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Decreasing emissions

CASE STUDY

Reducing NOx emissions at Stockstadt Mill

The project team at Stockstadt Mill worked to find an innovative solution to reduce NOx emissions: They examined the combustion process of thick liquor in the utilities department. Working inside one of the boilers, they experimented with the fuel injectors. By altering the fuel injectors to have an asymmetrical combustion gradation, the team observed greater efficiency in reducing NOx levels. The modified fuel injectors improve the temperature profile in the combustion chamber and reduce the formation of thermal NOx. After a 12-month trial period, the results are excellent. At all operating parameters the new NOx target figures are fulfilled. The mill's self-developed new fuel injectors will enable it to meet new regulatory limits at every load range of combustion without further investments in fluegas purification plants. The mill has been able to reduce NOx figures by over 18% from below 400 mg/Nm3 to below 325 mg/Nm3.

Specific NOx air emissions (kg/adt)

Comment:

Globally, there was a decrease. There was a slight increase in SEU due mainly to an increase in the fluegas at boiler no 2 at Kirkniemi Mill. Rebuild of PM8 at Lanaken Mill which resulted in significantly lower levels of production and the integration of Rockwell Solutions also contributed to the slight increase. There was a slight decrease, in SNA, due mainly to increased production levels at Cloquet and Somerset Mills. There was a decrease across all mills in SSA.

Specific SOx air emissions (kg/adt)

Comment:

Globally, there was an increase. The slight increase in SEU was caused by the higher sulphur content of coal used in boiler no 9 at Stockstadt Mill (low-sulphur coal is not available on the market). The increase was also driven by lower levels of production at Lanaken Mill due to the rebuild of the PM8. These impacts were only partly compensated for by a decrease in the other mills in the region. There was a slight decrease in SNA given less coal combustion at Westbrook Mill. Emissions went up in SSA due to an increase in high concentration low volume (HVLC) gas volumes attributable to higher levels of dissolving wood pulp production. The situation should improve once maintenance on the Tugela Copeland scrubber is completed in 2020.

Specific particulate matter air emissions (kg/adt)

Comment:

Globally, there was an increase. The increase in SEU was very low, due mainly to increased operating time of the recovery boiler at Stockstadt Mill. The measured concentration at the bark boiler fluegas at Ehingen Mill was also higher. There was a decrease in SNA where emissions at Cloquet Mill were significantly lower due to recovery boiler precipitator upgrades. Emissions at Somerset Mill increased due to an unfavourable stack test as the result of mechanical issues with the east precipitator on the no 1 boiler. The precipitator has now been rebuilt and a new test scheduled. The situation at Westbrook Mill improved due to reduced coal burning. There was an increase in SSA, due mainly to the performance of the grit collectors on boilers no 3 and no 4 at Stanger Mill. However, these will be replaced in 2020 while maintenance and optimisation of boilers no 1 and no 2 at the same mill, and of the boilers at Tugela Mill, is in progress and will continue through 2020.