Stora Enso eyes dissolving pulp growth

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Stora Enso eyes dissolving pulp growth

September 22, 2014 - 07:06

Hard by the Russian border in eastern Finland, the history of Stora Enso's Enocell mill is intertwined with the politics of the region. Now the mill adds another chapter to its history. Its original fiber line (No. 1) has just been converted to produce dissolving pulp, although it had gone through various upgrades before the 2012 conversion project.

The onsite sawmill was constructed to help Finland pay reparations to the Soviet Union after World War 2. When the sawmill was opened in early 1950's, Enocell pulp mill's current managing director, Sauli Purho, says there were many joint ventures between the Soviet Union and Finland, most of which did not work and were bought back by the Finns. Enso-Gutzeit bought the sawmill some years later. The pulp mill was constructed in 1967. Originally, the mill could produce 100,000 tonnes/yr, using an 80-90% birch furnish.

In 1989, the decision was made to build a new fiber line using the recently introduced Sunds Defibrator SuperBatch pulping process. A saw and a planning mill were also included in the project as well as a new waste water treatment system that features an aerated lagoon and biological treatment.

The project created Enocell, which was a subsidiary of Enso (99%). The other 1% was owned by the Soviets. At startup in 1992, the mill was using a vast amount of Soviet birch. Purho adds that 20% of the equipment was supposed to be supplied by the Soviets but the 1991 breakup of the Soviet Union put an end to those plans.

In 1999, Swedish-based Stora and Enso Gutzeit merged to form Stora Enso. In 2005, Stora Enso bought back the Russian shares. (Purho says it took until then to find out who held the shares.)

In 2006, the mill produced its 10 millionth tonne of pulp. However, darker days lay ahead. Between March and September 2009, the mill closed due to poor market conditions.

Up until 2009, Enocell produced about 67% birch pulp and 33% softwood pulp. However, new Russian wood duties and increasing costs meant Enocell produced only softwood pulp in 2010 and 2011.

This brings the mill to its present day status. Fiber Line 2 is the 300,000-tonne/yr SuperBatch line producing softwood market pulp using a mostly pine furnish although some spruce is used. "We are very flexible with the batch cooking, "Purho says. "We can run softwood or hardwood chips - pine, spruce, aspen, eucalyptus. Batch cooking has definitely helped us adapt to market conditions."

About 30% of the paper pulp is sold as pure market pulp to Europe, China or other areas. The rest is destined for other Nordic paper mills.

A new era

Fiber Line 1 has been converted to produce 150,000 tonnes/yr of dissolving pulp from a 100% birch furnish, something Purho says is very rare. "It can be tricky because of birch's high hemicellulose content." The birch is sourced domestically and from Russia and supply is very secure, he adds.

Purho estimates it is probably a three-year project to successfully convert to dissolving pulp and bring it to market. To convert, the mill needed a lot of help from Stora Enso R&D division which is based in nearby Imatra. "We still work daily with them. Dissolving pulp is a totally different world from paper pulp and there is a learning curve," Purho says.

Converting market paper pulp mills to dissolving pulp has been a familiar story recently. For Stora Enso, it was really a step into the unknown. Purho notes that the company had no experience in the sector. "We learned together with the operators."

Market conditions made the step easier. The textile pulp market is growing 5-7%/yr, according to Purho. "Cotton cannot grow at the same rate. Other cotton fiber substitutes are growing too but these are mostly for sports gear, he adds. Also, future cotton production will not be able to meet demand growth due to land constraints, environmental protection, and the cost of fertilizers and pesticides.

"We are confident dissolving pulp will be a good, expanding market for us. It comes down to production costs and fiber costs."

In­house cooperation leads to success

Although startup was January 2012, work started much earlier. Working with Imatra's R&D staff, laboratory trials preceded six months of pilot studies. After six months, we were ready to start commercially," Purho adds.

The actual financial investment was relatively low. Most of the money went to equipment such as steam pipelines and control valves. The digester controls needed to be rebuilt. As Purho explains, the steam lines were needed for the additional access points in the digesters.

