Maximization of electricity generation or pelletization of the surplus bagasse in a Cuban

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Bachelor of Science Thesis

KTH School of Industrial Engineering and Management

Maximization of electricity generation or pelletization of the surplus bagasse in a Cuban

sugar mill: A comparative analysis

Amanda Öhman Linnea Lundberg

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Keywords: sugar cane industry, sugar cane, bagasse, excess bagasse, cogeneration,

bagasse pellets, unplanned production stoppages, electricity generation, pelletization

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Bachelor of Science Thesis EGI-‐2016

Maximization of electricity generation or pelletization of the surplus bagasse in a Cuban sugar mill: A comparative analysis

Amanda Öhman

Linnea Lundberg

Approved

2016-‐06-‐15

Examiner

Anders Malmquist

Supervisor

Catharina Erlich

Commissioner

UCLV, Santa Clara, Cuba

Contact person

Idalberto Herrera Moya

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Acknowledgement

The Linnaeus-‐Palme programme has with their scholarship made this bachelor thesis possible and for that we would like to express our great appreciation. Also, we would like to thank the ÅForsk Foundation for their generous travel grant. Our greatest thank you to the people who have helped us along the way, especially Professor Idalberto Herrera Moya who with great enthusiasm contributed with his knowledge regarding the sugar cane industry and Cuba. In addition to this, we would like to thank the employees at Carlos Baliño who showed us their factory and helped us collect the required data.

Furthermore we would like to thank our travelling partners, Åsa and Therése, whose company truly has made this time unforgettable. Lastly, we would like to thank Catharina Erlich for opening our eyes to Cuba and the sugar cane industry.

Linnea Lundberg and Amanda Öhman Santa Clara, Cuba, 2016-‐05-‐26

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Abstract

The Cuban energy sector today is highly dependent on fossil fuels, oil in particular which amounts for around 85 % of the power generated in Cuba. Only 4.3 % of the electricity is generated from renewable energy sources. In order for Cuba to increase the amount of renewable energy, the sugar industry can play an important role. The residue from sugar production, bagasse, has historically been seen as a disposal problem for many sugar mills but with a suitable application, it could be a valuable energy resource. The bagasse is used in the cogeneration unit in the sugar mills to generate electricity and heat but it can also be used for pellets production. The pellets could thereafter be used as fuel in industries or other power plants, replacing oil.

The aim of this project is to perform a comparative analysis between two possible applications for the excess bagasse at Carlos Baliño, a Cuban sugar mill situated in the province Villa Clara. The investigated applications are maximization of electricity generation and maximization of pelletization, with the aim to determine the best alternative from both economic and environmental points of view. The influence of unplanned stoppages in the production is also analysed since the many stoppages strongly affects the amount of excess bagasse available for these applications.

The method in order to fulfil the project aim is divided into four main parts. The first part is an evaluation of the current energy performance at Carlos Baliño. The second part investigates how the production stoppages influences the energy performance and the results are partly used in part three and four, where the two investigated applications for the excess bagasse are evaluated.

The results show that the biggest causes for production stoppages are interruptions caused by nature phenomena, operational interruptions and failure in equipment and that the amount of bagasse used during stoppages is approximately 7700 ton per season. Considering the extent of stoppages as well as the limitations in current machinery the maximum excess electricity generation that can be exported to the grid amounts to 2.58 GWh per season. This entails approximately 387 000 USD in revenues from sold electricity for Carlos Baliño and 1080 ton of oil that can be reduced in Cuba.

The amount of pellets that can be produced from the current amount of excess bagasse amounts to 6200 ton per year, which entails a yearly economic gain from pellets of approximately 653 000 USD for Carlos Baliño. This amount of pellets can be used to generate 8.79 GWh of net electricity per year and the amount of oil that can be reduced in Cuba is 3700 ton. It is concluded that maximization of pellets is the best application for the excess bagasse, both from an economic and an environmental point of view.

Since the investigation of production stoppages in this report is based on several assumptions, future work should mainly focus on further investigation within this area.

Inter alia, it should be investigated which types of stoppages that are likely to be reduced and to what extent they can be reduced.

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Sammanfattning

Den kubanska energisektorn är idag mycket beroende av fossila bränslen, speciellt olja vilket står för omkring 85 % av den el som genereras på Kuba. Endast 4,3 % av elen kommer från förnyelsebara energiresurser. För att Kuba ska kunna öka mängden förnyelsebar energi kan sockerindustrin spela en viktig roll. Restprodukten från sockerproduktionen, bagass, har historiskt setts som ett avfallsproblem för många sockerfabriker men med ett lämpligt användningsområde kan bagassen vara en värdefull energiresurs. Bagassen används i kraftvärmeprocesser i sockerfabriker för att generera elektricitet och värme men kan också användas för att producera pellets.

