Policy Analysis pubs.acs.org/est
Mapping the Global Flow of Steel: From Steelmaking to End-Use Goods Jonathan M. Cullen,*,† Julian M. Allwood,† and Margarita D. Bambach‡ †
Department of Engineering, University of Cambridge, Trumpington Street Cambridge, CB21PZ United Kingdom Institute of Ferrous Metallurgy, RWTH Aachen University
‡
S Supporting Information *
ABSTRACT: Our society is addicted to steel. Global demand for steel has risen to 1.4 billion tonnes a year and is set to at least double by 2050, while the steel industry generates nearly a 10th of the world’s energy related CO2 emissions. Meeting our 2050 climate change targets would require a 75% reduction in CO2 emissions for every tonne of steel produced and finding credible solutions is proving a challenge. The starting point for understanding the environmental impacts of steel production is to accurately map the global steel supply chain and identify the biggest steel flows where actions can be directed to deliver the largest impact. In this paper we present a map of global steel, which for the first time traces steel flows from steelmaking, through casting, forming, and rolling, to the fabrication of final goods. The diagram reveals the relative scale of steel flows and shows where efforts to improve energy and material efficiency should be focused. IEA estimates that only 5% of the total “best practice” reduction potential is available from steel finishing improvements (ref 7, Figure 16.8). However, although energy use and environmental impacts are concentrated in upstream steelmaking processes, CO2 emissions can be thought of as “embodied” in the steel and then “thrown away” when scrap steel is generated in downstream fabrication processes. Recycling does help, but the melting of scrap steel is still energy intensive and generates additional CO2 emissions. The starting point for allocating environmental impacts to final steel products is to accurately map the global steel supply chain. Quantifying the flows of steel through the supply chain is critical to understanding what drives emissions, and allows strategies that deliver more final goods with less steel, to be found. Despite the importance of steel as an engineering material and the significant fraction of global emissions arising from steel production, an accurate map of the global steel supply chain does not exist. This paper aims to collate the best available data for steel and trace the flow from liquid steel to final products, in an accessible visual form. 1.1. Steel Production Statistics. A number of different organisations collect steel statistics for different geographical regions, focusing on certain sections of the supply chain, and reporting these in various formats. The World Steel Association (worldsteel) publishes the most comprehensive view of global steel production. This is complemented by production statistics derived from: regional associations (e.g., The European
1. INTRODUCTION Steel is arguably the most important engineering material in use today. Ashby states that “Steel may lack the high-tech image that attaches to materials like titanium, carbon-fibre enforced composites, and (most recently) nano-materials, but make no mistake, its versatility, strength, toughness, low cost, and wide availability are unmatched” (ref 1, p 18). In little over 150 years, since the Bessemer process allowed steelmaking at the industrial scale, global annual demand for steel has risen to almost 1.4 billion tonnes (Gt) per year2that is 200 kg per year for every person on earth. This demand is driven by the need to create and maintain stocks of steel products, which Müller et al.3 calculate to have reached 8−12 tonnes per person in many developed nations. Our society is addicted to steel. Demand for steel products creates carbon emissions. Onequarter of all industrial CO2 come from steelmakingequal to 9% of global CO2 from energy and industrial processes making steel’s carbon footprint larger than any other industrial sector. Furthermore, the historical demand for steel has increased 4-fold since 19604 and is expected to rise to between 2.3 Gt and 2.8 Gt by 2050 according to predictions by the International Energy Agency (IEA) (ref 5, p 49). Increasing demand adds to the carbon problem for steel: to achieve the targeted 50% reduction in carbon emissions, against a doubling in steel demand, requires the CO2 emissions per tonne of steel to be reduced by 75%. The majority of the industry’s CO2 emissions associated with steel result from the steelmaking, from ore and from melting scrap.6 For this reason, plans for reducing CO2 emissions have focused on the upstream steelmaking processes (including mining, reduction, steelmaking, casting, and bulk forming) rather than downstream fabrication processes. For example, the © 2012 American Chemical Society
Received: Revised: Accepted: Published: 13048
