Bioethanol

Bioethanol in the world

Bioethanol is probably the most widely used alternative automotive fuel in the world, mainly due to Brazil’s decision to produce fuel alcohol from sugar cane, but also due to its use in North America as octane enhancer of gasoline in small percentage. The world’s largest ethanol producers are Brazil and the USA, which together account for more than 65% of global ethanol production; the figure for Europe is 13%. Fuel ethanol is produced in Brazil mainly from sugar cane and in the USA from corn, accounting for 11.9 and 7.6 million m3 respectively in 2001.

In Brazil, 60% of the produced ethanol is sold in hydrated form (93 vol-% ethanol and 7 vol-% water), which completely replaces petrol in vehicle engines. The remaining 40% ethanol is applied in water-free form in a mixture with petrol up to 24%.

Bioethanol production in the EU

The European bioethanol production amounted to 1,592 m litres in 2006. With 431 m litres, Germany is the leading producer in Europe.  However Spain is a close second with 396 m litres.  The sector’s success in Spain can be explained by the fact that Spain does not collect tax on ethanol. France was the third largest European producer in 2006 with 293 m litres.  Spain and France transform part of their bioethanol production into ETBE.

Feedstocks

Sugar is required to produce ethanol by fermentation. Plant materials (grain, stems and leaves) are composed mainly of sugars, so in principle almost any plants can serve as feedstock for ethanol manufacture. In practice, the choice of raw material depends on what grows best under the prevailing conditions of climate, landscape and soil composition, as well as on the sugar content and ease of processing of the various plants available. The result is a wide variety of ethanol feedstocks, and hence production processes.

Worldwide, most bioethanol is produced from sugar cane (Brazil), molasses and corn (USA), but other starchy materials such as wheat, barley and rye are also suitable. Crops that contain starch have to be converted to sugars first. A feedstock of around 3 tons of grains is needed for the production of 1 ton of ethanol. In Europe, the main crops for the production of bio-ethanol are starch crops (such as common wheat) and sugar beet. Sugar beet crops are grown in most of the EU-25 countries, and yield substantially more ethanol per hectare than wheat.

Potential bioethanol yields from common wheat and sugar beet in some of the EU-25 member states

At present, R&D activities in the field of bio-ethanol focus on using lignocellulosic or woody materials as a feedstock (see dedicated section). These include short rotation energy crops (for example willow, popular, miscanthus and eucalyptus), agricultural residues (e.g. straw and sugar cane bagasse), forest residues, waste woods, and municipal solid wastes. About 2 - 4 dry tons of woody or grassy material is required for the production of 1 ton of ethanol. With a total sugar content of 60–70% (40% glucose as cellulose and 25% xylose as hemicellulose), wheat straw can produce around 230 kg of ethanol per ton of dry material.

There are several reasons for shifting to ethanol production from lignocellulosic biomass. Lignocellulosic biomass is more abundant and less expensive than food crops, especially when it concerns a waste stream with very little or even negative economic value. Furthermore, it has a higher net energy balance, which makes it more attractive from an environmental point of view.  Indeed, ligno-cellusolic bioethanol has the potential to accrue up to 90% in greenhouse gas savings, well ahead of first generation biofuels.  However, these kinds of biomass are more difficult to convert to sugars due to their relatively inaccessible molecular structure.

Production process

The predominant technology for converting biomass to ethanol is fermentation followed by distillation. Fermentation is a bio-chemical conversion process in which the biomass is decomposed using micro- organisms (bacteria or enzymes). This technology can be used for various types of biomass feedstocks.

Practically all ethanol fermentation is still based on Baker’s yeast (Saccharomyces cerevisiae), which requires simple (monomeric) sugars as raw material. Conventional yeast fermentation produces 0.51 kg of ethanol from 1 kg of any the C6 sugars glucose, mannose and sucrose. However, not all feedstocks contain simple sugars. Starch and lignocellulose are polymers, and an hydrolysis is required to break the bonds between monomers and produce simple C6 sugars for fermentation.

	Hydrolysis of starch and cellulose followed by fermentation of glucose to ethanol

The first step in this conversion process comprises milling or grinding of the grain so as to release its starch. Then this material is diluted in water to adjust the amount of sugar in the mash. This is necessary to maintain the yeast and make the mash easier to stir and handle. Then this mixture is cooked to dissolve all the water-soluble starches. The starch is converted to sugars simultaneously. This can be done by enzymes or acid hydrolysis. In the case of acid hydrolysis, dilute mineral acid is added to the grain slurry prior to cooking. The short carbohydrates resulting from these processing steps can be fermented by micro-organisms. For growing of the yeast needed for the fermentation process, the solution must be slightly acid, namely a pH between 4.8 and 5.0. During fermentation, ethanol is produced, which is diluted with water. This process also results in the formation of CO2. Through a series of distillation and dehydration steps, the ethanol concentration can be increased.

