Installation of an Extensive Vapour Bleeding System at the Evaporators

Background

The sugar industry is a major consumer of thermal energy in the form of steam for the process. The steam consumers in the process are - evaporators and juice heaters (mixed juice, sulphited juice and clear juice).

Out of these consumers, the evaporators which concentrate the juice, typically from a brix content of 10 - 11 to about 55 - 60 brix, consume the maximum steam. The evaporators are multiple effect evaporators, with the vapour of one stage used as the heating medium in the subsequent stages.

In the older mills, the evaporators are triple/ quadruple effect and the vapour from the first effect is used for the vacuum pans and from the second effect for juice heating. The other requirements were met through usage of exhaust steam.

In the modern sugar mills, efforts have been taken to reduce the steam consumption. The following approach has been adopted in the boiling house for reducing the steam consumption:

• Increasing the number of evaporator effects - the higher the number of effects, the greater will be the steam economy (i.e., kilograms of solvent evaporated per ton of steam). Typically, the present day mills use a quintuple effect evaporator system.
  

• Extensive vapour bleeding - the extensive use of vapour coming out of the different effects of the evaporators are used for juice heaters and vacuum pans. The later the effect, the better is the steam economy in the system.
  

Additionally, the following aspects were also considered in the cane preparation section and milling section:

• Installation of heavy duty shredders, to achieve better preparatory index (> 92+ as compared to the conventional 85+) for cane
  

• Installation of Grooved Roller Pressure Feeder (GRPF) for pressure feed to the mills. This allows for better juice extraction from the cane.
  

• Lesser imbibition water addition, on account of the better juice extraction by the GRPF, resulting in reduction of boiling house steam consumption
  

This case study pertains to a sugar mill of 2500 TCD, where the above approach has been adopted at the design stage itself, resulting in lower steam consumption.

Conventional system

In a typical sugar mill, the most commonly used evaporators are the quintuple effect evaporators.

The typical vapour utilisation system in the evaporators comprises of:

• Vapour bleeding from II- or III- effect for heating (from 35 °C to 70 °C) in the raw (or dynamic) juice heaters
  

• Vapour bleeding from I- effect for heating (from 65 °C to 90 °C) in the first stage of the sulphited juice heater
  

• Exhaust steam for heating (from 90 °C to 105 °C) in the second stage of the sulphited juice heater
  

• Exhaust steam for heating (from 94 °C to 105 °C) in the clear juice heaters
  

• Exhaust steam for heating in the vacuum pans (C - pans)
  

The specific steam consumption with such a system for a 2500 TCD sugar mill is about 45 - 53 % on cane, depending on the crushing rate. However, maximum steam economy is achieved, if the vapour from the last two effects can be effectively utilised in the process, as the vapour would be otherwise lost. Also, the load on the evaporator condenser will reduce drastically.

Many of the energy efficient sugar mills, especially those having commercial cogeneration system, have adopted this practise and achieved tremendous benefits. The reduced steam consumption in the process, can result in additional power generation, which can be exported to the grid.

Present system

In a 2500 TCD sugar mill, the extensive use of vapour bleeding at evaporators, was adopted at the design stage itself. The plant has a quintuple-effect evaporator system.

This system comprises of:

• Vapour bleeding from the V- effect, for heating (from 30 °C to 45 °C) in the first stage of the raw juice heater
  

• Vapour bleeding from the IV- effect, for heating (from 45 °C to 70 °C) in the second stage of the raw juice heater
  

• Vapour bleeding from the II- effect, for heating in the A-pans, B-pans and first stage of sulphited juice heater
  

• Vapour bleeding from the I- effect, for heating in the C-pans, graining pan and second stage of sulphited juice heater
  

• Exhaust steam for heating in the clear juice heater
  

However, to ensure the efficient and stable operation of such a system, the exhaust steam pressure has to be maintained uniformly at an average of 1.2 - 1.4 ksc.

In this particular plant, this was being achieved, through an electronic governor control system for the turbo-alternator sets, in closed loop with the exhaust steam pressure. Whenever, the exhaust steam pressure decreases, the control system will send a signal to the alternator, to reduce the speed. This will reduce the power export to the grid and help achieve steady exhaust pressure and vice-versa.

