Environmental Engineering Reference
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densities or a factor of 12 assuming typical herbaceous biomass densities respectively (Richard
2010). In the U.S. context, the 1.3 billion Mg of biomass expected to meet domestic biofuel targets
by 2030 (Perlack et al. 2005) exceeds the 800 million Mg of total material flow of the U.S. food
system (Heller and Keolian 2000) by a factor of 1.6. The material handing equipment and logistics
needed to deliver this biomass to energy facilities represents a great opportunity for agricultural
equipment and truck manufacturers, but an equally great challenge for rural business development
and transportation infrastructure.
The current generation of biomass harvest and logistics systems has been adapted from those
developed for agricultural (food, animal feed) and forest (lumber, paper). Although existing harvest
technologies and procedures for hay and straw in agricultural production systems can be utilized
for herbaceous biomass, they have not been optimized for the distances and scales that bioenergy
facilities are expected to require. To supply sufficient feedstock for large facilities, bioenergy crops
are likely to be transported over longer distances than traditional on-farm uses such as animal
feeding or bedding; these materials may also need to be stored in large centralized facilities for
future use. Integrated systems that minimize material handling steps and maximize feedstock
flexibility are needed to reduce the costs of harvesting, storing, and transporting agricultural
feedstocks (Hess et al. 2007).
Such integrated systems are beginning to be established for woody biomass energy, building on
technologies and equipment developed for timber management and the pulp and paper industry.
This sector has long dealt with very large volumes as well as long-distance transport, including
intercontinental shipments of pulp, sawlogs, and woodchips for manufacturing paper and lumber
products. With expanding bioenergy markets, chips are now hauled hundreds of kilometers to
biomass power facilities, whereas pellets are shipped from North America to Europe. Typical
woody biomass operations include felling, delimbing, debarking, chipping, and transporting.
For short-rotation trees such as willow and poplar, agricultural machinery manufacturers have
developed oversized forage harvesters and wood choppers to harvest short-rotation trees. Woody
biomass materials are naturally denser than herbaceous plants, so woody biomass tends to store
more easily and is less expensive to transport. Woody biomass materials also have less need for
storage because of a longer harvest season relative to agricultural crop residues and herbaceous
energy crops. These advantages simplify woody biomass systems relative to herbaceous biomass
management, as illustrated in Figure 7.1.
7.2 herBaceous BIomass harvest
Herbaceous biomass refers to lignocellulosic plant matter that is nonwoody and includes agricultural
residues such as corn stover and wheat straw and dedicated energy crops such as switchgrass and
miscanthus. Dual-use crops such as alfalfa have a relatively short harvesting window if used as
forage because of changing levels of protein during growth (Muck et al. 2010). When forages
are fed to livestock, nutrient levels are of utmost importance. Although the criteria will likely be
somewhat different, crop maturity and quality will also be critical for biochemical conversion to
energy. But for combustion and thermochemical conversion, biomass crops can be harvested based
upon convenience and yield. Depending on the location, weather greatly affects the harvesting
window, crop quality, and mass loss of herbaceous biomass feedstocks. Thus, the harvest and
logistics systems must take into account the crop, its intended use, and local conditions to maintain
an efficient and cost-effective value chain.
7.2.1 m owing or S wathing
Mowing is the initial step in harvesting herbaceous crops. Mowing can be performed with a trac-
tor mounted implement or a self-contained unit. In the United States, disc mowers are common
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