PULP AND PAPER INDUSTRY

 

Raw material preparation


Wood based mills - selection of chippers & conveying equipment

Chippers are one of the major consumers of power in a wood based paper mill. It is suggested that energy efficient, high capacity, chippers be selected. The unitisation of equipment (select only one high capacity chipper) results in obtaining good operating efficiency.
Another factor to be seen in achieving optimum efficiency at chipper house is the optimal feed rate. To achieve the optimal feed rate, mechanised conveying to chipper is to be planned at design stage. This will result in better capacity utilisation of the chippers.

Conveying of chips also consumes considerable energy. The different methods of conveying chips are pneumatic conveying, screw conveying or belt conveying. The most energy efficient chips conveying is through belt conveyor. During design stage consideration should be given for layout to minimise transportation. This will result in power savings in chipper house. Planning of belt conveying systems for chips transport will also reduce maintenance cost (due to lower wear and tear compared to pneumatic conveying systems) and reduce atmospheric emissions from chippers.

During design stage, the paper mills should plan to burn chip-dust in the paper mill's power boilers. This will reduce consumption of purchased fuels in the paper mill's power boilers and eliminate environmental problems associated with chip-dust disposal.

Waste paper based mills

In countries like India, where there is a shortage of conventional raw material, every effort should be made to encourage utilisation of waste paper. It can be either imported or indigenous.
Bagasse processing

The general practice followed in the paper industry is to carry out moist depithing in the paper mill. Instead, the following can be thought of at the design stage itself.
  1. Plan moist depithing operations at sugar mill instead of paper mill. Also, plan to compact the moist depithed bagasse at the sugar mill, before transportation to the paper mill. This will eliminate transportation of pith from the sugar mill to the paper mill, and also reduce the volume of bagasse to be transported to the paper mill.
  2. Plan for burning the pith, generated by moist depithing, in the sugar mill boilers. This will reduce the consumption of purchased fuels in the sugar mill boilers.
  3. Plan a thickening system for pith generated at wet cleaning, and burn the thickened pith in the paper mill's power boilers. This will reduce the consumption of purchased fuels in the paper mill's power boilers.

Pulp mill

Pulping converts wood chips into fibres by the chemical reaction between lignin and the active chemicals in the cooking liquor.

Extended delignification

Wood pulping

The advanced cooking process, with extended delignification based on displacement heating in batch digesters or continuous vertical digesters, has given good results.
The benefits of such process sequences are multi-fold.
  1. Saves energy Energy recycling saves upto 75% of steam demand. The steam consumption can be brought down from 2.0 t/t of unbleached pulp to 0.65 t/t of unbleached pulp.
  2. Uniform and better pulp quality/properties, resulting in better machine runnability and efficiency
  3. Higher brightness levels can be achieved due to low kappa from digesters
  4. Considerable savings in bleaching chemicals, and less pollution

The following table gives an idea of the present chemical consumption pattern with conventional and that anticipated on installing extended delignification.

Chemicals used % of unbleached pulp

Present		 With extended delignification

Chlorine
6
3.85

Caustic
3		 1.6

Hypo (as chlorine)
7		 4.4

Peroxide
.60
.40

  1. Stronger Pulp Due to greater selectivity of this reaction and more uniform reaction medium it produces 15-20% greater tear/tensile strength properties at lower kappa numbers when compared to conventional batch digester pulp. The yield will also be more by about 2%
  2. Environmental friendly Due to low kappa number for hard wood (10), the demand of bleaching chemicals is reduced. This represents a substantial reduction in AOX, BOD, COD, and colour discharges in the bleach plant effluent.
  3. Lower effective alkali at equivalent kappa number
  4. Lower screen rejects due to uniform cooking

Hence, new plants should be planned with extended delignification.

