The depletion of non-renewable resources gained focus in the 1970s, with heated debates fueled by the advancing oil crisis and influenced by publications such as “The Limits to Growth” (Club of Rome, 1972). It was during this time that the first generation of biofuels emerged, produced from agricultural products and, therefore, based on renewable resources. With the Proálcool program, Brazil boosted the production of ethanol from sugarcane, which today replaces about 40% of the gasoline consumed in the country. In the United States, tons of corn starch are transformed into ethanol every year, while Europe has chosen wheat for this purpose.
Renewable sources, yes, but that does not mean they are completely sustainable. It is enough to remember that, in Brazil, the area cultivated with sugarcane increased a remarkable 9.2% in just one year (between 2010 and 2011). Furthermore, first-generation ethanol competes with food production, requiring increasingly larger cultivation areas. The current concern with the reduction of renewable resource stocks – such as forests – has led to the development of a new generation, bioethanol, this time based on an abundant resource that is wasted every day: cellulose. This chain formed by glucose molecules is the most abundant natural polymer on the planet. And it is found in large concentrations precisely in the waste generated by agriculture and forestry: sawdust and wood chips, sugarcane and orange bagasse, straw from sugarcane/corn/wheat/rice, etc... Tons of biomass with great potential to turn into bioethanol.
Cellulose is found in the cell wall that surrounds plant cells. It is usually associated with hemicellulose (which can also be utilized for ethanol production) and lignin, together forming the lignocellulosic matrix. The first step in ethanol production consists of separating these substances. The next step involves breaking down the polysaccharides into smaller parts, which will enable the action of the yeasts responsible for fermentation. Cellulose breaks down into hexoses (molecules with 6 carbons, such as glucose), while hemicellulose yields pentoses (5-carbon molecules). This may seem like just a detail, but it makes all the difference in the fermentation process. The fermentation of hexoses is reasonably well known, a technology retrieved from the first generation of biofuels. On the other hand, the possibility of using pentoses in ethanol production is a significant novelty, which in the future will greatly increase the productivity of the process, generating less waste and reducing costs.
The separation and fragmentation of cellulose/hemicellulose can be carried out through heat associated with the action of chemical agents. However, heating the mixture may require such high energy expenditure that it is not worthwhile. Purifying the final product by removing the applied chemical agents is also a complicated process. A new line of research is seeking enzymes that can perform this task. The inspirations are found in nature: fungi found on fallen logs literally feed on wood, just as termites digest cellulose with the help of microorganisms that live in their digestive systems. Enzymes isolated from these and other living beings have shown promising results, but transitioning to industrial scale is difficult. Nevertheless, this technology is beginning to bear fruit. GraalBio has just announced that it will start commercial production of second-generation bioethanol in 2013, at a facility in the state of Alagoas.
As cellulose ethanol technology advances, a new parallel direction for biofuels is beginning to emerge. The third generation focuses on the application of microorganisms and will rely on genetic engineering. One possible path is to use biomass from microalgae as a substrate (carbon source) for bioethanol production. Its cultivation is simpler than that of conventional agricultural products, and it requires much smaller areas, as its productivity is significantly higher. The other steps are similar to the processes already used in bioethanol production. Another possible alternative, even more elegant, is the development of microalgae that can produce bioethanol on their own. Experiments are already being conducted with blue-green algae (cyanobacteria).
Indeed, there is no single solution. But it is this flexibility that makes bioethanol such a promising technology. Its production is possible from multiple biomass sources and can be adapted according to local conditions and needs. By utilizing waste that would otherwise be useless, more value is added to agriculture. Meanwhile, the cultivation of microalgae could expand the range of options in regions less suited to conventional agriculture. All of this without the need to expand cultivation areas and, consequently, reducing deforestation. Thus, second and third-generation bioethanol proves essential in developing a new energy matrix, cleaner, and capable of driving sustainable development.
Consulted bibliography:
AHMAD, A. L. et al (2011) Microalgae as a sustainable energy source for biodiesel production: a review. Renewable and Sustainable Energy Reviews 15: 584-593
EMBRAPA (2011) Lignocellulosic ethanol (folder). Available at: http://www.infoteca.cnptia.embrapa.br/bitstream/doc/887226/1/Etanolcelulosico.pdf
