Abstract
Pretreatment of lignocellulose has received considerable research globally due to its influence on the technical, economic and environmental sustainability of cellulosic ethanol production. Some of the most promising pretreatment methods require the application of chemicals such as acids, alkali, salts, oxidants, and solvents. Thus, advances in research have enabled the development and integration of chemical-based pretreatment into proprietary ethanol production technologies in several pilot and demonstration plants globally, with potential to scale-up to commercial levels. This paper reviews known and emerging chemical pretreatment methods, highlighting recent findings and process innovations developed to offset inherent challenges via a range of interventions, notably, the combination of chemical pretreatment with other methods to improve carbohydrate preservation, reduce formation of degradation products, achieve high sugar yields at mild reaction conditions, reduce solvent loads and enzyme dose, reduce waste generation, and improve recovery of biomass components in pure forms. The use of chemicals such as ionic liquids, NMMO, and sulphite are promising once challenges in solvent recovery are overcome. For developing countries, alkali-based methods are relatively easy to deploy in decentralized, low-tech systems owing to advantages such as the requirement of simple reactors and the ease of operation.
Highlights
Cellulosic or second generation (2G) bioethanol is produced from lignocellulosic biomass (LB) in three main steps: pretreatment, hydrolysis, and fermentation
Pretreatment involves the use of physical processes, chemical methods, physicochemical processes, biological methods, and several combinations thereof to fractionate the lignocellulose into its components
Concentrated H3PO4 is effective at low temperatures, dissolves cellulose in the presence of water, possesses no inhibitory effects on hydrolysis and fermentation, and gives high sugars [93] compared to dilute acid pretreatment [89]
Summary
Cellulosic or second generation (2G) bioethanol is produced from lignocellulosic biomass (LB) in three main steps: pretreatment, hydrolysis, and fermentation. Pretreatment involves the use of physical processes (e.g., size reduction, steaming/boiling, ultrasonication, and popping), chemical methods (e.g., acids, bases, salts, and solvents), physicochemical processes (e.g., liquid hot water and ammonium fibre explosion or AFEX), biological methods (e.g., whiterot/brown-rot fungi and bacteria), and several combinations thereof to fractionate the lignocellulose into its components It results in the disruption of the lignin seal to increase enzyme access to holocellulose [1, 2], reduction of cellulose crystallinity [3, 4], and increase in the surface area [5, 6] and porosity [7, 8] of pretreated substrates, resulting in increased hydrolysis rate. The methods discussed are centred on the use of acids, bases, oxidants, and solvents
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