Benefits achieved

The installation of the extensive vapour utilisation system at the evaporators has resulted in improved steam economy. The specific steam consumption achieved (as % cane crushed) at various crushing rates are as follows:

• At 2500 - 2700 TCD : 41% on cane
• At 2700 - 2800 TCD : 40% on cane
• At 2800 - 3000 TCD : 39% on cane
• At 3000 TCD and above : 38% on cane
  

Thus, the specific steam consumption (% on cane) is lower by atleast 7%. This means a saving of 3.5% of bagasse percent cane (or 35 kg of bagasse per ton of cane crushed).

Financial analysis

The annual benefits on account of sale of bagasse (@ Rs.350/- per ton of bagasse and 120 days of operation) works out to Rs.4.50 million. This project was installed at the design stage itself. The actual incremental investment, over the conventional system, was not available.

Note :

In another sugar mill of 5000 TCD, the same project was implemented. The annual saving achieved was Rs.11.00 million. This required an investment of Rs.6.50 million, which had an attractive simple payback period of 8 months.

Replacement of Steam Driven Mill Drives with Electric DC Motors

Background

Conventionally, steam turbines are used as the prime movers for the mills, in a sugar industry. These steam turbines are typically, single stage impulse type turbines having about 25 - 30% efficiency.

The recent installation of commercial cogeneration system, with provision for selling the excess power to the grid, has made the generation of excess power in a sugar mill, very attractive. One of the methods of increasing the cogeneration power in a sugar mill is to replace the smaller low efficiency mill turbines, with better efficiency drives, such as, DC motors or hydraulic drives.

The power turbines (multi-stage steam turbines) can operate at efficiencies of about 65 - 70%. Hence, the equivalent quantity of steam saved by the installation of DC motors or hydraulic drives can be passed through the power turbine, to generate additional power.

This replacement can aid in increase of net saleable power to the grid, resulting in additional revenue for the sugar plant. This case study highlights the details of one such project, implemented in a 5000 TCD sugar plant.

Previous Status

A 5000 TCD sugar mill had six numbers of 750 HP mill turbines and one number of 900 HP shredder turbine. The average steam consumption per mill (average load of 300 kW) was about 7.5 TPH steam @ 15 Ata. The steam driven mill drives had an efficiency of about 35%, in the case of single-stage turbine and about 50%, in the case of two stage turbines.

The plant team was planning to commission a commercial cogeneration plant. This offered an excellent opportunity for the plant team to replace the low efficiency steam turbine driven mills, with DC motors or hydraulic drives and maximise the cogeneration potential.

Energy saving project

The plant team contemplated the replacement of the steam driven mills with electric DC motors, along with the commissioning of the cogeneration plant.

Concept of the project

The conventional single stage impulse type steam turbines have very low efficiencies of 35%. Hence, the steam consumption per unit of power output is very high. A single high capacity steam turbine is more efficient as compared to multiple numbers of smaller capacity steam turbines. Hence, the steam can be passed through the larger capacity steam turbine to generate more saleable power.

The latest drives, such as, DC drives and hydraulic drives have very high efficiencies of 90%. The steam saved by the installation of DC drives, can be passed through the larger capacity power turbines of higher efficiency (about 65 - 70%), to generate additional saleable power.

Implementation methodology, problems faced and time frame

The steam turbine mill drives were replaced with DC drives, once the cogeneration plant was commissioned. The modifications carried were as follows:

Four numbers of 900 HP and two numbers of 750 HP DC motors were installed in place of the six numbers of 750 HP mill turbines

Two numbers of 1100 kW AC motors were installed for the fibrizer, in place of the single 900 HP shredder turbine

There were no major problems faced during the implementation of this project. The implementation of the project was completed in 24 months.

Benefits achieved

The comparative analysis of the operational parameters before and after the modification is as follows:

The equivalent power saved (850 kW/mill) by the implementation of this project, could be exported to the grid, to realize maximum savings.

Financial analysis

The annual energy saving achieved was Rs.62.37 million. This required an investment of Rs.42.00 million, which had an attractive simple payback period of 9 months.

