SSAB steelworks coking plant in Raahe, Finland

The SSAB steelworks in Raahe, together with subcontractors, employs more than 4 000 people and uses over a million tons of coal per year.

Filtrabit’s flow-dynamic filter is installed inside the 15-metre-high filtration unit of the coking plant, where a traditional textile filter can reduce only part of all the microparticles below 2.5 micrometres in diameter.

Air flows through the flow-dynamic filter at a rate of 2 400 cubic meters per hour. From the air, 150 kilograms of coal dust per hour is separated directly into a container. Most of the dust collected is in the sub-2.5 micrometre range, that is especially harmful to breathe.

The flow-dynamic filter has markedly improved the working conditions of process workers at the filtration unit.

The system has been in continuous use since 2020.

Problem description

SSAB has a baghouse of 40,000 m3/h at their coking plant. The major problems were situated in the sieving area after the coke ovens and the Coke Dry Quenching (CDQ) system. The dust created in the coarse coke sieving unit during the sieving process, went to the main bag house together with dust from other parts of the plant and overloaded the bag system.

Several operating parameters were considered unsatisfactory:

• Lifetime of the bag filters were sometimes only about six months due to wear and tear caused by coarse coke particles.

• Replacing bag filters meant costly production breaks. In some cases, the plant was operational only 60% of the time. The replacement work was dirty and a health hazard to workers.

• Existing dust suction capacity was not sufficient to handle exceptional dust bombs. As a result, the coke dust entered into the production facilities.

• The traditional textile filter used was able to filter off only approximately half of the microparticles below 2.5 micrometers in diameter, leaving a lot of the health hazardous particles behind.

• Additional problems were frequent costs of replacing bag filters. The old filter bags were hazardous waste that had to be disposed according to the regulations, and with some disposal fees. The performance of the bag filters was not satisfactory, they sometimes could not handle the heavy dust load, and clogged.

Solution

SSAB partnered with Filtrabit to evaluate the dust collection system and identify new solutions to solve the problem in a completely new way, without capital project costs, and taking into account the space limitations at the site.

A solution was developed to keep the existing extraction ducting and the old baghouse, but to relieve the dust load in the baghouse system by using a new dust separation system, which was developed by Filtrabit together with the University of Oulu. The new system removes the main part of the dust before the air goes into the old bag system.

Filtrabit’s flow-dynamic filter was installed inside the 15-metre-high filtration unit of the coke sieving plant. Filtrabit’s system was connected to only one of the existing filtration system dust collection points, which was considered one of the dustiest locations, where high dust content coke was just arriving to the sieving unit. The existing bag filter system continued to also collect dust from nine other locations in later phases of the sieving process.

The common inlet duct for Filtrabit and the old bag filter system, is installed inside the hood that covers the coarse coke sieve. A valve is installed at the extraction duct, and it directs dust-loaded air either to Filtrabit’s dust separation system or to the old bag filter system in order to have the alternative system available at any time.

The outlet duct of Filtrabit’s flow-dynamic filter is connected to the factory’s main bag filter system, but only for purposes of venting out the cleaned air. Filtrabit is in this case working in series with the old system, helping the old bag filter to cope with the dust load by pre-filtering most particles. Filtrabit also occasionally runs in parallel with the old system by venting cleaned air out directly, when efficiency measurements are made. The average dust output from the Filtrabit system meets the technical requirements from SSAB, as well as current environmental regulations.

The basic cleaning device in the new Filtrabit system is a dust separation cone that operates in a fixed flow mode of 1,200 m3/h. The system implemented at SSAB has two cones making the total flow 2,400 m3/h. The fluid dynamic principles utilized in the system, require that the flow of the dusty air is constant.

Filtrabit’s flow-dynamic filter is embedded into the process line at SSAB’s request. The system separates around half of the dust that previously used to go to the 40,000 m3/h bag filter.

The flow-dynamic filter can in SSAB’s case, replace a traditional 20,000 m3/h bag filter’s capacity, using only 2.400 m3/h flow volume. It’s possible because the dust at SSAB’s sieving station can be collected at the spot location where it is created, before it is diluted with a much larger volume of ambient air.

An upgrade of Filtrabit’s flow capacity to replace the bag system completely, is currently in the engineering phase.

Dust removal is a two-stage process: the pre-processing cyclone separates coarser particles and Filtrabit’s cone separator removes the very fine dust. The fall-out from the coarse cyclone and the fine dust that is extracted in the cone separator, is directed to a dust conveying system and subsequently further to the dust silo.

The solid particles in the dust are very small and extremely abrasive. The median particle size in the client process line is about 200 microns. The smallest particles can even be under 1 micron. The concentration of the dust particles is approximately 15 g/m3 in average, but occasionally ten times higher. The air temperature can be up to 200 C.

The energy consumption of the blower in the client installation cannot be compared directly with the energy consumption of the old bag filter, because the operational principles are very different. Fabrics in a bag filter typically fill up with dust during the operation, causing increased pressure drop and energy consumption. In contrast, the pressure drop in Filtrabit’s system is always constant. Energy consumption in the old system is variable to a certain degree.

