Replacement of Air Inter-coolers in the Ammonia Plant

Background

The Ammonia is synthesised by reacting Hydrogen generated by reforming of hydrocarbons and Nitrogen in the atmospheric air in the presence of a catalyst at higher pressure to synthesise Ammonia. The atmospheric air is supplied to the reactor by a battery of air compressors. These compressors are very important for the operation of the plant and hence are rightly referred to as the heart of a fertiliser plant. The efficiency of these compressors therefore play a very important role in the efficiency of the whole plant. This case study describes about a project implemented in the compressor section of a fertiliser plant.

Previous status

In a 1,00,000 ton per annum capacity Ammonia plant, the air requirements of the Ammonia converter were being met by two numbers of oil lubricated 4 stage reciprocating compressors. The compressors were provided with inter-coolers with finned tubes and were laid in a horizontal fashion. The oil in the air from cylinders used to plug the gap between the fins and reduce the heat transfer. The exit air from the inter-cooler used to be at 55 – 58°C as against the design of 42°C. The capacity of the subsequent stages was getting reduced leading to loss of Ammonia production.

Energy saving project

The inter-coolers for the compressor was replaced with finless tubes and laid in a vertical fashion.

Implementation methodology & time frame

The implementation of this project was taken up parallely while the plant was operating. The replacement was done for one compressor first and the second compressor was taken up subsequently. The implementation and the subsequent operation did not pose any problem.

Benefits of the Project

The replacement of the horizontal fin type cooler with vertical finless cooler resulted in reduction of exit air temperature to around 45°C. There was a reduction of power to the extent of 45 kW.

Financial Analysis

This amounted to an annual monetary saving of Rs 0.85 million. The power saving alone has been considered. The investment made was around Rs 2.0 million. The simple payback period for this project was 28 months.

Benefits of vertical finless cooler

• Exit air temperature reduces
• Reduction in power consumption

Cost benefit analysis

• Annual Savings - Rs. 0.85 million
• Investment - Rs. 2.0 million
• Simple payback - 28 months

Installation of Make-up Gas Chiller at Suction of Synthesis Gas Compressor at Ammonia Plant

Background

The compressor is the heart of nitrogenous fertiliser plant and is used for various purposes such as compressing the synthesis gas, air, re-cycle gas and ammonia. The compressor capacity is also one of the important parameters controlling the capacity of the plant. Hence, the design of the compressor and its effective utilisation is essential for achieving higher production and lower energy consumption.

The compressor is a constant volume equipment and hence the capacity of the compressor can be increased by increasing the density of the gas at the suction of the compressor. As the gas density is inversely proportional to the temperature, there is a good possibility of increasing the capacity of the compressor by cooling the inlet gas.

This case study describes one such project taken up and implemented successfully in a fertiliser plant.

Previous status

This case study pertains to a ammonia fertiliser complex producing 900 tons per day of Urea. The plant was operating at about 920 TPD of ammonia production. The synthesis gas was entering the compressor at about 39°C.

Energy saving project

The plant installed a vapour absorption refrigeration system with LP steam for cooling the synthesis gas.

Implementation methodology & time frame

The implementation of this project was taken up when the plant was in operation. The hooking up of the new system with the existing was done during the planned shut of the plant. The installation of the new system and successful commissioning took about 18 months. No problem was encountered during the implementation and subsequent operation of the plant.

Benefits of the project

The implementation of this project resulted in the following benefits.

Parameter - Before Implementation - After Implementation

Ammonia Production - 920 TPD - 944 TPD
Syn. gas temperature - 39°C - 13°C
Syn. gas compressor speed - 13,142 RPM - 13,071 RPM

The implementation of this project resulted in a saving of 28,035 GCal per year, which amounted to 0.09 GCal / ton of ammonia.

Financial analysis

The implementatio n of this project resulted in a net annual saving (@ Rs. 350 / GCal) of Rs. 9.8 million. The investment made was about Rs. 22.0 million, which got paid back in 27 months.

