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Chemical engineering

Enzymatic route

New sugarcane bagasse pre-treatments make production of second generation ethanol easier

EDUARDO CESARAfter hydrolysis, bagasse sugar is ready for fermentationEDUARDO CESAR

The fermentation of sugarcane juice continues to be the best way to obtain fuel alcohol, a product whose supply may be rising even if the area of plantation has not increased. This is a challenge that Brazil must overcome in order to meet rising foreign demand for ethanol. It is also a possibility that is now looking more hopeful thanks to a study announced in early February by the United States’ Environmental Protection Agency (EPA), which showed that sugarcane ethanol is an advanced type of fuel that can lower the emission of harmful greenhouse gases by as much as 61% vs. gasoline. One solution is to make use of the sugar found in sugarcane bagasse and in its straw (the leaves left in the field during the harvest), in spite of the existing juice that is currently used in ethanol production, in order to make the so-called cellulose ethanol. The chief technological barrier to overcome is that the cellulose sugar in bagasse and in other biomasses is organized in large structures called polysaccharides, which the yeasts are unable to ferment directly in order to transform them into ethanol. To make it easier to convert cellulose into glucose chemically, in a process called hydrolysis, two Brazilian research groups, one coordinated by professors Rubens Maciel Filho and Aline Carvalho Costa, from the School of Chemical Engineering (FEQ) at the State University of Campinas (Unicamp), and the other, by professor Adilson Roberto Gonçalves, from the Lorena Engineering School (EEL) of the University of São Paulo (USP), in collaboration with the Federal University of Pernambuco (Ufpe), developed different processes for pre-treating the biomass – one at room temperature and the other requiring steam heating – to separate the three components that form the cellular walls of the plants (cellulose, hemicellulose and lignin). The three are interlinked and contribute to the rigid texture of the plants.

In the process developed by Rubens Maciel’s research group, which includes professor Aline Carvalho da Costa and PhD candidate Sarita Cândida Rabelo, both from FEQ-Unicamp, the separation of the components is conducted with a chemical product, hydrogen peroxide, at ambient temperature. “The hydrogen peroxide is put in contact with the bagasse, which doesn’t require any treatment prior to being used,” says the researcher. The product attacks the vegetable structure in such a way as to release in liquid form, cellulose and hemicellulose, in addition to dissolving the lignin, which is later recovered to be used in other ways, ranging from the manufacturing of chemicals to the generation of energy by burning it in boilers. Cellulose is a polysaccharide formed by glucose monomers, which are structures with six interconnected carbon atoms. When the cellulose undergoes hydrolysis, it releases these monomers, which yeasts can then easily ferment. The hemicellulose has pentose monomers in its structure, i.e., sugars with five carbon atoms. These are far harder to convert into ethanol with the microorganisms currently available for fermentation. As for lignin, it is a complex organic macro-molecule that binds the cellulose fibers, increasing the rigidity of the vegetable wall.

“The advantage of the process that we developed is that it is conducted at room temperature and it is very fast, taking about one hour,” says Maciel. Moreover, it does not leave any environment polluting waste. “It’s a low-cost process, thanks to the speed with which the peroxide decomposes the lignin-cellulose structure, with no energy expenditure.” The pre-treatment is an auxiliary operation for the dismantling of the vegetable structure, thereby making the cellulose material available to be hydrolyzed by those microorganisms that are able to extract glucose from cellulose in order to make ethanol. “It is very important to prepare this cellulose material in a way that in the hydrolysis phase one can use the lowest possible quantity of enzymes, in order to keep the costs of this stage down, for the process to be commercially acceptable,” says Maciel. The enzymes are proteins made by fungi, bacteria and plants capable of producing specific chemical reactions, with no composition change. One of the enzymes most often used to make ethanol, which was chosen for its efficiency in the pre-treatment process trials, is produced by the firm Novozymes, a Danish multinational that makes industrial enzymes used in detergents, biofuels, and food manufacturing, among other products. The production of enzymes for the manufacturing of ethanol is also part of one of the lines of research conducted at the university’s School of Chemical Engineering. The objective is to develop an enzyme from the bagasse itself, in order to eliminate the purification stage, which makes the end product more expensive.

