Bagasse Sistem Drying

Training Workshop on: “Utilization of By-Products and Waste Materials in the Sugar Industry”

BAGASSE DRYING: A REVIEW Ali K. Abdel-Rahman Professor, Dept. of Mech. Eng., Faculty of Eng., Assiut University Chairperson of Energy Resources Engineering, E-JUST

Abstract The utilization of bagasse as a fuel in sugar cane based industry is well known. The moisture content of fresh bagasse is relatively high which lowers the total heat available from bagasse and affects its combustion efficiency. Therefore, bagasse drying has become a necessity in order to improve its combustion efficiency and the environmental conditions, and to reduce the bagasse quantities used as a fuel. In the last three decades, several research projects have been directed towards the process of bagasse drying. Several drying techniques and dryer designs have been tested. The geometric properties, chemical composition and physical properties of the bagasse have been experimentally determined. Yet, a comprehensive study of bagasse drying is not available in the published literature. Moreover, the existing data and results suffer from the lack of consistency in carrying the research work. This review paper is carried out as a necessary preliminary step for such a study. The main objective of this review paper is to collect and review the available studies in order to shed light on the points which need extra studies and those which have not been studied yet. Moreover, this review will try to find an answer for the question of: which type of bagasse dryer is the most efficient? KEYWORDS: Bagasse, Bagasse drying, Drying, Dryers, Bagasse composition.

1. Introduction The down-ward trend of sugar prices in the world market, and the uncertainties of fuel cost, highlights the importance of energy 2-14 May 2015, Assiut, Egypt

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saving devices. The idea of bagasse drying can contribute to the reduction of sugar production cost through energy saving devices thereby guaranteeing competition in international market and regular margin of profit locally. Sugar cane based industry has a tremendous potential and its future is bright in many countries including Egypt. It is an old idea to concentrate on producing sugar alone from sugar cane. The byproducts of sugar industry (such as bagasse, filter cake and molasses) today attract as much attention as the conventional main product, sugar. The trend in the sugar producing nations now is to concentrate more and more on by-products - thus rendering sugar ironically, as the by-product. Bagasse, the fibrous residue of the sugar cane stalk coming from the mills after crushing and extraction of juice, is the natural fuel for steam generation in the cane sugar factories. Its composition varies with the variety of cane, its maturity, the method of harvesting and the efficiency of the milling plant. It consists of woody fibre, water, soluble solids and ash in varying percentages. In the past, bagasse was considered as waste material. Today, bagasse has become valuable material. Aside from being used as a fuel, it is a starting material for different products such as paper, paper board, fiberboard, particle board, animal feed, etc. [1-3]. In Brazil, today’s leader of sugar industry, the emphasis is on developing a variety of cane having up to 25 % of fibre content since more bagasse signifies higher profits. Moreover, in Pakistan, the price of bagasse per ton is higher than the price of sugar cane [3]. The value of bagasse may be more than the value of sugar. So it is necessary to optimize the use of bagasse, because of the competition between its use as fuel in boilers and as starting material. The moisture content in bagasse varies considerably with the degree of extraction, but under average conditions it may be taken as 50 % of the total weight leaving the mills [1,4,5]. (The average value of the moisture content in bagasse reaches 52 % in almost all the cane sugar factories of the Sugar and Integrated Industries Company SIIC of Egypt in the 1998 harvesting season [6].) The higher the moisture content, the lower is the amount of combustible material per unit 2-14 May 2015, Assiut, Egypt

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weight of bagasse and the total available heat, therefore, varies inversely with the moisture content. Furthermore, the portion of heat which is used to evaporate the bagasse moisture and to superheat the resulting steam to the temperature of the flue gases must be deducted from the total available heat. (Each kg of water in bagasse requires 418 kJ (100 kcal) to reach its boiling point and 2,259 kJ (540 kcal) to convert it into steam [3].) The increase in moisture content of bagasse with relatively more excess air is not only increasing the heat loss through flue gases but also lowers the combustion temperature in the furnace. Therefore, at present many of the sugar cane factories consumed their entire production of bagasse as fuel for boilers and still have to purchase considerable amount of oil fuel. (The cane sugar factories of the Sugar and Integrated Industries Company SIIC of Egypt, consumed a total sum of 69,881 metric ton of oil fuel worth of 12,788,274 Egyptian Pound in the 1998 harvesting season [6].) With increasing of the energy costs and decreasing the sugar price in addition to the increasing demands for the use of bagasse as a raw material for the production of different products, it becomes necessary to search for a mean to utilize bagasse efficiently. This is the role of bagasse drying. In the following, the advantages of the drying process of bagasse and a review of the available literature dealing with the subject will be considered.

2. BAGASSE DRYING - WHY? During the combustion process in a boiler considerable amount of furnace energy is wasted in evaporating the water contained in the fresh bagasse. The basic principle of bagasse dryer is to dry the fresh bagasse by utilizing the flue gases waste heat content. Lesser moisture content in bagasse would simply mean higher energy density and better combustion quality resulting in the improved thermal efficiencies of the concerned boilers. In the following, the main advantages of bagasse drying will be summarized as: 1- Increasing the calorific values: The quantity of heat produced per unit weight of fuel is termed the calorific value. The higher or gross calorific value (GCV) of a fuel is the amount of heat energy 2-14 May 2015, Assiut, Egypt