Benefits of chiller at suction of synthesis gas compressor

Increase in capacity of compressor
Increased ammonia production
Reduction in specific energy consumption

Cost benefit analysis

Annual Savings - Rs. 9.8 million
Investment - Rs. 22.0 million
Simple payback - 27 months

Modernisation of the Ammonia Converter Basket – a case study on fertilizer industry

Background

The Hydrogen generated by reforming of hydro-carbons is reacted with Nitrogen in the atmospheric air in the presence of a catalyst at higher pressure to synthesise Ammonia. The synthesis of Ammonia occurs in the Ammonia converters. The older Ammonia converters were all of axial type which required higher pressure and resulted in lower conversions. These have been replaced in some of the plants with radial type / axial-radial system with considerable benefits. A case study on the modernisation of the Ammonia converter basket taken up in a plant is described below.

Previous status

In a 357 TPD Ammonia plant, the Ammonia converter basket had a conventional axial type basket, as shown in the figure. This needed an operating synthesis loop pressure of 300 bar. The catalyst used was Topsoe supplied of 10 mm size with a pressure drop of 5 bar. The conversion per pass was around 16 %. In 1992, the bottom exchanger developed a leak, leading to further reduction of ammonia conversion and increased loop pressure. The total production loss was around 30 %.

Energy saving project

The converter basket was modified to a axial-radial type system. The modified system is indicated in the diagram.

Implementation methodology & time frame

The implementation of this project was taken up as part of the Revamping exercise and hence a separate stoppage of the plant was avoided. The implementation and consequently the operation did not pose any problem.

Benefits of the project

The replacement of the old axial type converter basket with the modern axial-radial system resulted in the following benefits:

Loop pressure reduced to 250 bar – reducing compression energy
Lower pressure drop in converter beds – 3 bar as against 5 bar before
Higher Ammonia production ( about 10 TPD )

The above benefits resulted in the reduction of energy consumption by 0.35 Gcal / MT of Ammonia.

Financial analysis

This amounted to an annual monetary saving (@ 1,00,000 MT production of Ammonia & Rs 1000 / Gcal) of Rs 20 million. The energy saving alone has been considered. The investment made was around Rs 50 million. The simple payback period for this project was 30 months.

Benefits of axial-radial type ammonia convertor basket

Lower loop pressure - reduces compression energy
Lower pressure drop in convertor beds (3 bar against 5 bar)
Higher Ammonia production by 2 to 3%

Cost benefit analysis

Annual Savings - Rs. 20 million
Investment - Rs. 50 million
Simple payback - 30 months

Replacement of the Air-lift with Bucket Elevator for Kiln Feed Transport to the Silo

Background

The kiln feed after blending in the silo is conveyed to the top of the pre-heater for further processing. The transport of kiln feed is normally done through pneumatic conveying systems such as air-lift. The pneumatic conveying system consumes more power, nearly 3 to 4 times that of the mechanical conveying system. Also, the pneumatic conveying system adds additional cold air to the pre-heater system, thus reducing the thermal efficiency of the system. Conventionally, the pneumatic conveying system was being preferred as the mechanical system (particularly the Bucket elevator) was not very reliable. In the recent years with the improvement in the metallurgy, bucket elevators that can operate continuously in a reliable manner have been developed. These have been installed in many plants with substantial benefits.

Previous status

In a million tonne dry process pre-calciner plant, the Kiln feed was being conveyed with the help of an air-lift.

Energy saving project

The air-lift was replaced with a bucket elevator. The air-lift was retained to meet the stand-by requirements.

Implementation methodology & time frame

The installation of the Bucket elevator took about 6 months. There was no stoppage of the plant, and the installation of the Bucket elevator was done parallely. The bucket elevator was hooked up during the regular stoppage of the Kiln and a separate stoppage was avoided.

