Barata, J.C.F. et al. Proc. Int. Soc. Sugar Cane Technol., Vol. 26, 2007 ________________________________________________________________________________________________

FROM BI-DRUM TO MONODRUM™ STEAM BOILERS—A STEP FORWARD INTO CO-GENERATION IN BRAZILIAN SUGAR MILLS

By

J.C.F. BARATA1, J.E. GOMES2, M.G. SANCHEZ1 and H. VERBANCK3

1Caldema Equips. Inds.Ltda. engenharia@caldema.com.br

2Thermocal, gomes.thermocal@boilers.com.br 3Alstom John Thompson hansv@jtafrica@.co.za

KEYWORDS: Boiler, Bi-drum, Monodrum™, Bagasse, Energy.

Abstract

THIS PAPER presents an assessment on the evolution of the bagasse-fired boilers to meet the current cogeneration trends of Brazilian sugar and alcohol plants. For this purpose, a single pass bi-drum Brazilian bagasse-fired boiler design is depicted through its constructive and operational features to meet higher capacities and steam generation parameters. The proposed comparative scenario contributes to illustrate the evolving process, highlighting the recent introduction of the Monodrum™ boiler. The description shows the progress and limitations of each development stage according to one key aspect: the suitability of the boiler in the context of modern expanded high steam generation parameters for cogeneration plants to match the ever growing tendencies of the Brazilian sugar industry. The paper shows how the increased power generation trend in Brazilian sugar and alcohol plants has successfully corroborated the use of Monodrum™ boilers for this application, as the only boiler capable of efficiently working at higher capacities and steam generation parameters.

Introduction

Sugar and alcohol plants in Brazil are increasing in size, and annual crushing capacities of 4.0 – 6.0 × 106 tonnes of cane are becoming more frequent. On the other hand, the government is promoting cogeneration plants based on biomass, to help match electricity demands. Twenty-year contracts for sales of electricity linked with more favourable finance are the elements to make cogeneration feasible on a large scale at sugar mills.

The association of large crushing capacities with optimisation of cogeneration for electricity sales purposes, together with the necessity to reduce operational costs, has pushed the demand for boiler capacities up to 350 t/h and pressure/temperature range 92 bar/520°C according to Caldema Equips. Inds. Ltda (2006).

In parallel, the operation of traditional single pass two drum boilers, sizes 150–175 t/h and pressure 63 bar/480°C, has shown problems related to tube leakages at the main tube bank, on expanded area into drums, as discussed by Barata et al. (2005). These leakages are associated with fast start-ups and shut downs, due to differential expansion between drum plate (~100 mm thick) and tubes (~4 – 5 mm thick). On cogeneration plants for sugar mills, it must be taken into account that rain periods force the mill to shut down at times due to the lack of sugarcane.

All the above factors expose the market to the Monodrum™ boiler technology, frequently used in north Europe for utility boilers, as the predominant design for power plants all over the world. The application of this technology started to be developed for sugar mill applications several years ago, being successfully introduced into the Brazilian market by 2004.

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The main goal of this paper is to discuss how the shift towards increased power generation in the Brazilian sugar mills has led to the development of a Monodrum™ boiler for this application

The bi-drum Brazilian bagasse-fired boiler in the sugar mill scenario Two drum single pass bagasse fired boilers traditionally used in sugar mills, usually in the

range from 100 to 200 t/h steam, have basically a fixed geometry, where the evaporator is located between the drums, and the superheater is located up stream to the flue gas flow, taking up some space in the furnace. The superheater can be directly exposed to the furnace radiation or, alternatively, can be protected by one or more rows of furnace tubes usually called screens. Either the air heater or the economiser, or both, can be located down stream to the evaporator.

The single pass bi-drum boilers were specifically developed to overcome the tube erosion problems and to eliminate structural refractory, a prevailing characteristic of many former boiler designs, which led to higher maintenance costs (Alstom John Thompson, 2003). On the other hand, as remarked by Sánchez and Barata (2006), quite high furnace designs aimed at promoting the stability of the heat released in the combustion chamber started to be considered in this type of design used by boiler manufacturers, in order to reach higher combustion efficiency.