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released by the complete combustion per unit mass of that fuel taking into consideration that all the products being cooled down to the original temperature of 20 oC. A lower or net calorific value (NCV) is also used. The difference between the two values being the latent heat of the water vapor which is not included in the NCV because the boiler plant cannot recover it due to the high temperature value of flue gases leaving the boiler. It has been shown, many times, that bone dry bagasse from different sugar cane varieties grown in different countries have very similar calorific values in spite of their rather considerable differences in physical appearance. Calorimetric measurements carried out suggest that a gross calorific value (GCV) of combustibles of 19,400 kJ/kg (4,605 kcal/kg) would be an appropriate figure for bone dry bagasse. Because of the moisture in the bagasse, the net calorific value (NCV) as it comes from the mills (50 % moisture content) is only about 7,563 kJ/kg (1,808 kcal/kg). The bagasse drying process increases the gross (GCV) and net (NCV) calorific values of bagasse [2-5,7] with the respective decrease in moisture content. (GCV is increased by 2 % for a one percent drop in moisture content [4].) Table (1) [3,5] shows the above mentioned increase in both GCV and NCV which are calculated by Hugot’s formula [8]: GCV = 4.1839 [4600 - 12 s - 46 w]

kJ/kg

NCV = 4.1839 [4250 - 12 s - 48.5 w]

kJ/kg

where (w) is the moisture content (%) of the bagasse (wet basis), and (s) is the sugar content (%) of bagasse. When bagasse flows out of the mills, it has a moisture content (w) of the order of 47.5 to 50 %, and a sugar content (s) which of the order of 1.5 to 2 % by weight [9].

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Table 1: Dependence of GCV and NCV on the bagasse moisture content [3,5] MC (w), %

GCV, kJ/kg

NCV, kJ/kg

% increase of NCV

50 40 35

09,498 11,422 12,385

07,531 09,623 10,565

0 27.8 40.3

2- Increasing the furnace temperature: When the moisture content in bagasse is in the range of 47 to 56 %, it will have a difficult time in reaching the ignition temperature, having expended much of its heat of combustion in evaporating the bagasse contained moisture. The vapor generated from the evaporation of bagasse moisture tends to shield the air from the surface of the bagasse and the flame suffocates from lack of oxygen. To maintain combustion, more excess air is blown into the furnace, with the net result that the combustion temperature is again lowered. Although bagasse contains 44 % by weight of oxygen as one of its basic components, it does not contain enough to ensure that its carbon component burns to CO2 or CO or both and its hydrogen component, to H2O. In fact, from a strict stoichiometric point of view, extra oxygen is required, which is usually obtained from air blown into the furnace. But air obviously brings in nitrogen, in ratio of 3.774:1 by volume with respect to the oxygen component. The ratio by weight, of excess air actually blown in, under factory conditions, to that theoretically necessary is usually represented by m (excess air factor, ratio) [9]. The induced draft (ID) fan speed needs to be increased to cope with the increase in moisture content as more excess air is needed for complete combustion. (Fresh bagasse needs 50 to 60 % excess air, while dry bagasse requires 20 % only [3].) The increase in moisture content of bagasse with relatively more excess air does not only lower the combustion temperature in the furnace but also increases the heat loss through flue gases [4]. Therefore, dry bagasse will increase the furnace temperature to about 20.5 to 30 2-14 May 2015, Assiut, Egypt

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% [2-5] as shown in table (2). This table illustrates the fact that 1 % drop in moisture content of the bagasse induces a theoretical rise of about 11.5 C in the combustion temperature [9]. Careful monitoring of the excess air is necessary to ensure a maximum furnace temperature to enable the flue gases to carry enough heat for the proper operation of heat recovery devices such as air heaters, economizers and bagasse dryers. Table 2: Effect of bagasse drying on the furnace’s temperature [3,5] Excess air factor (m), ratio MC (w), % 50 40 35

m = 1.5 1,040 oC 1,165 oC 1,210 oC

m = 1.3 1,120 oC 1,254 oC 1,280 oC

m = 1.2 -1,300 oC 1,350 oC

3- Increasing the energy production: As the combustion gas temperature is increased due to dryness of bagasse, the heat absorbed by the water walls of the boiler is increased which results in raising the amount of transferred heat for generating steam as shown in table (3) [3,5,10]. Hence, as a result of bagasse drying, the steam-to-bagasse ratio will improve considerably, leading to increased energy production [9]. Table 3: The amount of heat transferred to steam per kg of burnt bagasse, kJ/kg [3,5]. Excess air factor (m), ratio Temperature of flue gases (t), oC MC (w), % 50 40 35

1.5 220

1.3 150

1.2 140

5,332.4 7,292.6 8,145.3

-7,832.3 8,725.2

-7,933.9 8,834.8

4- Increasing the vaporization coefficient: Vaporization coefficient is the weight of steam generated per kg of burnt bagasse. Bagasse drying increases the vaporization coefficient considering the feeding water is at a temperature of 90 oC, and discounting the loss of weight due to the bagasse drying. The characteristics of 2-14 May 2015, Assiut, Egypt

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the steam and their corresponding coefficients are presented in table (4) [3,5]. 5- Increasing the stability of boiler’s operation: Drying increases the burnability of bagasse which increases the stability of boiler’s operation due to the low moisture content of the supplied bagasse [4,10,11]. The boiler could tolerate moisture contents up to 56 % with deteriorating performance, but combustion will probably be unstable above that point [12]. The improvements found in operating with dry fuel are: increased stability and decreased fuel requirements per unit of steam generated [11]. Table 4: Weight of steam produced per kg of bagasse, for different moisture and excess air factors, kg-steam/kgbagasse [3]. MC (w), % Excess air factor (m), ratio Temperature of flue gases (t), oC Steam conditions 1.6 MPa, sat. 2.0 MPa, 300 oC 3.0 MPa, 350 oC 3.0 MPa, 400 oC

50 1.5 220

50 1.3 220

40 1.3 220

40 1.2 220

35 1.2 220

2.21 2.02 1.95 1.87

2.25 2.06 1.99 1.91

2.59 2.37 2.29 2.20

2.62 2.39 2.31 2.22

2.72 2.48 2.40 2.30

6- Increasing the combustion velocity: The dry bagasse introduced to the furnace is at high temperature, which facilitates combustion and rapid ignition [8]. Velocity of combustion is definitely increased with a better heat absorption in water walls, resulting in a higher heat transfer rate. More heat transmitted to boiler-water means an efficiency increase in the boiler [3,5,8]. Moreover, burning of bagasse is completed while it is falling down when the moisture content is 38 % or less. 7- Increasing the speed of boiler’s response to load changes: One of the very important improvements found in operating boilers with dry fuel is the faster boiler response to load changes. This is due to the increase in the net calorific value (NCV) and furnace temperature of the dried bagasse [4,11].