Benefits of the project

The implementation of this project resulted in reduction of power from 146 kW for the air-lift to 51 kW for the Bucket elevator. The air handled by the pre-heater fan also got reduced resulting in pre-heater fan power consumption by 14 kW. The thermal saving was about 3.2 kCal/kg of clinker.

The saving annually amounted to

• Electrical saving - 8.6 lakh units / year
• Thermal saving - 2112 MMkCal / year
  

Financial analysis

The total benefits amounted to a monetary annual savings of Rs. 34.0 lakhs (@ Rs.3.1/unit & Rs.350/mmkCal) The investment made was around Rs. 75.0 lakhs. The simple payback period for this project was 27 months.

Benefits of installing bucket elevator for Kiln feed

• Reduction in conveying energy & PH fan power
• Reduction in thermal energy
  

Cost benefit analysis

• Annual Savings - Rs. 34.0 lakhs
• Investment - Rs. 75.0 lakhs
• Simple payback - 27 months
  

Usage of Cheaper Fuels for Calciner Firing

Background

The Kiln and the Calciner are major consumers of fuel in a Cement plant. The fuel cost amounts to nearly 20 % of the manufacturing cost. The increasing cost of fuel and the competition among the units have made the Cement units to take up many thermal energy saving projects. The plants are also looking for avenues for reducing the cost by replacing the costly fuels with cheaper fuels. The possible fuels that have been tried by the Cement units include Lignite, Rice husk and Ground-nut shell.

Previous status

In a million tonne dry process pre-calciner plant, Coal was being used as fuel for firing in both the Kiln and Calciner. The Coal was having a Calorific value of about 5900 kCal / kg with a cost of about Rs. 2000 / MT.

Energy saving project

A provision was made to utilise Rice husk in the Calciner. With the new system it was possible to replace part of the coal fired in the Calciner with Rice husk.

Implementation methodology & time frame

A hopper was installed by the side of the pre-heater building for storing the Rice husk. The rice husk was fed to this hopper with the help of front end loaders.

The Rice husk was conveyed to the Calciner with the help of a Rotary blower of 32 m³ / hour capacity. The whole system was fabricated with the waste material available in the plant. The system was hooked up with the main system during a brief stoppage of the plant. The system could be operated for about 8 months of non- rainy dry season.

Benefits of the project

The implementation of the project resulted in the reduction of the cost of fuel used in the Calciner. The cost comparison of Coal and Rice husk are as below;

Parameter -- Coal – Rice husk

Cost – Rs.2000/MT – Rs.750/MT

Calorific Value – 5900 kCal/kg – 2900 kCal/kg

Energy Cost – Rs.340/MMKCal – Rs. 260/MMKcal

The rice husk was used for replacing about 10% of the total coal used for firing in the calciners. This resulted in reduction of the total thermal energy cost, with the other conditions such as output, temperature, pressure etc. remaining the same. There was also a marginal reduction of the power consumption in the coal mill, as the rice husk was used directly without grinding. The rice husk becomes wet and handling becomes difficult during the rainy season. Hence, the usage of rice husk was restricted to the non-rainy and dry season (about 8 months in a year).

Financial analysis

The annual benefits (in the form of reduction in thermal energy cost) was about Rs. 35 lakhs. The equipment required for conveying and firing in the pre-heater was fabricated in-house with available material and hence the investment was negligible.

Benefits of using cheaper fuel

• Reduction in thermal energy cost
• Marginal reduction in coal mill power consumption
  

Cost benefit analysis

• Annual Savings - Rs. 35.0 lakhs
• Investment – Negligible

Parallel Pre-heater Fan for Increasing the Production

Background

The equipment in a Cement plant are all designed based on a certain load factor of the Kiln. Normally the pre-heaters, coolers and other major equipment are always designed with good margins. Many companies have made use of these margins and achieved higher levels of production with small modifications. In some of the plants all the equipment might have excess capacity and only the PH fan might be the bottle-neck for increasing production. In such cases the pre-heater fan is replaced or modified to increase the output of the plant. The increase in output also aids in reducing the power consumption.

