Transportation - Engineering and Science of Biomass Feedstock Production and Provision - page 149

pelletization has been removed. This effect can amount to over 30 %. Taking both
post-compression rebound and porosity into account, to achieve a desired bulk den-
sity of pelletized material, the “in-mold” density of the material must be over twice
as high. In addition, when biomass material flows through channels at extreme pres-
sure levels, the material can potentially self-combust [ 35 ]: The compressed sample
shown on the right side of Fig. 6.4 exhibits areas where the biomass was charred due
to excessive heat during compression. This heat caused localized pyrolysis in which
volatile gases are produced. These gases subsequently ignited, which led to an
explosion.

6.3.5 Storage

Lignocellulosic biomass is typically harvested in a short-time window of about 2-3
months. Since the aim of the conversion plant is to produce fuel year round, storage
of biomass is needed. The issues of the location of the storage operation within the
provision chain, the conditions of storage, and the infrastructure required are still
being addressed in research. Due to the low energy density and monetary value of
the material, uncovered outdoor stacking of bales may be most economical, but
long-term uncovered storage will incur quantity losses and possibly the emergence
of fungi and molds that are detrimental in the conversion process. Bales, either
round or square, may be a viable option before comminution, since inexpensive
twine is sufficient for containment. Wrapping bales in plastic for long-term storage
may prevent biomass loss, while allowing a high bale density and stacking on
unprepared grounds, but it requires low moisture content and is expensive. If stor-
age is to take place after comminution, the biomass must be preprocessed into a
self-contained compressed form such as pellets or briquettes, since storage of
powdered low-density biomass would be very inefficient. This is the idea behind the
Advanced Uniform Format as proposed by the Idaho National Laboratory [ 6 ].

6.4 Transportation

The success and sustainability of the biofuel industry depend largely upon an effi-
cient feedstock provision system, in which transportation plays a key role [ 6 ].
While it will provide a huge economic opportunity for communities across the
United States, harvest, preprocessing, storage, and transportation of massive
amounts of biomass will be challenging [ 38 ]. Dependent upon the biomass densifi-
cation level and transportation mode, transportation represents between 13 and
28 % of the feedstock provision costs, which limits the collection area. Taking into
account the US Department of Transportation's legal load limit of 21.8 tons for on-
road transportation, at least 150,000 road trips will be required per day by 2030 to
transport three million tons of biomass feedstock from farms to biorefineries.