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8- Reduction of the losses: This is due to the losses by incomplete combustion of wet bagasse while the dried bagasse burns completely leaving almost no residues [3,5]. Moreover, this is due to the recovery of 50 % from the whole soot, made by the cyclones of the dryer (an increase of the burning efficiency). 9- Increasing steam production: Drying of bagasse theoretically increases the steam production to about 15.03 % or, saving about 12.7 % of bagasse for same steam production [3,5]. (The power consumption of the bagasse dryer is already considered in this calculation.) Moreover, steam generation increases from 2.25 to 2.59 kg-steam/kg-bagasse due to bagasse drying (50 % basis) [13]. 10- Decreasing the flue gases volume: This is due to the fact that dry bagasse is using a lower excess air for combustion. The amount of flue gases may be calculated by Hugot formula [2,4,5,11]: VgN = [4.45 (1.0 - w) m + 0.572 w + 0.672] t

Nm3/kg bagasse

where t is the temperature of flue gases , oC. Pollution is diminished due to the smaller gases volume and the smaller amount of soots. Bagasse drying reduces air pollution from 10,000 mg/Nm3 of ash to less than 300 mg/Nm3 [13]. 11- Reducing the losses of sensible heat: An increase in furnace temperature is noted, but at the same time there is an important decrease of heat losses in flue gases, because the dried bagasse needs lesser excess air in furnace (m = 1.2 as compared to m = 1.5 for wet bagasse) and also 50 % of the flue gases are diverted towards bagasse dryer and leave the dryer at 100 oC (instead of 220 oC or 300 oC) [3]. Losses of sensible heat in the flue gases is also reduced due to decrease in its volume. Moreover, the reduction of the specific heat of the flue gases by the elimination of a part of the water enclosed in the bagasse is responsible for most of the reduction in the losses of the sensible heat in the chimneys [3-5]. Table (5) shows the values of sensible heat losses in the flue gases after they have been utilized in the bagasse drying process (kJ/kg) as calculated according to Hugot formula [8]: q = 4.1839 [(1.0 - w) (1.4 m - 0.13) + 0.5] t 2-14 May 2015, Assiut, Egypt

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kJ/kg

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Table 5: Loss of sensible heat in the flue gases, kJ/kg [3,5]. MC (w), % 50 40 35

m = 1.5 t = 220 oC 1,367.89 1,548.22 1,638.89

m = 1.4 t = 170 oC 1,006.44 1,136.61 1,201.67

m = 1.3 t = 150 oC -0,950.17 1,003.18

m = 1.2 t = 140 oC -837.62 883.02

3. BAGASSE DRYING Recovery of waste heat (e.g. boiler flue gas) from industry is in coping with energy conservation measures of today, as industry is the largest user of fuels. Using waste heat to create additional fuel is doubly important, particularly as applied to materials which are often a disposal problem (e.g. bagasse). Most waste organic materials are combustible if enough moisture can be removed. Today, waste heat from combustion processes is the exclusive drying media in raw sugar mills using boiler flue gas to dry bagasse for boiler fuel. Bagasse drying becomes a vital component of the sugar factory power house, if energy considerations become compelling. The first interest shown in bagasse drying with boiler flue gases dates back to 1910, when Professor E. W. Kerr [7] showed that it was impossible in some Louisiana mills at that time to cover the sugar mill’s energy demand with bagasse alone, owing to its high moisture content ranging from 54.47 to 44.45 %. His dryer was a square tower (similar to the one shown in Fig. 1) with bagasse descending and flue gas rising in a countercurrent manner. The tower is equipped with deflectors to promote better gas-solid contact. According to a report Kerr made of his work, the dryer was not used commercially [2]. However, the first commercial installation on bagasse drying using the flue gases was carried out in “Atlantic Sugar Association” by Vincent Processes INC in 1970s [5]. Between 1910 and 1975, only a few number of papers appeared in the literature and did not contain material and energy balances [2]. The reasons for the lack of interest in bagasse drying during this period are clear. First is the low cost of fossil fuel and, second is that the wide range of bagasse by-products were not fully utilized.


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Owing to the first oil crisis in 1970s, a number of technical reports, both theoretical and practical, have appeared. These reports can be classified into three major groups according to the main objectives of each. The first group includes the published papers which study the geometrical and physical properties of both the raw and dry bagasse. These properties are essential in providing the accurate and correct data needed for the correct design of bagasse dryers. The second group deals with the published reports that study the combustion of bagasse. At last, the third group of published papers deals with the drying systems. 3.1 PHYSICAL PROPERTIES OF BAGASSE The efficient utilization of bagasse as a fuel or as a raw material requires a complete knowledge of the properties of the bagasse particles which determine settling velocities, residence times in gas streams and cyclone efficiency [14]. The basic linear dimensions (length, width and thickness), volume, surface area, shape factor and density of the particles are used for mathematical modelling and design calculations for drying process. Raw bagasse is divided into three size classifications: bagasse, greater than 5 mm; bagacillo, between 0.3 and 5 mm, and pith, less than 0.3 mm. A typical distribution by size fraction is shown in table 6 [14]. Table 6: Bagasse size distribution at Pablo Noriega sugar mill, [14] Size, mm

0.18

0.48

0.72

1.02

1.78

3.18

4.80

6.80

9.60

13.6

19.2

Weight, %

4.0

11.8

14.1

11.2

17.8

7.4

13.5

4.5

7.0

4.2

2.0


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Fig. 1: Schematic diagram of tower dryer [7].

