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The world’s largest LC refiner: The TwinFlo 72
“We have a goal in this development project of 750 kWh/t,” says Christer Sandberg, Development Project Manager for Holmen Paper. “Our old TMP line consumed roughly 2,250 kWh/t.”
It should be possible, says Göran Korsfeldt,
Production Manager at Holmen’s Braviken mill in Sweden. Braviken is the site of a new TMP production line that is unique in the world. “Separating fibers in the refining process consumes roughly only one-third of the total energy. The rest is just producing heat. If we can reach something like 800-1,000 kWh/t, then we will significantly lower our cost of producing pulp.”
The concept for the new line at Braviken is a combination of three process steps: RT-pretreatment, high consistency (HC) mainline refining, and low consistency (LC) secondary refining. Not only is the process unique, but the huge ANDRITZ TwinFlo LC refiner (72-inch) is a totally new machine and the largest in the world.
Laying the groundwork The groundwork for the development project started 10 years ago, according to Sandberg, when Holmen experimented with HC mainline refining and LC secondary and reject refining in the same line. “We began by using a mobile rig from the Swedish research institute here many years ago,” says Sandberg.
The results were so good that Braviken installed its first ANDRITZ TwinFlo refiner for LC in 2003. “The big advantage to LC is the low energy consumption,” Sandberg says. “It has about half the energy consumption for a given increase in strength properties compared to HC single-disc refining. Another advantage is that when we put the TwinFlo in the mainline before the screening room, our problems with screen plugging stopped.”
“The whole industry can benefit”The TMP development project received a grant of SEK 40 million (EUR 3.8 million) from the Swedish Energy Agency. According to Thomas Korsfeldt, Director General of the Agency at the time of the grant, “If this project gives successful results, the technology could be employed at other TMP mills in Sweden and produce annual electricity savings of around 1.5 TWh, which is between 1-1.5% of Sweden’s total electricity use.”
Because of the grant from the Swedish Energy Agency, most of the results Braviken obtains from the project will be public. “That is fine with us,” says Rikard Wallin, Mill Manager. “We built bypasses in this line that gives a highly scientific level to our work that you would not normally find in a running mill. I am really proud that the Holmen board of directors had the vision and courage to invest the money to do these extra things.”
Three industrial postgraduate students are also working on the project, each with a focus on a key part of the process: pretreatment, HC refining, and LC refining. “It is quite unique to have three industrial researchers of this caliber working on the same TMP line,” Sandberg says.
The new lineBraviken was one of the first mills to go completely with TMP technology when it was built in 1977. Formerly, there were three lines producing 1,300 t/d for three paper machines producing newsprint and directory paper. Now there are two with only a slight increase in production capacity. Capacity of the new line is 800 t/d. The mainline started in August 2008 and the LC refiner was brought online earlier this year.
“If you look at our installations, the main driving forces have been production increases and energy savings,” says Korsfeldt. “This development project with ANDRITZ is very much focused on quality and energy, not so much for a production increase.”
Matching size with a new designHolmen began discussions with ANDRITZ to inquire about a new LC refiner design. “The general trend for HC refiners is that they have become bigger and bigger,” Sandberg says. “Why not match the LC refiner size with the HC refiner and reduce the overall cost of the installation?”
ANDRITZ has been active in developing LC refining technology and gladly accepted the challenge to work with Holmen on producing a large LC refiner. “Energy will not be cheaper,” says Thomas Paar, ANDRITZ’s Project Manager, “so what is necessary is to drive energy consumption down. We were very positive and very excited to work with Holmen on this project.”
Producing the world’s largest LC refiner was not as simple as just scaling up a smaller design. “Each requirement such as gap measurement, rotor stability, sufficient mass, and precision hydraulic control had to be taken into account,” Paar says.
Adaptations to the massive (46 tonne) refiner are ongoing as the unit gains operating time. “ANDRITZ has listened to our comments and our input,” Sandberg says. “There is work to optimize the refiner plate design and to ensure rotor gap stability. Since this is the first LC refiner with gap measurement, we have a PhD student helping learn how to effectively utilize this measurement.”
“It’s possible that the TF 72 will produce even greater benefits on the rejects line than the mainline,” says Lennart Nilsson,Production Manager for TMP and Wood handling. “The refiner is very good, very stable, but I think it needs higher throughput. You can put much more load on reject fibers in LC than is possible on mainline fibers.”
“We have the flexibility to try these different ideas,” Sandberg says. “We will also look very carefully at chemical pretreatments. We are not in a hurry and will do this step-by-step.”
Pretreatment optionsAccording to Nilsson, one step that holds promise is the ANDRITZ RT-pretreatment process although it has never been operated on mill scale before with Nordic Spruce. “We have to investigate what benefits we can achieve with it,” he says.
According to Paar, the main benefit of pretreatment is an increase in long fiber content and tear strength. ANDRITZ delivered the entire pretreatment system including variable speed conveyors, presteaming bin, rotary valve, Impressafiner MSD, impregnator, and a buffer bin.
The goals of pretreatment are to soften the cell walls of the wood chips to ensure a gentle opening of the fibers and to eliminate potential damage to the fibers before processing the chips in the Impressafiner MSD. The Impressafiner MSD macerates the chips (to expose more surface area for chemical impregnation) and squeezes out pitch and other extractives.
“We will make a lot of trials with different chemicals,” Nilsson says. “ANDRITZ is modifying the screw in the Impressafiner MSD to increase the compression rate, which should improve pitch reduction and also energy savings.”
Impressive results already“I have been involved in seven big TMP installation projects in my career,” Nilsson says. “This was the best one as it was so easy to start. Within two hours from start-up the paper machine operators could see the difference in pulp quality. So, it was a success from the very beginning.”
“We knew that we would get a big boost in energy recovery, but we also had some pleasant surprises with the new line,” Wallin says. “The light scattering of the pulp is so high that we have been able to reduce the use of special pigments. We have been able to reduce retention aid chemicals on the paper machine. And, we have reduced the use of kraft pulp to almost nothing in the mill.”
Cut it in half?“My view is that we must find a way to drastically cut energy consumption – at least by half – to be profitable in the future,” Wallin says. “I don’t know how we’re going to do it, but we have to do it. We have to set aggressive goals – not 2% here and 3% there.”
Wallin finds it positive that ANDRITZ wants to be part of the solution. “We really want to work with them to make mechanical pulping competitive in an energy sense,” he says. “In this economy, it’s not a walk in the park for anyone, so we really appreciate the commitment and support.”
Platform for the future“This investment gives us freedom to develop this mill,” says Wallin. “We have a plant that can support conversion into any mechanical printing grade: news, improved news, SC paper, or coated mechanical if we want to go that way.”
Wallin thinks that from a practical point of view, “the front line of mechanical pulping research is in Braviken for the next two or three years. If the technologies here can be applied in other places, we can contribute to lowering energy consumption and emissions globally.”
Wallin’s team of managers and operators find this development work fun and exciting. “We all have children and grandchildren,” he says. “We think it’s a noble goal to produce desirable products at a profit, without harming anyone or harming the environment. If we can’t do that, we are not very good engineers or business people.”
