Bioethanol is a biofuel, a renewable energy produced from organic matter such as sugar crops, cereals and agricultural or forestry waste. There are three generations of bioethanol, corresponding to different types of raw materials and production processes. The first generation comes from sugar- or starch-rich food crops (corn, sugar cane, sugar beet), while the second generation is obtained from lignocellulosic biomass (agricultural residues, forestry waste). The third generation is derived from algae or microorganisms genetically modified to enhance ethanol production.
Bioethanol is mainly used as a gasoline additive to reduce greenhouse gas emissions and improve engine performance. The United States and Brazil are the biggest producers and consumers of bioethanol, using corn and sugarcane respectively as the main raw materials.
The benefits of bioethanol include reduced dependence on fossil fuels, lower greenhouse gas emissions and the valorization of agricultural waste. However, first-generation bioethanol production raises environmental and social concerns, including competition with food crops and pressure on water and land resources.
First-generation bioethanol
Origin
These sources of bioethanol come from food crops rich in sugars or starch. The main feedstocks in this category include
- Sugar cane: widely grown in tropical regions, notably Brazil, sugar cane is a major source of bioethanol due to its high content of easily fermentable sugars.
- Corn: corn is a common source of bioethanol in the United States. The starch contained in corn kernels is transformed into fermentable sugars by an enzymatic hydrolysis process.
- Sugar beet: grown mainly in Europe, sugar beet is another source of bioethanol. Like sugar cane, it contains a high proportion of fermentable sugars.
The production process
irst-generation bioethanol is produced from sugar- or starch-rich feedstocks such as sugar cane, corn, sugar beet and wheat. Here are the key stages in the first-generation bioethanol production process:
- Feedstock preparation : Raw materials are received, cleaned and crushed to facilitate extraction of soluble sugars and starch.
- Liquefaction and saccharification : For starch-rich feedstocks such as corn and wheat, the starch is first mixed with water and heated to form a mixture known as “mash”. Enzymes (alpha-amylases) are added to break down the starch into the simpler sugar maltose. Then other enzymes (glucoamylases) are added to convert maltose into glucose.
- Sugar extraction : In the case of sugar-rich raw materials such as sugar cane and sugar beet, soluble sugars are extracted directly by diffusion or pressing, then dissolved in water to form a sweet juice.
- Fermentation : Glucose from starch or sweet juice extracted from raw materials is fermented into ethanol by micro-organisms (usually yeast). The fermentation process generally lasts 24 to 72 hours, producing ethanol and carbon dioxide.
- Distillation : The fermented mixture, called “vinasse”, is heated to separate ethanol from water and other residual compounds. The ethanol is vaporized and condensed to a concentrated product of around 95% by volume.
- Dehydration : Concentrated ethanol is dehydrated, usually by adsorption on a molecular sieve, to remove residual water and obtain an anhydrous product containing at least 99.5% ethanol.
- Denaturation and blending: Anhydrous ethanol is often denatured with an additive to render it unfit for human consumption. Finally, it is blended with gasoline to produce the final bioethanol blend, such as E10 (10% ethanol, 90% gasoline) or E85 (85% ethanol, 15% gasoline), which is used as fuel in compatible vehicles.
Second-generation bioethanol
Origin
Second-generation bioethanol sources are derived from non-food feedstocks, such as agricultural residues, forestry waste and dedicated energy crops. The main feedstocks in this category include
- Straw and crop residues: residues from corn, wheat, rice and other crops can be used to produce bioethanol.
- Forestry waste: wood chips, bark and branches from forestry operations can be converted into bioethanol.
- Dedicated energy crops: certain plants, such as miscanthus or switchgrass, are specifically cultivated for their lignocellulosic biomass.
The manufacturing process
Second-generation bioethanol production uses lignocellulosic raw materials such as agricultural residues (straw, stalks), forestry residues (branches, bark) and non-food energy crops (miscanthus, switchgrass). Here are the main stages in the production process for second-generation bioethanol:
- Feedstock preparation: Lignocellulosic feedstocks are received, cleaned and ground into small pieces to facilitate extraction of the sugars contained in cellulose and hemicellulose.
- Pre-treatment : The ground material is subjected to pre-treatment (thermal, chemical or biological) to separate the lignin from the polysaccharides (cellulose and hemicellulose) and make the sugars accessible to the enzymes. This step is crucial to improving the efficiency of subsequent enzymatic conversion.
- Enzymatic hydrolysis: Specific enzymes, called cellulases and hemicellulases, are added to the pre-treated mixture to break down cellulose and hemicellulose into simple sugars, mainly glucose and xylose.
- Fermentation : The simple sugars obtained are fermented into ethanol by micro-organisms, usually yeasts or bacteria. Some strains of micro-organisms are capable of fermenting both glucose and xylose, while others require separate co-fermentation.
- Distillation : As with first-generation bioethanol, the fermented mixture is heated to separate ethanol from water and other residual compounds. The ethanol is vaporized and condensed to a concentrated product of around 95% by volume.
