Production of Ethanol from Lignocellulosic Biomass

Bioconversion of lignocellulosics to ethanol consists of four major unit operations:

• Pretreatment

• Hydrolysis

• Fermentation

• Product separation/distillation.

3.1.3.1 Pretreatment

Pretreatments employed can be divided into physical, chemical, and biologi­cal methods, but there is a strong inter-dependence of these processes. There is not a perfect pretreatment method employed and remaining bottlenecks include generation of inhibitory chemicals (acids, furans, phenols), high particle load, high energy input and efficient separation of soluble sugars from solid residues. Specific pretreatment conditions are required for individual feedstocks, and mech­anistic models can help in the rational design of such processes (Zhang et al. 2009; Eggeman and Elander 2005). It is especially important to optimize lignocel — lulose pretreatment methods because they are one of the most expensive steps in the overall conversion to bioethanol. For example, Mosier et al. (2005) reported that pretreatment accounts for ~30 US cents/gallon of cellulosic ethanol produced. Pretreatment is required to alter the biomass macroscopic and microscopic size and structure as well as its submicroscopic chemical composition and structure so that hydrolysis of carbohydrate fraction to monomeric sugars can be achieved more rapidly and with greater yields (Sun and Cheng 2002; Mosier et al. 2005; Tucker et al. 2003). Pretreatment affects the structure of biomass by solubilizing hemicellulose, reducing crystallinity and increase the available surface area and pore volume of the substrate. Pretreatment has been considered as one of the most expensive processing steps in biomass to lowering the cost of pretreatment process through extensive R&D approaches. Pretreatment of cellulosic biomass in cost — effective manner is a major challenge of cellulose to ethanol technology research and development. Native lignocellulosic biomass is extremely recalcitrant to enzy­matic digestion. Therefore, a number of thermochemical pretreatment methods have been developed to improve digestibility (Wyman et al. 2005). Recent stud­ies have clearly proved that there is a direct correlation between the removal of lignin and hemicellulose on cellulose digestibility (Kim and Holtzapple 2006). Thermochemical processing options appear more promising than biological options for the conversion of lignin fraction of cellulosic biomass, which can have a detrimental effect on enzyme hydrolysis. It can also serve as a source of process energy and potential co-products that have important benefits in a life-cycle con­text (Sheehan et al. 2003). Pretreatment can be carried out in different ways such as mechanical combination, steam explosion, ammonia fiber explosion, acid or alkaline pretreatment and biological treatment, organosolv pretreatment (Cadoche and Lopez 1989; Gregg and Saddler 1996; Kim et al. 2003; Damaso et al. 2004; Kuhad et al. 1997; Keller et al. 2003; Itoh et al. 2003).

To asses the cost and performance of pretreatment methods, technoeconomic analysis have been made (Eggerman and Elander 2005). There is huge scope in lowering the cost of pretreatment process through extensive R&D approaches. Pretreatment of cellulosic biomass in cost-effective manner is a major challenge of cellulose to ethanol technology research and development. Native lignocellulosic biomass is extremely recalcitrant to enzymatic digestion. Therefore, a number of thermochemical pretreatment methods have been developed to improve digestibil­ity (Wyman et al. 2005). Recent studies have clearly proved that there is a direct correlation between the removal of lignin and hemicellulose on cellulose digest­ibility (Kim and Holtzapple 2006). Thermochemical processing options appear more promising than biological options for the conversion of lignin fraction of cel — lulosic biomass, which can have a detrimental effect on enzyme hydrolysis. It can also serve as a source of process energy and potential co-products that have impor­tant benefits in a life-cycle context (Sheehan et al. 2003).

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