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Only a few kilometers beneath
our feet – three, for example – rocks can reach temperatures of over 200 degrees. More than enough to turn water into vapor, which can be used to operate a turbine, which activates an electric power generator. Using geothermal power is one of Luiz Limaverde´s projects. Luiz is the former secretary of Energy, Naval Industry, and Oil and former sub secretary of Energy of the State of Rio de Janeiro. Now President of the Eletrobras Social Security Foundation (Eletros, Fundação Eletrobrás de Seguridade Social), Limaverde studies the various sources of alternative energy. In Brazil, he defends the use of biomass and wants the babaçu to feed a plant for providing fuel for the rocket launch base in Alcântara, in the state of Maranhão.
Following the example of other countries, he also wants Brazil to widely employ the power obtained through the burning of methane obtained from garbage decomposition and from garbage itself. Having this in mind, he visited several European countries to understand technologies that might be employed, with advantages, in Brazil. Simultaneously, depending on the power generation method adopted, the issue of methane emissions could be solved. In terms of the greenhouse effect, methane causes 21 times more harm than carbonic gas, and it also affects the ozone layer. Limaverde’s work started with solar power, and there are functioning photovoltaic panels installed in fuel stations, assuring pump operation.
In regards to geothermal power, which is already used in several countries (there are about 100 geothermal plants in the world), Limaverde said that the first step was to perform a survey about Brazil´s geothermal gradients. Such gradient is determined by the increase in temperature per depth, in kilometers. The Brazilian average corresponds to about 35 degrees Celsius per kilometer, which means that at a 3,000-meter depth, it would barely be possible to boil water. However, this isn’t the scenario over the entire country. At some points of the Brazilian territory, such as the Potiguar Basin (that goes from Pernambuco to Ceará), the geothermal gradient is about 70 degrees per kilometer. Petrobras has already performed drillings in this and other sedimentary basins searching for oil, and collected information that includes well temperature profiles.
The first step of a geothermal power project is to obtain a “radiography” of the soil, provided by magnetometers, gravimeters, and other pieces of equipment installed in airplanes. This allows a subsoil profile to be determined and the identification of granite rocks, which accumulate most heat from the Earth’s depths. Limaverde explains how a geothermal plant is built.
Naturally, the first step is to locate a granite rock at a spot with high geothermal gradient. Then, drillings are made for installing wells in the soil. Such wells shall have a distance of about 500 meters between them, according to the technology to be used and the intended power. Water will be injected into one of the wells, and vapor will come out of the other well. Then the rock needs to be hydraulically fractured for creating a heat exchanger. The deep liquid column receives repeated impacts from a pump, and the liquid cracks and fractures the rock. “Of course the water looks for a way out. What is it? The other well! When it goes up the other well, water temperature is very high, and turns into vapor,” says Limaverde. He adds that, going down the first well, the water has a temperature of about 20 degrees Celsius, and when it touches the hot rock, a thermal shock is provoked. The sudden increase in the temperature of water or any other geothermal fluid causes a sudden volume expansion, making it go up the other well under pressure. A valve prevents the water injected in the well to flow back, explains Limaverde.
According to Limaverde, geothermal power is fully renewable, for it will be feasible while the Earth’s depths constitute a heat reservoir. Except for small amounts that are lost and need to be restored, the water used in the process is recycled and goes back into the first well after going through a common cooling process that uses heat exchangers. Limaverde explains that the water needs to be cooled to allow for the thermal shock. Surface facilities, he says, are the same as in a regular thermoelectric plant, having a vapor-fueled turbine and a generator activated by it. The highest cost, amortized over time, as power is produced by the plant, corresponds to well drilling, totaling about R$ 6 million for a 3-kilometer depth. The initial capacity of a geothermal generation unit may vary between five and 25 megawatts, depending on geothermal conditions and the number of wells. However, much higher power ratings could be obtained. Limaverde points out that carbon emission reduction certificates can be deducted from a geothermal plant’s costs. As a matter of fact, the water that circulates around the rocks doesn’t come back completely free of pollutants (such as sulphur), which, however, can be removed, although this entails an additional expense.
The French and Canadian, says Limaverde, are very advanced in terms of geothermal power technologies. But it was from Austria, more precisely Innsbruck, that he brought a model for using garbage biogas. It’s a process that can be applied to existing deposits with high waste volumes. The first step is the construction of an isolated, waterproof reservoir where garbage will be compacted, with mechanical blades, so as to reduce the volume originally used up. Then concrete can be poured to seal the surface of the full reservoir, which will be completely waterproofed in the end. To help the gas formation process, a highly problematic waste is used: tires that are dumped in the bottom of the deposit allowing for the creation of air pockets. Pipelines are installed for removing drippings (treated and dumped in a clean manner), and perforated pipelines are installed for capturing gases that come out as a mixture, including the toxic and corrosive hydrogen sulfide. Gases are separated and methane corresponds to about 55% of the total volume.
