2 Steel Industry: Production Processes

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2 Steel Industry: Production Processes 2.1 Introduction The chapters 4 through 7 highlight various aspects of the introduction of new technologies. This chapter presents the technology data on existing
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2 Steel Industry: Production Processes 2.1 Introduction The chapters 4 through 7 highlight various aspects of the introduction of new technologies. This chapter presents the technology data on existing and new technologies. After a description of present technologies, and those available in the near future, the chapter gives an overview of production routes, based on a selection of the indicated technologies. Tables present the data on the included processes, while the main text provides more detailed, often technical information. In addition, text boxes provide background information on some processes. 2.2 Basics of iron and steel production Secondary, or scrap based, production of steel is very straightforward. Melting steel scrap results in liquid steel, which can be shaped in the desired forms by casting, rolling and finishing. However, scrap steel, both quantitatively and qualitatively, cannot meet the entire demand for steel. Therefore, there is a continuing demand for primary steel. In essence, all primary iron and steel production processes are based on the same chemical reactions. In iron ore, iron is presents in its oxidised form. In iron reduction processes, a reducing agent, based on carbon or hydrogen, removes the oxygen from the iron. This removal can take place either above or below the melting point of the ore and the reduced iron. Reduction above the melting point results in pig iron. Above the melting point, most of the gangue materials segregate from the liquid iron, and float upon it. Because of the high temperature and the liquid state of the iron, carbon readily dissolves in the pig iron, resulting in typical carbon contents between 4 and 6%. Reduction and dissolution of some of the gangue material may result in silicon and manganese contents of about 1%, depending on the ore composition and process conditions. In addition, pig iron contains some sulphur and phosphorus. Oxygen blowing converts the pig iron to steel by oxidation of the carbon, silicon, and phosphorus. The carbon escapes mainly as carbon monoxide, while the oxidised silicon, sulphur 4 and phosphorus segregate into the slag. This segregation process is enhanced by addition of flux materials, such as lime. Reduction below the melting point results in directly reduced iron (DRI), which retains the original shape of the ore, and includes the gangue material present in the ore. At the temperature of reduction, the carbon of the reducing agent hardly dissolves in the iron, therefore DRI contains virtually no carbon. For conversion of DRI to steel, removal of the gangue material is necessary. Moreover, the carbon content has to be adjusted to the required amount. Therefore, melting the DRI is necessary to convert the DRI to steel. Although the chemical reactions involved in the various steel production methods are very similar, steel producing installations and the flows therein are very different. Therefore, the various production routes have different demands with regard to resources. Iron ore occurs as lump ore and fine ore, and the latter is often processed into sinter or pellets. Both natural gas and coal are used as reducing agents. The blast furnace requires 4 Nowadays, removal of the sulphur generally takes place prior to the oxygen blowing, as the reduced state of the pig iron promotes the removal of the sulphur. 21 the conversion of an important share of the coal to coke. The demands with regard to the resources, and necessary preprocessing steps strongly influence costs, energyuse and emissions of the various routes leading from ore to steel. 2.3 Production processes This section gives an overview of some production technologies relevant for steel production. Besides established technologies, it includes recently introduced technologies and those likely to be available within the next 25 years. The technologies have been categorised according to their positions in the production chain, shown in figure 2.1. The categories are material preprocessing, iron reduction, steel production and casting, rolling and finishing. Special attention is given to those process characteristics that play an important role in the choice between different production processes. Apart from the basic data on costs and energy consumption, these characteristics include the dependence on products of other production processes. Such dependence has important consequences for the overall plant layout, and for the attractiveness to switch to other processes. The technologies that are the basis for the analyses in chapters 4 through 7 receive a detailed presentation in process tables. These present the labour input and the investment per production capacity, and show ranges of input and output data. In addition, the proces tables specify the typical values used in the analysis in chapter 4, which is based on fixed process characteristics. The status of the typical values may differ strongly from values found in specific plants. The processes included are representations of types of production processes, and subdivisions of these have not been listed separately. Where possible and relevant, the text pays attention to the range of process characteristics found in actual installations. 