guia do novo coronavirus
Imprimir Republish

Energy

Sustainable balance

Study by Embrapa updates the advantages of ethanol in combating greenhouse gases

EDUARDO CESARProducing and consuming ethanol made from Brazilian sugar cane emits 73% less carbon dioxide (CO2), one of the main greenhouse gases, than the processes for obtaining and burning the gasoline sold in the country. Researchers from Embrapa Agribiology, a unit of the Brazilian Agriculture and Livestock Research Company in Seropédica (RJ), arrived at this result when they updated an assessment of the amount of fossil energy necessary to produce fuel alcohol, which considered new variables, like the substitution of land use and the degree of mechanization used in harvesting.

“Energy balance assessments are important because they give the measure of the sustainability of a fuel”, observes biologist and agronomist, Luis Henrique de Barros Soares, from Embrapa Agribiology, who was responsible for the study along with researchers Bruno José Rodrigues Alves, Segundo Urquiaga and Robert Michael Boddey. “In the case of ethanol from sugar cane there is evidence of a significant advantage relative to gasoline and diesel oil. Considering the total amount of ethanol produced in the last harvest, the country contributed towards mitigating 50 million tons of CO2, or 13.4% of all emissions from greenhouse gases derived from the use of fossil fuels”, he says. The study was developed from data compiled by the U N’s Intergovernmental Panel on Climate Change (IPCC) and field measurements. The researchers took into account emissions coming from a Chevrolet S-10 truck, with a flex-fuel engine, on a 100 km journey, using either pure gasoline or pure ethanol. They repeated the analysis with another similar truck with a diesel engine.

The sustainable nature of sugar cane ethanol has been well-known for several decades. In addition to the fact that it is a renewable fuel, Brazilian alcohol has an advantage over ethanol extracted from other plants, like corn and sugar beet, both in terms of productivity as well as in its capacity to generate electricity from its waste. But energy balances take into account a myriad of topics, like gases emitted in the production and application of herbicides and fertilizers, the construction of alcohol refineries, the manufacture of agricultural machinery and transporting the fuel to the end consumer and other aspects.

Embrapa’s work is the most recent of a series of ethanol energy balance assessments and its merit is that it fills in gaps and provides answers to the doubts raised by previous studies. One of the first studies of the type was published by physicist José Goldemberg, in 1978, in Science. He concluded that approximately one tenth of a liter of fossil fuel was used to produce a liter of ethanol. In recognition of this pioneering study Goldemberg was included in Time magazine’s list of world heroes of the environment, which it published last year. “The Embrapa study will be very useful, because it’s extremely detailed and up-to-date”, says Goldemberg in its praise.

In the 1990’s, engineering professor Isaias de Macedo, from the Interdisciplinary Nucleus for Energy Planning of the State University of Campinas (Nipe-Unicamp), prepared a new balance sheet using more up-to-date data and concluded that the advantage of ethanol over gasoline was 8 to 1, a result compatible both with the findings of Goldemberg and with those of Embrapa.

The issue is that other studies arrived at various results, depending on the variables that were taken into consideration in the equation. In 1988, for example, research led by David Pimentel, a professor of Ecology from Cornell University, arrived at a result that was 50% lower, but took into account that generating energy in the refineries was based on the use of fossil fuels. Today, this practically hardly ever happens in Brazil, because the bagasse from the sugar cane is used by the refineries to produce the electricity they consume.

A more recent question raised has to do with the change in land use. In a study published in February 2008 in Science, a group of researchers led by Timothy Searchinger, from Princeton University, proposed a new methodology that also includes the indirect effects of land use, caused by the increase in the interest in biofuels, such as expansion of the area planted with soybeans in Mato Grosso,  attributing to the growth in the planting of corn for producing ethanol in the United States, or the advance of livestock farming in the Amazon forest, supposedly driven by the growing of sugar cane in areas where cattle used to be raised in São Paulo. “Studies of this type ignore the particular aspects of the production of ethanol in Brazil and were clearly used to discredit the fuel”, says José Goldemberg.

Conversion of pasture
According to Searchinger’s model, ethanol from North American corn, instead of bringing about a 20% reduction in emissions, would double the emission of gases over 30 years and increase emissions for 167 years. Although he admits that ethanol from sugar cane is very much more productive than that from corn, Searchinger issues a warning: if the increase in the cultivated area is based on conversion of tropical pasture land the emission of gases may be compensated for in four years. But if it implies cutting down tropical forests the compensation period would rise to 45 years.