Non-wood fibres

Continuous digesters may be planned for cooking non-wood fibres instead of using rotary spherical batch digesters.
The following are the benefits realised by using continuous digesters.
  1. Uniformity in pulp quality
  2. Flexible production through controlled retention time
  3. Optimum heat economy
  4. Incorporation of cold blow system prevents damage to pulp fibres during blowing and eliminates the need for blow heat recovery system
    Washing

Though washing is not a major consumer of energy, there still exists a scope for energy saving and environmental impact reduction.
The suggested black liquor concentration leaving the washing section is 18% for wood pulp and 12% for agro residues.
Some of the proven equipment in washing are
  1. Flat belt/wire washer
  2. Double wire press
  3. DD washer
  4. Twin roll press

The above are advanced types of washing equipment and have several advantages. Some of them are
  1. Less energy intensive when compared to conventional drum washing.
  2. Operate at higher pulp consistency levels; pulp enters at 3.0% consistency and leaves at 32% consistency. BOD and COD discharged with the pulp, is less. Hence, less bleach chemical is required and colour of the bleach plant effluent is significantly reduced.
    l Increased black liquor solids concentration to evaporators and reduced steam demand in evaporators. The black liquor concentration at inlet to evaporators increases from 15% to 18%.
  3. Chemicals loss is only 10 kg/t of pulp when compared to 20 kg/t of pulp in conventional drum washing.
  4. These equipment operate with a dilution factor of 1.5 as against a dilution factor of 3.0 for conventional drum washers. Hence, water consumption is only 50% as compared to conventional drum washers. This increases the weak black liquor (WBL) concentration and results in substantial reduction in steam requirement in black liquor evaporation plant.

Hence, it is recommended that such energy efficient washing systems are installed, instead of conventional drum washers.

Screening

Many improvements have taken place in screening. The installation of pressure screens can result in good energy savings. The pressure screens can be operated at a higher consistency, typically 3.5%, against the conventional screening at 1.5% consistency. This results in enormous energy savings in pumping energy.

The separation efficiency of the pressure screen is 95%, while that of the conventional screens is only 65% to 70%. Another advantage is that the centricleaners can be completely avoided, if high pressure screens with slotted baskets of proper slot size are installed.

Therefore, pressure screens should be installed and centricleaners should be avoided in pulp mills.

Oxygen delignification

With oxygen delignification, it is possible to reduce the kappa number of the pulp by nearly 50%, thus resulting in savings in bleach chemicals. It also results in reduced effluent load from the pulp mill, with reduction in the formation of organic material, like COD and AOX.
The additional black liquor solids generated in oxygen delignification stage is sent to chemical recovery for recovery of heat and chemicals in black liquor. This stage not only recovers chemical in an environmental-friendly manner but also recovers energy, which otherwise adds to COD, BOD and AOX load of effluent from bleach plant.

Bleaching

The type of bleaching sequence significantly impacts environmental aspects. In India, most of the plants still use elemental chlorine as a bleaching chemical. In this era of strict environmental regulations, usage of elemental chlorine is frowned upon. For wood based paper mills, use of elemental chlorine does not permit compliance with the tolerance limits for total organic chlorides in the mill effluent.

Hence, new plants have to consider Elemental Chlorine Free (ECF) or Totally Chlorine Free (TCF) bleaching sequences. In the long run, all plants would be would require to be TCF, if environmental considerations become more stringent.

Medium consistency pumping (8 to 12%) is an established practice in developed countries while developing countries normally use low consistency (3 to 4%). Themedium consistency pumping brings down the energy consumption for pumping considerably.

Hence, ECF/TCF bleaching, with medium consistency (8 to 12%) operation, is suggested.

Black liquor evaporation plant

Black liquor evaporation plant is one of the major consumer of steam in paper mill. The steam consumption depends on the number of stages at the evaporation section. During design, provision of a seven stage falling film evaporator with a steam economy of six is an ideal choice to optimise the steam consumption. These plants operate with a minimum steam economy of 6.0 compared to the normal steam economy of 4.5. This results in reduction in steam consumption by about 25%.

In the conventional systems, forced circulation consumes a lot of power in terms of pumping. In the new systems, forced circulation is completely avoided with considerable reduction in power consumption. The additional cost for the new system will be paid back in two (2) years.

In Indian mills efficient screening of weak black liquor is not practised. This results in down time of evaporation plant in the form of water boiling and steam energy requirement for water boiling. Installation of pressure filters for weak black liquor ensures removal of fibres prior to feeding weak black liquor to multiple effect evaporation plant. This results in reduced 'scaling' of evaporator heat transfer surfaces and, hence, higher availability and capacity. This in turn reduces the steam requirement for water boiling.

