Cellulosic ethanol

from Wikipedia, the free encyclopedia
Cellulosic ethanol
other names

Second generation bio-ethanol

Brief description Petrol for adapted engines
origin

biosynthetic

Characteristic components

Ethanol (containing water)

properties
Physical state liquid
Octane number

approx. 104 RON

safety instructions
GHS labeling of hazardous substances
no classification available
H and P phrases H: see above
P: see above
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Ethanol made from vegetable waste is known as cellulosic ethanol or lignocellulosic ethanol . Like conventional ethanol fuel , it is a petrol that can be obtained by fermenting vegetable waste ( bio-ethanol ).

Bioethanol from plant biomass

Bioethanol is alcohol that is obtained through fermentation from sugars with the help of microorganisms. In general, the yeast with the scientific name Saccharomyces cerevisiae is used for this. The sugars come from plants, which use the energy of sunlight through the process of photosynthesis to build their organic components from carbon dioxide (CO 2 ). The sugars can be stored in the form of starch (e.g. cereal grain, potato) or sucrose (e.g. sugar beet, sugar cane), or they are converted into structural components (e.g. cellulose ) that give the plant its shape and shape Give stability. At present, bioethanol is mainly obtained by fermenting sucrose (Brazilian sugar cane) or hydrolyzates from starch (corn, grain). After distillation and drying, the ethanol can be used as fuel. However, this type of production creates a competitive situation with the food market. In addition, the limited cultivation areas and the ecological problems associated with the necessary intensification of agriculture stand in the way of large-scale production of starch-based ethanol. The use of low-cost plant residues such as straw, wood scraps and landscape maintenance or energy crops such as The aim is therefore increasingly switchgrass (including switchgrass, Panicum virgatum ) or Miscanthus , which do not require intensive agricultural cultivation and grow on poor quality soils.

Plant residues or energy plants have little starch or sucrose , but contain carbohydrates in the form of lignocelluloses stored in their cell walls. Lignocelluloses consist of cellulose , hemicelluloses and the non-fermentable lignin ("wood pulp"). Like starch, cellulose is a polymer made of sugar molecules with six carbon atoms, the glucose , which are linked to form long chains. Both differ only in the type of links. Hemicelluloses consist mainly of sugars with five carbon atoms, xylose and arabinose , which are attached to one another in branched chains.

Cellulosic ethanol production process

In order to be able to produce bioethanol from lignocellulose, the cellulose and the hemicelluloses must first be split into the individual sugars. This is done with acids and special enzymes. Then the yeasts have to ferment the mixture of glucose, xylose and arabinose into ethanol. The fermentation, distillation and drying take place in the same way as the classic ethanol-fuel process. Since the end of 2013, cellulosic ethanol with more than 75 million liters annually has been produced commercially in a plant operated by the company "Beta Renewables" in northern Italy. However, the company was restructured in 2017. Current companies with appropriate procedures include Iogen, POET and Verbio. Further companies in Germany and Austria are in the overview of the Federal Association of the German Bioethanol Industry eV

Pre-treatment and saccharification of the plant material

Despite the great similarities in starch and lignocellulose fermentation, the latter has some difficulties. First the lignocellulose has to be liquefied and saccharified. This is much more difficult than with starch, since the sugar chains are difficult to access. The plant material must therefore first be chemically or thermally pretreated. Only then can the saccharification take place with the help of special enzymes ( cellulases , xylanases , glucosidases ) which, like the amylases in starch, split the cellulose chains into glucose. These enzymes are obtained from fungi, which in nature are involved in the rotting of plant debris. Since significantly more enzymes are required than for starch saccharification, this leads to increased costs. However, research efforts have led to a reduction in costs in recent years.

Fermentation of the sugar mixture from hexoses and pentoses

The second major difference is that in lignocellulose, as in starch, there is not only glucose as a sugar component, but also other sugars such as xylose and arabinose (= C5 sugar or pentoses). However, these cannot be used by the yeasts used to produce ethanol. So specially grown yeasts have to be used which, in addition to the glucose, can also ferment the other sugars to form ethanol.

Only yeasts of the Saccharomyces type are used in traditional ethanol fuel production . These are the same yeasts that are used to make bread , beer and wine . Yeasts have the advantage over bacteria that their handling in industrial processes has been established for centuries. For this reason, they are ideal for the production of ethanol from lignocellulose. Their major disadvantage, however, is that they can only ferment the C6 sugars (= hexoses) but not the C5 sugars (= pentoses).

In recent years, various research groups from Europe and the USA have been able to produce yeast strains that also ferment C5 sugars into ethanol. The genetic material of the yeast shows that it was once able to utilize C5 sugar. However, they have lost this property again in the course of their evolution . With the help of genetic engineering it was possible to restore this property to the yeast cells or even to improve them significantly. For this purpose, the relevant genetic material from other yeasts, fungi and bacteria was introduced into them. This resulted in yeast cells that can ferment both C6 and C5 sugars.

