In a globally competitive environment, North American mills are challenged to keep pace with newer mills that constantly raise the bar defining a "world class" facility. Preserving world class status means investing in technologies that ensure low-cost, efficient operations providing the highest product quality for customers, while also addressing regulatory and environmental concerns.
According to Bowater Newsprint vice president and resident manager Howard Johnson, this effort to achieve world class status led to the latest round of modernization projects at the Calhoun, Tenn., newsprint and market pulp mill. Since 1997, the mill has invested approximately $180 million for three major projects: a wood yard upgrade (see Pulp & Paper, October 2000), conversion of an idle recovery boiler into a low-odor, bubbling fluidized bed boiler, and the addition of two thermomechanical pulp (TMP) lines.
"This mill needed these projects to establish us as a world class facility, and that is what we certainly consider ourselves today," states Johnson. "Since 1987, we have invested more than $1 billion to modernize the mill. We make a high quality product at competitive prices. We are in a commodity market business, and we have to remain competitive to stay in business long term."
With the new projects, Johnson reports that the mill was able to restructure, meaning savings in labor costs. Also, the renovated boiler allowed burning of sludge that previously was landfilled. However, the two new TMP lines, supported by a better, more consistent chip from the wood yard, have lowered production costs while maintaining high product quality, fulfilling the world class definition.
Bowater's two new TMP lines, sized for 407 a.d. tpd each, utilize Andritz retention-temperature-speed (RTS) technology for primary refining.
DEFINING AND EXECUTING THE TMP PROJECT. As the largest North American producer of newsprint, Bowater's Calhoun mill makes 2,150 mtpd of paper, along with some specialty products and 545 mtpd of bleached hardwood market pulp. Built in 1954, the mill now has five paper machines.
Although Bowater had six TMP lines installed in 1979 to provide around 45% of its newsprint furnish, it was still using about 20% groundwood, along with 18% pine kraft and 15% recycle. In late 1997, Bowater conducted a definition study for a TMP project that would replace the remaining groundwood content in the sheet. At that time, the mill pulled together an engineer-procure-construct (EPC) package for release to potential contractors. Simultaneously, the mill was researching equipment vendors for the TMP equipment.
One of the companies receiving the EPC bid packages was Kamtech, a construction firm Bowater had worked with in the past. To bid the contract, Kamtech formed a joint venture with Simons Engineering (now AgraSimons), another firm that the mill had worked with previously. In March 1998, Bowater awarded the EPC contract to the Simons/Kamtech joint venture. By that time, the mill had decided on Andritz as the supplier of TMP equipment, stipulating its designation as a subcontractor in the EPC contract.
Manager of pulping operations Randy Brock notes that the choice to use Andritz TMP technology was a "close decision" resulting from visits to mills in North America and Europe to view different technologies. He reports that the decision was ultimately based on the "mechanical integrity" of the Andritz design, where the refiner shaft is "simply supported by a bearing on either end."
Scope of project and execution. In conjunction with Andritz, the Simons/Kamtech joint venture provided all EPC activities for the project, including layout and sizing of the equipment. This equipment included the high-density storage tanks, chemical supply system, tanks, pumps, DCS, and electrical supply, as well as a Sunds bleach plant and Krupp-Robbins tube belt conveyor for chips. Also, an Andritz twin wire press was installed as part of the EPC contract, outside of Andritz's work as subcontractor on the TMP equipment.
Construction began with the demolition of a parking lot in April 1998 and ended with startup of the second TMP line-No. 8-in July 1999. With the wood yard upgrade occurring at the same time, coordination between the mill and its contractors was crucial to completing the project on time and not interfering with existing operations. However, according to Johnson, the project "started up extremely well." He also notes that Bowater was able to "hold the line very well on costs."
Training. Bowater's existing TMP plant (lines No.1 through No. 6) was operated by a crew of six (one shift leader and five technicians). For the new lines No. 7 and No. 8, the mill added two technicians to the existing crews so that all eight lines could be operated by a crew of eight (one shift leader and seven technicians). These eight additional technicians (two per crew) were taken from the old groundwood operations.
Bowater utilized a company that specializes in developing training programs, Claymore and Assoc., to work with three of its TMP technicians to create training documents for the new TMP process. Technicians and vendor representatives used this documentation, along with vendor information, to conduct classroom training in March, April, and May of 1999, prior to startup of the first new line in June.
According to Brock, the mill's TMP technicians had used DCS controls since the mid-1980s, and training focused mainly on the new equipment and process. He notes that "automation was not a new issue for our technicians."
TMP LINES. The two new TMP lines were sized for a design rate of 814 a.d. tpd (a little more than 400 a.d. tpd each). Target freeness for TMP is 85 to 100 Csf, while target brightness is around 59 ISO.
