HIGH-EFFICIENCY CANE SUGAR FACTORY IN A COGENERATION CONTEXT. A. Belotti & B. Moreau FCB,.FRANCE

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1 HGH-EFFCENCY CANE SUGAR FACTORY N A COGENERATON CONTEXT A. Belotti & B. Moreau FCB,.FRANCE Products: Energy ABSTRACT This paper presents a simulation of what could be achieved today in a high-efficiency cane sugar factory, starting from a concrete example and applying to it new improvements. A simulation was made of a high-performance sugar factory with full cogeneration based on the following conditions: The use of proven equipment and simple process flow-sheets; very low steam consumption and a much higher electric production; and despite this low steam consumption with higher than traditional sugar extraction. ts appears possible to reach 235 kglt cane steam consumption and 732 kw/t of fiber delivered to an external network eventhough we maintained a classical 40-bar boiler. Keywords: Sugarcane, sugar factory, thermodynamics, cogeneration, bagasse. NTRODUCTON More and more cane sugar factories plan to use surplus bagasse to generate electricity. Today's target is to produce as much electric energy as possible to cover the sugar factory's needs and provide a surplus to feed into an external electricity network. To achieve this, steam consumption must be reduced as much as possible. This was our objective in designing the Borotou-Koro sugar factory (vory Coast, 3600 TCD). The objectives of this sugar factory, commissioned in 1979, were originally 405 kg steam/t of cane; the measured consumption was 380 kg steamlt of cane. Thanks to this design, the Borotou sugar factory delivered 45 kwh/t of cane to an external electric network for field irrigation purposes. Nowadays, the use of improved technologies such as heavyduty shredders, pre-extractor and four-mill tandem, 100 % continuous crystallization, 100 % continuous centrifugation, mechanical steam recompression on evaporation and heat recovery on condensates permits further improvements of these results. This paper shows the energy balance that will be achieved if we apply all these technologies at the same time. CANE CRUSHNG The tendency is toward optimum cane preparation followed by a pre-extraction device consisting of one preextractor coupled to the first mill p h a three-mill tandem. Our choice of milling tandem is justified by the following two points: The high sugar extraction level obtained with modem tandems with low water consumption and production of a cold source (i.e., the extracted juice) will always be better for the overall thermodynamic efficiency of the plant. Specifically this allows recovery of waste energy and optimization of the heat balance of the overall plant. The mills would be driven by DC motors instead of the conventional mill turbines for the following reasons: Mill reaction time is shorter and control is more flexible around set point values; thermodynamic efficiency of turbines is optimum for a given speed, which is oren different from the desired speed; electric motors can easily be reversed; and maintenance costs of electric motors are low. Therefore, from an efficiency standpoint, it is better to produce the milling energy by big fixed-speed turbines (turbo-alternators) and then operate the mills with electric power. The heavyduty shredder, the rotating speed of which is fixed, would be driven by an AC motor. Milling house The improvement in the existing milling house efficiency with respect to our cogeneratioh goal led to the elimination of knives and installation of a heavyduty shredder on the existing cane carrier, thereby increasing the overall extraction, using less water, with a pre-extracting device coupled to the first mill. The heavy-duty shredder superimposed on the existing cane carrier gives excellent cane preparation with the lowest energy input and the lowest installation cost (Fig. 1). This heavyduty shredder produces long fibers, which facilitates uniform percolation during the extraction process. Cell rupture reaches 90-92% with a low energy consumption compared to equivalent systems. A preextractor coupled to the first mill (Fig. 2) increases the overall extraction rate. t increases the dry pressure extraction. ts higher efficiency compared to a conventional mill lowers energy consumption; pol extraction for 15 %

2 A. Belotti & B. Moreau Figure 1. Heavyduty shredder, superimposed installation on an existing cane carrier. The present invention relates to presses designed to express a liquid out of a material, in the present case, a vegetal material, consisting of two rolls with their axes parallel to each other, between which the material to be processed is squeezed to extract the liquid it contains. The press is characterized by the fact that the plane (P-P) of the roll axes is inclined downstreamward with reference to the material displacement, that circumferential, narrow and deep grooves (36) are machined in~the lower roll (12), and that a chute (14) inclined downwards is installed at the outlet of the rolls nip, and that this chute is provided with throttling means (16) generating a pressure applied to the material inside this chute, between the rolls and the said throttle. Figure 2. Pre-extractor principle.

