Steel is by far the world’s most important metal, with a global production of 1120 Million Metric ton (MMt) in 2009 (Worldsteel, 2010). In september 2010 the most important steel producers were China (42, 90 %), EU-27 (12, 79 %), Japan (8, 26 %), USA (5, 95 %) and India (5, 05 %) (Worldsteel, 2010). Figure 1 shows the development of world steel production since 1999. Clearly, China has become the dominant steel producer.
With steel being an essential aspect of many economies around the world, and being the most important metal, it is important to efficiently use and produce steel. The technology described here allows for more efficient use of steel scrap in the production process of new steel. As such, it is essentially a recycling technique of old steel. The technology allows for lower energy use in the production of steel, and has a variety of socio-economic and environmental protection benefits.
According the the IPCC (2007) and Worldsteel (2010), three main routes can be discerned in steelmaking:
1) The primary route. This route, used in almost fifty countries, reduces iron ore to iron in blast furnaces using mostly coke or coal. The iron is then processed into steel (IPCC, 2007). This route accounts for approximately 70 % of world steel production (Worldsteel, 2010).
2) The Direct Reduced Iron (DRI) production route, which accounts for approximately 5 % of worldsteel production. This route uses natural gas to produce DRI which is mainly used as an alternative iron input in electric arc furnaces. The use of this route can result in a reduction of up to 50 % in CO2 emissions compared to the primary route of steelmaking (IEA, 2006a). Use of DRI is expected to increase in the future.
3) The third route, consisting approximately 25 % of the iron and steel industry, melts scrap steel in Electric Arc Furnaces (EAFs) to produce crude steel that is further processed (IPCC, 2007). According to De Beer et al., this process uses only 30 to 40 % of the energy of the primary route (De Beer et al., 1998).
The three routes are summarized in Figure 2.
Since the EAF process can use up to 100 % scrap for the production of crude steel, it is the main process in which the technology of scrap preheating applies. However, the primary route can use up to 30 % scrap in its process, and because it is the dominant route, the technology of scrap preheating is also valuable for this route. This description will mainly focus on the EAF process.
The EAF process has four main outputs:
a) the liquid steel itself;
b) cooling losses;
and d) hot waste gases which account for approximately 15 to 20 % of the output.
Essentially, scrap preheating is a technology that uses the hot waste gases of the furnace to preheat the scrap charge (SOACT, 2007, p. 68). The scrap charge is the scrap that is the input into the EAF process. Preheating the scrap with the hot waste gases lowers the power consumption of the EAFs, as it removes the need for combusting fuel to heat the scrap.
The three main technologies, corresponding to the three main production routes, are Basic Oxygen Furnace (BOF), EAF, and Open Hearth Furnace (OHF) (Worldsteel, 2009). Of these technologies, the EAF accounts for 30, 8 % on a global basis while OBC accounts for 66, 7 % and OHF for 2.4 % (Worldsteel, 2009). Therefore, it is clear that the EAF technology itself is widespread.
Due to the large quantities of hot combustion product gasses generated in the modern EAF, accounting for approximately 15 - 20 % of the output, the development of several novel scrap preheating systems occured (Manning & Fruehan, 2001). Three different modalities of the scrap preheating system are discussed in this section: a) the conventional scrap bucket charge preheating system; b) the Consteel process; and c) the Fuchs shaft furnace. The Consteel and Fuchs processes are the main modalities in preheating systems that have reached the commercially mature stage (IPPC, 2001). Please note that there are other preheating systems which have other characteristics and operational necessities. Therefore, which preheating system is most suitable for a particular steelmaking facility should be assessed per individual case.
Scrap bucket charge process
In figure 3, a conventional scrap charging bucket preheating system is schematically illustrated. Preheating scrap has several advantages: a) reduced energy consumption; b) removal of moisture from the scrap; c) reduced electrode consumption; and d) reduced refractory consumption. However, the method of scrap charging bucket preheating has some disadvantages limiting its use: a) inconvenient to operate as the scrap sticks to the bucket; b) a short bucket life; c) poor controllability of preheating; d) for tap-to-tap times of less than seventy minutes the logistics of this method of preheating lead to minimal energy savings (EPRI, 1997).
