TECHNIK + TRENDS Metallurgie Novel raw materials for

stahl und eisen 137 (2017) Nr. 6 33

TECHNIK + TRENDS

Metallurgie

Novel raw materials for ironmakingNeuartige Rohstoffe für die Roheisenerzeugung

This contribution summarizes new developments on non-conventional iron carriers and reducing agents. The background for development and application of such substances is relating to main challenges in the steel industry – increasingly scarce resources and climate change. Studies presented on the use of biomass and waste plastics, self-reducing pellets and ore-carbon composites, nut coke, granular and pre-treated coals, briquettes for Corex and DRI (Direct Reduced Iron) and LRI (Low Reduced Iron) in the BF, as well as mini-pellets in sintering, and re-use of top gas are discussed considering recent and current research activities of the IEHK at RWTH Aachen University.

Dieser Beitrag fasst neue Entwicklungen unkonventioneller Eisenträger und Reduktionsmittel zusammen. Der Hintergrund der Entwicklungen und des Einsatzes dieser Rohstoffe ist die Herausforderung an die Stahlmetallurgie bezüglich der immer knapper gewordenen Ressourcen und des steigenden Drucks aufgrund von Umweltbelastungen. Die hier diskutierten Studien zum Einsatz von Biomasse, Kunststoffabfällen, selbstreduzierenden Pellets und Erz-Kohlenstoff-Kompositen, Brechkoks, granularen und vorbehandelten Kohlen, Brikettes für Corex und DRI/LRI für den Hochofen, Minipellets für Sinterverfahren und Gichtgasrecycling basieren hauptsächlich auf jüngsten Forschungsaktivitäten des IEHK an der RWTH Aachen University im Bereich der Ressourcen und Umweltproblematik.

Alexander Babich and Dieter Senk

Injection test at the IEHK, RWTH Aachen University | Einblasversuch am IEHK, RWTH Aachen University

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stahl und eisen 137 (2017) Nr. 634

TECHNIK + TRENDS

Metallurgie

O riginally, three input materials were applied in the blast furnace: iron ore, charcoal (later coke) and air. In the 20th century, further substances became state-of-the-art input materials: sinter,

pellets, oxygen, fossil injectants (mostly pulverized coal, natural gas and oil). The development and usage of these materials were directed to raise the process efficiency (increase of productivity, decrease of slag volume and coke rate). Nowadays, a number of unconventional input materials are introduced or being investigated, among them biomass and products of their treatment, organic wastes, metal-lurgical off-gases (e. g. top gas, BOF gas, Corex export gas or coke oven gas), nut coke, anthracite, self-re-ducing pellets (SRP), iron ore-carbon composites, mini-/micro-pellets for sintering, hybrid pelletized sinter (HPS), DRI / HBI / LRI. The key reasons for this development are increasingly scarce resources and climate change. These trends seems to be major challenges in view of sustainable mass production of high-quality steel at acceptable costs.

Since many decades, CO2 emissions in the steel industry have reflected primary energy consump-tion. This dependence makes it hardly possible to mitigate further carbon dioxide emission in the most energy-intensive ironmaking sector. The best performing blast furnaces operate with energy consumption close to the so-called thermodynamic limit while using conventional input materials of high quality. The usage of secondary raw materials and recycling (e. g. organic wastes, specially treated coal and coke rests, briquettes and off-gases from

metallurgical process), application of renewable energy sources (bio-based products), materials low-ering energy demand (e. g. high reactivity carbon carries and pre-reduced iron carriers, granular coal) and combined sintering-pelletizing technologies are ways to break the direct dependence of CO2 emissions on primary energy consumption and to counteract the lowering quality of raw materials.

The scarcity of primary raw materials in general is with regards to the steel industry not a correct term. Material scarcity is controlled by supply versus de-mand. There are sufficient reserves and resources of iron ores and coals [1]. The problem is scarce resources of high-quality materials in terms of their physical, mechanical and chemical properties. Thus the iron content in iron ores decreased from 56.5 % in the year 2000 to about 52.7 % in 2012 and is expected to drop further [2]. As a result of such a development, an average reducing agent consumption in German blast furnaces increased by about 20 kg/t HM during the last ten years.

