Title: Opportunities to Improve Energy Efficiency on paper machine Dr. Vivek Kumar Department of Paper Tech
1 Opportunities to Improve Energy Efficiency on paper machineDr. Vivek KumarDepartment of Paper TechnologyIIT Roorkee Saharanpur Campus 2 Introduction
The paper mill area is a major energy consumer in most integrated pulp and paper mills.
Energy consumption and cost per ton vary widely on paper machines.
Mills have different energy costs based on mill configuration vintage of papermaking equipment geographic location and other parameters.
Each mill needs to develop an understanding of energy cost factors before preparing an effective energy reduction program.
Techniques for improving energy efficiency on paper machine i.e. Forming Section Pressing section Drying section.
Optimize performance of equipment.
Use steam water air and vacuum pressures appropriate to processes.
Minimize energy losses.
Minimize water evaporated in the dryers.
4 Forming section 5 Vacuum system
The vacuum system is among largest process in the paper mill with respect to space requirements piping and energy usage. Vacuum systems can have installed and required energy of 500 to 8000 hp.
Several steps are required to guarantee the vacuum system is operating as efficiently as possible.
Vacuum system audits or surveys.
Routine maintenance of vacuum pumps and auxiliary equipment including belt and gear drives and motors
Replacement and calibration of gauges and process instrumentation
6 How do we control or minimize wasted energy
Vacuum system audits
Once identified some vacuum system problems may cause higher energy usage costs and inefficiencies for the entire system.
Hot seal water.
High seal water flow due to unregulated or high seal water pressure.
Back pressure on the vacuum pumps due to system design or operational problems.
Synchronous vs. induction motors which may affect the power factor.
7 Vacuum system........
Elevated seal water will reduce vacuum pump efficiency and will lower vacuum system capacity.
However there are methods to reduce seal water consumption without sacrificing vacuum pump performance. This can involve a water recovery plan and/or cooling the water with a cooling tower. It is important to note that special dirty-water cooling towers have been successful in this application.
8 Vacuum system........
We must have to maintain a vacuum system in such a way that it should not have more than 1 psig of back pressure measured at the pump discharge.
Also we have to maintain the system clean and the scale build up as minimum as possible.
In systems where a discharge sump is used high water levels within the sump can cause back pressure as well as contribute to poor air/water separation within the sump.
9 Vacuum system........
MOTOR SIZE/VACUUM PUMP HORSEPOWER
Another issue involving motors is the selection of the motor size with respect to the vacuum pump horsepower. Many mills elect to install oversized motors on equipment to reduce spare parts and storeroom inventory. Many vacuum pumps are installed and driven with a motor from an old refiner that may have 50 more horsepower than the vacuum pump requires.
A frequent practice is to replace a correctly sized drive motor with a larger size to eliminate overloading and trip-out problems. This is an example of correcting the symptom rather than solving the problem.
10 Vacuum system........
MOTOR SIZE/VACUUM PUMP HORSEPOWER
Older inefficient vacuum pumps versus new modern designs
Process carryover and high seal water flows
Piping losses or poor system design.
11 Vacuum system........
Newer larger pumps can reduce the quantity of pumps on a paper machine by one half.
Scale or calcium carbonate buildup will increase vacuum pump horsepower requirements by as much as 20 to 30 and often locks up the rotor within the vacuum pump.
Water treatment and descaling programs are useful in the prevention of scaling. Recirculated seal water systems with cooling towers have experienced better control of the water quality for the vacuum pumps.
13 Vacuum system........
In one audit it is mentioned as the total vacuum system power consumption was 3040 hp. Allowing for performance tolerances and drive losses the system should require only 2775 hp. This excessive power consumption of 265 hp was due to several factors affecting. The problems included overloading due to high seal water flows and internal scaling.
Vacuum pump operated at 11.3 in. Hg
Highest vacuum level of the was 7.0 Hg. The extra horsepower to operate the vacuum pump at 11.3 in. Hg required an additional 34 hp
Another vacuum pump was requiring 36 hp more than necessary due to the incorrect model being installed. This was a high vacuum model designed for more efficient operation at above 15 in. Hg. However the pump was on the low vacuum press zone and operated at only 7 in. Hg. A low vacuum model needed to be installed to reduce horsepower.
