ArcelorMittal Gent tackles dustSecondary dedusting unit in the steel shop
Thanks to a series of major investments, the capacity of the steel shop was increased by 25%. It has now a production capacity of up to 5 million tonnes of steel per year. The capstone of the investment programme was the new dust removal unit, which reduced the dust emissions of the converters in the steel shop by more than 95%, to less than 1 mg of dust/ m3 of expelled air. With this performance, ArcelorMittal Gent more than meets the Flemish environmental standards, which permit a maximum of 20 mg of dust/ m3 of expelled air.
A sketch of the situation
This investment, which cost about 29 million euros, was not only good for the environment, but it also improved working conditions in the steel shop.
In the steel shop - the heart of ArcelorMittal Gent - hot metal is converted into liquid steel through the oxidation of carbon and other elements and through the removal of impurities. Pure oxygen is therefore blown onto the hot metal bath. The combustion process takes place in two gigantic tanks, called ‘converters’. During this process, a significant amount of dust is generated.
Each converter is equipped with a primary dust removal installation. It removes the flue gases that are developed when oxygen is blown onto the hot metal bath. The primary dust removal is a wet process because the dust particles end up in water. The unit was completely revamped in 2001.
The primary dust removal installation cannot extract all of the dust-rich flue gases. The gases that are developed while loading the converter with scrap and hot iron and while casting the liquid steel – at these moments, the converter is tilted - used to be washed with water before being discharged via the chimneys. Now, our secondary dust removal installation extracts these flue gases and cleans them.
The secondary dust removal installation investigated
For the secondary dust removal installation, a special aerodynamic exhaust hood was developed. The exhaust hood extracts all the flames and flue gases that are released when hot metal and scrap are loaded into the converter. It has an extraction rate of 2.4 million m³ an hour. That is the same capacity as 4,000 cooker hoods together. The flue gases are cooled by recuperators afterwards.
Through one major collector pipe with a length of 160 metres and a diameter of up to 6 metres, the flue gases are then sent through a battery with 7,200 filter sleeves.
The filter sleeves have a fabric surface area of 26,000 m², which is more than the total surface area of five soccer fields. For the sound insulation, the filter building is mainly executed in concrete and coated with sound-absorbing panels. Eventually, the purified flue gases disappear up the 45 metre-high chimney.
Dust is tackled in other departments as well
Also the raw materials, harbour, transport and recycling department puts in efforts to continue reducing dust emissions and has, for example, commissioned a dedusting unit for the supply lines for sinter to the blast furnaces. The dust on those lines is mainly generated at the dropping points, where the material is dropped from one conveyor belt to another.
Before the new dedusting unit was commissioned, the existing dedusting system used a water spray, which had drawbacks. Therefore we built a central dedusting system that removes the dust at the dropping points. The purified air contains less than 0,5 kg/h of dust, which is well below the threshold value of the measurement requirements.
Dedusting at the dropping points on the sinter and coke lines has paid off.
In addition, our company also tackles the dust on roads and the stacks of raw materials that is blown up during dry spells. Stacks of raw materials are being sprayed, the transport on unpaved roads is limited and the existing road infrastructure is improved.
What about fine dust?
Fine dust is an umbrella term for fine particles with various compositions which are so small that they can be inhaled. Large-scale epidemiological studies carried out in America and Europe have revealed the health impact of this problem. As a consequence, European air quality targets were set in early 2000 and became official in 2005.
Flanders is in a part of Europe that is characterised by relatively high dust concentrations. In the specific case of Flanders, which is sandwiched between the industrialised areas of Holland, Germany and France, the authorities have calculated that 70 to 80% of the measured dust concentrations come from outside Flanders.
The Gent canal area was found to be one of Flanders’ hot spots. In practice, this means that the quality standards for PM10 (particles having a granularity of less than 10 micrometres) were not always met in the government’s measuring stations in the Gent canal area. Generally speaking, there is no problem respecting the annual average threshold value (40 µg/m³) but the daily average (50 µg/m³) is exceeded more often than allowed (35 times). Given the high background concentrations in Flanders, the weather conditions play an important role in the number of violations of the standards. For example, periods characterised by high PM10 values are often marked by predominantly dry weather, the presence of continental air coming from Eastern Europe or temperature inversion.
The activities of ArcelorMittal Gent have a certain impact on the environment. Research carried out by the Flemish Institute for Technological Research (VITO) confirms that it is especially the overall diffuse emissions (=not through a chimney) that affect the air quality in the vicinity of the company. On the other hand, the impact of guided sources (= chimneys) is negligible. This means that future investments will be aimed especially at controlling dust emissions from diffuse sources.