Environmental Engineering Reference
lignin in this study, it is possible that chemical alteration could be part of the lignin
removal and transport process because lignin can partially dissolve and react in acid
solutions under appropriate conditions . It is further possible that part of this
mobilized lignin could contain lignin-carbohydrate complexes that might sequester
cellulases as observed in some studies [34, 36].
Another recent study  showed that purified lignin preparations as well as
native lignin from corn stover could be redeposited onto clean cellulose surfaces
such as filter paper. More severe pretreatments (higher temperature or acid concen-
trations) resulted in finer redeposited droplets. Under these conditions, digestibility
of filter paper was lower by up to 15% in comparison with treatments that did not
contain lignin. Since these digestions were performed at very high enzyme load-
ings to circumvent issues related to non-productive binding to lignin, it appears that
physical blockage of the cellulose surface by lignin resulted in lower digestibility.
Although redeposited lignin inhibited digestion of pure cellulose substrates in the
study by Selig and coworkers , it is also probable that the mass transport of
lignin could enhance enzymatic cellulose degradation in biomass. For example, we
could visualize that as a result of lignin mass transport, the lignin sheath coating
cellulose surfaces gets concentrated into droplets rendering a greater cellulose sur-
face area available for enzymatic attack. Removal of lignin could also improve cell
wall porosity allowing enzymes better access for penetration. Much work needs to
be done to completely understand the nature and implications of lignin transport.
5 Rheology of Biomass Slurries and Implications for Mixing
Uniform distribution of heat, chemical catalysts, and enzymes as well as absence
of product gradients within conversion reactors are all dependent on the mixing
properties of biomass slurries being processed, which in turn are determined by
rheological characteristics. Biomass rheology poses several challenges because of
the fibrous nature of the particles, their ability to absorb water and become unsatu-
rated at relatively low solid concentrations of 25-35% (w/w), and the continually
changing particle chemical/physical properties during flow through the process.
Free water content appears to be the largest factor contributing to slurry rheology.
This is especially true at the high solid concentrations that are desired to make the
overall process economical by lowering equipment volume and thereby cost .
At solid concentrations beyond the point of unsaturation, the slurries become wet
granular material that agglomerate and can compact under their own weight if not
adequately mixed. At lower concentrations, adequate mixing is still required to pre-
vent settling. To further complicate matters, as biomass gets broken down into its
constitutive sugars, changes occur in particle size as well as chemical properties.
Water retaining polymers, such as hemicellulose and pectin, are broken down and
the previously hygroscopic biomass has lower capacity for water absorption result-
ing in an increased amount of free water, and thereby altered slurry rheology. These
dynamic changes in solid properties necessitate studies to understand rheological
behavior of slurries through various process treatments.