2.4 Process data As far as possible, process data originate from public data sources. For processes currently in use, Eurofer [10] has been an important source of economical and consumption data. Table 2.33 at the end of this chapter gives an overview of depreciation data according to Eurofer. In case of contradictions between Eurofer and other sources, Eurofer data generally have prevailed, because of the depth of coverage of Eurofer and its West European focus. The Making, Shaping and Treating of Steel [39] has been a useful and important source for background information on processes and of more precise consumption data. For new processes, various more specialised sources have been important. It was often necessary to fill gaps in the data by estimates based on the available information on the respective processes and on comparable data from other processes. The process data of all processes follow the format shown in box Preprocessing of raw materials The preprocessing of materials involve various processes that modify the characteristics of materials such that they meet the demands of the main processes for iron reduction and steel production. The reasons of preprocessing follow in the description of the main processes. Preprocessing steps included are coke production, ore agglomeration and oxygen production. 22 Figure 2.2 Overview of steel production routes 23 Box 2.1 Explanation of process table format Table number installation name Product: product Name Unit Minimal Maximal Assumed Input name dimension value value value Output name dimension value value value Investment Euro /. year O&M Euro/t Labour manhours Lifetime years Scale Megas per year Status specification Other remarks Remarks 6 Flow dimensions in the respective units:, GJ, GJ e. Electricity specified in GJ e. All flows exclude energy required for energy. 6 Minimal and maximal values only specified if various values were found and if these were substantially different from each other 6 The tables specify financial data in Euro. As most sources do not specify the year for which cost data apply, it is not possible to convert cost data to the Euro equivalent of a certain year. 6 O&M: operation and maintenance 6 The status indicates the current availability or state of development of a process. Coke production [10, 48, 39] Table 2.1 shows the characteristics of the coking process. It involves the conversion of coal into a volatile fraction and the desired coke. Coke is a solid material, consisting mainly of carbon. It does not liquefy at high temperatures. The volatile fraction consists of coking gas and coal tar. Coke is essential in the blast furnace. Some cokebreeze is applied in sinter production but can be substituted by anthracite. Table 2.1 Coke oven [10] Product: Coke Name Unit Minimal Maximal Assumed Input Coal Gas t GJ Output Coke Breeze* Gas byproduct t t GJ GJ Investment Euro/t/y O&M Euro/t Labour 0.55 mh Lifetime 40 y Scale 1.34 Mt/y Status established, final stage Other Coke input output characteristics depend heavily on both process properties and properties of the coking coal. The steel producer may extend the life of the installation considerably. Traditional coke production is a very polluting process (table 2.2), and the byproducts from the volatile fraction contain many carcinogenic substances, which may result in problems with the sale of these byproducts. New coking processes avoid part of these problems, but they do not produce the valuable coking gas. Conventional coke ovens that meet presently acceptable environmental standards are very expensive. Table 2.3 gives an impression of coke oven specific investments for some cases. 24 Table 2.2 Conventional coke oven emissions Category g/ coke TSM (total suspended matter) 680 Particulate matter 10 µm 100 SOx 1420 NOx 860 VOC (volatile organic compounds) 340 CO 180 Table 2.3 Some investment data for conventional coke ovens [48] Specification investment capacity per annual Euro Euro 1987 tall oven, Germany short oven, Germany Kaiserstuhl III coke works, 120 tall, wide ovens, dryquenching, 650 million 2 million 323 byproducts plant Rebuild US, Great Lakes Division of National Steel 360 million 0.9 million 404 Estimate Nippon Steel Corporation, Japan 320 million 1.2 million 269 The coke yield depends on the characteristics of both the coal and the process itself. The suitability of coals for coking does not only depend on a sufficient yield, but also on their dilatation behaviour during the coking process. During the coking process the coal volume first increases, to shrink again towards the end of