Installation of 30 MW Commercial Co-generation Plant

Background

The Indian sugar industry by its inherent nature can generate surplus power, in contrast to the other industries, which are only consumers of energy. This is mainly possible because of the 30 % fibre content in the sugar cane used by the sugar mills. This fibre, referred to as bagasse, has good fuel value and is used for generation of the energy required, for the operation of the sugar mill.

The bagasse is fired in the boiler, for producing steam at high pressures, which is extracted through various back-pressure turbines and used in the process. This simultaneous generation of steam and power, commonly referred to as Co-generation. Conventionally, the cogeneration system was designed to cater to the in-house requirements of the sugar mill only. The excess bagasse generated, was sold to the outside market.

In the recent years, with the increasing power Demand-Supply gap, the generation of power from the excess bagasse, has been found to be attractive. This also offers an excellent opportunity for the sugar mills to generate additional revenue. Co-generation option has been adopted in many of the sugar mills, with substantial additional revenue for the mills. This also contributes to serve the national cause in a small way, by bridging the Demand - Supply gap.

This case study describes the installation of a commercial co-generation plant in a 5000 TCD mill.

Previous status

A 5000 TCD sugar mill in Tamilnadu operating for about 200 days in a year had the following equipment:

Boilers - 2 numbers of 18 TPH, 12 ATA

2 numbers of 29 TPH, 15 ATA

1 number of 50 TPH, 15 ATA

Turbines - 1 number 2.5 MW

1 number 2.0 MW

1 number 1.5 MW

Mill drives - 6 numbers 750 BHP steam turbines

1 number 900 BHP shredder turbine

The plant had an average steam consumption of 52%. The power requirement of the plant during the sugar-season was met by the internal generation and during the nonseason from the grid.

Energy saving project

The plant went in for a commercial co-generation plant. The old boilers and turbine were replaced with high pressure boilers and a single high capacity turbine. The new turbine installed was an extraction-cum condensing turbine.

A provision was also made, for exporting (transmitting) the excess power generated, to the state grid. The mill steam turbines were replaced with DC drives. The details of the new boilers, turbines and the steam distribution are as indicated below:

Boilers - 2 numbers of 70 TPH, 67 ATA

Multi-fuel fired boilers

Turbines - 1 number of 30 MW turbo-alternator set

(Extraction-cum-condensing type)

Mill drives - 4 numbers of 900 HP DC motors for mills

2 numbers of 750 HP DC motors for mills

2 numbers of 1100 kW AC motors for fibrizer

Implementation methodology, problems faced and time frame

Two high capacity, high-pressure boilers and a 30 MW turbine was installed in place of the old boilers and smaller turbine. While selecting the turbo-generator, it was decided to have the provision for operation of the co-generation plant, during the off-season also. This could be achieved, by utilising the surplus bagasse generated during the season, as well as by purchasing surplus bagasse, from other sugar mills and biomass fuels, such as, groundnut shell, paddy husk, cane trash etc.

The shortfall of bagasse during the off-season was a problem initially. The purchase of biomass fuels from the nearby areas and the use of lignite solved this problem. The entire project was completed and commissioned in 30 months time.

Benefits

The installation of high-pressure boilers and high-pressure turbo-generators has enhanced the power generation from 9 MW to 23 MW. Thus, surplus power of 14 MW is available for exporting to the grid.

The following operating parameters were achieved:

Typical (average) crushing rate = 5003 TCD

Typical power generation

During season = 5,18,321 units/day

During off-season = 2,49,929 units/day

Typical power exported to grid

During season = 3,18,892 units/day

(13.29 MW/day)

During off-season = 1,97,625 units/day

(8.23 MW/day)

Typical no. of days of operation = 219 days (season)

= 52 (off-season)

The summary of the benefits achieved (expressed as value addition per ton of bagasse fired) is as follows:

Financial analysis

The annual monetary benefits achieved are Rs.204.13 million (based on cost of power sold to the grid @ Rs.2.548/unit, sugar season of 219 days and off-season of 52 days). This required an investment of Rs.820.6 million. The investment had an attractive simple payback period of 48 months.