This contribution summarizes new developments on unconventional raw materials, mostly based on research activities of the IEHK/RWTH Aachen Uni-versity (further IEHK) and its partners.

Unconventional forms of coal and coke

Nut coke. The use of nut coke meanwhile became state of the art of the blast furnace technology aiming at lower coke costs by the application of under-sieve fraction and can per se hardly be referred to as un-conventional material. Operation of many blast furnaces has proved the possibility of coke saving and increase of furnace productivity when using nut coke in mixture with burden (usually with sinter) but reasons and mechanism of this phenomenon were not very clear until recent times. Consequently, its use still is not optimized.

Basically, three reasons may affect the decrease in coke rate [3]:

▷ Improvement of gas permeability in the “dry zone” of a blast furnace

▷ Improvement of reduction conditions of iron burden

▷ “Protection” of metallurgical coke from the solu-tion loss reaction in the shaft due to higher reac-tivity of nut coke.

Comprehensive studies on all phenomena mentioned are summarized briefly below.

Streaming conditions in the “dry zone”: A positive effect of nut coke on gas permeability in the BF “dry zone” has been proven. The pressure drop in shaft decreases as the nut coke rate increases and the nut coke effect becomes more remarkable at higher flow rates and Reynolds numbers. The big-ger the grain size of nut coke is, the stronger is its influence on the change of the gas permeability (graph in figure 1 ) [4].

1

Graph: decrease of pressure drop in blast furnace “dry zone” when using nut coke of various size; photos: sinter bed reduced without nut coke (a) and with nut coke (b) [4; 5]

Diagramm: Abnahme des Druckverlustes in der „Trockenzone“ des Hochofens mit Brechkokseinsatz verschiedener Koksgrößen; Fotos: Sinterbett reduziert ohne (a) und mit (b) Brechkoks [4; 5]


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Metallurgie

Reduction conditions in the shaft: The improvement of reduction conditions of iron burden while using nut coke has been proven [5]:

▷ The sinter/pellet reducibility increases in the temperature range of 900 − 1 100 °C as the nut coke ratio increases although the mechanism for sinter and for pellets is partly different.

▷ Nut coke in the sinter layer prevents the sticking of the sinter pieces (photos in figure 1 ) and in the pellet layer it promotes direct reduction of fayalite phase.Nut coke reactivity: The supposed preferential con-

sumption of nut coke by solution loss reaction was not proved by using coke traced with zirconium oxide but the results might depend on the nut coke grain size. To increase nut coke reactivity, activation agents such as a solution of Fe(NO3)3, slurry of Ca(OH)2 and iron oxides (Fe2O3 and Fe3O4) were tested [6].

High reactivity blast furnace coke. The use of coke with high CRI value was proposed for de-creasing the carbon consumption in the blast furnace. The idea behind, which is derived from the Rist’ diagram, is to shift the wustite reduc-tion equilibrium point to lower temperatures by decreasing the starting temperature of solution loss reaction. Such a coke can be produced e. g. by adding Ca-rich non-caking coal or reactive biomass (see section “Biocoke” below) or catalyst compounds (e. g. CaO or Fe2O3) to a coal blend, or coke pushed out of the coke oven can be treat-ed by coating the coke surface with catalyst or by impregnation of coke in catalyst solution [7]. Conditions for realization and consequences of the use of high-reactivity coke are discussed in [1]. As mentioned above, high-reactivity coke might be efficiently applied in the case of nut coke use, which can be catalysed/activated to reach high re-activity. Blast furnace coke, on the contrary, could

be even passivated e. g. by a composite colloid, while using activated nut coke [6]. It has also to be considered that coke reactivity might be affected by PC or other solid carbonaceous injectants. An experimental study testified that char and ash, generated from PC or waste plastics, may also influence coke properties and its reaction and degradation behaviour [8].

Coating the breeze coke to control its reactiv ity can also be applied to decrease NOx emissions in the sintering process. Thus a coating layer of CaO-Fe2O3 on coke surface promotes high-tempera-ture combustion and acts as catalyst for the reduc-tion of nitrogen oxides. Also coating of coke with CaO has been studied for decreasing NOx emissions by sintering [9].

