Bioethanol

Bioethanol is actually ethyl alcohol (or also referred to as ethanol), identical to drinking alcohol by its composition. There are mainly two ways of producing ethanol, namely by synthesis of hydrocarbons and from biomass. Only the second approach deserves the terminology "bioethanol".

Production pathways

All fermentable sugars (i.e. glucose, sucrose, etc.) may be converted to ethanol by fermentation. Such sugars are present in a more or less polymerized state in many species of the plant world such as sugarbeets, sugarcane, wheat, corn, potatoes, but also in grass or wood. Some waste products (e.g. cheese whey, waste paper, etc.) and various residues may also be converted into bioethanol.

Figure : Bioethanol production process

Depending on the state of polymerization, the sugars have to undergo one or several treatment steps, with the aim of transforming the various polymer chains in simple fermentable sugars. After fermentation by means of micro-organisms (yeasts, bacteria, etc.), ethanol is recovered by distillation (hydrous ethanol at 92-96% vol.) followed by dehydration (anhydrous ethanol at 99.7% vol.). The above figure shows the schematic process of bioethanol production from cereals (source: DesMoinesRegister.com).

The main bioethanol production pathways are described in more detail below.

Figure : Production of bioethanol from sugarbeet

The idea of using bioethanol as a fuel is not new. In the early 20th century indeed, Henry Ford had imagined using ethanol to fuel its legendary "FORD Model T." Historically, the production of ethanol from biomass has been limited (mainly for technical reasons) to the conversion of simple sugars readily available in the soluble form (sugarcane, beets, fruit) or starch (cereals).

Bioethanol production from lignocellulosic biomass

More recently, new technologies have emerged to enable the conversion of lignocellulosic biomass (wood, grass, agricultural wastes and residues, etc.) into ethanol. For instance, if one hectare of sugarcane produces about 10 tons of sugar and 3 tons of molasses, it also produces 20-25 tonnes of biomass, inedible but nonetheless potentially convertible into ethanol. These few remarks highlight the relevance of lignocellulosic biomass in terms of both cost and availability, but also with regard to the potential competition with food. Given the current price of crude oil and the difficulty of the political world to implement some form of CO2 tax, bioethanol is likely to keep suffering from the law of fossil fuels as long as significant cost reductions cannot be done. In this context, lignocellulosic biomass offers significant prospects in terms of production cost reduction in the short-to-medium term, by its abundance and its potentially lower procurement costs with respect to other feedstocks.

Lignocellulosic biomass is composed mostly of cellulose (a polymer of glucose) and hemicellulose (a polymer of xylose and other C5-C6 sugars). To produce bioethanol, these components must first be converted into simple sugars by hydrolysis (reaction aimed at breaking the long chains of polymers). If acid hydrolysis (treatment with sulphuric acid at high temperature) is already applied at a commercial scale in various areas, enzymatic hydrolysis seems however to present a greater potential (in terms of both cost and sustainability) although the technology is today limited to demonstration/pilot scale. There are today only four demonstration/pilot facilities worlwide, applying this technology to the production of ethanol. The National Renewable Energy Laboratory (USA) has built a pilot plant capable of producing 120'000 litres per year. Since 2003, the company Iogen Corporation (Canada) has been operating a demonstration plant with a capacity of 320'000 litres of ethanol per year, using wheat straw as the main feedstock. In 2004, the company Sekab (Sweden) has developed a pilot plant of 120'000 litres of bioethanol per year from soft wood. Since 2006 finally, the company Biogasol (Denmark) has been operating a pilot plant with a capacity of 15'000 litres of bioethanol per year.

Most recently (read the news on the subject), the US Department of Energy announced its decision to invest up to 385 million US$ in 6 projects biorefineries over the next 5 years. Once operational, these biorefineries are expected to produce about 450 million litres of bioethanol per year.

Specifications of bioethanol (European Norm)

In recent years, the Technical Committee 19 (TC 19) CEN has been working on the development of a new standard for fuel-ethanol. The European standard for ethanol is currently being prepared (pr EN 15376 final draft), and is expected to be adopted in 2008. This standard should apply to bioethanol used as a fuel in a mixture with conventional gasoline at up to 5% vol. Regarding E85 (a mixture of 85% ethanol and 15% gasoline), a draft standard (CWA 15293) was developed and accepted, and should soon lead to a proper standard.