Developments Toward an Intelligent Electric Arc Furnace at - Tenova

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OVERVIEW The CMC Texas EAF is a 22’ diameter EAF, 120 short ton eccentric bottom tapping, AC furnace powered by an 80 MVA transformer. Chemical energy is provided by three PTI JetBox modules, three 3.5 MW burners (for carbon, oxygen and methane) and one 1 MW EBT conventional burner. The Goodfellow Expert Furnace System Optimization Process (Goodfellow EFSOP ® ) is a dynamic control and optimization system for electric arc steelmaking furnaces (EAF) based on real-time measurements of off-gas composition. The system uses state-of-the-art off-gas analysis combined with process data acquisition and real-time closed loop control to optimize the operation of the EAF. Typically, the objectives for optimizing the EAF operation are to reduce conversion costs (energy and material), increase production and improve safety. A schematic of the EFSOP ® system is shown in Figure 1. It includes the following components: • A water-cooled sampling probe • Gas analysis system • Supervisory Control and Data Acquisition (SCADA) system Figure 1: Schematic of the EFSOP ® System
A patented, water-cooled sampling probe, custom designed for use in the harsh environment of the EAF, is installed through a port in the water-cooled ductwork, at the entrance to the fixed duct and after the combustion gap. The location and positioning of the probe is such that the gases are sampled before introduction of combustion air from the gap and so are representative of the composition of gases in the freeboard of the furnace. The extracted gases are transported via a heated sample line to the EFSOP ® gas analysis system where the continuous stream of sampled gases are analyzed for oxygen (O2), carbon dioxide (CO2), carbon monoxide (CO), and hydrogen (H2). A secondary function of the analyzer unit is to periodically back-purge the sampling line and probe to remove accretions inside the probe and dust build-up from the various internal filters employed. The system is only purged during natural breaks in the process (e.g. during charging, tapping or other power-off times) so as to ensure reliable and continuous off-gas composition measurements. Composition measurements, as well as operational alarms and outputs from the analyzer are sent to the plant’s PLC network. The EFSOP ® HMI is linked to the same network. The Human-Machine Interface (HMI) reads and logs the off-gas data, as well as all relevant process data at a frequency of 1 second. In total over 200 furnace parameters are sampled and logged. Both historical and real-time plots of the data are available. In addition to real time process data trending and historical data mining, the HMI allows regular data transfer to TGTI’s office in Mississauga. The EFSOP ® SCADA system consists of a high-speed computer and GE’s iFIX SCADA software. Furthermore, the SCADA system performs control calculations, in response to furnace off-gas data and sends oxygen and methane flow setpoints PLC. EAF Optimization Furnace optimization via EFSOP ® , typically, follows two parallel paths: 1) closed-loop control of chemical energy and postcombustion (oxygen, methane and carbon rates) in response to off-gas composition measurements and 2) adjustments to the EAF process from off-line analysis of furnace operation using a variety of proprietary tools such as advanced regression methods; and TGTI’s DECSIM electric arc furnace modeling software ([1] and [2]). The first is the implementation dynamic control of post-combustion in the furnace freeboard in response to off-gas composition while working within the mechanical limitations of the existing hardware. During periods when the furnace off-gas is reducing, additional oxygen may be added or methane flow reduced; alternatively, during periods when the furnace off-gas is oxidizing, oxygen flow maybe reduced and/or methane increased. The challenge is to maintain a slightly reducing atmosphere in the furnace freeboard. In practice, the implementation of closed-loop control requires that constraints be placed flow set-points of oxygen/methane at different stages of the heat. For example: directly after charging the flows may be limited so that the flame does impinge on the scrap and rebound back onto the panel; during melting the flows are constrained so that the injectors operate around stoichiometric or slightly super-stoichiometric; and during refining, where supersonic oxygen flow is required, the constraints on oxygen flow are increased. The operating profiles for oxygen and methane delivery, definition of constraints and timing as well as the ability to save different programs are possible through customized screens developed within the EFSOP ® HMI. The second aspect of optimization via the EFSOP ® system is in the form of process adjustments (e.g. timing and intensity of oxygen lancing, carbon injection, lime practice, fume system parameters, etc.). A holistic approach to optimization is taken and adjustments to the process are made according to what is observed in the off-gas. To aid in this process, the EFSOP ® system includes proprietary software that forwards operating data directly to TGTI (on a daily basis). This data includes not only dynamic operational information from the process, but also alarms and operating status of the EFSOP ® analyzer. Any operational issues with the EFSOP ® analyzer are known directly and can be addressed quickly by feedback to the plant personnel. During the optimization stage of the EFSOP ® implementation, TGTI engineers are able to follow plant performance remotely and communicate adjustments to the plant. The EFSOP ® system (probe, analyzer and SCADA system) was installed at CMC Texas in December, 2005. Preliminary observations of off-gas composition revealed that CMC’s operation resulted in a significantly reducing furnace off-gas with relatively high levels of hydrogen and carbon monoxide and that there was the opportunity to improve the practice by reducing the amount of chemical energy usage in the furnace. A different approach would have been taken if the process was determined to be oxidizing. An example off-gas profile before optimization is included (see Figure 3). The profile shows that carbon monoxide concentration (before optimization) was typically in the range of 20%-30%, during melting, and 30%-40% during refining. The noted observation was typical during the preliminary data collections and monitoring period that extended into January and part of February of 2006. A strategy for optimization was developed over that period and implemented during the later part of February 2006. Using CMC’s typical burner and power programs as a starting point, the following adjustments were made to the process: • Carbon injection was reduced during the melting stages of the process; • Closed-loop control was implemented to automatically adjust the stoichiometric ratio of oxygen to methane in response to off-gas composition measurements provided by the EFSOP ® system; and
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Developments Toward an Intelligent Electric Arc Furnace at - Tenova

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