Vol.2 No.4 2010

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Research paper : Bioethanol production from woods with the aid of nanotechnology (T. Endo)−278−Synthesiology - English edition Vol.2 No.4 (2010) (Fig. 9). The cell structure of wood is 20-500 m, and the wood structure is destroyed by the powerful shear force of the wet cutter mill. The microfibrils that correspond to the hoop are partially broken. Next, the hemicellulose that glues the microfibrils together is partially hydrolyzed (few % of all hemicelluloses) by the autoclave treatment. When the autoclaved product was observed at high magnification by SEM, a large number of pores of tens of nm were observed. These pores are thought to be the holes after the hemicellulose dropped out. As the wood structure weakens after these steps, the structure can be unraveled into microfibrils easily by the final disk milling, and the enzymatic saccharification is improved greatly.So far, the explanation was centered on the mechanism for the improvement of enzymatic saccharification of the cellulose component. However, in bioethanol manufacturing, the saccharification of hemicellulose is also important to increase the yield of sugar as fermentation material. In the wood structure, hemicellulose covers the surface of the cellulose microfibril, and in our pretreatment process where the microfibrils are separated from each other, the saccharification of hemicellulose progresses alongside the saccharification of cellulose. Since drastic chemical reaction does not take place in our pretreatment, the structure of lignin does not change from the structure in the untreated wood. After enzymatic saccharification, the lignin remains as residue.7 Advantage of the mechanochemical treatment7.1 Cost reduction of enzymatic saccharificationAs mentioned earlier, the disk mill treatment has high efficiency and relatively low cost. However, compared to the simple heat treatment, dramatic cost reduction is not possible since the driving power of motors cannot be recovered or recycled. Yet, the advantage of the mechanochemical treatment is that the costs in other processes of the bioethanol production can be reduced greatly. In the mechanochemical treatment, enzymatic saccharification and fermentation progress efficiently without the chemical treatments such as lignin removal. Also, dependency on raw material is low, and the process can be applied as the pretreatment of hardwoods, softwoods, straws, and others. The greatest advantage is the reduction in the cost of enzymes such as cellulase, which in some cases dominate over half the cost of bioethanol production. The mechanochemically treated products can be sufficiently saccharified with relatively small amount of enzymes. Figure 10 shows the relationship of the amount of enzymes and the degree of saccharification of the products of various pretreatments[14]. Compared to the mechanochemical treatment, the degree of saccharification falls greatly when the amount of enzymes is small in the 200 ºC high-temperature hydrothermal process. This phenomenon is thought to be due to the production of inhibitor from the transformation of wood components in the high-temperature hydrothermal process. Similar results are expected in the acid treatment that follows the similar reaction as the hydrothermal treatment. Relatively low temperature (160 ºC) treatment reduces the time for mechanochemical treatment, and saccharification progresses even with small amount of enzymes.7.2 Comparison with conventional technologyThe comparison of the characteristics of the conventional technology against the pretreatment process for enzymatic saccharification we developed is as follows (table at bottom of Fig. 2). The degree of saccharification of the treated product is about the equivalent to that of the classic mechanochemical treatment. Advantages are: there is no need of advanced reaction control as in the chemical process; and since only water is used for the process, recovery of chemicals is not necessary and waste liquid management is easy. With the same amount treated, the electricity consumed by our process is about 1:10~20 or less of the conventional ball mill process. The autoclave treatment also has low energy consumption since it is not a high-temperature high-pressure process, and the equipment does not have to be high-pressure resistant. As mentioned earlier, the dependency on raw material biomass species is low. Also, our pretreatment process is basically a wet process, and the raw material does not have to be dried. Raw materials with high water content can be used as is. In the concentrated sulfuric acid saccharfication, it may be necessary to dry the raw material to prevent heating due to dilution of the sulfuric acid, and therefore, the efficiency is poor.The pretreatment process we developed overcomes the various issues of the conventional technology, and can be applied without loosing the advantages. The new issue is Fig. 10 Relationship between amount of enzyme added and saccharification by different pretreatment.Amount of enzyme added (FPU/g substrate weight)*FPU:Filter paper unitGlucose yield (%)Ball milling (120 min)Hydrothermal treatment (160 ºC, 30 min) → Ball milling (40 min)Hydrothermal treatment (200 ºC, 30 min)00101003040405060607080802020

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