Pelletsen kan därefter användas som bränsle i industrier eller andra kraftverk för att ersätta olja.

Syftet med detta projekt är att göra en jämförande analys mellan två möjliga användningsområden för överskottsbagassen på Carlos Baliño, en kubansk sockerfabrik belägen i provinsen Villa Clara. De undersökta användningsområdena är maximering av elgenerering och maximering av pelletering, med syftet att bestämma det bästa alternativet, både från ett ekonomiskt samt ett miljömässigt perspektiv. Inverkan av oplanerade produktionsstopp analyseras också eftersom de många stoppen påverkar mängden överskottsbagass som kan användas för andra tillämpningar.

Metoden för att uppfylla projektets syfte är uppdelad i fyra huvuddelar. Första delen är en utvärdering av Carlos Baliños nuvarande energiprestation. Andra delen undersöker hur produktionsstoppen påverkar energiprestationen och resultaten används delvis i del tre och fyra där de två användningsområdena för överskottsbagassen utvärderas.

Resultaten visar att de största orsakerna till produktionsstopp är avbrott orsakade av naturfenomen, driftsstörningar och fel i utrustning och att mängden bagass som används under stopp är ungefär 7700 ton per säsong. Med hänsyn till produktionsstoppen samt begränsningar i nuvarande maskineri uppgår den maximala överskottselektriciteten som kan exporteras till det nationella elnätet till 2,58 GWh per säsong. Detta medför intäkter på 387 000 USD från såld el för Carlos Baliño och 1080 ton i minskad oljeanvändning för Kuba. Mängden pellets som kan produceras från den nuvarande mängden överskottsbagass uppgår till 6200 ton per år vilket medför en årlig ekonomisk vinning från pellets på cirka 653 000 USD för Carlos Baliño. Denna mängd pellets kan användas för att generera 8,79 GWh elektricitet, netto, per år och mängden olja som kan reduceras i Kuba uppgår till 3700 ton. Slutsatsen är att maximering av pellets är det bästa användningsområdet för överskottsbagass, både från ett ekonomiskt och ett miljömässigt perspektiv.

Eftersom undersökningen av produktionsstopp i denna rapport baseras på många antaganden bör framtida arbete fokusera på ytterligare utredningar inom detta område.

Exempelvis bör det utredas vilken typ av produktionsstopp som är möjliga att reducera samt till vilken grad de kan reduceras.

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Table of contents

Acknowledgement ... 4

Abstract ... 5

Sammanfattning ... 6

Nomenclature ... 11

1 Introduction ... 14

1.1 Problem formulation ... 15

1.2 Aim ... 15

2 Sugar cane production ... 16

2.1 Milling ... 16

2.2 Clarification and filtration ... 17

2.3 Evaporation ... 17

2.4 Crystallization ... 17

2.5 Centrifugation ... 17

2.6 Drying ... 18

2.7 Bagasse ... 18

2.8 Unplanned stoppages in the sugar production ... 19

2.9 Cogeneration in the sugar cane industry ... 20

2.9.1 The cogeneration process ... 21

2.9.2 How to increase the generation of surplus electricity in cogeneration ... 23

2.9.3 Market for electricity generated to the national grid ... 24

2.10 Pelletization of bagasse ... 25

2.10.1 Pelletizing technology ... 26

2.10.2 Market for bagasse pellets ... 27

3 Carlos Baliño ... 28

3.1 Excess bagasse at Carlos Baliño ... 29

3.2 Unplanned stoppages at Carlos Baliño ... 31

3.3 Previous work at Carlos Baliño ... 31

4 Method ... 33

4.1 Model overview ... 33

4.2 Evaluate current energy performance ... 34

4.2.1 System boundary ... 34

4.2.2 Numerical calculations ... 35

4.2.3 Sensitivity analysis ... 36

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4.4 Electricity generated from bagasse ... 40

4.4.1.1 Part 1 ... 41

4.4.1.2 Part 2 ... 42

4.4.1.3 Part 3 ... 44

4.5 Pelletization of bagasse ... 46

4.5.1 System boundary ... 47

5 Results and discussion ... 51

5.1 Current energy performance ... 51

5.1.2 Discussion ... 51

5.2 Investigation of stoppages ... 52

5.3 Maximizing electricity generation ... 56

5.3.1 Part 1 ... 56

5.3.2 Part 2 ... 56

5.3.3 Part 3 ... 57

5.4 Maximizing pelletization ... 59

5.5 Final discussion ... 61

6 Conclusion and future work ... 64

6.1 Conclusions ... 64

6.2 Future work ... 64

References ... 66

Appendix A – Operational data from sugar mill Carlos Baliño ... 70

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Appendix B – Unplanned stoppages 25^th Dec 2015 to 12^th Apr 2016 ... 71