By making some changes in felt conditioning there was an opportunity to save an additional 108 hp.
15 Replacement of Existing Vacuum Pumps
In Paper machine No. 1 two vacuum pumps (capacity 1800 m3/hr rated vacuum 400 mm Hg each) are provided and having common header (i.e. for Flat Boxes uhle Boxes) and consuming 107 kWh
As the suction vacuum requirement is only 200 mm Hg. It is recommended to install a new vacuum pump of higher capacity (rated suction vacuum 300 mmHg capacity 3500 m3/hr) in place of existing 2 vacuum pumps which will consume about 65 kWh. Saving potential is 107 65 42 kWh or Rs. 8.05 lacs/annum against investment of Rs. 3.00 lacs.
In Paper machine No. 2 two vacuum pumps (capacity 1800 m3/hr rated vacuum 400 mmHg each) are provided and having common header (i.e. for Flat Boxes uhle Boxes) and consuming 77 kWh.
As the vacuum requirement is only 200 mmHg. It is recommended to install a new vacuum pump of higher capacity (rated vacuum 300 mmHg rated capacity 3200 m3/hr) in place of existing 2 vacuum pumps which will consume about 56 kWh. Saving potential is 77 56 21 kWh or Rs. 4.02 lacs/annum against investment of Rs. 3.00 lacs.
In paper machine No.4 one vacuum pump (KVM 900 capacity 3600 m3/hr Rated vacuum 500 mmHg) has been installed for uhle Box which is consuming 104 kWh.
As the requirement of suction vacuum in uhle Box is around 200 mmHg. It is suggested to convent it as per the specification i.e. Capacity 4125 m3/hr Rated vacuum 300 mmHg. At these specification the power consumption will be about 70 kWh. Therefore saving potential is 104 70 34 kWh resulting in saving of Rs. 6.5 lacs/annum against investment of Rs. 5.00 lacs.
In paper machine No.5 one vacuum pump (7LV Capacity 1410 m3/hr Rated vacuum 660) has been installed for flat boxes which is consuming 48.7 kWh power.
As the requirement of suction vacuum is only 200 mmHg in flat boxes it is suggested to replace existing vacuum pump with KVM 300 (Capacity 1500 m3/hr Rated vacuum 300 mmHg) which will consume only 24.5 kWh power. Therefore saving potential is about 48.7-24.5 24 kWh resulting in saving of Rs. 4.6 lacs/annum against investment of Rs. 2.00 lacs.
In the paper machine No.7 four vacuum pumps (all KVM 600) are running continuously and having common header for Couch suction press flat boxes uhle box. Requirement of suction vacuum in Couch and suction press is high i.e. upto 400 mmHg and in Flat Box and uhle Box it is less i.e. 200 mmHg. All the four pumps are consuming about 273 kWh power.
As the requirement of suction vacuum is less in case of Flat Boxes and uhle box it is recommended to fabricate separate lines for two vacuum pumps and reduce the rpm 500 from present 550 rpm. A saving potential of 14 kWh is estimated against reduction of rpm resulting in saving of Rs. 2.7 lacs with an investment of Rs. 50000.
Improvement Effect(A) Rs. 32.96 lacs/annum
Investment (B) Rs. 13.50 lacs
Simple Pay Back (B/A) 5 months
21 Energy Efficient Formers
Twin wire forming is new principle had the advantages of avoiding the free surface between mix and air of increasing the dewatering capacity by allowing dewatering capacity through two wires.
The forming sections is very short and the formation takes place in a fraction of the time it takes for a Fourdrinier machine.
Produces a paper of equal and uniform quality at a higher rate of speed. Coupling the former with a press section rebuild or an improvement in the drying capacity increases production capacity by as much as 30.
Energy savings come from reduced electricity consumption . The technology also may improve quality and electricity savings of 41 kWh/t of paper.
23 High consistancy forming
In high consistency forming the furnish (process pulp) which enters at the forming stage has more than double the consistency (3) than normal furnish. This measure increases forming speed and reduces dewatering and vacuum power requirements.
Electricity savings are estimated at 8 that is about 41 kWh/t of paper.