Previous status

A million tonne dry process pre-calciner plant was operating at a capacity of about 3000 to 3100 TPD. The major equipment had additional margin for increasing the capacity by another 10 %, but the pre-heater fan was operating at its full capacity.

Energy saving project

An additional small pre-heater fan equal to about 15 % of the capacity of the existing fan was installed parallel to the existing PH fan. The head of the new fan was the same as that of the existing fan.

Implementation methodology & time frame

The new PH fan was installed parallel to the existing fan. The fan was installed during running of the plant. The new parallel fan was hooked up during a small stoppage of the plant.

Benefits of the project

There was an increase in the output of the Kiln, reduction in pressure drop of the pre-heater, reduction in Kiln section power consumption and reduction in Kiln specific thermal energy consumption. The comparison of the conditions and the energy consumption before and after installation of the Parallel Pre-heater fan are as below:

Parameter Before Implementation After Implementation

Clinker Production 3000 TPD 3200 TPD

DP across Pre heater 880 mmWg 860 mmWg

Pre heater fan Power 12.67 kWh /ton<>

Heat Consumption 790 kCal / kg 780 kCal / kg

The implementation of this project resulted in a power saving of 0.6 units / ton of Clinker, which annually amounted to 10.80 lakh units / year. Additionally there was also the thermal energy reduction of about 10 kCal / kg. The increased output of 200 TPD of clinker also aided in reducing the fixed cost component.

Financial analysis

The total benefits amounted to a monetary annual savings of Rs. 50 lakhs. The investment made was around Rs. 10 lakhs. The simple payback period for this project was 2 months.

Benefits of additional P.H. fans

• Increased clinker production
• Lower P.H. fan (total) power consumption
• Reduction in thermal energy

Cost benefit analysis

• Annual Savings - Rs. 50.0 lakhs
• Investment - Rs. 10.0 lakhs
• Simple payback - 2 months

Replacement of Existing Cyclones with Low Pressure Drop (LP) Cyclones

Background

The Pre-heaters comprising of 4/5/6 stages of cyclones is an important part of the Kiln section in a Cement Plant. In the pre-heaters the waste gas coming out of the Kiln system is used for pre-heating the kiln feed material. With increased focus towards more heat recovery from the waste gas, the number of pre heater stages have been increased from 4 to 5 / 6. The increase in the number of stages however led to increase in the pressure drop across the system and hence higher fan power. This led to the development of cyclones, which have a lower pressure drop. The low pressure drop (LP) cyclones have the advantage of

• Low pressure drop. Hence, lower Pre-heater fan power consumption.
• Higher output rate with the same Pre-heater fan
• Reduction in thermal energy consumption

Previous status

In a million tonne dry process pre-calciner plant, there were 4 stages of conventional cyclones with a twin cyclone at the top. The pressure drop across the top twin cyclone was about 100 – 125 mmWg.

Energy saving project

The existing top stage twin cyclone was replaced with a low pressure drop cyclone.

Implementation methodology & time frame

The top cyclone was at a height of nearly 106 metres. The implementation of this project involved removal of the existing cyclone and fixing of the new LP cyclone. The normal procedure involves the following steps:

• Removal of the bricks inside the existing top cyclone
• Removal of the old cyclone
• Installation of the new cyclone
• Refractory lining of the new cyclones
  

This procedure however needs a stoppage of the plant of more than 90 days. The plant could not afford such a long stoppage and the consequent loss of production.

Hence, the procedure was improvised to reduce the plant stoppage time. The improvised procedure adopted by the plant is as below:

• The entire cyclone was assembled at the ground floor
• The inside brick lining was also done at the ground floor only
• The plant was then stopped and the existing cyclones removed
• The entire twin cyclone along with brick lining was lifted to the top and fixed. A special crane was used for lifting the cyclones of about 150 MT to a height of about 106 metres. In this manner, the project could be implemented with a stoppage of only 20 days.

