Dynamic Modeling of the Main Blow in Basic Oxygen Steelmaking Using Measured Step Responses
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RODUCTION
TO improve control of basic oxygen steelmaking and to enable dynamic optimization, the process should be modeled dynamically. To develop a dynamic model, the influence of changes in control variables, such as the lance height, the oxygen blowing rate, and the addition rates, on the process should be known. How the behavior of the process changes with time under the influence of changes in control variables can be investigated using an experimental approach.[1,2] In this approach, the value of control variables is deliberately changed. One possibility is to change the control variables by making a step change and monitoring the process variable response. In the literature, little information is published on step responses in basic oxygen steelmaking.[3–6] It is however known that an increase in oxygen input, either due to an increase in oxygen blowing rate[3–5] or due to the addition of ore,[6] increases the decarburization rate. Furthermore, Anderson et al.[6] published the delay between the occurrence of a change in a control variable such as the iron ore addition rate and a change in decarburization rate. In this article, the step response to changes in control variables is determined experimentally in the first section. Using the measured step responses, a simplified C. KATTENBELT, Researcher, is with the Research Group Control Systems Engineering, Hogeschool Arnhem-Nijmegen, Karveel 67, 7429 BK Colmschate, The Netherlands. Contact e-mail: c.kattenbelt@alum nus.utwente.nl B. ROFFEL, Professor, is with the Hogeschool ArnhemNijmegen, University of Twente, The Netherlands Manuscript submitted March 6, 2008. Article published online October 10, 2008. 764—VOLUME 39B, OCTOBER 2008
process model is developed in Section II. In that section, the measured and simulated step responses are compared. Finally, the conclusions are summarized.
II.
EXPERIMENTAL
Due to the high temperatures and dusty environment involved in basic oxygen steelmaking, important process variables such as the steel composition and steel temperature can not be measured continuously. Therefore, in most steel plants, indirect measurements, such as waste gas flow, temperature, and composition, are used to monitor the process. The decarburization rate and the accumulation rate of oxygen inside the converter, which are commonly derived from waste gas measurements, can be used as an indication of the change in composition of the steel and the slag, respectively.[7,8] The decarburization rate dC dt can be given by dC /wg ðCOwg þ CO2;wg Þ ¼ dt VM
½1
where /wg is the measured waste gas flow, COwg and CO2,wg are the measured percentage of carbon monoxide and carbon dioxide in the waste gasses, and VM is the molar volume. The measured accumulation rate of oxygen inside the converter dO dt can be given by dO dOlance dOadditions dOwg dOair ¼ þ ½2 dt dt dt dt dt where dOdtlance is the measured rate at which oxygen is is the measured rate in blown into the converter, dOadditions dt dO which oxygen in additions enters the converter, dtwg is the mea
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