Gas production starts within about 60 days, and the amount of methane generated depends on the amount of organic waste. A landfill with about 500 tons/day available over a period of more than six months is interesting for project development. The capacity of the landfill Limaverde visited in Innsbruck is 20 tons of methane per day, generating 3 MW. The injection of more organic matter can increase its useful life. “How do we inject such matter in the reservoir? By using sewer mud, which today is dumped into the Guanabara Bay, for instance. The mud is dried and pellets are made out of it, then it is introduced in the reservoir through pipes,” explains Limaverde, adding that this mud is richer in methane than regular garbage. He points out that, in Austria, as in the rest of Europe, generally speaking garbage has a low percentage of organic materials – about 14%. In Brazil, the percentage is much higher, and a plant such as Innsbruck’s would produce about 10 megawatts. We should remember that the plant works as any other gas-fueled thermal plant, meaning there are no additional equipment costs.
Since, when this technique is adopted, the waste reservoir is permanently sealed, it’s possible to urbanize the former waste deposit – creating, for instance, a park or another kind of leisure area.
Limaverde brought information from England about another process employed by the Thermsave Company that also uses garbage as raw material. In this case, the waste is not sent to deposits, but directly to the treatment station. Free from wires, cables, PVC pieces, and other materials that might lock the rotating blades of the silos, the in natura garbage is dumped in containers with capacity for 20-25 tons. Vapor at six bar (six times the regular air pressure) and 170 degrees Celsius is injected into the silos for 50 minutes. “It’s a cooking process, similar to that of a pressure cooker,” says Limaverde. After this thermal treatment, the material is screened twice. The organic matter and light plastics are transformed into fibers, which have two main applications: they can be burned for generating energy or used for producing highly resistant blocks. The value of cans and other metallic objects that can be recycled is increased, for they come out of the silos without paint and impurities, which are removed by vapor, that circulates in a closed circuit. After use, it goes through a cleaning process and is reused. From the total garbage, 12% to 15% is left (including fabrics, etc.), which may be used as debris. PET containers and other plastics are partially melted in blocks, making its reutilization easier. The remainder can be used, for instance, for making road foundations and level terrains. The process has a proven economic feasibility, and there’s the advantage of reducing the amount of waste sent to the deposits. Furthermore, there’s no dripping production nor methane emission.
Limaverde says that burning fiber is much more efficient than burning biogas. With 500 tons of garbage/day, it’s possible to generate two or three megawatts with biogas, and with the same amount of garbage, in Brazil, we could generate about 12 or 13 megawatts. Since the plant itself consumes about 2.5 megawatts, net generation would total about ten megawatts. A project of this kind will be implemented in the Caju district, in Rio de Janeiro. There are two explanations for the high percentage of organic matter in Brazilian garbage: the first one is sheer waste, the second one is the fact that Brazilians consume large amounts of non-industrialized food products.
Luiz Limaverde is a fierce advocate of the use of biomass for electric power generation. “We have serious power issues in system extremities because of the lack of resources for greater investments. One of them is Alcântara, our rocket launch base, which needs at least 20 megawatts for its recovery. However, the line that feeds it bears a maximum of 2 megawatts,” says Limaverde, adding that Alcântara is the world’s biggest babaçu producer. He explains: “The babaçu shell has more heat power than the sugar cane bagasse. And the level of pollutant emissions from shell burning is lower than that of the bagasse. A higher heat power naturally entails lower emissions, since the burning process is more complete. Therefore, I think it’s absurd that today oil is transported to Alcântara, since it’s a risky operation... High investments in a huge transmission line are being considered. This line would have to be implemented by Cemar (the power company of the state of Maranhão), which is going through a troubled period. This line would feed, with only 20 megawatts, a base that won’t use this potential all of the time. This power rating is used only during launch times. Therefore, the biomass solution, in addition to everything, fosters agriculture, generates jobs in the region, and uses what’s thrown away – the babaçu shell. And there’s an increase in the production of nuts, whose oil has a wonderful world market and which is an absolutely natural, native resource. Therefore, several improvements can be obtained with a perfect power input. The launch base would have steady energy. I always defend the biomass proposal.”
The CEO of Eletros believes wind power is important, despite the fact that the factors (period in which the wind blows) in Brazil are low, ranging between 25% and 40%. However, there’s the visual pollution issue, and offshore wind parks are being created. “It’s the best solution, because there are factors of 70%, 75%. In the Northeast, we have a factor of about 40%, meaning that 40% of the time there’s enough wind for power generation. During 70% of the time, the propellers are idle or operating with insufficient speed. Out of 24 hours, we obtain less than 10 hours of generation. Over this time, water is being saved in the reservoirs of the hydroelectric plants. But it’s a complicated process, because it requires high investments. Although the Incentive Program for Alternative Electric Power Sources (Proinfa, Programa de Incentivo às Fontes Alternativas de Energia Elétrica) determines high wind power rates, it’s not a steady power source. For this reason, I have been dedicating my time to biomass and, above all, to urban garbage biomass,” says Limaverde. He adds: “Why? There’s a basic reason. I totally agree with the model that’s being used here, but I think that, little by little, we have to move towards the model adopted in Europe today. It involves the following: power rates are the same for any source. But bonuses are given for those that bring an external factor – social or environmental – important for the community at stake. Therefore, if power is generated from garbage for instance, a crucial problem is being solved, reducing landfill size, methane emissions (methane is 21 times more harmful than CO2), drippings (that contaminate the water table), in addition to generating carbon credits and jobs, because recyclable materials will obviously be used. Thus, the external factors are so great that they entitle the undertaker to fiscal bonuses, such as tax reduction or bonuses granted by the government itself.”•
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