3 Products: Energy fiber in cane is 55 % minimum, and we obtain approx. 80 % with the first mill just behind the pre-extractor; wear and maintenance costs are lower than for equivalent mills. Comparison of tandem performance Results obtained on industrial installations are given in Tables 1 & 2 and compared with the ones in Borotou. For a slightly higher electric consumption (= 12 %) we gain 4 points in extraction. mbibition is reduced by more than 20 % 4 final moisture of bagasse is reduced by more than 4 percentage points. The advantage from the process standpoint is double: first we have an important increase in the bagasse calorific value; secondly juice flow to purification and evaporation is lower. For similar or slightly higher extraction in cane sugar, factories commonly use double the quantity of imbibition water, which has a direct influence on the overall energy consumption of the factory. THE CRYSTALLZATON PLANT Fully continuous crystallization The choice is toward conthous operation of vacuum pans, centrhgals and crystallizers. Continuous vacuum pans offer higher thermal and crystallization efficiency than batch ones. The steam for continuous vacuum pans is extracted from a lower evaporation effect. One of the advantages of continuous crystallization is either the smaller size of equipment for the same capacity or the possibility of using lower steam pressure. t is interesting to note that, with a view to moving steam bleedings to lower effects, we introduced pre-evaporation of syrup before the A pan. This can be achieved either with a small evaporator or with a recirculating flash tank. As a result we are able to partially pre-evaporate using a lower vapor level VP4, fourth effect vapor. We maintained a C-B-A process (Fig. 3). High-efficiency continuous centrifugals For decades the sugar industry has accepted the disadvantages of batch operation for producing high-grade sugar. Nowadays with efficient continuous centrifugals (STG) it is possible to have continuous centrifugation in all strikes in cane sugar factories and refineries. The advantages of such continuous centrifuging are that a steady continuous flow reduces upstreamldownstream plant and maintenance costs; lowers electric consumption with the STG continuous centrifugal on B and A sugars ( kwhttmc) compared to batch centrifugals (1-1.2 kwh/tmc); fewer machines are required (5 instead of 8 in our case); the molasses purity from the high-efficiency continuous centrifugals is points lower than from a batch machine with equivalent capacity (this value tends to be more pronounced on B massecuites); and safe, dependable, unattended operation is possible. All these advantages are obtained while maintaining a granulometry equivalent to the one obtained in batch centrhgals without lump formation. n our application the use of such centrifugals will reduce electricity consumption 35 %, from 200 to 130 kwh. Consequences of these new technologies for the crystallization plant (Tables 3-5). The higher extraction rate in the milling station results in a higher dry matter content in the crystallization plant. Because of the 100 % continuous design we will consider a rather constant raw syrup Brix of 70 at the evaporation station outlet. A standard liquor concentrator before the A continuous pan will give a Brix of Continuous crystallization and centrifugation enable us to decrease thermal losses by dilution or crystal remelt in centrifhgals and power consumption. All this results in a decrease of % in steam bleedings to crystallization and the use of low level steam (from 95 to 105 C) in crystallization while maintaining the same volume of pans (Table 6). Only the C footing pan will be fed with first effect vapor (VP1); sugar yield is 88 %, higher than at Borotou.

4 A. Belotti & B. ~oreau Table 1. Milling house. Borotou 1979 Nominal power consumption High-efficiency - cane sugar factor simulation Nominal power consumption Equipment &w> Equipment &w> 1 first knive cane levelers second knive feeding drum 90 1 self-setting mill, x 2100 mm 1 self-setting mill, 0920 x21oomm 1 self-setting mill, x 2100 mm 1 self-setting mill, x 2100 rnm shredder pre-extractor self-setting mill self-setting mill self-setting mill self-setting mill 250 Overall kw 2160 Overall kw 2430 consumption consumption Table 2. Tandem results. Borotou Simulation Pol extraction % mbibition % fiber Cell rupture P % Bagasse final moisture % ~mbibition temperature "C

