As established in Chapter 5, iron ore, coke (processed coal) and limestone are the main natural resources required to make steel. Iron is a chemical element that, because of its natural affinity for oxygen, appears naturally in the form of iron oxide. The most common forms of iron oxide are haematite and magnetite, which both have low metallic concentrations such that good quality ores consist of about a quarter of pure iron. The extracted rock is crushed first and the iron ore separated from the quartz by use of magnets for magnetite and floatation for haematite (Worldsteel, 2011a).
Iron ore is one of the earth’s most abundant materials (USGS, 2012) but at the current consumption rate the world will eventually run out of this natural resource. The current operations are targeting the best deposits in terms of quality and ease of extraction. Future ore deposits will be less convenient and may lead to greater energy requirements and more carbon emissions during their extraction.
The uneven geographical distribution of iron ore has adverse environmental and economic impacts on steel production. For instance, cheap hydro-electricity is largely available in Canada, while iron ore is abundant in Australia, Russia and Brazil. This means that raw materials are being transported long distances or a more carbon-intensive power supply is being used for processing steel. Currently China acquires the majority of its imported iron ore from Australia. In future, this uneven distribution of iron ore may cause political conflicts as the resources start to deplete (Allwood and Cullen, 2012).
Although not to the same extent as during the steelmaking phase, the mining stage emits greenhouse gases through the combustion of fuels used by the mining equipment and for the transportation of the ores. The extraction process requires the use of chemicals some of which are harmful. Some of the material production processes are water intensive and, depending on the location of the mining site, may cause local water stress (Worldsteel, 2011a).
A tonne of steel requires much more than a tonne of feedstock material, with the majority of impurities removed during the steelmaking process. Furthermore, to yield a tonne of ore
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typically requires 10 tonnes of the rock to be extracted. The band of iron ore is usually not on the ground surface and so much more land stress and environmental degradation occurs during the mining process. Rio Tinto iron ore expansion in Western Australia gave them access to 71 thousand square kilometres of land, an area larger than half of England (Allwood and Cullen, 2012).
8.2
Steel production
The steel industry produces liquid steel from the ores and recycled steel and casts it into stock products. The major impact of the steel manufacturing process is the use of virgin materials and energy, which results in the emission of greenhouse gases. The materials include: iron ore; coal; limestone; and recycled steel; but other materials are added in small amounts as outlined in Chapter 5. The main emissions include: carbon dioxide; sulphur oxides; nitrogen oxides; and dust (Allwood and Cullen, 2012).
Over the years, the steelmaking industry has developed technology that uses raw materials efficiently. According to Worldsteel (2011a), a tonne of crude steel produced through the BF-BOF route requires 1.4 tonnes of iron ore, 0.77 tonnes of coal, 0.15 tonnes of limestone and 0.12 tonnes of recycled steel. The EAF typically requires 0.88 tonnes of recycled steel, 0.15 tonnes of coal and 0.043 tonnes of limestone to produce a tonne of crude steel.
Converting iron ore into steel is an energy-intensive process, especially the liquid metal phase. Energy is required to melt the materials in order to facilitate casting into different shapes, to energise the atoms leading to diffusion and to encourage chemical reactions between atoms (Allwood and Cullen, 2012).
In compliance with well-established international standards, steel products from different steel mills cannot be easily distinguished and so consumers have the freedom to buy the cheapest stock on the market. Thus, it is difficult for the steel mills to dictate the selling price of their stock, although they can control operating costs. With energy use accounting for up to 40% of operating costs (APPCDC, 2010), this has been the principle cost-cutting initiative in the industry for years as highlighted in Figure 8.4 below. The graph shows that, on average, energy consumption has been halved over the past 35 years.
