Biomass Feedstock Storage for Quantity and Quality Preservation - Engineering and Science of Biomass Feedstock Production and Provision - page 177

7.5.1.1 Plant Structure

Kenney [ 35 ] described the components of dry lignocellulosic biomass, their typical
composition, and expected variability. The first is cellulose, 30-50 % of dry matter,
which is hydrolyzed to C6 sugars. The second is hemicellulose, 20-40 % of dry
matter, hydrolyzed to the C5 sugars xylose, arabinose, mannose, and galactose. The
third component is lignin, 15-25 % of dry matter. Lignin coats and protects
the cellulose-hemicellulose complexes from degradation [ 16 ]. These complexes are
the building blocks of secondary plant cell walls, and they consist of cellulose-based
microfibrils coated with hemicellulose and lignin. Chains of microfibrils are pro-
duced six at a time by protein complexes (rosettes) embedded in the cell wall. These
microfibrils have a semicrystalline structure due to the bonding across the chains.
The fibrils are insoluble oligosaccharides, with more than five molecules per poly-
mer. Some amorphous (easy to digest) regions in the fibrils are due to faults in the
order of strands produced by the rosettes. Hemicellulose molecules coat theses
microfibrils made of β-1,4 glucan strands, connected by extensive hydrogen bonds.
Hemicelluloses plasticize the cellulose strands apart to allow for flexibility in the
cell wall. Hemicelluloses also bind with lignin, which covers the fibrils, protects
them from water, gives mechanical reinforcement, and acts as a barrier to microbial
digestion. It is this complex structure, shown in Fig. 7.6 , adapted from Gomez et al.
[ 16 ], that makes the lignocellulosic biomass (plant cell walls primarily) resilient to
break down. According to Kurasawa et al. [ 36 ], NDF (neutral detergent fiber) from
a food sample contains cellulose, hemicellulose, and lignin as cell wall constituents.
ADF (acid detergent fiber) contains most of the cellulose, lignin, a portion of the
pectin substances, and variable but small amounts of the hemicellulose.
Hemicellulose is obtained by subtracting the ADF from the NDF, and the value of
cellulose is estimated by the difference between the values of ADF and lignin.
Since cellulose is a portion of dry matter,

( )

Celluloseloss = % drymatterloss ´ % cellulose db ..

7.5.1.2 Moisture

Higher moisture content increases the elasticity of the biomass, making cutting or
shredding, as part of preprocessing, less effective. In contrast, compaction treat-
ments to increase material density before packing are less effective when biomass is
too dry. A range of biomass moisture content, which does not impede preprocessing
yet facilitates compaction, needs to be determined. In the case of corn stover, rela-
tively dry biomass also causes dry matter losses due to wind erosion and scatter,
compared to wet biomass [ 37 ]. A study by Chaoui et al. [ 12 ] showed that as mois-
ture content decreases, the stiffness of shoot segments from Miscanthus plants
increases. Stiffness affects milling of the harvested biomass.