EDUARDO CESAREthanol produced by enzymatic hydrolysisEDUARDO CESAR

“Our process enables one to obtain ethanol from biomass with a low enzyme load, considerably lowering production costs,” says Maciel. As a result of all these innovations, such as using an inexpensive raw material for deconstructing the vegetable structure, the process led in Inova, the Unicamp Innovation Agency, to submit a patent request to INPI, Brazil’s National Institute of Industrial Property. “The advance of our pre-treatment process lies in using hydrogen peroxide, besides the operating conditions, such as temperature and the operation’s time, which are protected by the patent,” he says. With this process, which up to the current phase has proved itself viable for large-scale use, the group managed to make all the sugar in the bagasse available for fermentation. “Without the pre-treatment process, only about nine percent of the bagasse sugar is transformed into ethanol.”

Semi-industrial scale
For the time being, the trials are being conducted on a laboratory scale. “But as this is a process that uses a reactor, a tank which spins, with which we are highly familiar from other processes carried out in chemical, petrochemical and biotechnology industries, we foresee no problem in switching to a larger scale.” One of the next stages will consist of testing the process on a semi-industrial scale in a pilot plant. This is already being built at CTBE (the Center of Bioethanol Science and Technology), which is part of the Ministry of Science and Technology, in the city of Campinas, inner-state São Paulo. The same facilities are to be used by the research group coordinated by professor Adilson Roberto Gonçalves, research leader of the Vegetable Biomass Conversion group at USP’s Lorena Engineering School (EEL), to test on a large scale their process for extracting ethanol from cellulose, based on a thermal treatment with steam, which breaks down the rigid structure of the biomass, making the polysaccharides available for the yeasts.

The bagasse placed inside a closed reactor is impregnated with steam at temperatures ranging from 170 to 190 degrees Celsius, for seven minutes. This system has an opening valve at one of its exits. When this valve is opened quickly it results in sudden decompression, a process called a steam explosion. The process is employed by a mill in the Ribeirão Preto region, in inner-state São Paulo, to make cattle feed from sugarcane bagasse. “The destructuring of the bagasse, in this case, is conducted to aid the animals’ digestion,” says Gonçalves. However, this system was yet to be used in the production of ethanol. Professor Ana Maria Souto Maior, from the Federal University of Pernambuco, one of the project collaborators, is testing certain conditions of the process adapted for ethanol in a reactor used in the Bioethanol project research, financed by CNPq (the National Council for Scientific and Technological Development) and by Finep (the Studies and Projects Finance Agency). One of the PhD students under Gonçalves, Priscila Maziero, is to do a traineeship at the University of Lund, Sweden, to study the processes of agricultural waste hydrolysis. She recently spent two weeks at UFPE monitoring the trials.

What is left after the heat pre-treatment is a solid mixture of lignocellulose, a combination of lignin and cellulose with liquid (a water solution of the hemicelluloses components). After this, the lignocellulose goes through a chemical extraction stage to remove the lignin, leaving only the cellulose. Studies conducted by the group with electron and a light refraction microscopy at LNLS (the National Laboratory of Synchotron Light) in Campinas, with the collaboration of professor Igor Polikarpov, from USP’s Physics Institute in São Carlos, indicated (though this is still subject to final analysis) that the direct hydrolysis of the lignocellulose material jeopardizes the work of the enzymes. “So we decided to incorporate an alkaline dis-lignification stage, which results in removing the lignin. This is akin to what is used in the processes that obtain pulp for making paper,” says Gonçalves. To achieve this, one uses a base, which in a laboratory is sodium hydroxide; however, in industry, this can be replaced by lime or sodium oxide, depending on costs. “However, our alkaline dis-lignification system is mild, with solutions that contain about 1% of sodium hydroxide, while in the cellulose pulp making process this can go as high as 20%,” says Gonçalves, who had collaboration from professor George Jackson Rocha, also from EEL. Once the lignin has been eliminated, the cellulose is ready for enzymatic hydrolysis. This group also used Novozymes enzymes. “The origin of the cellulose that is to be degraded makes little difference in terms of the end result, but the enzyme used makes a big difference,” says the researcher.

The initial focus of both projects was the bagasse that results from crushing sugarcane. Although the mills currenly burn part of it, calculations indicate that there is still a 30% surplus of this biomass. However, nothing stands in the way of the two pre-treatment processes also being used to process the sugarcane straw that is left on the ground during the harvest. “As burning is prohibited, the tendency is for the straw also to be added to the bagasse and for this biomass to be used to complement the production of liquid fuels,” says Maciel. For the time being, there is no scheme for collecting the straw from the field. Part is chopped up and used to cover the soil and the rest has no practical use. “There is a technological challenge to be overcome for the straw not to rot away in the field,” says Gonçalves.