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Figure 8.4: Indexed energy consumption/t crude steel produced in North America, Japan and Europe (1975 = 100%) (Worldsteel, 2008)
According to Allwood and Cullen (2012), the theoretical minimum energy for an ideal steelmaking process from ore is 6.7GJ per tonne of liquid metal. A current best practice of about twice this absolute limit demonstrates the maturity of the steelmaking technology. The savings are realised through process efficiency, improvements in electricity generation and developments in technology. A single continuous thermal cycle in the furnace helps to further reduce heat losses and start-up energy consumption. However, there are processes such as tempering (high temperature after quenching) and annealing (high temperature after cold forming) that still require a second thermal cycle.
The high temperatures involved in steelmaking mean that intricate cooling techniques are necessary. About 80% of the water used in steelmaking is for once-through cooling and the remaining amount is used for cleaning and cooling other areas of the process, as well as in heat processing equipment. Sea water is preferred as it can be returned to the source with nominal change in quality (for example, the water may be slightly warmer than the original state) but this depends primarily on availability and national legislation. Water consumption is very low and recycling rates of up to 98% in advanced technologies are possible, with the main losses being through evaporation (SCI, 2003).
Emissions into the air are monitored by the industry, enabling process improvements to be investigated and implemented. Some of the control methods include dust suppression, thermal oxidation, chemical treatment, scrubber and filtration systems (SCI, 2003).
Considering that extensive research and marked progress has been made over the years, can the environmental footprint of the steelmaking process be reduced any further? The parameters for an efficient furnace have been long established and these include using pure
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oxygen instead of air, increasing capacity to benefit from economies of scale, continuous operation of furnaces to minimise stabilisation energy, properly sealed and insulated furnaces to reduce heat losses, maintaining the optimum air-fuel ratio, heat recovery from exhaust gases and better programming and control of processes (Allwood and Cullen, 2012).
The answer, however, is a resounding yes! Although advanced technologies are now very efficient, steelmaking companies are at different maturity levels. The average energy consumption in primary steel production is estimated as 25 GJ/tonne compared to the theoretical minimum of 6.7 GJ/tonne (Allwood and Cullen, 2012). This average figure is also more than double that of the current best practice. There are still significant improvements to be brought to some sites through technology transfer and sharing of best practice in order to achieve optimum operating levels.
According to the IEA and OECD (2009), if all sites have the latest steelmaking technology installed, there will be a saving of about 14% in the current emissions. The industry’s target is to reduce the use of primary energy in steel production (see Figure 8.4 above) to 19% (Worldsteel, 2012b). Some of the areas of improvement include increased recycling rates and the utilisation of by-products, energy efficiency and process automation to enhance precision.
Most of the carbon dioxide is generated from the ore reduction process within the blast furnace as a result of the chemical reaction between iron ore and coke. Whilst coke cannot be completely substituted because of the structural role it serves in the BF, there is already replacement (by pulverized coal or natural gas) of up to 50% in some applications (SCI, 2003).
Furthermore, there has been large-scale research in low-carbon technologies across the globe. These programmes include the EU’s ULCOS (ultra-low CO2 steelmaking) and Japan’s
Course 50. Other programmes, largely sponsored by the steelmaking industry, are taking place in the USA, Canada, South America, South Korea, China and Australia. Technologies that reduce GHG emissions by 50% have been identified and a number of experiments are now in the feasibility stages. For example, a demonstrator for the ULCOS-BF has just been completed in the Netherlands. This process relies on the separation of the top gas and the
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Carbon Capture and Storage (CCS) principle to recycle the useful gases back into the blast furnace as a reducing agent (Worldsteel, 2009).
Although steel already has a superior strength to weight ratio when compared with other construction materials, there is on-going research on improving both its strength (in order to reduce material weight through dematerialisation in designs that are governed by strength) and ductility (which improves flexibility and the manufacturing process) properties (SCI, 2003).
The two most common grades of steel in building design, S275 and S355, cannot be distinguished by visual inspection. There has been a long standing debate on whether it would be beneficial to only maintain the higher grade S355 steel (Needham, 1978). Not only does this prevent on-site errors of inadvertently incorporating an S275 section where S355 grade was specified, but it facilitates dematerialisation and encourages disassembly and re- use (no verification is required for re-use if a single grade is adopted).
