Biomass Feasibility Project Final Report - Xcel Energy

PHYSICAL FACTORS AFFECTING BIOMASS PROCESSING
Depending on cost factors, processing may be done at the source, at the plant, or somewhere
in between. But there are physical and logistical factors to consider as well.
Moisture Content
Biomass fuels exist in a wide range of moisture contents which have a substantial effect on
biomass fuels’ transportation costs, degradation in long-term storage, processing, and
conversion into energy. Therefore moisture content underlies decisions in all those areas.
Agricultural biomass. The timing of the harvest affects its processing after harvest. A biomass’s
moisture content fluctuates throughout the year. Herbaceous crops, like switchgrass, and grain
crops, like corn, begin to dry even before harvest. Switchgrass can drop up to 15% of its moisture
late in the season and be ready to bale immediately after harvest. Switchgrass cut earlier than
that has to be dried before baling.
The moisture content of any biomass fuel drops after harvest. Crops like corn stover or
switchgrass can dry enough to bale within hours or days of harvest regardless of the date. The
biomass’s original moisture content, the weather conditions, and the arrangement of the harvest
in the field all affect drying time. Material spread evenly over a field typically dries much faster
than it does in windrows.
Woody biomass. Wood can be passively or actively dried. Logs left to dry in the open air
typically drop to 30% to 50% moisture content within a year after harvest. Active methods, like
blowing air through logs under shelter or placing them in rotary drum driers like those used to dry
alfalfa, can lower moisture more quickly. Rotary drum driers usually burn natural gas, but some
newer models run on electricity or waste process heat. Some even pull a vacuum. Infra-red
drying, a technology used in the lumber and paint industries, also has come into use for logs.
Infra-red drying is most effective for products that already are at less than 30% moisture.
The cost of drying will vary considerably depending on material, initial moisture content, and
many other factors. Drying costs range widely from $0.50 to $15 per ton.
Wood Particle Size
Each biomass power plant will specify the particle size needed for efficient and safe operation.
Particles too large will burn incompletely, thereby increasing costs of fuel and disposal. At the
other extreme, particles too small – dust – can be explosive in boilers and processing and
handling equipment. Hence most facilities remove dust. District Energy St. Paul, for example,
screens feedstocks with a ¼” trommel to remove all sawdust and grit. Some facilities dispose of
collected dust while others carefully remix it with fuel to burn it safely.
The two most common size-reduction machines used on logs are chippers and tub grinders.
Chippers cut consistently sized angular chips with roughly equal sides. Tub grinders produce
long, stringy particles, usually about 1” by 2” by 6”, that almost always have “tails” on their ends.
Grinders usually are used to make wood mulch because the tails hold it in place on a planting
bed. But tails can bridge in storage bins and jam bucket elevators and auger conveyors.
Identifying Effective Biomass Strategies: Page 41
Quantifying Minnesota’s Resources and Evaluating Future Opportunities