In 2014, Brazil generated 280 billion liters of vinasse, a waste byproduct of ethanol production. Almost all of it (97%) was used as fertilizer and in the process of irrigating the sugarcane itself. The problem is that this practice harms the environment and wastes the potential for better uses of vinasse, such as, for example, to generate electricity. Transforming vinasse into biogas with biodigesters can solve this problem, as demonstrated by two projects under development: one at the São Carlos School of Engineering at the University of São Paulo (EESC-USP), and the other at the Bioethanol Science and Technology Laboratory (CTBE) of the Brazilian Center for Research in Energy and Materials (CNPEM).
Vinasse is an ethanol waste byproduct that grew in importance after the National Alcohol Program, better known as Pro-Álcool, was created in 1975. Sugarcane juice is used to produce alcohol. What remains is the vinasse, an organic material rich in potassium, calcium and magnesium. Since the amount of waste coming from approximately 400 plants operating in Brazil is very large, for economic reasons, it is overused as fertilizer. Such excessive use causes environmental damage, such as groundwater contamination with potassium, soil salinization, leaching of metals and sulphates, the release of noxious odors and the emission of greenhouse gases such as nitrous oxide (N2O), which is about 300 times more polluting than carbon dioxide (CO2).
Marcelo Zaiat of EESC-USP coordinates the project whose goal is to make better use of vinasse. The research began in early 2011 and involves nine researchers from a number of institutions, including the EESC, the Federal University of São Carlos (UFSCar), São Paulo State University (Unesp) and the Mauá Institute of Technology (IMT), located in São Caetano do Sul. “Our main objective is to develop a new generation of anaerobic biodigesters, which are more compact, robust and stable, and highly efficient in converting the organic matter of vinasse into biogas,” says Zaiat. He notes that this type of equipment is designed to produce reactions catalyzed by bacteria and archaea microorganisms. The anaerobic process takes place in the absence of oxygen through the self-regulated fermentation of organic matter caused by a group of microorganisms living in these environments.
“What we want to do is transform the organic matter of vinasse into biogas with a culture of microorganisms,” says Zaiat. The primary component of biogas is methane, in addition to small quantities of carbon dioxide and other gases. After being treated, biogas can be used to generate sufficient electricity to drive the turbine of a generator. Furthermore, biogas production minimizes the environmental impact of the use of waste as fertilizer in the cultivation of sugarcane, since biodigested vinasse contains less organic matter.
There are no bioreactors on a large scale in Brazil, only a small one from the 1980s found in a plant in Ribeirão Preto (São Paulo State), which biodigests a small amount of vinasse, producing biogas used in yeast drying. As far as the equipment that his team is developing is concerned, Zaiat says that the progress in scientific knowledge on the fundamentals of anaerobic processes made in the last 30 years helped in designing the bioreactor. “Although the essential process is the same, today’s reactors are much more technologically advanced, enabling higher conversion efficiency, and greater process stability, in more-compact and secure systems,” says Zaiat.
The group is working with several biodigester configurations. “There are several techniques for this, but the one most used in our area provides a surface of inert material to which the bacteria and archaea adhere, forming what we call biofilm,” notes Zaiat. “We took advantage of their natural ability to adhere to surfaces with vinasse as a culture medium.”
The biogas produced in the bioreactor, with lower concentrations of CO2, can be used, for example, for energy cogeneration in the plant’s boilers, releasing the bagasse—now used to burn and generate electricity—for the production of second-generation ethanol. The gas can also be used to replace part of the diesel fuel used in trucks and tractors, thereby making the sugarcane production process more sustainable. The biodigested vinasse, in liquid form, can also be used as fertilizer, which is low in organic matter but preserves almost all of the original waste nutrients. Or it can be concentrated and used as a base to formulate an organomineral fertilizer for cultivating sugarcane, based on the needs of the sugarcane plant. In the latter case, the water withdrawn from the concentration process can go back to the production plant for various uses. “That’s what I call integration: the waste is used in the same production process,” says Zaiat.
