Cellulosic ethanol
Second generation bio-ethanol
Biosynthetically
Ethanol ( hydrous )
About 104 RON
Ethanol, which is manufactured from plant wastes is referred to as cellulose - ethanol or lignocellulosic ethanol. Like the conventional fuel ethanol it is a gasoline fuel that can be produced by fermentation of plant waste materials (bio - ethanol). The production of lignocellulose -ethanol processes, however, is still under development and has been realized only in the test scale.
- 2.1 pretreatment and saccharification of plant material
- 2.2 fermentation of the sugar mixture of hexoses and pentoses
- 2.3 fermentation inhibitors
- 2.4 logistics
Bioethanol from plant biomass
Bioethanol is alcohol, which is produced by fermentation of sugars using microorganisms. In general, the yeast is used for this purpose with the scientific name Saccharomyces cerevisiae. The sugar derived from plants that exploit through the process of photosynthesis, the energy of sunlight to (CO2) from carbon dioxide build their organic components. The sugar may be in the form of starch (eg corn grain, potato) or sucrose (eg sugar beet, sugar cane ) are stored, or they are converted into structural components (eg, cellulose ) that the plant its shape and provide stability. Currently, bioethanol is produced (maize, cereals) primarily by fermentation of sucrose ( Brazilian sugar cane ) or starch. By distillation, and drying, the ethanol can be used as fuel. However, this method of production produces a competitive situation the grocery store. In addition, is hampered by limited arable land and the ecological problems with the necessary intensification of agriculture, a large-scale production of starch-based ethanol. The aim is therefore increasingly the use of low-cost crop residues such as straw, wood residues and landscape management or energy crops such as switchgrass (including switchgrass, Panicum virgatum ) and miscanthus, require no intensive agricultural management and also grow on poor quality soils.
Composition of plant biomass
Plant residues or energy crops have little strength or sucrose, but contain sugar in the form of lignocellulose stored in their cell walls. Lignocelluloses consist of cellulose, hemicelluloses and non-fermentable lignin ( " pulp "). Cellulose, such as the strength, a polymer of glucose molecules with six carbon atoms of glucose are linked together to form long chains. They differ only in the type of links. Hemicelluloses consist mainly of sugars with five carbon atoms, xylose and arabinose, which are juxtaposed in branched chains.
Overview of the cellulosic ethanol production process
In order to produce bioethanol from lignocellulose, cellulose and hemicelluloses, the first must be broken into the individual sugars. This is done with special acids and enzymes. After that, the yeasts have to ferment the mixture of glucose, xylose and arabinose to ethanol. The fermentation, distillation and drying is analogous to the classical fuel ethanol process.
Pretreatment and saccharification of the plant material
Despite the great similarities in the starch and fermentation Lignocellulosic the latter has some difficulties. First, the lignocellulose must be liquefied and sugared. This is much more difficult than with the strength, because the sugar chains are difficult to access. The plant material must therefore first be pretreated chemically or thermally. Only then will the saccharification with the help of special enzymes ( cellulases, xylanases, glycosidases ) to happen, the split analogous to the amylases in the starch, the cellulose chains into glucose. These enzymes are obtained from fungi that are involved in the nature of the decomposition of plant residues. Since substantially more enzymes needed for starch saccharification as this leads to increased costs. Research efforts have resulted here in recent years to a significant cost reduction.
Fermentation of the sugar mixture of hexoses and pentoses
The second main difference is that, as in the strength of only glucose is not available as the sugar unit in the lignocellulose, but other sugars such as xylose and arabinose ( = C5 sugars or pentoses ). However, these can not be used by the yeasts used for ethanol production. It should specially bred yeasts are used, which are able to ferment the different sugars to ethanol in addition to the glucose.
In the traditional fuel ethanol production exclusively yeast Saccharomyces type are used. These are the same yeasts that also serve for the production of bread, beer and wine. Yeasts have the advantage that their use is well established in industrial processes for centuries to bacteria. In addition, they are also much more resistant and robust. For this reason, they are the ideal in the production of ethanol from lignocellulose. Their big drawback is that they can ferment only the C6 sugars ( hexoses = ) but not the C5 sugars ( pentoses = ).
Various research groups from Europe and the U.S. have been able to grow yeast strains in recent years, also ferment C5 sugars to ethanol. From the genetic material of the yeasts can be seen that this used to be able to utilize C5 sugars. However, they have lost this property again in the course of its evolution. With the help of modern biological techniques to lend to yeast cells this property again and they even improve significantly succeeded. These them the appropriate genetic material from other yeasts, fungi and bacteria was specifically introduced. In this case, yeast cells have emerged that are able to ferment both C6 and C5 sugars.
