Historically, the production of ethanol was developed thousand years ago when it was produced as wine from grapes. Ethanol was also produced from grains. For many years ethanol has been produced by the catalytic addition reaction of water to ethene which is a fraction from oil refining. But a sustainable ethanol production needs renewable raw materials (feedstock) and cost-efficient methods to be able to replace ethanol made from a fossil precursor. There are some differences in the processing technology between first generation feedstocks (sugar feedstock, starchy feedstock) and second generation feedstocks (lignocellulosic feedstock).

Generally, commercial bioethanol production requires several steps:

• Preparation of the feedstock to achieve maximum yield of the feedstock and also its sugar content.

• Preparation (actually size reduction) of the feedstock to achieve the right (optimal) physical size and form of the raw material in the ethanol production process. This also reduces the transport cost of the feedstock.

• Pre-treatment of the feedstock to release cellulose, starch or sucrose from lignin, fibre and other biological parts of the raw material.

• Hydrolysis of the feedstock to achieve partial or complete hydrolysis of the simple and complex polymeric molecules to produce sugar units. This hydrolysis might be either thermochemical hydrolysis or a combination of thermochemical and biochemical hydrolysis.

• Fermentation of the sugar units from the hexose fraction to ethanol by yeast.

• Fermentation of the sugar units from the pentose fraction to ethanol by other microorganism or enzymes.

• Several purification and distillation steps.

A schematic figure of different steps in bioethanol production is shown in Fig. 9.5.

Preparation of the feedstock is an important part of ethanol production since a substantial part of the ethanol production cost is the price of the feedstock, depending on what feedstock is used. Therefore, it is essential to optimise the yield of the feedstock with a high amount of fermentable sugar content. Preparation of the first generation feedstock usually includes cutting the material to proper size and form, e. g. sugarcane chopped and milled (dry or wet milling) and corn or woody materials chopped as chips. An optimum size of the feedstock reduces the transport cost and thereby production cost of bioethanol. Size reduction also increases the contact surface of the feedstock with the pre-treatment catalysts.

In the pre-treatment, the raw material is subjected to a mechanical or thermochemical treatment to make the carbohydrate polymers (cellulose and hemicellulose) available for hydrolysis. In the hydrolysis process, the polymers are hydrolysed into fermentable sugar units. Depolymerisation of lignocellulosic feedstock releases both pentoses and hexoses, depending on what type of feedstock is used. When the sugars have been released, fermentation takes place. The fermentation is an anaerobic catabolism of sugar by one or several microorganisms. After the fermentation step the ethanol concentration varies between 4% and 15% depending on what kind of feedstock is used (first or second generation) and what process. The ethanol is then purified by filtration and/or distillation steps. The ethanol concentration is increased to a maximum of 95% after distillation and after the absolutisation (drying) step it is more than 99% pure ethanol. The distillation and absolutisation steps require lots of energy. There are both similarities and differences between ethanol production techniques using first generation feedstocks and second generation feedstocks, and therefore the processing technology for ethanol production is separated into different sections.