Benefits of the project

There was an increase in the output of the Kiln, reduction in pressure drop of the pre-heater, reduction in Kiln section power consumption and reduction in Kiln specific thermal energy consumption. The comparison of the conditions and the energy consumption before and after installation of the LP cyclones are as below:

Parameter Before Implementation After Implementation

Clinker Production 2650 TPD 2850 TPD

DP across Top Cyclone 100 – 125 mmWg 70 – 90 mmWg

Kiln section Power 30 kWh /ton 28.5 kWh / ton

Heat Consumption 830 kCal / kg 810 kCal / kg

The implementation of this project resulted in a power saving of 1.5 units / ton of Clinker, which annually amounted to 14 lakh units / year. Additionally there was also the thermal energy reduction of about 20 kCal / kg. The increased output of 200 TPD of clinker also aided in reducing the fixed cost component.

Financial analysis

The total benefits amounted to a monetary annual savings of Rs. 240 lakhs. The investment made was around Rs. 220 lakhs. The simple payback period for this project was 11 months.

Benefits of low pressure drop cyclone

• Lower pressure drop across P.H.
• Reduction in P.H. fan power consumption
• Increase in clinker production
• Reduction in thermal energy consumption.
  

Cost benefit analysis

• Annual Savings - Rs. 240.0 lakhs
• Investment - Rs. 220.0 lakhs
• Simple payback - 11 months

Blending Control System for Maintaining Consistent Kiln Feed Quality

Background

The Kiln is the heart of a Cement plant. The steady and continuous operation of the Kiln is essential for producing good quality Clinker, higher level of output and lower energy consumption. To ensure this, consistent quality of kiln feed is a pre-requisite. This can be achieved only if the raw meal fed to the silo is consistent. The raw meal is produced by grinding various raw materials such as limestone, bauxite, iron-ore, etc,. The quality of these raw materials varies from time-to-time. Hence, the quality of the raw meal is analysed every hour and the percentage of the mix is varied to maintain the raw meal quality. In the old plants, the mix percentage is varied manually from hour to hour. This manual method of adjustment is inaccurate and generally leads to fluctuation in the quality of raw meal and hence the kiln-feed.

In the latest plants, the hourly chemical analysis is performed and the data is fed to the computer. A Software included in the system, varies the raw mix proportion so that the required mix quality is maintained. This has been retrofitted in many plants with substantial benefits.

Previous status

In a 3000 TPD dry process pre-calciner plant operating with a VRM and a continuous blending cum storage silo, the raw meal was being produced by grinding Limestone, bauxite and iron-ore. The raw meal was being analysed every hour through an X-ray analyser and the mix varied manually.

Energy saving project

A new blending software based control system was introduced. The new system had a separate PC which could be linked to the existing control system. The X-ray analysis was fed to the blending control system, which automatically varied the raw mix proportion.

Implementation methodology & time frame

The raw mix proportioning was done manually in the initial days and the blending control was put into operation in a phased manner. The system was checked for the extreme conditions (e.g. limestone of maximum and minimum LSF) by both the supplier and the plant team, so that the system is functional at all conditions.

Benefits of the project

There was a marginal increase in the output of the Kiln, reduction in feed cuts on account of quality of kiln feed, better quality of clinker and steady operation of the Kiln.

The benefits achieved are as below.

• Increase in kiln output by 10 to 15 tpd.
• Reduction in LSF variation in Kiln feed from 0.4 % to 0.2 %.
• Reduction in Thermal Energy Consumption
  

Financial analysis

The implementation of this project resulted in an annual saving of Rs. 18 lakhs (Increased Production and thermal energy saving). The investment made was around Rs. 15.0 lakhs. The simple payback period was 10 months.

