Fermentation is an anaerobic process  that breaks down the glucose within organic materials. It is a series of chemical reactions that convert sugars to alcohol or acid. Yeast or bacteria are added to the biomass material, which feed on the sugars to produce ethanol and carbon dioxide. The ethanol is distilled and dehydrated to obtain a higher concentration of alcohol to achieve the required purity for the use as automotive fuel. The solid residue from the fermentation process can be used as cattle-feed and in the case of sugar cane; the bagasse can be used as a fuel for boilers or for subsequent gasification.

Source: http://www.biomassinnovation.ca

Fermentation of forest and industrial residues
Raw materials containing sugars or be transformed into sugars can be used as fermentation substrates. The fermentable raw materials can be grouped as directly fermentable sugary materials, starchy, lignocellulosic materials and industrial wastes. Sugar containing materials require the least costly pretreatment, where starchy, lignocellulosic materials and urban wastes needed costly pretreatment, to convert into fermentable substrates.

Fermentation of agricultural wastes
Conventional crops such as corn and sugarcane are unable to meet the global demand of bioethanol production due to their primary value of food and feed. Therefore, lignocellulosic substances such as agricultural wastes are attractive feedstocks for bioethanol production. Agricultural wastes are cost effective, renewable and abundant. Bioethanol from agricultural waste could be a promising technology though the process has several challenges and limitations such as biomass transport and handling, and efficient pretreatment methods for total delignification of lignocellulosic. Proper pretreatment methods can increase concentrations of fermentable sugars after enzymatic saccharification, thereby improving the efficiency of the whole process. Conversion of glucose as well as xylose to ethanol needs some new fermentation technologies, to make the whole process cost effective. Goals of an effective pretreatment process are:

(i) formation of sugars directly or subsequently by hydrolysis
(ii) to avoid loss and/or degradation of sugars formed
(iii) to limit formation of inhibitory products
(iv) to reduce energy demands
(v) to minimize costs.

Source: Khan and Dwivedi,2013. Fermentation of Biomass for Production of Ethanol: A Review. Universal Journal of Environmental Research and Technology.