Both lines are similar in their pulp making processes. There are four digesters in Fiber Line 1 and six in the SuperBatch system. There is one recovery line. Following the cook, the dissolving pulp process has washing, oxygen delignification, screening, a D0-EOP-D1-P bleaching sequence and drying.

What makes dissolving pulp different from conventional paper pulp is the elimination of hemicelluloses (hemicellulose makes up 18% of the birch Enocell uses.). Purho explains, "Hemicellulose has five carbon sugars which in paper pulp provide good properties so you want to keep them in. In dissolving pulp, they are a negative."

Paper pulp can be converted to dissolving pulp using caustic but then there is the question of what to do with the caustic-containing hemicelluloses. "You can't send it to landfill," Purho adds.

Enocell uses steam to penetrate the chips which releases organic acids that hemicelluloses can't tolerate. No chemicals are needed. Then, a mostly conventional cooking process is used to remove lignin. In dissolving pulp, lower viscosity is the goal as is good chemical reactivity and high quality.

For dissolving pulp, wood yield is about 40%. Although there tends to be a higher load on the recovery boiler, Enocell had the capacity to handle it. There was no effect on the mill's effluent treatment system. "For dissolving pulp, kappa levels to the bleach plant are very low so it actually eases up the COD," Purho says. "There is also less production wastewater."

One step at a time

Purho is satisfied with what the mill has accomplished thus far and with the quality. But this does not mean the mill is standing still. "We are getting ready to go further and expand the offering. We are looking at other applications. We feel it will take two to three years for each step."

Currently the mill makes dissolving pulp (viscose staple fiber) for clothing and some sausage casing. As Purho says, there are opportunities for other applications but these tend to be smaller, niche markets with stricter quality demands. Most of the pulp is sent to Asia.

As well as its pulp, Enocell also produces and sells 17,800 tonnes/yr of tall oil and 140,000 tonnes/yr of turpentine. It has a 105-MW back pressure condensing turbine. As the mill is rural, there is no district heating in the area so the excess power, about 200 GWh, is sold to the grid.

The mill itself is now part of the company's Biomaterials Division and as such, its headquarters is now São Paulo, Brazil.

There are no plans at present to convert Fiber Line 2 to produce dissolving pulp. It is one option but Stora Enso wants to retain the flexibility to be able to adapt to different markets. "Flexibility is all important," Purho states. "It has saved our lives, but there is no easy business anymore."

He adds that the mill can serve its customer by making pulp to their exact specifications, using spruce, and pine logs or chips, to give the properties they are looking for on the paper machines they are running.

Another type of conversion

While many North American mills convert to natural gas as an environmental and cost measure, some European mills, particularly in the Nordic region, are trying to wean themselves off fossil fuels altogether.

Stora Enso's Enocell mill in eastern Finland is another example. It is installing a turnkey unit from Valmet that will eliminate heavy oil in firing the lime kiln and replace it with sawdust. Startup is expected in early 2015.

The sawdust will be sourced from Stora's own onsite sawmill as well as from another in the area. These two should supply all the sawdust needed although there are several sawmills in the region if other sources are needed.

The equipment will include a 100-m long double layer belt dryer, what Purho calls a slow moving wire, with four circulating fans. That is the biggest issue: to sufficiently dry the sawdust. The energy to heat the dryer air will come from the pulp mill's recovery boiler flue gases. In 2012-13, the mill started to burn all its high-volume, low-concentration odorous gases in its recovery boiler.

There is also metal separation and screening (capacity of 120 m3/hr; oversize pieces or go to the power boiler).

The feed capacity of the system will be 10 tonnes/hr. Incoming moisture level of the sawdust is about 50-55%; the dried sawdust moisture level is 5%.

The dried sawdust is then sent to the hammer mill to pulverize it to a flour-like consistency (less than 1 mm in size). There is a screening system after the hammer mill that can recirculate oversize particles.

The material is sent to a buffer silo and then to a feeding station. This is a critical part that must be well-designed because it is hard to get sawdust to flow. A rotary weight feeder measures fuel flow to the burner. Dust is blown into the burner and a very consistent flow is needed as is a constant thermal load.

Purho notes that the entire system will work under very low oxygen levels because of the risk of explosion. Spark detectors are a must.

