The soot that rises into the sky in the countryside when sugar cane straw is being burnt at harvest time, and lands on the ground in the form of fine dark flakes, consists of some 70 chemical compounds that have a negative impact on the environment due to the emission of greenhouse effect gases, and cause serious respiratory problems to those who are exposed to them. Although this practice is yet to be permanently eliminated from sugar cane farming, a number of research groups are focusing on studying better uses for this material that has a huge potential for electric energy generation, biofuel production and the manufacture of products such as bioplastics, charcoal for the steel industry and even cement. The possibilities for making use of the fragments of sugar cane straw, a material that is left over in the fields after the harvest and consists of green leaves, plant ends, straw and remains of the stems, include a number of potential applications in the productive sector. One of the lines of research, under the stewardship of the Materials Engineering Department at the Federal University of São Carlos (UFSCar), resulted in silicon carbide, a versatile synthetic material, which was produced from the silica in the sugar cane straw.
The innovation in the choice of the raw material and the process used to produce the silicon carbide led the university to file a patent request. Properties such as excellent resistance to wear and tear, thermal shock and acids mean that this material, which is also a semiconductor, can be used in abrasives, refractory products, aircraft shielding, microeletronics and other applications. The discovery evolved out of a project designed to produce silicon carbide from rice straw, which had previously been developed by the same research group. “When we did the chemical analysis of the residue of the burnt sugar cane straw, we realized that it was also a rich source of silica,” explains Professor Ruth Kiminami, the project’s coordinator. The silica was then mixed with a source of carbon such as lead and placed in a special, controlled atmosphere oven, with no oxygen, for the silicon carbide to form. The material is obtained by the carbothermal reduction reaction, which occurs at a high temperature. “Within four to five hours, we produced very fine silicon carbide particles of one to five micrometers, which are used in more advanced applications,” says Ruth.
The method currently used on an industrial scale relies on a mixture of silica with carbon. The composite material is placed in an electric oven at a temperature higher than 2,400°C for 32 to 40 hours. This results in silicon blocks that need to be processed mechanically by crushing and grinding. “The process we used eliminates the added stages of crushing and grinding, which increase the product’s cost,” she emphasizes.
In another survey, chopped sugar cane straw was placed in a closed circuit at a high temperature, and at the end of the process this resulted in three products with applications in different areas – a bio-oil with potential to be used in the chemical industry, a fine charcoal powder that can be used in steel production, and a gas with high calorific power, made up of carbon monoxide, methane and hydrogen, which could be used both to feed the reactor and for power generation. The thermal conversion process used is called fast pyrolysis. “This is a rapid molecular decomposition achieved in a few seconds using high temperatures,” explains Professor Luis Augusto Barbosa Cortez, from the Agricultural Engineering School of the State University of Campinas (Unicamp), who is the project’s coordinator. Started ten years ago, the project resulted in the company Bioware, hatched at the Think-Tank in the university’s Technology Center with FAPESP’s support as part of the program Innovative Research in Small and Micro Companies (Pipe).
To obtain a high bio-oil yield, the researchers use a technique known as a bubbling fluidized bed, which results from a combination of air and sand at average temperatures of around 550ºC. Charcoal is placed in the reactor’s entrance or bed to initiate the heating process. When the temperature reaches 600ºC, the sand is put into the reactor and blown in order to form the fluidized bed, on which the dried chopped up biomass is placed to be degraded and transformed into products such as dark colored and highly viscous bio-oil, which can be used as a chemical raw material, fuel for turbines and boilers, a substitute for petrochemical phenol in resins and an additive in the formulation of cellular concrete in the building industry. “When mixed with the charcoal powder obtained in the process it has characteristics required for use in steel mills, such as high mechanical resistance, with 75% carbon, and a low content of volatile vapor, at most 25%,” says the researcher Rodrigo Jordan, who is taking part in the project on a post-doctoral scholarship.