Pre-treated anthracite. A technology for the pro-duction of thermo-anthracite and for charging this material with increased mechanical strength into a blast furnace for partial replacement of coke was de-veloped and realized in Ukraine [10]. It includes two steps: anthracite treatment in an electromagnetic field to remove water, followed by covering coal pieces with a polymer film (by means of spraying) to protect them from secondary moisture penetration in micro- and macro-fissure, and to further improve its strength and other properties. The anthracite treatment with magnetic field instead of time-con-suming heating enables significantly lower energy consumption for dehydration and, consequently, cost-efficient production.

The blast furnace technology with charging up to 40 − 70 kg/t HM of treated anthracite has been implemented at several blast furnaces including one with an inner volume of 5 000 m3. Thermo-anthra-cite is charged in mixture with iron burden. The coke rate was decreased and hot metal production costs were reduced [10].

2

Raw (left) and pyrolysed (right) briquette | Roh- (links) und pyrolysiertes (rechts) Brikett


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Metallurgie

Granular coal. Pulverized Coal (PC) became a stand-ard term for fine coal used for the shaft furnace injection (mainly, blast furnace but also Finex, Corex, OxiCup etc.). Fine coal grinding intensifies its combustion due to rising the specific surface of coal particles, but increases costs for the coal grind-ing due to additional electrical energy consumption and reduces mill productivity. PC with main mass fraction of grain size of <0.1 mm is typically used for blast furnace injection. Contrarily to PC, grain size of granular coal (GC) injected into a few blast furnaces in the UK and in the USA is coarser, e. g. 100 % <5 mm, 98 % <2 mm, 30 % <0.063 mm or 100 % <1 mm and 50 % <0.250 mm. The usage of the wider range and larger particle sizes results in less energy to mill coals [1; 11]. Reasons for the successful conversion of GC in a blast furnace and mechanism of this process are still not very clear and are being studied with the scope of cooperation between Tata Steel Europe and the IEHK.

Briquettes for Corex. The background for this appli-cation is similar to the nut coke use in a blast furnace. The smelting reduction processes Corex and Finex require mainly lumpy coal with well-defined prop-erties. Coal grain size of bigger than 8 mm (>95 %) is required to maintain sufficient char bed permea-

bility in a smelter gasifier [12]. Coal briquetting makes it possible to use fine coal and to reduce in such a way operational costs. To evaluate the con-version behaviour of briquettes, figure 2 , produced from mixtures of a low volatile and weak-baking coal and a high volatile and baking coal in differ-ent ratios with various binders, lab and pilot tests were performed under conditions simulating the reactions in different zones of the smelter gasifier: in the dome, in the fixed bed upper region and in front of the raceway [13].

The briquettes react similar to lumpy coal, ex-hibit sufficient mechanical strength and can suc-cessfully be used in the Corex process. The biggest effect of the briquette chemical composition and, particularly, binder type on their reaction kinetics occurs during the gasification in a CO2 atmosphere related to the fixed bed upper region.

Biocoke. Biomass into a coal blend can only be added in a small amount without negative effect on the blend fluidity and consequently on coke strength and reactivity. Until now, a maximum ratio of raw biomass in a coal blend was set below 2 − 3 % mass content. Recently, several types of biomass – both raw and pyrolysed ones at various tempera tures – were mixed in different proportions with coal and carbonized in a laboratory furnace [14]. Results showed that by means of biomass pre-treatment, and namely by pyrolysis at a temperature of about 400 °C, the biomass product ratio in the coal blend can be increased to some 5 % mass content, and in some cases at a higher pyrolysis temperature even more without remarkable negative change of CSR and CRI values of the coke produced. This phenom-enon can be explained by the coke microstructure matrix. In figure 3 boundaries between coal- and biomass-based structures in coke are coalesced, which means that biomass is integrated into the coke structure. Therefore, the coke strength is only slightly influenced. Broken grain boundaries be-tween coal and biomass with a weaker connectivity would lead to a bigger specific surface, thus result-ing in a higher reactivity. The connectivity of coal and biomass particles was improved by increased pre-treatment temperatures. The effect of biomass type on coke quality becomes smaller with a rising degree of pyrolysis [14].