He adds that hammer milling will be as efficient as possible as it is a maintenance-intensive operation and uses power. The mill will have a buffer silo of fuel is case a maintenance break is needed.

If necessary, the burner will be able to operate on flue gases, oil and/or methanol. Purho says the new burner will save the equivalent of two truckloads per day of oil. It is part of the effort to make Enocell a fossil fuel-free mill. This will lower its CO2 emissions.

Best job I ever had

When she was an employee of Stora Enso for 10 years, Sirpa Välimaa did not envision the position she would come to hold.

Long a dominant figure in the graphic paper grades and market pulp, the company has had to redefine itself as markets changed. "I did not realize that Stora Enso would ever go into dissolving pulp," says the TCS manager, dissolving pulp, pulp sales & marketing. She adds that it is "only the first step in the direction of regenerated cellulose and forest products."

Välimaa came back to Stora Enso in 2011 after working for a small consultancy for a few years, calling it a "fantastic opportunity. We have plenty of pulp mills serving printing and writing grades, we knew there were new opportunities for our pulp mills."

She credits Stora Enso's former CEO Jouko Karvinen with the turnaround. "He did a splendid job of adjusting the company to find new opportunities, being flexible, able to take decisions and make change quickly."

Välimaa says that despite Stora Enso's size and reputation in graphic paper, it was basically an unknown quantity in the dissolving pulp sector. Another unknown was the company's source of fiber for dissolving pulp, birch. However, birch brings good reactivity and brightness. The metal content of birch is also extremely low compared with other hardwood species.

When meeting with potential clients before startup, Välimaa says she would first concentrate on the reputation of Stora Enso as a reliable supplier and its ability to listen to customers as Stora Enso did not have any background in dissolving pulp."I was like Alice in Wonderland at first. They were all new customers. I tried to do my homework. You have to listen more than speak. I had to learn my customers' processes and bring their demands back to the mill."

It was a new process for the mill as well. A completely new set of property specifications had to be mastered. Välimaa, the mill engineers and operators spent a lot of time in the control room to accomplish such process conditions that would produce the desired quality.

Working with the mill and the company's R&D personnel, after laboratory trials, the right process conditions for making dissolving pulp were established. Välimaa served as project manager, ably assisted by Jukka Kahelin.

Through their efforts, many potential clients expressed an interest in Stora Enso's dissolving pulp. Trials had to be conducted and the results assessed vigorously to see what had to change. A series of three trials were done before Enocell went fulltime to dissolving pulp in 2012.

"Being on dissolving pulp constantly was appreciated," Välimaa notes. "Customers could see we were serious about the business. We can do what the customer needs."

In the future, Välimaa sees Stora Enso going into what she calls new products, new such that the company will need to define the needs to the customer. She likens it more to "regenerated cellulose". The basic idea is to get more high-value products out of the mill whatever they might be in the future. I see it more as a biomaterial business area that we are in."

Stora Enso has even specified specific products for specific mills. Enocell will concentrate in dissolving pulp; Sunila on lignin and Skutskar on fluff pulp.

"It's the best job I have ever had," she exclaims. "Enocell is a great mill. I have spent a lot of time with the production people. They want to know how the customer feels and what they can do to improve."

A time capsule

Long before Enocell was created, there was a German company, Haukivuoren Puutavara Oy, in Uimaharju, which ownership was transferred to the Soviet Union's after the second world war.

1955 Enso-Gutzeit bought the sawmill from Soviet Union

1967 Enso-Gutzeit's pulp mill started April 16

1989 Decision of Enocell project and contract with the Soviet Union

1991 Enocell started operating; pulp mill, sawmill and planing mill. Enocell was Stora Enso's subsidiary company: 99% owned by Stora Enso and 1% by Russian interests.

1992 Renewed pulp mill started.

1996 Sawmill started to operate under Enso Timber Ltd Uimaharju Saw Mill

1999 Swedish Stora Kopparberg and Finnish Enso-Gutzeit merged creating one big company: Stora Enso started.

2005 Stora Enso bought shares back from the Russians. Fortek contracted to take care of technical planning, mechanical and automation maintenance.

2006 10 millionth tonne of pulp was made in March.

2009 New maintenance company is Efora.

2012 Dissolving pulp production started.

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