7.4 Chemical Pre-treatment Methods
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overcome this drawback and to enhance the utilization of lignocellulosic feedstock,
effective fractionation of biomass is of great importance. Chemical pre-treatments
employ chemical reactions to change the structure of lignocellulosic biomass. As
they demonstrate highly efficient biodegradation of complex materials, compared
with other methods (biological or physical pre-treatments) [20, 21], they are widely
used in the fractionation of agricultural and forestry residues into their components
(cellulose, hemicellulose and lignin) for biorefining the abundant and inexpensive
bioresources into biofuels and bio-based chemicals/materials. Agricultural residues
are the byproducts and remnants of harvesting grain crops, including mainly corn
stover, cornstalk, wheat straw, rice straw/husk. Crop residues contribute to at least 50
wt% of the biomass in grown crops [24]. On one hand, they provide essential environmental benefits such as improving or maintain soil quality and moisture when
retained on a field [25], while on the other hand they can be an interesting resource
for biofuel (bio-ethanol) production. Forestry biomass or woody biomass involves
all parts of trees such as trunk, bark, branches, leaves, and even the roots. Since the
forestry biomass contains dominantly carbohydrates (mostly cellulose and hemicellulose) and lignin in its cell wall, it is a promising feedstock for the production of
biofuels and bio-chemicals and bio-based materials via forest biorefinery.
The most commonly used chemical reagents in chemical pre-treatments are
oxidizing agents, acids, alkali, organic solvents, ionic liquids and salts, as detailed
below.
7.4.1 Acidic Pre-treatment
Acidic pre-treatment involves the use of concentrated or diluted acids to disrupt the
recalcitrant structure of the lignocellulosic biomass. Acid pre-treatments are one
of the most effective methods of solubilizing hemicellulose and making cellulose
more accessible [20], hence improving the enzymatic hydrolysis of lignocellulosic
biomass to release fermentable sugars [17, 21]. Hydrolysis of hemicellulose, especially xylan, and condensation and precipitation of solubilized lignin are the main
reactions during the acid pre-treatment process [20]. The process operates under
constant mixing at a temperature range of 130 °C to 210 °C for a time period from a
few minutes to hours. The most commonly used acids for pre-treatments are mineral
acids such as sulfuric, phosphoric, nitric and hydrochloric acid as well as organic
acids such as formic, maleic and oxalic acid [18]. Concentrated acids (30–70%)
are usually used at low temperatures (<160 °C), while dilute acids (0.1–10%) are
usually used at both high temperatures (160–250 °C) for low biomass loadings (5–10
wt%) and low temperatures (<160 °C) for high biomass loadings (10–40 wt%) [17,
18]. The advantages of concentrated acid treatments are high efficiency and high
sugar yields at mild temperatures. However, they are highly corrosive and can affect
the biomass processing cost due to the requirement of corrosion-resistant reactors.
They can also lead to the formation of inhibitory compounds such as furfurals, 5hydroxymethylfurfural (5-HMF), phenolic acids and aldehydes during the treatment.
187
overcome this drawback and to enhance the utilization of lignocellulosic feedstock,
effective fractionation of biomass is of great importance. Chemical pre-treatments
employ chemical reactions to change the structure of lignocellulosic biomass. As
they demonstrate highly efficient biodegradation of complex materials, compared
with other methods (biological or physical pre-treatments) [20, 21], they are widely
used in the fractionation of agricultural and forestry residues into their components
(cellulose, hemicellulose and lignin) for biorefining the abundant and inexpensive
bioresources into biofuels and bio-based chemicals/materials. Agricultural residues
are the byproducts and remnants of harvesting grain crops, including mainly corn
stover, cornstalk, wheat straw, rice straw/husk. Crop residues contribute to at least 50
wt% of the biomass in grown crops [24]. On one hand, they provide essential environmental benefits such as improving or maintain soil quality and moisture when
retained on a field [25], while on the other hand they can be an interesting resource
for biofuel (bio-ethanol) production. Forestry biomass or woody biomass involves
all parts of trees such as trunk, bark, branches, leaves, and even the roots. Since the
forestry biomass contains dominantly carbohydrates (mostly cellulose and hemicellulose) and lignin in its cell wall, it is a promising feedstock for the production of
biofuels and bio-chemicals and bio-based materials via forest biorefinery.
The most commonly used chemical reagents in chemical pre-treatments are
oxidizing agents, acids, alkali, organic solvents, ionic liquids and salts, as detailed
below.
7.4.1 Acidic Pre-treatment
Acidic pre-treatment involves the use of concentrated or diluted acids to disrupt the
recalcitrant structure of the lignocellulosic biomass. Acid pre-treatments are one
of the most effective methods of solubilizing hemicellulose and making cellulose
more accessible [20], hence improving the enzymatic hydrolysis of lignocellulosic
biomass to release fermentable sugars [17, 21]. Hydrolysis of hemicellulose, especially xylan, and condensation and precipitation of solubilized lignin are the main
reactions during the acid pre-treatment process [20]. The process operates under
constant mixing at a temperature range of 130 °C to 210 °C for a time period from a
few minutes to hours. The most commonly used acids for pre-treatments are mineral
acids such as sulfuric, phosphoric, nitric and hydrochloric acid as well as organic
acids such as formic, maleic and oxalic acid [18]. Concentrated acids (30–70%)
are usually used at low temperatures (<160 °C), while dilute acids (0.1–10%) are
usually used at both high temperatures (160–250 °C) for low biomass loadings (5–10
wt%) and low temperatures (<160 °C) for high biomass loadings (10–40 wt%) [17,
18]. The advantages of concentrated acid treatments are high efficiency and high
sugar yields at mild temperatures. However, they are highly corrosive and can affect
the biomass processing cost due to the requirement of corrosion-resistant reactors.
They can also lead to the formation of inhibitory compounds such as furfurals, 5hydroxymethylfurfural (5-HMF), phenolic acids and aldehydes during the treatment.