The steam used for bio-oil production, after undergoing a washing process, results in acid water that can be used as bio-stimulant for plant growth and a bio-insecticide in bean cultivation. The gases emitted by the pyrolysis process can be used to fuel boilers or in the reactor’s own combustion process. Thus, the maximum possible use is made of the sugarcane straw. The pilot plant has a processing capacity of 200 kilos of sugarcane straw an hour, resulting in 80 kilograms of oil and 50 kilograms of charcoal.
Cortez, who for more than a decade has been studying other uses for sugarcane apart from sugar and alcohol, along with other projects, is coordinating a survey into the state of São Paulo?s sugar cane agribusiness, under FAPESP’s Program of Research into Public Policies, in partnership with the state of São Paulo’s Agribusiness Technology Agency (Apta). “Under the production system currently used, sugarcane’s efficiency stands at around 28%,” he says. This calculation is based on the energy contained in sugarcane divided into equal parts between sugar, pulp and straw; in other words, one third for each one of them. “Using the pyrolysis system to make the most out of the sugarcane straw, I believe that this percentage could rise to 50% or 60%.”
Although the full energy generation potential in the fragments of sugar cane straw is not known precisely, because no agronomic research has been carried out that indicates the ideal amount of sugar cane straw that should be left in the fields after the crop has been harvested, a study coordinated by Professor Nilson Augusto Villa Nova, from the Exact Sciences Department of USP’s Luiz de Queiroz School of Agronomy (Esalq), in Piracicaba, shows that it is possible to keep a hydroelectric power station similar to the one at Itaipu, in Foz de Iguaçu, working during the dry season between May and October just using the energy drawn from fragments of sugarcane straw and bagasse.
“Fragments of sugarcane straw, which are currently an environmental problem because of the burning that takes place in the countryside, have excellent power generation potential due to their high calorific value,” says Villa Nova. “As we cannot remove all the fragments of sugar cane straw from the fields because we have to maintain the farming quality of the soil, our proposal is to remove about 50% of this material for power generation purposes,” states professor Tomaz Caetano Cannavan Rípoli, from Esalq’s Rural Engineering Department, who, for the past 18 years, has been researching this material, its characteristics and how to handle it. The remaining 50% would be left in the fields to improve the soil’s physiochemical properties, i.e., to help in the carbon-nitrogen ratio, to increase organic matter content, to improve the soil’s microbiotic activity and protect it from the erosion caused by rain.
The paper, entitled “The future of fragments of sugarcane straw as a major source of electric energy in Brazil”, which was first presented at an international agriculture engineering conference in 2007, indicates that owing to possible water restrictions in the main water basins, due to climate change, the use of biomass should be the main source of renewable energy. Thanks to this study and all the other projects in which he has been involved during his 52-year long career, Villa Nova was considered for the Bunge Foundation’s 2008 Prize under the category Life and Work. The calculations to show the electric energy generation potential from the burning of fragments of sugar cane straw during the 200-day long harvest period were made on the basis of 100 tons of sugarcane an hour, but a plant can grind 500, 1,000 or even as much 1,200 tons an hour.
Energy during the dry season
The equation’s result pointed to a figure of 76,800 megawatts/hour (MW/h) each harvest. “Bearing in mind that Brazil’s annual sugarcane production is in the order of 386 million tons at each harvest, the annual energy expectation with the biomass would total some 122,800,000 MW/h a year, which would make it possible to supply Itaipu’s requirements during the dry season,” says Villa Nova.
Eight plants in the state of São Paulo, including Equipav, Quatá, Cosan, Rafard and Santa Elisa, are already mixing bagasse with fragments of sugar cane straw for co-generation of energy in the boilers. “The trend today is toward an integral harvest,” says Rípoli. Instead of picking the sugarcane and leaving the sugarcane straw on the ground, to be carried to the plant later, everything is taken together to pre-cleaning stations, where the materials are separated. “This is the cheapest solution for collecting the fragments of sugar cane straw, because the harvester is used with the cleaning system switched off or running slowly,” he stresses. The process is working on a commercial scale, but still experimentally. This is because we still do not know exactly what proportion of the fragments of sugar cane straw and bagasse should be used in the boilers during the harvest in order not to interfere in the efficiency of the process. “I’m absolutely sure that in 15 years’ time we will no longer have sugar mills but, rather, power plants,” says Rípoli.
