By Deborah Jackman, PhD, PE, LEED AP™ - originally posted on 09/18/2012
The Painting and its Historical Significance:
No series of essays featuring the works from the Man at Work Collection would be complete without including one of the seminal works in the collection—the 1609 oil painting by Flemish artist Marten van Valckenborch, “Fantastic River Landscape with Ironworks”. Not only is it one of the oldest works in the Collection, but it depicts fledgling elements of the steel industry, an industry that underpinned much of the Industrial Revolution and the development of modern society. It is also an industry that has enormous environmental impacts, and which, in recent years has undergone significant changes to improve its sustainability.
On the left bank of the river is the ironworks. The ironworks is comprised of groups of workers who offload iron ore from boats; a blast furnace used to convert iron ore to elemental iron; a forge shop used to produce wrought iron implements from the elemental iron; and lime quarrying on the hillside above the ironworks. The blast furnace is the structure in the middle left foreground with the large, rectangular stone chimney and one water wheel. The forge shop is located just to the left of the blast furnace in the structure without walls, with the pyramidal thatched roof, attached to the building with two water wheels. (The water wheels in both the case of the blast furnace and forge were used to power air bellows which blew the air required for both processes.) The lime being quarried on the hill side was used in the blast furnace during the chemical reduction of iron oxide to elemental iron. On the right bank of the river, farming activities are depicted. The contrast between the activities on the opposite banks of the river is evidence that the time period in which the painting was created was one of transition. The early 17th century represents the start of the Industrial Revolution and a move away from an agrarian society. For the purpose of this essay, we will focus on the activities on the left bank of the river—those associated with iron making.
In 1609, iron produced in blast furnaces was transformed into wrought iron implements such as plowshares, horseshoes, and other hardware items, by blacksmiths at the forge. In 2012, blast furnaces are still used, but the iron produced in them is nearly all used for the subsequent manufacture of steel, as discussed in the section below. The blast furnace depicted in the painting is thus a focus of our study because it is a technology that has endured over the last 400 years (albeit with improvements along the way) and because its operation has the greatest environmental impacts of any process associated with steel production.
An Introduction to Steel Production:
Modern steel plants vary from plant to plant in their details of operation. But, the general process used to produce steel is common to nearly all modern plants . This general process includes the following steps:
- Various material handling practices and technologies to bring the needed raw materials to the plant and prepare them for further processing. The raw materials include iron ore, limestone, and fuel, usually coking coal. Depending on the exact processes used in a given plant, the iron ore is processed before being introduced into the blast furnace by crushing, pelletizing, and/or concentration. The general term used to prepare the iron for the blast furnace is beneficiation. Different plants use different beneficiation processes, which vary in complexity and which depend, in part, on the type and quality of iron ore being used. Limestone—also used as a reagent in the blast furnace—must be crushed and screened. Coal brought into the steel plant must be converted to coke in order to prepare it properly for the chemical reaction for which it will be used when introduced into the blast furnace. Therefore, most modern steel plants incorporate coking ovens. The coking process introduces additional process steps, and consumes some of the energy originally stored in the coal. Hence, coking, while necessary in conventional steel making, lowers the overall energy efficiency of the steel production process, and, from a life cycle assessment standpoint produces additional environmental impacts.
- Chemical reduction of the iron oxides contained in the iron ore to elemental iron. This is most commonly accomplished through the use of a blast furnace, although recent advances in technology, discussed below, have produced alternate methods of achieving reduction of the iron oxides to elemental iron which are less energy intensive and less polluting. The elemental iron produced in the blast furnace is commonly called pig iron. Upon leaving the blast furnace, the pig iron typically contains too much carbon content to be used in steel products. It is suitable only to be used as cast iron.
- Controlled oxidation of the pig iron in a steelmaking furnace to lower its carbon content and to produce carbon steel with the desired metallurgical composition. Steelmaking furnaces typically fall into one of three categories—a basic-oxygen furnace (BOF), an open-hearth furnace (OHF), or an electric-arc furnace (EAF). The basic-oxygen furnace is the type most commonly found in large, integrated steel making plants in the U.S. In the steelmaking furnace, molten pig iron is exposed to air in a controlled, basic (i.e. high pH) environment and various trace elements such as chromium, manganese, nickel or molybdenum may be added to the molten metal to produce various steel alloys. Since most steel recovered from demolished buildings and other objects is recycled today, scrap steel is also commonly introduced to the steelmaking furnace, to replace all or part of the pig iron used. The processing of recycled scrap steel, as opposed to virgin pig iron, favors the use of an EAF, for reasons described below.
- Various forming and heat treating processes to produce the desired end products. These processes include rolling, casting, forging, drawing, and extruding the molten steel into such products as bars, plates, wire, tubular products like pipe, and other structural shapes.
