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biological process for converting the lignocellulose to fuel ethanol requires the following: delignification to liberate cellulose and hemicellulose from their complex with lignin, depolymerization of the carbohydrate polymers to produce free sugars, and fermentation of mixed hexose and pentose sugars to produce ethanol. Among the key processes described above, the delignification of lignocellulosic raw materials is the rate-limiting and most difficult task to be solved. Another problem is that the aqueous acid used to hydrolyze the cellulose in wood to glucose and other simple sugars destroys much of the sugars in the process. Extensive research has been carried out in this field for decades (Yu and Zhang, 2004), and the first demonstration plant using lignocellulosic feedstocks has been in operation in Canada since April 2004 (Tampier et al., 2004). It is expected that the cost of lignocellulosic ethanol can undercut that of starch-based ethanol because low-value agricultural residues can be used.
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2.7.1 Bio-ethanol from sugar feedstocks
Both raw juice and molasses from sugar cane and sugar beets can be used for ethanol production. The juice is extracted from sugar cane by either squeezing (roll mills) or diffusion (diffuser). Part of the juice is used for sugar manufacture while the remaining is used for ethanol production. Molasses, which is a low-value by- product, is also used for ethanol production. The solid residue from the extraction step, which is referred to as biogases, is burned to generate energy for use in the plant. Ethanol is normally obtained by fermentation of cane juice or a mixture of cane molasses and juice. Before putting into the fermenters, the sugar solution must undergo purification and pasteurization. Purification normally involves treatment with lime, heating and later decantation similar to treatment use in sugar manufacture. Pasteurization involves heating and immediate cooling. The cooling typically includes two stages. In the first stage, the hot sugar solution is passed through a heat exchanger in counter- current flow to the cold solution. At the end of this stage, the hot solution is cooled to about 600C. In the second stage, the sugar solution is cooled further to 300C using water as the cooling fluid. The sugar concentration normally is adjusted to approximately 190C (Drapcho et al., 2008).
Today the processes of milling (cutting of cane into regular pieces) and raw sugar refining are usually done together on one site. During the milling the sugar cane is washed, chopped and shredded by revolving knives. The shredded cane (20-25cm) is fed into mill combinations which crush and extract the cane juice. The juice is filtered and pasteurized (treatment of heat to kill micro-bacterial impurities) along with chemicals.
2.7.2 Cereal crops
For starch (cereal) based crops the procedure is similar to sugar crops but with the added process of hydrolysis to break down the polymers into monomers which can then be broken down into simple C6 sugars. From the milling of the
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grain to the release of the starch, it is then diluted into water to adjust the volume of sugar in the mash. The mixture is cooked with yeast and all the water soluble starches dissolve into the water. And through either acid hydrolysis or enzymes, the starch is converted into sugars. The unrefined fermented liquid known as
―beer‖, is produced and through various evaporation and distillation stages fuel grade ethanol can be produced.
They can be naturally divided into cereal crops, sugar crops and woody/lignocellulosic biomass. Any sort of wood, crop residues or forestry waste like sawdust and chips can be used for 2nd Generation bio-ethanol.
Miscanthus and the other examples below are some fast growing grasses which are proving more and more popular for heating fuel. They could also be used for lignocellulosic bio-ethanol.
2.7.3 Lignocellulosic bio-ethanol
Lignocellucosic feedstocks consists of three main components namely cellulose hemicelluloses and lignin. Lignocellucosic materials are more complex than starch.
Generally, the percentage composition of lignocellucosic biomass is as follows:
40-60% cellulose, 20-40% hemicelluloses and 10-25% lignin depending on the biomass (Mattocks, 1987). Lignocellucosic biomass are cheap renewable resources and available in large quantity that can be used for the production of ethanol. They can be obtained at low cost from a variety of resources such as wood, grass, bagasse, waste paper, municipal solid waste and stalks of cereals (Kalman and Reczey, 2007). Technologies for conversion of these feedstocks to ethanol have been developed on two plant forms which can be referred to as the sugar platform and the synthesis gas (or syngas) platform.
