Fundamental investigation into the formation of nitrogen oxides in the electric arc furnace
With regard to NOx emissions from electric arc furnaces, there are almost no studies in Europe and very few published scientific studies in the USA. The NOx emission of the electric arc furnace depends on the one hand on rapidly changing operating parameters (e.g. arc length, switch-on processes, composition of the atmosphere in the electric arc furnace) and on the other hand on the mode of operation (e.g. use of fuels such as natural gas). The effects of process conditions and arc parameters such as arc length, arc current, direct current (DC) or alternating current (AC) operation of the arc furnace on NOx emissions have not yet been investigated.
The composition of the furnace atmosphere changes constantly with the operating state of the arc furnace and generally deviates significantly from the known exhaust gas compositions during the combustion of fossil fuels, for which the NOx formation mechanisms have been intensively investigated in recent years. Thus, the research results from this area cannot be transferred to the conditions in the electric arc furnace. The high gas temperatures of up to 1800 °C in the furnace chamber and the electrically conductive arc with currents of up to 100 kA and temperatures of the gas plasma of 8000 to 10000 K provide good conditions for the formation of nitrogen oxides in gas mixtures containing N2/O2/CO2.
In order to investigate the formation of nitrogen oxides (NOx), a series of tests were carried out on a gas-tight laboratory arc furnace, which had to be retrofitted for these tests. The aim of the tests was to determine the NOx quantities in the furnace exhaust gas as a function of the process parameters to be varied. Therefore, a number of parameters such as the composition of the furnace atmosphere, the gas volume flow through the furnace as well as the arc length and the arc flow were changed during the experiments.
The tests carried out and theoretical work have shown that the operating parameters of the arc, which above all influence the arc volume (arc length and power) and the gas mass flow (arc flow) through the arc range, have a relatively small influence on the amount of nitrogen oxides formed in the range of the technically possible variations.
The furnace chamber atmosphere represents a significantly larger influence, which is more relevant for possible emission reduction strategies. Their quantity and composition depend on the tightness of the furnace and the amount of exhaust gas extracted in the individual process phases of the arc furnace, the reaction of carbon from the melt and the graphite electrodes with oxygen and the operating conditions of the auxiliary burners. Important parameters here are the air mass flow through the furnace chamber, which on the one hand determines the residence time of the gas in the furnace chamber and on the other hand dilutes the nitrogen oxides formed in the plasma range, as well as the furnace chamber temperature and the oxygen supply or the content of NOx-reducing gas species such as CO, which occur in the furnace as a result of the process.
During the tests on the 200 kg pilot arc furnace, typical operating conditions of the arc furnace process were simulated (e.g. arc ignition in air, systematic reduction of the oxygen content and CO2 addition in the gas atmosphere).
The thermodynamic calculations carried out are calculations of the thermodynamic equilibrium by minimizing the Gibbs energy (free enthalpy) ΔG for a given composition of the gas phase and temperature and modelling the influence of the reaction kinetics on NOx formation in the arc furnace using a reactor network based on the Cantera software.
In the experiment as well as in the thermodynamic calculations and simulations, consistent results were achieved. In particular, the modelling of the furnace in the form of a reactor network, including the reaction kinetics, has already yielded good results despite various simplifications.
Since so far only few data for NOx emissions are available and the cause-effect relationships for their formation were little known, these investigations provide important information for environmental legislation.
The project was funded by the Deutsche Forschungsgemeinschaft (DFG) under the reference number PF 394/10-1.