Note :

Critical factors affecting power generation

The efficient operation of a co-generation system depends on various factors. This has a direct bearing on the loss in power generation and the power exported to the grid. Some of these critical factors affecting the power generation (quantified as loss in generation per day) are as follows:

1% drop in bagasse % in cane : 18300 units

1% increase in moisture content of bagasse : 6800 - 10200 units

1% increase in process steam consumption : 4200 units

1% drop in crushing rate : 5000 - 7400 units

1 hour downtime : 20600 units

Drop in 1 ton of cane availability : 60 units

The above figures are based on the following operational parameters:

Crushing rate : 5000 TCD

Steam . bagasse ratio : 1 : 2.2

NCV of bagasse (50% moisture) : 1804 kCal/kg

Bagasse content, in % cane : 27%

Sugar Manufacturing Operations

2.1 Cane handling
2.2 Milling
2.3 Clarification/evaporation
2.4 The pan stage
2.5 The fugal stage
2.6 Final sugar
2.7 Energy supply systems
2.8 Associated operations

A sugar mill is a large factory used to produce raw sugar and other products from sugar cane. Mills are made up of a range of industrial plant such as boilers, storage and processing vessels, crushing and hammer mills and a large range of maintenance equipment. Mills operate in two distinct modes, crushing and non-crushing, both of which introduce a range of specific and general hazards to employers, workers and others. In essence, a sugar mill can be broken into the following processes (see Figure 2 for a diagram that shows the sugar milling process).

2.1 Cane handling

Cane handling describes the methods used to move cane billets into the milling section of the process. Billets are transported and stored using items such as:

1. cane railway bins
2. road transport systems (such as multi-lifts and semi trailers)
3. in field transporters.

The cane billets are then transferred into the milling system by:

1. trans-loaders (such as from road to rail)
2. tipplers (tipping cane bins into carriers)
3. direct tip into the carrier (by infield transporters and road transport).

Rail transfer methods use large hydraulic systems to push or pull rakes of bins into the tippler which tips them onto a ‘carrier’ (a moving floor conveyor). Most mills have storage yards for excess bins. Tipplers are a rotary device which hold the rail bin in place and turn it 180 degrees to empty its contents into the main conveyor (carrier).

2.2 Milling

The milling process involves the initial breakdown of cane into its primary fibres by a large hammer mill (shredder). Shredders consist of a number of large hammers (usually around 12 kg in weight) attached to a rotor by swing rods which are then driven at around 1200 revolutions per minute (rpm) by mechanical means (either by steam turbine or electric motor). The billets are shredded by smashing them between the hammers and the grid bar (a hard set of plates on one side of the shredder) breaking them into individual strands of fibre. This fibre is then processed through a series of crushing mills to extract juice. Mill rollers exert huge forces on the shredded cane which is fed through them via a vertical chute. The pressure between the rollers is large enough to break down the cell structure of the fibres so that the sucrose can be extracted within the juice. Juice contains a large amount of water which is removed or reduced in subsequent processes. The remaining fibre is then burned in a boiler to produce steam which drives most mill processes in a typical factory.

Extraction of as much of the sucrose as possible is a key element in milling. Mills use a number of methods to aid sugar extraction which include the application of hot water (around 95ْ C) to the fibre within the mill set, a series of crushing mill sets (the milling train) and reapplication of mixed juice and water (maceration) throughout the milling process.

2.3 Clarification/evaporation

The clarification/evaporation stage executes a number of functions such as:

• mixed juice incubation
• adjusting PH by addition of lime
• heating
• addition of flocculant (a product which assists contaminants to subside)
• addition of anti-scale chemicals
• removal of mud and heavy contaminants
• reduction of water levels in the juice.

Heating is completed using shell and tube heaters that are normally either cylindrical units with multiple passes for juice in tubes surrounded by steam (allowing thermal transfer between the two products) or multi-path plate and frame commercial units that are smaller than conventional heaters and are constructed from pressed SS sheets separated by gasket material.

Lime and flocculant are usually added to the juice as a slurry. A subsider then removes heavy contaminants from the juice. Subsiding, the process of allowing heavy materials to sink or fall to the bottom, usually removes the majority of dirt and the chemical mud formed from the reaction between the phosphate in the juice and the added lime from juice. The mud is then spread across a moving filter (a rotary drum filter) and ‘washed’ to leech out any remaining sucrose before removal from the factory. Mill mud is a nutrient rich product which is normally returned to the field.

