Cellulosic ethanol

Summary: Cellulosic ethanol, based on the hydrolysis of woody or fibrous biomass, is the second generation of biofuels, attractive because it does not use the food from crops, such as corn or sugarcane, and is more sustainable because it can be grown on degraded land.

Cellulosic ethanol is produced through the hydrolysis of woody or fibrous biomass. Cellulosic ethanol is often referred to as a second-generation biofuel. Feedstock for cellulosic biofuels is generally divided into two categories: dedicated energy crops (such as willow, switchgrass, and eucalyptus) and agricultural residues (the nonfood parts of crops, such as stems, husks, leaves, and forestry waste).

Second-generation biofuels are more environmentally sustainable than first-generation biofuels because they can make use of abandoned and degraded land. First-generation biofuels are produced primarily from food crops (including sugarcane and vegetables such as corn), and there have been ongoing concerns about the displacement of food crops and competition over scarce resources, particularly the land and water used for those food crops. Second-generation biofuels make use of residues from agricultural and forestry production and can thereby assist in creating additional revenue streams in rural areas without requiring increased use of arable land. Constraints occur in areas where rural populations are dependent on agricultural residues for animal fodder or domestic fuel.

Production Processes

Cellulosic materials, such as straw and wood, are much cheaper and often more readily available than starch- and sugar-based feedstocks. However, the fermentation of cellulosic raw material to ethanol requires complex and costly processes. Cellulosic material contains cellulose and hemicellulose, bound together by lignin, which makes the fermentation of ethanol difficult, because the lignin protects the cellulose from breakdown by enzymes; moreover, when the enzymes are able to reach the cellulose, they are hindered by the crystalline structure of the molecules. The process therefore involves a high-temperature pretreatment, followed by treatment with enzymes and a two-stage fermentation process.

Cellulosic ethanol is currently not economically cost-competitive with gasoline (a fossil fuel) or corn-based ethanol. More research is required to reduce the costs and improve the efficiency of commercial enzymes, including onsite production of enzymes as part of the ethanol production process. Currently, no large-scale cellulosic ethanol production facilities are operating or under construction. However, technological advancements in the processing could soon make cellulosic ethanol a more economically viable and sustainable option for expanded ethanol production.

Growth

Although second-generation biofuels are not yet commercially produced on a large scale, several pilot plants have been established. In 2007, the U.S. Department of Energy (DOE) invested $385 million for six biorefinery projects, which were expected eventually to produce more than 130 million gallons of cellulosic ethanol per year. Several demonstration plants for production on a commercial scale are under development in Denmark, France, and Europe. Projections published in the World Energy Outlook 2009’s 450 Scenario estimated that biofuels could provide 9 percent of total transport fuel demand by 2030, which could grow to 26 percent by 2050. In 2050, second-generation biofuels are likely to make up 90 percent of all biofuels, with more than half of the production estimates to occur in countries that are not members of the Organization for Economic Co-operation and Development (OECD), mainly China and India.

In 2022, the US was the largest producer of ethanol, accounting for roughly fifty-five percent of the global output. Though scientists were initially hopeful about the widespread adoption of cellulostic ethanol, they overestimated the technology available. Though DuPont, Poet-DSM, and Aberngoa built commercial-scale cellulosic ethanol plants during the 2010s, none remained in operation in the 2020s.

Impacts

Second-generation biofuels have potential to reduce impact on water quality significantly over their first-generation counterparts—largely because second-generation crops generally are more tolerant of dry weather and drought and can more easily be cultivated on degraded land. However, they may require more of this marginal land to achieve necessary levels of productivity. Furthermore, much like first-generation crops, advanced biofuel crops need to be sustainably cultivated to reduce soil erosion and depletion of natural resources.

More research is required to understand the impacts of cellulosic ethanol production on the hydrologic cycle. In some instances, the complete removal of agricultural residues for biofuel production may lead to erosion and nutrient runoff to water supplies, as well as soil impoverishment and concomitant requirements for additional nutrient application for future crops. As cellulosic bioenergy crops become more viable, policy instruments will be required, based on local conditions, to ensure sustainable production and reduce nonpoint pollution.

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