Finally, it should be stressed that a decrease in the threshold temperature of the Boudouard reaction (see section “High reactivity blast furnace coke”) is more important than a direct replacement of coal with biomass in terms of CO2 emissions.

Biomass

Structural ingredients, conversion steps and op-tions for use. The thermal conversion behaviour of biomass (pyrolysis, gasification, combustion) depends

3

Surface microstructure of a) reference coke, b) bio-coke 400K10*, c) bio-coke 600K10, d) bio-coke 800K5; LOM, 200x [14] (*400K10: “400”: biomass pre- treatment temperature, “10”: biomass ratio in coal blend, % mass content; “Biomass” and “coal” areas in the images mean biomass-originated and coal-originated structures in bio-coke)

Oberflächenmikrostruktur des a) Referenzkokses, b) Biokokses 400K10*, c) Biokokses 600K10, d) Biokokses 800K5; LOM, 200x [14] ( *400K10: „400“: Vorbehandlungstemperatur der Biomasse, „10“: Biomasseanteil in der Kohlemischung, % Massenanteil; „Biomass“- und „Coal“-Bereiche in den Bildern bedeuten entsprechend aus Biomasse und Kohle stammende Strukturen)

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strongly on organic compounds, figure 4 , and inor-ganic matter (mineral phases forming ash). Structural ingredients of organic matter are primarily cellu-lose (e. g. (C6H10O5)n), hemicellulose (e.g. (C5H8O4)n) and lignin (e. g. C6H9O5 or [C9H10O3(OCH3)0.9-1.7]n). The shares of these organic compounds is varying in dif-ferent types of biomass, e. g. woody, herbaceous or animal ones.

Quality characteristics and requirements on bio-mass properties that should be considered by their metallurgical applications are discussed in [16].

Biomass can mainly be used in the steel industry by mixing or embedded in raw materials, by gasifi-cation with a syngas generation and by injection. With the exception of gasification, solid products of biomass thermal treatment such as charcoals, semi-charcoals or torrefied materials are rather suit-able than raw biomass [16]. In this chapter, only the direct use of biomass products by injection into a blast furnace and further metallurgical aggregates is shortly discussed. Biomass mixing in coal blend for biocoke production was already introduced, further applications will be presented later.

Direct use by means of injection. Results of recent studies at the IEHK on BF biomass injection can be summarized as follows. Conversion behaviour of charcoal and torrefied biomass is comparable or even better than that for PC under raceway-simulating conditions; oxygen in front of the tuyère disappears a little bit quickly, CO2 and temperature peaks are somewhat higher, figure 5 . Consequently coarser grinding of biomass products compared to PC might be sufficient. This could lead to an increase of the dust yield and energy and cost saving. BF operation parameters are strongly affected by biomass product chemistry. In general, PC can be replaced completely with charcoal; the coke replacement ratio around 1.0 kg/kg can be achieved. Concerning the torrefied materials, only “highly” torrefied biomass might be suitable for BF injection. Injection of torrefied biomass characterized by very high VM and very low carbon content would strongly affect BF oper-ation; therefore only partial PC replacement might be reasonable.

4

Conversion behaviour of biomass organic components depending on the temperature and the air-fuel ratio [15]

Umsetzungsverhalten der organischen Komponenten von Biomassen in Abhängigkeit der Temperatur und der Luftzahl [15]

5

Evolution of gas analysis and temperature during the injection of pulverized coal (PC) and charcoal (CC) in a pilot plant [17]

Gasanalyse und -temperatur während des Einblasens der Kohle (PC) und Holzkohle (CC) in eine Pilotanlage [17]


Biomass products can also be injected via tuyères in further shaft furnaces, e. g. into the Melter Gasifier of Corex or Finex plants, cupola and OxiCup furnaces. Furthermore they can be injected into the electric arc furnace, e. g. to control characteristics of foaming slag which increases the efficiency of energy transfer from graphite electrode to the steel bath.

