Environmental Engineering Reference
In-Depth Information
pelletized for fuel, although on a much smaller scale [12]. Pelletizing new forms of
biomass is challenging, as non-woody biomass generally has more hemicellulose,
less cellulose and lignin than wood, giving it less tensile and compressive strength.
One of the ways to overcome this problem is through specific combination of dif-
ferent types of biomass in a proportion which best suits pellet production [13].
The main issue with the future of the biomass pellet sector is the sustainable and
price-competitive production of pellets for a growing industrial and domestic pellet
market, which place high demands on pellet quality and combustion properties.
Current trends are to combine torrefaction (thermal treatment of biomass) and pel-
letization. Moreover, introducing a broader base of raw materials (i.e. agricultural
residues, energy grass, and mixed biomass resources) would help tackle this issue [9].
3.2.2
Physical Pretreatment of Biomass
3.2.2.1 ScCO
2
and Natural Solvent Extraction
Liquid or supercritical CO
2
supercritical carbon dioxide (ScCO
2
) extractions
have been proposed as alternative techniques [14] for plant oil extraction. ScCO
2
extraction is based on the solvating properties of supercritical CO
2
, which can
be obtained by employing pressure and temperature above the critical point of a
compound, mixture, or element. Extraction by ScCO
2
depends on the intrinsic
tunable nature of supercritical fluids such as temperature and pressure, and some
extrinsic features such as the characteristics of the sample matrix, interaction with
targeted analytes, and many environmental factors [15]. To optimize the process of
ScCO
2
extraction, a large number of variables such as pressure, temperature, flow
rate, and modifier used need to be investigated. Proper control of these parameters
allows the extractability of ScCO
2
to be modified, enabling the process to be applied
in different industrial fields from food to pesticide manufacturing [16]. Moreover,
a broader range of controllable parameters makes the ScCO
2
extraction process more
unique, sensitive, and specific compared to conventional extraction methods [17].
Due to its unique properties, effective isolation and fractionation of valuable
components with low environmental impact can be achieved. Furthermore, ScCO
2
has another advantage in that compounds extracted in this way can be classified
as 'natural' under EU regulations in contrast to those extracted with conventional
organic solvents. In addition, because CO
2
is a major product of fermentation, inte-
grating its use in a biorefinery is advantageous. Moreover, the ScCO
2
extraction
method leaves no solvent residues in the product(s), making the product suitable
for use in food, personal care, or pharmaceutical applications. Products resulting
from this extraction method therefore have major commercial advantages over tra-
ditional extraction methods. Industrial-scale use of ScCO
2
should be applicable
within the biorefinery as it has already been employed commercially for hop extrac-
tion, decaffeination of coffee and dry cleaning. The utilization of ScCO
2
technology
within the biorefinery also enables the recycling of the internally generated CO
2
from the thermal treatment of the biomass residues.
