Iron, Steel and Cement Production
Iron and steel manufacturing is the largest industrial source of greenhouse gases, contributing about 7% of the world’s emissions from human activities. This stems from the vast amounts of energy required for iron and steel smelting (purifying iron or steel from iron ore) and from several chemical reactions that directly generate CO2. Earth’s crust contains deposits of iron ore, which is composed of iron combined with oxygen (e.g., iron hematite, Fe2O3) or sulfur (e.g., iron pyrrhotite, FeS, and iron pyrite “fool’s gold,” FeS2).
Blast furnace for iron and steel smelting Iron ore (primarily iron oxide), coke (purified coal), and limestone (primarily CaCO3) is fed into the top of the furnace, where iron forms from the reaction of the ore with carbon monoxide (CO). In the middle of the furnace, a reaction of carbon dioxide (CO2) with the carbon in the coke produces CO. Oxidation of the coke near the air inlets heats the oven and generates CO2. Limestone removes sulfur and other impurities from the ore and coke but releases CO2. Molten pig iron flow out from the bottom of the furnace. Slag (a mixture of iron, oxygen, sulfur, silicon, and other impurities from the ore) is less dense and drains out from a tap hole at a higher point.
Heating iron ore in a low-oxygen environment and in the presence of carbon from charcoal, coal, or coke expels the oxygen or sulfur and produces a carbon–iron alloy: wrought iron, carbon steel, or pig iron, depending on the proportions of iron and carbon. Higher carbon content yields a harder but more brittle metal. Stainless steel is a carbon–steel alloy that contains substantial amounts of chromium, nickel, and molybdenum to improve its corrosion resistance. Today, iron and steel production follow any of three pathways. The principal pathway is the blast furnace, which produces about 60% of the world’s iron and steel. A blast furnace, in produces a steady stream of pig iron, which requires further processing to reduce its carbon content. Several reactions in a blast furnace convert iron ore into iron which generate CO2.
Blast furnaces, on average, emit 534 kg of carbon equivalents of greenhouse gases per metric ton of steel, whereas more energy efficient designs emit as little as 400 kg of carbon equivalents per metric ton. [1] Most blast furnaces use coke as their primary carbon and energy source. Preparing coke accounts for between 4% and 24% of the greenhouse gas emissions from the iron and steel industry (International Energy Agency 2006a).
The second pathway, one that produces 35% of the iron and steel globally, is recycling of scrap metal. Recycling is relatively energy efficient: Electric arc furnaces that melt the scrap consume only 30% to 45% of the energy of blast furnaces per unit of iron or steel processed. Also, electric arc furnaces do not directly emit CO2; rather, their greenhouse gas emissions depend on the source of electricity.
The third pathway, which produces about 5% of iron and steel, is the direct reduction of iron (DRI). This process first converts the methane (CH4) in natural gas to carbon monoxide (CO) and hydrogen gas (H2)
Cement Production
Globally, greenhouse gases from cement production have risen to about 5% of human emissions and soon will surpass iron and steel manufacturing as the largest industrial source. Half of cement emissions derive from energy use during processing. The other half is CO2 released during the chemical reactions. The chemistry of cement involves heating limestone and small amounts of clay to 1450°C to make clinker (mostly crystals of calcium oxide silicates)
Grinding the clinker to a fine powder and adding about 5% gypsum yields Portland cement, the most common type of cement. Portland cement plus gravel and sand make concrete. Greenhouse gas abatement during cement production entails either decreasing the amount of limestone or conserving energy. Conserving energy may require purchasing new clinker kilns or grinding materials less finely. In addition, the United States uses mostly coal power for cement production, whereas some other countries substitute alternative sources such as nuclear power for a substantial portion of the energy and thereby emit less CO2 per amount of cement produced.
[1] Anderson, S. H., G. E. Metius, and J. M. McClelland (2008) Future Green Steelmaking Technologies, Midrex Technologies Inc., Charlotte, NC, http://www.midrex.com/uploads/documents/FutureGSTechnologies-Final2.pdf.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
©2010 Sinauer Associates and UC Regents