The conversion process of lignocellulosic biomass to ethanol only differs from the process described above with respect to the break down, or hydrolysis, of the raw material to fermentable sugar. This hydrolysis process is more difficult than the hydrolysis of starch. Lignocellulosic biomass contains carbohydrate polymers called cellulose (40-60% of dry weight) and hemicellulose (20-40% of dry weight) that can be converted to sugars. Cellulose is composed of glucose molecules bonded together in long chains that form a crystalline structure. Hemicellulose consists of a mixture of polymers made up from xylose, mannose, galactose, or arabinose. It is much less stable than cellulose. Both materials are not soluble in water. The remaining fraction, a complex aromatic polymer called lignin (10-25% of dry weight) cannot be fermented because it is resistant to biological degradation. This material can be utilised for the production of electricity and/or heat.

For fuel applications, the purity of the ethanol must be almost 100%. This means that the water content must be much lower compared to ethanol produced by current industrial technology. For the dehydration of ethanol several technologies are available, such as the use of molecular sieves and membrane separation, which can still be improved. The power and heat production from the non-fermentable fraction of the biomass and the overall process integration can also be developed further, which will lead to an increase of the energetic efficiency and economic performance of the process.

Fuel properties
Bioethanol has much lower energy content than gasoline (about two-third of the energy content of the latter on a volume base). This means that, for mobility applications, for a given tank volume, the range of the vehicle is reduced in the same proportion.

The octane number of ethanol is higher than that for petrol; hence ethanol has better antiknock characteristics. This better quality of the fuel can be exploited if the compression ratio of the engine is adjusted accordingly. This increases the fuel efficiency of the engine. The oxygen content of ethanol also leads to a higher efficiency, which results in a cleaner combustion process at relatively low temperatures.

The Reid vapour pressure, a measure for the volatility of a fuel, is very low for ethanol. This indicates a slow evaporation, which has the advantage that the concentration of evaporative emissions in the air remains relatively low. This reduces the risk of explosions. However, the low vapour pressure of ethanol, together with its single boiling point, is disadvantageous with regard to engine start at low ambient temperatures. Without aids, engines using ethanol cannot be started at temperatures below 20ºC. Cold start difficulties are the most important problem with regard to the application of alcohols as automotive fuels.

Fuel properties of gasoline, bioethanol and ETBE

Applications
Ethanol can be used :

• as a transport fuel to replace gasoline
• as a fuel for power generation by thermal combustion
• as a fuel for fuel cells by thermochemical reaction
• as a fuel in cogeneration systems
• as a feedstock in the chemicals industry

Ethanol is best used in spark-ignition engines because of its high octane rating. Due to its poor ignition quality (low cetane number), it is less suitable for diesel engines.

It is generally impractical to use neat ethanol in spark-ignition engines due to its low vapour pressure and high latent heat of vaporisation which make cold start problematic. The most cost-effective aid is the blending of ethanol with a small proportion of a volatile fuel such as gasoline. Thus, various mixture of bioethanol with gasoline or diesel fuels have been used. The most well-known blends are (by volume):

• E5G to E26G (5-26% ethanol, 95-74% gasoline)
• E85G (85% ethanol, 15% gasoline)
• E15D (15% ethanol, 85% diesel)
• E95D (95% ethanol, 5% water, with ignition improver)

Bioethanol has been extensively tested in light duty flexible fuel vehicles (FFV) as E85G. ETBE is also used in blends of 10-15 % with gasoline to enhance its octane rating and reduce emissions. Blends of gasoline with up to 22% ethanol (E22G) can be used in spark ignition engines without any material or operating problems. Blends of diesel with up to 15% ethanol (E22D) do not introduce any technical engine problem and require no ignition improver.

The introduction of E85 in Europe started in Sweden around the year 2000.  Only in the last 2 years has the E85 infrastructure expanded to other countries in the EU such as Germany, France and Ireland.  The map below shows the E85 infrastructure density in Europe as of April 2007.

Click here to download the map

Economics
Fermentation of sugars to ethanol is a mature technology, which is applied commercially on a large scale. There is a little chance of technological improvements that may significantly reduce the current production costs. These costs are largely determined by biomass feedstock prices, which can account for 55 - 80% of the final price of ethanol.

According to an ECN report, present production costs for ethanol derived from sugar and starch crops are 20 €/GJ (corn, USA–0.42 €/L, or 834 €/toe) and 15-25 €/GJ (sugar beet, North West Europe). This is about 0.32-0.53 €/litre, or 625–1040 €/toe.

Another source (BTG, 2004) presents the following bioethanol production costs analysis. Prices of 140 EUR/ton for common wheat, and 26.2 EUR/tonne for sugar beet are assumed. The co-product credit reduces the production costs of ethanol. In case of ethanol from wheat, the co-product is Dried Distillers Grains Soluble (DDGS), while the by-product of beet sugar ethanol is sugar beet pulp.

Bioethanol production costs in the EU-25 + Bulgaria, Romania