Transportation - Engineering and Science of Biomass Feedstock Production and Provision - page 138

The production of liquid fuels is achieved through thermochemical/hydrothermal,
biochemical, and/or chemical processes, with or without pretreatment. These pro-
cesses typically produce ethanol or bio-oil as the fuel. The liquid biofuel production
chain consists of three sequential parts: (1) feedstock provision, (2) pretreatment,
and (3) conversion. If the complete liquid biofuel production chain is to be opti-
mized, the biomass material must be preprocessed into a form that optimizes each
of the three parts, but unfortunately they are not necessarily in agreement: The opti-
mal form of the material during the feedstock provision phase is dominated by
handling and transportation requirements. For instance, to optimize the provision
phase, the material should ideally be preprocessed into a form that minimizes losses
(e.g., by limiting dust), flows under gravity to allow the use of traditional conveying
equipment such as augers, chutes, and conveyor belts, and have a sufficiently high
bulk density to ensure that the transportation equipment reaches its weight and vol-
ume limits simultaneously. From this viewpoint, stable biomass consisting of flow-
able particulates of consistent size and shape with a high material density would be
ideal. This concept has been captured in the Uniform Format as defined by Idaho
National Laboratory [ 6 ]. The aim here is to gradually transition from “Conventional
Bale” systems through a “Pioneer Uniform” system, which uses mainly existing
equipment, to the futuristic “Advanced Uniform” system, which provides stable
solid biomass in a blendable, tradable commodity form. The target is to reduce the
cost of delivered biomass from US$100 ton −1 in 2007 to US$30 ton −1 , in 2017 [ 6 ].
To optimize the complete biofuel production chain, the provision phase must pro-
duce materials in a form that are well suited for pretreatment, which must transform
them into a form that allows for optimization of conversion. Pretreatment is a process
in which the structure of the lignin, cellulose, and hemicellulose matrix is broken
down to enable enzymatic activity during hydrolysis. This can be achieved by chem-
ical methods using acids and ionic liquids, using enzymes, using physical methods
(such as the classical steam explosion process used in corn ethanol production), or
by using a combination of chemical and physical methods, such as the ammonia
fiber explosion (AFEX) method [ 7 , 8 ]. The extent to which the pretreatment method
is robust with respect to the biomass form is not known for most bioenergy crops.
Although AFEX employs a physical explosion process, it is sensitive to particle size
in the case of corn stover [ 9 ]. In general, for pretreatment methods that do not incor-
porate physical separation processes, the ideal particle size may be as small as 80 μm.
This is achievable by ball milling the material for a rather long time. However,
research on Miscanthus giganteus has shown that size reduction to such a small size
requires 100 % of the inherent heating value (PIHV) of the material. Therefore, the
optimal particle form for conversion is to a large extent determined by the trade-off
between the energy requirement for comminution and the increased conversion effi-
ciency for smaller particle sizes. As a general rule, the feedstock must be commi-
nuted into particle sizes ranging from 9.35 to 25.4 mm with pretreatment and smaller
than 1 mm without pretreatment. Table 6.1 shows an overview of typical particles
sizes as a function of conversion technologies and feedstock.
The chapter is arranged as follows: Firstly, various types of feedstock currently
either employed or under investigation are addressed. Secondly, preprocessing