the batch. Conventional coke ovens cannot withstand the dilatation force of many coals. The specific characteristics of coking coals are the cause of their scarcity and relatively high prices. Sinter and pellet production Ore in its natural state occurs as lump ore and fine ore. Lump ore is a suitable feed material for many processes. A main drawback is the price of lump ore, resulting from its wide applicability. Another may be the ore grade, as it is not possible to increase the iron content of the ore while conserving its lump form. For most processes, fine ore is not a suitable feed material. However, because of its low price it is attractive to process fine ore into larger aggregates. These aggregated forms are often even better feed materials than lump ore. Sintering and pelletising are two common ore agglomeration processes. Iron reduction processes that can consume fine ore have an important cost advantage. They avoid the additional costs of either ore agglomeration or of the purchase of more expensive lump ore. Sinter production [10] Sintering (table 2.4) involves the heating of fine ore, causing it to agglomerate into larger granules. The structure of the ore mass becomes more open, facilitating the passage of gasflows. Sinter is the most important ironcontaining feed material for the blast furnace. In sinter production, ore, coke breeze or anthracite and flux materials are intimately mixed and fed uniformly onto a travelling grate. Near the feed end, the charge is ignited. Air is pulled downwards through the feed to burn the fuel (coke breeze or anthracite) by downdraft combustion. As the mixture moves towards the discharge end of the bed, the combustion front moves progressively downward, creating sufficient heat to sinter the fine ore particles into porous clinker. The incorporation of blast furnace flux in the sinter saves fuel in the blast furnace. Data 25 indicate 201 kg of saved coke for each metric of limestone charged into the sinter plant instead of in the blast furnace. Use of sized sinter further increases iron production rates in the blast furnace Pellet production [39, 30] Pellet production (table 2.5) also changes the structure of the ore. Rolling of fine ore, mixed with a binder, produces green pellets. Baking stabilises these pellets. Pellets, just as sinter, provide a more open structure to the ore mass. Pellets are used in the blast furnace, but they are more important as a feed material for COREX and direct reduction processes such as Midrex. Pellet production takes place by rolling of ore fines to balls, green pellets, and subsequent hardening to baked pellets. Ore fines, sometimes after additional grinding, are mixed with a binder (bentonite, clay, hydrated lime) and optional moisture. The mix is fed into a ball drum or disc pelletiser for the formation of the green pellets. Baking of these takes place in a travellinggrate system, a gratekiln system, or a shaft furnace. In general, fuel consumption depends on both the furnace design and the feed material. Of the various systems, the shaft furnace has the lowest energy consumption, but its reliability is lower than that of the other types, due to the occasional formation of large aggregates in the shaft. New developments include selffluxing and prereduced pellets. Around 1985 these were becoming accepted on a commercial scale. Table 2.4 Sinter production [10, 30, 34] Product: sinter Name Unit Minimal Maximal Assumed Input fine ore additives breeze/anthracite gas electricity GJ GJe Output sinter 1 Investment Euro/t/y O&M 4.8 Euro/t Labour 0.14 mh Lifetime 30 y Scale 3 Mt Status established Other Life may exceed 50 years Table 2.5 Pellet plant Product: pellets Name Unit Minimal Maximal Assumed Input fine ore gas electricity binders GJ GJe Output pellets 1 Investment 45.3 Euro/t/y O&M 4.8 Euro/t Labour 0.14 mh Lifetime 30 y Scale var Status established Other Oxygen production [8, 10, 3, 39, 62] Traditionally, in the steel industries cryogenic separation of air has been the predominant process for oxygen production. The required purity of the oxygen depends on its application. Highpurity oxygen ( 99.5%) is essential for the basic oxygen steel production and for improvement of the steel quality in electric steel production. Around 95% purity 26 oxygen may be used as oxygen enrichment in the blast furnace. Many new production processes allow 95% lowpurity oxygen, such as COREX, CCF and Circofer. For low purity oxygen other air separation processes are available, with lower energy use. With membrane processes and Vacuum Swing Adsorption or Pressure Swing Adsorption processes, oxygen production for these applications may be much cheaper. Cryogenic oxygen production optimised for these applications will be much cheaper, too. There is no lack of data on oxygen production, but most sources are incomplete. Moreover, some sources specify contradictory data and the incompleteness of the data prevents insight in the causes thereof. Table 2.6 gives an overview of data on oxygen production from various sources. Oxygen production costs and energy use will become more important, as many future iron production processes are likely to use large amounts of oxygen. Electricity costs have the largest impact on oxygen prices [62]. Table 2.6 Data on oxygen production. Economic data in Euro per oxygen, energy in GJe/ oxygen, assumed energy price 11.1 Euro/GJe Source system purity energy, incl costs [10] costs investments remarks % GJe Euro Euro Euro/t cap [10]???? 