At present, in Indian mills, the average black liquor concentration leaving the evaporators is about 50%. A few mills have achieved a black liquor concentration at the outlet of evaporators as high as 65% (abroad, it is 75 to 85%). For the new mills, the black liquor concentration is to be planned at 65% leaving the evaporation plant.
Present day Indian mills do not practice condensate stripping. This results in venting out of obnoxious malodorous mercaptan gases. From an environmental angle, segregation, collection and utilisation of non-condensable gases must be thought of during the design stage, even though energy available in non-condensible gases is recovered in rotary lime kiln/chemical recovery boiler. Design should consider effective thermal insulation of black liquor system and black liquor tankages.

Recovery boiler

At recovery boiler, the steam generation per tonne of black liquor solids depends on the concentration of black liquor entering the boiler. The present average steam generation is only 3 t/t of black liquor solid (with 45% concentration black liquor entering the boiler). The steam generation increase with increased concentration of black liquor is as follows

Black liquor concentration Steam generation
(in percentage) (t/t of black liquor solid)
45 3.0
65 3.4
85 3.8

Therefore, there exists a good potential for increasing the steam generation by at least 13% (from 3 to 3.4 tonnes of steam per tonne of fuel) by improving the black liquor concentration at the evaporator.
The advantages of high solids liquor firing are increased capacity of the boiler, increased steam generation, very low SOx emission, low power consumption and high sodium sulphate reduction efficiency.
It is suggested recovery boilers be installed with
  1. A large economiser (to recover the waste heat from flue gas and to eliminate direct contact secondary evaporator of black liquor by flue gas) increase steam generation and increased availability of boiler
  2. A two stage air preheater
  3. An ID fan with variable frequency drive
  4. FD fans with variable frequency drive
  5. Environmental protection equipment such as Electro Static Precipitator (ESP)
    Causticiser and rotary lime kiln
    Installation of lime kiln is imperative for the new plants, from environmental considerations of eliminating solid-wastes disposal problems. Kiln is also a major fuel energy consumer. The following points can be considered while designing the lime kiln system
  6. The dryness of lime mud feed is to be increased by installing snap blow filters. The suggested dryness is 70-75%. For example, by increasing the dryness from 50% to 70%, the fuel consumption comes down from 220 l/t of burnt lime to 160 l/t of burnt lime.
  7. Kiln has to be provided with in-built preheating, double layer (with insulating layer) refractory and a high efficiency, low NOx, burner for higher fuel efficiency.
  8. Installation of ESP for dust recovery and environmentally friendly operation.
    Design should also consider good thermal insulation of white liquor tankages.


Refiners

Refiners are the major consumers of power in stock preparation section. It is possible to save a considerable amount of energy (20-25% savings) by installing energy efficient refiners. Energy efficient refiners consume very low 'no-load' energy. 'No-load' energy refers to the power consumed by the refiner with pulp stock passing through the refiner when refiner tackles are moved apart. The ratio between net refining energy to total refining energy, should be as high as possible. This should be the selection/sizing criterion for refiners.

Centricleaners

There is a good scope for optimising the power consumption by considering the following points at design stage

  1. Installation of low pressure drop centricleaners (maximum 15 m drop).
  2. The operating consistency should be 1% (against the present average consistency of 0.7%). However, this could be over-ridden by the paper machine headbox consistency, which is configuration-dependent.
  3. Installation of double dilution system. It gives better control and saves considerable energy, particularly when machine headbox consistency requirement is low. Centricleaners can then be run at optimum consistency independent of head box consistency.
  4. The water cycle has to be closed as far as possible, to minimise fresh water consumption. The water consumption at paper machine can be brought down to 12 m3/t of paper at paper machine and approach system.
    Agitators
Agitators also consume considerable energy. There are two types of designs in agitators, namely, mixing type and suspension type. The suspension type agitators are suggested where the pulp has to be kept in suspension for pumping. On the other hand, mixing type agitators are suggested to mix different types of pulp. Therefore, mixing type agitators generally require high torque motors and with higher motor capacity. Proper selection of agitators appropriate to the duty conditions can save upto 50% of power consumption in agitators.

Hence, proper type of agitators as per application should be selected.

Paper machine

Forming Area

Design formers with low power consumption and low friction suction boxes.
At the forming area, drainage provisions have to be made with less frictional losses. The flat boxes can be of ceramic material. This saves 5-7% energy.
Evaporation from free wet surfaces is to be avoided by optimising ventilation at the wet end of paper machine.

