Sugar cane bagasse drying: a review

Sugar cane bagasse drying - a review By Juan H. Sosa-Arnao1, Jefferson L. G. Corrêa2, Maria A. Silva3 and Silvia A. Nebra4 1 Mechanical Engineering Faculty, State University of Campinas - Unicamp P.O. Box 6122, CEP 13083-970, Campinas - SP, Brazil, E-mail: jhsosa@fem.unicamp.br 2 Food Science Department. Federal University of Lavras - UFLA, CEP 37200-000, Lavras – MG, Brazil, E-mail: jefferson@ufla.br 3 Chemical Engineering Faculty, State University of Campinas - Unicamp, P.O. Box 6066, CEP 13083-970, Campinas - SP, Brazil, E-mail: cida@feq.unicamp.br 4 Interdisciplinary Center of Energy Planning, State University of Campinas, Rua Dr. Shigeo Mori, 2013 - Cidade Universitária - 13083-770 Campinas, SP, Brazil, E-mail: silvia.nebra@pesquisador.cnpq.br Abstract Sugar cane bagasse is the only fuel used in the sugar-alcohol industry in Brazil, the world's leading sugar cane producer. The sugaralcohol industry produces cogenerated electrical energy for its own consumption and the surplus is sold to the market. Improving the use of bagasse in furnaces is currently an important industrial strategy. The topic has aroused great interest due to an increase in the cogeneration level in recent years. This work reviews the state of the art of sugar cane bagasse drying. Introduction The export of surplus electricity from sugar industry cogeneration is becoming the norm in many parts of the world. Cogeneration systems are composed of a steam generation system, steam turbines and, of course, the process plant that acts as the condenser for the LP exhaust steam. The need to keep generating beyond the end of crop means that many systems also have water cooled condensers, typically integrated with a turbine which is then a pass-out condensing machine. The steam generation system is the principle source of losses from modern cogeneration stations. It has a boiler, an air pre-heater, an economiser and sometimes a bagasse dryer. The bagasse dryer, like the pre-heater and economiser, increases the steam generation efficiency but it only becomes a significant influence when flue gas is used as the heat source for drying, the steam generation efficiency being directly related to the final gas temperature. It must be remembered that, even though bagasse drying is a means of making more energy available from the cogeneration station, the net increase in electricity generation depends on the characteristics of the entire plant, which is also a function of mainly the turbine inlet steam condition, the temperature and pressure of the exhaust and the turbine efficiency. Arrascaeta and Friedman1,2 have presented the state of art of sugar cane bagasse drying until 1987. However, despite the importance of bagasse drying in recent years, it has not been sufficiently discussed. The aim of this work is to present a review of the most important drying systems (whether reported in the literature or not) and a discussion of their main characteristics. Advantages of drying sugar cane bagasse When the boiler efficiency is determined based on the Higher Heating Value - HHV, it is possible to perceive that a critical aspect is the bagasse moisture content.3 This value represents about 14.79% of total heat losses, since the typical boiler efficiency is about 65.83%. Don et al.4 according Upadhiaya5 reported that the HHV of brixfree, ash free and moisture-free bagasse is 19605 kJ by kg of dry bagasse, but if its moisture content is 50%, in the same conditions of brix and ash, the LHV becomes 7929 kJ by kg of wet bagasse. The reduction of cane bagasse moisture increases the LHV, and at the same time, the volume of the boiler exit gases is reduced. Table 1. Industrial bagasse drying systems Type and size Capacity [t/h] Year Plant, location Counter current flow Rotary dryer Rotary dryer 3.6mx12 m Rotary dryer Pneumatic dryer Pneumatic dryer Pneumatic dryer Pneumatic dryer Pneumatic dryer Pneumatic dryer Pneumatic dryer Rotary dryer 3.6mx9m 1.4 (three) 30 50 35 (five) 5 12 30 25 (six) 4.7 (six) 5 (six) 6.1 65 10.7 72 1910 1976 1976 1979 1980 Palo Alto Sugar Factory Donaldsonville, Louisiana Atlantic Sugar Association, Florida St. Mary Sugar Co., Louisiana Waialua Sugar Co., Hawaii Açucareira Santo Antonio, Brazil Açucareira Santo Antonio, Brazil Açucareira Santo Antonio, Brazil Barra Grande sugar factory, Lençois Paulistas, SP, Brazil Cruz Alta Plant, Olímpia, SP, Brazil Cruz Alta Plant, Olímpia, SP, Brazil Cruz Alta Plant, Olímpia, SP, Brazil 6 10 2 16 12 12 12 7 12 