Appendix C – Calculations for current energy performance ... 72

Appendix D -‐ Calculations for maximizing electricity generation, Part 1-‐2 ... 73

Appendix E -‐ Calculations for maximizing electricity generation, Part 3 ... 75

Appendix F – Calculations for maximizing pelletization ... 76

Appendix G – Sensitivity analysis for current energy performance ... 78

Appendix H – Sensitivity analysis for investigation of unplanned stoppages ... 79

Appendix I – Sensitivity analysis for maximizing electricity generation ... 80

Appendix J – Sensitivity analysis for maximizing pelletization ... 81

List of Figures Figure 1. Sugar cane production process (Erlich, 2009) ... 16

Figure 2. Cogeneration unit (Erlich, 2009) ... 22

Figure 3. a) Back-‐pressure turbine (BPST) b) Extraction condensing turbine (CEST), modified from Energy Industry Challenges (2016) ... 23

Figure 4. Steps in the production of pellets ... 26

Figure 5. Pelletizing of biomass (Erlich 2009) ... 26

Figure 6. Excess bagasse stored at Carlos Baliño (Carlos Baliño 2016) ... 30

Figure 7. Functions for the bagasse at Carlos Baliño today ... 30

Figure 8. Model over the method ... 34

Figure 9. Model for evaluating the current energy performance ... 34

Figure 10. System boundary for current energy performance ... 35

Figure 11. Model for investigation of production stoppages ... 37

Figure 12. Model for maximizing the electricity generated from bagasse ... 41

Figure 13. Model for maximizing the pelletization of bagasse ... 46

Figure 14. System boundary for pelletization of bagasse ... 47

Figure 15. Number of stoppages at Carlos Baliño ... 52

Figure 16. Total duration of stoppages at Carlos Baliño ... 52

Figure 17. Stoppages caused by operational interruptions at Carlos Baliño ... 53

Figure 18. Variation of amount of bagasse used during stoppages depending on SGI ... 55

Figure 19. Variation of amount of bagasse used during stoppages depending on power generation during operating time ... 55

Figure 20. a) Revenue from sold electricity and b) Amount of oil reduced, depending on mass flow of cane during stoppages ... 59

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List of Tables

Table 1. Composition of different types of bagasse, modified from Hallersbo and Onoszko

(2015) ... 18

Table 2. Chemical components of dry bagasse, modified from Hallersbo and Onoszko (2015) ... 19

Table 3. Operation parameters of Carlos Baliño, Carlos Baliño (2016) ... 29

Table 4. Types of stoppages at Carlos Baliño, Carlos Baliño (2016) ... 31

Table 5. Equations for calculating the current energy performance ... 36

Table 6. Parameter in sensitivity analysis for current energy performance ... 36

Table 7. Equations used to compile data regarding the production stoppages ... 38

Table 8. Equations for calculating the amount of bagasse used during stoppages ... 38

Table 9. Parameter in sensitivity analysis for unplanned stoppages ... 40

Table 10. Equations for calculating the electricity generated from bagasse ... 42

Table 11. Equations for calculating maximum energy performance considering limitations in machinery ... 43

Table 12. Equations for calculating maximum energy performance considering production stoppages ... 45

Table 13. Parameters in sensitivity analysis for maximum electricity generation ... 46

Table 14. Equations for calculating revenue and electricity generation from pellets ... 48

Table 15. Parameters in sensitivity analysis for maximizing pelletization ... 50

Table 16. Results from numerical calculations for the current energy performance ... 51

Table 17. Result from sensitivity analysis for current energy performance ... 51

Table 18. Result from numerical calculations for the used amount of bagasse during stoppages ... 53

Table 19. Result from sensitivity analysis for the used amount of bagasse during stoppages ... 53

Table 20. Results from numerical calculations when maximizing electricity generation 56 Table 21. Results from numerical calculations when maximizing electricity generation, considering limitations in machinery ... 57

Table 22. Results from numerical calculations when maximizing electricity generation, considering stoppages in production ... 57

Table 23. Results from sensitivity analysis when maximizing electricity generation ... 58

Table 24. Results from numerical calulations regarding maximizing pelletization ... 60

