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Electricity from waste

São Paulo landfill site produces biogas for lighting and for engines

Eduardo CesarClosed cylinder in which biogas from the landfill is burnedEduardo Cesar

Methane gas released by the decomposing garbage in sanitary landfills may be used for producing electricity and reducing the environmental impact of greenhouse gases, as shown by a project developed by the National Biomass Reference Center (Cenbio), a bioenergy research group from the Electrotechnical and Energy Institute of the University of São Paulo (USP). What is particularly new about this initiative is its detailed survey of the volume of gas annually emitted by the waste treatment center in Caieiras, which is controlled by the group Essencis Soluções Ambientais. The center receives some 10,000 tons of garbage daily, 75% of the amount collected by the city of São Paulo. Based on these data, one can find out the energy power available in the site and analyze the possibility of replicating the initiative in small sanitary landfills throughout Brazil.

“In the Caieiras landfill site, we’ve implemented a pioneering gas lighting system, which is still in the test phase, as well as an electricity generating system that is already in operation,” explains chemical engineer Vanessa Pecora, who is taking part in the project, coordinated by Professor Geraldo Francisco Burani and funded by the Ministry of Mines and Energy. The biogas is produced by the biodigestion of the anaerobic kind (without oxygen) of the organic waste in the garbage. The Caieiras treatment center, on km 33 of the Bandeirantes Highway, has a collection and burning system, including wells for the extraction of biogas from deep in the landfill site by means of blowers (which draw up the biogas) and a network of tubes that carry the gas, sending it to a very large closed cylinder, called a flare. Here the methane is burned, to transform it into carbon dioxide, a gas with less potential to generate greenhouse effects.

Carbon credits
“Methane is 21 times more harmful to the environment in global warming terms than carbon dioxide” says Vanessa. When the methane from the decomposition of organic waste stops being released into the atmosphere, the company can sell the carbon credits on the international market, in accordance with the directives of the Kyoto Protocol, the goal of which is to reduce greenhouse gases by 2012. In the older landfill sites with no gas extraction plant, drains spread across the site capture the gas that rises to the surface, where it is burned manually. “The efficiency with which the methane is burned in this system is only 20%,” explains Vanessa. When all the gas produced is captured by tubes and sent to the flare, the burn efficiency rises to 90%. “It is this 70% gain that goes into the carbon credit account.” If in addition to burning, one makes use of the energy from the biogas, the efficiency calculation reaches 100%.

In the Cenbio project, a Brazilian-made 200 kW Otto cycle internal combustion engine, which works largely like an automotive engine, was used to convert biogas into energy. “We chose this engine because it is the most powerful one manufactured here,” explains the researcher. The more powerful engines are all imported, which would raise the project’s cost. The energy produced in the process is used by the equipment blower. “As a result, there’s a saving in electricity from the national grid,” says Vanessa. The current production of the Caieiras landfill is 12,600 cubic meters of biogas an hour, but only some 200 cubic meters an hour are used. “If all the biogas produced was used , one could generate almost 15 MW of power,” says Vanessa, enough to supply a 250,000-inhabitant town. In the project’s current phase, which began in January 2006 and will end in December of this year, the researchers are finalizing a technical and financial analysis of the undertaking.

The study for using the biogas from the sanitary landfill was born out of two other projects previously developed by the researchers from Cenbio. The objective of the first of these, implemented by the Companhia de Saneamento Básico do Estado de São Paulo (Sabesp) in Barueri, was to compare two different conversion technologies – a 30 kW micro-turbine and an Otto-cycle engine that was adapted to use biogas and that had the same power – for generating electricity from the sewage treatment biogas. Biogas is produced from sewage that is treated in biodigestors, closed equipment in which the anaerobic digestion of organic matter by bacteria takes place. In this process, the sewage ferments and releases biogas. “The energy generated by the two conversion technologies were interlinked in the Sabesp network,” says Vanessa.

During the same period, another project for generating energy from biogas was taking shape at USP itself, as part of the Program for the Rational Use of Energy and Alternative Sources (Purefa). “Cenbio took part in this project with the goal of using the gas produced in the biodigestor that collected some of the sewage from CRUSP, the USP student halls of residence, and transforming it into energy,” explains Vanessa. The researchers developed a sewage purification and treatment system. The biogas produced was stored and sent to an engine that converted it into energy sufficient to light a demonstration panel of the experiment.