64? presumably purchase costs for steel producers [62]? 1Mt/y 9599?? up to 80.8 oxygen supply for synthesis gas production [39] C ~ ?? + compression and storage C ~ ?? only air separation C ?? only air separation [3] C? ?? oxygen for coal gasification [8] VSA ?? gaseous oxygen PSA ?? gaseous oxygen C Cryogenic VSA Vacuum Swing Absorption PSA Pressure Swing Absorption The total costs according to Eurofer [10] seem very high, compared to both the total production costs of oxygen for synthesis gas, and the energy costs for the other sources. It is difficult to explain the gap between the energy costs and the total or attribute this to the various cost factors. Presumably, the Eurofer costs are based on the purchase costs for oxygen. Especially small scale producers may have to pay high prices. Table 2.7 Cryogenic oxygen plant, highpurity oxygen for steel production, derived from table 2.6 Product: oxygen % Name Unit Minimal Maximal Assumed Input air electricity ton GJe Output oxygen nitrogen argon ton ton ton Investment 80.8 Euro/t/y O&M 11 Euro/t Labour 0.2 mh Lifetime 20 y Scale var Status established Other Pressurising and storage. Assumed total costs per, ex depreciation, 34 Euro 27 However, primary plants will usually have an onsite oxygen plant. The energy consumption data from all sources are in the same range, with the exception of the Pressure Swing Adsorption process, which operates on smaller scales. The other sources specify an energy consumption between 1.08 and 1.8 GJ e per of oxygen. The differences show a reasonable match with the specifications of purity, pressure and the presence of storage facilities. The process characteristics used for the analyses have been estimated from the above data. The cost and energy data of the high purity oxygen production (table 2.7) for steelmaking assume highpressure storage, to guarantee the oxygen supply of the oxygen steel converter, a batch process. Total costs are assumed to be about 12 Euro per higher than those of the oxygen for synthesis gas production [62], because of the required purity and the additional costs of pressurising and storage equipment. Investment costs have been based on the highest costs specified for the oxygen production for synthesis gas, 80.8 Euro per capacity. Cost and energy data for the low purity oxygen (table 2.8), for use in the blast furnace and some new iron reduction processes, assume gaseous oxygen production, without storage. The consumption of oxygen in continuously operating installations will probably not require storage facilities. The costs are assumed to be in the range of those of the oxygen production for synthesis gas production [62]. As the required purity, 95%, is in the lower boundary of the specifications from this source, investment costs have been assumed to equal the lowest costs specified. Total costs are estimated based on the relatively high electricity costs in the EU Iron reduction The production of reduced iron involves the separation of the iron from the oxygen in the ore. There are two main forms of raw reduced iron, namely hot liquid pig iron and solid direct reduced iron (figure 2.1). Their characteristics determine the subsequent steel production processes. The basic chemistry of all iron reduction processes is the same, only the rates of the relevant chemical reactions are different. Table 2.8 Low purity oxygen plant, oxygen for iron reduction processes, derived from table 2.6 Product: oxygen ~95 % Name Unit Minimal Maximal Assumed Input air electricity ton GJe Output oxygen nitrogen argon ton ton ton Investment 48.5 Euro/t/y O&M 8 Euro/t Labour 0.1 mh Lifetime 20 y Scale var Status existing technology Other No pressurising and storage. Assumed total costs per, ex depreciation, 22 Euro Table 2.9 gives an overview of the selected iron reduction processes. Table 2.10 lists iron reduction processes based on the reducing agent, steel process and required ore form. The choice of processes for the analyses in chapter 4 through 7 is based on data availability, category representation, current importance and estimated future potential. 28 Table 2.9 Processes for iron reduction included in the analysis, with required fuel, iron feed material, product and oxidant Process Product Fuel Iron feed Oxidant Blast furnace pig iron cokes/coal sinter/pellets/lump air (+oxygen) COREX pig iron coal lump/pellets oxygen CCF pig iron coal fine ore oxygen MIDREX DRI gas lump/pellets air Circored DRI gas fine ore air Circofer DRI coal fine ore oxygen Table 2.10 Primary steel production routes by the listed name of the iron production process, categor
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