7.5 Physico-chemical Methods
199
that catalyzes the further hydrolysis of hemicellulose [18, 20]. The moisture content
of biomass, particle size, residence time and temperature are important factors that
affect the efficiency of the SE process [21, 26]. The pre-treatment is commonly
performed in the presence of a catalyst such as H 2 SO 4 , CO 2 , and SO 2 , resulting in
higher sugar recovery, minimal formation of inhibitory compounds and improved
enzymatic hydrolysis [26, 27].
The main advantages of the SE method include low environmental effects, short
processing time, lower energy consumption and limited chemical use. However, it is
associated with poor lignin removal and possible generation of inhibitors at higher
temperatures. In addition, this method was found to be effective for agricultural
residues and hardwood, but less effective for softwood [18, 20, 21].
7.5.2 Ammonia-Based Pre-treatment
The ammonia-based pre-treatments, using liquid ammonia, include Ammonia Fiber
Explosion (AFEX), Ammonia Recycle Percolation (ARP) and Soaking Aqueous
Ammonia (SAA). The AFEX process is similar to the steam explosion method. In this
process, lignocellulosic biomass is treated with liquid ammonia at moderate temperature (60–100 °C) and high pressure (0.7–2.7 MPa). The depressurization of the
system results in the rapid expansion of the ammonia gas, disruption of the biomass
structure and partial de-crystallization of cellulose [11, 18, 20, 26]. The process efficiency strongly depends on the operating conditions, e.g., temperature, blowdown
pressure, moisture content and ammonia loading. The AFEX process results in negligible inhibitor formation compared to other pre-treatment methods. Other advantages of the AFEX process include lower moisture content and complete recovery of
solid material. However, it has low efficiency for high lignin-content biomass (e.g.,
woody biomass) and has a corrosive reaction environment. The ammonia used in the
process needs to be recovered and recycled to make the process more economical
and environmentally friendly [18, 20, 26].
In an ARP process, aqueous ammonia at 5–15 wt% concentration is passed
through a packed-bed reactor containing biomass. A higher temperature (140–
210 °C) and longer retention time are employed in this method compared with
AFEX. The ARP method is capable of removing the majority of lignin and solubilizing more than half of the hemicellulose, while keeping high cellulose recovery.
Compared with the AFEX process, the main disadvantage of the ARP method is its
high energy requirements due to the higher operating temperatures [17, 26, 27]. The
SAA method treats biomass using aqueous ammonia in a batch reactor at low temperatures (30–60 °C). This process decreases the liquid throughput of the pre-treatment
and requires less energy due to its low operating temperatures, but has much lower
treatment effect if compared with AFEX and ARP processes [27].
199
that catalyzes the further hydrolysis of hemicellulose [18, 20]. The moisture content
of biomass, particle size, residence time and temperature are important factors that
affect the efficiency of the SE process [21, 26]. The pre-treatment is commonly
performed in the presence of a catalyst such as H 2 SO 4 , CO 2 , and SO 2 , resulting in
higher sugar recovery, minimal formation of inhibitory compounds and improved
enzymatic hydrolysis [26, 27].
The main advantages of the SE method include low environmental effects, short
processing time, lower energy consumption and limited chemical use. However, it is
associated with poor lignin removal and possible generation of inhibitors at higher
temperatures. In addition, this method was found to be effective for agricultural
residues and hardwood, but less effective for softwood [18, 20, 21].
7.5.2 Ammonia-Based Pre-treatment
The ammonia-based pre-treatments, using liquid ammonia, include Ammonia Fiber
Explosion (AFEX), Ammonia Recycle Percolation (ARP) and Soaking Aqueous
Ammonia (SAA). The AFEX process is similar to the steam explosion method. In this
process, lignocellulosic biomass is treated with liquid ammonia at moderate temperature (60–100 °C) and high pressure (0.7–2.7 MPa). The depressurization of the
system results in the rapid expansion of the ammonia gas, disruption of the biomass
structure and partial de-crystallization of cellulose [11, 18, 20, 26]. The process efficiency strongly depends on the operating conditions, e.g., temperature, blowdown
pressure, moisture content and ammonia loading. The AFEX process results in negligible inhibitor formation compared to other pre-treatment methods. Other advantages of the AFEX process include lower moisture content and complete recovery of
solid material. However, it has low efficiency for high lignin-content biomass (e.g.,
woody biomass) and has a corrosive reaction environment. The ammonia used in the
process needs to be recovered and recycled to make the process more economical
and environmentally friendly [18, 20, 26].
In an ARP process, aqueous ammonia at 5–15 wt% concentration is passed
through a packed-bed reactor containing biomass. A higher temperature (140–
210 °C) and longer retention time are employed in this method compared with
AFEX. The ARP method is capable of removing the majority of lignin and solubilizing more than half of the hemicellulose, while keeping high cellulose recovery.
Compared with the AFEX process, the main disadvantage of the ARP method is its
high energy requirements due to the higher operating temperatures [17, 26, 27]. The
SAA method treats biomass using aqueous ammonia in a batch reactor at low temperatures (30–60 °C). This process decreases the liquid throughput of the pre-treatment
and requires less energy due to its low operating temperatures, but has much lower
treatment effect if compared with AFEX and ARP processes [27].