21.2.1 Bioethanol

Current bioethanol production technologies are based on the conversion of carbohydrates derived from sugar cane, sugar beet, maize or cereals (i. e. wheat, barley) into ethanol. In addition, bioethanol can be derived from a number of other agricultural commodities such as cassava, or from residues or waste streams from other agro-industrial processes, including cane or beet molasses and starchy residues.

A number of by-products or co-products are produced during the conversion of biomass to ethanol. Most prominent by-product from ethanol production from corn, wheat or barley is so-called DDGS, which is a protein-rich fibrous residue that is primarily sold as animal feed. DDGS is formed by combining insoluble residues from the fermentation step with soluble residual streams from the distillation step, and drying the combined product. The market price of DDGS devaluated in the last 20 years due to increased production volumes saturating the feed market. Other high added-value products need to be found for DGGS to maintain its co-product status.

A common by-product of sugarcane derived ethanol is bagasse, which is the fibrous residue of the sugar cane stem after extraction of soluble sugars. Bagasse is commonly used to generate electric power and heat at the sugar mill facility to supply the energy needed for the bioconversion process.

Upgrading of process residues like DDGS and bagasse to higher added-value bio-based products (i. e. chemicals, materials) — turning the processes into biofuel — driven biorefineries — maximises the sustainable valorisation of the raw biomass materials, increasing the market competitiveness of the bioethanol produced. DDGS is high in protein content (over 30%) which, if isolated, can be used potentially for the production of chemical precursors (Brehmer, 2008). Bagasse can also have many other applications such as the production of fibre boards or the production of high added-value specialty chemicals, i. e. xylitol from xylose-rich effluents from acid hydrolysis of sugarcane (Baudel, 2005).

Another by-product of bioethanol production is CO2, which in certain cases is marketed as gas for industrial use.