Environmental Engineering Reference
In-Depth Information
7.3 BIomass storaGe
The moisture content of agricultural biomass will greatly influence storage options as well as
appropriate strategies for subsequent transportation. Wet materials can be ensiled and stored
anaerobically in bunker silos or bags, whereas dry material can be baled and stored like straw
and hay. For dry storage, hay tools and hay harvesting technologies are available in most agricul-
tural operations and can be easily adapted for herbaceous biomass harvest. Typical on-farm hay
harvest procedures include mowing, raking and/or tedding, merging, baling, and bale handling.
Bale moisture content must be maintained below 20% to minimize dry matter loss as well as
self-heating and the potential for spontaneous combustion. Since indoor storage is expensive,
in humid climates bales can alternatively be stored outdoor in plastic bale wraps to minimize
moisture absorption, whereas in more arid climates bales can simply be stacked outside for the
most part of the year. For wet materials, (i.e., >35% water on a wet basis) ensilage can be used.
Ensilage has been used to store chopped moist biomass such as sorghum, corn stover, and wet
chopped grasses as bioenergy crops (Montross and Crofcheck 2004; Ren et al. 2006; Shinners
et al. 2007b; Bennett and Anex 2008) and has been used for other forages crops and foods for over
a thousand years. At moisture levels above 35% (wet basis), biomass feedstocks need to be stored
in a bunker silo, plastic “ag bags,” or another anaerobic environment immediately after harvested
to ensure quality. Intermediate moisture levels between 20% and 40% can be dried or moistened,
with an aim to achieve appropriate dry or wet storage conditions for safe and stable storage.
Although dry handling and storage has clear advantages for energy systems based on combustion
or thermochemical conversion, wet storage may be more cost-effective for biochemical platforms
(Bennett and Anex 2008).
7.4 Forest BIomass harvest
As previously indicated, there is considerably more experience today with the harvesting, handling,
and managing logistics of forest-based biomass at scales much greater than 500,000 Mg/year than
there is with agricultural energy crops and perennials. Forest biomass can come from logging oper-
ations, forest thinning for wildfire prevention, or forest improvement projects. Typical feedstocks
are limbs, tops, and other byproducts of forest management. Along with being suitable for biofuels,
forest biomass can be used for engineered wood products and composite lumber (Dooley et al.
2006). Woody biomass harvest processes may include felling, piling, forwarding, chipping and/or
bundling, and trucking. Products could be woodchips, bundles, round wood logs or a combination
there of.
7.4.1 f
fElling,
, d
ElimBing
,
and
d
EBarking
In a typical forest harvest operation, round wood products for building materials are cut from a forest
as large whole trees. Trees are cut and piled and then forwarded by a feller/buncher to a location
where they are delimbed and debarked. Woodchip production for pulp mills has very similar field
operations to those described, although pulp-mill procurement specialists are more willing to
purchase woodchips and small-diameter “low-use” wood that loggers could collect and sell. These
pulp and paper industry employees and former employees represent a legacy of experience and often
know of idle equipment in the many communities where the pulp and paper industry is in decline.
After typical timber harvest operations, 13% of total softwood volume and 24% of total hardwood
volume is left as logging residue, most of which (75%) is in tops and limbs (Hartsough 1992). This
residue may be left in the forest or harvested for use as a biomass product. Before removing this
material it is important to understand the carbon and nutrient cycle dynamics of the site because
up to 30% of the thinnings should be left on the forest floor to replenish soil carbon and nutrients.