5 Table 3. Boiling house, size comparison between batch and continuous vacuum pan, example for 3600 TCD with C - B - A-process. Borotou 1979 Classical factory High-efficiency factory existing batch continuous vacuum simulation, continuous Strike alternative pan alternative vaccum pan alternative A 3x500hl 450 hl 640 hl B 1 x 500 hl 450 hl 640 hl C 380 hl 450 hl + footing 250 hl + footing 250 hl batch batch Total V, = hl V, = 1530 hl V, = 1980 hl vol. Required calandria 115 C 110 C C 95 C C Products: Energy Table 4. Type and power consumption of centrifugals. Borotou 1979 High-efficiency Classical factory simulation alternative STG alternative batch centrifugals 2 (40 kw) 0 number continuous centrifu- 6 5 gals number kw consumption Table 5. Boiling house, comparison of process results. Sugar - Losses and yields Highefficiency sugar Borotou factory 1979 simulation Sugar content in cane (kglt cane) Sugar losses in bagasse (kglt cane) Sugar losses in scum (kglt cane) ndetermined sugar losses (kglt cane) Sugar losses in molasses (kglt cane) Overall sugar losses (kglt cane) Sugar production with 0.5 % moisture and purity (kglt cane) Sucrose extraction (kg % kg sucrose in 84 88

6 A. Belotti & B. Moreau For sugar drying we use a multi-tube dryer; this clean, compact equipment requires much less electric power than other systems such as fluidized bed dryers. TEE EVAPORATONaSTATON (Table 6, Fig. 4) General organization We will keep a 5-effect evaporation but with minor changes in the vapor bleedings. Condensates from the first body will be sent directly to feed the boiler'in order to maximize heat recovery. One mechanical compressor is installed on the first evaporator body to maintain a condenser escape of about 1 tlh. This choice must be evaluated case by case as it depends on available recompression ratio; kw price and cane harvesting period; mechanical vapor recompression (MVR) installation cost and thermodynamic yield of condensation turbines. Steam pressure before evaporation was 1.5 bar gauge to prevent sucrose degradation caused by overheating of juices during evaporation. This classical choice is partially justified if we consider the amount of color formation in our evaporators. Juice heating and water balance The aim was to move bleedings on evaporation as far back as possible and also to recover all the enthalpy possible on condensates. n comparison with Borotou, we made the following modifications: maximum use of the last 2 effect vapors to heat jcuces and addition of a hot water condenser to recover usually wasted heat. This hot water is used as a heating medium for the process. ENERGY ANALYSS OF THE COGENERATON SYSTEM (Fig. 5) n the Borotou sugar factory project, we made use of high-pressure boilers with superheaters (47 bar, 420 C); the global yield was 83 %. =our simulation we did not consider very high-pressure boilers (80 bar) because they are expensive to install, operate and maintain; and the supply of feed water requires installation of a sophisticated water treatment plant. Furthermore such a boiler would not permit any steam recompression because of sugar traces in condensates. The maximum pressure allowed for steam recompression was 47 bar. We slightly increased boiler yield from 83 to 86 %, but did not exceed this value so as not to multiply auxiliary equipment. For this purpose we shall add a second economizer to bring gas temperature down to 160 C. Electric consumption of the Borotou cane factory was 28 kwlt of cane. n this simulation, we estimated it at 32.3 kwlt of cane, which means a slightly greater consumption due to MVR installation, more heat exchangers and a slight increase in the milling tandem. Comparative analysis of the two steam balances shows a very important gain in the electric power delivered although we maintained a classical boiler. t appears possible to deliver about 732 kw/t of fiber in cane with these proven technologies, which can be summed up in three points: mprovement of the milling house efficiency in order to use less water a mprovement of the crystallization house in order to be as continuous as possible mprovement of plant thermal efficiency with welldesigned evaporating station and hot water balance. CONCLUSONS The results obtained with such proven technologies are of course not the maximum achievable. Nevertheless further steam savings would require more equipment and control and make the factory more difficult to operate. n this paper we restricted our studies to, the technical aspects, without taking into account the economic ones; return on investment of these technologies has to be studied case by case, taking into account the specific aspects of each factory, the characteristics of the electric network outside the factory, and the energy policy of each country.