Nebra and Macedo [15] made an experimental study of the typical shapes and size of bagasse particles and on their free-settling velocity. The drag coefficient for fibre shape particles as function of Reynolds number was obtained for 10< Rep <2000. These data are very important for the design of systems for pneumatic transport and drying of bagasse. Upadhiaya [4] studied both the geometrical and physical composition of bagasse. He also evaluated the gross calorific value (GCV) and the net calorific value (NCV) of bagasse through the study of bagasse as a fuel. A new approach to judge the bagasse quality has been proposed in this study. 2-14 May 2015, Assiut, Egypt

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Duggal et al. [16] studied the drying characteristics of bagasse in thin layers in relation to air temperature (from 50 to 90 oC) and velocity (from 0.1 to 0.3 m/s). Desorption equilibrium moisture curves (shown in Fig. 2) were developed for the bagasse in the temperature range of 50 to 90 oC. These equilibrium moisture content data of bagasse can be described by Henderson’s model. The results also showed that the drying behavior of bagasse can be explained by grain drying theories. However, drying rates of bagasse are relatively higher than that of grains.

Fig. 2: Equilibrium moisture content of bagasse [16].

3.2 BAGASSE COMBUSTION Harel and Baguant [9] reviewed the theory of bagasse combustion, highlighting the necessity for the correct dosage of excess air. A theoretical model was proposed for a typical bagasse furnace fully equipped with positive flue gas heat feedback loops. The model has been simulated numerically to show that the performance of the 2-14 May 2015, Assiut, Egypt

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furnace is a multivariable function of bagasse moisture, excess air factor, steam to bagasse ratio and live steam conditions. Kinoshita [17] analyzed the combustion efficiency and flue gas drying of solid fuels. He developed a simple arithmetic expression for combustion efficiency. This expression involves four primary dimensionless parameters which relate to (and fixed for given) fuel and ambient conditions, and three secondary dimensionless parameters which relate to (and vary with) fuel moisture content, excess air, and flue gas temperature. Additional expressions involving the same primary parameters are developed to calculate the decrease in fuel moisture content due to flue gas drying with and without entrainment of air into the dryer system and the decrease in flue gas temperature with air entrainment. Values for the four primary parameters are presented for various fuels; their values do not vary much for most biomass fuels. Cruz et al. [11] found that when the bagasse feed in a traditional boiler was changed from 50 % to 25-30 % moisture content, the system’s efficiency increased substantially but operating problems resulted from the formation of slag which caused blockages in the furnace and stoppages for cleaning. The boiler was remodeled by: (i) elimination of the radiant arches, (ii) increase and redistribution of the heat transfer surfaces, and (iii) adjustment of the combustion air flow. Thus, the pneumatic bagasse dryer-modified boiler system is a very cost-effective alternative to a more expensive modern hightechnology boiler. Lora et al. [18] presented the results of thermal tests carried out in the reconstructed Cuban boiler RETO CV-25-18 for bagasse combustion. The main task of the reconstruction was the introduction of suspension burning of this fuel, using two different systems: vertical and horizontal swirl furnace. In conclusion a comparative analysis was made of performance and behavior of these systems. Luo et al. [12] in their study on modelling combustion variables in bagasse-fired furnace found that the addition of secondary air has little effect on the overall performance. Moreover, their study showed that the boilers can tolerate bagasse moisture content up to 56.0 % with an accompanying loss of performance, but combustion will be 2-14 May 2015, Assiut, Egypt

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probably unstable above that point. The transfer of half the bagasse feed to injectors on the rear wall will give approximately a 20 % increase in heat extraction rates. Dixon [10] has summarized the concept of swirl burner combustion system for bagasse firing application and concluded that the swirl burner system is suitable for application on all bagasse boilers. The study showed that the swirl burner firing allows increased steam capacity of boiler and increased efficiency. The level of steam capacity upgrading that can be achieved, within the limits of boiler design that are currently adopted, will depend on the particular design of the boiler to be upgraded. In general a capacity increase of 50 % can be expected. 3.3 BAGASSE DRYING SYSTEMS Boulet [19] in his article shows why a properly designed bagasse drying system is the best solution to both the auxiliary fuel problem and the air pollution problem. Moreover, the study shows that a drying system using flue gases can pay for itself (out of fuel savings) every 2.5 to 4 months. Bailliet [20] studied both the bagasse drying and the combustion air preheating as means to improve the bagasse combustion and efficiency of existing boilers. For mills unable to attain a bagasse moisture below 52 % and developing a substantial excess bagasse problem, the study showed that the bagasse drying is definitely recommendable. Bailliet [20] found an increase of 8.9 % in boiler efficiency when fired with bagasse reduced from 52 to 32 % moisture. Young [21] proposed the use of a boiler in conjunction with a direct fired rotary dryer (as shown in Fig. 3) to improve the boiler efficiency. The operation of the direct fired rotary dryer is controlled by varying the firing rate in the furnace to maintain the set temperature and by controlling the rate of gas flow through the dryer drum. The study showed that the capacity of a rotary dryer will vary as the square of the diameter and the retention time of the bagasse being dried determines the efficiency of the dryer. The rotary dryer is equipped with internal baffles that distribute the bagasse across the 2-14 May 2015, Assiut, Egypt

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drum and act as a heat transfer media between gases and the moisture in the bagasse. The study also suggested that the dryers are well suited for operation with steam boilers, as not only does the boiler use the fuel, but it also produces the waste heat (as shown in Fig. 4). Arrascaeta and Friedman [2] reviewed the status of bagasse drying up to 1984. A critical analysis was made of the literature, and the advantages of bagasse drying were shown. Different types of dryer were considered, and the use of a combined fluidized-pneumatic transport dryer was suggested. This type of dryer also separate the bagasse into different fractions that would be advantageous for handling, storing and burning bagasse in an integrated system. Several possibilities for the use of such an integrated system are suggested. Two of such possibilities are reproduced in Figs. 5 and 6 [2].

Fig. 3: Direct fired rotary dryer with exhaust gas recycle [21].


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Fig. 4 : Rotary dryer with steam boiler [21].