Virtual production plant
The other project, coordinated by the CTBE and also including Zaiat, seeks, through mathematical models, to create a more efficient production plant in all its operations. “We are developing mathematical models for various plant operations, such as delivery and preparation of the sugarcane, juice extraction, fermentation, crystallization [sugar], distillation and dehydration [ethanol], energy cogeneration and biodigestion of the vinasse,” notes Antonio Bonomi, a chemical engineer and the project’s coordinator. “These models make it possible to optimize the process, that is, to define the operating conditions under which the biorefinery must work to maximize its yield and economic return.”
What Bonomi’s group is doing in this project is building a virtual first-generation production plant. To do this, simulation software known as Environment for Modeling, Simulation and Optimization (EMSO) is used to run mathematical models of each operation within the plant. “In the case of the vinasse biodigestion operation, for example, the mathematical model will indicate which potential flow into feeding the biodigester will produce the highest amount of biogas,” says Bonomi. One of the additional goals is to produce a gas with a higher methane content, that is, upgraded to 96.5% to produce biomethane. Today the maximum achieved in common reactors is 60%. With biomethane, it is possible, in addition to replacing diesel fuel in trucks and agricultural machinery, to inject it into the public natural gas distribution network.
The mathematical models and evaluations of the biodigestion unit are developed by Bruna de Souza Moraes, a CTBE researcher, and coordinated by Professor Rogers Ribeiro of the University of São Paulo, Faculty of Animal Science and Food Engineering (FZEA-USP). “Our job is to evaluate the inclusion of biogas production in production plants, beginning with waste and the potential for its use to promote energy optimization and environmental sustainability in the sector,” says Moraes. “The idea is to present the advantages of these scenarios with numbers, through technical, economic and environmental assessment, in order to simulate the actual application of this new biorefinery configuration.”
Moraes notes that so far the results have indicated that biogas production from vinasse is more advantageous when it is converted into biomethane (gas containing at least 96.5% methane). “Of these scenarios, its sale to the public natural gas network and use as a diesel fuel replacement showed the best economic and environmental indicators,” she says. “The latest assessment showed that it is possible to obtain up to a 42% reduction in greenhouse gas emissions in sugarcane production through partial replacement of fossil fuel.” The annual internal rate of return on investment for this scenario is 25% for a biorefinery with a processing capacity of 4 million metric tons of sugarcane per harvest.
Moraes has also crunched the numbers to show the potential for generating electric power from vinasse. In a plant that produces 50% ethanol and 50% sugar, with a capacity to mill 4 million metric tons of sugarcane per year, it is possible to produce about 2 million cubic meters (m3) of waste per year. Considering that 1 m3 of vinasse has the potential to generate up to 14 m3 of biogas, that same biorefinery would be able to provide 28 million m3 of this gas per year.
This amount represents an annual capacity of 65,000 megawatt hours (MWh) of electricity. “This means that the energy generated from biogas production from vinasse at a plant could supply the electricity needs of a city of about 260,000 inhabitants,” says Moraes. “If all of Brazil’s vinasse were biodigested, the potential for domestic electricity generation would be equivalent to 7.5% of the hydroelectric capacity of Itaipu Dam.”
The Brazilian Sugarcane Industry Association (Unica), which represents sugar and ethanol producers, supports the use of vinasse in irrigation as a way to save water with fertilization. “Fertigation (applying soluble fertilizers through an irrigation system) with vinasse also saves on the use chemical fertilizers,” says Alfred Szwarc, Unica’s consultant on emissions and technologies. “There are new ways of using vinasse, but they are still on a small scale. Other projects are looking at ways to concentrate and turn vinasse into commercial fertilizer,” says Szwarc.
1. Bioenergy production from wastewaters and environmental suitability of liquid and solid wastes generated (nº 2009/15984-0); Grant Mechanism Thematic Project; Principal Investigator Marcelo Zaiat (USP); Investment R$1,855,372.36 and $US428,142.36.
2. Simulation of 1st generation sugarcane biorefinery on Emso platform (nº 2011/51902-9); Grant Mechanism Thematic Project; Principal investigator Antonio Bonomi (CTBE); Investment R$2,410,414.09 and $US926,930.50.
3. Anaerobic digestion of vinasse and pentoses liquor in integrated biorefineries of 1st and 2nd generation (nº 2012/00414-7); Grant Mechanism Postdoctoral Research Grant; Grant Recipient Bruna de Souza Moraes (CTBE); Principal Investigator Marcelo Zaiat (USP); Investment R$302,095.11.