Of the four steps outlined above, the two which have the largest environmental impacts are the iron reduction step involving the blast furnace and the operation of the steelmaking furnace. These environmental impacts arise from the enormous amounts of energy consumed during the two processes and from the generation of large amounts of toxic exhaust gases, contaminated wastewaters, and solid waste. We will therefore focus on understanding these two processes in more depth, so as to be able to understand ways to minimize the environmental impacts associated with them.
The blast furnace, contrary to what its name suggests, is more than just an oven or furnace for heating the raw materials. It is a chemical reaction vessel in which iron oxide is reduced to elemental iron. The term commonly used for this reduction reaction of iron oxides to iron is smelting. A charge containing iron ore, flux (usually limestone), and fuel (usually coke in modern plants) is introduced to the top of the blast furnace, while air (sometimes enriched with oxygen) is blown into the lower portion of the furnace. The chemical reaction takes place through out the body of the blast furnace. The material charge moves downward, reacting with the hot combustion gases, rich in carbon monoxide, moving upward. The end products are molten pig iron and ****, which exit the bottom, and flue gases which exit the top of the furnace. The chemical reactions involved can be summarized as follows:
|2 C(s) + O2(g) → 2 CO (g)||(1)|
[Coke and oxygen are converted to carbon monoxide in an incomplete combustion reaction]
|3 Fe2O3(s) + CO(g) → 2 Fe3O4(s) + CO2||(2)|
|Fe3O4(s) + CO(g) → 3 FeO(s) + CO2(g)||(3)|
|FeO(s) + CO(g) → Fe(s) + CO2(g)||(4)|
[Multi-step chemical reduction of iron (+3) oxide to elemental iron using carbon monoxide as the reducing agent]
The net reaction is:
|Fe2O3 + 3 CO → 2 Fe + 3 CO2||(5)|
Careful inspection of the above chemical equations indicates that carbon monoxide is the limiting reagent. Consequently, there needs to be a way for more carbon monoxide to be generated in the body of the reactor (blast furnace) as the material charge moves through in order to keep the reaction going. That is the purpose of the flux (limestone):
|CaCO3(s) → CaO(s) + CO2(g)||(6a)|
|CO2(g) + C ↔ 2 CO||(6b)|
In reactions (6a) and (6b), the limestone is decomposed into calcium oxide and carbon dioxide, and then the carbon dioxide reacts with the carbon from the coke to generate more carbon monoxide. The calcium oxide generated by the decomposition of the limestone typically reacts with impurities in the iron ore such as silica to form various types of mineral ****, which are byproducts of the blast furnace process. The most common **** produced is calcium silicate:
SiO2 + CaO → CaSiO3
Calcium silicate has properties similar to Portland cement and is used as a replacement for a portion of Portland cement in some concrete mixes. This use provides a natural route for reclaiming and recycling the **** produced in the blast furnace reaction, thereby minimizing the impact of this particular solid waste byproduct on the environment. (The interested reader may wish to read Installment Three of this essay series, “ ’The Experience of the German Autobahn’—A Discussion of Sustainable Pavement Technologies,” for a more in-depth description of how the use of **** in concrete mixes used in highway construction can minimize the environmental impacts of road construction.)
It is apparent that the blast furnace process is incredibly energy intensive, requiring large amounts of carbon-based fuels, and producing significant amounts of greenhouse gases in the form of carbon dioxide. According to Rubel, et al, , 75% of all energy consumed in an integrated steel plant is consumed in the form of coke used during the blast furnace process. 15 gigajoules of energy are used in the blast furnace process required to produce one ton of steel. In 1609, the blast furnace depicted in our painting probably used charcoal, produced by burning wood in an oxygen-starved environment. Coke, derived from coal, is used today. Both charcoal and coke burn at temperatures higher than the materials from which they are derived. This higher temperature promotes a more efficient reduction reaction. In addition to carbon monoxide and carbon dioxide, the exhaust gases from the blast furnace contain significant amounts of particulate matter, heavy metals, and other pollutants. In the time of van Valckenborch, no one worried about air emissions, but today, increasingly stringent environmental regulations require steel producers to employ various pollution abatement technologies, which are expensive and which add complexity to the process.
