Energy and emission issues of the FerroAlloysMetalist
2.5 Energy and emission issues of the FerroAlloys
2.5.1 Energy Demand for Ferroalloys Making
Today’s ferroalloys are almost always produced by smelting in electric submerged arc furnaces (SAFs); only a comparatively small amount of ferromanganese is still produced in blast furnaces. A conventional SAF is an open furnace from which off-gas is mixed and combusted with a large amount of air. A “closed furnace,” however, is designed to maintain CO-rich off-gas by collecting, cleaning, and storing it for further utilization. An intermediate type, referred to as a “semi-closed” furnace, is still common for FeSi and special alloys production. It is still possible to recover off-gas from this type of furnace, but it is less calorific. There is a strong trend toward closed furnaces with gas recovery and utilization; for instance, in ferrochrome production closed furnaces have rapidly increased (Holappa, 2010).
2.5.2 CO2 Emissions from Ferroalloys Production
Ferroalloys production is an energy-intensive industry with a high consumption of electricity but only a moderate consumption of coke and minor consumption of other fuels and reductants. This affects direct CO2 emissions. For these ferroalloys, different grades (high, medium, low carbon; different silicon content) with different emission factors were integrated. Carbon dissolved in ferroalloys was not included as an emission in ferroalloys production.
As shown, the total emissions from four main ferroalloys with 32 MT annual production were about 63 MT, from which an approximate figure about 87 MT CO2 for total ferroalloys production (44 MT) can be given as a rough estimate. One has to add indirect emissions generated in electricity production so, depending on the method of electricity generation, the indirect CO2 emissions can be large.
Countries that have plenty of renewable energy (including nuclear power) naturally have low emission factors when compared with countries that are using coal power. In such conditions the total emission factor can be doubled compared to the value of the direct emission factor. The emission factor should be one decisive criterion when erecting and operating ferroalloys plants.
Although more data of energy and materials consumption in different processes/furnaces can be found in the literature, any direct comparison is impossible because of the use of different raw materials, process conditions, products, definitions of terms, and so on. A new approach for process evaluation is the best available technology (BAT) procedure.
The most effective and advanced stage in the development of activities and their methods of operation which indicates the practicable suitability of particular techniques for providing in principle the basis for emission limit values designed to prevent, and where that is not practicable, generally to reduce the emissions and the impact on the environment as a whole.
This definition implies that BAT not only covers the technology used but also the way in which the installation is operated, thereby ensuring a high level of environmental protection as a whole. BAT takes into account the balance between the costs and environmental benefits. The following information summarizes the most effective and advanced technologies for use on the ferroalloy production line (Holappa, 2011; Worrell et al., 2008):
l Concentrate sintering by utilizing CO gas from a smelting furnace.
Preheat charge material for the smelting furnace by utilizing off-gas from the SAF.
l Pre-reduce the charge before smelting in the SAF. This is a potential but unestablished sub-process for certain ferroalloys.
l Smelt in closed SAFs with efficient off-gas recovery, filtering, and energy utilization in-plant or in neighbor users.
l Use of a semi-closed furnace (for FeSi) is acceptable if energy can be recovered from CO gas.
l Utilize latent heats of liquid ferroalloy and slag from the smelting furnaces.
l Improve the recovery of metals. Cr yield into FeCr is typically 90% to 95%, and for Mn in FeMn production the yield ranges from less than 80% to more than 90% depending on the slag practice and recycling.
l Integrate ferroalloy production with steel production, with other industries and for use within society. This might give excellent possibilities for energy recovery, electricity production, CO gas utilization, heat recovery, and utilization.
l Apply efficient gas cleaning for dust, heavy metals, and toxic emissions.
l Use a closed water system to remove particulates and harmful components.
l Recycle, reuse, and utilize solid wastes like slags as by-products.
From the viewpoint of energy, the first five aspects are most significant. Today only in one plant is the heat content of liquid FeCr directly utilized in stainless steel making (Holappa, 2010). Improvement of metals yields is a metallurgical problem but is also connected with recycling and process integration. The last three aspects listed are environmental issues and thus extremely important.
When operating at high temperatures, a lot of dust is generated via vaporization and other mechanisms. In addition to valuable and harmless components, the dust often contains harmful, even poisonous, components. Therefore, the dust must be carefully collected and prevented from ending up in air, water, or soil.
It is reasonable to expect an energy-saving potential of 20% to 30% on a global level by adopting the best available techniques as widely as possible. With efficient integration, the benefit can be even higher. Another option is to introduce renewable biomaterial (i.e., charcoal) as a substitute reductant for coke and to mitigate green-house gas emissions by CO2 recovery and sequestration.
In large production units, this might be economically reasonable and realizable.