Biomass Feasibility Project Final Report - Xcel Energy

(Photo by Dave Parsons (NREL, n.d.))
Figure IV-5: Wood chips stored at the McNeil Generating Station in Burlington, VT
Fuel Degradation and Decomposition
Moisture content, odor, particle size, and fire hazards all are considerations in storing biomass
fuels. Storing grasses or plant leaves with moisture content above 15% leads to partial
biodegradation or composting of the biomass. Freshly cut biomass with green foliage holds
substantial moisture within cell walls. Microbes can thrive in this moisture and decompose the
stored biomass, reducing its energy content and its structural integrity, making piles unstable and
bales difficult to move.
Some biomasses present fire hazards. Dusts from wood and grains are explosive. Brush chips,
with small particle sizes and a 30:1 ratio of carbon to nitrogen, are highly combustible. Nitrogen
is contained in green leaves or green stems in the form of proteins, and carbon is concentrated
in the woody stems. The mix of chips and leaves in brush piles makes for rapid composting at
temperatures as high as 120° to 180° F. But since those high temperatures can lead to
spontaneous combustion, long-term storage of brush chips presents a serious fire hazard. Corn
stover can pose the same problem in storage. Drying is the best preventative. Lowering
Moisture content to 15% limits the microbial action that causes high temperatures. In addition,
rules have been developed to provide standards and specifications pertaining to chip and
sawdust pile geometries that minimizes combustion problems.
Odors
Wet biomass can give off unpleasant smells from the breakdown of protein and fats. When the
interior of a biomass pile lacks oxygen, anaerobes release ammonia and/or sulfur compounds.
The buyer can minimize those emissions by checking moisture content at delivery and rejecting
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Identifying Effective Biomass Strategies:
Quantifying Minnesota’s Resources and Evaluating Future Opportunities