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8 'C TO 801 LERS JAEO:LGHT JUCE HEATNG PDCJ:C FOOTNG SECH:DRYER JAE1:LGHT JUCE HEATNG MC1:A CONTNUOUS PAN MB:MBlElTON JAE2:LCHT JUCE HEATNG JCW :LMED JUCE HEATNG MC2:B CONTNUOUS PAN JVRT:RAW JUCE MC3:C CONTNUOUS PAN RDV:OTHER HEATERS CL5:SYRUP CONCENTRATOR Figure 4. Evaporation house: high-efficiency sugarcane factory simulation.

9 Products: Energy Table 6. Evaporation house: overall comparison. Main results and equipment High thermal- Classical cane efficiency cane factory, Borotou factory 1979 simulation Mechanical recompression none 1 small compressor with 1 steel wheel Areas 1st vessel nd vessel rd vessel th vessel 888 5th vessel 888 Light juice flow (kglt cane) 943 Light juice (O Brix) 15.5 Thick juice flow (kglt cane) Thick juice (O Brix) 70 Temp. (C) 1st vessel 120 2nd vessel rd vessel 104 4th vessel 92 5th vessel 69.4 Direct steam needs (kglt cane) Recompressed steam (kglt cane) 90 Total to evaporation (kglt cane) 324.5

10 A. Belotti & B. Moreau p OACASSE H,Ot:45 UNTS:T/H FRGU 1st EFFECT 'C WATER WATER PRODUCED KY;23060 KW 2.5 CONSUuMEO KW: 5300 Kw S OTHERS mrotou SUGAR FACTORY 197% BAGASSE (EX~ESS 2.1T/H) Ha01 : 50 T/H:47.2 u PC1 :850 kcof/kg BOLER 32! WA~ER : ~ WATER 4 WATER HEATNG 4.4 OESUPERHEATNO - EVAPWATON OTHERS PRODUCCO KW: KW CONSllMMEO KW; 4591) KW Figure 5. Cogeneration balance.

11 Products: Energy REFERENCES De Cremoux, J. & Lombard, P. (1980). Complex sucrier de Borotou Koro: Un exemple deconomie d'energie en Sucrerie de Cannes. n AA, pp 7-8. Belotti, A. & Journet, G. (1994). The FCB continuous vacuum pan. n Roc. SSCT Combined FactoryEnergy Workshop, Pune (ndia). Sug. Technol. Ass. of ndia, pp Greig, C.R. & Belotti, A. (1994). The successful development of a continuous centrifugal for high grades sugars. n Proc. SSCT Combined Factory/Energy Workshop, Pune (ndia). Sug. Technol. Ass. of ndia, pp ' Sullivan, M.D. (1993). Reabsorption control. nt. Sugar J. 95: ALTA EFClENCA EN LA FABRCA DE AZUCAR EN EL MARC0 DE LA CO-GENERACON A. Belotti & B. Moreau FCB - FRANCE RESUMEN El presente trabajo muestra mediante simulaci6n, lo que se puede lograr actualmente en una fhbrica de &car de cafh de alta eficiencia. Comenzando con un ejemplo concreto, y haciendole modificaciones para lograr mejoras sucesivas. Sobre esta base, se realiz6 la simulaci6n de una fhbrica con comportamiento de alta eficiencia con cogeneraci6n alta, orientada a: a) Utilizaci6n s610 de equipos probados y con diagramas de flujo sencillos. b) Bajos consumos de vapor y una alta generaci6n electrica. c) A pesar de este bajo consumo de vapor, la extracci6n de adcar calculada result6 superior a la original. Se hizo aparente la posible obtencidn de consumos de vapor tan bajos como 235 kg por toneladas de c&, asi, como la entrega a redes electricas externas al ingenio, de 732 kw-h por tonelada de fibra procesada, y esto considerando la operaci6n con una caldera conventional de 40 bar.