In the case of the C5 sugar xylose , two different strategies were used. Scientists at Lund University in Sweden used a two-step mechanism ( xylose reductase / xylitol dehydrogenase from the yeast Pichia stipitis ) to introduce xylose into the metabolism of the Saccharomyces yeast. Scientists from the University of Frankfurt and those from the Technical University of Delft in the Netherlands were recently able to successfully breed yeasts that integrate xylose directly into their metabolism in one step with the help of the enzyme xylose isomerase and ferment it 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 the C5 sugar arabinose , the 5-stage degradation path in the Saccharomyces yeasts , which is often found in fungi, turned out to be unsuitable . On the other hand, a 3-step metabolic pathway was successfully established at the University of Frankfurt , which is otherwise only found in bacteria. If this metabolic pathway was integrated into the yeast and then forced to use arabinose as the only source of energy for several months, yeast strains actually developed that could ferment arabinose in addition to glucose. Together with researchers from Lund University, a yeast was then grown that can ferment all sugars, i.e. glucose, xylose and arabinose, into ethanol.

Fermentation inhibitors

A third difference between the classic ethanol-fuel process and cellulose-ethanol are toxic substances that arise during the chemical and thermal pretreatment of the plant material (e.g. furfurals ). These inhibitors damage the microorganisms used in fermentation. They must therefore be removed before fermentation, which, however, causes additional costs.

logistics

A fourth key difference is the lower density of plant waste, i.e. H. the lower energy density compared to grain or corn kernels . This means increased transport costs and increased storage space requirements. Therefore, more efficient pressing techniques , the transport of already shredded material and smaller, decentralized production facilities are being investigated.

Economic consideration

The implementation of all sugars can significantly improve the profitability of the fermentation of plant biomass. Straw contains around 32% glucose, 19% xylose and 2.4% arabinose. So 1  t of straw contains 320  kg of glucose. With complete fermentation, around 160 kg of ethanol are produced, which corresponds to a volume of 200  l . The complete fermentation of the pentose sugar xylose results in an additional 124 liters of ethanol per ton of straw.

In a study published in 2009 ( Biofuels - A Comparative Analysis ), the Agency for Renewable Raw Materials (FNR) estimated the cost of lignocellulosic ethanol from waste straw to be around € 24 / GJ for 2020, while this figure was € 30 / GJ in 2007. With a calorific value of 23.5 MJ / l for bioethanol, this corresponds to around 56 cents / l (2020) or around 70 cents / l (2007). This means that the costs are higher than the costs for starch- ethanol. Against this background, the study comes to the conclusion that bioethanol made from lignocellulose is unlikely to be competitive without subsidies. However, it must be taken into account that the real costs only show a commercially operated system. The greatest costs are caused by the enzymes for the saccharification of cellulose. However, enzyme manufacturers point out that there are already inexpensive processes for more effective enzymes, but it is not worth making them because there is no demand. In the long term, cellulosic ethanol will probably only be a temporary solution. The third generation biofuels, such as B. Biobutanol show better properties, but only if they are obtained from lignocellulose.

literature

Individual evidence

  1. ↑ This substance has either not yet been classified with regard to its hazardousness or a reliable and citable source has not yet been found.
  2. ↑ MR Schmer, KP Vogel, RB Mitchell, and RK Perrin: Net energy of cellulosic ethanol from switchgrass . In: PNAS . 105, No. 2, 2008, pp. 464-469. doi : 10.1073 / pnas.0704767105 . and a lot of bioethanol for little use. In: Wissenschaft.de. January 8, 2008, accessed on September 8, 2019 (German summary).
  3. ↑ Beta Renewables in cellulosic ethanol crisis, as Grupo M&G parent files for restructuring: Biofuels Digest. Retrieved May 13, 2020 (American English).
  4. ↑ Iogen Corporation. Retrieved May 13, 2020 .
  5. ↑ Biofuel - POET. Retrieved May 13, 2020 (English).
  6. ↑ German Tele Markt GmbH Internet and advertising agency: VERBIO acquires a cellulose ethanol plant from DuPont in Nevada / IOWA, USA. Retrieved May 13, 2020 .
  7. ↑ Manufacturer. Retrieved May 13, 2020 .
  8. ↑ Fachagentur Nachwachsende Rohstoffe e. V .: Biofuels - A comparative analysis (PDF; 2.0 MB), Gülzow 2009, p. 64/65, accessed on March 5, 2010.
  9. ↑ Jens Lubbadeh: Fuel made from straw: Brewing fuel with super yeast. In: Spiegel Online . August 18, 2008, accessed March 5, 2010 .