This is the main finding of a study performed by researchers at the University of São Paulo’s Center for Nuclear Energy in Agriculture (CENA-USP) and Luiz de Queiroz College of Agriculture (ESALQ), Brazil, in collaboration with colleagues at the National Bioethanol Science & Technology Laboratory (CTBE).
The results of the study, which was supported by FAPESP, were published in the journal Atmospheric Environment.
“The ethanol industry has introduced new technologies for the transportation and storage of vinasse, contributing to a reduction in the emission of methane [the main greenhouse gas produced by the residue],” said Bruna Gonçalves de Oliveira, a postdoctoral fellow at the Agronomy Institute (IAC) with a fellowship from FAPESP and first author of the study.
The researchers quantified methane emissions from the two main systems that mills currently use to transport and store vinasse, a liquid residue formed during the distillation of sugarcane molasses to produce ethanol. The traditional system consists of open channels, which may or may not be lined. A new method comprises closed tanks and pipes.
An analysis of comparative data showed that emissions from open channels were 620 times greater than emissions from closed tanks and pipes.
“The difference is due to the characteristics of the transportation and storage system consisting of tanks and pipes,” Oliveira toldAgência FAPESP. “Transporting vinasse from one tank to another in high-pressure pipes tends to oxygenate the residue, modifying the anaerobic conditions [lack of oxygen] and making them favorable for the production of methane.”
To reduce the environmental impact of vinasse, which contains high concentrations of organic matter, potassium and sulfates, the Brazilian ethanol industry decided some 30 years ago that the cheapest and simplest solution would be to recycle the vinasse for use as a fertilizer on irrigated sugarcane plantations.
To store vinasse and transport it to plantations, the mills initially used a system of open channels lined with plastic, cement or concrete, along which the residue is transported by gravity and then pumped to cane fields.
However, Oliveira noted, the amounts of greenhouse gases (GHGs), such as methane, emitted by the vinasse during storage and transportation had never been quantified and are not included in the ethanol industry’s GHG inventories.
“The industry’s GHG inventories take into account only emissions from the use of vinasse in fertirrigation,” she said.
During her master’s research supervised by Brigitte Josefine Feigl, a researcher at CENA-USP and a principal investigator for the project, Oliveira quantified methane emissions from vinasse after it left the distillery in a 40-km open channel, which was mostly unlined (a small stretch was lined with cement).
The analysis showed that 98% of the total emissions of methane and other GHGs from vinasse occurred during storage and transportation via this system of open channels.
“Nitrogen emissions in the field during fertirrigation with vinasse are accounted for by GHG inventories, but we found that these emissions contributed to only 2% of total emissions,” Oliveira said.
Comparison of systems
In compliance with rules issued by the CETESB, São Paulo State’s environmental regulator, sugar plants in the state began lining most of the open channels used to transport vinasse to prevent the residue from infiltrating the soil and contaminating the water table.
Some plants, such as those Oliveira studied during her PhD research, switched to underground tanks and pipes as a more sustainable solution.
No research had ever been conducted to evaluate whether these improvements could alter the conditions for methane production and significantly reduce GHG emissions during ethanol production.
“Our goal was to find out how advances in vinasse storage and transportation technology affected GHG emissions,” Oliveira said.
In pursuit of this goal, from 2012-13 (a period corresponding to two harvests), the researchers monitored methane emissions from the vinasse storage and transportation systems used by two sugar mills in the Piracicaba and Bauru regions of São Paulo State.
One used open channels measuring 1.5 m in width, 0.6 m in depth and 60 km in length, of which 40 km were cement lined. In the 20 km of unlined channels, vinasse came into direct contact with the soil.
The other mill used a system comprising ten polyethylene-lined tanks that pumped vinasse at a high speed to sugarcane plantations through pipelines.
The analysis showed that in general, the intensity of methane emissions was approximately 1.36 kg of CO2 equivalent per cubic meter of vinasse transported in open channels, 620 times greater than the emissions from vinasse stored in tanks and transported by pipelines.
Roughly 80% of the methane emissions from the open channel system came from the unlined section.
“Vinasse supplies nutrients and ideal anaerobic conditions and temperatures for soil microorganisms in the vicinity of the unlined channels to emit methane. This doesn’t happen in the system of tanks and pipelines,” Oliveira said.
In closed systems, the vinasse reaches a temperature of 60 °C, which inhibits microbiological activity. In addition, the residue is pumped at a high pressure, which reduces decantation to the bottom of the tanks and methane-producing decomposition by microorganisms.
The high pressure at which vinasse is pumped in the system of closed tanks and pipes also oxygenates the residue, diminishing anaerobic conditions for microorganisms to produce methane (methanogenesis).
“Oxygenation limits the potential for vinasse redox [oxidation and reduction] and anaerobiosis. Methanogenic microbial activity is therefore limited or even non-existent, reducing methane emissions,” Oliveira said.
Based on these findings, the researchers concluded that systems of tanks and pipes for vinasse storage and transportation could effectively mitigate methane emissions during the production of ethanol from sugarcane.
They also concluded that the use of new technologies, combined with improvements to vinasse storage and distribution systems, would significantly reduce GHG emissions.
“In addition to these factors altogether, a number of innovations that the sugar and fuel ethanol industry is starting to introduce, such as vinasse concentration and biodigestion, will reduce GHG emissions even further, so that ethanol from sugarcane can become an even cleaner and more sustainable biofuel,” Oliveira said.
Source : By Elton Alisson | Agência FAPESP