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•Yellishetty et al.20 reports on the historical flows of the iron ore and steel industry from 1950 to 2005 and the associated increase in steel demand and associated CO2 emissions during this period. They discuss the implications in ore reserves, the ore versus scrap mix, and energy intensity of the industry, up until 2030. Müller et al.3 examine in more detail the patterns of iron and steel use for six industrial countries: U.S., France, U.K., Japan, Canada, and Australia. They use a dynamic material flow model to estimate the saturation level for in-use stocks for each country, and discuss the implications on scrap availability and developing countries. A 2012 review paper by Chen and Graedel summarizes anthropogenic cycles of elements, listing 200 different iron cycle studies across a range of geographical scales and life-cycle stages.21 1.3. The Need for a Detailed Map of Global Steel Flows. Previous attempts to quantify the flows of iron and steel at country, regional, and global levels are useful for collating data, developing methods for structuring data, and understanding what drives demand in the sector. Together, they provide an important backdrop for this paper, which maps global flows of steel, but is different from previous attempts in three ways. First, previous studies fail to trace the production of steel all the way from liquid steel to final goods. Mapping material flows in and out of a single integrated steel plant, or even a wider envelope of all steel production facilities in a country, can only ever identify efficiency options within that envelope. In a previous paper,6 we showed that steelmaking facilities, because they are energy intensive, are already relatively energy efficient in comparison to other sectors such as transport and buildings. Significant opportunities remain for improving energy efficiency, mainly related to improving the average plant performance to Best Available Technology. However even a perfect pursuit of all process efficiency options will not be sufficient to achieve an absolute 50% cut in CO2 emissions for the sector, relative to 2000 levels, given anticipated growth in demand. The paper suggests a range of material efficiency strategies, which if implemented alongside process efficiency options, would be sufficient to meet this 2050 climate change target. Product life-cycle analysis (LCA), which assigns environmental impacts to end-use products, is designed only to be used only for relative comparison between products, rather than prioritisation of actions, and the sum of the impacts of all LCAs never adds up to the sum of the whole system due to the impossibility of specifying complete system boundaries. A complete understanding of the entire supply chain, from liquid steel through to final goods, it is imperative for understanding the impacts of energy and material efficiency options and choosing the best emission reduction strategies for steel. Second, most attempts to describe the structure of the steel industry will aggregate data and in doing so lose the detailed breakdowns for intermediate and final steel goods. Important features of the supply chain that are recorded in tabulated statisticssuch as the breakdown between billets, blooms and slabs, or the specific flows of reinforcing bar or galvanized coldrolled coilare lost in the flowchart representations. In this paper, such detailed features remain visible. Third, this paper attempts to present the global flows of steel in an easily accessible form. Many of the data points used to construct our map are available in tabulated form from a range
Confederation of Iron and Steel Industries (EUROFER)), national statistics offices (e.g., U.S. Geological Survey (USGS) and UK Steel), trade organisations (e.g., International Iron Metallics Association (IIMA)), steel companies (e.g., Tata Steel) and private media groups (e.g., Steel Business Briefing (SBB)). These organisations collect steel data across regional, country, and company levels, and publish in annual reports, magazines, and online statistics. A full listing of important data sources for the steel industry is provided in the Supporting Information (SI) (Section 1, Table S1). Not surprisingly, more comprehensive steel data can be found for the upstream steelmaking processes, where larger firms make data collection easier. In contrast data about the fabrication of steel into end-use goods is scarce due to the many different types of products and number of commercial players involved. The challenge is to bring this data together, from various sources, into a global picture of the steel industry that is consistent and adds up. 1.2. Attempts To Structure Steel Production Data. The U.K. has a rich history of research into iron and steel flows, with three major studies published over the past decade: Michaelis and Jackson8,9 track