Encyclopedia of Renewable Energy. James G. Speight

Acid hydrolysis is the means by which cellulosic material can be converted to lower molecular weight products and thence to fuels. Although the chemical transformation steps can be complex, using polysaccharide derivatives (such as starch) The overall process can be represented by a series of simplified steps (Table A-3):

Table A-3 Simplified steps that represent the conversion of polysaccharide derivatives to methane.

      Large amounts of inexpensive and renewable lignocellulose waste are generated each year from forestry and agricultural production. These include fruit processing residues, dairy industry wastes, corn and sugar by-products, and paper industry wastes. Lignocellulosic biomass can be converted to biofuels such as ethanol and hydrogen via simple sugars. Lignocellulose is a composite of cellulose fibres embedded in a cross-linked lignin-hemicellulose matrix. It gives structure and strength to plants. Although abundant, lignocellulosic waste is difficult to convert to fermentable sugars because of its complex chemical structure: its three major components (cellulose (crystalline and amorphous) hemicellulose and lignin) must be processed separately (Lee et al., 2007).

      Cellulose consists of linear, highly ordered chains of glucose and is one of the most abundant biopolymers on the planet. When produced by plants, it has both highly amorphous regions containing large voids and other irregularities as well as tightly packed crystalline regions. It also accumulates in the environment because it is resistant to most forms of degradation. Hemicellulose is a complex polymer of a variety of sugars. This mix of sugars mainly consists of six-carbon and five-carbon sugars. Lignin is a poly-phenolic polymer that fills the spaces in the cell wall between cellulose, hemicellulose, and pectin components. It is covalently linked to hemicellulose and crosslinks different plant polysaccharides, conferring mechanical strength to the cell wall, and by extension, the plant as a whole. The content of cellulose, hemicellulose and lignin in common agricultural residues depends upon the source and origin of the feedstocks. However, because lignocellulosic biomass is so complex, it is difficult to use as a feedstock for biofuel. A variety of physical, chemical, and enzymatic processes have been developed to fractionate lignocellulose into the major plant components of hemicellulose, cellulose, and lignin.

Ethanol is produced from lignocellulose via pre-treatment, hydrolysis, fermentation, and distillation. The goal of pre-treatment is to increase the surface area of lignocellulosic material, making the polysaccharides more susceptible to hydrolysis, by separating the xylose and lignin from the crystalline cellulose. Pre-treatment must also avoid the degradation or loss of precious carbohydrates and avoid the formation of by-products that can inhibit subsequent steps. However, pre-treatment often produces biological inhibitors, which affect fermentation. A large variety of pre-treatment processes have been developed. Common pre-treatments are steam-explosion, acid treatment, biological methods, and comminution. These methods can be used singly or in combination.

      Steam explosion involves saturation of the pores of plant materials with steam followed by rapid decompression. The explosive expansion of steam reduces the plant material to separated fibres, increasing accessibility of polysaccharides to subsequent hydrolysis. Ammonia fibre explosion (AFEX) is similar to steam explosion except that liquid ammonia is used. It is effective on agricultural residues but has not been successful in pre-treating woody biomass. Biological pre-treatments employ microorganisms that produce lignin-degrading enzymes (ligninase). Comminution is an integral part of pre-treatment and uses a hammer mill to produce particle sizes that can pass through 3 mm screen openings.

      The mechanisms by which pre-treatments improve the digestibility of lignocellulose are not well understood. Pre-treatment effectiveness has been correlated with removal of hemicellulose and lignin (lignin solubilisation is beneficial for subsequent hydrolysis but may also produce derivatives that inhibit enzyme activity). Some pre-treatments reduce the crystallinity of cellulose, which improves reactivity, but this does not appear to be the key for many successfully pre-treatments.

      Hemicellulose is readily hydrolysed to pentoses (5 carbon sugars) but pentoses are difficult to ferment. The cellulose hydrolyses to hexoses (6 carbon sugars). Crystalline cellulose is difficult to hydrolyse but the resulting hexose derivatives are readily fermented. The three basic methods for hydrolysing structural polysaccharides to fermentable sugars (glucose, xylose, arabinose) are concentrated acid hydrolysis, dilute acid hydrolysis and enzymatic hydrolysis.

      Acid treatment is the use of acid to hydrolyze cellulosic materials. Two acid processes hydrolyze both hemicellulose and cellulose with minimal pre-treatment beyond comminution of the lignocellulosic material to particles of approximately 1 mm in size. These are concentrated and dilute acid hydrolysis. Concentrated acid hydrolysis dissolves carbohydrates in woody biomass to form a homogeneous gelatine with acid where cellulose is extremely susceptible to hydrolysis. Typically, this is achieved with 70 to 90% v/v sulfuric acid (H₂SO₄) at room temperature, leaving lignin. Before fermentation, the solution of oligosaccharides is diluted to 4% v/v sulfuric acid and (i) heated (at the boiling point) for four hours or (ii) processed using an autoclave at 120°C (248°F) for one hour to yield monosaccharides. Following neutralisation with limestone, the sugar solution is fermented. The procedure takes 10 to 12 hours. Concentrated acid hydrolysis is attractive because it is relatively simple, and high sugar yields (approach 100% of theoretical hexose yields in pure samples and 90% in mixed samples which replicate municipal waste mixtures) are achieved. However,

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