Depending on their origin, the most promising lignocellulosic materials regard­ing their conversion into fuel ethanol can be classified into seven big groups (see Table 3.10):

1. Agricultural residues

2. Agro-industrial residues

3. Hardwood

4. Softwood

5. Herbaceous biomass

6. Cellulosic wastes

7. Municipal solid waste

The agricultural residues comprise those lignocellulosic materials derived from the cropping and harvesting of plant species with economic importance. In particular, the exploitation of cereal plantations implies the generation of a great

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FIGURE 3.9 Schematic diagram of the structure of a lignocellulosic complex.

Classification of Some Materials with High Content of Lignocellulosic Biomass

volume of residues, among which the straw should be highlighted. The straw is the dried, crushed or not crushed material coming from plants of the family Gramineae once it is separated from the grain. The straws most evaluated for ethanol production purposes are wheat straw, rice straw, and barley straw. The agro-industrial residues refer to the by-products and wastes generated during the commercial transformation of agricultural crops. The bagasse should be listed among these residues. The most studied agro-industrial residue is the sugarcane bagasse, the fibrous residue obtained after juice extraction during the milling step of sugarcane. Other promising residue is the sweet sorghum bagasse. Other mate­rials belonging to this category are corn stover and olive stone and pulp.

The materials having origin in hardwood constitute a separate group of ligno — cellulosic feedstocks. These materials comprise not only the wood itself, but also its derivatives like sawdust, shavings, and the collected biomass resulting from forestry activities, such as branches, stalks, and trunk pieces. The wood obtained from trees of the angiosperm species—poplar, eucalyptus, aspen, oak, maple, birch, rosewood, and mahogany—belongs to hardwood. The softwood, in turn, comprises the wood of conifers. The wood from trees of the gymnosperm spe­cies, such as pine, fir, spruce, larch, and cedar, can be included in this group. Softwood has higher lignin content than hardwood.

The herbaceous biomass refers to the materials coming from herbaceous plants, i. e., those plants not generating wood. The grasses are plants that present neither woody stems nor woody roots. In general, their stems are green. The most studied herbaceous biomass for ethanol production purposes comprises differ­ent types of grasses like switchgrass, used in North America for hay production, alfalfa hay, reed canary grass, coastal Bermuda grass, and timothy grass that covers almost two-thirds of the livestock meadows in the United States. These herbaceous plants have a great importance because they grow very fast and have reduced nutritional requirements. Thus, these plants are excellent candidates for their exploitation as crops dedicated to bioenergy production.

Among the cellulosic wastes are residues resulting from industrial activities, mostly related to paper processing, which have an elevated content of cellulose compared to other types of lignocellulosic biomass. As examples of this group, newspaper, waste office paper, and paper sludge, one of the effluents of plants for paper recycling, should be highlighted. Finally, the organic fraction of munici­pal solid wastes are composed of materials with high lignocellulosic content like wasted paper, cardboard, fruit and vegetable peels, garden residues, and wood items, among others.

The composition of lignocellulosic biomass, expressed as the proportion of cellulose, hemicellulose, and lignin, depends on its origin, though some simi­larities can be observed independent on the group to which all material belongs. The composition of several representative lignocellulosic materials is presented in Table 3.11. As can be observed, hardwood presents high cellulose content mak­ing it very promising regarding the production of second generation bioethanol. In this sense, the hydrolysis of cellulose allows the formation of glucose, which in turn can be transformed into ethanol by fermentation. Likewise, the cellulosic

Percentage Composition (Dry Basis) of Some Lignocellulosic Materials

wastes have high cellulose contents. On the other hand and due to its high lignin content, softwood has a more difficult transformation process increasing the com­plexity of fuel ethanol production.

From the large variety of lignocellulosic materials that have been proved for fuel ethanol production, those with higher availability and production volume have been defined as the most promising. Logically, this selection is made based on the context of each region and country. Thus, the corn stover along with pop­lar wood has become the most potential feedstocks for bioethanol production in the United States. In Europe, the straw of different cereals is the most promising material. In fact, wheat straw and barley straw are the feedstocks defined for the first commercial plant for production of cellulosic ethanol whose construction started in 2005 and will open soon in Salamanca (Spain; Abengoa, 2008). In the case of South American countries like Brazil and Colombia, the most promising feedstock for second generation (lignocellulosic) ethanol is cane bagasse because it has great availability and production volumes. In addition, the participation of the sugar sector could provide the necessary financial resources needed for the implementation of these technologies at industrial level.