12 12 Pilot scale GIT = 218 ºC GIT = 315°C GIT = 244°C GIT = 300°C GIT= 330°C GIT= 300°C GIT = 300°C GIT = 259°C GIT = 239°C GIT = 257°C Davies Hamakua Sugar Co., Paauilo - Hawaii Hilo Coast Processing Co., Pepeekeo, Hawaii Paia Factory of HC&S Co., Maui, Hawaii Central Azucarero Don Pedro, Batangas, Philippines Central Aidsisa, Bacolod, Philippines Central Victoria, Bacolod, Philippines 16 16 16 2 2 2 Pellets 11 2 Pilot scale Pilot scale Rotary dryer 4.2mx9 m Pneumatic dryer Pneumatic dryer Rotary dryer 3.6mx12 m. Rotary dryer 2.4mx15.7 m 1981 1980 1980 Reference Remarks 24 45 13 1982 Pneumatic dryer 2 Pneumatic dryer GIT = 140°C Pneumatic dryer 0.5 GIT = 200°C Through circulation (moving) 3.8 1980 1983 Sugar Research Inst., Mackay, Queensland, Australia Chun Cheng Sugar Factory, China 1983 Central Pablo Noriega, Quivicán, Cuba 1983 Usina Paraiso Alagoas Pernambuco - Brazil 20 Pneumatic dryer Solar active/passive system Pneumatic dryer Rotary Dryer 3mx 16m Pneumatic dryer Pneumatic dryer Pneumatic dryer scale Prototype Pneumatic 7 2.7 1983 1983 Central Pablo Noriega, Quivicán, Cuba Consuelo factory, Dominican Republic 2 17 Industrial prototype GIT = 300°C H y b r i d 7.5 14 1984 1985 Usina Itajubara – GIJS, Maranhão – Brazil Industry Zaio, Morocco 41 26 GIT = 330°C GIT = 220°C 10 22 13 1985 1986 1989 ICINAZ CAI Jesus Rabi, Calimate – Matanzas, Cuba Ingenio Ñuñorco, Tucuman, Argentina 21 25 28 GIT = 250°C 17 1993 Ingenio Ñuñorco, Tucuman, Argentina 13 Industrial prototype 38 2003 Ingenio Ñuñorco, Tucuman, Argentina 29 Industrial prototype 2003 Ingenio Leales, Tucuman - Argentina 30 Industrial prototype 2003 Cia. Agroindustrial de Goiana - CAIG - Açúcar e Álcool – GIJS, Brazil dryer Pneumatic dryer Hybrid dryer Pneumatic dryer 28 Remembering that the specific heat of water vapour is almost twice that of other gases, the heat losses by exit flue gas are also reduced. Water vapour reduction also decreases the load of induced draft fans5. Boulet6 stated that drying the sugar cane bagasse could reduce air pollution and air demand in the furnace. Nebra7 concluded that pneumatic drying is a good choice even when combustion gases coming from an air pre-heater are used (GIT 180°C). Bailliet8 stated that the main advantage of sugar cane bagasse drying over the air pre-heater is the substantial increase in bagasse "burnability". Hence, bagasse 2 GIT=258°C Pilot scale Industrial 41 building drying is definitely a good course of action for mills producing large amounts of it with moisture above 50%. Marquezi and Nebra9 have shown that bagasse drying can save more energy than the air pre-heater. Drying sugar cane bagasse in an integrated system makes possible to obtain exit gases from the steam generation system at a lower temperature1,11. The use of bagasse dryer could reduce the temperature of the boiler exit gases from 300 to 140°C, increasing the steam generation system efficiency from 54% to 69%10. Currently there are bagasse dryers with even better Figure 1a. Pneumatic dryer (Santo Antonio Factory)12 performances, capable of reducing the GOT to about 75°C12, 13, 14. Energetic and exergetic analysis of a steam generation system has shown that bagasse drying improves the energetic efficiency from 71.44% to 84.98%, simultaneously increasing the exergetic efficiency by 3.14%15. State of the art of sugar cane bagasse drying Industrial-scale systems Professor Kerr6, in 1910, was the first researcher to report sugar cane bagasse drying using boiler exit gases. The dryer was of the countercurrent type. Hot drying gas entered the dryer at the bottom, travelling along a zig-zag path to the top, where it was drawn off by a fan. Bagasse entered at the top and worked its way down a series of inclined pivoted shelves. The dryer was made of steel and measured 1.2 m x 1.8 m in cross-section and 6.0 m in height. Moisture content was reduced from 54.3% to 46.4% (all moisture content figures are wet basis). Gas inlet temperature (GIT) was about 245°C, and gas Figure 1b. Pneumatic dryer (Barra Grande Factory)7 outlet temperature (GOT) 104°C. Few articles about bagasse drying were published from 1910 to 1970. Alternative energy sources such as sugar cane bagasse were not regarded as important in Brazil during that period due to the low price of fossil fuels. The energy crisis of the 1970's compelled plants to focus on further reducing bagasse moisture by drying it with flue gases. Consequently, boiler efficiency was improved, and emissions of particulates and CO2 were reduced. Table 