Waste plastics

In 2013, worldwide plastic production reached nearly 300 million t/a [18]. In the European Union, plastic recycling and energy recovery reached 62 % while aiming for zero plastics to landfill by 2025 [18]. Use of waste plastics (WP) in the steel industry may contribute to both waste recycling and carbon dioxide emission mitigation due to additional hydrogen input. On the other hand, the inhomogeneity of the physical and chemical properties of WP has to be considered. In addition to different polymers such as PVC, PE, PP, PS and PET, WP can contain various metallic and mineral impurities including harmful elements. WP of both municipal and industrial origin, can be used in the steel industry similar to biomass: by injection, gasification and incorporation into raw materials. Prior to its use in metallurgical processes, it can undergo mechanical or/and thermal processing. In this chapter only direct WP use by injection into a blast furnace is presented.

Blast furnace injection. There is industrial experience of WP injection into blast furnaces in Germany, Japan and in Austria.

Recent study on reaction kinetics of waste plastics showed that despite extremely high volatile matter content of about 80 − 90 % its conversion degree under the racewaysimulation conditions is

rather low resulting in residues entering the bird’s nest and shaft [19].

No supposed circulation of coarse plastics particles was observed by WP injection into the pilot coke bed simulator. Unburnt in the raceway residues can be consumed under conditions of bird’s nest and shaft, see e. g. figure 6 ; extension of reactions of secondary gasification depends among others on residue burn out rate and size [19].

Unconventional forms of sinter, pellets and composites

Biosinter. The effect of biochar substitution for coke breeze on sintering performance and sinter quality was investigated; biochar from woody pellets was applied [20]. The sintering process was characterized by a lower vertical speed and a lower production rate as the biochar substitution ratio increased. The tumbler index decreased while the biochar rate increased.

The reducibility of biosinter was increased compared to that of coke breeze. Under the examined conditions, the replacement of 25 % coke breeze with biochar seems to be an appropriate level for optimum sinter productivity and quality, figure 7 [20].

Hybrid sinter-pellets and mini-pellets for sinter-ing. Due to the rising amount of ultra-fines in the sinter mix, new sintering technologies for formation of micro/minipellets have been and are being developed. A Hybrid Pelletized Sinter agglomeration process was proposed and tested in Japan, which includes the pelletizing of fine ores prior to the sintering process [21]. At the IEHK, the minipellets (4 − 9 mm) were composed of ultra-fine iron ore (−0.2 mm) and calcium hydrate while coke fines was used as coating layer [22]. The curing of green

6

Plastic residues on coke surface (in red circles), screen shot by thermo-vision camera; above: plastic pellets before injection [19]

Kunststoffreste auf Koksoberfläche; Screenshot von einer Thermovisionskamera; oben: Kunststoffpellets vor dem Einblasen [19]

7

Effect of biochar on sinter process [20]

Einfluss des Biochars auf den Sinterprozess [20]


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Metallurgie

mini-pellets with CO2 improved their strength. The utilization of cured mini-pellets improves the sinter bed permeability and results in shorter sintering time. Currently a one-step approach avoiding the mini-pellet production step is being investigated at the IEHK in co-operation with industrial partners. Sinter mixing and granulation of ultra-fine iron ore (pellet feed, about 90 % below 0.1 mm) takes place simultaneously [23].

Self-reducing pellets. The use of self-reducing pellets (SRP) with cold-embedded carbonaceous materials targets at the lowering of the thermal reserve zone temperature and, consequently at decreasing the coke and total reducing agent con-sumption in the blast furnace. Furthermore, some pellets have the tendency to swell enormously in a reducing atmosphere at the certain temperature range; this phenomenon can lead to various oper-

ational problems and loss of productivity e. g. in the rotary hearth furnace and shaft furnaces for direct reduction. The use of SRP can hinder or reduce their swelling and even lead to shrinking [1].

The laboratory reduction experiments with SRP produced from hematite iron ore testified that the starting temperature of the solution loss reaction can be decreased by 50 − 100 K depending on the reducing agent type and amount (coal, charcoal or coke were examined in the range of 10 to 18 % mass content) compared to that while using conventional pellets [24]. Charcoal has the strongest effect.

The test results also showed that SRP are charac-terized by higher reducibility. The SEM images of the samples after reduction are shown in figure 8 (phase analysis is obtained from SEM combined with EDX). Furthermore, the volume of SRP is de-creased after reduction; it means that no swelling but shrinking occurs [24].