In addition, there have been discussions on whether there is any benefit in liquid steel being cast in the shape of the final product. Allwood and Cullen (2012) highlighted that it will be difficult to control the grain sizes, defects and cooling rate of the steel. This is likely to lead to a less strong and less tough product, with defects that are difficult to remove through further processing.
This efficiency in the steel-making process has been extended to recycling rates and co- product use. The main by-products of the steelmaking process include slag, dust, sludge and gases. These are recycled back into the process or sold under the “industry symbiosis" programme whereby companies trade in by-products. Not only does this arrangement reduce landfill waste and generate revenue for the steel producers but it conserves natural resources and reduces GHG emissions (Allwood and Cullen, 2012).
In modern steelmaking processes, nearly 100% of iron making (blast furnace) slag is recovered. However, the recovery rate of steelmaking (ferro-lime) slag is slightly lower at approximately 80% owing to its high free lime content. Research on the separation of this
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free lime is underway, which has ready applications in fertilisers and cement production (Worldsteel, 2012b).
Clinker, the main ingredient of cement, produces about 0.52 tCO2 emissions per tonne of the
material. Cement itself is known to contribute to 2.4% of global CO2 emissions (Gibbs et al.,
2001). In the UK, ground granulated blast furnace slag (GGBS) can be used to replace up to 85% of normal cement in concrete. Supplies of GGBS in the UK can be limited and is rarely disposed of on waste sites. Although concrete with GGBS suffers from a slightly longer setting time and an increased rate of carbonation, it benefits from a considerably low carbon footprint and reduced heat of hydration. Slag is also used as a substitute for primary aggregates in construction (Hammond and Jones, 2011).
Exhaust gases contain 80% of chemical energy lost from the furnace. These gases can be captured, cleaned and combusted to recover the energy through innovative technologies such as the CCS system mentioned above. The recycled energy can be used as fuel in the furnace and other downstream processes or for heating up the coke-making ovens, thereby reducing the need for primary energy. Research on the capture of hydrogen from coke gas is also underway (Worldsteel, 2008).
Whilst the dust and sludge are fed back into the steelmaking process, the iron oxides are sold for the manufacture of electric motor cores and cementitious products. Zinc oxides from the EAF and coke oven gas from the BF-BOF are raw materials to be sold to the fertilisers and plastics industries. According to Worldsteel (2012b), steel waste rarely finds its way to landfills and the industry target is zero waste. Worldsteel (ibid.) claims that an average of 98% of raw the materials currently being used in the production of crude steel are being converted into useful products or by-products.
Steelmaking involves high economies of scale, meaning that the more volume that each site can produce, the cheaper the processing costs. As a consequence, most of the steel is produced from relatively few locations across the globe (Worldsteel, 2012b). Whilst the large conglomerates rely on this benefit, developing economies are likely to emit less carbon as they can provide newer technologies and cheaper labour costs.
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The level of demand for steel products, and the efficiency in their use, are the result of large investments by the industry in the training and education of its consumers, employees and the general public. This includes the training of professionals such as structural engineers who help to create the demand for steel through building designs and specifications. The following subsection will therefore look at this design phase prior to moving on to the fabrication stage.
8.3
Design
Even an aggressive approach to the reduction of energy consumption in material production will not meet the 2050 carbon emission target set in the Climate Change Act without a reduction in material demand (Gutowski et al., 2013).
Whilst design is not a phase of steel production per se, it is a vital stage that generates the demand for steel products. Prior to ordering material from the steel mill, the fabricator would have received an order from a client to construct the building structure. An engineer would be involved, either from the client or fabricator side, to carry out the structural design and prepare the relevant technical specifications.
The material to be used for the building frame is decided during feasibility studies as informed by a structural appraisal (BS 5950-1; BS EN 1993-1-1). Many factors, such as aesthetics and energy consumption, are simultaneously considered by the delivery team of professionals involved in order to achieve a balanced design across building quality, functionality, cost and environmental impact.
