|
| Chemical | Mode of action | Advantages | Disadvantages/limitation | Remarks |
|
| Dilute acid |
| Sulfuric acid, phosphoric acid [36] | (1) Removal of hemicellulose | (1) Higher reaction rates
(2) Increase the accessibility of cellulose | (1) Form by-product (fermentation inhibitors)
(2) High cost and expensive construction material due to acidic environment
(3) Corrosive to reactor | Minimal degradation of lignin and cellulose |
|
| Concentrated acid |
| Sulfuric acid, phosphoric acid [37] | (1) Solubilisation of hemicellulose and direct hydrolysis of cellulose to glucose | (1) Suitable to all types of biomass | (1) Uncontrolled hydrolysis process
(2) Corrosive to reactor | Suitable for the glucose synthesis (saccharification of biomass) |
|
| Alkaline hydrolysis |
| Sodium hydroxide, calcium hydroxide [38, 39] | (1) Removal of lignin (major)
(2) Removal of hemicellulose
(3) Cellulose swelling | (1) High solubilisation of lignin
(2) Low formation fermentation inhibitors | (1) High cost of chemical
(2) Alteration of lignin structure | Suitable to use prior to direct fermentation of carbohydrates |
|
| Organosolv |
| Mixture of organic solvent and water [40] | (1) Extraction of lignin
(2) Complete solubilisation of hemicellulose | (1) High recovery of lignin
(2) Organic solvent used can be recycled and reused
(3) No grinding/milling of biomass feedstock
(4) Selective pretreatment method for lignin extraction | (1) High cost of solvent
(2) High energy consumption during solvent recovering process | Suitable for lignin fractionation process where high content of lignin can be recover for specialty chemical synthesis |
|
| Ionic liquid |
| Imidazolium salts [41] | (1) Extraction of lignin
(2) Decrease the cellulose crystallinity index
(3) Carbohydrate dissolution | (1) IL is high thermal stability and low volatility | (1) High cost of chemicals. | The effects towards hemicellulose and lignin are depending on the nature of ionic liquid used |
|
| Oxidative delignification |
| Hydrogen peroxide [42] | (1) Solubilisation of lignin and hemicellulose.
(2) Bleaching effect to the pulp | (1) Efficient in removal of lignin
(2) Increase biomass digestibility | (1) High costs of chemicals | Suitable for cellulose bleaching where lignin and hemicellulose will degrade in the presence of alkali |
|
