200
7 Resource Utilization of Agricultural/Forestry Residues …
7.5.3 Supercritical Fluid Pre-treatment
Supercritical fluids are either liquid or gas above their critical temperature and pressure where distinct liquid and gas phases do not exist. At these conditions, they have
unique properties such as liquid-like density and gas-like diffusivity and viscosity.
Thus, they are suitable for penetrating the crystalline structure of lignocellulosic
biomass and overcoming the mass transfer limitations associated with other pretreatment methods [26]. Supercritical carbon dioxide (CO 2 ) has been commonly
used as a solvent for pre-treatment of various lignocellulosic biomass. It diffuses
through the interspaces of biomass like a gas and dissolves material like a liquid
solvent. Compared to the steam explosion and ammonia explosion methods, supercritical CO 2 is less corrosive and has a lower energy requirement [18]. Once dissolved
in water, CO 2 forms carbonic acid which flavors hemicellulose hydrolysis. Explosive
release of CO 2 disrupted the hemicellulose and lignin structures and improved the
accessibility of cellulose fibers [18, 26]. The lower temperatures of the pre-treatment
resulted in the stability of sugars and prevented further degradation by acids [26].
The advantages of supercritical CO 2 treatment include low cost, low environmental impact, no toxic material formation and easy recovery. However, the process
is yet to be applied on industrial scale [18, 26, 27].
7.5.4 Hydrothermal Processes
Hydrothermal processes use water for pre-treatment of lignocellulosic biomass.
No chemicals or catalysts are needed during the process. The hydrothermal treatments lead to dissolution and degradation of hemicellulose and removal of lignin,
thus, making the cellulose more accessible. Depending on the operating conditions,
these processes are divided into: (1) Hot Water Extraction (HWE), Pressurized Hot
Water Extraction (PHWE), Liquid Hot Water Pre-treatment (LHW), Hydrothermal
Carbonization (HTC) and Hydrothermal Liquefaction (HTL) [20]. A short description of each process is as following: HWE is operated at temperatures lower than
100 °C and is used to extract water-soluble components of biomass such as pectin and
tannin. PWHE is operated at 150–180 °C and used for reducing the hemicellulose
content of forestry biomass as a pre-treatment for pulp-dissolving processes. The
LHW process is carried out at 140–230 °C and used for partial dissolution of hemicellulose and lignin, which results in a reduction of the structure stability. During
the HTC process, operated at 180–250 °C, the biomass is converted into carbonaceous materials with enhanced heating values. The reactions taking place in HTC
treatment include hydrolysis, dehydration, decarboxylation, polymerization, aromatization, and condensation. The HTL process is operated at temperatures above
250 °C and converts the biomass into biocrude oils [20].
The LHW process is very much alike to the steam explosion method; however,
rapid depressurization is not required and the pressure applied in the process is only
7 Resource Utilization of Agricultural/Forestry Residues …
7.5.3 Supercritical Fluid Pre-treatment
Supercritical fluids are either liquid or gas above their critical temperature and pressure where distinct liquid and gas phases do not exist. At these conditions, they have
unique properties such as liquid-like density and gas-like diffusivity and viscosity.
Thus, they are suitable for penetrating the crystalline structure of lignocellulosic
biomass and overcoming the mass transfer limitations associated with other pretreatment methods [26]. Supercritical carbon dioxide (CO 2 ) has been commonly
used as a solvent for pre-treatment of various lignocellulosic biomass. It diffuses
through the interspaces of biomass like a gas and dissolves material like a liquid
solvent. Compared to the steam explosion and ammonia explosion methods, supercritical CO 2 is less corrosive and has a lower energy requirement [18]. Once dissolved
in water, CO 2 forms carbonic acid which flavors hemicellulose hydrolysis. Explosive
release of CO 2 disrupted the hemicellulose and lignin structures and improved the
accessibility of cellulose fibers [18, 26]. The lower temperatures of the pre-treatment
resulted in the stability of sugars and prevented further degradation by acids [26].
The advantages of supercritical CO 2 treatment include low cost, low environmental impact, no toxic material formation and easy recovery. However, the process
is yet to be applied on industrial scale [18, 26, 27].
7.5.4 Hydrothermal Processes
Hydrothermal processes use water for pre-treatment of lignocellulosic biomass.
No chemicals or catalysts are needed during the process. The hydrothermal treatments lead to dissolution and degradation of hemicellulose and removal of lignin,
thus, making the cellulose more accessible. Depending on the operating conditions,
these processes are divided into: (1) Hot Water Extraction (HWE), Pressurized Hot
Water Extraction (PHWE), Liquid Hot Water Pre-treatment (LHW), Hydrothermal
Carbonization (HTC) and Hydrothermal Liquefaction (HTL) [20]. A short description of each process is as following: HWE is operated at temperatures lower than
100 °C and is used to extract water-soluble components of biomass such as pectin and
tannin. PWHE is operated at 150–180 °C and used for reducing the hemicellulose
content of forestry biomass as a pre-treatment for pulp-dissolving processes. The
LHW process is carried out at 140–230 °C and used for partial dissolution of hemicellulose and lignin, which results in a reduction of the structure stability. During
the HTC process, operated at 180–250 °C, the biomass is converted into carbonaceous materials with enhanced heating values. The reactions taking place in HTC
treatment include hydrolysis, dehydration, decarboxylation, polymerization, aromatization, and condensation. The HTL process is operated at temperatures above
250 °C and converts the biomass into biocrude oils [20].
The LHW process is very much alike to the steam explosion method; however,
rapid depressurization is not required and the pressure applied in the process is only