Bioethanol (from sugar and starch crops)

On a world level, bioethanol is the most used biofuel. It is produced from sugar-containing agricultural products such as sugar cane (Brazil), corn (United States), wheat, sugar beet, waste from sugar refineries, or sweet sorghum. The predominant technology for converting biomass to ethanol is fermentation, which is a mature bio-chemical technology. In this process, the biomass is decomposed using micro organisms (bacteria or enzymes). Then, yeast converts the sugars present in the biomass to alcohol. Finally, the ethanol is distilled and dehydrated to obtain a higher concentration of alcohol to achieve the required purity to make the bioethanol suitable for the use as automotive fuel. Ethanol is best used in a spark ignition or Otto engine because of its high octane rating, implying very good anti-knock characteristics. However, when applying bioethanol as a petrol substitute, the lower vapour pressure and volumetric energy content (ca. two-third of that of petrol) should be taken into account. Moreover, ethanol is corrosive towards certain kinds of plastics and metals, but this does not cause problems in low-proportion blends with petrol. At present, bioethanol is applied in several EU Member States, however mostly in the form of its derivative ETBE (Ethyl Tertiary Butyl Ether), for example, in Spain and France. The reason for this is to avoid corrosiveness problems as a result of the presence of water in ethanol/petrol blends. ETBE is perfectly mixable with petrol (ETBE is allowed in blends up to 15% in European petrol) and improves the combustion properties of petrol. Bioethanol can be used in blends up to 20% with fossil petrol without necessary engine modifications, but currently only blends up to 5% in European petrol are allowed. In Sweden, a high-proportion bioethanol blend E85 (85% ethanol and 15% petrol) is being used in Flexible Fuel Vehicles (FFVs) with modified engines that are able to run on either E85 or petrol, or any mixture of the two.

Bioethanol (from lignocellulosic biomass)

Lignocellulosic or woody biomass is considered a future alternative for the agricultural products that are currently used as feedstock for bioethanol production, because it is more abundant and less expensive than food crops, especially when waste streams are used. Furthermore, the use of lignocellulosic biomass is more attractive in terms of energy balances and emissions. Lignocellulosic biomass consists of three main components, i.e. carbohydrate polymers called cellulose and hemicellulose that can be converted to sugars, and a non-fermentable fraction called lignin that can be utilised for the production of electricity and/or heat. Although the decomposition of the material into fermentable sugars is more complicated, the fermentation, distillation and dehydration process steps are basically identical for bioethanol from either agricultural crops or lignocellulosic biomass. Various process configurations are possible for the production of cellulosic ethanol, however, the most common method combines cellulose hydrolysis and fermentation of five- and six-ringed sugars in the same reactor (Simultaneous saccharification and co-fermentation, SSCF). In a more advanced process called Consolidated Bio-Processing (CBP), which will take long development, enzyme production, hydrolysis and fermentation all take place in the same vessel.

One of the main challenges for the production of ethanol from woody biomass is the development of an efficient pre-treatment process in order to break up the fibre structure of the biomass. There are several methods being developed: mechanical, thermal, chemical and biological processes and combinations thereof. However, none of them has proven to be suitable so far, due to their high costs, low yields, produced waste or undesired by-products. Another important research topic concerns the current high costs and low productivity of the enzyme cellulase, which is needed to convert cellulose to glucose in enzymatic hydrolysis. An alternative, acid hydrolysis, is available but for the hydrolysis of cellulosic materials it is capital intensive and has a negative effect on sugar yield. Although the performance of enzyme cellulase has improved rapidly over the past years, further improvement is still needed. Finally, a suitable organism for the fermentation of five-ringed sugars, which are present in hemicellulose, needs to be found or developed. Recently a breakthrough has been achieved by genetic modification of industrial yeast, but its robustness needs to be improved for industrial process conditions. At present, most research on cellulosic ethanol takes place in the US. There is one pilot production facility, which is located in Sweden.

Another technology for producing bioethanol from woody materials is the gasification of biomass, followed by fermentation to ethanol using anaerobic bacteria. This eliminates the need for hydrolysis to break up the cellulose and hemicellulose fractions of the biomass. Furthermore, in this process also the lignin fraction can be converted into ethanol. The development of this process is still at lab scale. Another process still under development is the gasification of biomass combined with catalytical processes to produce bioethanol, which especially gains more attention in the United States. For gasification to produce bioethanol the gasification process and the development of catalysts especially need more research.