Steam energy has its rewards

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Steam energy has its rewards

July 16, 2012 - 04:09
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BRUSSELS, July 16, 2012 (RISI) -Using steam energy in the right place gives back a lot of rewards, as the WEPA Leuna tissue mill near Merseburg, Germany, has discovered. The installation of a Metso IQ Steam Profiler on it's tissue machine has resulted in significant production increases on towel grades with no increase in energy consumption, a reduction in electrical energy to drive the drying hood fans, and tissue quality improvement as an added benefit. The Leuna mill, built as a greenfield project in 2004, has a single 5.4-m wide crescent-former machine with a production capacity of 64,500 tonnes/yr of toilet tissue and towel grades.

As described by Dirk Barten, technical manager, the installation of the steam profiler was justified by an expected improvement in cross-direction product quality, a production increase for the drying- limited towel grades and lower drying energy requirements for the toilet tissue grades. These interrelated benefits are possible with a steam profiler, since it provides an even CD sheet moisture profile and raises the overall sheet temperature for cost-effective water removal by pressing and more efficient use of drying energy.

With 10 WEPA sites located in Germany, Italy, France, Spain and Poland manufacturing 600,000 tonnes/yr of hygiene paper, the WEPA Group is one of the leading companies in the European hygiene paper industry.

The steam profiler is located over the suction zone of the pressure roll immediately before it is applied to the Yankee surface. Mikko Talonen, Metso's product manager, explains why this location is an ideal place to influence the thermodynamics of water removal: "There are several factors which contribute to energy savings in tissue machines. First, mechanical dewatering is improved in the press nip because of lower water viscosity at higher temperatures. Then, there is less water need to remove in Yankee section. Next, with higher web temperatures immediately before the Yankee, the energy required to heat the web to drying temperature is less. Furthermore, better moisture CD-profile provides more even contact to the Yankee surface. Heat transfer from the Yankee to the web is more even."

He adds, "Good contact between web and Yankee cylinder is one of the key elements of tissue making. It affects energy consumption and tissue quality as well. The web is easier to doctor from the Yankee surface if the surface contact is even."

How does steam condense in the porous tissue sheet and why it does not blow right through? The answer is in the steam delivery and diffuser design. The new Metso profiler operates at much lower steam pressure, temperature and velocity than previous steambox designs in which some of the steam condensed before it reached the sheet and caused water dripping and sheet runnability problems. Hence, the older units were run at high steam pressures and temperatures. With lower steam pressure, temperature and velocity the steam is delivered gently so it condenses in the sheet where it releases its latent heat and raises sheet temperature.

The steam profiler unit on the WEPA Leuna machine uses moisture profile measurements from the existing ABB quality control system. The Metso-implemented control determines the cross-direction actuator positioning which is visible to the operators on a control room monitor. Keeping the unit clean in a notoriously dirty environment was a challenge that was met successfully. When the steam profiler is retracted during a creping blade change a spray shower is automatically activated so the steam diffuser plate is kept clean. No operator actions are required.

Positive response, lasting results

After the initial steam profiler commissioning in mid- 2010, the distance between the diffuser plate and the suction pressure roll was optimized to provide the best steaming efficiency and to ensure safety. The optimum superheat in the steam supply was also determined. On-line step tests in August 2010 indicated the profile correction capabilities of the profiler and the effects on drying energy consumption.

The moisture profile improved by 60% when the profiler was activated in a step test

Figure 1 shows the moisture profile improved by 60% when the profiler was turned on in a step test. Longer term statistics from the mill's records indicate a consistent improvement of about 30% in moisture profiles. Figure 2 shows how gas flow to the air hood gas burners was reduced considerably when the profiler was activated in a step test while running a toilet paper grade.

With the profiler activated, the gas consumption to the air hood burners was reduced significantly. The wet end and dry end hood temperatures decreased by 33 C and 43 C respectively

The wet end and dry end hood temperatures decreased to maintain the same final sheet moisture. This reduction in drying energy requirements permitted an increase in speed for the toweling grades. Since the production rate increased with no more net energy input, the specific energy (per tonne) was decreased.

After the profiler operation was optimized, the mill conducted a three-month study to determine the lasting results of the profiler installation. These included: • 8% increase in production on towel grades. This was achieved by a significant reduction in specific gas energy consumption. The production speed was previously limited by Yankee/air cap drying energy capacity. • 5% reduction in electrical energy by driving hood fans at slower speed. • 30% improvement in CD moisture profiles.

Although the steam consumption of the machine was raised 10 to 11% by implementing the steam profiler, this increase in energy was offset by the reduction in air cap natural gas energy consumption.

More uniform consumer rolls

Quality improvement was also one of the major objectives of the project and that has been achieved conclusively. The improvement on the machine extends right through the production chain, as explained by Barten. "There is less CD variation in parent reels, so that means our finished consumer roll weight is also more even. It is clearly better," he concludes. The total variation of weight was decreased by about 15% for towel grades whereas the total weight variation of toilet paper grades was decreased by a very significant 48%. The changes in average weight were small.

Sven Gattke, process engineer, sums up the successful outcome of the project: "The profiles have improved a lot so the operators use the profiler all of the time. They would not run without it. In summary, I would say this is one of the best projects we have had in this mill."