1 presents a comprehensive list of bagasse drying systems installed in various sugar industries around the world. These systems are also discussed below. During the fuel crisis of 1972-73, Furines10 studied the viability of bagasse pre-drying by boiler exit gases. This author tested three rotary-drum type dryers using boiler exit gases at a GIT of 218°C to process bagasse obtained from 8000 t/day of sugar cane. Moisture content was reduced from 54% to 46%. Total fuel oil consumption, originally 1.62 gal/t of sugar cane, dropped to 1.01 gal/t in 1976. Kinoshita16 reported four systems using boiler exit gases to dry cane bagasse in Hawaii. Three of them were rotary-drum type dryers, and the fourth was a flash dryer composed of a pneumatic dryer and a cyclone. The author compared the increase in electric energy brought by the dryers and the electric energy consumed by them, and found a ratio of 2.7 to 3.7 for the rotary drum type and 1.5 for the flash type. Correia12 described the use of pneumatic dryers in Brazil and their advantages over rotary drum dryers (Figure 1a). He reported an increase of 16% in steam generation resulting from a decrease in moisture content from 50% to 38%. Soon afterwards, the Cruz Alta sugar cane company installed new sugar cane mills using that system. Even though waste gases are the main heat source for drying sugar cane bagasse, a solar dryer has been reported in the scientific literature as also adequate. It had a low initial cost, and low operating and maintenance requirements17. This belt dryer processed 2.7 t/h of bagasse and reduced moisture from 50% to 33%. Massarani and Valença18,19,20 studied bagasse drying in a moving bed dryer which they developed from laboratory scale to pilot scale. The laboratory dryer had a cross-section of 0.40 m x 0.50 m, and 2 m height; the pilot moving bed dryer had a cross section of 2 m x 0.5 m and 6.0 m height. The pilot dryer processed 3.8 t/h achieving a moisture reduction from 55.0% to 35.2%. Salermo and Santana21 worked with a dryer made up of a fluidized bed, a pneumatic duct and a cyclone. The system processed 10 t/h of sugar cane with exit gases at 250°C. The moisture content was reduced from 47% to 35%. Nebra and Macedo7 studied an industrial pneumatic dryer (Figure 1b), and found that the most significant moisture content reduction took place in the cyclone. This dryer processed 20.4 t/h of bagasse, and reduced moisture content from 50% to 23.2%22, with a GIT of 300°C. Nebra23 was the first researcher in Brazil to study a cyclone as a dryer. It is important to note that the cyclones in systems such as that of Salermo and Santana21 were designed to work as gas cleaners, not as dryers. Arrascaeta et al.24 patented a dryer that elutriates sugar cane bagasse and separates the particles into coarse and fine particulate fractions. This classification allows the use of feeders and burners specifically adjusted to each size fraction. Consequently, different sizes of bagasse could be used as fuel and as raw material for paper production, particle boards, furfural and other products. This dryer had a capacity of 7 t/h. In 1986, Arrascaeta et al.25 described an indus- Figure 1c. Pneumatic dryer (Ñuñorco indutry)29 pneumatic dryer reported so far. This dryer was also installed at Ñuñorco mill, in Argentina. Saab30 reported a hybrid drying system composed of three types of dryers -- rotary, pneumatic and cyclonic -- installed at Leales Sugar mill in Tucumán, Argentina (see Figure 1d). Technical data about this system are not available. Morgenroth and Batstone31 proposed an innovative technology: steam drying. Drying by superheated steam is a known process, but it had not been applied for cane bagasse previously. The authors presented calculations about their proposal, adapted from beet pulp drying, to be introduced in a factory, showing the theoretical possibility of an attractive increase in boiler efficiency and cogeneration. Afterwards they reported data about a prototype steam dryer tested with whole sugarcane with 70% moisture to be used for animal feed. The prototype was of fixed bed type, with evaporation capacity of 0.3 t/h and inlet superheated steam at a temperature of 180ºC. The outlet bagasse had 10% of moisture content. The feeding and discharge systems, key design aspects, were carefully developed in the drying system. Experimental systems trial-size pneumatic dryer that processed 22 t/h of bagasse with a moisture content of 48%. Outlet moisture content was 21%. Gamgami26 