Induration of SRP by firing causes the significant carbon loss due to reduction. Therefore they can be used e. g. as a direct reduced iron (DRI) in different processes but not suitable for BF charging as an ore burden. For this purpose, the SRP were hardened by cement-bonded curing. The crushing strength of these pellets is sufficient for the usage in low-height blast furnaces. During the reduction experiments, no effect of cement on the SRP reducibility was found [24].

Other experiments simulating the conditions of DR processes, were performed with SRP produced from various magnetite and hematite ores and con-centrates by embedding high and low volatile coals, charcoal, unburned carbon from fly ash and waste plastics. Optimal shrinkage can be controlled by the type and rate of embedded reductant as well as by reduction time and temperature [25]. An ex-ample of the influence of reduction temperature and of embedded carbonaceous material is shown in figure 9 .

Iron ore-carbon composites and ferro-coke. Similar to SRP, pressed iron ore-carbon composites might be used in the blast furnace burden. A study on carburization behaviour of iron ore − charcoal composites showed that charcoal ash prevents the reaction between iron and carbon. Ash has a strong-er effect on the carburization behaviour than car-bon crystallinity. The thermal-treated charcoal (at 1 000 °C) has the advantage for carbon dissolution in molten iron [26].

Iron ore-carbon composites (hematite powder was applied) with embedded pulverized Refuse-De-rived Fuel (RDF) made from municipal solid waste were produced and examined as well [27]. Main contents of the RDF are waste plastics mixed with paper and wood. The RDF sample used in this study contained 76.4 % C, 9.8 % H, 12.6 % O, 0.3 % N, 0.1 % S; ash content was 4.9 %. Results obtained

8

SEM micrographs of pellets after reduction [24]; phase identification: metallic iron (white zones), FeO (grey zones), mixing phase of Fe-Si-Al-O (dark grey zones); black zones are porous

REM-Aufnahmen von Pellets nach Reduktion [24]: metallisches Eisen (weiße Zonen), FeO (Grauzonen), Mischphase von Fe-Si-Al-O (Dunkelgrauzonen), Poren (Schwarzzonen)

9

Volume change of hematite pellets with embedded low volatile coal (LVC), reduction time 30 min [25]

Volumenänderung der hämatitischen Pellets mit eingebetteter niedrigflüchtiger Kohle (LVC), Reduktionszeit 30 min [25]


indicate that proper gases for reduction, mainly consisting of H2 and CO, were generated [27].

Ferro-coke, which is obtained by combining iron ore with coal at the time of manufactur-ing, can also be considered as a kind of a highly reactive coke. The use of hyper-coal (HPC) as a binder to increase the strength of ferro-coke has been studied. HPC is an ashless coal obtained via fractionation. In particular, it is achieved by gravitational separation of the solvent-insoluble portion of ash and unreacted products from coal liquefied oil (coal to liquid). HPC liquefies at a low temperature and shows high fluidity. Because of these properties, it may be possible to increase coal particle adhesion of coke which is made of slightly caking coal [28].

Metallurgical off-gases and syngas

Free resources of off-gases from metallurgical pro-cesses such as BF top gas, BOF gas, coke oven gas (COG) or Corex-export gas can be used in raw state or after a certain physical, thermal and/or chemical preparation as reducing media in the BF and DR pro-cesses. Furthermore, reducing gas can be generated by gasification of biomass and secondary resources, e. g. waste plastics.

Top gas recycling. The development of a BF tech-nology with injection of hot reducing gases, gen-erated by gasification of low grade coal, natural gas or waste as well as by top gas recycling has a long history. In the scope of the European program ULCOS (Ultra-Low CO2 Steelmaking), so-called TGRBF (Top Gas Recycling Blast Furnace) was de-veloped and tested aiming at a drastic reduction of carbon dioxide emissions. The key feature of this approach consists of the CO2 removal from the top gas using the CCS (Carbon Capture and Storage) technology. During the pilot trials at an experimental blast furnace, the carbon input could be reduced by about 25 % [29].