Table 25. Results from sensitivity analysis when maximizing pelletization ... 60

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Nomenclature

Symbol Description Unit

𝐶_!,! Cost of imported electricity for pellet production USD

𝐷_!" Duration of a stop h

𝐷!",!"! Total duration of stoppages h

𝑒_!,! Amount of electricity per unit oil kWh/kg

𝐸!",!" Excess electricity generation during stoppages kWh

𝐸!",!" Excess electricity gen. during operating time kWh

𝐸!",!"! Total maximum excess electricity generation kWh

when considering stoppages

𝐸_!" Daily electricity gen. during operating time kWh

𝐸_!,!"# Electricity generated from total amount of pellets kWh

𝑒!,!"#!!"#$% Electricity needed per kg pellet kWh/kg

𝐸_!,!"# Net electricity generated from pellets kWh

𝐸_!,!"#$ Electricity needed in pellet production kWh

𝐸_!" Daily electricity generation during stoppages kWh

𝐸_!"! Total daily electricity generation kWh

𝐺_! Yearly economic gain from pellets USD

ℎ_!" Enthalpy after turbine kJ/kg

ℎ_!" Enthalpy before turbine kJ/kg

𝐻𝑉_! Heating value oil kJ/kg

𝐻𝑉_! Heating value bagasse pellets kJ/kg

𝐿_!" Average length of one stoppage h

𝑚_!,!",! Amount of excess bagasse available for kg

pelletization

𝑚_!,!" Mass of bagasse used during stoppages kg

𝑚!,!",!"#!$% Mass of bagasse used during stoppages per season kg

𝑚!,!"#!$% Total amount of canes milled during a season ton

𝑚!,!,!"#$%"# Amount of oil reduced when generating kg

electricity with bagasse as direct fuel

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𝑚!,!,!"#$%"#,! Theoretical amount of oil reduced when kg gen. electricity with bagasse as direct fuel

𝑚!,!,!"#$%"#,!" Amount of oil reduced, considering stoppages kg

𝑚_! Mass of pellets kg

𝑚!,!,!"#$%"# Amount of oil reduced when use of pellets kg

𝑚_! Mass flow of bagasse kg/s

𝑚_!,!"# Mass flow of bagasse when maximization kg/s

𝑚!,!",!"# Mass flow of excess bagasse when maximization kg/s

𝑚!,!"!#$!%$& Mass flow of available bagasse kg/s

𝑚_!,!" Mass flow of bagasse during stoppages kg/s

𝑚_! Mass flow of canes kg/s

𝑚_!,!"# Maximum mass flow of canes kg/s

𝑚!,!"#,! Theoretical maximum mass flow of canes kg/s

𝑚_!"#$% Mass flow of steam kg/s

𝑚!"#$%,!"# Maximum mass flow of steam kg/s

𝑚!"#$%,!"#,! Theoretical maximum mass flow of steam kg/s

𝑚!"#$%,!" Mass flow of steam during stoppages kg/s

𝑚!"#$%,!",!! Mass flow of steam during stoppages – Turbine 1 kg/s 𝑚!"#$%,!",!! Mass flow of steam during stoppages – Turbine 2 kg/s

𝑁 Number of days during a season days

𝑁_!" Unplanned stoppages in production daily -‐

𝑁!",!"! Total number of stoppages -‐

𝑂𝐻 Operating hours per day h

𝑝_!,!" Export price for electricity USD/kWh

𝑝_!,!" Import price for electricity USD/kWh

𝑝_! Price for pellets USD/kg

𝑃_!" Excess power kW

𝑃!",!"# Maximum excess power kW

𝑃!",!"#,! Theoretical maximum excess power kW

𝑃_!" Electrical power generation during operating time kW

𝑃!"#$%&& Electrical power demand by process kW

𝑃!"#$%&&,! Theoretical electrical power demand by process kW

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𝑃!"#$%&&,!" Electrical power demand by process kW

during stoppages

𝑃_!" Electrical power generation during stoppages kW

𝑃!",!"# Average electrical power gen. during stoppages kW

𝑃_!",!! Average electrical power generation during kW

stoppages – Turbine 1

𝑃_!",!! Average electrical power generation during kW

stoppages – Turbine 2

𝑃_!"! Total electrical power generated kW

𝑃!"!,!"# Maximum total electrical power generated kW

𝑃!"!,!"#,! Theoretical max. total electrical power generated kW

𝑅!,!"#$%! Yearly revenue for sold pellets USD

𝑅_!,!"# Maximum revenue sold electricity USD

𝑅_!,!" Revenue sold electricity, considering stoppages USD

𝑠𝑡 Stoppage time per day h

𝑠𝑡_!"# Average stoppage time per day h

𝑥_!"# Steam consumption index tst/tc

𝑥_!"# Steam generation index tst/tb

𝜂_! Generator efficiency %

𝜂_! Mechanical efficiency %

𝜂_! Electrical efficiency when combusting oil %

𝜂_! Electrical efficiency when combusting pellets %

∆𝑥_!,! Factor describing the reduction of water when %

drying bagasse for pelletization

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1 Introduction

The usage of electricity is increasing in the world and it is important to improve the utilization of renewable energy sources in order to reduce the effects on the world’s climate change.Access to renewable energy is important for the developing countries, as it is a chance for them to develop in a sustainable and climate friendly way.