The pig iron produced by smelting in the blast furnace typically has around 4% by weight carbon content. The desired carbon content of carbon steel is around 0.5% to 1%, depending on the particular type of steel . The pig iron also contains contaminants such as sulfur, silica and phosphorus. The steelmaking furnace incorporates a process whereby excess carbon is oxidized and removed, in the form of carbon dioxide, and where other impurities can be reacted and removed as ****. It also provides a convenient point in the process where various metal alloys can be added to produce steels with enhanced chromium, molybdenum, or nickel content (alloy steels). As mentioned above, the three styles of steelmaking furnaces encountered today are (1) the Open Hearth Furnace (sometimes called the Siemens Process); (2) the Basic Oxygen Furnace (BOF) (sometimes called the Bessemer Process); and (3) the Electric Arc Furnace (EAF). In the Open Hearth Furnace (OHF), the molten pig iron is introduced into a vessel lined with a basic refractory material, such as magnesite brick, a material capable of withstanding the high temperatures and one that does not introduce acidic byproducts into the process. Into the refractory vessel gasified fuel and excess air is introduced, along with a limestone charge. During the ensuing combustion process, carbon residing in the carbon steel is oxidized and other trace impurities react with the limestone to produce various types of ****. The Open Hearth process requires a fuel gasifier if coal is to be used as the primary fuel source. A distinguishing feature of the OHF is a series of passageways in the refractory brick to allow the brick to be preheated by hot exhaust gases. This allows the OHF to reach very high temperatures. In the Basic Oxygen Furnace (BOF), the molten pig iron enters a reaction vessel into which pure oxygen is blown through the molten iron mass. A limestone charge is also added. Oxidation of carbon to carbon dioxide occurs and other impurities are converted to **** in the presence of the limestone. Because pure oxygen is used instead of air, no external fuel source is needed to promote the oxidation (combustion) of carbon to carbon dioxide. The oxidation reaction of carbon to carbon dioxide is exothermic, so the heat generated by the reaction itself is sufficient to sustain the temperature in the reaction vessel. In this way, the BOF process is different from the OHF process. Because BOF does not require an external fuel source and doesn’t require a fuel gasification system, it has largely replaced the OHF process in most modern, integrated steel production facilities. However, because BOF does not have an external fuel source, it is limited as to how much scrap steel it can process, unless that scrap steel is first melted. The BOF process is optimized around the use of molten pig iron generated by the blast furnace. Thus, in the modern steel plant, the use of a blast furnace and a BOF are tightly linked. The third type of steelmaking furnace is the Electric Arc Furnace, (EAF). It uses electrical current introduced into the iron or recycled steel by large electrodes to melt the metal. The metal charge is spiked with burnt lime prior to being placed in the furnace. The lime acts as flux, promoting the conversion of impurities in the iron or steel into ****, which can subsequently be separated from the steel. Oxygen is blown into the furnace during operation to convert carbon into carbon dioxide, just as in the case of the BOF and the OHF processes. Because the EAF is designed to be able to melt its charge before oxidation of excess carbon occurs, it is uniquely capable of handling 100% recycled steel and does not need a supply of pig iron in order to produce steel. Because the energy source is 100% electrical energy, rather than coal or natural gas, the EAF process can also theoretically be run using electricity generated from nuclear or renewable sources, thereby allowing it to operate with a much smaller carbon footprint.
Recent Technological Developments to Increase the Sustainability of Steel Production:
The steel industry has always known that its processes are energy intensive and as a result have significant negative environmental impacts. Consequently, incremental improvements to both the blast furnace and BOF processes have occurred through out the last century in an effort to recover energy. These efforts have not, until quite recently, been directed at reducing the processes’ carbon footprints, but rather at reducing production costs. Nonetheless, a number of energy recovery strategies have been employed, particularly with blast furnace processes to reduce the cost of their operations, (and incidentally to also reduce their carbon footprint). One of the most basic strategies has been to use the hot exhaust gases from the blast furnace and waste heat from other places in the steel plant to preheat the air entering the furnace. Recently, a new design called the Top-Gas Recycling Blast Furnace has been pilot tested in the EU. This technology employs carbon capture and storage of exhaust gases from the blast furnace. It is projected to be fully commercialized by 2020 .
Since the Clean Air and Clean Water Acts were first passed by the US Congress in the early 1970’s, during the administration of President Richard M. Nixon, US steel makers have had to also employ various pollution abatement technologies. These technologies are well documented in Reference . European and other first world countries have had similar pollution control regulations in place for steel manufacturers for much of the last half century. However, treating contaminated air and water streams after they have been generated in the steel making process is inherently less sustainable than preventing that pollution in the first place. Therefore, the newest and most innovative steel production processes seek to minimize energy use and prevent pollution, over the life cycle of the process, rather than treat and remediate pollutants after they are generated.
One such process has been developed by Siemens—a patented process known as the COREX® process. This process involves a modified blast furnace process, the details of which are proprietary. COREX® eliminates the need for coking the coal prior to the iron reduction step and also eliminates the need to sinter the iron ore prior to reduction. By eliminating the need for coke ovens and sintering plants, overall environmental impacts are reduced on a life cycle assessment basis. Overall energy consumption of the steel making process is also reduced because conventional coking reduces some of the useable chemical energy originally stored in the coal, in exchange for providing the higher combustion temperatures, needed in a conventional blast furnace. Reference  provides additional information on the life cycle assessment analysis performed by Siemens on its COREX ® process.
