Control of the gas system for a rotary kiln reduction plant

摘要

In Part 1 of this thesis a set of generic models suitable for the modeling of gas systems found in metallurgical and other industrial processes and power plants are developed and analyzed. This set includes models for fan/duct, structural volume, thermal capacity, quencher and combustion of gaseous reactants. These control system design models give explicit relations between control inputs like the speed of process fans and measurable outputs like pressure and temperature. The models are used for modeling the gas system part of the rotary kiln reduction process for ilmenite ore at Tinfos Titan & Iron, in Tyssedal, Norway. A main reason why control performance is important for this system, is that the gas in the grate-kiln system is not well sealed from the surrounding atmosphere. Hence, it is important that the pressure in this system is kept as close as possible to the atmospheric pressure. Positive differential pressure leads to dust and toxic gases being discharged into the factory premises. On the other hand, negative differential pressure influences the energy balance, expecially in the kiln, where it can lead to local overheating of the ilmenite pellets. Hence, it influences the quality of the final product. This leakage of air into the kiln also influences the energy economy. In Part 2 a procedure for automatic tuning of proportional-integral-derivative (PID) controllers based on transfer function estimation is developed. A main feature of the tuning procedure is a method for the excitation of a closed-loop system in the most important frequency range with respect to control system performance. The closed-loop system is excited by generating limit cycle oscillations in the system at two different frequencies, the crossover frequency and the critical frequency for the feedback loop. 68 refs., 121 figs.