The majority of the pulp from the new TMP lines is used to make newsprint on the No. 1, No. 3, No. 4, and No. 5 paper machines. However, 300 tpd is bleached to higher brightness and used to make Bowater's specialty newsprint insert grades on the No. 2 machine.
Chip washing system. Bowater's primary fiber source is southern/loblolly pine, which is mixed with around 24% Virginia pine and 9% short leaf pine. The mill's modernized wood yard produces 100% of the chips for the new TMP lines (see Pulp & Paper, October 2000).
Chips are transported by a new tube belt conveyor to two 600-ton capacity silos. Chips from the No. 1 silo flow into one chip washing system, while chips from the No. 2 silo flow into another. Each Andritz R60 rotary chip washer/trash separator removes sand, stones, metal, and other contaminants. Metered chips are sprayed with hot (140º F) recycled wash water as they enter the washers, which normally operate at 4% to 5% chip/water consistency. Using such hot wash water elevates the temperature of chips, softening them for grit removal. Bark, knots, and other heavy materials migrate into the waste chamber of each chip washer.
As each chip waster discharges chips and wash water, a sump feeds them to a chip pump of recessed impeller (vortex) type. This pump takes the chips to an Andritz inclined twin screw chip drainer, located at the top of the TMP building, which eliminates the use of a pneumatic blowing system or belt conveyors.
Each chip drainer has two parallel screws in close clearance with a screen plate so that excess water can drain through the screen and collect in the base. To avoid plugging, the screen back is periodically washed with a small amount of fresh water.
Dirty wash water from both chip drainers passes through Andritz Hydrasieves mounted above the wash water sand settling tank. Each of the seven Hydrasieves functions to recover fibrous material from the wash water and features a three-slope, self-cleaning system with a 0.040-in. screen. Recovered material is discharged from the screen face into a junk thickener.
From the Hydrasieves, screened wash water flows by gravity to a sand settling tank, which is designed to facilitate grit and sand removal from the filtrate. Wash water overflows from this tank into a standpipe and is pumped from there to the chip wash water Centri-Cleaners for removal of the remaining contaminants. This debris is discharged via the rejects headers for effluent treatment, while the cleaned water returns to the chip washer wash water system. Thickened fines from the Hydrasieve screens proceed to a junk thickener for removal of solid waste material. After chip washing, chips are at around 130º F to 160º F and have gained approximately 7% in moisture content.
Pre-steaming of chips. There are two lines of mainline refiners fed from two lines of chip washing. Chips discharge from two twin screw drainers into a chip bin that is designed with 15-min. retention time for atmospheric preheating and uses two variable speed discharge screws. The bin bottom allows direct injection of low pressure clean steam, or dirty refiner-generated steam, to heat the chips to around 180ºF and remove entrained air. The level control on the bin controls the rate of chip feed into the chip washing system.
The presteaming bin discharges into two light-duty plug screw feeders, which provide the system a low pressure seal and reduces the entrained air content of chips, squeezing out some water as well. This air removal reduces brightness loss during chip heating and improves heat transfer to them. Discharge from the feed is at 20 to 25 psig.
From each plug screw feeder, chips drop into a pressurized leveling conveyor that controls retention time, which can vary from five to 20 seconds. This low pressure retention heats the pine chip matrix so cell walls soften, providing resilience to compression after they are discharged by conveyor into the Impressafiners.
Each pressurized Impressafiner is a multiple-screw device that slightly macerates chips to delaminate and separate cell walls while squeezing out water. Its axial compressive action reduces entrained air content to around 2% by volume.
From the Impressafiners, an atmospheric inclined drainer conveyor discharges chips into the plug screw feeder, where they are again compressed, dropping through an inlet tee into a stream splitter conveyor. The stream splitter is a dual screw conveyor that divides the incoming chip flow into two equal rates to feed the refining zones of each refiner. At the primary refiner inlet, Brock says that chips enter the refiners at 65 to 90 psi and around 240º F.
Refining. The primary refiners are Twin 66 refiners, direct coupled to 34,000-hp, 1,800-rpm Siemens motors with a speed increasing gearbox for 2,300 rpm refiner operation. This refiner type combines two refining zones within a single frame and is equally loaded on each side of a single refiner disc by an adjustable hydraulic force applied to the movable plate holders on each side of the machine.
Chips enter each primary refiner via load sensing conveyors, are conveyed down the refiner's shaft mounted ribbon feeders, and enter the refining plates. The ribbon feeders convey the chips to the refining zone, allowing a portion of the refiner-generated steam to exit back through the incoming stock flow without upsetting the incoming chip feed.
