Biomass Feedstock Storage for Quantity and Quality Preservation - Engineering and Science of Biomass Feedstock Production and Provision - page 172

gas-powered fans and by exposing the biomass to an air flow at either ambient
temperature or heated conditions. Alternatively, the exhaust gas from harvesters,
other vehicles, or other engines used in biomass handling operations can be injected
into a biomass container with an interior structure designed for maximum exposure
of biomass to air. Such an exhaust gas would be both hot and rich in CO 2 , which
inhibits microbial oxidation. Drying is facilitated as particle size is reduced, due to
higher surface area to volume ratio in the particles from which water is to be evapo-
rated and more disrupted plant tissue from which water can evaporate.
Typically, the targeted moisture level commonly reported in literature for safe,
long-term storage of biomass is 15 % on a dry basis. Additives such as salts may
also be added to allow reaching the inhibiting water activity level while maintaining
higher moisture content. However, drying can be quite expensive, especially at the
scale envisioned for the lignocellulosic biomass feedstock system. The energy cost
depends on the heat capacity of the biomass pile, its moisture content, the ambient
temperature, and relative humidity. Thus, achieving 15 % moisture content may not
always be feasible. Moreover, the relative humidity and temperature have a signifi-
cant impact on the equilibrium moisture content of the biomass. A more fundamen-
tal understanding of the drying process by developing the moisture isotherms as a
function of these factors thus becomes important. Recent literature shows some
interest in generating these moisture isotherms for the novel energy crops [ 25 - 27 ].
A thermodynamic calculation can then be made to compute how much energy is
needed to evaporate enough water from biomass in order to reach a target moisture
level. Well-known sorption/desorption models such as the Chung-Pfost model or
the modified Oswin model may also be fitted to the experimental data. This facili-
tates the drying facility design calculations.
It is important to note that drying also causes dry matter losses before having a
preserving effect on dry matter by inhibiting microbial activity [ 28 ]. This is due in
part to the volatilization of volatile organic compounds during drying. If heated air
is used for drying, the dry matter loss could also be due to the increase in cellulose
hydrolysis into water-soluble sugar as temperature increase catalyzes the activity of
the cellulose enzyme-producing Trichoderma sp., up to a limit. Bacterial oxidation
of sugars also increases with temperature up to a certain limit, adding to the losses
in dry matter. Oxidation rate doubles with every 10 °C increase in temperature [ 22 ].

7.4.2 Compaction and Sealing

Oxidation by aerobic bacteria can be prevented by rapid packing, dense packing,
and sealing [ 29 ]. In the case of pre-compacted silage, the model by Mani et al. [ 20 ]
assumed 1-day pre-seal phase, 1- to 3-month fermentation phase, and 6- to 9-month
oxygen infiltration phase. The maximum dry matter losses were 3 %. When samples
with varying dry matter content were compared, losses increased as silage % dry
matter increased. In Mani et al. [ 20 ] dry matter losses were up to six times higher in
the oxygen infiltration phase than in the fermentation phase. In the case of