Benefits of blending control system

• Fine and accurate control of raw mix
• Reduced Kiln feed - L.S.F. variation
• Lower thermal consumption

Cost benefit analysis

• Annual Savings - Rs. 18.0 lakhs
• Investment - Rs. 15.0 lakhs
• Simple payback - 10 months

From the Energy Management Discussion Group (http://in.groups.yahoo.com/group/aipnpc/ )

“Is it advisable to install DF/DT relay for load shedding during power failure”- A technical discussion

1.	Ours is a chemical plant of home load 2.7MW. Our capitive power plant capacity and generation both is 2.3MW(92% actuator opening). We are synchronising the DG power (11/22KV) with EB Grid. During any heavy fault in EB side we are facing block out condition (i.e) due to O/Fin powerplant. So we decided to install DF/DT relay for load shedding during power failure. Is this advisable to install this relay . Can anyone give suggestions or any comments. Kandangowri via email
2.	Dear Mr. Kandangawri, Would you pl. elaborate the type of problem as what means 'Blocking'. Does it go Islanding the own Generation ? I wish you all a happy inteaction and great success for ur. objectives. Thanks N L Singh
3.	Hi Regarding incorporating df/dt relay for islanding operation of your TG from the grid is very much recommended way of reliable operation. The df/dt does function very well with the following conditons i.e In export conditon, under frequency (say 47.8 Hz) and rate of change of freq with time. Balaji.M
4.	Blockout means all the breakers (i.e)our EB,TIE,TG breakergot opened.After islanding,within few seconds blockout happens.
5.	Your df/dt relay shall initiate a logic to trip selected load of your Plant so that balanced load comes within your capacity of your generation and that should be immidiate. Then your Plant will survive. Selection of Load Shedding shall be proper i.e. Lighting,Emergency Loads of selected drive,Auxiliaries of your own Generation,Control & Instrumentation Power. Relay also shall be provided with two stage of U/Frequency operation and in extreme condition Aux.of Generation and Lighting & Control. Thing will work like magic and you can forget Black out. The same is experienced by me. Amar Nath Mukherjee
6.	Dear Sir/Madam, While the engine is running in parrelel to grid and whenever there is disturvance on grid, the engine should get Island mode which can be done by installing the vector sum relay as well all Ht, medium breakers in the distribution should be taken as grid failure contact to Island the own generation. Once the engine goes on Island mode with respect to its capacity load has to arranged by load shedding deciding the critical v/s non critical load. When the grid runs in parrelel the frequency is normaly found lower but in Island mode it will take standard frequency and to avoid any problem the own generation can be put on constant power mode. Regards N L Singh
7.	Yes it is better to put dV/dt and dF/dt relay to isolate the generator from grid during sudden change in grid frequency / voltage. Also since your Plant power requirement is 2.7MW and your generation is 2.3MW , it is good to put AUTO LOAD SHEDDING of 0.4-0.5MW load during islanding with import power condition. regards, Ananth
8.	Dear Sir, Calculate the sustainable capacity after islanding & then incorporate islanding scheme. Regards, vijay
9.	DF/Dt protection will be suitable for your requirement but you may think also vector surge protection relay (mains decoupler) which includes the protection of vector surge (andle) in addition of DF/Dt. Furhter vector surge protection relay sensing is fastet than Df/Dt. Prabhu

From the Energy Management Discussion Group (http://in.groups.yahoo.com/group/aipnpc/ )

A technical discussion on “Parallel Operation of Transformers”