24 Pressing section 25 PRESSING ARRANGEMENTS
Drying the sheet of paper using steam heated dryers is a lot more expensive than using efficient presses. The primary function of the press section therefore is to remove as much water from the sheet as possible consolidate the sheet and improve the surface smoothness.
26 Extended nip press (Shoe press)
The additional pressing area allows for greater water extraction (about 5-7 more water removal) to a level of 35-50 dryness.
This technology reduces the load on the dryer it allows plants to increase capacity up to 25 in cases where the plant was dryer limited.
Extended nip pressing also increases wet tensile strength.
27 Hot pressing
Pre-heating the water in the paper sheet before pressing can reduce the evaporation load.
Use of steam showers has been estimated to reduce the steam requirement by 1 kg of steam per kg paper. We estimate steam energy savings of 0.61 GJ/t paper through hot pressing.
Installation of Steam Boxes in the Press Sections
There are no steam boxes installed in the press section
With the Installation of steam boxes the following optimizations are possible
Increase of production capacity or reduction of steam consumption in the dryer section
Improve moisture profile
In general the design data of steam boxes are
Steam consumption approx. 80 - 120 kg steam / tonne paper (the value depends on the dryness of the paper)
Potential of warming up the paper web from 10 to 15 K
10 K increase of paper web temperature corresponds with an increase of dryness after the press section of about 1
1 increase of dryness after press section leads to 5 higher production or 5 less steam demand in the drying section
With the installation of steam boxes in the press sections of PM 4 and PM 5 the following steam reduction is possible-
PM 4 (3.26 t paper / hour)
Steam consumption 3.4 t/t 11 t/h
Steam saving in drying section 6 0.66 t/h
Steam demand steam box 0.3 t/h.
PM 5 (26 t paper / hour)
Steam consumption 3.9 t/t 10 t/h
Steam saving in drying section 6 0.6 t/h
Steam demand steam box 0.23 t/h.
Furthermore after installation of a steam box less evaporation (0.1 to 0.2 m³/h) occurs in the drying section which can reduce condensation in the pre-drier section of PM 5.
Improvement Effect (A) Rs. 21.0 lacs/annum in PM 4
(Steam reduction 2880 t/a)
Rs. 21.6 lacs/annum in PM 5
(Steam reduction 2960 t/a)
Investment (B) 100.00 lacs / steam Box
Simple Pay Back (B/A) 5 years
34 DRYING ARRANGEMENTS
To optimize energy use in the drying process it is essential to clearly identify the areas and actions that offer measurable opportunities for improvement. Reducing the evaporation load to the dryer section is a smart starting point. Pressing improvements are the best way to reduce dryer section steam use.
Improving machine efficiency is another obvious path.
Identifying losses from the dryer drainage system is essential.
The cost of high-pressure steam should be factored into dryer energy cost.
Minimizing the dryer air heating requirement.
36 Improper siphon size
Replacing rotary siphons with stationary siphons in dryers that discharge directly to the condenser offers potential for improvement. Stationary siphons are designed for a blowthrough steam flow of 10 versus 20 to 25 often required with rotary siphons.
Proper siphon design is a key to keeping the system tight.
37 Maximize the Use of Low-Pressure Steam
This allows the most electricity to be generated at the boiler house using the high-pressure steam to power an electricity-generating turbine.
Using dry low pressure steam is advantageous when considering energy efficiency due to high condensing heat rate.
38 Minimize Dryer Air Heating Requirement
Adjust the pocket ventilation air temperatures to match the drying requirements and hood design.
The hood must be well sealed and have good insulation properties to operate with low pocket ventilation air temperatures.
Air to air heat recovery systems on existing machines recover only about 15 of the energy contained in the hood exhaust air. This percentage could be increased to 60-70.
Paper machines with enclosed hoods require about one-half the amount of air per machine reduces thermal energy demands since a smaller volume of air is heated. Electricity requirements in the exhaust fan are also reduced. Steam savings of 0.76 GJ/t paper and electricity savings of 6.3 kWh/t paper by installing a closed hood and an optimized ventilation system is estimated.
Stopping of steam condensate leakages from the pocket ventilation system
The Pocket ventilation system for dryer group (II) (III) consists of steam heaters for heating the inlet air. There are two rows of steam battery which operates on the steam pressure of 3.5 Kg/ cm2 .
