Hallsta paper mill's experience with air blowers for water treatment
Hallsta Pappersbruk is located in Hallstavik outside Norrtäljein Sweden. Hallsta paper mill develops and produces quality magazine and book paper.
Together, we enabled the journey towards a more efficient facility with new technology.
Holmen : a strong focus on sustainability
The paper production process
At Hallsta Pappersbruk, the process starts with pulpwood from sustainably managed forests. First, the bark is removed in a bark drum and the logs are then chopped into chips. The wood fiber is extracted and separated by grinding the chips in large mills, refiners, in a so-called thermomechanical process. The fibers are then mixed with large amounts of water to be cleaned and to get the pulp to the right consistency so that it can be pumped to the large paper machines. The paper machines remove the water and ensure that the fibers are assembled correctly. The finished paper is rolled up on a large steel cylinder. The paper is cut to the right width and rolled up on smaller rolls, packaged and delivered to customers.
Energy consumption and recycling
Holmen's paper mill in Hallstavik is one of Sweden's largest energy consumers. They constantly review how they can reduce their energy consumption and recycle more, including:
- The factory does not use solid fuel boilers and has no fossil emissions.
- The refiners in the TMP (Thermo Mechanical Pulp) plant generate a lot of heat and steam, 80% of which they recycle. They are self-sufficient in the steam that is used, among other things, to heat the water needed in the manufacturing process and to dry the paper in the paper machines.
- By making district heating of the cooling water from the compressed air compressors, they contribute approximately 600-700 kW/h which is used to heat Hallstavik's preschools and schools.
- Sludge with fiber residues from the sewage treatment process is composted and becomes fiber soil that contains a lot of nutrients and is sold on as soil improvement.
- Sawdust and bark are sold as biofuel to external heating plants where it is used to produce district heating.
- In order for the forest to continue to grow, Holmen plants at least two new trees for every tree that is felled.
Within the entire paper mill, compressed air is used for a variety of processes such as control, cleaning, instrument air and producing paper through a machine known as tip drawing.
Several screw compressors (ZR) from Atlas Copco in various sizes have been used for these processes since the 80s. For the biological water treatment process for process wastewater, blowers ( ZS and ZB ) from Atlas Copco are used for oxygenation in the basins.
Air blowers: an important part of the water treatment process
In the production of pulp and paper, large amounts of water are used. At Hallsta paper mill, approximately 24,500 m3 of process water is cleaned per day. The used process water is treated in several stages, in three basins with biological tr of the MBBR type and activated sludge. It is important to have enough air to accommodate the treatment process, to maintain the right quality and the right type of bacteria to achieve optimal function.
The largest pool has a depth of ten meters and the two smaller pools have a depth of five meters. In the past, mainly lobe blowers were used to oxygenate the water in the pools. The results varied, but ultimately, the lobe machines did not fare well. There were several failures of lobe machines, especially in the deepest basin, because there was too much back pressure. Hot air was pushed back, the machine overheated and one machine even caught fire. From an operational engineering perspective, it was necessary to find blowing machines that can withstand a back pressure of more than 1 bar(e).
Potential energy savings of 25-30% and better pressure in the deep basin
Previously, only lobe blowers were used for oxygenation of the deep basin, and when it was time to replace one of them, a screw blower (ZS) from Atlas Copco was chosen instead.
On five occasions, Hallsta paper mill conducted tests where they compared technology and energy consumption between lobe blowing machines and screw blowing machines. The tests showed a potential energy saving of 25-30% by using the Atlas Copco screw blower.
Another advantage is that the screw blower is able to push the air down to the bottom of the basin, which was a problem previously with just lobe machines. Gradually, all lobe blowers have now been replaced by screw blowers (ZS) in the deep basin.
Increased capacity and oxygenation in the shallow pools
For the two shallower basins, lobe machines and two smaller turbo machines were previously used. These were replaced by turbo blowers (ZB) from Atlas Copco with more power and greater increase in air volume.
As the production at Hallsta paper mill has developed and changed over time while the emission requirements have been tightened, more blowing machine capacity was needed to be able to clean the water in a good way.
These turbo blowers are connected to the Optimizer 4.0 control system, a central control system to control and optimize the machines' operating time and energy consumption.
You must have the correct capacity and a high availability of the air blowers, to be able to have full production in the factory.
Waste water management
Sedimentation basins at Hallsta paper mill. Photo: Holmen
With increased production, the organic load on sewage treatment increases. Organic material such as cellulose residues are broken down in the water with the help of microorganisms that need oxygen.
During the sedimentation process, it is important to have good and the right type of bacteria, which you get from having the right temperature, pH value, nutrients and oxygen levels.
During sewage treatment, they look at the amount of TOC (total organic carbon) in the water. The emissions requirements for TOC are high, but by achieving 90% efficiency, seen as TOC reduction, this is met at Hallsta Pappersbruk.
It costs not to have the right capacity in the water treatment process
Results after 10 years with screw and turbo blowers
After replacing all the lobe machines over a ten-year period, you now see lower maintenance costs, higher reliability and increased availability.
The life of the screw blowers in the deep basin is much better than that of the lobe machines. Hallsta paper mill's first screw compressor was purchased in 2012 and it is still in operation. It has a higher investment cost compared to a lobe machine, but the profitability over time is better.
The reason why Atlas Copco's screw blowers and turbo blowers were chosen is due to a combination of energy consumption, price, technology and service agreement.