Generally, bioethanol production can be classified into three main types, depending on used/converted feedstocks (figure 1):
Bioethanol production of 1st generation is made on the one hand from starchy feedstocks, e.g. seeds or grains such as wheat, barley and corn. Those feedstocks contain starch, which is then fermented into bioethanol. On the other hand, sucrose-containing feedstocks are used to produce bioethanol of 1stgeneration, e.g. sugar beet, sugar cane and sweet sorghum. Sugar is then converted into bioethanol through fermentation with yeast. But those feedstocks also could enter the animal or human food chain. Therefore bioethanol production of 1st generation is critically regarded by global population, worrying about food shortages and price rises.
Bioethanol production of 2ndgeneration can use non food crops (not counted in animal or human food chain), including waste and remnant biomass e.g. stalks of wheat and corn, wood, straw and grass. Those feedstocks are composed of lignocellulose (cellulose, hemicellulose and lignin), which is problematically to be converted into bioethanol. The sugars, which are necessary for being fermented into bioethanol are embedded inside the lignocellulose. Therefore, lignocellulose has to be decomposed, in order to extract the fermentable sugars. Another problem concerning bioethanol production of 2nd generation is the high percentage of pentoses (C5-sugars) in the hemicellulose, e.g. xylose, which are difficult to be fermented by yeast.
Bioethanol production of 3rd generation uses algal biomass. Algae are photosynthetic microorganisms that convert sunlight, water and carbon dioxide to low-input, high-yield algal biomass. For example, macroalgae (seaweed) have a great potential for bioethanol and biogas production. Additionally, many microalgae are exceedingly rich in oil, which can be converted to biofuel using existing technology. Moreover, microalgal biofuel has the potential to be able to completely displace petroleum-derived transport fuels without adversely impacting supplies of food and other agricultural products.
Figure 1: Schematic classification of used/converted feedstocks for bioethanol production