Cellulosic materials are significantly more resistant to hydrolysis than starchy materials. Hemicellulose is a branched heteropolymer of not just glucose but multiple five and six carbon sugars; xylose, L-arabinose and hexose sugars;
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galactose, D-glucose, D-mannose, L-rhamnose and L-fructose. The structure of hemicelluloses varies with the particular biomass, but generally xylose constitutes a relatively large percent of the composition. Lignin is not composed of sugars, but it is instead a complex aromatic polymer. As such, lignin cannot be used to make ethanol it can however be utilized as a fuel source (Zaldivar et al., 2001).
In the sugar platform, cellulose and hemicelluloses are first converted to fermentable sugars, which then are fermented to produce ethanol. The fermentable sugars include glucose, xylose, arabinose, galactose and mannose. Hydrolysis of cellulose and hemicellulose to generate these sugars can be carried out by using either acids or enzymes. Pre-treatment of the biomass are needed prior to hydrolysis. The main objectives of the pre-treatment process are to speed up the rates of hydrolysis and increase the yield of fermentable sugar. In all pre-treatment processes, these goals are accomplished by modifying the structure of the polymer matrix in the biomass thus making the carbohydrate fractions more susceptible to acid attack or more accessible to enzymes action.
The difference in process steps between starch and lignocellulosic feedstocks is that lignocellulosic biomass requires a more complicated hydrolysis stage. The reason for this is that cellulose in the wood containing carbohydrate polymers called cellulose. Cellulose is made up of long chains of glucose and a more complex set of enzymes are required to break the long chains. Therefore lignocellulosic bio-ethanol is technically more demanding and thus more expensive. Work at the moment ongoing to enhance the pre-treatment methods such as steam explosion, ammonia steam explosion, acid processing and synthesizing more efficient enzymes. Another area for development is fractionation technology so one can use more variable biomass, such as agriculture and forest crop residues and urban waste. The chemical structure
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of the crop and forest residues are highly variable which creates added complexity compared to the homogeneity of starch or sugar crops.
2.7.4 Bagasse
Bagasse is the primary by-product from sugar cane production. Bagasse is commonly combusted in boilers or cogeneration systems in the sugar industry for the production of heat in the mill for sugar refining processes and for the production of electricity for either direct use by the plant or to
sell to the national grid which can increase their overall profit. About 35% of the weight of sugar cane becomes bagasse. Brazil, India, China and Thailand are the largest producers and utilisers of bagasse.
Bagasse is also a straw like material left from cane sugar. It can also be used for making agro-pellets which can be exported as a feedstocks for home pellet boilers or co-firing.
2.7.5 Straw
Straw is another important co-product from cereals and has been used for centuries for various uses. Straw is the waste part of the plant that does not contain the grain and it makes up around 50% of the plants weight. Historical uses include use for rope, paper, packaging, hatching and bedding. It has mostly been used for animal feed although recent uses include bio-fuels in the lignocellulosic path to biogas production through anaerobic digestion. Straw has mainly been somewhat of a burden for farmers as they had to dispose of it some way but its application for bio-ethanol or biogas means they can sell this waste as a marketable by-product.
2.7.6 Fuel cells
Fuel cells are another potential area for ethanol use to produce heat and power.
Fuel cells function by combining the fuel hydrogen with oxygen from the air to produce electrical energy, with water vapour and heat as by-products. Fuel
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Cells have a typical electrical efficiency of between 30 and 60 % and an overall efficiency, if using the heat by-product, of 70-90 %. The units run with very low noise emissions and pollutant gas emissions are also reduced considerably. Its disadvantages are its relatively high cost and their short life span (regular replacement of components). They are however, regarded as very reliable for the duration of their lifespan and are often used for emergency power. Some uses of fuel cell systems include providing heat and power for hospitals, university campus‘, remote telecommunication stations as well as for transport, stationary power generation and residential buildings. The recent growth in small residential (0.5 to 10 kW) fuel cell CHP is based on natural gas fuelled units. A number of fuel cells can use bio-ethanol as well as fossil fuels, sometimes with, sometimes without the need for a reformer (to convert it to hydrogen). Acumentrics (USA) and Ceramic Fuel Cells (Australia) manufacture such fuel cells.