The condensate of steam is collected through bottom line in separate tank. It was found that there was lot of steam and condensate leakages and all area below Pocket ventilation system nearby is flooded with condensate.
It is recommended that steam condensate leakages should be stopped immediately.
ECONOMICS Cost of 1 m3 condensate Rs. 60 (including heat value) Improvement Effect Saving by stopping condensate draining (5m3 daily) Approx 60 5 Rs. 300 wastage Annual Saving 300330 Rs. 99000/- Investment Negligible Pay back Immediate 43 Closing of side panels doors of dryer hoods of both machines
It was observed that the lifting doors of both machines are always partially lifted and all basement area is open.
The closed hood is therefore working as semi closed hood and lot of air infiltration is takes place from machine house.
This reduces the drying efficiency of dryers and hence more steam is consumed for drying the same quantity of paper. It also reduces the efficiency of pocket ventilation system and also increases steam consumption in PV system.
Beside this the condition of side panels of hood was not proper resulting in heavy Infiltrations from sides .
The insulation of side panels were also in bad shape. All these reduce the efficiency of hood resulting in more power consumption at exhaust fans.
It is recommended that machine should run with lifting door in down position.
The doors should be lifted only at the time of paper break and should be brought down after rethreading of paper.
Also all infiltration should be stopped and insulation of side panels and roof should be repaired / replaced.
By proper insulation repairing and stopping infiltration of air in hood efficiency in drying will be improved.
By assuming 1 improvement in hood efficiency mill can save 0.24 t/hr steam resulting in saving of Rs. 15.20 lacs/annum as shown below.
1 improvement in drying 0.24 t/hr after reducing in steam consumption.
5.76T/day or 1900 T/annum
Savings Rs. 1520320/-
Investment Rs. 500000
Pay back 3 months
46 Insulation of Hot Surfaces in Paper Machine
During the audit at mill it was observed
that various pipelines and Paper machine
dryers having higher temperature are not
insulated resulting in high heat losses
through these uninsulated insulated
surfaces in the form of radiations.
It is suggested to put proper insulation on all bare surfaces to reduce the heat loss through radiation and save the energy wastage.
Detail of Uninsulated lines
By putting the insulation on the uninsulated surfaces enormous calories of heat energy can be saved which is equivalent to a saving of 2127 ton of steam per annum resulting in saving of Rs.8.51 lacs per annum with a payback period of 2-8 months for putting the insulation.
Improvement effect (B) Rs. 8.51Lacs/annum
Investment Amount (A) Rs. 4.28 Lacs
Simple pay Back (B/A) 2-8 months
49 Replacing of the missing air filter panels and cleaning of air filters .
It is recommended that air filter panel of PV system should be repaired and maintained in proper condition regularly. All missing filter panels should be placed in position and should be cleaned from dust.
By adopting this proposal clean air will be purged in pocket ducts of dryer reducing chance of dirty air to bleached paper.
This would improve the paper surface quality. Also the energy consumed by PV fan would reduces marginally.
51 Use of flash steam of main condensate tank in pocket ventilation system
The condensate of last group dryers are collected in the main condensate tank from where it is pumped to boiler house.
It was observed that the flash steam from main condensate tank is vented to atmosphere.
Without recovering its heat value. About 80 kg/hr flash steam is vented.
It is recommended that this flash steam should be used in the steam battery of PV system. Since the flash steam is not sufficient for PV system the live steam will also be used but the use of flash steam would reduce the use of live steam in PV system.
Assuming 2 tonne flash steam per day utilized in the PV system mill can save Rs. 3.96 lacs/annum by using it in PV system.
Installation of Heat Recovery System in Paper Machine 5
The pre-dryer section of PM 5 is equipped with a closed hood
54 Current Situation 17.530 kg/h 683C/512 14.250 kg/h 714C/435 29C / 333 Supply air for the hood coming from the hall 18.200 kg/h Filter by-passed Leak air from the hall into the hood external air polluted Exhaust air from the hood in the hall Exhaust air from the vacuum system into the hall 50C/ 99/ 853 g/kg 55
There is no heat recovery system installed in the exhaust air system. The supply air (120C) is heated by live steam.
The supply / exhaust air proportion (66 ) is within the range of recommended values.
