Page 93 - Biofuels Refining and Performance

76 Chapter Three

In the wet-milling process, grain is steeped and separated into starch,
germ, and fiber components. Wet milling is capital intensive, but it gen-
erates numerous coproducts that help to improve the overall production
economics [2]. Wet mills produce corn gluten feed, corn gluten meal, corn
germ, and other related coproducts. In this method, after the grain is
cleaned, it is steeped and then ground to remove the germ. Further grind-
ing, washing, and filtering steps separate the fiber and gluten. The starch
that remains after these separation steps is then broken down into fer-
mentable sugars by the addition of enzymes in the liquefaction and sac-
charification stages. The fermentable sugars produced are then subjected
to fermentation for ethanol production, like the other fermentable sugars.

3.5 Characterization of Lignocellulosic
Materials
Lignocellulosic materials predominantly contain a mixture of carbohy-
drate polymers (cellulose and hemicellulose) and lignin. The carbohydrate
polymers are tightly bound to lignin mainly through hydrogen bonding,
but also through some covalent bonding. The contents of cellulose, hemi-
cellulose, and lignin in common lignocellulosic materials are listed in
Table 3.2. Different types of carbohydrates (glucan, xylan, galactan,
arabinan, and mannan), lignin, extractive, and ash content of many lig-
nocellulosic materials have been analyzed and are available in the lit-
erature [2, 11–14] (see Table 3.2).

3.5.1 Cellulose
Cellulose is the main component of most lignocellulosic materials.
Cellulose is a linear polymer of up to 27,000 glucosyl residues linked by
-1,4 bonds. However, each glucose residue is rotated 180 relative to

TABLE 3.2 Contents of Cellulose, Hemicellulose, and Lignin in Common
Lignocellulosic Materials
Lignocellulosic Cellulose Hemicellulose Lignin
materials (%) (%) (%)
Hardwood stems 40–75 10–40 15–25
Softwood stems 30–50 25–40 25–35
Corn cobs 45 35 15
Wheat straw 30 50 15
Rice straw 32–47 19–27 5–24
Sugarcane bagasse 40 24 25
Leaves 15–20 80–85 0
Paper 85–99 0 0–15
Newspaper 40–55 25–40 18–30
Waste paper from 60–70 10–20 5–10
chemical pulps
Grasses 25–40 25–50 10–30