- Dehydration : Concentrated ethanol is dehydrated, usually by adsorption on a molecular sieve, to remove residual water and obtain an anhydrous product containing at least 99.5% ethanol.
- Denaturing and blending: Anhydrous ethanol is denatured with an additive to render it unfit for human consumption. Finally, it is blended with gasoline to obtain the final bioethanol blend, such as E10 (10% ethanol, 90% gasoline) or E85 (85% ethanol, 15% gasoline), used as fuel in compatible vehicles.
The main challenge in producing second-generation bioethanol lies in the complexity of pre-treatment and enzymatic hydrolysis, which are costly and technically demanding steps. However, this technology has the advantage of valorizing non-food resources and reducing greenhouse gas emissions compared with first-generation biofuels.
Third-generation bioethanol
Source
Algae are a promising source of bioethanol. They can produce large quantities of biomass quickly and do not require agricultural land for cultivation. Algae can also be grown in saline or waste water, reducing the impact on freshwater resources.
Production methods
Third-generation bioethanol production relies on the use of microalgae or macroalgae (seaweed) as feedstock. These photosynthetic organisms can produce biomolecules, including sugars and lipids, used in bioethanol production. Here are the main steps in the third-generation bioethanol production process:
- Harvesting algae: Algal biomass is harvested by various methods, such as centrifugation, flotation or filtration, depending on the size and density of the algae.
- Sugar extraction : The cell walls of algae are broken down by mechanical, chemical or enzymatic methods to release the sugars contained in the biomass. In the case of macroalgae, polysaccharides (alginate, laminarin, fucoidan) are hydrolyzed into simple sugars (mainly mannitol and glucose) by specific enzymes.
- Fermentation : The extracted sugars are fermented into ethanol by micro-organisms, generally yeasts or bacteria, adapted to the specific composition of the sugars. Research is currently underway to develop strains of micro-organisms capable of efficiently fermenting sugars from algae.
- Distillation : The fermented mixture is heated to separate ethanol from water and other residual compounds, as in first- and second-generation processes. The ethanol is vaporized and condensed to a concentrated product of around 95% by volume.
- Dehydration : Concentrated ethanol is dehydrated to remove residual water and obtain an anhydrous product containing at least 99.5% ethanol.
- Denaturation and blending: The anhydrous ethanol is denatured with an additive to render it unfit for human consumption, and blended with gasoline to obtain the final bioethanol blend for use as fuel in compatible vehicles.
The main challenge in producing third-generation bioethanol lies in controlling the cost and efficiency of the algae cultivation, harvesting and extraction stages. Nevertheless, this technology has the advantage of not competing with food resources and of helping to reduce greenhouse gas emissions. Algae can also be grown on marginal land and use wastewater as a nutrient source, offering opportunities for waste recovery and impact mitigation
Bioethanol production and consumption worldwide
Bioethanol production and consumption have risen considerably in recent decades, driven by growing demand for renewable energy and the need to reduce greenhouse gas emissions. Here’s an overview of bioethanol production and consumption worldwide:
Production
In 2020, global bioethanol production was around 100 billion liters. The United States and Brazil are the two largest bioethanol producers, together accounting for around 85% of global production. Other major producers include the European Union, China, Argentina and Canada.
- United States: The United States is the world’s largest producer of bioethanol, with an annual output of over 50 billion liters. Most of this bioethanol comes from corn, a first-generation source.
- Brazil: Brazil is the second largest producer of bioethanol, with annual output of around 30 billion liters. Sugar cane is the main raw material used in bioethanol production in Brazil.
Consumption
Bioethanol consumption varies from country to country, depending on policies and regulations, as well as infrastructure and demand for renewable fuels. The United States and Brazil are also the two biggest consumers of bioethanol.
- United States: The United States consumes a large proportion of its bioethanol production. Ethanol is generally blended with gasoline at concentrations of up to 10% (E10) or 15% (E15) for light vehicles, and up to 85% (E85) for flex-fuel vehicles.
- Brazil: Bioethanol consumption in Brazil is also high, with common blends of ethanol and gasoline ranging from 18% to 27.5% (E18 to E27.5) for internal combustion engine vehicles. The country also boasts a large fleet of flex-fuel vehicles capable of running on blends of up to 100% ethanol (E100).
The European Union, China, Argentina and Canada are also major consumers of bioethanol, although their consumption is lower than in the USA and Brazil. Biofuel policies and regulations in these countries have a significant impact on bioethanol demand and consumption.
What is cellulosic bioethanol?
Cellulosic bioethanol is a type of second-generation bioethanol produced from lignocellulosic feedstocks. These feedstocks are composed of cellulose, hemicellulose and lignin, and are generally derived from agricultural residues (e.g. straw, corn stalks), forestry residues, or non-food energy crops specifically grown for biofuel production, such as miscanthus or switchgrass.