historical material and energy flows in the steel sector 1954−1994 and predict future flows using a dynamic scenario analysis for 1994−2019, including a summary level flow diagram for the U.K. steel sector in 1994. Dahlstrom et al.10 provide a more detailed flowchart of U.K. iron and steel flows in 2001, including trade movements and an estimated breakdown for steel in final products. Geyer et al.11 and Davis et al.12 update the MFA data for U.K. steel supply chain 1970− 2000, exploring the “more elusive flows of scrap generation and recycling”. They use simple flowcharts to compare the 1970 and 2000 steel flows and provide a dynamic stock analysis to forecast future scrap flows. At the wider European level, Moll et al.13 have published a working paper to “determine the environmental implications of the EU iron and steel life-cycle system” with the aim of directing policy intervention. Iron and steel flows are tracked from ore and scrap through to final products and presented in a simple flowchart form. Müller et al.14 trace the “anthropogenic” iron cycle in the United States from 1900 to 2004, developing a stock model to understand the demand for ore and scrap based steel. There is a significant body of research into iron stocks and flows in Japan, for example Nakajima et al.,15 which has also more recently has been extended into Asia.16 Pauliuk et al.17 perform a dynamic material flow analysis of the Chinese steel cycle and forecast demand for iron, scrap and steel products until 2100. Examples of mass flow accounting for the global steel industry are listed below: •Wang et al.18 provide a snapshot of metallurgical iron cycles across production, fabrication, use, and waste management and recycling for the year 2000, based on a 68 country/nine region model. Stock and flow data for each country or region is presented in a standardized flowchart form, along with a “bestestimate” flowchart for the world. Their steel products-to-uses matrix (PTUM) for 2000 is particularly useful, mapping a selection of intermediate products onto five end-use categories: construction, consumer durables, industrial machinery, transport equipment, and others. •Hatayama et al.19 use a dynamic material flow analysis to calculate in-use steel stocks for 42 countries from 1980 to 2005, and then forecast future demand until 2050. 13049
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•The worldsteel value for concrete reinforcing bar in 2008 is 147 Mt (ref 2, p 48), whereas SBB suggest it could be as high as 210 Mt.26 For the Sankey diagram, balancing our total for hot rolled long products to equal the worldsteel value of 526 Mt (ref 2, p 41), gives a solved value for concrete reinforcing bar of 174 Mt, about halfway between the two literature values29 2.2. From Intermediate Products to End-Use Goods. The steel industry does not report the flows of steel beyond intermediate products, so finding data for this section of the Sankey diagram is more difficult. However, worldsteel does report intermediate product sales by end-use sectors for 2007 (ref 31, p 7), into construction (50%), mechanical machinery (14%), metal products (14%), automotive (12%), electrical equipment (3%), other transport (3%), and domestic appliances (3%), but a detailed mapping of intermediate products to end-use goods is missing. Wang et al.18 publish a product-to-uses matrix (PTUM) which maps seven intermediate products onto a limited set of five end-use sectors, using global 2000 data. However, their global breakdown by end-use sectors, when scaled up to 2007 production for intermediate products, does not match the worldsteel end-use sector breakdown, probably due to the increase of steel production from 848 Mt to 1351 Mt (ref 26, p 7) between the years of the two studies. To improve this mapping we create two matrices. The allocation matrix A, maps the metal flow from each intermediate product (19 rows) onto each end-use product (10 columns). The fabrication yield matrix Y accounts for the steel lost in transforming intermediate products into end-use goods, e.g. cold rolled coil (CRC) has a fabrication yield of 90% when used in construction (large flat rectangular sheets), but a lower value of 60% for car body panels (shaped panels with cut-out holes). The two matrices are of equal size and multiplying the corresponding cells in each matrix allows us to link the output of end-use goods to the input of intermediate products. Written formally:
of sources, but bringing this data together to make a visual map allows the user to instantly compare the scale of material and product flows. The Sankey diagram has been chosen as the visual format to structure the data. Schmidt22,23 provides an excellent overview of the history and methodology for using Sankey diagrams, and the Sankey diagram format has been used at similar scales by Cullen and Allwood24,25 to map global energy flows.