Even more promising than COREX®, from an energy conservation and environmental impacts perspective, are Direct Reduced Iron (DRI) processes . This family of processes allows for the direct reduction of iron ore (in the form of lumps, pellets, or fines) by a reducing gas (either a mix of hydrogen and carbon monoxide) or by non-coking grades of coal. The process occurs in the solid phase– the iron ore is not melted, as in a blast furnace process. Because there is no phase change during the process (i.e. no melting occurs), the process is inherently less energy intensive. Furthermore, the solid-phase product produced from the DRI process can then be fed directly to an EAF furnace, further reducing the need for primary fossil fuel use. A final advantage of DRI is that because a source of coking-grade coal is not required, it can be conducted in geographical locations where low grade coal or other fuels are available. The ability to make steel in the same region where it will be used by combining DRI and EAF technologies reduces transportation costs and thereby the embodied energy and environmental impact of the product over its life cycle. Reference  provides a detailed Life Cycle Assessment (LCA) analysis of the material flows involved in the conventional steel industry. It shows that the traditional methods of making steel which involve transporting raw materials and finished product across large geographical distances is unsustainable, and that the steel industry must move to a more localized production model. Such localized steel production has taken the form of “mini-mills”. Mini-mills require far less capital investment, and unlike a blast furnace, which cannot be shut down for years at a time (because the start-up energy demand is so high), mini-mills are able to be operated on-demand. DRI is somewhat less rich in elemental iron—88% as opposed to 95% iron content from blast furnace processes and it contains more silica impurities than pig iron from a blast furnace because blast furnace **** is not removed. But, these disadvantages are largely outweighed by other factors and can be compensated for by various pre- and post-treatments. One economic driver for the increased use of DRI processes is the growth of the scrap steel market. Scrap steel is now highly sought after for use in EAF- based mini-mills and the price of scrap steel has increased significantly over the last several decades. DRI is therefore desirable as an alternate feedstock for these mini-mills.
Final Thoughts– The Connection between Economic Competitiveness and Sustainable Production Methods:
Over the last 30 years, the amount of energy required to produce a ton of steel has been reduced by 50%, and today, over 70% of scrap steel is recycled . However, until quite recently, these improvements in energy efficiency and recycling were not due to environmental consciousness on the part of steel companies so much as due to a desire to reduce production costs. They were largely market-driven. This illustrates a key point regarding sustainability– that it often can be accomplished in parallel with cost reductions and increased profitability, and not in opposition to them. An older and largely obsolete view of environmentalism is that it is always an added cost, over and above other production costs. This is clearly not the case in the steel industry, which has embraced sustainability as a means to remain viable into the 21st Century. Reference  provides a detailed analysis of how sustainable practices can be used to improve profitability within the steel industry over the next decades. Such industry trade groups as the World Steel Association (www.worldsteel.com ) have devoted considerable resources to developing more efficient and sustainable models for steel production and to sharing them with their member companies. Just as the blast furnace technology represented in van Valckenborch’s 1609 painting helped to lead humanity into the modern industrial age, efforts at increased sustainability by the modern steel industry can help forge an integrated, green economy of the future.
- The Making, Shaping and Treating of Steel, 10th Edition, United States Steel Corporation, edited by William T. Lankford, Jr., Norman L. Samways, Robert F. Craven, and Harold E McGannon, 1985.
- “Sustainable Steelmaking—Meeting Today’s Challenges, Forging Tomorrow’s Solutions”, Rubel, H.; Wortler, M.; Schuler, F.; and Micha, R. ; The Boston Consulting Group, July, 2009.
- http://www.ulcos.org/en/research/blast_furnace.php, information provided by the Ultra Low CO2 Steelmaking initiative.
- Pollution Prevention Technology Handbook, edited by Robert Noyes; p. 168-192, Noyes Publications, Park Ridge, New Jersey, 1993.
- Siemens: A better ecobalance in steel production – Pig iron production with COREX® and FINEX®
- “The Increasing Role of Direct Reduced Iron in Global Steelmaking,” Grobler, F., and Minnitt, R.C.A., The Journal of the South African Institute of Mining and Metallurgy, March/April, 1999, p. 111-116.
- “Iron Ore and Steel Production Trends and Material Flows in the World: Is this Really Sustainable?”, Yellishetty, M., Ranjith, P.G., and Tharumarajah, A., Resources, Conservation and Recycling, Volume 54 (2010), p. 1084-1094.
- World Steel Association: Sustainable Steel: At the core of a green economy
Coming in January 2013, is an essay based on Johvi Schulze-Görlitz’s 1951 oil-on-canvas painting, “Trümmerfrauen” (Rubble Women). The essay will look the evolution of materials recycling in the construction industry and the growth of the green building movement.?