The effet stage consists of a number of evaporators (large kettles) in series that boil the juice to reduce the water content. Effets are constructed in a particular pattern using multiple effet evaporation. Vapour produced from each vessel is used to boil the juice in the subsequent vessel at a lower pressure making maximum use of the energy initially put into the first vessel as low pressure steam. The latter effets in the set are operated at a vacuum in order to reduce the boiling point. The final product from the effet stage is usually known as ’liquor’ or ‘syrup’ and is a dark gold coloured liquid.

Dependent on juice properties heating surfaces within the effets and contact heaters are prone to contaminant build up (scale) which reduces heating efficiencies and after a period needs to be removed. Most factories use a chemical process to remove scale build up, normally by boiling caustic soda in the vessels or other chemical means such as sulphamic acid or rarely EDTA. On some occasions manual cleaning is required and is completed by blasting with high pressure water or mechanical brushing.

2.4 The pan stage

The pan stage is a similar process to the effets in that a pan boils off additional water. The main function of the pan stage is to produce sugar crystal from the liquor. In order to increase the speed of this process the pan stage operates in a manner which utilises ’seed crystal’ and a combination of products with varying levels of sugar content to produce a range of crystal sizes and hence qualities. The pan stage has many storage tanks such as receivers (tanks which receive product from the pans), crystallisers (a series of tanks and stirrers which cool the product from the pan stage resulting in additional crystal growth before fugaling) and large transfer pipes and valves.

2.5 The fugal stage

A fugal is a large electric centrifuge which spins up to 1200 revolutions per minute (rpm) dependent on its function and stage of operation (while filling batch fugals only turn at around 50 rpm). There are two types of centrifuge in use within sugar mills, high grade centrifuges (usually batch, but sometimes continuous) and low grade centrifuges which are continuous. Continuous fugals maintain a constant flow of product through them while batch fugals fill, operate and then discharge the final product. The fugal stage removes the remaining liquid product which surrounds the crystal, washes the crystal and delivers it into the final sugar system through a series of conveyors and a drier. The material removed during the centrifuge process is known as molasses and has a range of uses including sale as stock feed, fermentation for distillery production and as a component of cattle licks.

2.6 Final sugar

Finally, the sugar crystal is dried and moved to large storage bins awaiting transport to sugar terminals or other areas (such as refineries). Driers are large cylinders which are fluted and rotate to pass the crystal through at an even rate whilst dry air is applied via ducted fans or large air conditioners. Moisture levels and sucrose purity are important measures for sugar quality. Storage bins hold large amounts of raw sugar and the conveyor system supplying them can be directed into different bins dependent on the product type. Low moisture levels in final sugar product and atmospheric conditions can create a risk of sugar dust explosion. Sugar dust explosions are rare, however, they have caused significant damage and loss of life in sugar mills overseas.

2.7 Energy supply systems

Mills are usually powered by steam and subsidised by electrical devices, however in recent years a number of factories are moving to predominantly electric powered equipment. A standard sugar mill will still include equipment such as suspension or multiple fuel boilers, steam turbines, electrical generators and all of the associated distribution equipment for electric and steam power. A range of equipment is associated with steam and electric energy including transformers, high and low voltage distribution systems, protection devices such as circuit breakers, steam relief valves, expansion joints and water traps.

Mills also have extensive air distribution systems supplying general and instrument air.

2.8 Associated operations

A range of facilities associated with sugar production are located on site including:

• laboratory and associated processes
• packaging lines
• engineering workshops covering areas such as rolling stock repair, general engineering and fabrication, and electrical
• administration areas
• molasses storage and distribution systems
• water supply and effluent systems
• mud, ash, bagasse and other by-product handling and storage.

Figure 2 The sugar milling process

1. Cane marshalling yard
2. Cane receival
3. Weight bridge tippler and empty bin return system
4. Shredder
5. Milling train
6. Juice heater
7. Evaporator station
8. Filtration
9. Crystallisation and separation
10. Bulk sugar handling
11. Bagasse storage bin
12. Boiler station