The Automation System (PLC) is based on SIEMENS TIA-portal v15.1 PLC System, which consists of the following system components and software:

• Operation Station HMI / configuration and TIA-portal software;
• Process Station (PLC) including I/O’s with PLC-cabinet;
• Communication networks (Industrial Ethernet and PROFIBUS-DP);
• Uninterrupted Power Supply (UPS) for automation system devices.

Results

Both SSAB and Filtrabit were most satisfied with the outcome of the project. Filtrabit technology proved to perform extremely well as a supporting system for the existing bag filtration unit. Filtrabit’s system received SSAB’s final acceptance certificate on the 30^th of May 2022.

Filtrabit’s system, which is only connected to a single point in SSAB’s existing dust removal system, still manages to significantly relieve the load on the existing bag filters, making the coke sieving unit operation more sustainable and economical.

• Previously planned multimillion euro investment for renewing the existing bag filter system could be postponed or even possibly canceled, with the help of Filtrabit dust removal technology.

• The maintenance interval of the existing bag filter extended from 6 months to 2 years, reducing significantly the costs related to servicing the bag filters, excess filter waste and cleaning the dust conveying systems, thus improving the productivity of the sieving unit.

• Overall, more coke dust could be collected from the sieving process, meaning that:
  • The coke out of the sieving unit was cleaner.
  • More of the valuable coke dust could be collected and sold to customers.
  • The working environment in the coke plant has become visibly cleaner.

• Filtrabit’s system is able to extract 150 kg/h of coke dust in continuous operation. In addition, the system is able to work efficiently in the times of exceptional dust bombs, where even up to 300 kg/h of coke dust can be collected. This helped significantly the existing bag filter system to continue working properly.

• Most of the time, the dust extraction efficiency of the Filtrabit system is close to 100%, which helped SSAB’s filtering system to meet SSAB’s and general industrial requirements for particulate matter emissions.

• Filtrabit’s system proved to be very reliable, and the plant had no unplanned stops during the planned operation periods.

Interview with Timo Vierimaa, Technology Manager at SSAB Raahe

”There’s certainly too much dust in this world. It should be collected and not allowed to be spread around, regardless of whether it’s from a steel mill or another facility.”

– What challenges led SSAB Raahe to start innovating on dust removal?

”Some recirculating gas needs to be removed from the process, and dust was escaping the facility along with the gas. There was enough of it to be visible. At some point we started to look at how we could capture that dust, so that it wouldn’t burden the surrounding area. That’s the first time we thought of a solution like a bag filter.

The challenge for the device and process is that the gas is very corrosive. There are chlorides in it. The chlorides have an effect after reaching a point of acidity in temperature, where points of corrosion appear in the steel. From there we started to look for solutions that could withstand such an environment.”

– How did Filtrabit’s device help answer those challenges?

”Hard particles inside the dust result in a lot of wear, which has resulted in rather significant expenses through maintenance. The device that was started to be tested and developed, was meant to support a conventional dust removal system. We deployed the device developed by Filtrabit as a first pass filter, which resulted in the traditional filter to be under less strain.”

– What are the main benefits of Filtrabit’s system?

”The big advantage of this device is that currently we can achieve in the tests some extremely clean results. Results that would be hard to reach by bag filter technology.

This technology provides a lot of added value, when we go to hot particles and eroding particles.

The durability of this device is much improved over a conventional filter.

Bringing a device like this in front of an older facility reduces maintenance costs, which is a big thing as well. This we are investigating as we speak, to see the level of impact it has on the maintenance costs.”

– How has air quality improved at the coking plant?

”The coking plant is perceived around the world as a very dirty, dusty and difficult place but so many of whom have visited have said: ‘You’ve managed to keep the nature around here in a pretty clean state.’ And that it’s nice to come and see that there are foxes, moose, rabbits and even a white-tailed eagle along with everything else.”

As we improve this a little further, I’m sure the environment around the steelworks will remain in good condition. And our people who work in the facility, they have noticed that the air is much cleaner inside as well. Which is something to be happy about on its own.”

– Can other SSAB plants and processes utilize the technology?

”This container-based system developed by Filtrabit should be fairly easy to utilize in any of our processes that need dust control.

The steelmaking process is evolving, and especially in Raahe, we at SSAB are at the forefront of changing our process towards meeting future challenges of fossil-free steel and so forth. At that point, when there are some of these dust control units in use at sites that likely will not remain in use in the new process, that’s where this container-based technology will help in a way where you don’t need to invest in a more effective dust control facility, but instead this is helping alongside.

And if that moment comes when some of these processes are being deprecated, that use the container-based technology, those containers can be moved to our new, upcoming production line.”

– Do you see benefits beyond the current use case?

”In our process, we don’t utilize the dust, but in many other processes it can be utilized, at which point it constitutes direct raw material savings. It’s one of the good things you can do with this dust control system. By raising or lowering the airflows, you can control how much, and what type of material to collect from the raw material base, that is desirable to collect, and would otherwise be lost from the production process, along with the dust. That’s where this device is of primary importance.

For us, it’s more important that we save on maintenance costs, and can stay within the limits set by environmental regulations.”

– How has it been working with Filtrabit?

”Filtrabit has had, from the start, a very innovative model. And it has developed all the time.

Working with Filtrabit has been very… How would I put it best… It’s both warm and flexible. Co-operatively we’ve solved any problems. If we’ve faced problems, no one has looked at who should solve the problem, just that it gets solved together.”