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Integrated steelmaking sites on the basis of blast furnace technology still account for 58% of steel production within the European Union (28) and even 73% of the worldwide steel is provided via the blast furnace route [3]. About 26% of the worldwide steel is produced by scrap recycling via an electric arc furnace (EAF) [3]. In sum, the energy-intensive steel industry is a large emitter of CO2 emissions accounting to about 7% of total worldwide anthropogenic emissions [4]. Although steel is a material with a highly effective recycling loop, the predicted worldwide demand of steel until 2050 and beyond needs considerable input of iron ore, since the increasing demand cannot be filled by scrap recycling alone [4].
Integrated sites persist to incorporate iron ore into the production cycle of steel.
Integrated sites will continue to produce high purity steel qualities with superior surfaces, which set the standards in premium flat products.
The coal-based metallurgy of blast furnaces within the integrated sites causes an inacceptable high carbon footprint. Coal-based reduction of ore needs to be replaced by carbon reduced techniques.
DR technology and direct reduced iron (DRI) material can be included into the existing material streams of existing plants in different ways. Figure 1 shows possible outbound material streams of DR plants. Several possible paths are described: the first one, DRI or in form of hot briquetted iron (HBI) material can provide feedstock to an existing blast furnace (BF), see arrow 1. HBI would be the natural choice in this case as DRI usage bears the risk of re-oxidation in the upper parts of the BF. Although the required carbon input into the blast furnace can be reduced by HBI input, the energy for melting still originates from coal. Subsequently reduction in carbon dioxide emissions is not complete [6].
Possible Flow schemes for Direct reduced Iron/Hot briquetted Iron (DRI/HBI) at integrated sites
Path 2 in Fig. 1 uses DRI or HBI as a scrap substitute at the BOF. Reduction in CO2eq emissions are limited as is the scrap rate in BOF steelmaking. Path 3 overcomes the limitations of path 2 by pre-melting DRI or HBI in an electric melting unit. This melt replaces hot metal and therefore makes blast furnaces obsolete. The melting unit process will still require some metallurgical carbon, which needs additional attention to reach decarbonized steel production. Path 4 uses a classical electric arc furnace (EAF) to melt DRI/HBI and scrap. This straightforward concept replaces not only blast furnaces but also BOFs. Some metallurgical carbon might be required here as well to preserve advantages of a foaming slag within the EAF [7].
In order to produce high quality steel grades lowest levels of nitrogen, phosphor, or carbon can be mandatory [8, 9]. Murphy discusses various aspects of nitrogen control in EAF steelmaking and concludes, "Technological solution is required to enable EAF to compete with BOF route on all grades" [8]. The problem to reach lowest nitrogen contents becomes even more difficult when lowest carbon content is simultaneously necessary [8, 9]. So far, no economically reasonable solution is available, while such steel grades are widely used in automotive applications, electro-mobility and deep drawing [9]. This can be a limitation for path 4 in Fig. 1 (EAF steelmaking).
In a direct comparison of converter vs. EAF steelmaking the following matters: The integrated steelmaking based on BOF process reaches nitrogen values between 20 and 40 ppm even in final products [9]. The BOF vessel shields the melt well against the surrounding atmosphere and it takes additional high-volume streams of carbon monoxide to keep nitrogen low throughout the blowing process. EAF modules do not present a similar air tightness and reach typical nitrogen values between 40 and 90 ppm [9].
Focus of this paper is the environmental evaluation of a DR plant combined with an electric melting unit (Fig. 1, path 3). The life cycle assessment (LCA) according to the international standards ISO 14040/44 [10, 11] is an established standardized methodology to determine the environmental influence of products. Within an LCA material and energy-related flows as well as environmental impacts are assessed in a holistic approach. LCAs for the current steel production are already widely applied in steel industry:
Norgate et al. [12], Burchart-Korol [13], Renzulli et al. [14], Chisalita et al. [15], and Backes et al. [16] presented LCAs for conventional steel production via the currently most common BF-BOF route. The presented product carbon footprints range from 1.6 kg CO2eq/kg steel up to 2.3 kg CO2eq/kg steel. Besides the product steel, some studies relate the environmental impact to the product hot-rolled coil. Different scrap rates, quality of raw materials, technical production sites, and methodological assumptions explain the differences.
Although most of the studies are comprehensive studies, none of these follow the LCA or product carbon footprint (PCF) methodology according to ISO 14040/44 [10, 11] and ISO 14067 [26], respectively. The presented study fills this gap by providing a holistic carbon footprint assessment according to ISO 14067 for this innovative steel production route and all environmental impacts from raw material acquisition to the product hot-rolled coil are included. In addition, the novel concept of incorporating an electric melting unit into integrated sites is discussed and analyzed, whereas the focus of the available literature is on classical EAFs.
