Increased Use of Natural Gas in Blast Furnace Ironmaking: Mass and Energy Balance Calculations
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TRODUCTION
STEELMAKING is a highly carbon-intensive process with approximately 1.8 tons of CO2 emitted per ton of hot metal (THM) produced.[1] Most of this carbon is consumed by the blast furnace in the form of coke. Coke acts as the fuel and reductant, and provides structural support to the furnace. Coke can be partially replaced with other fuels—such as injected pulverized coal or natural gas. The recent increase in natural gas production in the US, and lower natural gas prices, prompted this study on maximizing natural gas usage in blast furnace ironmaking. In North America, the intensity of natural gas injection through blast furnace tuye`res (expressed as the volume of natural gas per THM) has increased in response to the lower natural gas prices.[2,3] However, the amount of natural gas that can be injected through the tuye`res is limited by the endothermic effect of natural gas in the raceway; although the flame temperature can be increased by oxygen enrichment of the blast air, oxygen enrichment causes the top gas temperature to decrease. The overall
MEGHA JAMPANI is with Praxair, Inc., 175 E Park Dr, Tonwanda, NY 14150 and also with the Center for Iron and Steelmaking Research, Department of Materials Science and Engineering, Carnegie Mellon University, 5000 Forbes Ave, Pittsburgh, PA 15213. JORGE GIBSON and PETRUS CHRISTIAAN PISTORIUS are with the Center for Iron and Steelmaking Research, Department of Materials Science and Engineering, Carnegie Mellon University. Contact e-mail: [email protected] Manuscript submitted August 14, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
effect is that the maximum amount of cold natural gas that can be injected through the blast furnace tuye`res is limited to around 150 kg/THM (kilograms of natural gas per THM).[4] (‘‘THM’’ refers to tonnes of hot metal.) The quantitative origin of this limit is revisited below. As an alternative, shaft injection of preheated and partially combusted natural gas has been proposed (see Figure 1)[4]; this work evaluated the feasibility of that approach, based on mass and energy balances and estimates of the kinetics of partial combustion. Shaft injection of hot reducing gas has been proposed before, although the focus of that work was on injection of recycled top gas and of syngas (from gasification of low-grade coal).[5–7] It is proposed to meet the energy requirements of increased natural gas utilization by preheating and partially combusting the gas. This is similar to the approach used in the Energiron ZR (‘‘zero-reforming’’) direct-reduction (DR) process, that uses natural gas to reduce iron oxide in a shaft furnace.[8] Figure 2 schematically shows gas flow in the zero-reforming DR process, emphasizing top gas recycling and the reductant stream. The reducing gas that enters the DR shaft contains CH4, CO, H2 and H2O to reduce and carburize the iron ore pellets in the shaft furnace. The top gas from the furnace is recirculated after condensation of water, CO2 removal, and addition of natural gas. The mixed gas stream is preheated to app
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