2. CONSTRUCTING A MAP OF GLOBAL STEEL FLOWS To construct the Sankey diagram, steel flows are tracedfrom steelmaking processes, through intermediate products, to enduse goodsusing a range of data sources, mass balancing and estimation. Full details of the calculation of global steel flows are given in the 25 page Supporting Information, derived from 59 data sources. 2.1. From Steel Making to Intermediate Products. The steel industry transforms iron ore and scrap steel into a range of intermediate products: beams, bars, tubes, wire, sheets, and plate. Production tonnages for most of the key processes and products in 2008including steelmaking, casting, rolling, and coatingare sourced from the worldsteel reports: Steel Statistical Yearbook 20092 and World Steel in Figures 2009.26 Barrington27 provides a global metal balance for electric arc furnace (EAF) steel in 2008, including a breakdown of iron sources and scrap steel entering EAF furnaces. Data for some intermediate products is sourced from EUROFER28 and SBB.29 The overall map structure, and in particular the interconnections between the rolling and forming processes is influenced by the SBB29 global flowcharts of steel flow in 2008. Global production statistics are combined with process yield data from the report Yield Improvement in the Steel Industry30 to balance the flows entering and leaving each process. The blast furnace yield includes the sintering processes and an additional 1.4 Mt from smelt reduction processes. Yields for the steelmaking processes (oxygen blown furnaces, open hearth furnaces, and electric furnaces) include secondary metallurgy and measure the loss of non-iron materials (carbon and elements in the dross), whereas all other yields consider only the loss of iron content, with the result that the iron ore input to the blast furnace excludes the oxygen content and the input of alloying elements to the oxygen blown furnace is not shown. Given the number of data points and different sources used to create the Sankey diagram, the level of agreement between data is surprisingly consistent. Only four discrepancies were found, requiring minor corrections to the worldsteel numbers to balance the flows: •Adding up our totals for continuously cast steel, cast ingots and cast steel products gives 1316 Mt in 2008, which is slightly less than the reported worldsteel total of 1324 Mt (ref 2, p 9). •The worldsteel 2008 global total for hot rolled steel is 1315 Mt (ref 2, p 39). It is difficult to see how this can be the output of hot rolling processes as it is only 13 Mt less than their crude steel production of 1329 Mt (ref 2, p 5), giving as implausible combined casting and rolling yield of 99%. The value 1315 Mt is more likely to be the input steel to hot rolling processes which is closer to our input of 1299 Mt. •The worldsteel value for welded tube is increased from 44.5 Mt (ref 2, p 62) to 62.4 Mt, so that our total for intermediate products matches the worldsteel’s value for finished steel products of 1207 Mt (ref 2, p 96) (excluding cast foundry iron products). The higher value is closer to the comparable SBB value for welded tube of 65 Mt.
Y = (A◦Y)X
(1)
where Y is a vector of end-use goods, X is a vector of intermediate products, and the ◦ symbol indicates the Hadamard product (element-wise) of the two matrices. An equivalent loss vector YL can be defined as: YL = (A◦(J − Y))X
(2)
where J is a matrix of ones, of the equal size to Y and A. The model is not complicated in itself, however no data set exists to populate the allocation and yield matrices, so the complex web linking intermediate products to end-use goods emerges from a process analogue to solving a Sudoku puzzle involving 28 separate calculation steps and using data from 20 different sources. In the simplest cases, published breakdowns of intermediate products are used to estimate the allocation, e.g., 53% of reinforcing steel is used in buildings, based on an analysis of U.K., U.S., and Turkish data. For reinforcing bar, a fabrication yield of 95% is used based on data from a U.K. steel fabricator. Thus, the annual mass of steel used for reinforcing in buildings is calculated as 174 Mt × 53% × 95% = 87 Mt. In other cases, only a steel breakdown for the end-use good was available, necessitating a back-calculation (dividing each end-use value by the fabrication yield) to populate the allocation matrix. For example, we calculate that 61% of the steel in a car is sheet metal (54 Mt globally), using a breakdown of materials for a typical car. The comparatively low fabrication 13050
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Figure 1. Global flow of steel from liquid metal to end-use good.