Besides the injection of decarbonated and pre-heated top gas in the lower shaft, the TGR-BF technology includes injection into the hearth of recycled reducing gas (primarily carbon mon-oxide) together with two further substances: PC and oxygen. The interaction between these three co-injected materials is of great importance in view of coal conversion. Results of the study using a multifunctional injection plant MIRI at the IEHK showed that the mixing of PC and oxygen before entering the furnace reaction zone (scenarios 2 to 4 in figure 10 ) does not essentially improve the coal conversion degree while co-injecting reducing gas. The presence of carbon monoxide worsens the coal conversion, even when the PC stream is separated from CO gas by oxygen (scenarios 1 and 2 in figure 10 ) [30].

Coke oven gas. The injection of H2-rich COG is prac-ticed in a number of blast furnaces. Apart from the known effect of COG on combustion conditions, gas characteristics, direct reduction rate and other oper-ation parameters, it affects the reduction behaviour of iron burden. Sinter reducibility under the BF shaft simulating conditions was examined at the IEHK for three scenarios: reference case without COG (gas com-position corresponds to the BF operation with about 145 kg/t HM PCI and about 3 % oxygen enrichment), injection of 150 m3/t HM COG (115 kg/t HM PCI and 19 % O2 enrichment) and injection of 300 m3/t HM (85 kg/t HM PCI and 38 % O2). The test results testified the enhancement of the reduction process and con-sequently the decreasing of coke consumption and CO2 emissions while injecting COG. For example, at the non-isothermal reduction the lowest reduction degree (~50 %) was observed in the reference case without COG while it increased to 75 and 95 % in cases with 150 and 300 m3/t HM COG respectively [31].

The application of COG for DRI production in the integrated steelmaking route is being discussed in-tensively. An experimental study at the IEHK on iron ore pellets reduction with simulated original and reformed COG was conducted under the temperature conditions in the Midrex and HyL processes [32]. The results were compared with those gained by pellets reduction with the original and reformed natural gas. The highest reduction degree was obtained by using the reformed COG. A slowdown phenomenon at 900 − 980 °C, figure 11 , obviously appeared due to

10

Injection test scenarios [30]: 1: individual entering of three components into the reaction zone (simulated raceway) via separate inlets, 2: pre-mixing of PC and oxygen using a short mixing area, 3: pre-mixing of PC and oxygen using a long mixing area, 4: pre-mixing of PC and reducing gas

Szenarien der Einblasversuche: 1: Eindüsung der drei Komponenten in die Reaktionszone durch separate Zuleitungen, 2: Einblasen von PC und Sauerstoff mit einer kurzen Mischungszone, 3: Einblasen von PC und Sauerstoff mit einer langen Mischungszone, 4: Vormischen von PC und Reduktionsgas [30]


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higher CaO content in pellets which act as catalyst for methane decomposition.

Syngas generation. Gasification of biomass in a flu-idized bed can be used to produce synthesis gas for different applications, including the reduction of iron ore. The High-Temperature-Winkler-gasifier (HTW) at the IEHK is used for the production of reducing gas with high amounts of CO and H2 and lower amounts of CO2, H2O and tars [33]. Air, oxygen, carbon dioxide, water vapour and their mixtures can be injected into

the pilot plant. By gasification of biomass (different woody pellets and fibrous biomasses like miscanthus), a degree of conversion over 95 % is realized.

DRI / HBI / LRI for a blast furnace

The usage of partially reduced iron burden is targeted at the increase of blast furnace productivity, flexibil-ity, and the decrease of the reducing agent rate and CO2 emissions. These materials can be introduced into the blast furnace in two ways: charging on the top and injection via tuyères.

Charging. The charging of sponge iron into a blast furnace in the form of HBI (Hot Briquetted Iron) is preferable because the structure of HBI is, compared with the very porous DRI (Direct Reduced Iron), much more compact. The main reason of the HBI charge into the blast furnace is the use of H2-containig low-priced reducing agents for their production, typically natural gas, particularly, in form of shale gas, and saving in such a way the metallurgical coke for hot metal production. Total reducing agent consumption for both processes – pre-reduced burden production and their melting in the blast furnace – remains roughly the same as in the case of conventional bur-den use or might be even higher.