The Embrapa study looks in detail at this questioning and comes up with a response that reiterates the advantage of ethanol. Taking into account the expansion of sugar cane planting in Sao Paulo the researchers show that this has not been at the expense of the tropical forest, the extent of which has remained steady for the last 40 years. In the recent past the impact of sugar cane on other crops has been small. During the period there was a small reduction in the area planted with soybeans and corn, while oranges and coffee retained the same area. While sugar cane made inroads into livestock farming, it did not, however, suffer as a result, since the productivity of pastures in Sao Paulo increased. “We can say that sugar cane advanced mainly in degraded areas, because pasture, when it is well-managed, is profitable”, says Luis Henrique, according to whom it is reasonable to transpose this type of change in land use to the new fronts on which sugar cane is advancing. The Cerrado region, for example, has a similar profile, with extensive, but not very profitable livestock farming.

In the assessment of meteorologist Carlos Nobre, coordinator of the FAPESP Global Climate Change Research Program and the Terrestrial System Science Center (CCST) of the National Space Research Institute (Inpe), the Embrapa study does justice to the capacity of ethanol to mitigate the effects of climate change. But, he observes, there is still a challenge to add ecological value to Brazilian biofuel. “Other monocultures, like eucalyptus, for example, have managed to claw back some of the ecological functions of the ecosystems they substituted. They have recuperated river-bank vegetation, for example. We still don’t see this happening with sugar cane plantations”, he says. “This is a debt the sector owes to the environment and society. Researchers have a role to play in this task, helping to produce scientific studies and find technological solutions for this issue. This recomposition would be highly beneficial, even for the sugar cane business”, says Nobre.

Sugar cane is a R$ 40 billion a year business in Brazil. The 2007/2008 sugar cane harvest was some 550 million tons, 15.2% more than the previous year. Half of it is used for making ethanol, which makes Brazil the second biggest producer of the fuel in the world. First place belongs to the United States, which extracts ethanol from heavily subsidized corn. Two thirds of Brazil’s production comes from the State of São Paulo.

Mechanization
According to the work produced by Embrapa, a hectare of sugar cane produces 3,244 kg a year of greenhouse gases, in CO2 equivalents, while soybean and corn crops emit on average 1,160 kg and pastures, 2,840 kg. But a hectare of sugar cane substitutes 4,500 liters of gasoline, the combustion of which emits 16.4 tons of CO2 per year into the atmosphere. The result is that each hectare of sugar cane that is transformed into alcohol and used in substitution of gasoline reduces annual emissions of CO2 by more than 12 tons a year.

A fact that favored the balance was the growing mechanization of cane harvesting, which has been substituting the use of burning  used to clean the soil, but which emits greenhouse gases, like CO2 and methane (CH4) into the atmosphere. Methane, in fact, has a greenhouse gas potential 21 times greater than carbon dioxide. Today, 60% of the planted area in the State of São Paulo is mechanized and, because of the law, this is likely to reach almost all of the sugar cane farms by 2022 – the exception will be areas with slopes over 12%, on which only manual harvesting is possible. According to the study, if the sugar cane harvest was totally mechanized, without resorting to burning, the advantage of alcohol would be even greater: 86% greater than gasoline and 78% greater than diesel oil. “The harvesting of raw sugar cane eliminates the emission of methane and nitrous oxide and also reduces emissions linked to the use of labor. In compensation the sugar cane harvesting machine consumes 40 liters of diesel in the same period”, says Luis Henrique. “But the comparison between the emissions of the two cutting systems makes it much clearer that, despite the heavy consumption of the harvester, the elimination of burning reduces the total emissions that occur during harvesting by almost 80%.”

The Embrapa study mentions two recent studies, one by Robert Boddey and his colleagues from the institution, and another by Carlos Cerri’s group, a professor from the Center for Nuclear Energy in Agriculture (Cena) of the “Luiz de Queiroz” USP campus in Piracicaba, according to which the change in the use of land under pasture for sugar cane crops collected raw causes an increase in carbon stocks in the soil. The use of fertilizers was also highlighted in the study. Every kilo of nitrogen in the form of fertilizer emits 4.5 kilos of CO2 into the atmosphere as it synthesize. But Brazil uses less nitrogenated fertilizer in sugar cane relative to other countries, thanks to the capacity of the crop to fix the nitrogen from the air through the action of bacteria. After comparing ethanol from sugar cane with gasoline and diesel, Embrapa Agribiology is going to prepare studies with data from the Department of Agriculture in the United States (USDA) to compare the saving in CO2 in the production of ethanol from North American corn.

Republish