The Cuban energy sector is today dominated by fossil fuels; especially oil which amounts to around 85% of the electricity generated (IEA, 2013). Today, only 4.3% of the power comes from renewable energy sources (IBR, 2015). Since the use of fossil fuels entails emissions of greenhouse gases and has a negative impact on the environment, the large amount of fossil fuel is today a big challenge for Cuba to develop sustainably.

The Cuban electricity generation is today very costly partly due to inefficient processes, with high costs for electricity as a consequence (Herrera Moya, 2016). The large amount of fossil fuels used along with the high costs for electricity have affected the government into stating a new policy for the development of renewable energy (Herrera Moya, 2015).

A growth in renewable energy would benefit the country both from an environmental and an economic perspective.

One of the biggest and most important industries in Cuba is the sugar industry. In fact, during the first half of the 20^th century, Cuba was the world’s leading exporter of sugar.

Before the collapse of the Soviet Union in 1991, Cuba and Soviet Union had a beneficial exchange agreement including sugar and oil. After the collapse, the Cuban sugar cane production declined drastically when the expensive and fuel oil-‐dependent Cuban sugar could not compete worldwide. The long economic and commercial blockade on Cuba imposed by the United States has also affected the decline and has made it hard for the industry to recover (Alonso-‐Pippo et al, 2008). However, since 2010 the sugar industry has experienced a rebirth and is nowadays ranked as number eight in Cuba’s foreign currency earnings (Reuters, 2015).

Most of the sugar mills in Cuba use one of the by-‐products from sugar production, bagasse, as an energy source for their own energy consumption (IPS, 2014). Since there has been a recovery and awakening in the sugar industry, the production of sugar has increased. With an increased production comes more by-‐products, and one of them is, as previously mentioned, bagasse (IPS, 2013). The majority of the bagasse is used to fuel the boilers in the cogeneration unit of the mills, where thermal heat and electricity is generated. The increase of this by-‐product is thus seen as a potential fuel for electricity generation. In 2006 the Energy Revolution was launched, with aims regarding energy efficiency, better availability and reliability in the national grid, and also increasing the amount of renewable energy. In this case, the sugar industry is seen as the main source for renewable energy, both previously and in the future (Vazques et al, 2015; IPS 2014).

However, Cuba lack foreign currency for investments in the machinery of the sugar mills, which leads to bagasse mainly being a disposal problem (IPS 2014; Erlich, 2016).

Though, if investments were to be made, 20 sugar mills in the country should generate 755 MW to the national grid by 2030, increasing the amount of electricity generated from

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bagasse to 14% of the total amount of electricity generated in Cuba. The first-‐hand choice for Cuba regarding the surplus bagasse is thus to generate excess electricity to support the national grid (IPS, 2014). Another alternative is to produce biomass pellets and then convert it into electricity. Combustion of bagasse pellets are far higher efficient than combusting wet bagasse directly. A large advantage for electricity generated from bagasse pellets is also that bagasse pellets can be stored and then be used when necessary, which is not possible with other renewables such as solar power and wind power.

1.1 Problem formulation

Carlos Baliño is a recently modernized sugar mill situated in the province Villa Clara, Cuba. The residue from the production, bagasse, is used as fuel in the cogeneration process generating electricity and heat, making the sugar mill completely self-‐sufficient in energy, on a yearly basis. The modernization done in 2009 has resulted in large amounts of surplus bagasse being produced and the bagasse is now a disposal problem. Bagasse has a lot of potential as a renewable energy source and is suitable for many different applications. One application is as direct fuel for excess power production in the cogeneration process, which enables export of electricity to the national grid, and another is pelletization for use in high efficient combustion and possibly sold to another power plant.

Unplanned stoppages in the sugar production lead to surplus bagasse being used as maintenance fuel, generating heat or electricity to the processes that are working despite the stoppage. The extent of stoppages affects the amount of bagasse available for other applications.

1.2 Aim

The aim of the project is to evaluate current energy performance at the sugar mill Carlos Baliño and identify whether maximization of electricity generation or pelletization of the surplus bagasse is the best option. The stoppages in the production will also be investigated.

The study will determine:

• An evaluation of the current energy performance

• What events that are causing unplanned stoppages in the production and how they influence the energy performance

• The best option, electricity generation or pelletization of excess bagasse, from an economic point of view

• The best option, electricity generation or pelletization of excess bagasse, from an

environmental point of view

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2 Sugar cane production

In Figure 1 the sugar cane production process in a sugar mill is illustrated and below the different processes in the production are further described.

Figure 1. Sugar cane production process (Erlich, 2009)

2.1 Milling

Before the sugar cane enters the mills it is washed and prepared for the milling process.

