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Adding value to gas

Project is to take advantage of CO2 to grow microalgae and cyanobacteria

João Carlos Monteiro de Carvalho / USPUSP reactors produce Spirulina with carbon dioxide from sugar mills and ethanol plantsJoão Carlos Monteiro de Carvalho / USP

After the sugarcane straw and bagasse burnt in boilers was used to generate energy, it is now the turn of the carbon dioxide (CO2) from the alcoholic fermentation in sugar mills and ethanol plants to be used as a byproduct with high added value. Research at the School of Pharmaceutical Sciences (FCF) of the University of São Paulo (USP) showed that this gas could be recycled to grow photosynthesizing microorganisms, such as microalgae and cyanobacteria, which can be used as raw materials in several production processes in the food, energy, medical drug, and cosmetic industries. One example is the Spirulina platensis, a cyanobacterium that can be used as a food complement and a source of protein and vitamins or included in grain and animal feed. These microorganisms can also be used as pigment to create natural colors, such as phycocyanin or chlorophyll.

These photosynthesing microorganisms also have a high fatty acid content and might help the national power generation system in the production of biodiesel – there are studies in several countries on obtaining biodiesel from microalgae. Other applications include the use of the molecules by the pharmaceutical, cosmetic and chemical industries. The research, under the coordination of the pharmacist João Carlos Monteiro de Carvalho, from the Department of Biochemical-Pharmaceutical Technology, led to a patent request and was conducted with the collaboration of professor Sunao Sato and several students, along with the researcher Attilio Converti, from the University of Genoa, Italy.

“Our work deals with using CO2 immediately in growing these microorganisms, which use light as a source of energy, or with storing it for future use,” explains Carvalho. Globally, the firms that currently produce these microorganisms use compressed and purified CO2 in cylinders to render their production feasible. However, the USP study showed that the gas produced in the alcohol fermentation reactor in the mills could be injected by means of a flow of bubbles directly into other reactors, where the microalgae and cyanobacteria grow. The CO2 has two functions: “It replenishes the carbon consumed by these microorganisms in the photosynthesis process while also maintaining the pH for their growth,” explains. This is the way to use the CO2 directly. Alternatively, the gas could be purified and stored for future use. The storage would be handled as follows: the CO2 captured from the alcohol fermentation equipment would be put through an alkaline environment – such as sodium hydroxide (caustic soda) – and upon reacting with it would form sodium bicarbonate or sodium carbonate, used to grow microalgae. “Thus, the carbon dioxide could be held in the form of a liquid alkaline solution to be used later, for instance, during the sugarcane off-crop season, when there is no cane processing nor sugar and alcohol production and therefore no CO2 at the mills,” explains Carvalho.

Ethanol in grams
The growth of photosynthesizing microorganisms occurs within reactors, which can be closed or open. At the USP laboratories, the tests used 3.5 liter closed reactors, but the scientific literature does report open reactors with about 5,000 sq. m. “The type of reactor in which cyanobacteria or microalgae grow doesn’t affect the process, because, essentially, the working principle of the carbon dioxide is the same.” The potential use of this gas as a raw material for growing these microorganisms is huge. According to the researcher, for each molecule of glucose consumed in the alcoholic fermentation of sugarcane, two molecules of ethanol and two of CO2 are formed. In other words,  each kilogram of ethanol produced yields about 0.96 kilogram of CO2. As Brazil’s total production of ethanol in the 2008/2009 crop amounted to 27.5 billion liters (or 21.7 billion kilograms, as one liter of ethanol weights 0.789 kilogram), 20.8 million tons of carbon dioxide were released into the atmosphere. Although almost all this gas is consumed by the sugarcane plantation, including the gas released by automobiles, in the photosynthesis process one can compare these figures with the emissions of a diesel bus operating in a major city, which is 100 tons of CO2 a year. In São Paulo city, for example, some one million tons of CO2 are generated a year by its 10 thousand buses.

Besides the CO2 from alcoholic fermentation, the project, which was financed by FAPESP, also involved reusing the gas generated by the mills when they burn bagasse. In this process, even more CO2 would be generated if all the bagasse were burnt to produce energy: some 83 billion kilograms of gas. However, the resulting CO2 is not as pure and would require scrubbing and purifying before its injection into the microalgae and cyanobacteria reactors. The project, according to Carvalho, would lead to the reduction of CO2 emissions throughout Brazil.

The idea of making use of gases containing CO2 to grow microorganisms was researched, back in the 1980’s, by professor Eugênio Aquarone, who was also from FCF-USP. His group, in a study conducted jointly with the University of Florence, Italy, assessed the effect of the CO2 from alcoholic fermentation in the production of Spirulina maxima. “In our patent request, however, we present methods that help to make it feasible to use carbon dioxide from fermenting sugarcane or burning bagasse to grow photosynthesizing microorganisms,” says Carvalho.

The project
Growing Spirulina platensis (Arthrospira) in a tubular reactor using urea as a source of nitrogen and pure CO2 or CO2 from alcoholic fermentation (nº 2006/56976-2); Type Regular Research Awards; Coodinator João Carlos Monteiro de Carvalho – USP; Investment R$70,656.98 and US$37,145.92 (FAPESP)

Scientific article
RODRIGUES, M.S. et al. Fed-batch cultivation of Arthrospira (Spirulina) platensis: Potassium nitrate and ammonium chloride as simultaneous nitrogen sources. Bioresource Technology. v. 101, p. 4.491-98. 2010.