The Consteel process consists of a conveyor belt which carries the scrap through a tunnel, down to the EAF through a hot heel (SOACT, 2007). The technology is in a mature stage with 8 installations in the U.S. and 35 installations worldwide (Tenova, no date). The conveyor belt continuously transports the scrap charge to the EAF, while the charge is preheated by off gases leaving the furnace through the preheat conveyor (Tenova, no date). The continuous feeding of the preheated scrap to the EAF is one of the main differing characteristics with other methods. Figure 4 schematically illustrates the Consteel process.
The Fuchs shaft furnace
In contrast to the continous feed system of the Consteel process, the Fuchs shaft furnace is a batch-feed system. Within the shaft, scrap is preheated by low-velocity gases from the EAF and then dropped into the EAF. Like the Consteel and scrap bucket charge, this technique is in its mature stage (IPPC, 2001, p. 277).
The majority of crude steel production is located in four countries and the EU-27. Together they constitute 74, 95 % of the global production of crude steel, as illustrated in Table 1.
|Country||Crude steel production (thousand metric tons)||Percentage of global production||Percentage use of EAF processa|
|China||47945||42, 90 %||9, 1 %|
|EU-27||14288||12, 97 %||41, 4 %|
|Japan||9233||8, 26 %||24, 8 %|
|USA||6645||5,95 %||58, 1 %|
|India||5640||5,05 %||58, 2 %|
|Total share of production of top-5 |
producers of crude steel
|83751||74, 95%||30, 6 %|
a= in 2008
The iron and steel sector is the second-largest industrial consumer of energy, after the chemical sector (ETSAP, 2010). Therefore, it is important for the iron and steel sector to reduce the energy needs of the production process. The technology of scrap preheating is capable of reducing the energy needs. As can be seen from Table 1, China has a low penetration rate of the EAF process technology. Since China is the major producer of iron and steel, there is still a large potential for EAF implementation. Since the EAF process is highly suitable for scrap preheating there is also a huge potential for the implementation of scrap preheating technology in China.
The iron and steel industry is faced with a wide range of environmental concerns that are fundamentally related to the high energy requirements, material usage, and the byproducts associated with generating enormous quantities of steel (Manning & Fruehan, 2001). While the iron and steel industry has succeeded in reducing the energy intensity of steel production over time, as illustrated in Figure 6, it is still a very energy intensive industry. Therefore, it is a priority of the iron and steel industry to reduce the energy requirements.
The use of an EAF, already uses approximately 30 to 40 % less energy compared to the primary route (De Beer et al., 1998). Partial scrap preheating generally saves approximately 60 kWh/ton, while total scrap preheating saves up to 100 kWh/t of liquid steel (IPPC, 2001). Since lower electricity use leads to lower CO2 emissions the technology supports environmental protection.
However, preheating the scrap also results in environmental concerns. For instance, scrap preheating may result in higher emissions of aromatic organohalogen compounds (IPPC, 2001). This category includes substances such as polychlorinated dibenzo-p-dioxine and -furans and PCBs (IPPC, 2001). The emission of these substances depends on the scrap material used. For example, scrap contaminated with paints, plastics, or lubricants result in higher emissions of these substances. Therefore, the suitability of scrap preheating depends on the charge scrap.
The applicability of scrap preheating depends on the local circumstances and has to be determined on a plant by plant basis (IPPC, 2001). However, next to capital costs, the raw materials costs for scrap and the cost of electricity are major components of the financial balance of a steelmaking plant (ETSAP, 2010). Therefore, scrap preheating should be able to substantially reduce operational costs. This reduction is due to the reduced need for large quantities of electricity to heat the scrap in the EAF. Partial scrap preheating generally saves approximately 60 kWh/ton, while full scrap preheating saves up to 100 kWh/ton of liquid steel (IPPC, 2001).
General information about how to apply CDM methodologies for GHG accounting can be found at: http://cdm.unfccc.int/methodologies/PAmethodologies/approved.html .
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