Second-generation ethanol with biogas

A study proposes combining cellulose ethanol production with biogas production and using the residue from the process as a source of energy for plants.

By Karina Toledo

Agência FAPESP – Second-generation ethanol, made from the cellulose found in sugarcane bagasse, is an important alternative for increasing biofuel production without jeopardizing food crops or environmental preservation areas.

However, because its production process is more expensive than that of first-generation ethanol – obtained through the fermentation of sucrose in sugar cane juice – scientists must find alternatives to make it economically feasible.

A group of researchers from Universidade Estadual de Campinas (Unicamp) have proposed that ethanol production be combined with biogas production and that the residue obtained in the process be used as a source of energy for plants.

The research project “Optimization of biomass pre-treatment and hydrolysis to maximize production of biogas based on agroindustrial residue” was funded by FAPESP and developed in partnership with researchers from the Institut National de la Recherche Agronomique (Inra) in France.

“At present, the sugarcane bagasse left over from making first-generation ethanol is burned and used by the industry as a source of electrical energy or thermal energy as steam. When we use this bagasse to make second generation ethanol, we manage to recover only 32% of the energy that would be obtained by burning it in a boiler,” says chemical engineer Aline Carvalho da Costa, coordinator of the study.

Under the model proposed by the researchers, some 65% of the energy is recoverable. The advantage is the increased production of liquid biofuel, which can be used for transportation. For this reason, the process has greater economic appeal. “Additionally, biogas and other residues can be used as sources of energy for industry, substituting bagasse,” stresses Costa.

In addition to the cellulose used in second-generation ethanol production, sugarcane bagasse contains hemicellulose – a substance comprising five-carbon sugars called pentoses – and lignin, the plant’s structural material, which imparts rigidity, impermeability and resistance to the vegetable tissue.

For the biomass to be transformed into biofuel, it must undergo a pre-treatment that separates the cellulose from the lignin, a substance that impedes hydrolysis. This is one of the most expensive and least technologically mature steps in the second-generation ethanol production process.

In the next step, the cellulose must be treated with enzymes that will break it into several glucose molecules so that microorganisms can begin fermentation. This procedure is known as hydrolysis.

“The lignin that is left over from pre-treatment can be burned and used as a source of energy. The same can be done with solid residues from the hydrolysis process. But when we speak of second-generation ethanol, the big question is: what do we do with the pentoses? That’s when we came up with the idea of turning them into biogas,” explains Costa.

The researcher explains that this type of sugar cannot be used in ethanol production because microorganisms cannot ferment it efficiently.

“Genetically modified organisms could force fermentation, but this would require biosafety infrastructure at plants, which would make production unfeasible in the current Brazilian scenario, although this could change in the long term,” she says.

Cane straw

Through an anaerobic digestion process conducted by a set of bacteria capable of degrading the organic matter, the researchers managed to transform these pentoses into biogas.

“This stage of research was conducted in France, a country with broad experience in biogas production from several residues, and had the participation of my doctoral student Sarita Cândida Rabelo,” said Costa. Rabelo received a FAPESP fellowship for her doctoral studies.

With the goal of transforming cellulose to ethanol in a more inexpensive and efficient manner, the researchers also compared two types of pre-treatment: one with lime and another with alkaline hydrogen peroxide. The latter proved to be more promising because it took less time and did not leave residue in the biomass.

“The step still needs improvement in order to make second generation ethanol more competitive,” stresses Costa. She suggests that the use of all residues in the production process is probably the only way to make the product economically feasible and environmentally sustainable. “Our major contribution was to show that the pentose-rich pre-treatment liquor has major potential for biogas production.  Although several alternatives for utilization of pentoses have been studied, none is definitive.”

The researcher stressed that second-generation ethanol will make it is possible to increase the country’s biofuel production without increasing the area where sugarcane is planted.

Although it is possible to obtain biofuel from practically any type of vegetable biomass, Brazil has invested in sugarcane biomass because it is an abundant input that is already at the processing plants, eliminating transportation outlays.

“We also researched ethanol production using sugarcane straw as a raw material. Straw makes up one third of the plant and is currently not used for anything. The partial results are similar to those obtained with ethanol production from bagasse,” says Costa.

This part of the study resulted in a master’s thesis that will be defended this March. Three other dissertations are also part of the project. The results of the study have been presented at national conferences and several indexed magazines, including Bioresource Technology and the Journal of Chemical Technology and Biotechnology.