Biomass Feedstock Storage for Quantity and Quality Preservation - Engineering and Science of Biomass Feedstock Production and Provision - page 176

Table 7.3 Effect of pH
on the production of ethanol
from glucose as catalyzed
by an enzyme a

Ethanol production (g/l)
with β -glucosidase in 3-4 days
3.5 0.5
4.5 9.1
6.0 7.6
a Adapted from Christakopoulos et al. [ 34 ]

pH

7.4.6.3 Carbon Dioxide

Carbon dioxide (CO 2 ) is an additive that can be obtained from the exhaust of engines
used in harvesting, transporting, and preserving biomass. Such exhaust gases would
have high temperature that can dry biomass. However, CO 2 is also an inhibitory
agent to aerobic respiration. This exhaust gas can be routed from the engines into
closed storage containers. A pressure-release valve can be added to the conduits to
prevent exhaust gas backflow into the engine when gas pressure becomes too high
in the storage containers. Organic matter-based biofilters can also be added to the
storage container to allow CO 2 gas to precipitate or be fixed into the organic matter
as the excess gas exits into the atmosphere. The economic feasibility of such a sys-
tem, especially for large scale biomass storage, will need to be studied.

7.5 Reduction of Biomass Recalcitrance to Breakdown

As mentioned in the introduction, the goal of biomass feedstock storage is not only
to preserve total biomass dry matter and the carbohydrates, but also to effect
changes that aid further processing of biomass to ethanol or other products. The
proposal of regional or centralized storage and preprocessing centers uses this con-
cept to combine preprocessing with storage. Therefore, it is important to summa-
rize the main factors that make biomass recalcitrant, thereby creating barriers for
decomposition. A better understanding of the recalcitrance will enable selective
breakdown of biomass to recalcitrance while preserving the total dry matter and
carbohydrate content. This section presents the important source of recalcitrance
and the options to reduce it.

7.5.1 Sources of Recalcitrance

Recalcitrance is strongly affected by plant structure with reference to lignin, cellu-
lose, and hemicellulose components. In addition, moisture content and particle size
both play an important role in both material compression and preprocessing.