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Training Workshop on: “Utilization of By-Products and Waste Materials in the Sugar Industry” 1000 Sugar cane

(i) Raw material to by-product plant

250 182 Bagasse 50% water

Surplus bagasse 68

Mill train

Compacter

Gases to stack

Bagasse dryer

Pneumatic 130 Bagasse transport 30% water

Transport

65 Fines 65 Coarse

Fines silo

Coarse silo

Storage

Pneumatic transport and feeder


Boiler stack gases

Fig. 5. Case A (Flows in tonnes per day) [2]

By-product factory

Fines burner

Coarse burner

168

(iii) Electricity to neighbouring plant or electrical grid system

168

Steam

400

1300 kWh

Surplus bagasse 168

(ii) Steam to neighbouring plant

Boiler

400

Steam to sugar mill

Electricity to sugar mill

Ali K. Abdel-Rahman

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Training Workshop on: “Utilization of By-Products and Waste Materials in the Sugar Industry” 1000 Sugar cane

64 Fibre

91

Depithing

Surplus bagasse

Mill train

250 159 Bagasse 50% water

Gases to stack

Fine dryer

Bagasse dryer

52 Coarse

52 Fines

Compacter

27 Pith

Pneumatic 104 Bagasse transport 30% water

Fines silo

52

79

Transport

Coarse silo



Boiler stack gases

Fig. 6. Case B (Flows in tonnes per day) [2]

Storage

Fines burner

Coarse burner

By-product plant

Boiler

400

Steam

To

sugar mill

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In a report published by the Dominican Republic [22], it was found that a passive collector drying facility is highly suitable for bagasse drying purpose as it has a relatively low initial cost as well as low operating and maintenance requirements. Hogot [8] stated that, there is no advantage in drying bagasse below 10 % moisture, as it would be liable to increase to 10 % by absorption of atmospheric humidity. Progress in the industry and in technique may one day make it possible to dry bagasse to zero moisture. However, for combustion of bagasse in the spreader-stoker furnaces used at present, drying is generally not taken below 30 % in the dried bagasse, since a dried fuel would involve risk of higher combustion temperatures which could cause deposit of fused ash on the boiler tube [8]. He also found that the volume of the dryer is approximately 12-15 m3/(t/hr) of water to be evaporated. Arrascaeta and Friedman [13] reported the results of bench and pilot scale studies on the terminal velocities and classification of bagasse particles. An industrial prototype bagasse dryer (shown in Fig. 7) of 7 tonnes/hr wet feed is described and the operational results were reported, the most significant being: 6 tonnes/hr bagasse at 46 % dried to 28 % moisture content; increase in boiler efficiency from 72 to 82 % and in steam generation from 2.25 to 2.59 kg steam/kg bagasse (50 % basis). The investment was paid off in less than one harvest. Friedman et al. [23] carried out a study in order to increase the energy efficiency and produce surplus bagasse. They could achieve their goal through the installation of: (i) a sextuple effect evaporator station with vapor bleeding from four effects, and (ii) a bagasse dryer using waste heat from the flue gases to dry the bagasse fuel. A surplus of 34.8 % of the bagasse produced is reported for the entire 1985 harvest and about 40 % when the bagasse dryer was in operation.


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Fig. 7: chematic diagram of the AZ industrial prototype dryer (Combined fluid bed-pneumatic transport dryer) [13].

Nebra and Macedo [24] developed a simulation model of drying in pneumatic bagasse dryers (shown in Fig. 7). This dryer comprised a particles injector, a main column and a cyclone. The equipment pressure drop and heat loss were evaluated. Theoretical equations were used to simulate the operation of an existing system and results were compared with data available from the industrial system. The influence of different parameters on the system operation (size of material, mass rate of gas and solids flow, gas inlet temperature, etc.) was analyzed. They suggested that their model must and can be improved through the modification of the equations utilized for drying in the cyclone. These improvements are made possible through the support by a lot of new experimental data. Improvements in correlations for particle-gas heat and mass transfer in the cyclone would be necessary too. 2-14 May 2015, Assiut, Egypt

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The study of Duggal et al. [16] and other research on the drying process of many biological materials (such as bagasse) have revealed the proportionality of the drying rate and moisture content of the material being dried. This relation can be expressed as one of the two models [16]: Moisture Ratio = exp(-k)

exponential model

Moisture Ratio = exp(-kn)

Page model

where Moisture Ratio = (w – we)/(wo – we) wo w we



k, n

= initial moisture content of the material, % (db) = moisture content of the material, % (db) = equilibrium moisture content of the material, % (db) = drying time, s = constants

both models were further analyzed by taking into account the dependence of rate constant on drying air temperature (t), oC. Arrhenius type relationships are obtained for each model as: k = 760.54 exp(-3167.62/t)

exponential model

k = 1474.13 exp(-3530.20/t)

Page model

The results showed that the drying constant vary exponentially with temperature and the drying rates of bagasse can be closely predicted by Page’s model with an Arrhenius type dependence of k on temperature and a constant n of 1.14 [16]. McGaw and Pilgrim [25] carried out an experimental study on bagasse drying to determine the drying characteristics of bagasse using the single layer drying technique. Their results showed that the standard drying equation: Moisture Ratio = a exp(-k) was valid, with a = 1, and k a function of the system variables as follows:


