More power in Scana’s furnaces

#1/2009
Text: Susanna Lidström

Modern heating technology has led to increased efficiency and environmental gains at Scana Steel’s historical forge in Björneborg, Sweden. Oxyfuel burners were introduced in 1997, followed by flameless combustion in 2005. Then in summer 2008, a new flue gas system was installed.

The first forge was built in Björneborg, in Värmland, Sweden, in 1656. Now, some 350 years later, modernisation efforts by the current owner, Norwegian industrial concern Scana, have put the forge at the cutting edge of modern technology. In 1997, 1998 and 2000 the company installed REBOX oxyfuel solutions for reheating technology in three of its large bogie furnaces, and the rest is history.

“Oxyfuel has given us additional heat that has been critical in helping us achieve the volumes we are achieving in the furnaces,” says Hans Joelsson, project manager and previously forge and maintenance manager at Scana Steel Björneborg. “Without it we wouldn’t be where we are today.”

Forge manager Leif Näsman agrees, adding that Björneborg, like many others in the steel industry, in recent years has experienced strong demand from the market. Production has been running at full speed to fulfil orders for the large rotationally symmetrical parts that Björneborg specialises in, such as propeller axles for ships and rotor axles for wind power stations.The company has its own steel plant and forge as well as a heat treatment facility and machine shop. It employs 350 people and reports sales of more than 100,000 euros.

Ingots weighing five to 75 tons are cast in the steel plant, coming into the forge with a surface temperature of around 650°C. To be able to forge them into the desired shape, the ingots are heated to 1250 degrees – a process that, before the installation of REBOX, took place at a rate of 10 to 15 degrees per hour.

“When we installed the oxyfuel burners, we achieved a test speed of 50 degrees an hour with the same effect installed,” says Joelsson. “That equates to an enormous time savings, and we also save quite a bit of energy.”

In oxyfuel technology, pure oxygen is used instead of air in combustion. That means the nitrogen in the air doesn’t need to be heated, which increases furnace efficiency. The process demands significantly less fuel than conventional technology, which also means reduced carbon dioxide emissions.

Even though the oxyfuel burners use pure oxygen, they do generate a fair amount of nitric oxide emissions. This is partly because the furnace lets in air and partly because the flame in conventional oxyfuel combustion becomes very hot, which in combination produces nitric oxides.

Since 2005, the forge in Björneborg has used AGA’s flameless burners. This results in a wider flame that is diluted by furnace gases, resulting in a lower maximum flame temperature without compromising the heating effect.

Pressure regulation is also important in reducing the risk of leakage along with undesirable air intake. By switching from air to oxygen, flue gas volumes are reduced by around 80 percent, although it then becomes necessary to reduce the flue gas channels by the same degree to prevent low pressure, which sucks heat out of the furnace, pulling in cold air. Initially the forge in Björneborg regulated this by throttling the flue channels, but in the summer the company installed a completely new flue system from AGA.

“Previously the flue gas channels exited high up in the furnaces,” says Näsman. “With the new system we have moved them down so that they are now at hearth level. This makes it easier to maintain uniform pressure in the whole furnace, and it has led to a fuel savings of 5 to 10 percent.

To further improve the seal, the hatch closing was changed in one of the furnaces to a modern variant from AGA.

“Our forge furnaces are from 1980 and are starting to show signs of wear,” says Näsman “Sealing is important to gain the full benefit of oxyfuel combustion.”