Presses
Paper machine is the major steam consumer in a paper mill. The steam consumption in paper machine area depends mainly on the off-press dryness. The percentage dryness vs. the specific steam consumption is shown below.

DRYNESS VS. SPECIFIC STEAM CONSUMPTION


Off-press
Web dryness		 Water to be
evaporated
in dryness
(t/t of paper) 		Specific steam consumption
(t/t of machine production) 		Remarks
With closed
hood & good
steam and
condensate
system 		With open
hood & good
steam and
condensate
system

35
1.72
2.24 		2.75 Present

40 		1.38 1.80 2.21 Scenario

3 		45 1.11 1.45 1.78 Recommended
50 0.90 1.17 1.45 Scenario


The off-press dryness of 50% can be achieved by extended nip press/shoe press. Pulp or paper web should not be overdried by evaporation rather it should be dried maximum by efficient pressing.
Hence, new plants should plan with extended nip/shoe press.
Condensate removal system

Proper condensate removal system with the latest design, high speed separation efficiency, stationary siphon improves the evaporation rate at the paper machine drying section.

Selection of rotary or stationary siphons is dictated by the machine operating speed. Modern high speed paper machines are with stationary siphons. Design of stationary siphons has advanced to such levels that differential pressure can be kept to the minimum for efficient evacuation of the dryer cans. The amount of blow through steam is dependent upon the siphon differential pressure required for efficient evacuation. The lower the differential pressure, lower the blow through steam and higher the overall thermal efficiencies. By choice of the right stationary siphons with low differential pressure requirement, thermal efficiencies can be improved.

At higher machine speeds, the thickness of the condensate rim dictates the heat transfer rates for drying. Lower the rim thickness, higher the heat transfer rates and lower the steam consumption. Siphon clearance dictates the rim thickness. Modern stationary siphons can operate at low siphon clearance.

Breaking the condensate rim by means of spoiler bars can significantly improve thermal efficiencies in drying. With a good steam and condensate system and good siphon design, steam requirements can be brought down to 1.3 kg steam for every kg water evaporated from the paper web. Inefficient systems can be as high as 1.8 kg steam for every kg of water evaporated.

Hence, it is recommended to install stationary siphon with high separation efficiency condensate system.

Closed hood system

A well designed hood system will reduce the heating requirements at the drying section as high as 35%. An automatic hood control system can be installed at the suction of the exhaust fan to have better results.

Modern totally closed hoods can operate with dew point as high as 65oC. This would mean that the exhaust air and supply air volumes can be brought down quite substantially, thereby saving on drive power for the fans. Further, since the same amount of heat is available in a smaller quantity of air, this exhaust air is better utilised for heat recovery because of higher differential pressure at the heat recovery units.

So, a well designed closed hood system and automatic hood control at the suction of the exhaust fan is recommended.

Type of drive
In the present context of highly reliable AC drives in the market, it is suggested to install A/c drives for the paper machine main drive.

Vacuum pump

Vacuum pumps also consume considerable energy in paper machine area. Some of the simple aspects to be taken at design stage are

  1. Selection of energy efficient vacuum pump.
  2. Segregation of low vacuum and high vacuum levels.
  3. Correct sizing and routing of pipes to minimise losses.

Vacuum pumps shall be limited to minimum possible numbers and to be provided with large capacity motors with double ended shaft and of synchronous motor design.

Hence, correct size vacuum pumps have to be selected to optimise power consumption.

In recent times, mills abroad are using centrifugal exhausters for vacuum generation at paper machine area. These centrifugal exhausters are energy efficient by 30% compared to water ring vacuum pumps.

Conventional liquid ring vacuum pumps are fixed volume/variable vacuum intensity devices and, therefore, have constant power characteristic.

Centrifugal exhausters are variable volume/fixed vacuum intensity devices and, therefore, have a varying power characteristic.
Paper machine vacuum systems are characterised by variable air flow requirements in the life cycle of the machine clothing. Liquid ring vacuum pumps will consume the same quantity of power through this life cycle. Centrifugal exhausters follow the system characteristics and, thereby, offer savings in power (lower power when air flow requirements are on the lower side during the felt life cycle).

Hence, a centrifugal exhauster can be selected, instead of vacuum pumps.