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Training Workshop on: “Utilization of By-Products and Waste Materials in the Sugar Industry” k = 0.0019 exp(0.0073t) + 0.0292 exp(-0.89D) + 0.00078V + 0.00057H-0.57 – 0.00088V exp(-0.89D) – 0.314

where V is the gas velocity m/s, D is the particle size (cross section diameter) m, and H is the gas humidity kg-water vapor/kg-dry air. The results also showed that the rate of drying increases significantly with increasing gas inlet temperature and gas inlet velocity and with decreasing the gas humidity. On the other hand, the strong effect of gas humidity and gas velocity on the drying rate demonstrate a high degree of external control. The effect of reducing the humidity of the drying gas will to increase the humidity driving force. The effect of increasing gas velocity will be to increase the relevant transfer coefficients. The drying time would only be of the order of a minute or two even for the larger particles, and of the order of seconds for smaller particles [25]. This explains the potential for the use of pneumatic dryers more particularly in sugar factories. In order to obtain a better use of bagasse, the dryer developed by Alarcon and Justiz [1] has the possibility of pneumatically classifying it into two fractions: a coarse fraction and a fine fraction. The fine fraction is dried by using the flue gases and its moisture decreased from 50 to 30 % and then sent to the boiler furnace to be used as fuel. The coarse fraction is the surplus which is an excellent product to be used as raw material in other industrial processes such as paper pulp production. Stanmore and Arici [26] studied the convective drying of bagasse in boiler. They found that the drying of bagasse in a boiler strongly influences the temperature distribution and the location of ignition in the furnace due to the high initial moisture content of bagasse. Two mathematical models of the drying process were evaluated by experimental study of cylinders of fresh bagasse in a stream of hot, dry air. The first model, which was empirical, assumed an exponential fall in water content. The second assumed that drying takes place at the wet bulb temperature on a shrinking moist core, with a conductive resistance to heat flow through the dry outer shell. For commercial dryers and boilers which handle run-of-mill bagasse, the empirical model is recommended. 2-14 May 2015, Assiut, Egypt

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SIIC, has recently realized the importance of bagasse drying for both energy saving and efficiency improvement of the steam generation equipment. Starting from this point, the company has built a bagasse dryer to be used in one of its factories (Guirga Sugar Factory) and to serve as a pilot plant for study and performance evaluation before extending its usage to other factories. The dryer is of the rotary drum type with a diameter of 4 m and a length of 16 m as shown in Fig. 8 [27].

Fig. 8: A schematic diagram of the SIIC rotary drum dryer [27].

The thermal design calculations of the existing bagasse rotary dryer has been carried out under the following conditions: (i) flue gases inlet temperature is 195 oC (it should not exceed 200 oC to avoid the self ignition of fine bagasse), and (ii) fresh bagasse temperature is 25 oC and moisture content is 52 %. In this study calculations of the maximum possible load in ton bagasse per hour at 45 % moisture content (mB45) obtained by the dryer under the operating conditions at Guirga Factory were carried out. These calculations were carried out for two cases. In the first case (case I), one boiler uses some of the output of the dryer and the rest is directed to the second boiler together with the 52 % bagasse (mB52). The proposed flow diagram in this case is shown in Fig. 9. In the second case (case II), the output of 2-14 May 2015, Assiut, Egypt

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the dryer is mixed together with the 52 % bagasse and then each boiler uses 50 % of the dried bagasse (mB45) and 50 % of the wet bagasse (mB52). The proposed flow diagram in this case is shown in Fig. 10. Based on this study, it was found that case II provides higher boiler capacity and a higher annual saving than case I. So, the study recommended the implementation of case II for operation.

4. BAGASSE DRYERS 4.1 PROCESS There are a number of methods for drying the bagasse. These methods can be classified according to the drying medium to the followings: (a) bagasse drying by using flue gases from boilers; (b) bagasse drying by using heated air [1,2,4,5]; (c) bagasse drying by using solar energy [23]; and (d) bagasse drying by using high pressure superheated steam [28].

Fig. 9: A schematic diagram of case I [28]. 2-14 May 2015, Assiut, Egypt

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The cheapest and most efficient means are to bring the bagasse in direct contact with the flue gases from boilers and this can be done without setting the bagasse on fire in the dryer. Theoretically [7] the wasted heat in stack is more than sufficient to evaporate all the moisture from the bagasse. The drying process requires approximately 60 % of the hot gases generated in the furnace [3]. These 60 % of the gases after drying the bagasse are relatively cooler and attain a higher degree of humidity. These are then mixed with rest of 40 % of hot gases. The resulting mixture has an average temperature between 130–140 oC, which is well above the low corrosion point of carbon steel.

Fig. 10: Flow diagram of case II [28].

The advantages of using the flue gases in drying of bagasse can be summarized as: (1) reduction of the air pollution from values of about 10,000 mg/Nm3 of ash to less than 300 mg/Nm3 [2,5,13]; and (2) reduction of the heat losses in the flue gases due to decreasing the exit temperature of gases. (The bagasse dryer allows cooling of the 2-14 May 2015, Assiut, Egypt

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gases to 90 oC, the only limit being imposed by the necessity to avoid cooling to the dew point of 60 - 70 oC.) The types of dryers which can be used for bagasse drying by using the flue gases are given below. 4.2 TYPES OF DRYERS 4.2.1 Tower Dryers Tower dryer is the oldest type (see Fig. 1) which was built in 1910 by E. W. Kerr [2,7]. It is a square tower with bagasse descending and stack gas rising in a countercurrent manner. The tower is equipped with deflectors to promote better gas-solid contact. This dryer was not used commercially [2]. 4.2.2 Rotary Drum Dryers The principle of single or multiple passage of hot gases contact in parallel with the bagasse to be dried inside a rotary drum type is similar to the sugar dryer [3,5]. The dry bagasse is then separated from the wet gases in large diameter cyclone [3] as shown in Fig. 4. Rotary drum dryers have the following advantages: (i) they can give uniform final moisture content with non-uniform particle size and inflow; (ii) they are more suitable for medium and large units; (iii) they tend to discharge less dust to atmosphere; (iv) its specific energy consumption is less than pneumatic dryers (it is 20 kWh/t-water evaporated) [29]. But they are big dryers, of low efficiency, occupying large areas [3,5]. Their construction and installation are quite complex. They need an extra conveyor to transport bagasse to the drums and then back to the boilers. 4.2.3 Fluidized-Pneumatic Dryers Fluidized-Pneumatic dryers are much more efficient than rotary dryers due to the fact that they utilize drying columns where the bagasse is raised by a stream of hot gases and is separated from the wet gases after drying by one or two cyclones [3,5]. Their cyclones are comparatively smaller and need less investment and operation costs [2,3,8]. Luiz Maranhao [5] reports that the cost of a pneumatic transport dryer is 47 % less than that of the rotary drum dryer. They are very safe as far as fire-hazards are concerned [3]. Another important advantage is that during the drying, size classification of particles can be achieved [1,2]. Such dryers can be classified into 2-14 May 2015, Assiut, Egypt

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three types as: (a) fluidized bed dryers such as the one shown in Fig. 11; (b) Pneumatic transport dryers such as the one shown in Fig. 12; and (c) Combined fluid bed-pneumatic transport dryers such as the one shown in Fig. 7.