yield of 60% for sheet metal in cars means that 91 Mt is required out of a total 127 Mt of sheet metal, giving a fraction of 71% in the allocation matrix A. To complete the two matrices we have worked through many such data sourcesmarket breakdowns for intermediate products, compositions of end-use goods, and fabrication yieldsand resolved any conflicting estimates, to create a detailed mapping of intermediate products to end-use goods. Fabrication yields were taken from literature where possible for example Milford et al.32 calculate yields along the steel supply chain for several case studiesor estimated by considering the complexity of the shape of the end-use good, such that the total scrap from fabrication balanced with the known demand for scrap in steelmaking. This paper is the first known attempt to map intermediate steel products onto end-use goods in detail and to quantify the yield for each fabrication process. The data for this allocation and yield matrices is given in the SI (Section 4, Figure S3). 2.3. Balancing the Steel Scrap Flows. Steel scrap is produced in casting, forming and fabrication processes, when steel is cut away to reveal the final products. The resulting flows of steel scrap travel in the opposite direction to the steel products, as return loops to earlier melting process in the supply chain. The balancing of this scrap creation with steelmaking demand for scrap requires some additional explanation. An estimated 476 Mt (ref 26, p 26) of scrap steel was consumed in 2008 in steelmaking processes, according to worldsteel. This is purchased scrap, consisting of fabrication and postconsumer scrap, but excludes forming scrap, which is recycled internally and omitted from recycling statistics. The three types of scrap are estimated as follows: Forming Scrap. This is calculated from the steelmaking mass balance, which results in 98 Mt scrap from rolling and forming
returned to steelmaking, plus an additional 40 Mt recycled internally in steel casting and 35 Mt in iron foundry casting. Fabrication Scrap. World Steel Dynamics (WSD) (ref 33, p 19) calculate that “new steel scrap” (i.e., from fabrication processes) equals 14% of global ‘apparent steel consumption’. At the global level, steel consumption and crude steel production are almost equal (ignoring any stock changes), so using 1329 Mt (ref 2, p 5) of crude steel gives an estimate of 186 Mt for fabrication scrap. Postconsumer Scrap. Data for postconsumer scrap is not collected directly, and estimating the available scrap dynamically is complex, as it requires detailed knowledge of historical production volumes and as accurate estimate of the average lifespan for each product type. Therefore, in our study postconsumer scrap is calculated as the balance for overall scrap demand, and is calculated by subtracting the fabrication scrap from the scrap demand for steelmaking (476 Mt − 186 Mt = 290 Mt). For comparison, using dynamic stock based calculation of postconsumer scrap by Hatayama et al. (ref 19, Figure 2) and extrapolating to 2008 gives 370 Mt of discards, which when multiplied by the recovery rate of 80% estimated by worldsteel34 gives a comparable value of 296 Mt. 2.4. Drawing the Sankey Diagram. The steel Sankey diagram is drawn using the graphical software program Adobe Illustrator, to produce a clear and visually pleasing image. The width of each horizontal line on the diagram is proportional to the flow of steel. The manufacturing processes in the supply chain are organized into six stages, which form the vertical “slices” in the Sankey diagram: •Reduction/Scrap Preparation where iron ore is reduced to pig iron in the blast furnace and direct reduction processes, and scrap steel is prepared for melting. 13051
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•Steelmaking where pig iron and scrap steel are converted to liquid steel in oxygen blown, open hearth, and electric furnaces. •Casting where liquid steel is continuously cast into slabs, billets, and blooms, and batch cast into ingots and steel products, while liquid iron is cast into foundry products. •Rolling/Forming where castings are hot rolled into of strip/ sheet, plate, rod/bar and sections, followed by cold rolling, forming and coating process which are used to create a range of intermediate products. •Fabrication where intermediate products are cut, joined, machined and assembled into end-use goods. •End-use Products which are grouped into vehicles, industrial equipment, construction and metal products. Each major process step is shown on the Sankey diagram as a vertical black line, where the input steel flow is split into three possible outputs: useful metal (shown in color); losses, typically dross and oxidized steel (shown black); and process scrap steel which loops back to a melting step for recycling (shown in gray). A preliminary version of the Sankey diagram was presented in the book, Sustainable Materials: with both eyes open. (ref 35, p 54). In this paper, we update the map of global steel flows and for the first time provided the detailed analysis used to create the diagram.
