Secondly, the expansion will be fueled by new opportunities to produce energy from two currently underutilized waste materials in ethanol production: stillage, a liquid byproduct from the ethanol distillation process (also known as vinasse); and filter cake, a solid material with high moisture content that is produced during the purification of sugarcane juice. Both substances are rich in organic matter that can be transformed into biogas and a liquid biofertilizer using anaerobic biodigestion\u2014a fermentation process that occurs in the absence of oxygen.<\/p>\n
The biogas produced by biodigestion is approximately 62% methane (CH4<\/sub>), 37% carbon dioxide (CO2<\/sub>), and 1% other gases, explains Julio Romano Meneghini, scientific director at the University of S\u00e3o Paulo\u2019s (USP) Research Center for Gas Innovation (RCGI). Thermal power stations can use this biogas as fuel to produce electricity.<\/p>\n Eliminating CO2<\/sub> through an additional purification step generates a biomethane product that meets the National Petroleum, Natural Gas and Biofuels Agency (ANP) specifications for piped gas, which can be used as fuel in some industries, as a replacement for compressed natural gas (CNG), or as a fuel for thermal power stations to produce electricity. Sugar and ethanol plants can also use biomethane to fuel their trucks and tractors in replacement of diesel. Some of the leading truck and agricultural machinery manufacturers in Brazil have announced plans to launch heavy vehicles powered by natural gas in the near future.<\/p>\n RCGI, an Engineering Research Center (CPE) created by FAPESP in partnership with Shell, estimates in a report published in August that the stillage, cane leaves, and filter cake currently available in the 10 major sugar- and ethanol-producing cities in S\u00e3o Paulo State would be enough to generate 3 billion normal cubic meters<\/a> (Nm3<\/sup>, a measure of gas volume) of biogas per season.<\/p>\n Lead author Suani Teixeira Coelho, a researcher at USP\u2019s Institute for Energy and the Environment (IEE), says current biogas potential could be used to generate almost 32,000 GWh of electricity, equivalent to around 80% of residential consumption in the state of S\u00e3o Paulo, according to data from the S\u00e3o Paulo State Energy Budget. Alternatively, if upgraded into biomethane, it could be used as a replacement of 30% of the natural gas consumed in the state.<\/p>\n Biodigestion of stillage<\/strong> Andr\u00e9 Elia Neto, an environmental and water resource consultant at UNICA, notes that the industry has been in search of better applications for its stillage byproduct for several decades. Generating biogas through anaerobic biodigestion in covered ponds or cylinders has been attempted multiple times since the 1980s. \u201cWhile the experiments were technically successful, they were ultimately abandoned because the process was sown not to be economically feasible,\u201d he says.<\/p>\n Geo Energ\u00e9tica<\/span><\/a> A Geo Energ\u00e9tica plant producing biogas, biomethane, and electricity in Tamboara, Paran\u00e1Geo Energ\u00e9tica<\/span><\/p><\/div>\n Recently, however, ethanol plants have revived their efforts to produce bioenergy. \u201cThere is now greater pipeline coverage to collect the produced biomethane. Diesel prices are also higher than they were in previous decades, providing an incentive for ethanol plants to invest in natural gas\u2013fueled fleets. There is also increasing pressure on the industry to implement environmentally compliant practices for the disposal of waste materials, including stillage,\u201d explains Neto. Using the effluent to produce biomethane would prevent 90% of the greenhouse gas emissions released from stillage when it is disposed of in the environment.<\/p>\n Another recent enabler of bioenergy production is a new biodigestion system that can be fed with a combination of stillage, filter cake, and cane leaves. Developed by Geo Energ\u00e9tica, a firm based in southern Brazil, the process uses two biodigesters\u2014one for stillage and the other for other waste materials. Managing Partner Alessandro Gardemann explains that the organic load and energy density in stillage is variable from one season to another, and this makes the biodigestion process spasmodic. The large volumes of stillage that are produced make long-term storage prohibitive, requiring the biodigestion process to be located immediately downstream of the sugarcane crushing operation.<\/p>\n Sugarcane leaves and filter cake have a steadier organic load and higher solid content, so they can be stored in buffer stockpiles and later sent for biodigestion throughout the offseason. \u201cCombining the two waste streams allows plants to generate electricity steadily all year round,\u201d says Gardemann. Geo Energ\u00e9tica built its first process in 2012 at Cooperativa Agr\u00edcola Regional de Produtores de Cana (COOPCANA), in Tamboara, northwestern Paran\u00e1, with a capacity to generate 4 MW of electricity. Toward the end of 2019 the power plant will be expanded to produce 10 MW.<\/p>\n Two other projects are due to start operation next year: one at ethanol giant Ra\u00edzen\u2019s Bonfim plant in Guariba, S\u00e3o Paulo, with an estimated installed capacity of 138,000 MWh\/year, in an investment of R$153 million; and the other at Grupo Cocal\u2019s Narandiba plant in Presidente Prudente, S\u00e3o Paulo, in an investment of R$160 million, with a daily output of 67,000 m3<\/sup> of biomethane that will be collected by a distribution pipeline network operated by project partner GasBrasiliano. Two biomass power plants secured contracts for their power output in a recent power-purchase auction organized by the Brazilian Electricity Regulatory Agency (ANEEL), demonstrating the economic feasibility of this energy source.<\/p>\n New technologies<\/strong> Another project is testing membranes made of carbon nanostructures, such as graphene and nanotubes. Physicist Caetano Rodrigues Miranda, of the Physics Institute at USP, explains that much of the research involving carbon nanomembranes is concerned with finding ways to separate methane and carbon dioxide molecules, which have similar diameters. It is difficult to separate them when they pass at high flow rates across conventional polymer membranes.<\/p>\n
\nEvery liter of ethanol produced generates 12 liters of stillage. In the 2018\/2019 season the industry produced more than 390 billion liters of this effluent, which can contaminate groundwater with potassium and the air with greenhouse gases such as CO2<\/sub>, CH4<\/sub>, and nitrous oxide (N2<\/sub>O). Much of the stillage produced by ethanol plants is now treated and used as a biofertilizer in sugarcane fields.<\/p>\n![\"\"](\"https:\/\/revistapesquisa.fapesp.br\/wp-content\/uploads\/2020\/02\/060-067_bioenergia_286-2-1140.jpg\")
\nMeneghini says that the growing interest in biomethane has provided a stimulus for research into more efficient technologies for biogas purification, a process known as \u201cupgrading.\u201d The conventional technology is filtration using polymer membranes. Two ongoing projects at RCGI are evaluating the potential of new materials, such as ceramic membranes made of zeolite\u2014a porous mineral that does not require the use of chemicals in the cation exchange process (which neutralizes positively charged ions).<\/p>\n
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