Fig. 11: Fluidized bed dryer [5]

On the other hand, fluidized-pneumatic dryers require uniform particle size and uniform inflow of wet bagasse and their specific energy consumption is more than that of the rotary dryer types [29]. It is about 35 kWh/t-water evaporated [29]. Moreover, they need conveyors for wet and dry bagasse [3,5]. The most significant results of the industrial prototype dryer which is shown in Fig. 7 are summarized as follows [13]: 1- 6 tone/hr bagasse of 46 % is dried to 28 % moisture content. 2-14 May 2015, Assiut, Egypt

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2- 70 % of stack gases cooled from 300 OC to 100 OC. 3- Boiler efficiency is increased from 72 % to 82 %. 4- Steam generation is increased from 2.25 to 2.59 kgsteam/kg-bagasse (50 % basis). 5- Dust in flue gases is reduced from about 2000 to 300 mg/Nm3. 6- The investment cost is low and the payoff time is about half a harvest season (fuel oil is at $175 per ton). With low sugar prices and high energy costs it seems that the pneumatic dryer-modified boiler system (ICINAZ) shown in Fig. 7 would, in many cases, be the preferred alternative to a higher cost new modern boiler as shown in table 7 [11]. Table 7: Comparison between modified boiler and new modern boiler [11] System Level of technology Investment cost Operating cost Efficiency

Bagasse dryer + modified boiler Medium Low Medium High

New modern boiler High High High High

Fig. 12: Pneumatic transport dryer [5]. 2-14 May 2015, Assiut, Egypt

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4.2.4 Individual Dryers Luiz Maranhao [5] reported that Zanini and Cemasa of Brazil have developed an individual dryer for each furnace similar to the one shown in Fig. 13. The hot gases are sucked before the air preheater and pulled by an auxiliary fan until the front side of the boiler by overhead or an underground duct. The wet bagasse falls down by gravity from the normal conveyor passing by a feeder that blocks the inlet of false air instead of going to the spreaders and diverted by a bypass porthole to the bagasse blending chamber with the hot gases. Thus the bagasse half spread is sucked by an auxiliary fan that raises the mixture through the dryer columns until the cyclone where the wet gases are apparted from the dry bagasse. The wet gases go towards the chimney through big diameter ducts with an average temperature of 90 oC. The dry bagasse falls down by gravity into the cyclone and goes down by an air locker through the spreaders and feeds the furnace. The dryer utilizes only 60 % of total flue gases to dry bagasse. The remnant 40 % of flue gases are used in the air heater to warm the burning air. The final temperature of flue gases is about 135 o C. This dryer can be mounted in front of the boiler individually to each furnace. It does not need any other installation of conveyors to bring and to take back the bagasse to the dryer.

Fig. 13: Individual dryer [5]. 2-14 May 2015, Assiut, Egypt

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Luiz Maranhao [5] carried out experimental tests on an industrial scale using an individual dryer developed in 1979 (such as the one shown in Fig. 13). The obtained results can be summarized as [5]: 1- Bagasse drying was done utilizing flue gas temperature of 220 o C and 300 oC. The efficiency of drying was increased by 68 % using hotter gases. 2- Using flue-gas temperature 300 oC, the dried bagasse left the dryer at 40 oC and the wet gases at 100 oC. 3- The system can be controlled easily, because it only requires a depression of air on the top exit of the cyclone. 4- The total investment cost is 50 to 60 % of the existing fluidizedpneumatic dryers and the total power consumption is roughly 55 %. The efficiency is the same as with others. 5- The individual dryer can be installed on any existing boiler, and it is possible to dry a portion of the bagasse, or all the bagasse of the boiler, and so auxiliary bagasse conveyor is unnecessary. 6- It consumes 50 to 60 % of the flue gases from the same boiler, in which it is installed. The rest 40 to 50 % is used to mix with the wet gases to avoid corrosion in the ducts and chimney. 7- If the hot gases are taken after air preheater at 220 oC, the moisture can be decreased by 10 points. But, if the gases are taken before the air preheater which has a temperature of 300 oC, the moisture will be decreased by 15 points. Luiz Maranhao [5] reported that the payback period for a dryer would be 5 months, if the excess steam produced by the dried bagasse is totally utilized.

5. CONCLUSIONS An extensive review for the subject of bagasse drying has been made in this review paper. The objective of this review is to introduce the subject of bagasse drying to new comers and to supply a state of art review to those who are interested in the subject. The presentation methodology has given a priority for the achievement of the above objective among the other objectives stated before. 2-14 May 2015, Assiut, Egypt

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The review covers the subject of bagasse drying from different points of view such as the necessity and advantages of bagasse drying. Drying process basics, required data, and related topics such as bagasse combustion have been reviewed. Moreover, an extensive review for the published papers and reports in bagasse drying is made. The types of the available bagasse dryers are also reviewed and discussed. The review pointed to the following facts: 1- It is important for all sugar mills to install bagasse dryer. 2- The bagasse drying by using the flue gases is an economic necessity for cane sugar factories where it can realizes self sufficiency of fuel in additional to the achievement of surplus of bagasse to be used in by-product industry. 3- It is an environmental necessity where it reduces the air pollution. 4- Bagasse has not become by-product of sugar industry, but it has become main product. 5- Many types of bagasse dryers have been installed in different countries. 6- Rotary drum dryers have become seldom used now where they left their position for a more efficient and economically attractive pneumatic dryers. 7- Future research is needed to develop an optimum bagasse dryer through deep research work on the theory and modelling of bagasse drying process. With low sugar prices and high energy costs it seems that the pneumatic dryer-modified boiler system (ICINAZ) shown in Fig. 7 would, in many cases, be the preferred alternative to a higher cost new modern boiler.

