By Per-Ivar Nikolaisen, Teknisk Ukeblad
This did not add up. Their jaws dropped at Elkem Salten that day in 2009. Measurements showed that the NOx emissions were almost twice as high as expected. The combustion plant with panoramic view to the mountains Middagsfjellet and Straumklompen was suddenly a large red circle on the environmental authorities emission map.
They decided to redo the measurements. The result was the same – Elkem Salten was Norway’s single largest polluter of nitrogen oxides (NOx). Only the Oseberg and Troll platforms in the North Sea were worse on Norwegian territory.
The discouraging emission data was the start of a process in which the silicon plant in the small community of Straumen was going to make a discovery that would not only take the combustion plant down from the top delinquent list, but eventually help smog-filled cities in China.
White and hasty
Flames are dancing over piles of coal. The window between the control room and furnace number two are made to withstand a forceful explosion.
Going back in Norwegian combustion plant history, reaction between vapor and liquid metal has occurred and led to great damage to both people and equipment.
Through the special designed telescope with dark lenses, we can see the difference in the intensity of the flames. Dark fields imply that everything is in order, but an area left of the furnace is burning more powerfully and whiter than elsewhere.
– It has become fierce. It is impossible to kill all the fire under the current conditions, says Finn Harald Olsen, Operator at Elkem Salten.
Olsen wants to “kill” the strong and white flame on the left wing because it is in extreme heat pockets like these that NOx is formed.
The winter has been cold and dry. Small amounts of snow have fallen, only to disappear again. Inside by the furnaces it is hot. However, a few meters further out on the premises the cold is finding its way through “country of the permafrost”, according to engineer Nils Eivind Kamfjord.
Barely a year has passed since he came back after finishing his PhD at the Norwegian University of Science and Technology, NTNU in Trondheim. As part of his studies, Kamfjord measured emissions from the furnaces at Salten in 2009.
These measurements showed that NOx emissions were far greater than what they previously had calculated and reported to the authorities.
At NTNU, Kamfjord and his supervisor, Halvard Tveit, professor in the Department of Materials Science and Corporate Specialist in Elkem, started the difficult task of solving the mystery of high emissions at Salten.
They concluded that the surprisingly high NOx emission was caused by both airflow and the manner in which raw materials were fed into the combustion chamber.
The supply and exhaust caused extremely hot “pockets” in the furnaces. In the white flames, the air became so hot that the silicon gas began to split oxygen molecules and serve themselves with atoms – which meant dramatically higher emissions of NOx when the lonely and unstable oxygen atoms hunted new partners, and eventually mated with nitrogen atoms.
– The design of the furnace was very unfortunate, says Professor Halvard Tveit.
By increasing the frequency of supplying raw materials from every third quarters to two times a minute, Tveit and Kamfjord demonstrated how to avoid the hot pockets.
The fact that there were four outlets on the furnace caused the air to pile up and become hot. Kamfjord’s thesis concluded that one outlet gave more efficient airflow.
All this was theory and pilot trials, but they lacked large scale testing. They were extremely excited when Elkem chose to rebuild one of the furnaces at Salten according to Kamfjord’s recipe.
An operator pushes an iron pipe with ceramic coating several times into one of the drain channels on furnace number two, with sparks flying. It may look dramatic, but the operator blowing away the slag from the channel is wearing three layers of flame resistant clothing and double stitches.
The challenge with silicon production is that the raw materials have a tendency to get lumpy.
Challenges are not only related to the fact that channels for silicon can clog and must be blown clean. Clogging can also lead to raw materials not falling down through to the furnace, and therefore not achieve the proper combustion in moderate temperature.
Operators and metallurgists are seeking the optimal operation – the golden balance between too much and too little raw materials, just the right heat, the best results and lowest possible emissions.
It was not a proud moment when Elkem Salten appeared on top of the NOx-statistics.
– We got very negative results. As a company, we did not want to have a reputation that our point emissions were among the worst of its’ kind, says Plant Manager Arve Ulriksen.
150 000 diesel cars
The doctoral dissertation by Nils Eivind Kamfjord may have set a national record in the implementation of research into practice, as Elkem fired up the converted furnace last summer.
– It was absolutely amazing. Not many go straight from work on a PhD exam onto implementing in full scale what you learned in the thesis, says Kamfjord.
The results were equally fantastic. By feeding the raw materials in a different way than before, and making the suction of air to go smoother, Elkem Salten is preventing the fire of becoming more aggressive resulting in significantly higher emissions. After producing silicon in the new furnace for half a year the NOx emissions are reduced by approximately 40 percent, equivalent to the emissions from 150 000 diesel cars.
– The new furnace will never receive stagnant air. The airflows are different now. The extremely hot air pockets were like backwater in a river, and made the emission of NOx explode, explains Kamfjord.
Plant manager Arve Ulriksen cannot praise Kamfjord, Tveit and the rest of the development environment at Elkem enough.
– It took exactly four years from the starting point of the research until we pressed the button on the furnace, says Ulriksen.
Elkem will now upgrade the two other furnaces in the same way. At the combustion plant in Bremanger, one of the furnaces will undergo the same upgrade.
– This is a small revolution. I am proud on behalf of Elkem, says Ulriksen immodest.
A quarter billion
The upgrade of the furnace in Salten has cost 250 million NOK. Of the total amount, the NOx fund within the business community has contributed with 54 million NOK.
– The financial risk was great. We would never have started this project without the NOx funding, says Ulriksen.
The makeover of Elkem’s furnace at Straumen is significant in a national context, according to general manager in the NOx fund, Tommy Johnsen. When Elkem rebuilds the remaining furnaces in Salten, the upgrade will be the largest single measures to reduce NOx known today. A reduction of 494 tons in emissions was more than expected.
– It is not often we get results that are better than what was predicted in the application. I am very pleased, says Johnsen.
The NOx fund has already decided to support further reconstruction of the furnaces in the Elkem system with 180 million NOK.
Consequences for China
NOx can cause health damages when the concentration in the air reaches a certain level. The combustion plant in Salten is situated in a sparsely populated area with little traffic emissions. The situation had been more dangerous to public health if emissions had come on top of NOx emissions from a large fleet of vehicles – such as smog that troubles Chinese cities.
The breakthrough in Salten can have global effect, especially for air pollution in China. Elkem’s Chinese owners, Blustar, have shown great interest in the environmental effect of the reconstruction in Salten. Simultaneously, Elkem has promised to share the research results globally.
– In a global perspective, it is a great advantage when this is passed on to China. We have not calculated what it might mean for the total emissions, but a reduction of 40 percent was better than expected and above our targets, says Ulriksen.
China has many smaller combustion plants in and near cities. The long-term goal is to replace these with larger and less polluting units. Then the knowledge of how we tamed the fierce flames in “permafrost country” will be vital and important to millions of people in the “Middle Kingdom”.
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