Table 1. Annual Scrap Flows Entering the Steel and Iron Industries Mt/year
steel
forming scrap internal casting scrapa fabrication scrap end-of-life scrap total scrap liquid metalb industrial scrapc fraction lostd
99 39 186 290 614 1367 324 24%
cast iron 35 34 69 104 35 33%
total 99 74 186 324 683 1471 359 24%
a
Run-around scrap generated in the casting processes. bCalculated as the input to all casting processes, including the internal casting scrap (run-around). cThe sum of forming, fabrication and internal casting scrap. dThe ratio of industrial scrap to liquid metal.
greater than 80%.34 The low fraction of end-of-life scrap in the recycling stream results from the historical growth in steel demand. The end-of-life scrap collected today is related not to the steel goods being produced today, but to the demand for steel at the time when today’s discarded goods were first produced. The remaining scrap is generated within the industry during forming, casting, and fabrication processes: 74 Mt of internal casting scrap (known also as run-around scrap) and 99 Mt of forming scrap is generated while transforming liquid steel and iron into intermediate products, with an overall material yield of 88%. Industry efforts to improve yieldfor example the worldsteel report, Yield Improvement in the Steel Industry30 have focused on reducing scrap from forming and casting, the upstream steelmaking processes where large concentrated flows allow for simpler implementation. For example, a significant improvement in casting yield has been achieved with the introduction of continuous casting machines for slab, blooms and billets since the 1970s. Today’s average yield for continuous casters is 97%, a vast improvement over the displaced technology of solid ingot casting followed by an extra rolling step, which has a yield of about 91%. The majority of steel products are now continuously cast eliminating an estimated 75Mt of scrap a year. In contrast, the yields for casting steel product and iron are much lower, and there is significant opportunity to minimize the generation of casting scrap in these sectors. Further gains are available in the downstream fabrication processes, where intermediate products are transformed into end-use goods with an average material yield of 85%, generating 186 Mt of scrap steel. Table 1 shows a wide range of yields across fabrication processesfrom 60% for galvanized cold rolled coil used in car manufacture, to 95% for sections used in building construction. The fabrication yields for long-products (sections, bar/rod, tubes) are on average much higher than for flat-products (strip, sheet, plate), as a single cut in onedimension generates less scrap than a complex cut-out shape followed by forming. So the simplest option for reducing industrial scrap is to make more steel goods with long-products rather than with sheet, for example, the structural strength in cars could be provided using a tubular frame covered with a light nonmetal skin, instead of current integrated sheet steel body designs. More challenging is to systematically improve the fabrication yields of each process, where the wide range of yields is influenced by the shape and complexity of each enduse good. Yet many technical options are available for
3. RESULTS Figure 1 shows our best estimate of global steel flow in 2008, from steelmaking (left), through intermediate products (middle), to end-use goods (right). The resulting map represents a major contribution to understanding the complex interactions of the steel supply chain and provides a baseline from which to project future resource requirements and product flows. 4. DISCUSSION The map of global steel flows presented in Figure 1 gives immediate insight into the relative scale of steel processes and flows, in a way which would be difficult to extract from report based statistical data tables: two-thirds of liquid steel comes from iron ore and one-third from recovered scrap; 93% of steel is cast continuously and 99% of steel is rolled after casting; more than half of the world’s steel is used in the construction of buildings and infrastructure with the remainder roughly shared between vehicles, industrial equipment, and metal goods. Delving deeper we see not all scrap is melted in electric furnaces as often assumed, with a third of all scrap flowing to oxygen blown furnaces. This translates to 20% scrap charge for the ore-based steelmaking route, helping to cool the furnace by absorbing heat given off in the exothermic oxidation of carbon, but also affecting the energy balance and generation of CO2 emissions significantly. In addition, directly reduced iron and pig iron (nonscrap inputs) make up nearly a quarter of the input to electric furnaces. Further insights about the size of steel flows in the global map are presented in the next sections, with specific focus on the yields of manufacturing processes and the potential of material efficiency options. 4.1. Analysis of Yield Losses. The breakdown of scrap flows entering the steel and cast iron industries is shown in Table 1. Surprisingly, less than half of our steel scrap is collected as end-of-life scrap, from used steel goods discarded at the end-of-life. However, it would be wrong to assume that collection processes are inefficient; the opposite is true with worldsteel estimating the recovery rate for end-of-life scrap is 13052