The first constructive single pass bi-drum design was the bottom supported boiler (no steel structure), with finned water wall furnace, proposed for capacities up to 165 t/h and with steam parameters around 65 bar and 490oC. Initial designs considered low height bagasse conveyor and, therefore, lower height bagasse feeders. Partial suspension firing (up to forty percent) with positive results started to be used in steam generation units, even without significant reductions in the thermal load of the furnace and the consequent increase in the residence time.

The furnace volume increase, mainly due to its height, had more significant results in the top supported bi-drum design, recommended for higher capacities (up to 200 t/h). Higher boiler heights allowed suitable conditions for the reduction of air excess coefficients to reach efficient combustion conditions, while very little change was experienced in the grate surface.

However, in such cases, the installation of supplementary burners for oil and/or gas is rather difficult, as its design has to consider certain precautions to protect the grate and superheater tubes from the direct radiation of these burners. Higher steam pressure results in much thicker drums (for example around 150 mm for 200 t/h boiler/92 bar) and higher degree of superheating results in rather large superheaters, using up furnace volume.

Figure 1 shows a diagram of a bi-drum top supported boiler.

The Monodrum™ bagasse-boiler Opposite to the two-drum design, the Monodrum™ is a very flexible design as the boiler

modules, like superheaters and evaporators, can be located anywhere in accordance with the convenience of the design engineer. As the main features of the Monodrum™ design, the following points can be highlighted:

All risers and down comers are welded to drum No expanded tubes. The drum is located out of the flue gas flow, which means no differential expansion

between plates and tubes welded to the drum. Thus, it is impossible to have tube leakages and, therefore, consequential damages due to lack of water in the drum.

Water/Steam circulation circuits very well defined, resulting in very good level stability

Possibility to have a cavity after the furnace, for further gas cooling before the superheater.

Superheaters placed out of the furnace are of convective type and with secondary superheater on parallel flow. Lower tube metal temperatures.

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The feature of convective superheater together with welded connections into the drum allows very fast start up and shut downs, usually ⅓ of conventional bi-drum boilers, as shown in Figure 2.

Fig. 1—Operational diagram of the bi-drum top supported design.

Fig. 2—Cold start-up curves for Monodrum™ and bi-drum bagasse boilers.

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6

Time (hrs)

Ste

am p

ress

ure

at d

rum

(bar

)

BI-DRUM COLD START-UP SINGLE-DRUM COLD START-UP

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Fully drainable superheaters. Possibility to carry on chemical or acid cleaning. Wet preservation during off-season.

Straight downward flow of gases at convection surfaces, with no hoppers and no change of flue gas flow in superheaters and evaporators.

For boilers around and over 200 t/h, the suspension burning technology shows some advantages when compared with combustion only on the grate. This technology reduces the grate surface almost by half of the normal size due to suspension burning. With this system, 50% or 60% of combustion air (secondary air) is injected at high pressure and temperatures (~350oC) tangentially at the corners of furnace in different rows, creating a swirling like movement in the flue gas. This results in fantastic fuel/air combination with good residence time for the fuel. Primary air is injected under the grate at temperatures around 250oC. Boiler load response for this type of boiler is similar to oil and gas boilers.

Due to the location of superheater out of furnace, supplementary oil or gas burners can be easily installed without any risks for either the grate or superheater, as shown in Figure 3.

Fig. 3—Functional diagram of the Monodrum™ boiler.

Pinhole grate has been the predominant combustion system in the Monodrum™ boilers. For top supported projects, the grate is incorporated into the furnace, resulting in no sealing problems.

Residence time at furnace is over 3 seconds and flue gas temperatures under 950oC at superheater inlet.

Two levels of bagasse feeders for units over 200 t/h.

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Conclusions With some units already in operation, it can be concluded that Monodrum™ boiler

technology is a very competitive and reliable technology available for bagasse boilers, with real advantages for units over 200 t/h capacity associated with high steam pressure and temperature. Only as an example of how this technology has successfully struck the market, a 300 t/h, 92 bar and 520oC unit is under construction at the moment with its start up scheduled for 2nd half 2008, which will be, as per available references, (Subramaniam 2005; Natu 2005), one of the highest associated steam pressure and temperature bagasse-fired boilers installed at a sugar mill worldwide.