1.	I need a technical advice from you or any of the experienced members of the group regarding parallel operation of transformers on LT side. I want two nos of 250 KVA transformers of 11 kV / 415 volts to run on parallel. BOTH THE TRANSFORMERS ARE CONNECTED TO A COMMON bUS BAR IN THE lt PANEL THROUGH INDIVIDUAL ACBs and Bus coupler. Kindly advice me the control equipments to be provided in the LT panel and wiring for parallel operation of these two transformers when the load exceeds 250 KVA. i will be much grateful if a suggestion can be given in this regard. with regards Narayanan
2.	Dear all, Very briefly the things to be considered are: 1. Check for short circuit currents during parallel operation to check if the fault level is within the limits of the switchgears selected. 2. Voltages for the side being paralleled should be same. 3. Impedance values should be almost same - it may be difficult to get exactly the same values. 4. Ideally have the same vector group for the transformers. 5. Phase sequence should be same. 6. Protection is another critical issue. It should be ensured that the overload relays are well set so that the full load current is not exceeded for any of the transformers. If one of the paralleled transformer trips (HT or LT side) the other transformer will take load of both the transformers. It will be ideal to have interlocks so that in case of one transformer failure the corresponding (predetermined) loads also get disconnected. Alternatively if one transformer fails ensure that the other also trips to avoid overloading condition. There may be some specific issues which may need consideration as well. Hope this will serve some purpose. Regards Arvind
3.	Dear Mr.Thukral, It is a good explanation. However, I would like to add two more additional points with you. 3) The purpose of impedance level matching is to withstand the circulating current effect under parallel operation. 6) Regarding the protection issue- separate CB should to provided to individual Tf and later connected to common bus bar in order to avoid the overloading of individual TF Kindly please give me feedback whether the two point are correct or not. Thanks & regards, Ananda Krishnan G M
4.	Dear Ananda, These points are valid. Thanks for the explanation. Regards Arvind
5.	Dear Anand / Arvind, I have some further clarifications: Item. # 3 The purpose of insisting equal % impedance value for the transformers for parallel operation is to minimize the circulating current (up to zero if the % impedances are exactly equal), not to withstand the circulating current, there by to achieve equal sharing of the total load by each transformers under parallel operation. Item # 4 If the vector groups of the transformers, (having almost same % impedance) are deferent, it should not be connected in parallel, unless the minimum requirement for parallel operation such as (1) Same voltage Ratio (2) Same phase angle deference between primary and secondary (3) Same Polarity and (4) same phase sequence can be achieved by suitably connecting the transformer terminals for paralleling. Item # 6 Tripping of pre-determined loads or tripping the healthy transformer in service, only because of tripping of one transformer, (without considering the total load) is not advisable. Pre-determined loads need be removed (by tripping) only if the total load exceeds the capacity of healthy transformer in service. This can be achieved by properly set over load relays, and by this tripping of healthy transformer also can be avoided. Please provide your valuables comments on these points. With Regards Johny.P.A
6.	Dear Mr.Pulikottil, Its a good explanation. However, i would like to excuse me for the mistake that you pointed .Actually i tried to mention Circulating current effect but i left the word effect hence, the whole meaning changed.i tried to convey the same as you did. Regarding the Item no 4: i think there is a contradiction in your statement.first you said if vector group is not matching parrelel connection should not be done and later you have explained if it satisfies the minimum requirements of 1)same voltage ratio 2)same phase angle diffrence between primary and secondary 3)same polarity4)phase sequence. If and only if it is of same vector group it satisfies same phase angle concept. I hope I am clear. Please feel free to correct the statement if i made mistake.
7.	Mr.Narayanan- The concept of phase sequence is the important issue. if the two TF are feed from different feeders from the distribution company make sure that under all condition the phase sequence should not be altered. For better aspect always try to get the supply from a single feeder. If you fail to maintain then you will face a serious impact. Thanks & regards, Ananda Krishnan G M
8.	The supply is only from single feeder. Thank you for all valuable advices. with regards narayanan
9.	Dear Ananda, First of all the point # 4 "ideally have the same vector group of the transformers" gives a possibility (not ideal) to connect two transformers having deferent vector groups. It is true, but should follow the requirement of parallel operation of the transformers. All vector groups of the transformers is grouped in 4 groups. Out of these 4 groups transformers grouped in two particular groups can be connected in parallel. For more details please refer to 'J&P Transformer Book". With Regards, Johny.P.A
10.	Dear jhony Thank you for the informations passed on regarding parallel operation of transformers. Narayanan

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.