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Table 2. Material Efficiency Strategies and Opportunities for Improving Material Efficiency strategies reducing yield losses diverting manufacturing scrap reusing metal components using less metal by design longer life products reducing final demand
opportunities for improving material efficiencya reducing the loss of scrap steel from fabrication processes could raise the overall yield from 85% to 90%, eliminating around 60 Mt of scrap steel every year approximately one-third of scrap steel from fabrication processes (∼65 Mt per year) could be diverted to produce alternative products, instead of being melted as scrap steel up to 30% of discarded steel products could be reused with only minor refurbishment, displacing 65 Mt of steel in new products every year making lighter-weight products has the potential to reduce the steel used in products by 25%, saving over 250 Mt of new steel every year the lifespan of end-use goods could be doubled with only minor design changes to the product, which in the long term would halve the projected annual demand for new products and halve steel demand. using products more intensely could reduce final demand by nearly 30% across all products, saving over 320 Mt of steel per year
a
The potential savings are weighted averages across all steel products for each strategy. The absolute reductions in steel demand are reduced if all strategies are applied simultaneously, as the savings are multiplicative rather than additive across the strategies.
improving fabrication yields (as described in SI Section 4.3) and 186 Mt of fabrication scrap demands attention. Overall, one-quarter of all liquid steel produced never makes it into end-use goods, but is instead discarded as scrap during casting, forming and fabrication processes. This loss of steel not only costs the industryscrap steel is valued at about one-third of new steelbut also represents a loss of the CO2 emissions embodied in the steel, as they can no longer be put to good use in a product. Industry produces so-called “clean” scrap that is recovered and recycled at near perfect efficiency, yet the cost of collecting, sorting, preparing and melting even clean scrap is high. Maintaining a never-ending internal remelting loop of some 360 Mt of steel is energy intensive, generates significant CO2 emissions, and takes considerable efforteffort that in our view would be better directed toward improving manufacturing processes to limit scrap production. 4.2. Material Efficiency Options. The Sankey diagram presented in Figure 1 allows us to focus on the biggest flows of steel, in the hope of using steel more efficiently. Material efficiency in this context means delivering the same services provided by steel while reducing steel demand and the environmental impacts of steel. It addresses our addiction to steel rather than the consequences of our addiction. Allwood et al. describe six strategies for improving material efficiency in their book Sustainable Materials: With Both Eyes Open.35 These are listed in Table 2 alongside estimates of the potential for reducing steel demand through pursuing material efficiency in the steel supply chain industry. The six strategies can be divided into two groups according to how they act on the Sankey diagram. The first three strategies involve minimizing or diverting scrap steel, which reduces recycled steel flows (the gray flows in Figure 1). Improving the material yield of the steel supply chain not only saves money by reducing scrap losses, but also reduces the energy and CO2 emissions linked to melting scrap steel at high temperatures. The sheer scale of industrial scrap generation some 360 Mt and 24% of liquid steel productionis a “wakeup call” to pursue improvements in the material yield of manufacturing processes. There is no single solution for reducing yield losses across the many different products and manufacturing processes, but a useful start is to apply the following seven principles: 1. avoid cutting 2. use straight perpendicular cuts 3. make goods from long-products rather than flat-products 4. design sheet steel components to tessellate, by optimizing steel plate and roll cutting patterns