The filter in the external supply air system is by-passed.
Furthermore wet exhaust air from the vacuum system is led into the basement. There is condensation in the hood when the hood is closed.
To make hood closing possible and thus increasing the quality of paper it is suggested to increase the supply and exhaust air mass flows step by step (target values supply air mass flow 28000 kg/h / exhaust air mass flow 42000 kg/h the target values might not be reached because of design of the existing air system).
This leads to an increase of electricity consumption but also to a reduction of steam consumption. It is estimated that these changes in energy consumption are not affecting the operational costs.
Furthermore it is recommended to lead the exhaust air from the vacuum system out of the hall and to lead the external supply air through the filters.
advances in drying
59 Impulse Drying
Impulse drying works by lowering the moisture content of the paper web entering the drying section by up to 38 percent. In a conventional papermaking operation the web has a moisture content of 50 to 60 percent as it enters the drying phase.
It is estimated that the operation of 65 impulse drying units in the year 2020 could save 13 trillion Btus of energy annually for the industry.
60 Infrared drying
Along with improved energy efficiency it increases the drying power output. Infrared dryers are powered by electricity and require about 4.08 GJ of electricity/t paper versus 8.16 GJ of steam/t paper for conventional steam dryers (Jaccard Willis.). We therefore estimate primary energy savings of 3.3 GJ/t paper.
Investment costs for infrared dryer installation are Rs.5203/t paper including OM costs requirements.
61 Condebelt drying
This drying technique has the potential to completely replace the drying section of a conventional paper machine with a drying rate 5-15 times higher than conventional steam drying.
For large machines savings of 10-20 steam are possible (1.6 GJ/t of paper) and a slight reduction in electricity consumption (20 kWh/t of paper). Capital costs are considered to be high although the size of the drying area can be reduced.
This provides opportunities to enhance energy economy. Energy for the Condebelt has three components primary steam used per kg of evaporated water recovery of the latent heat from the steam evaporated from the web and electrical energy.
Typical steam consumption is about1.31.5 kg per kg of evaporated water.
In the Condebelt process about 85 of the heat can be recovered. Thus the temperature of the pulp entering the headbox can be increased by 10C using the excess heat from the Condebelt.
The total electrical energy consumption in a Condebelt dryer is comparable to that found in conventional cylinder drying.
63 Air impingement drying
This technology can be combined with existing technologies. Heat input requirements have been modelled at 3 MJ/kg evaporated water or a 10-40 savings in steam requirements. Electricity requirements are expected to increase by 0-5.
This involves optimization of the geometry of the nozzle pattern nozzle type nozzle velocitydiameter nozzle to sheer distance.
64 Steam impingement drying
Steam requirements are estimated at 4.5 GJ/t paper with additional savings available if the latent heat from the purge steam is captured. De Beer estimates a savings of 10-15 with a slightly lower reduction in electricity requirements (5-10).
66 Avoid Idle running of motor of hood blower in paper machine
During the visit it was observed that motor of hood blower of paper machine was running idle without fan.
The shaft or connection between motor fan has been broken and motor runs continuously without fan for 24 hours.
It is recommended that proper maintenance should be taken care and all such system should be checked at least once a shift.
Assuming the motor running goes unnoticed for a day Loss in electrical energy 30kW X24 720kWh.
Assuming 30 load 216 kWh/ day
Annual saving 216 1.61 330
Savings Rs. 1.14 Lacs/annum
Pay back period with in one month
69 Motor considerations
Vacuum pumps are driven by induction and synchronous motors using v-belt drives and gear reducers as required.
The use of synchronous motors is based on reliability space considerations and elimination of the need for power transmission equipment (v-belt drives and gear reducers).
Also it is observed that some mills elect to use higher speed synchronous motors (1200 or 1800 rpm) and gear reducers that may offer cost savings over the larger slow speed synchronous motors.
In Paper Machine No.3 one vacuum pump (KVM-600) has been installed for S. Couch and drawing 90 kWh.
As per the manufacturer performance table motor should draw only 53 kWh against present 90 kWh. Therefore it is suggested to contact the supplier for the root cause analysis of higher Energy Consumption. Saving potential is 90 69 37 kWh resulting in saving of Rs.7.10 lacs /annum by maintenance of existing pump.
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