However, environmental impact is seldom considered because the structural arrangement is decided right at the outset of a project before any LCA has been undertaken (DEW, 2007). Yet the best opportunity to address environmental issues is at this early stage of design (Ding, 2008; Kohler and Moffat, 2003).
As a construction material, steel offers many benefits such as fast and safe assembly, less noise on site and manufacturing precision as it is fabricated off-site (SCI, 2003). Not only does the off-site manufacture reduce wastage on site, but the little waste collected by the fabricator goes straight to the furnace without the need for further processing.
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Structural engineers are best placed to complement the improvements being realised by the steel manufacturing industry through the design of robust and efficient buildings. Below are some of the areas where the structural engineer can contribute in the reduction of carbon footprint of building:
• Steel can span long distances due to its superior strength to weight ratio relative to
other construction materials such as concrete, leading to a reduced amount of supports. Furthermore, long span beam design is usually governed by deflection rather than bending capacity whereby pre-cambering of steel beams can be employed to maintain in-service deflections within limits and still use relatively lighter sections. The principle of steel-concrete composite action (use of concrete in compression and steel in tension zones) can also be employed to optimise the steel beam sizes. Less material and fewer columns translate to lighter foundations as well as reduce the effort required to transport and assemble the steel structure (SCI, 2003).
• The adoption of a repetitive structural grid and/or floor heights will minimise steel fabrication and construction effort. Often this leads to the specification of standard cladding panels and other follow-on trades, thereby reducing GHG and waste on downstream activities. This will also support the development of simple and efficient interface details that can be tightly sealed to avoid air infiltration and cold-bridging. • Buildings cannot be made of steel alone and the design of all associated elements
should be conducted with the minimisation of overall emissions in mind. For example, a steel frame is lightweight and requires relatively fewer and lighter foundations but the use of heavy partitions may negate this benefit.
• The more material used, the more embodied carbon and, therefore, the variety of other components that completes the building must complement the steel in order to achieve overall material efficiency. Wise (2010) refers to a building where only 40% of the concrete frame was provided for structural performance reasons (the remaining 60% as finishes). High carbon materials such as steel and concrete should only be used for structural purposes. Wise (2010) suggested that Building Regulation approval should only be granted if the utilisation ratio of structural elements is more than 90%, implying less than 10% waste.
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• Minimising the environmental impact of buildings begins with keeping the building simple and functional, right from the outset of the design phase. The energy that the building is likely to consume is directly proportional to the complexity of the technical building systems (Atkinson et al., 2009). Passive (solar heating and natural ventilation) control is one of the main techniques used in building system design to minimise operational energy consumption.
• With the correct building layout (for example, north facing) and structural
arrangement (for example, the provision of thermal mass), passive heating and cooling can be utilised to achieve thermal comfort during the building operating phase (DEW, 2007). The structural engineer needs to work closely with the rest of the delivery team and other stakeholders to develop the building shape, form and orientation that optimises passive control. Unfortunately, passive design is still being supplemented by mechanical plant in the UK, especially for those rooms located away from the building elevations.
• Not only does passive control reduces operation carbon emissions but also minimises
the size of the mechanical plant to be installed in the building, resulting in lower loadings to be applied to the structure. Recent design developments such as open plan offices and use of lightweight cladding systems have resulted in further reduction of structural loadings. With developments in 3D modelling technology, it should be possible to further evaluate and establish realistic building loadings and behaviours, in terms of both structural performance and energy needs. Thus, a lean and efficient structural design will result in the optimisation of structural materials, which translates to a low embodied carbon footprint (DEW, 2007).
• Often long after their design life, steel buildings can easily be adapted for change of
usage if the superior strength to weight ratio of steel is exploited in creating long spans and flexible spaces. A steel building should first be designed for interchangeable use in its primary location without the need for major renovation or