86
3 Advanced Technologies (Biological and Thermochemical) …
acid or base-catalyzed hydrolysis of cellulose, hemicellulose and lignin [40]. Most of
the hemicellulose could be removed through solubilization and only a small amount
of lignin remained, and neutralizing the pH would help in decreasing the inhibitor
formation [102].
3.2.3.2 Chemical Methods
Chemical methods include acid and alkaline hydrolysis, oxidation and ozonation,
treatment with ionic liquids and treatment with solvents (organic and other). Acid pretreatment increases the sugar substrate digestibility, which is defined as the concentration of reducing sugars after the hydrolysis. Common acids such as sulfuric (H 2 SO 4 )
and hydrochloric (HCl) are used to convert the feedstock into reduced sugars.
Dilute acid pre-treatments are usually preferable because high acid concentration
can degrade the fermentable sugars and form undesirable compounds. During alkali
pre-treatment, biomass undergoes swelling, the crystallinity index decreases and the
specific area of biomass increases, which would assist the hydrolysis and fermentation processes. Alkali solutions such as sodium hydroxide (NaOH), potassium
hydroxide (KOH), sodium carbonate (Na 2 CO 3 ), etc., are commonly used to pre-treat
the biomass. The efficiency of alkaline pre-treatment is higher than that of the acid
hydrolysis at the same temperature and hydrolysis time. Oxidizing pre-treatments
use peroxides or alcohol solution of peracids to dissolve the lignin and amorphous
cellulose in lignocellulosic biomass. Ozonation is another pre-treatment method,
which is more expensive than other methods. Ozone is a strong oxidizing agent
and removes lignin and partially the hemicellulose of the lignocellulosic biomass.
Ionic liquids (ILs) are a class of non-molecular compounds that are composed solely
of ions. Treatments with ionic liquids are very few as they are rather expensive
solvents. Treatments with organic solvents are to remove lignin and hemicellulose
of the lignocellulosic biomass. The common organic solvents are low boiling point
substances such as ethanol, glycerin, ethers, ketones, phenols, organic acids and
dimethylsulfoxide [98, 101].
3.2.3.3 Physical Pre-treatments
Physical pre-treatments increase the substrate accessibility to enzymes by reducing
the biomass particles and by breaking the cell walls. Since the surface area of biomass
increases by mechanical pre-treatments, they have the potential to improve the efficiency of the subsequent physical or chemical processes. Some types of physical
pre-treatments include the following:
– Fragmentation (grinding, milling, rolling) in which the biomass breaks into
smaller parts and the cellulosic structure is exposed to enzymes;
– Microwave radiation in the range of 300–700 W/m
3 which reduces the cellulose
crystal structure of lignocellulosic biomass;
3 Advanced Technologies (Biological and Thermochemical) …
acid or base-catalyzed hydrolysis of cellulose, hemicellulose and lignin [40]. Most of
the hemicellulose could be removed through solubilization and only a small amount
of lignin remained, and neutralizing the pH would help in decreasing the inhibitor
formation [102].
3.2.3.2 Chemical Methods
Chemical methods include acid and alkaline hydrolysis, oxidation and ozonation,
treatment with ionic liquids and treatment with solvents (organic and other). Acid pretreatment increases the sugar substrate digestibility, which is defined as the concentration of reducing sugars after the hydrolysis. Common acids such as sulfuric (H 2 SO 4 )
and hydrochloric (HCl) are used to convert the feedstock into reduced sugars.
Dilute acid pre-treatments are usually preferable because high acid concentration
can degrade the fermentable sugars and form undesirable compounds. During alkali
pre-treatment, biomass undergoes swelling, the crystallinity index decreases and the
specific area of biomass increases, which would assist the hydrolysis and fermentation processes. Alkali solutions such as sodium hydroxide (NaOH), potassium
hydroxide (KOH), sodium carbonate (Na 2 CO 3 ), etc., are commonly used to pre-treat
the biomass. The efficiency of alkaline pre-treatment is higher than that of the acid
hydrolysis at the same temperature and hydrolysis time. Oxidizing pre-treatments
use peroxides or alcohol solution of peracids to dissolve the lignin and amorphous
cellulose in lignocellulosic biomass. Ozonation is another pre-treatment method,
which is more expensive than other methods. Ozone is a strong oxidizing agent
and removes lignin and partially the hemicellulose of the lignocellulosic biomass.
Ionic liquids (ILs) are a class of non-molecular compounds that are composed solely
of ions. Treatments with ionic liquids are very few as they are rather expensive
solvents. Treatments with organic solvents are to remove lignin and hemicellulose
of the lignocellulosic biomass. The common organic solvents are low boiling point
substances such as ethanol, glycerin, ethers, ketones, phenols, organic acids and
dimethylsulfoxide [98, 101].
3.2.3.3 Physical Pre-treatments
Physical pre-treatments increase the substrate accessibility to enzymes by reducing
the biomass particles and by breaking the cell walls. Since the surface area of biomass
increases by mechanical pre-treatments, they have the potential to improve the efficiency of the subsequent physical or chemical processes. Some types of physical
pre-treatments include the following:
– Fragmentation (grinding, milling, rolling) in which the biomass breaks into
smaller parts and the cellulosic structure is exposed to enzymes;
– Microwave radiation in the range of 300–700 W/m
3 which reduces the cellulose
crystal structure of lignocellulosic biomass;