The preparation is done by knives, crushers and shredders, which facilitate the extraction of juice from the cane (Hugot, 1986). In the milling process the cane is separated from the sugar juice by applying different pressures while the cane passes between rollers. The efficiency of the separation is determined by a lot of different factors; number of squeezes and effective pressure among others. In order to make the extraction of sugar juice more efficient, imbibition water is added (Payne, 1982). After the mills, the imbibition water together with the sugar juice continue to the clarification process as mixed juice. The residual is bagasse and consists of fibre, unextracted brix and water (Abdulhadi and Larsson, 2014). If the sugar mill in question has a cogeneration unit installed, the bagasse continues to this unit and is used as fuel in the boilers (Hallersbo and Onoszko, 2015).

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2.2 Clarification and filtration

There are two main objectives in the clarification process. The first is to increase the pH-‐

value of the mixed juice to minimize sugar inversion, the undesired reaction when sucrose dissolves into glucose and fructose (Payne, 1982). The second is to remove impurities and undesired organic compounds left in the mixed juice after the milling process. This is done by heating the juice almost to the boiling point at 115 °C and then adding lime and sulfuric acid in the clarifier (Morandin et al., 2010). After this process, the juice is left in the clarifier to settle in order to separate the clarified juice from the impurities, which collect at the bottom as mud. The mud is then filtered and removed and the juice continues to the evaporation process (Hugot, 1986).

2.3 Evaporation

In this stage, the water contained in the clear juice from the clarification and filtration is evaporated with thick syrup as a result (Hugot, 1986). The thick syrup has a sucrose mass fraction of 62-‐69% (Morandin et al., 2010). The evaporation station consists of a set of connected vessels, which enable multiple-‐effect evaporation. In the first vessel the low-‐

pressure steam from the cogeneration is used as heating source (Hallersbo and Onoszko, 2015). By continuously lowering the pressure in the following vessels it is possible to use the vapor from the previous vessel as heating source in the next vessel (Hugot, 1986). The condensate from the first vessel is considered pure enough to be reused and is partly utilized as feed water in the boiler in the cogeneration unit (Morandin, 2010).

2.4 Crystallization

After the evaporation step, the thick syrup is led to a set of vacuum pans with low pressure. The pressures in the vacuum pans are lower than the pressure in the evaporators and the steam from the evaporators can therefore be used as heating source.

In the crystallization the thick juice is saturated and crystals begin to form (Hugot, 1986).

In this stage there are still some impurities in the mixture that affect the efficiency of the crystallization. Impurities affect the solubility of sucrose in water and therefore to what extent the sucrose can be dissolved from the by-‐product of the process, molasses (Abdulhadi and Larsson, 2014). The sucrose that is not crystallized exits with the molasses, which together with the evaporated water are the outputs of the crystallization (Hugot, 1986).

2.5 Centrifugation

After the crystallization process there is a mix of molasses and crystals. In order to separate these two and obtain sugar the mixture is centrifuged. The remaining molasses after centrifugation still contains crystals and is sent back to the vacuum pans in the crystallization process. This procedure continues until no more crystals can be obtained and the final molasses is removed from the process (Hugot, 1986). The removed molasses still contains some fragments of non-‐crystallized sugar and is possible to use as a source for ethanol production (Erlich, 2009).

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2.6 Drying

The quality of the sugar is highly correlated to the moisture content. The last step in the process, drying, is therefore very important to ensure high quality sugar. The sugar coming from the centrifugation process usually has a moisture content of 0.5-‐2% and is dried with hot air until the moisture content reaches the upper limit, which is about 1%.

The upper limit is determined by the proportion of impurities in the sugar (Hugot, 1986).

2.7 Bagasse

Bagasse is a fibre and a by-‐product of the sugar production. As previously mentioned, the canes are in the milling process crushed in order to extract the sugar juice. This process leaves the residue of the sugar cane, bagasse, which has a moisture content of around 45-‐

50% when leaving the process. The mass ratio between the amount of bagasse and the cane stalk is approximately 30% (Bilba and Arsene, 2008). In tons this means that roughly 3 tons of wet bagasse is produced when 10 tons of sugar canes have been processed (Hallersbo and Onoszko, 2015). The by-‐product is mainly used as fuel in the boilers in sugar mills with a cogeneration system (Alves et al, 2015). In order to expand the ways of application, bagasse can also be converted into biomass pellets.

Bagasse mainly consists of three components; cellulose, hemicelluloses and lignin. In percentage these three components stand for 40-‐50%, 25-‐35% respectively 15-‐20% of the bagasse (Wang et al, 2013). However, the composition differs based on the type of bagasse in question. The percentage of either component should be considered when examining where the bagasse should be applied. In the production of pulp and paper a high proportion of cellulose is preferable. Therefore, in order to use bagasse as fuel in the cogeneration process one should consider the composition of the bagasse, since these three components determine the heating value and the emission levels (Hallersbo and Onoszko, 2015).

