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Pre-treatment of lignocellulosic biomass for the production of second-generation bioethanol

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A procedure for making soluble the lignin and other polymers responsible for the low digestibility of lignocellulosic materials, in particular rice husks, has been developed by the University of Cadiz research group on "Food Engineering and Technology” (AGR-203). This new procedure utilizes hydrogen peroxide in conditions of high pressure for digesting the material. This pre-hydrolysis stage is essential for extracting maximum amounts of the sugars that are subsequently fermented to produce second-generation bioethanol.

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Background

Lignocellulose is the principal component of biomass, which is the collective term used for the most abundant, economic and renewable source of organic matter in the whole world. Lignocellulosic biomass includes the residues of many large-scale agricultural, forestry and industrial processes, and is generated in very large quantities. Usually these residues have been considered useless waste that must be eliminated, frequently by combustion in the open air. Such combustion is a cause of serious environmental problems.

Among the principal applications that have been found for lignocellulosic biomass is the production of second-generation bioethanol. The new Spanish Plan for Renewable Energy (PER) 2011-2020 contained some quantified estimates of the country's capacity for the production of bioethanol from lignocellulosic materials (what is now termed second-generation bioethanol). This product should become available on the Spanish market around the middle of the decade, and it has been forecasted that by the year 2020 it could account for 13% of the total potential production of bioethanol.

However, at the present time, one of the principal technological obstacles for producing bioethanol from lignocellulosic biomass is in the stage of enzymatic hydrolysis required to obtain the fermentable sugars, since the subsequent fermentation and distillation stages are processes that have already been optimized to a sufficiently high degree.

Description

The pre-treatment of the lignocellulosic material is considered the most costly stage of the process for converting this biomass into ethanol; this pre-treatment has been estimated to account for up to 30% of the total cost. In recent years various different types of treatment have been tried under different conditions, but new technological advances are needed to reduce the cost of the process and make it more competitive. For this reason, improvements in the efficiency of the pre-treatment may offer a significant decrease of the cost of producing lignocellulosic ethanol.

The invention consists of a simple procedure for delignifying lignocellulosic biomass of recalcitrant nature - a procedure that combines in one single stage the action of the alkaline peroxide at moderate concentration and temperature, with a high pressure and short reaction times. This proposed pre-treatment improves the accessibility of the polysaccharides to subsequent enzymatic attack, with the object of finally producing fermentable sugars. Hydrolysis yields of close to 100% are obtained.

The delignification procedure, which is the topic of the invention, can be complemented with a prior stage of conditioning of the raw material, by means of which the size of particle is reduced. This has the effect of making available a greater specific surface for contact with the hydrogen peroxide.

Advantages

• The action of the alkaline peroxide at moderate concentrations has been combined in one single stage.

• The reaction times are short and the operating temperature is moderate.

• The accessibility of the polysaccharides to the subsequent enzymatic attack is improved.

• Hydrolysis yields of close to 100% are obtained.

• Added value can be obtained from agricultural wastes rich in lignocellulose.

• Low cost of implementation.

• Simplicity of design.

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