figure IV-1: Tub Grinder
Log size is an important consideration in deciding which machine to use. Tub grinders can
handle trees and stumps up to 14’ diameter. But even the largest chippers do not work well on
trees larger than 2’ in diameter. Logs larger than that have to be split by shears into pieces with
a diameter less than 2’ in order to enter a chipper. Thus tub grinders reduce large logs more
cheaply than chippers because only one step with one machine is involved. But if the tub
grinder can’t make wood particles small enough to meet specifications in one pass, that
becomes a false economy because the wood has to run through the machine a second time,
adding cost. If possible, the plant should select a machine that produces its size specification in
a single pass.
In Minnesota, the usual cost of grinding or chipping logs and brush to a particle size of less than
2” would be $8.00 to $15.00 per ton, not including transportation and tipping fees. Adding
delivery would about double the cost. To grind or shred wood to 1/4” or 1/2” would probably
increase the cost by another $3 to $10 per ton. Therefore biomass facilities designed to accept
larger particle sizes can buy their feedstocks cheaper.
Anaerobic Digester Particle Size
Particle size also affects the performance of anaerobic digesters. A size of 1.5 inch or less works
best for high-solids anaerobic digesters. Besides traditional grinders and chippers, size reduction
drums and thermal treatments can be used to reduce feedstock sizes. There is a trend toward
doing more processing of municipal solid waste (MSW) in central facilities. Source-separated
organics (SSO) programs, like one in Hutchinson, Minnesota, are appearing in many U.S.
communities. Minneapolis and St. Paul are planning SSO programs.
Some European and Canadian SSO plants prepare waste for anaerobic digestion by rolling it in
drums 8 feet to 12 feet in diameter. Internal risers or shelves lift the waste until it tumbles down.
Liquids taken from anaerobic digesters are reintroduced into the drum to inoculate the waste.
The liquid combined with the tumbling produces 1.5-inch particles.
Page 42
Identifying Effective Biomass Strategies:
Quantifying Minnesota’s Resources and Evaluating Future Opportunities