The refiners utilize Andrtiz's RTS (retention-temperature-speed) TMP process. However, at startup, the primary refiner for TMP line No. 7 operated only in standard TMP mode. At that time, it was not equipped with the speed increasing gearbox that boosted motor speed to 2,300 rpm and allowed the RTS process as on line No. 8. Brock explains that, though pilot tests had been favorable, Bowater wanted to fully assess the effects of RTS refining on southern fiber-a new application of the technology-by comparing standard TMP versus RTS processing.
"Initially, we discovered that the RTS fiber length was shorter," describes Brock. "However, through experimenting with refining plate designs and by optimizing chip pretreatment, we were able to reduce this difference so that fiber lengths are now only a fraction shorter than in TMP mode. This September, we installed the speed increaser on the primary refiner for line No. 7 so that we could take advantage of the energy savings from the RTS process, as on line No. 8."
For chip pretreatment in the RTS mode, Bowater decided to elevate pressures to 5 to 15 psi in the presteaming tube prior to the Impressafiner. In addition, compression ratios within the Impressafiner were increased. The mill and Andritz also experimented with different refiner plate designs to minimize impact on fiber length.
"Higher pressures and temperature in the tube soften the chip so more work can be done in the Impressafiner," explains Brock. "Combining that with the increased compression ratio in the Impressafiner means that the chip is now more resilient to applied energy, so high-speed refining has less impact on fiber length."
For RTS mode, Brock reports that the primary refiners run at a pressure of 90 psi-about 25 psi higher than in TMP mode.
The primary refined stock and the balance of steam from the refiners are blown to a pressurized cyclone for steam/fiber separation. The pulp is then divided into two equal flows in the secondary steam splitter conveyor and fed into the secondary Twin 66 refiners via load sensing conveyors.
The secondary refiners and stream splitter conveyors are identical to the primary refiner systems with the exception of the speed increasing gearbox for RTS on the primary refiners. Steam again exits from the inlet chamber by passing back through the ribbon feeders.
As with primary pulp, secondary pulp discharges to a pressurized cyclone, mounted on a swept orifice discharger, for steam/fiber separation. Fiber is conveyed by steam through the discharger into the latency chest, where it is diluted to 4% consistency. Designed for a 45-min. retention time at the 814 a.d. tpd operating rate, the chest maintains a high temperature (176º F to 185º F). Pulp from this chest goes to the screening system.
Screening. Mainline screening is a P1/P2 configuration and the process is carried out at low (2.2%) consistency. There are two P1 screens in parallel with 0.075-in. conically-drilled holes. The two P2 screens have 0.010-in. wedgewire slotted baskets. The total reject rate by mass from the mainline screening system is designed for 50%, meaning that P1 screens will reject 35% by mass and the P2 screens will reject 25% by mass.
In each line, the refined stock is pumped to the two P1 primary screens. Accepts from the P1 screens flow to the two P2 primary screens. From there, P2 primary accepts feed forward to GL&V disc filters for thickening and storage. Rejects from both the P1 and P2 primary screens are collected and become part of the feed to the rejects refining system.
Rejects refining system. Unrefined rejects from the P1 and P2 screens, along with those from the R1 reject screen, are collected and then feed pre-thickening Hydrasieve screens. These sidehill screens are similar to those in the chip wash area, although the 0.020-in. screen plate is smaller.
Thickened pulp flows down a chute to the unrefined rejects chest, which collects approximately 529 a.d. tpd for a total reject rate of just over 65% According to Brock, Bowater's robust rejects system was needed due to pulp strength considerations.
"We sized the reject system for southern fiber, as ample specific energy is necessary to improve pulp strength," Brock describes. "The reject system was sized to handle 65% of the throughput through the main lines."
Stock is pumped at 4% consistency from the unrefined rejects chest to two dewatering screw presses where stock is dewatered to about 30% consistency. Each press is dedicated to a single rejects refiner. The thickened rejects are next conveyed to two parallel Andritz SB-170 refiners using an atmospheric leveling conveyor. These refiners are fed through side-entry plug screw feeders.
The SB-170 refiners are direct-coupled to 24,000-hp, 1,800-rpm Siemens motors and are designed for low-intensity refining of rejects pulp under pressurized conditions and at high consistencies. Refining consistencies are maintained at 40% to 45% at the discharge.
As with pulp from the primary and secondary refiners, rejects pulp is discharged from the refiner to a pressurized cyclone mounted on a swept orifice discharger and then to the refined rejects chest. There, it is diluted to about 4% consistency. This chest is designed for a 45-min. retention time and temperature of 176ºF to 185ºF. Pulp from this chest is pumped to rejects pressure screens. Dirty TMP steam generated by the rejects refiners is routed to the mill's heat recovery system.
New equipment included (clockwise from left) Twin 66 refiners for primary and secondary refining, a screening system with a P1, P2, R1 configuration, SB-170 reject refiners with 24,000-hp motors, and a 3.85-m twin wire press for post bleach washing.