The components in the bagasse differ based on geographical location, harvesting techniques, age of the cane and so on. In Table 1 and 2, the composition as well as the chemical components in bagasse is shown.

Cellulose Hemicellulose Lignin Ash Others* References

40 24.4 15 5 13.7 Vazgueq et al. 1999

40-‐43 28-‐30 9-‐11 5-‐6 11-‐18 Ramaraj 2007

46 24.5 19.5 2.4 7.6 Mulinari et al. 2009

69.4 21.1 4.4 0.6 4.5 Habibi et al. 2008

55.2 16.8 25.3 1.1 1.6 Trindade et al. 2005

56 6 29 7 2 Maldas and Kokta 1991

In percentage of dry mass *Protein, Fat and Waxes, Saccharose, Silica and Glucose

Table 1. Composition of different types of bagasse, modified from Hallersbo and Onoszko (2015)

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Carbon Hydrogen Oxygen Ash Others* References

45.2 5.4 41.8 7.4 0.25 Alves et al. 2015

48.2 6 43.1 2.7 -‐ Kelly, 1966

48.2 6.1 43.3 1.1 0.38 Cardozo et al, 2013

48.1 6.1 43.3 2.5 -‐ Gregory, 1944

47.4 5.9 44 2.5 0.29 Carlos Baliño, 2015

In percentage of dry mass *Nitrogen, Sulphur, Chlorine

Table 2. Chemical components of dry bagasse, modified from Hallersbo and Onoszko (2015)

Another factor to account for is the ash-‐content of dry bagasse. Considering the fact that bagasse is burned, when used as fuel in industries and homes, the ash-‐content plays an important part. The ash-‐content is linked to the first steps in the sugar cane production, i.e. the farming process. Based on the amount of chemicals used in this process, the ash-‐

content is affected. In use of fewer chemicals, as in ecological farms, the ash-‐content is lowered (Hallersbo and Onoszko, 2015). When gasification tests of high ash-‐content and low ash-‐content bagasse have been compared, it has been found that the time for conversion is longer for bagasse with a higher ash-‐content (Erlich, 2009). This implies a need of higher combustion temperature in order to burn bagasse with a high ash-‐content and vice versa. As a consequence of this, pellets with a low ash-‐content are more energy efficient, which also entails a higher market value (Hallersbo and Onoszko, 2015).

As seen in Table 1 and 2 above, the ash-‐content in the bagasse differs along with the other components. An ash-‐content around 0.6-‐2.7% would to be considered as low, while a content around 5-‐7.4% would be considered as high. The average ash-‐content in bagasse would be around 4%, which is relatively low in comparison with biomass of other kinds.

For example, the residue rice husk has an ash-‐content of approximately 17.5% (Ahmed and Gupta, 2011).

Table 2 also indicates other chemical components of bagasse, such as nitrogen, sulphur and chlorine. Higher amounts of sulphur and chlorine could cause corrosion in long-‐term operation (Erlich and Fransson, 2011). An experimental study, done in 2013, determined the percentage amount of sulphur in bagasse to 0.03 ± 0.007 and the percentage amount of chlorine to 0.05 ± 0.001 (Cardozo et al, 2013). In comparison, the residue EFB (empty fruit bunch) from the palm-‐oil production, showed higher rates of both components in a study by Erlich and Fransson. The amount of sulphur was determined to 0.12 ± 0.01 and the amount of chlorine 0.46 ± 0.1, indicating a greater risk for corrosion when using pellets made from EFB instead of pellets made from bagasse (Erlich and Fransson, 2011).

2.8 Unplanned stoppages in the sugar production

Sugar production is not a continuous process and unplanned stoppages due to technical or management problems are something that sugar factories must handle at a regular basis. Normally the cogeneration unit in the mill is still running during a stoppage, due to the fact that the heating of the boilers in the cogeneration is time consuming. The common case is that the stoppages last for a shorter period of time than it takes to heat

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the boilers. Therefore, bagasse is still being used during stoppages and can be seen as waste since the steam generated is not fully used in the production (Erlich, 2016). There are only some processes in the production that require steam during a stoppage, and the boiler is running at a lower load. In some sugar mills the surplus bagasse is not enough to run the boilers during stoppages, even though they are running at a lower load, and the use of other fuels, such as fuel oil and wood, are inevitable (Assefa and Omprakash, 2013).

