Laser-Emissionsspektrometrie für die Partikelanalyse von Prozessgasen bei der Roheisenerzeugung

Abstract

The industrial use of laser emission spectrometry for particle analysis in gas at overpressure is a metrological challenge due to a reduction of analytical sensitivity caused by a top gas pressure of up to four bar. In the scope of this work it is shown that an increase of the gas pressure affects the electron pressure as well as the electron density due to collison and relaxation processes. Plasma properties are changed which lead to a decrease of line intensity, line broadening and to a decrease of signal-to-backround ratio. Consequently the gas pressure has an impact on the plasma dynamics and must be considered as a relevant parameter for the analysis of gas and particles in gas. Model calculations proof that the observed change of the emission line profil can by described by Stark-Effekt-braodening and Stark-Effekt-shift using a plasma model with core- and shell- specific electron temperatures. Double pulses with interpulse separation in the ns region are used to increase the line intensity of the LIBS spectra and to avoid the impact of overpressure on the emission line profil, thus leading to an enhanced signal-to-backround ratio of the spectral emission lines. Based on the laboratory results a laser set-up for field tests in steel industry was developed and tested on-site for muli-element analysis at different European blast furnaces. Signal-to-background ratio was optimised by double pulse excitation to allow the analysis of low-concentrated cirulting elements in top gas covering a few ppm. Line broadening and spectral shift is in a way reduced which makes a re-profilation of the laser-based set-up due to variations in top gas pressure no longer necessary. A simultaneous and continous on-line monitoring of the circulating elements Na, K, Zn und Pb was demonstrated successfully by performing daily measurements at a maximum duration of up to 10 hours