In the case of C5 sugar xylose, two different strategies have been applied to. Scientists at the University of Lund in Sweden took advantage of a two-step mechanism ( Xylose-Reductase/Xylitol-Dehydrogenase from the yeast Pichia stipitis ) to inject the metabolism of xylose in Saccharomyces yeasts. Scientists at the University of Frankfurt and those of the Technical University of Delft in the Netherlands have recently also successfully breed yeasts that integrate xylose directly in one step using the enzyme xylose isomerase in their metabolism and are able to ferment to ethanol. The Delft scientists use a eukaryotic xylose isomerase whereas the Frankfurt scientists use a bacterial xylose isomerase, which has the advantage of being less strongly inhibited by the inhibitor xylitol.
In the case of C5 sugar arabinose to the frequently found in fungi 5-step pathway presented in the Saccharomyces yeasts out to be less suitable. By contrast, could at the University of Frankfurt successfully a 3 -step pathway to be established, which is usually found only in bacteria. Integrated to this pathway in the yeast and then forced them several months to use arabinose as a sole source of energy, then actually developed yeast strains that were able to ferment arabinose and glucose addition to the. Yeast was grown together with researchers from the University of Lund then that can ferment all sugars, ie glucose, xylose and arabinose to ethanol.
Fermentation inhibitors
A third difference between the classic fuel ethanol process and cellulosic ethanol are toxic substances produced by the chemical and thermal pretreatment of the plant material (eg furfural ). These inhibitors damage microorganisms used in the fermentation. They must be removed before fermentation, but this causes additional costs. One solution is to use inhibitor - tolerant yeasts.
Logistics
A fourth major difference is the lower density of plant waste, that is, the lower energy density compared to cereal or corn. This means increased transportation costs and increased storage space. This issue that could be solved by more efficient compression techniques, the transport of already crushed material and smaller, decentralized production.
Economic Consideration
The implementation of all sugar can greatly improve the efficiency of the fermentation of plant biomass. Straw contains about 32 % glucose, 19% xylose and 2.4% arabinose. Add 1 t of straw that is 320 kg glucose are included. For a complete fermentation incurring about 160 kg of ethanol, which corresponds to a volume of 200 l. The complete fermentation of the pentose sugar xylose yields corresponding additional 124 liters of ethanol per ton of straw.
Process Development and Implementation
As a next step is now to develop the successes achieved in the laboratory for industrial use. The various steps of the process must be transferred and implemented in an industrial scale. Similarly, the developed yeast strains have to be adapted to industrial conditions. Although the yeast strains used in daily laboratory are very well suited to explore the different fermentation strategies, but they are mostly for industrial applications less useful. Firstly, the laboratory yeasts are not stable enough and lose their acquired skills very quickly, on the other hand they are too sensitive to toxic substances ( furfurals ) arising in the chemical pretreatment of the plant material.
View
All the essential conditions for a lignocellulosic ethanol process are available. Now it is time to translate these research findings into large -scale production. The Agency for Renewable Resources ( FNR) has a published in 2009 Study ( Biofuels - A comparative analysis) estimated the cost of lignocellulosic ethanol from waste straw for 2020 to be about 24 € / GJ, while the rate in 2007 was still at 30 € / GJ. This corresponds with a calorific value of 23.5 MJ / l for bioethanol about 56 cents / l (2020 ) or about 70 cents / l (2007). Thus, the costs are but with the cost of starch ethanol. Against this background, the study comes to the conclusion that bioethanol from lignocellulosic not expected to be competitive without subsidies. However, it should be noted that the true cost will only show a commercially operated facility. The largest costs are still caused by the enzymes to Celluloseverzuckerung. Enzyme manufacturers point out, however, that there are already cost- effective processes for enzymes, but it is not worthwhile to produce them because there is no demand is there. Only when the first commercial plants are running, the demand will increase and the enzymes will be cheaper. Only then you will also see how the newly bred yeasts behave under these conditions. Although perhaps the C5 sugar fermentation initially not running optimally, all yeasts can already completely ferment C6 sugars. And once improved, cost-effective enzymes and yeasts are there, they can be replaced easily at any time in any system. In the United States of America, there are already some companies that are nearing commercialization of cellulosic ethanol processes. In the long term, however, cellulosic ethanol will represent only a temporary solution. The 3rd generation biofuels, such as Biobutanol show significantly better properties, but also only if they are derived from lignocellulose.