The presented study expands the study of Suer et al. [27], in which a PCF for hot-rolled coil produced via a conventional BF-BOF route is assessed. In Fig. 2, the results of the Base Case of the previous study are summarized.Footnote 1 The Base Case of an integrated steel production via BF-BOF route amounts an overall carbon footprint of 2.1 kg CO2eq/kg hot-rolled coil. Individual contributions are split in sub-categories:
Global warming potential (GWP) of hot-rolled coil, produced over a conventional BF-BOF route (Base Case). Data base 2018 [27]
The direct impact describes the processes of the integrated steel site and add up to 1.9 kg CO2eq/kg hot-rolled coil (HRC). The impacts are attributed to the processes, where the respective emissions are emitted and not where they are caused. E.g., the impact of the power plant is caused by the processes, in which the process gases are produced, which are incinerated in the power plant. Turning off the power plant could not eliminate the emissions resulting from the process gases, but these would have to be incinerated somewhere else.
Following the principle of system expansion credits are given for the co-products [11, 26], which reduce the global warming potential (GWP) to about 1.6 kg CO2eq/kg HRC. Especially, the use of the blast furnace slag within the cement industry is an environmental useful cross-functional cooperation. These benefits need to be taken into account to avoid unnoticed shift of environmental impacts. The impact of the upstream processes add up to about 0.56 kg CO2eq/kg HRC [27]. The result of the previous study of 2.1 kg CO2eq/kg hot-rolled coil [27] is consistent to the carbon footprint from the GaBi database of 2.0 kg CO2eq/kg slab.Footnote 2
In the previous study based on the results of the Base Case, modified BF operations are analyzed like the injection of hydrogen and the use of HBI in a BF. These measurements enable a reduced carbon input into the BF but the coke cannot be replaced, completely. Yet, the injection of hydrogen into existing blast furnaces can push the establishment of a hydrogen market and infrastructure and reduce the GHG emissions of the BF-BOF route. The use of HBI in a BF is a first step to integrate DR plants into an integrated steel site. [27]
Thus, these scenarios can function as intermediate scenarios towards a further CO2eq-reduced steel production. This goal is described in this paper by presenting a PCF for a natural gas-based and a hydrogen-based DR plant with an electric melting unit.
Since the data availability of future scenarios is not as technical mature as for conventional steel production, the focus of this paper lies on a single environmental impact category: climate change. Therefore the sum of greenhouse gas (GHG) emissions and removals of a product system, expressed as CO2eq are assessed. The mass of a GHG is converted into CO2eq by multiplying the mass of the GHG by the respective GWP. The GWP of a GHG characterizes its impact on the climate change in comparison to CO2. Since GHG have different life spans in the atmosphere a time horizon has to be defined. Within this paper the GWP 100 is used to represent the impact of the GHG emissions on climate change for a time horizon of 100 years. [26]
Within this paper a so-called cradle-to-gate approach is followed. Thus, GHG emissions of a life cycle from mining of raw materials and energy carriers, transport, and production processes are included, which are required to produce the considered product [26]. The declared unit is 1 kg of hot-rolled coil. Further downstream treatment of the hot-rolled coil and the use phase are consciously excluded because steel products have several applications. Since the downstream treatment and the use phase are not affected by the considered scenarios, the cradle-to-gate approach is adequate to evaluate the impact of the scenarios on climate change.
For assessing the production of co-products, which are used outside the integrated steel mill, the method of system expansion is chosen. Thus, it is assumed that the co-product substitutes a primary production of the product and therefore a credit is given [11, 26]. Since the given credits depend on the environmental impacts of the substituted primarily produced products, these values have a degree of uncertainty when considering future scenarios. Therefore, like in Fig. 2 the individual contributions of the processes are presented in this paper so that each impact is transparent. Thus, the communicated PCF can also be converted into a PCF without the consideration of credits, which is also done in this paper.
System boundary definition and major material streams of the future scenarios: natural gas (NG-Case; grey input) or hydrogen-based (H2-Case; green input) direct reduction with an integrated electric melting unit. White zone: processes of the integrated steel site. Grey zone: inputs and outputs of the integrated steel site. The process basic oxygen furnace (BOF) includes the secondary metallurgy. Not all considered inputs and outputs are listed in this figure for reasons of clarity
The product of the electric melting unit is an equal hot metal as the product from the BF. The hot metal is further refined within existing BOFs into crude steel. Thus, steel refining, secondary metallurgy, steel casting, and downstream processes do not need to change comparing a conventional integrated steel site.
The DR plant is fed exclusively with iron ore pellet feed. In general, the use of lump ore could be possible, as well. Subsequently, the DRI is charged hot into the electric melting unit. No co-product gas is generated from a DR plant. Although from the melting unit a carbon monoxide rich off-gas emerges, its amount is far below the range of the off-gases from the replaced BF. In sum, every DR plant in combination with an electric melting unit replacing a BF needs additional, newly generated electricity. Thus, the electricity surplus of the conventional BF-BOF route turns into a deficit for the DRI-based route.
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