There is a vast amount of lignocellulosic waste material from agriculture and the forest industry which can be used for ethanol production. Lignocellulosic biomass like wood and fast growing plants like switch grass, reed canary grass or crop residues from food production such as corn stover are cellulose feedstocks which can be used in bioethanol production.

Lignocellulosic biomass is composed of polymeric structures of cellulose, hemicellulose, lignin, other organic compounds (extractives) and inorganic salts. Cellulose is the major component in most lignocellulosic biomass. In fact, it is the most abundant polymer on earth. Like starch, cellulose is a polymer of glucose molecules and the chain length varies between 100 and 14000 units. However, in cellulose the glucose units are connected to each other by b-1,4-glycosidic bonds instead of a-1,4-bonds as in starch, the structure of cellulose is shown in Fig. 9.2.

This makes a crucial difference compared to starch. In cellulose the glucose polymer is linear giving the possibility for the cellulose chains to align with each other and form multiple hydrogen bonds between the chains. In this way, cellulose can form crystalline structures. These crystalline structures are very stable and they are the reason why it is so difficult to hydrolyse cellulose: the crystals are so tight that it is very difficult for the hydrogen ions and the water that is needed for the hydrolysis to actually get to the glycosidic bonds. In fact, although cellulose consists of very polar glucose units, the tight hydrogen bonds prevent water solvating the polymer and therefore cellulose is not soluble in water. This is fortunate because otherwise cotton clothes (cotton being pure cellulose) could not be washed and would not be so useful! However, not the whole portion of cellulose is in the crystalline form, in some locations, the crystal structure is disturbed and an amorphous form of cellulose is formed. This form is not as stable as the crystalline form and is more susceptible to hydrolysis.

The cellulose chains that are held together with hydrogen bonds form what are called fibrils and a bundle of these fibrils then forms the actual cellulose fibre. In order to ‘soften’ the cellulose, the hydrogen bonds must be broken and that is why the concentrated acid method is so effective: in such a high concentration of acid or also in fact strong base, the hydrogen bonds are broken and access to the glycosidic bonds is made. The double sugar units with a b-1,4-bond between the two glucose units is called cellobiose.

Hemicellulose is a branched polymer of both 6-carbon sugars (hexoses) like glucose, mannose and also 5-carbon sugars (pentoses) like xylose. In grasses and hardwoods, the pentoses in the form of xylans dominate, while in softwoods the major hemicellulose component is the hexosic glucomannan. Since the hemicellulose polymer chain is branched, the formation of hydrogen bonds creating the crystalline

structure of cellulose is prevented. This makes hemicellulose much more susceptible to the hydrolysis of the glycosidic bond. Actually, hemicellulose in solution is as easy to hydrolyse as starch. Two different structures of hemi-cellulose are shown in Fig. 9.3.

Lignin is a polymeric structure of aromatic units (p-hydroxy-phenyl-propanoid units) and the second most prevalent polymer on earth. Lignin functions as the glue between the cellulose fibres in the lignocellulosic biomass. The amount of lignin varies depending on the type of biomass; Table 9.1 shows the typical composition of cellulose, hemicellulose and lignin in different types of biomass.

The composition of lignin also varies between different types of biomass. The phenyl ring in the monomer structure of lignin can either have no, one or two

methoxy groups. In grasses the non-methoxy monomer is predominant, in hardwoods there is a mix of all three and in softwoods the one and two methoxy rings are predominant. Since lignin does not contain as much oxygen as cellulose and hemicellulose, the energy value is much higher. Cellulose and hemicellulose have an energy value (calorific value) of approximately 17 MJ/kg, while lignin has up to 25 MJ/kg. So although lignin is only around 25% of the dry solid content in wood for example, almost 40% of the heat value comes from it. A structure of a segment of lignin in softwood is shown in Fig. 9.4.

Historically, lignin has always been utilised as an energy source, for example in the energy recovery boilers of the pulp and paper industry. In a future bio-refinery process lignin may have a more important role as a feedstock for the production of several organic compounds, e. g. phenol. One problem chemically with lignin produced in a dilute acid or enzymatic process is that it is highly condensed which reduces the number of reactive hydroxyl groups and therefore there are problems to react it further.