REFERENCES Alstom John Thompson (2005). Cogeneration in the Sugar Cane Industry. Reference Boiler

Manual, AJT. Barata, J.C.F., Lopes, A.L.R. and Gomes, J.E. (2005). The evolution of the Brazilian bagasse

boiler: From the furnace to the single-drum boiler, SINATUB, Ribeirão Preto, São Paulo, Brazil, Dec. 2005. (In Portuguese).

Caldema Equips. Inds.Ltda. (2006). Manufacturer Data, Sertãozinho, São Paulo, Brazil. Natu, S.C. (2005). Bagasse based cogeneration, India marching ahead. International Sugar Journal,

107(1279): 416–424. Sánchez, P.M.G. and Barata, J.C.F. (2006). The evolution through the single-drum steam boiler

inside the expansion of cogeneration in the Brazilian sugar mill context, ENCIT 2006, 11th Brazilian Congress of Thermal Sciences and Engineering, Curitiba, Brazil, Dec 5–8.

Subramanian, A.K. (2005). High pressure multi-fuel co-generation grate boilers for the sugar industry. International Sugar Journal, 107(1279): 431–434.

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CHAUDIERES MONODRUM—PROGRES VERS LA COGENERATION AU BRESIL Par

J.C.F. BARATA1, J.E. GOMES2, M.G. SANCHEZ1 et H. VERBANCK3

1Caldema Equips. Inds.Ltda. engenharia@caldema.com.br 2Thermocal, gomes.thermocal@boilers.com.br

3Alstom John Thompson hansv@jtafrica@.co.za MOTS CLÉS: Chaudières, Bi-Drum, Tambour, Bagasse, Énergie.

Résume CE PAPIER présente l’évolution des chaudières a bagasse, particulièrement vers la cogénération, dans les sucreries et distilleries brésiliennes. On donne des détails de construction et d’opération d’une chaudière a bagasse pour atteindre des capacités plus hautes. On présente des scénarios et on discute la chaudière Monodrum. On décrit le progrès et les limitations du développement stage par stage, a partir d’un point central: une chaudière appropriée a l’industrie brésilienne en pleine expansion et avec des demandes de cogénération. Les chaudières Monodrum sont un facteur important pour la génération d’énergie au Brésil; elles sont les seules chaudières capables de produire les capacités nécessaires. DE CALDERAS CON DOS DOMOS A UN SOLO DOMO—UN PASO ADELANTE EN LA

COGENERACIÓN EN INGENIOS AZUCAREROS DEL BRASIL Por

J.C.F. BARATA1, J.E. GOMES2, M.G. SANCHEZ1 y H. VERBANCK3

1Caldema Equips. Inds.Ltda. engenharia@caldema.com.br 2Thermocal, gomes.thermocal@boilers.com.br

3Alstom John Thompson hansv@jtafrica@.co.za PALABRAS CLAVE: Caldera, Bi-drum, Monodrum™, Bagazo, Energía.

Resumen ESTE ARTÍCULO presenta un seguimiento a la evolución de las calderas bagaceras para cumplir con las tendencias actuales de cogeneración de las plantas brasileñas de azúcar y alcohol. Para este propósito se describe una caldera bagacera brasileña de un solo paso y dos domos desde los puntos de vista de sus atributos constructivos y operacionales tratando de satisfacer mayores capacidades y mayores parámetros de generación de vapor. El escenario comparativo propuesto contribuye a ilustrar el proceso evolutivo, señalando la reciente introducción de la caldera Monodrum™. La descripción muestra el progreso y las limitaciones de cada etapa de desarrollo de acuerdo a un aspecto clave: la idoneidad de la caldera en el contexto de los parámetros modernos de generación de vapor para plantas de cogeneración dentro de las tendencias cada vez crecientes de la industria azucarera brasileña. Este artículo muestra cómo la tendencia hacia mayor generación en las plantas brasileñas de azúcar y alcohol ha corroborado exitosamente el uso de calderas Monodrum™ para esta aplicación, como la única caldera capaz de trabajar eficientemente a las mayores capacidades y parámetros de generación de vapor.

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