according van der Poel et al.27 reported a rotary bagasse dryer installed in the Zairo sugar factory in Morocco. It processed 14 t/h of bagasse with moisture content of 53%. Outlet moisture content was 40%, and GIT was 220°C. Aralde et al.28 worked with a pneumatic dryer installed at Ñuñorco sugar cane mill in Tucumán, Argentina, in 1989. This dryer processed 13 t/h of bagasse with moisture content of 53%. Outlet moisture content was between 40% and 43%, and GIT was 291°C. Paz et al.13 also described an industrial-size pneumatic dryer. This was a modified version of the dryer described by Aralde et al.28 (Ñuñorco mill). It performed satisfactorily, processing 17 t/h of bagasse with an inlet moisture content of 54% and an outlet moisture content of 40%. GIT was 320°C. Colombres29 reported data of a pneumatic dryer (Figure 1c) with bagasse capacity of 38 t/h at a moisture content of 52%. Outlet moisture content was 38%, and GIT was 280°C. This was the biggest Figure 1d. Pneumatic dryer (Leales industry)30 Meirelles32 studied the use of a fluidized bed dryer for a sugar cane bagasse of unusual characteristics: very high inlet moisture (71 to 91%) and small particle sizes (0.51 to 1.02 mm). Meirelles found that a mixer was required to achieve fluidization. During the drying process, bagasse agglomeration was reduced and the dried particles were elutriated. Researchers at the Faculties of Mechanical Engineering and Chemical Engineering at State University of Campinas (UNICAMP) have been working on drying of agricultural residues in cyclones33. Nebra et al.34 presented a review of the drying process in cyclones, including part of the work by the UNICAMP researchers. Corrêa et al. 35,36,37 presented theoretical and experimental data on bagasse drying in a cyclone that underwent geometric changes in order to work as a dryer. These authors worked with bagasse with inlet moisture content from 48 to 78% (w.b.); bagasse mass flow in the range of 0.0017 to 0.012 kg/s; GIT 210°C; and air flow rate of 7.8x10-2 kg/s. They succeeded in reducing bagasse moisture content by 74% along a particle residence time of 5 to 23 seconds. A pilot-scale system was used in this experiment. The cyclone was 1.0 m in height. It must be pointed out that cyclonic dryers allow a longer residence time than other pneumatic types. Barbosa and Menegalli38 and Barbosa39 studied sugar cane bagasse drying kinetics in a pneumatic dryer (initial moisture content 36% to 82%). They found that the most significant moisture reduction happened in two places: the acceleration zone at the pneumatic duct, and the cyclone. This pilot-scale system measured 0.075 m in diameter and 3.0 m in height; bagasse mass flow varied from 0.0034 to 0.017 kg/s, air mass flow from 0.028 to 0.048 kg/s, GIT from 120 to 233°C, and final moisture content from 21.3% to 78.9%. Alarcón and Jústiz40 also worked with a pneumatic dryer. Coarse particles were removed in the beginning of the drying process, and the smaller particles remaining in the dryer had their moisture content reduced from 50 to 30%. GIT was in the range of 240 to 270°C. Bigger particles were used as raw material in the paper and pharmaceutical industries and smaller ones were burned to generate thermal energy. Conclusions The first industrial bagasse dryer was of the rotary type. Many other types have since been experimentally studied and used in industries, such as fluidized and solar dryers. At present, pneumatic dryers are most often used in factories because of their low price and small space requirements. Bagasse dryers undoubtedly promote energy savings that increase the efficiency of the steam generation system. With the current increase in export cogeneration in most Brazilian factories, the bagasse dryer could become an important element of the system, even though studies about trade-off between the air pre-heater, the economizer and the drier are necessary, taking into account costs and energy consumption, aiming to determine the best seating arrangement between these pieces of equipment. Acknowledgements The authors wish to thank CNPq (Proc. 142135-2003-8; and Proc. 305720/2003-1) and FAPESP (Proc. 2001/14302-1) for their financial support. References 1. Arrascaeta, A., Friedman, P. (1984), Bagasse drying: past, present and future International Sugar Journal, Vol. 86, no. 1021, pp. 3-6. 2. Arrascaeta, A., Friedman, P. (1987), Bagasse drying, International Sugar Journal, 89 (1060): 68-71. 3. Acosta J.C. 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