Moisture control system

An online moisture control system is very helpful to monitor the moisture to be maintained at the paper machine (immediately after the drying cylinders). The advantages are.
  1. Better control on the moisture.
  2. Avoids over drying, saves steam and increases the production.
Hence online moisture control is suggested.

General mill-wide design features

The following general mill-wide design features have to be considered while designing a new plant

Transport

The plant should be designed with minimum transport of raw material, chemicals, fuel, steam and water.
Pipe lengths

Steam water, chemicals and pulp pipe lengths should be minimum to minimise temperature loss and pressure losses. Plant layout should take care of this.

Source of Power for New Pulp and Paper Mill

Cost of electrical energy being about 25% of the manufacturing cost of paper, it is of vital importance to have a correct choice of inplant generation and utility drawal of power. At present, the grid power cost exceeds the inplant generation cost in most of the cases. The 100% inplant generation is a better option for mill design, considering the utility scenario obtained in the country, with its vagaries, interruption in availability and higher cost of grid power.

Pumps & fans

Over-sizing of pumps and fans should be minimised and energy efficient pumps and fans need to be selected. A detailed write-up on the selection and energy conservation aspects to be considered in pump and fans are highlighted in Chapter #6 and #7 respectively. A margin of 10% is considered adequate at design stage.

Installation of variable frequency drive (AC drive)

The concept of installation of variable frequency drive (VFD) is picking up in paper industry. VFDs are recommended wherever speed variation is required from 50 to 100% range. Several mills abroad are operating with VFD for all the pumps. Indian paper industry should also explore the possibility of installing VFD for different equipment.

The suggested areas where VFDs can be considered in paper industry are as follows

i) Recovery Boiler

a) Installation of VFD for forced draught fans
b) Installation of VFD for induced draught fan

ii) Washing, Screening and Bleaching

a) Replacement of eddy current drives with VFD for drum washers drives
b) Installation of VFD for primary, secondary and tertiary centricleaners, pumps of unbleached/bleached pulp.
c) Installation of VFD for all dilution pumps of unbleached and bleached sections.

iii) Stock Preparation

a) Installation of VFD for stock pumps to blending chest
b) Installation of VFD for machine chest pumps

iv) Paper Machine
a) Installation of VFD for fan pumps
b) Installation of VFD for secondary and tertiary centricleaner pumps
c) Installation of VFD for mould fan pumps
d) Installation of VFD for save-all clarified water pump
e) Installation of dual speed motors for couch pit and press pit agitators
f) Installation of VFD for MG machine/MF machine hood fans
g) Installation of VFD for coating knife edge blowers
h) Replacement of small steam turbines with DC drives or AC motors with VFDs

v) Raw Water/Recycle Water Pumps
a) Installation of VFD for raw water/recycle water pumps
vi) Effluent Treatment Plant
a) Installation of VFD for roots blower (for agitation purposes)
b) Installation of VFD for final effluent discharge pumps

Motors

Over sizing of motors to be minimised and energy efficient motors are to be planned at the design stage for the complete mill. Energy efficient motors will give a 2% saving in the energy input when compared to the motors that were hitherto being used.

High pulp consistency operations

Higher the pulp consistency, lesser will be the pumping requirements in the mill. Hence, it is advisable to design the mill with higher pulp consistencies wherever possible.

Insulation

Proper insulation of steam lines, condensate lines, digesters, paper machine sides, black liquor piping and tanks should be given due consideration at design stage to, to effect significant energy savings.

Condensate recovery

The plant should be designed and operated with maximum condensate recovery to improve the fuel economy and also reduce water consumption.

Trapping system

Selection of proper trapping system saves steam considerably. Selection of traps for a given application is highlighted in Chapter #4.

Utilise natural light

The building should be designed with adequate natural light to minimise artificial lighting during day time.

Use of energy efficient lamps like high pressure sodium vapour lamps will result in a 40% reduction in energy needed for lighting.
Power factor improvement|

The power factor in the industry should be maintained at 0.95 or better, to save the maximum demand drawn from the grid and also obtain effective usage of inplant generation. The reduced demand on the grid not only reduces energy bill but also saves the national grid from the "wattless" current and increased utilisation capacity of their equipment. The capacity of the electrical equipment in mills is utilised better with enhanced power factor. The cost of installation of capacitors to improve the power factor at 0.9 is paid back in less than a year.


Introduction

Target Energy Consumption
Specific water consumption break-up
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