Another future prospect for the use of sugarcane straw is that of ethanol production, which recently got a boost from the launch of FAPESP’s Bioenergy Research Program (Bioen). “We will use fungi that degrade the straw and bagasse to produce fermentable sugars from the cell wall,”says Professor Marcos Buckeridge, from the University of São Paulo’s (USP) Biosciences Institute and one of the program’s coordinators. It may seem simple, but the mechanisms for understanding how to carry out this cellular degradation demand that one first develops knowledge of the sugarcane genome. “We found 469 genes related to the metabolism of the sugarcane cell wall,” he explains. This broad study even included variables related to climate change, with a high level of carbon dioxide in the atmosphere. “In this scenario there will be changes in the composition of the cellular wall and we need to pay attention to this because, depending on the enzyme used, there can be alterations in the process of obtaining the ethanol.”
Finely chopped sugar cane straw is used as a substrate on which grain is sown to produce a large volume of food for cattle, swine and poultry in a few days
Integrating sugar cane and cattle raising on small farms by using sugar cane straw that is burned during harvesting is the proposal of José Luiz Guimarães de Souza, a retired professor from Paulista State University/Unesp in Botucatu, and economist José Abílio Silveira Cosentino. By using a technique called green hydroponic forage, or FHV, a production process that uses very little water and no soil, it is possible to harvest a significant volume of high quality animal feed in a short time. Sugar cane straw is used as the substrate.
The finely chopped sugar cane straw is placed on a piece of black, heavy-duty canvas; various kinds of grain seeds, such as corn, soy beans, wheat, guandu beans, oats, millet and sorghum are sown on the canvas. “Every 18 to 20 days I can pick, for example, 25 kilos of corn FVH per square meter,” says Souza. Corn FHV, or the FHV of any other planted grain, is harvested as if it were a carpet comprised of the sugar cane straw, the seeds that germinated and the respective leaves and roots, as well as the non-germinated seeds. This forage is used as a substitute for grass in animal feed. “The harvested quantity is the amount that a cow or a bull needs to eat once a day during the fattening period. This forage is complemented by a concentrate comprised of corn, soybean meal, wheat meal and mineral salt,” says the researcher. The objective of using this technique is to produce a huge amount of high-quality plant mass in a short time.
The only care required is watering the plant bed as necessary, as with a conventional vegetable patch. According to the researcher’s calculations, confirmed by field studies, 25 square meters can produce enough forage to feed a bull for an entire year; to this end, it is enough to plant and harvest one square meter a day. In Holambra II, in the municipal region of Avaré, São Paulo State, a rural landowner who raised Santa Gertrudes cattle maintained 18 beds of 60 meters each for two years. “He would harvest one bed every day to feed 60 to 80 head of cattle,” Souza reports. This process included data collection and the weighing of animals. The same method can be applied to feed swine and poultry.
Another proposal for small, sugarcane growing farms is to use the sugarcane straw to make handmade products, such as cachepots for vases, covers for bottles, hats, vases, plates and other objects. Thus, the sugarcane straw would also play a social role by generating income, instead of being burned in the fields. Several formulas have already been tested by researchers in partnership with a craftsman, who also tried dying this material. This experimentation resulted in products that are easy to make.
1. Biofuel obtained from the pyrolytic oil from sugarcane straw: technical, economic and environmental assessment of its use (nº 06/52843-8); Type Regular Research Awards; Coordinator Luis Augusto Barbosa Cortez – Unicamp; Investment R$ 35,056.65 (FAPESP)
2. Bio-oil obtained from fast pyrolysis of agricultural residues for use as fuel and materials (nº 02/02166-9); Type Pipe – Innovative Research at Small Companies; Coordinator Edgardo Olivares Gomez – Bioware; Investment R$ 67,733.61 (FAPESP)