Switch grass (Panicum virgatum L.) is a perennial warm-season C4 species (tolerant to heat and cold), which can be used in bioethanol production. This grass

is grown in Central USA as a fodder crop or for soil conservation and is a potential long-term bioethanol feedstock to replace corn. The composition of switch grass on a dry basis is about 30-36% cellulose, 24-27% hemicellulose and 16-18% lignin. From highly adapted switch grass varieties the theoretical ethanol production potential is about 5000-6000 litre/ha. Based on the technique used in ethanol production the ethanol yield is often high (72-92% of the theoretical value in labscale). The excess of switch grass can be used to produce Kraft pulp with short fibres (Keshwani and Cheng, 2009).

Reed canary grass (Phalaris arundinacea L.) is a perennial rhizomatous grass which is mainly used as a raw material for solid biofuel production in the Nordic countries. This grass grows naturally in Europe, Asia and North America, especially in wet and humus rich soil. The grass is about two meters tall with a sturdy, upright straw, broad leaves and a long panicle. The annual production yield is eight to ten tonnes dry solid/ha in Sweden (Xiong, Landstrom, and Olsson, 2009). The harvesting starts normally some years after establishment and growth persists for at least 12 years (Xiong, Landstrom, and Olsson, 2009). The grass is usually stored and transported as bales to increase the density and reduce production costs. Reed canary grass consists mainly of cellulose, hemicellulose and lignin, but there are also proteins, lipids and a relatively high content of inorganic material. The main sugars after hydrolysis of reed canary grass are glucose, xylose and also arabinose. In reed canary grass, the amount of hexoses in the stem varies between 38% and 45% of the dry weight of the material and the amount of pentoses about 22-25%. The lignin content varies between 18% and 21% of the dry weight. Therefore, the grass has a good potential as a feedstock for ethanol production in the future (Arshadi and Sellstedt, 2008).

Reed canary grass has also been found to be a useful complement to short fibre raw materials like birch in kraft pulp production (Paavilainen, 1996; Finell and Nilsson, 2004). Alfalfa (Medicago sativa L.) is usually used for the production of fuel, feed and other industrial materials. Alfalfa stems consist mainly of cellulose, hemicellulose, lignin, pectin and proteins. Therefore, the feedstock has the potential to be used for ethanol production and also other chemicals (Diena et al, 2006).

Previous work has shown that it is possible to produce ethanol from alfalfa either by separate hydrolysis and fermentation (SHF) or simultaneous saccharification and fermentation (SSF). The yield of fermentable sugars from hydrolysis or saccharification is an important response variable in assessing the value of the feedstock. Corn seed has been used as a starchy feedstock in bioethanol production but other parts of the corn plant have not been used until recently. The stalk and the leaves, which are called corn stover, can be used as a source of lignocellulosic material in ethanol production; also the corn cob can be used. The amount of corn stover is huge since for every kilogramme of produced corn, almost the same amount of corn stover is left. The amount of corn stover available for fermentation usage is estimated to be between 60 and 80 million dry tonnes per year (Kadam and McMillan, 2003). Some of the corn stover needs to be left in the field to prevent soil erosion and also corn stover may be needed as a feedstock for bio-based materials like composite products (Kadam and McMillan, 2003), but some part can be collected and used as a raw material in bioethanol production (Ohgren, Rudolf, Galbe, and Zacchi, 2006).

Rice straw is another lignocellulosic material that can be used as a raw material in bioethanol production, the annual world production of which is about 731 million tonnes. This amount of biomass has the potential to produce 205 billion litre of bioethanol (Balat, Balat, and Oz, 2008). Actually, the use of rice straw as a feedstock for bioethanol production will increase the income of farmers in many places with a gain in rice production which is an important carbohydrate source for many people in the world.

Sawdust and wood chips from softwoods (pine, spruce) are another important feedstock for ethanol production. Until now most of the excess of sawdust in some countries (e. g. Sweden, Finland) has been used as a raw material for wood pellets, a solid biofuel, for heating. The annual amount of sawdust used for the production of wood pellets is more than three million tonnes in Sweden alone. In fact the wood pellets production in North America has been increased drastically in recent years. However, for sustainable usage of the forest resources in a future bio-refinery, the extractives from the biomass can be extracted for the production of chemicals, with then the possibility of releasing the cellulose and hemicellulose components and converting them to ethanol. The residual, which contains mostly lignin together with additional sawdust and other biomass, can still be used as a feedstock for the wood pellet industry.