5. reduce the gripping border around drawn sheet parts 6. reduce machining by casting complex shapes close to their final shape (near-net-shape casting) 7. improve quality control to eliminate product defects After applying these principles, and where scrap losses are unavoidable, then manufacturing scrap should be diverted to new product uses, by cutting smaller components from larger pieces of scrap, and selling defective products to customers with lower expectations. Reuse of steel components at end-of-life offers a way to avoid energy intensive recycling of steel scrap. Large steel components, such as beams in construction, are rarely damaged in use and show great potential for reuse, but to be viable, require the development of nondestructive demolition techniques and procedures for recertifying used beams. The last three strategies in Table 2 involve reducing final demand for end-use steel products and are even more beneficial as they impact directly on the production of steel from iron ore, which carries approximately twice the energy and CO2 emission burden of scrap recycling. Using less steel by design means designing lighter-weight products which reduce the demand for steel. Efforts should be focused on the largest flows from Figure 1: construction of buildings and infrastructure, followed by industrial equipment and metal goods. Light-weighting vehicles not only leads to less material input in manufacturing but gives the cobenefit of reduced use-phase fuel consumption. Both longer-life products and reducing f inal demand for enduse steel products shrink the steel flows at right-hand end of the global steel map (Figure 1). Doubling the lifespan of a product leads to a halving of product demand, and will halve the lifecycle steel demand and energy input required to produce the product. Designing long-life products requires introducing resilience and adaptability into the product to allow for the changing needs and desires of the future. Adopting a modular based design strategy allows individual product components to fail and be replaced without compromising the operation of the whole product. Reduced demand for steel products does not necessarily imply austerity measures, but can be achieved through using steel products more intensely and at their full capacity. Car-pooling and office sharing are good examples of using steel products more efficiently. 4.3. Data Uncertainty. Steel production statistics are not published with error bands as is common when reporting experimental findings, making it impossible to perform an uncertainty analysis for our model. Errors in the data are likely to arise from the process of surveying production facilities and aggregating data, for example: the misunderstanding of survey terminology and questions; the accidental or deliberate 13053
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misreporting of data in the surveys; an incomplete coverage of production facilities requiring scaling to the global level; aggregation errors; miscommunication of data in published reports. Despite the best efforts of organisations like worldsteel to vet data sources and check data, the accuracy of the reported statistics is impossible to verify. We attempt to mitigate uncertainty by using data from a few trusted sources, which provides consistency and completeness, and by validating the data using alternative sources where available. Mass balance checks are employed at every slice and process along the Sankey diagram to ensure our mass flow calculations add up, and steel flows are reported to the nearest million tonnes (Mt) as a precaution. Throughout the mapping process we are required to choose between data sources, introducing further uncertainty, so full details are provided in the SI to enable to reader to judge the decisions, assumptions and calculations for themselves. Despite the limitations of using data of unknown accuracy, this type of whole system analysis at the global level is still useful for comparing the relative scale of steel flows for processes and products, and showing where action might be taken to reduce environmental impacts. 4.4. Future Work. The map of global steel flows presented in Figure 1 provides a static snapshot of the steel industry in 2008 and serves as a guide for directing actions to improve material efficiency and minimize environmental impacts. However, to set realistic targets and guide resource policy we also need to consider how the map will change in the future. Will demand for steel products increase? Will manufacturing yields improve? What new products will be required in new markets? Drawing future of maps of steel flow requires a dynamic model of steel stocks and flows which can deal with future behavior changes and predicts the future availability of steel scrap from discarded goods. Mapping CO2 emissions factors for each process onto an extended dynamic model will create a credible basis from which to forecast future steel demand and the long-term environmental impacts of the steel industry.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information for this paper describes in detail the assumptions, calculations, and references used to construct the map of global steel flows. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +44 1223 760360; fax: +44 1223 332 643; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The first two authors are supported by a Leadership Fellowship provided by the UK Engineering and Physical Sciences Research Council (EPSRC) reference EP/G007217/1. We thank Daniel Cooper, Muiris Moynihan, and Rachel Waugh for their generous contributions to the paper.
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REFERENCES
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