The largest Midrex module worldwide with a cap-acity of 2.0 million t/a was commissioned in the au-tumn of 2016 by voestalpine Stahl in Texas, USA [34]. It is planned to transport a part of the HBI produced to Europe and to charge it into the blast furnaces in Austria. An increase in blast furnace productivity by 7 − 15 % and decrease in coke rate by 25 − 35 kg/t HM is expected while charging 100 kg HBI/t HM [35]. This example shows that a BF can be operated flexibly according to the economic conditions. In general, the drawback of DRI/HBI charging onto the top consists of restriction of the blast furnace process

11

Reduction curves of pellets isothermally reduced with COG at different tempera tures [32]; in micrographs dense grains of metallic iron are seen in white

Isothermische Reduktion von Pellets mit Koksofengas [32]; dichte Eisenpartikel sind in der Aufnahme weiß dargestellt

12

Pilot DRI injection trial: image of thermo-vision camera during the injection (left) and iron-coke agglomerate extracted after the trial (right) [36]

Pilotversuch zum DRI-Einblasen: Aufnahme der Thermovisionskamera während des Tests (links); Eisen-Koks-Agglomerat, extrahiert nach dem Versuch (rechts) [36]


to a re-melting function due to shifting the indirect reduction to a DR plant.

Injection. Efficient use of sponge iron in the blast furnace targeting at an increase of its productivity and flexibility can be achieved by means of injection of DRI/LRI fines via tuyères (LRI: Low Reduced Iron is characterized by a lower reduction and metalliza-tion degree than that for common DRI). The reaction behaviour of pre-reduced iron ores during injection into BF was examined at the IEHK by means of la-boratory and pilot experiments, as well as mathe-matical modelling [36].

The extent of re-oxidation of DRI/LRI is primarily determined by its microstructure and reduction de-gree. The reduction degree also strongly affects the adiabatic flame temperature. The study on the DRI and LRI re-oxidation behaviour in the raceway under consideration of costs for their production allows for concluding that injection of LRI is more favourable. Furthermore, the injected DRI/LRI affects coke react-ivity and may cause a higher coke renewal rate and higher permeability in the bird’s nest, figure 12 . It was also found that co-injection of DRI/LRI with PC suppresses re-oxidation.

Outlook

Application of novel materials for hot metal and sponge iron production – renewable and secondary energy sources as well as unconventional forms of coal, coke and iron burden – can be considered as short and mid-term measures to mitigate climate and resources trends. To radically counteract these challenges, intensive use of hydrogen starting with available hydrogen-containing media such as original and reformed natural and coke oven gases or coal-

bed methane, over pure hydrogen produced e. g. by electrolysis, photo-catalysis or membrane processes, up to plasma hydrogen etc. will be unavoidable in future. Despite numerous efforts in all the mentioned fields, the status quo of hydrogen-based ironmaking technologies is still rather modest with the exception of natural gas use. The IEHK is designing and creat-ing a new lab for systematic study on application of hydrogen and hydrogen-containing synthetic and other media as well as bio-based sources in existing and new processes for iron extraction.

With regards to scarce resources of high-quality materials, the share of iron-containing compounds in iron ores will become smaller and smaller. In future the standard classification of ferrous and non-ferrous metallurgy will perhaps disappear. New technologies enabling the extraction of multiple metals from ores or even from rocks such as basalt containing e. g. Si, Al, Fe, Mg, Ca and other elements will probably be developed. The IEHK is working on the development of new methods, technologies and aggregates for the multivalent integrated use of various primary and secondary raw materials. The IEHK is also a mem-ber of the network Aachen Competence Centre for Resource Technology (AKR e. V., Aachener Kompe-tenzzentrum für Ressourcentechnologie).

This paper is mainly based on keynote lectures at the 10th CSM Steel Congress on 22 October 2015 in Shanghai and at the 2nd Int. Symposium on Development of the Blast Furnace Technology on 20 July 2016 in Dnepropetrovsk.

Dr.-Ing. Alexander Babich, Prof. Dr.-Ing. Dieter Senk, Dept. of Ferrous Metallurgy (IEHK), RWTH Aachen University, Aachen, [email protected]

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