There are normally two types of stoppages that occur in a sugar mill, those that occur from time to time and those that occur as a result of failures in the equipment. These inevitable stoppages in the sugar industry lead to thermal energy being wasted, and thus an increase in the production costs (Birru et al, 2015). Birru et al (2015) assesses a condition in the sugar industry referred to as transient condition, a condition where the unforeseen stoppages occur. A transient condition leads to some parts in the process being prevented from performing. In the case study of Birru et al, an unplanned stoppage in the production is assumed to last for 2 hours, on a daily basis. The stoppage assumed to occur is in the mill press of the factory. The first consequence of a stoppage in the mill press is that no bagasse will be produced. With the bagasse production shut down, there is no bagasse available as fuel to the boiler, i.e. the bagasse feeding system will also stop.

Also, when the mill press is stopped, the pressure is built up in the boiler, which leads to the opening of a pressure relief valve. The bagasse flow will then decrease, with the outcome of a decrease in the live steam flow. In order to continue the providing of electricity to the needing parts in the production, surplus bagasse and/or fuel oil can be used. In this particular case a bagasse feeding system is not implemented at the sugar mill, thus the use of bagasse is seen as a hypothetical use. However, in the case of bagasse and fuel oil used, the bagasse is introduced directly after the stoppage of the mill press.

The amount of bagasse used as fuel depends on the amount of bagasse saved for unplanned stoppages. When the saved bagasse is used, the fuel oil is introduced.

Gradually the mill starts to crush again and the stoppage is over (Birru et al, 2015).

As mentioned before, thermal energy, i.e. steam, is wasted during events like these.

According to the case study the amount of steam wasted varies, depending on the fuel in the boiler during the stoppage; only fuel oil or bagasse and fuel oil. With only fuel oil used in the process during the stoppage, the amount of steam wasted is 3 ton per day. With a bagasse feeding system, and thus both bagasse and fuel oil used in the process the amount of wasted steam is higher, 7 ton per day. Nevertheless, many mills only use bagasse as fuel during a stoppage, which has not been investigated in this case study. In order for sugar mills to be able to use only bagasse as fuel, 10 % of the bagasse produced is recommended to be stored in the sugar mill. This amount of bagasse is normally sufficient in order to start up the plant, in case of unplanned stoppages and other unforeseen events (Alves et al, 2015).

2.9 Cogeneration in the sugar cane industry

Cogeneration refers to the production of two useful forms of energy, all produced from the same energy source (Cengel and Boles, 2011). In the sugar cane industry, this means that electricity and thermal energy can be generated from the by-‐product bagasse. The

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sugar cane industry as a whole is seen as a major pollutant, mostly due to emissions and usage of fossil fuel during the life cycle stages, but has the ability to become more environmentally friendly with the help of a cogeneration process (Pérez et al, 2012).

When cogeneration is adopted in a sugar mill, the total efficiency increases by about 50 % compared to separate generation of electricity and heat (Birru, 2016).

Bagasse was in 2011 used as fuel in the boilers in 80 of the 115 countries that produce sugar from sugar cane (Chauhan et al, 2011). The mills in these countries are able to, with the help of the generated electricity from the cogeneration unit, meet their heat and electricity needs at a low cost. These needs are met in many factories worldwide, and when the generated electricity is more than satisfactory many sugar mills export the surplus to the national grid (Pérez et al, 2012).

However, the equipment in traditional sugar mills is usually not efficient enough to generate surplus power. Furthermore, many sugar mills are situated in areas with no connection to the national grid, which limits the possibilities for export to the grid, even if there is potential in the equipment (Birru, 2016). Cogeneration with bagasse as a fuel is also time dependent, since the sugar cane industry is seasonal and the cogeneration system usually operates during the harvest of the sugar canes (Hugot, 1986; Alves et al, 2015). This is known as intermittent electricity, i.e. that the flow of electricity fluctuates due to outside factors (Nylund and Puskoriute, 2015). This entails a necessity of storing bagasse during a period that is longer than the season for crushing. Unfortunately, storing of bagasse is costly and requires a large storage area due to the fact that there is a large volume in proportion to the weight (Hugot, 1986).

2.9.1 The cogeneration process

In Figure 2 below, a simplified drawing of a cogeneration unit is to be seen. The process begins with bagasse entering the boilers. The quantity of the boilers depends on the size of the mill, but usually consists of more than one (Hallersbo and Onoszko, 2015). In these boilers bagasse is burned in order to produce steam. Due to the high temperatures when burning, water can be converted into high-‐pressure steam (Nylund and Puskoriute, 2015). How much energy that is needed for this conversion is largely dependent on the moisture content of the bagasse. In order to receive high efficiency from the boilers, the moisture content should be low. In most cases, the moisture content of the bagasse is 50%

and has a net heating value of approximately 8800 kJ/kg. This can be compared to the net heating value of fully dry bagasse, which is approximately 17630 kJ/kg (Geethanjali and Vinod, 2010).