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Widening the bioloop

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Widening the bioloop

January 22, 2013 - 14:00
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BRUSSELS, Jan. 23, 2013 (RISI) -The Muppets' frog-philosopher Kermit once announced that it is not easy being green. However, to Swedish producer SCA, it seems to come as second nature. The company continues to reduce the carbon footprint of its Östrand pulp mill. Previous accomplishments have been well-documented in these pages (April 2005, August 2007, November 2008). Now, the mill has pioneered another environmental/technological breakthrough with the installation of a biofuel-powered lime kiln. In this case the biofuel is pellets supplied by SCA BioNorr pellet plant in northern Sweden.

Since the Östrand mill opted for totally chlorine-free (TCF) pulp bleaching in 1995, the goal has been for the mill to be among, if not the best environmental performer in its class. This desire to be an industry leader extends to the entire company. In 2011, SCA instituted what it calls the BioLoop.

To explain, start with the woodlands (SCA Skog). It provides the wood for the company's sawmills. The sawmills in turn provide chips for the pulp mills and sawdust for SCA's pellet plant (SCA BioNorr). Östrand purchases about 30% of BioNorr's production to feed the lime kiln and power boiler. Waste heat from the pulp mill is used for district heating that is used by, among others, the SCA tree nursery (Norrplant), which supplies SCA Skog.

The main project of BioLoop 2011 was the new lime kiln but included other sub-projects such as a remodeling of Östrand's chip screening facility to increase capacity, installation of four new burners in the No. 1 steam boiler to accommodate the use of pellets and construction of a new biofuel handling facility, which grinds the pellets that are used as fuel in the new lime kiln. This facility also has a silo because the pellets need to be kept dry.

Mill manager SCA Östrand, Ingela Ekebro, explains that the mill had two older, oil-fired lime kilns that were well past their expiry dates. "They had been running above capacity for several years."

The decision was taken to build a new lime kiln and change fuels. Natural gas is not an option in the area. It would have to be shipped in liquid form to the mill and it is a fossil fuel, which SCA wants to eliminate as much as possible in its operations.

On the day PPI visited, Östrand was enjoying an early snowfall

Striving for the top

Since the mill installed a totally chlorine-free (TCF) bleaching process in 1995, the objective has always been to be a top environmental performer. Although powering a lime kiln with pellets had not been done before, SCA was up for the challenge.

The decision to proceed was made in December 2009 with startup in October 2011. Andritz was chosen as the supplier for the kiln and this type of fuel handling and combustion system was also new to them. However, Ekebro notes with satisfaction that all went according to plan and the project came in on time and on budget despite dealing with the coldest winter in 20 years during construction.

Ekebro explains that one of the biggest challenges was to need to control the flame into the kiln. That is, Ekebro explains, "Get a good line. It's easy to control a liquid, but with a solid fuel transported in air, you can have density variations so you need a tight grip on combustion control."

The pellets, which are ground into a powder, can be an explosion risk. A lot of work was done with the Swedish explosive standards to ensure the safety of all concerned.

At present, the mill buys about 35,000 tonnes/yr of pellets. The storage silo can hold a two to four day supply. Two refiners grind the pellets and the powder is conveyed to a storage bin with a 10-hr capacity. Then, the fuel is fed to the lime kiln or boiler. The mill can run one refiner at a time when maintenance is needed.

With the increasing use of bioenergy, SCA designed the infeed system to the mill to handle other fuels such as branches. And, Ekebro, adds, it will be able to handle other fuels if something "interesting" comes up such as liquid biofuels.

There are five inlets to the lime kiln to handle various fuels and designing the system was a challenge, but Ekebro says that Andritz met the challenge even though it was new to them.

The powdered biofuel is fed into the lime kin via the angled pipe on the right

Need to pay attention to downstream processes

Changing a lime kiln's fuel source affects the makeup of the white liquor, which affects processes downstream. It can also lead to scaling problems in the digester and evaporators. Therefore, SCA installed pressurized white liquor filters when it installed the lime kiln. All this had to be accounted for when designing the new combustion system.

As noted, one of the concerns was the ability to control the flame. There needs to be a good spread, i.e., keeping the flame in the middle of the kiln so as not to damage the outside walls. Thus far, Ekebro feels mill personnel have been successful in this task although there is still room for improvement.

How successful? Between January and September 2012, the lime kiln's oil consumption was only 2% of its total fuel use, not 10% as budgeted. Oil is used for startups. In the bark boiler, it was planned that 50% of the fuel would be pellets but the mill has been able to increase that to 72%. Oil consumption has dropped by 17,500 m3/yr and the mill's carbon footprint has been reduced by more than 80%.

"When we built the lime kiln, we feared that the energy consumed to produce lime mud would rise," Ekebro says. "But we found that we had no rise in consumption compared with what we had in the two smaller lime kilns."

The success of the Östrand project had prompted SCA to invest in a similar biofuel-powered lime kiln at its Munksund paper mill in Piteå in northern Sweden. This should reduce the mill's carbon footprint by 75%.

Also, Östrand has supplied the district heating grid for the adjoining community of Timra. Now, it has entered into an agreement with the nearby larger city of Sundsvall. It needs more power for its district heating system. Plans call for SCA to build another pellet boiler at the Ortviken mill, which is close to Sundsvall. Östrand will also connect to the Sundsvall grid. However, these plans still have to be approved by SCA board of directors.

There was another benefit besides the environmental one. The mill was able to increase kraft pulp capacity by about 10,000 tonnes/yr because the availability of the new lime kiln is higher. Production capacity now is about 430,000 tonnes/yr. This is sold under the Celeste brand name. Östrand also produces 95,000 tonnes/yr of CTMP, marketed under the Star brand name.

But, looking further ahead, SCA hopes to increase production at Östrand to 800,000 tonnes/yr. Most of the equipment installed is designed for that level or can be modified. For example, the white liquor filter discs have capacity of 500,000 tonnes/yr, which can be easily expanded, according to Ekebro. Both the recovery boiler and lime kiln are dimensioned for 800,000 tonnes/yr.

The next step will be to replace the evaporation system, currently a bottleneck. This would bring capacity to 575,000 tonnes/yr. Work would also be needed on the chip infeed system as the impregnation bin is too small.

Beyond that though, Ekebro admits it would be difficult to find a step wise incremental increase. To reach 800,000 tonnes/yr, the optimum plan would see a new digester. A new bleach plant and pulp dryer would probably be needed as well.

To expand to 800,000 tonnes/yr would require a new environmental permit but Ekebro says that plans have been developed.