6. REFERENCES 1- G.A. Roca Alarcon and M.A. Boizan Justiz, “Industrial equipment for drying and classifying sugar cane bagasse”, Int. Sugar J., 1993, vol. 95, no. 1133, pp. 177-179.


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2- A. Arrascaeta and P. Friedman, “Bagasse drying: past, present and future”, Int. Sugar J., 1984, vol. 86, no. 1021, pp. 3-6. 3- Anwar S. Ansari, “Bagasse dryers: a study on the utilization of bagasse, part-1”, Proc. 22nd Annu. Conv. Pak. Soc. Sugar Cane Technol., 1986, pp. 304-319. 4- U.C. Upadhiaya, “Bagasse as a fuel”, Int. Sugar J., 1991, vol. 93, no. 1111, pp. 132-138. 5- Luiz E.C. Maranhao, “Bagasse drying”, Paper presented for the ISSCT combined factory/energy workshop, 1994, pp. 1/1051/117. 6- Private communication, “Annual Production Report”, Sugar and Integrated Industries Company SIIC, 1998. 7- E.W. Kerr, “Waste fuel drying and energy crisis”, Louisiana Bulletin, 1911, (128); quoted by W.P. Boulet, Sugar J., 1975, vol. 37, no. 10, pp. 40-47. 8- E. Hugot, “Handbook of cane sugar engineering”, 1986, Elsevier, Amsterdam, 3rd ed., pp. 922-988. 9- P. Harel and J. Baguant, “Bagasse combustion”, Int. Sugar J., 1992, vol. 94, no. 1117, pp. 11-15. 10- T.F. Dixon, “Boiler uprating and the SRI swirl burner”, Int. Sugar J., 1995, vol. 97, no. 1158E, pp. 248-252. 11- I. Cruz, J. Feria, A. Arrascaeta and P. Friedman, “Boiler remodelling for the combustion of dried bagasse”, Int. Sugar J., 1991, vol. 93, no. 1108, pp. 75-78. 12- M. Luo, B.R. Stanmore and T. Dixon, “Modelling combustion variables in a bagasse-fired furnace - The effect of air/fuel addition on performance”, Int. Sugar J., 1995, vol. 97, no. 1160, pp. 355-359. 13- A. Arrascaeta and P. Friedman, “Bagasse drying”, Int. Sugar J., 1987, vol. 89, no. 1060, pp. 68-71. 14- N. Ponce, P. Friedman and D. Leal, “Geometric properties and density of bagasse particles”, Int. Sugar J., 1983, vol. 85, no. 1018, pp. 291-294. 2-14 May 2015, Assiut, Egypt

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15- S.A. Nebra and I. de Carvalho Macedo, “Bagasse particles shape and size and their free-settling velocity”, Int. Sugar J., 1988, vol. 90, no. 1077, pp. 168-170. 16- A. Duggal, D.K. Gupta and B.P.N. Singh, “Drying characteristics of sugar cane bagasse”, The Journal of the Institution of Engineering (India) – Agricultural Engineering Division, IE(I) Journal-AG, 1988, vol. 68, pp. 96-98. 17- C.M. Kinoshita, “A theoretical analysis of predrying of solid fuels with flue gas”, J. Energy Resources Technology, Trans. of the ASME, 1988, vol. 110, no. 2, pp. 119-123. 18- E.S. Lora, P.B. Soler and G.L. Aguilar, “Swirl systems for the suspension burning of bagasse: evaluation of industrial results”. Int. Sugar J., 1994, vol. 96, no. 1141, pp. 36-38. 19- W.P. Boulet, “Waste fuel drying and the energy crisis”, Sugar J., 1975, vol. 38, no. 6, pp. 8-11. 20- V.J. Bailliet, P.E., “Bagasse drying versus air pre-heating”, Sugar J., 1976, vol. 38, no.10, pp. 52-53. 21- W.O. Young, “Biomass fuels dehydration with industrial waste heat”, Sugar J., 1981, vol. 43, no. 9, pp. 13-16. 22- “Interim report on bagasse solar drying system in the Dominican Republic”, Trident engineering report, Sugar J., 1985, vol. 47, no. 12, pp. 6-10. 23- P. Friedman, D. Leal and J. Feria, “Surplus bagasse”, Int. Sugar J., 1987, vol. 89, no. 1067, pp. 205-207. 24- S.A. Nebra and I. de Carvalho Macedo, “Pneumatic drying of bagasse”, Int. Sugar J., 1989, vol. 91, no. 1081, pp. 3-7. 25- D.R. McGaw and A.C. Pilgrim, “Utilization of flue gases in raw sugar factories for bagasse drying”, Drying 91, edited by A.S. Mujumdar and I. Filkova, 1991, Elsevier Science Publishers, Amsterdam, pp. 558-566. 26- B.R. Stanmore and P. Arici, “The convective drying of bagasse in a boiler”, Int. Sugar J., 1997, vol. 99, no. 1178, pp. 71-75.


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27- “Energy saving in the sugar production factories of the Sugar and Integrated Industries Company, SIIC”, Research project contract between the SIIC and Energy Research Center of Faculty of Engineering – Cairo University – First report, 1996. 28- B. Ramani and N.D. Kothari, “Drying of bagasse by exergy dryer”, Bharatiya sugar, 1990, vol. 16, no. 1, pp. 141-150. 29- J.H. Fonseca, “Drying of bagasse: comparison of drum dryers and flash-type dryers for use in the sugar industry”, Int. Sugar J., 1987, vol. 89, no. 1060, pp. 76-85.


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