Reject screening uses two low-consistency screens in parallel, each equipped with 0.010-in. wedgewire slotted baskets. Total reject rate by mass from this screening system is 15%. Accepts from the two R1 screens flow forward to the GL&V disc filter for thickening and storage. Rejects from these screens are returned to the rejects refiners. According to Brock, there are "no rejects to the sewer."
Heat recovery system. Dirty TMP steam from the primary and secondary refiners is routed to an Ahlstrom heat recovery system installed by Simons/Kamtech. As Brock explains, the steam first goes to a scrubber for steam/stock separation and on to a reboiler to produce clean steam for the black liquor evaporators.
"The stock goes one way and the dirty steam goes another, traveling out a large tube and shell heat exchanger called a reboiler," Brock describes. "We supply feed water to the reboiler and out of it comes clean steam, at 36 psi. That steam provides BTUs to our evaporators. We make 100,000 lb/hr of steam from the reboiler, and most of the evaporation is done by heat recovery from the TMP process."
In addition, a new Ahlstrom turpentine recovery system was installed by Simons/Kamtech. It is sized to recover 0.7 gal of turpentine per a.d. ton.
Pulp bleaching. Accepts from the screening system and the rejects system are sent at about 3% consistency and mid-50s ISO brightness to the deckers, where they are thickened to about 9% consistency and then sent to one of two 300-ton storage chests. Either storage chest can feed either the newsprint bleaching system or the bleaching system for specialty grades-both from Sunds.
For the newsprint bleaching system, pulp from a storage chest is sent through a retention tube, where it remains for around 15 min. and is bleached to 59 ISO with sodium hydrosulfite. It is then diluted to 4.5% consistency in a dilution tank and sent to a storage tank before transfer to the paper machines.
For the system that bleaches about 300 tpd of specialty grades, pulp proceeds through a retention tube, where hydrogen peroxide is applied, and then to a chemical mixer. It next goes through another retention tube, where peroxide is again applied, and then into the peroxide bleach tower. Pulp stays there around 3 hours to reach brightness levels of 65 to 73 ISO.
From the bleaching tower, stock moves to a 3.85-m Andritz twin wire press where pulp is thickened to 22%. A bleach press dilution conveyor takes the thickened pulp to a dilution tank, where consistency is reduced to 4.5 %. The pulp next flows into a storage tank before transfer to the paper machine.
Operations and controls. The new TMP process is controlled using a Moore distributed control system (DCS) running Intellution software, which was customized for Bowater by Simons/Kamtech.
A Pulp Expert fiber analyzer (automated pulp sample analysis) is used throughout the process to measure pulp properties, including freeness, fiber length, coarseness, and shives. This information feeds into a separate computer system, allowing historical analysis of properties.
RESULTS: LOWER MANUFACTURING COSTS. As a result of the wood yard upgrade and TMP project, Bowater has seen improvements in manufacturing costs, as well as overall process stability. The new TMP has allowed a reduction in kraft pulp use without compromising quality. Also, the mill has been able to substantially restructure its workforce with the new TMP plant as compared with groundwood operations. To Johnson, the recent $180 million dollar investment, as well as the other capital spent in the recent past, has brought the Calhoun mill to world-class status in terms of quality, productivity, and manufacturing cost.
Cost-effective furnish. Brock reports that the high quality of the new TMP pulp has allowed a cost-effective furnish mix for the mill that does not compromise newsprint sheet properties. For example, compared with the groundwood furnish, the new TMP's pulp burst factor is about 80% higher. Also, compared with the TMP from the older lines No. 1 through No. 6, TMP from lines No. 7 and No. 8 has a 15% higher burst strength and a 10 point higher tear strength.
The quality of the new TMP replacing the groundwood furnish has allowed the mill to cost-effectively reduce the amount of pine kraft pulp by 6% without impacting sheet properties.
"We are able to get a stronger furnish from the new mill, primarily because the of the reject system capacity, which allows more pulp to see refining," notes Brock.
Further, process stability as compared with the 1979-vintage TMP mill is much improved.
"The refining is much more stable with the new TMP versus the old," notes Brock. "With the old TMP plant, a refining line might trip out and go off production two or three times in an eight-hour shift. With the new mill, we might run for days without a trip. At times, we trip the new line purposely to stop production and blow the lines out with steam to remove pulp build up in the blow lines."
Productivity. Although eight technicians were added to overall TMP operations as a result of the two new lines, there had been a total of 64 employees working in groundwood prior to the project, so there was a total downsizing of 58 workers in mechanical pulp. With the wood yard, boiler, and TMP projects, the mill was able to downsize by about 190 full-time employees. However, Johnson points out that much of this was accomplished by attrition and by a hiring freeze, with only about 60 permanent people impacted in the end.
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