The production of steel, cement, and power are the most polluting industries in the world. The power sector is responsible for 25% of global carbon emissions, primarily from burning fossil fuels like coal, oil, and natural gas. In the US, the power sector accounts for 30% of total emissions, while in China it accounts for 50%. The cement industry, the second highest carbon emitting industry after power, accounts for 8% of total emissions. China, the largest producer of cement, is responsible for half of global cement-related emissions. The production of one ton of cement leads to the emission of 0.8 tons of CO2. The steel industry is also a significant source of greenhouse gas emissions, accounting for 25% of direct GHG emissions from industrial sectors and 7% of total direct emissions.
Reducing the Carbon Emissions in the Power Production
A major source of carbon emissions from the power sector is coal-fired power plants, which generate about 35% of the world’s electricity. The steam generator, also known as a boiler, is an important component in a thermal power plant. Upgrading a coal-fired power plant with a supercritical or ultra-supercritical steam generator can increase efficiency by 23% or 48%, respectively. These steam generators operate at higher temperatures and pressures, allowing the steam to produce more power in the turbine with less fuel and lower carbon emissions. Using low-moisture, low-sulfur coal can also reduce fuel use and CO2 emissions.
Co-firing, the process of using natural gas as a supplementary fuel in a coal-fired power plant, can be an effective way to reduce CO2 emissions. The extent of this reduction depends on the co-fire rate, the percentage of natural gas in the fuel mix, and the coal rank. Challenges in co-firing include the need for additional infrastructure and the possibility of combustion instabilities. An alternative is converting a coal-fired power plant to a natural gas combined cycle (NGCC) plant, which can achieve a reduction of up to 70% in CO2 emissions compared to the highest emission intensity of coal-fired power lants. In an NGCC plant, the waste heat from the gas turbine is used to generate additional electricity in the steam turbine, increasing the overall efficiency of the plant at lower carbon emissions.
Reducing the Carbon Emission of the Cement Industry
Alternative raw materials can help lower the carbon footprint further. Fly ash and slag, which are byproducts of coal combustion and steel production, respectively, can be used as a partial replacement for clinker, the main component of cement. Production of clinker is a highly energy-intensive process and is a major contributor to greenhouse gas emissions in the cement industry. On those lines, use of alternative cementitious materials can help to reduce the demand for cement, leading to lower emissions. Examples of such materials include rice husk ash from rice milling process, ground granulated blast furnace slag, and silica fume from ferrosilicon alloy production process. Use of alternative fuels is another approach, wherein fossil energy sources (coal and pet coke) can be substituted by alternative fuels (e.g., waste oil, tires, plastics, sludges, and municipal solid waste) to about 80%, which will in turn reduce carbon emission by 15.5%.
Implementing advanced efficiency technology can improve the efficiency of cement production and reduce energy requirements, leading to lower emissions. For example, using dry process technology to grind raw materials and form clinker using a dry process instead of a wet process can reduce energy consumption by up to 40%. Preheater-precalciner technology, which preheats and precalcines raw materials before they are fed into the kiln, can also reduce energy consumption by up to 25%. Additionally, it is important to consider carbonation effects, the uptake of atmospheric CO2 by cement during construction, in CO2 emission-mitigation inventories. During cement carbonation, CO2 reacts with calcium hydroxide, a byproduct of the cement hydration process, to form calcium carbonate. This process is a natural part of the life cycle of concrete and can improve the durability and longevity of concrete structures while reducing the carbon emissions.
Reducing the Carbon Emission of the Steel Industry
Hydrogen-based steelmaking technology uses hydrogen instead of coke to reduce iron ore to pure iron, which is then refined into steel. This process involves modifying coke oven gas (COG) to increase its hydrogen content for use in blast furnaces. COG is produced when coal is heated without oxygen and consists mainly of hydrogen, methane, and carbon monoxide. It can be steam reformed to produce hydrogen-rich gas using the waste heat of coke ovens. Reforming breaks down methane and other hydrocarbons in COG at high temperatures and pressure to produce hydrogen and carbon dioxide. Optimizing furnace chemical reactions, improving the quality of coke, sinter, and slag, and using enclosed, reductive/temperature-maintaining transportation technologies like tubular belt conveyors and pneumatic transport can also help reduce emissions in steelmaking.
The carbon reduction potential of hydrogen-based steelmaking depends on the source of hydrogen. Green hydrogen, produced from renewable energy sources, and blue hydrogen, produced from fossil fuels using carbon capture and storage (CCS) technology, are the most promising sources. CCS is expensive and energy-intensive, and the cost of capturing carbon from large power plants is lower than from smaller sources. The cost of hydrogen-derived iron and steel depends on the price of renewable energy used for hydrogen and hydrogen power generation. For ultralow carbon technologies in the iron and steel industry to be successful, the cost of electricity must drop below 25 USD/MWh. There is an investment opportunity in creating renewable energy facilities to produce green hydrogen, as demand for hydrogen is expected to reach 13-16 Mt per year.
About the Author:
This piece was written exclusively for ETEnergyworld by Dr. Siddharth Misra, an Associate Professor in the Harold Vance Department of Petroleum Engineering with a joint appointment in the Department of Geology and Geophysics at the Texas A&M University. He holds a Doctoral Degree from the University of Texas at Austin and B.Tech in Electrical Engineering from the Indian Institute of Technology Bombay. He has published two books and developed 4 patented and 5 patent-pending technologies on machine learning and electromagnetic sensing implementations for energy and earth resource exploration. In 2018, he was recognized as the U.S. Department of Energy Early Career Awardee. In 2020, for his significant contributions to exploration geophysics and subsurface engineering, he received SEG J. Clarence Karcher Award, SPWLA Young Technical Professional Award, EAGE Arie Van Weelden Award, and SPE Gulf Coast Formation Evaluation Award.