Transportation - Engineering and Science of Biomass Feedstock Production and Provision - page 150

With a throughput rate of 15-20 ton h −1 attainable by tub grinder hammer mills,
over 6,250 such machines would be required to comminute the three-million tons of
biomass feedstock to an average particle size of 25.4 mm. For a medium-sized
biofuel plant with a daily demand of 2,000 dry tons of feedstock, more than 100 trips
would be required per day [ 11 ].
In a typical bioenergy feedstock production process, the crop is collected and cut
using a harvester, conveyed into wagons that transport materials in fields, and trans-
loaded into trucks that transport the material either directly to a conversion plant or
to a CSP. To illustrate the logistics involved in the production of fuels from bioen-
ergy feedstock, a conceptual biomass provision system that includes a CSP is shown
in Fig. 6.6 . The sequence of events as illustrated is as follows:
1. Farmers deliver the biomass in bale form to the CSP using trucks, since the dis-
tances from the field to the CSP are relatively short. The advantage of using bal-
ers is that they are readily available, farmers are familiar with their workings, can
maintain them, and can capitalize on the advantage that the containment of the
biomass can be accomplished using inexpensive strings or netting and relatively
low-tech machinery. The density that is achievable using modern balers allows
flatbed trucks to reach their volume and weight limits simultaneously.
2. At the CSP, bales are stored and possibly dried using waste heat from the engine
that powers the comminution and pelletization operations. The road traffic from
farms to the CSP is seasonal and intermittent, similar to that occurring during the
harvest season of corn and soybean.
3. The task of the CSP is to preprocess the biomass feedstock through comminu-
tion. This operation runs continuously and is directly followed by a pelletization
operation; in fact, ideally the two operations are combined in a single large
machine, which allows containment of dust while preventing “dust explosions.”
Before pelletizing, the biomass could be treated with bonding agents and poten-
tially pretreatment agents. In addition, the CSP provides storage, loading, and
blending facilities for the pelletized material.
4. An elegant method of conserving energy is to use the biomass itself as the energy
source for the CSP. This would require direct combustion of the pelletized bio-
mass and employing energy conversion such as through a Stirling engine. This is
a constant-power machine, which runs on a temperature differential, where the
“hot” end is created by burning an arbitrary fuel (in this case biomass pellets)
and the “cold” end consists of a heat sink connected to the outside ambient tem-
perature. This is an advantage in colder climates, because the temperature dif-
ferential is naturally higher, compared to more temperate climates. The Stirling
engine also produces “waste heat,” which can be used for biomass drying. After
milling, the pelletized biomass is stored in large bins, similar to current storage
of corn and soybean.
5. The loading of the gravity-flowable pelletized biomass can take place using
classical handling equipment such as augers, chutes, and conveyor belts. In addi-
tion, the CSP can operate bins containing various biomass types and blend them