A measurement apparatus employing direct current (dc) plasma excited atomic absorption spectroscopy was developed and demonstrated for continuous measurement of toxic metals in process gases. Process gas is continuously sampled along a heated sample line. Metal compounds contained in the gas are thermally decomposed by mixing the gas with a plasma jet produced with a dc nitrogen plasma torch. Transmission of monochromatic light is measured through the gas jet, and absorbance caused by metal atoms is distinguished from the background by means of the Zeeman effect. The metal concentration in the sample gas is calculated from the measured absorbance with the known dilution and decomposition factors taken into account. The detection limits of the current prototype are 0.04 mg/m3 for cadmium and 0.4 mg/m3 for lead. The measurement accuracy is better than 20%, and the maximum measurement rate is about 100 values per minute. The instrument was designed to withstand wet, corrosive, and particulate-laden flue gases at temperatures up to 1100 °C. The instrument can also be used, after minor modification, for measurements at pressurized conditions. The performance of the instrument was demonstrated in connection with a real fluidized bed combustor.

In this work we have studied experimentally the influence of N-O chemistry on radiative emissions from a small DC plasma torch at 1 atm. The plasma torch has been designed for the atomizer of an on-line atomic absorption spectrometer, which is used for the detection of vaporized metals in various process gas flows. The plasma torch was burning in pure nitrogen and an oxygen bearing sample gas was introduced into the plasma jet. The temperature of the plasma jet was determined spectroscopically to be around 2000 K. At this temperature the equilibrium UV radiation of excited atoms or molecules is negligible, but molecules and atoms are effectively excited by collisions with metastable nitrogen molecules formed in the plasma. The emission spectra of the plasma jet were measured in the wavelength range 200-400 nm in various gas mixtures. In pure nitrogen the second positive system of N2was observed, the intensity of which exceeded the estimated radiation at thermal equilibrium even by 1013 times. With a trace amount of oxygen-containing species, such as O2, CO2 or H2O, very intense γ-band radiation of NO was observed. The intensities of the lines were calculated to be around 109 times greater than corresponding equilibrium intensities. High intensities were also observed in atomic resonance lines of several metals. The intensity of the Cu line at 324.75 nm was 5×103 higher than the estimated equilibrium emission at given conditions. Correspondingly, the emission intensity of Pb line at 283.3 nm was around 5×105 and Cd line at 228.8 nm around 2×107 times more intense than at thermal equilibrium. Our study indicated that the rate of the collisional excitation of metal atoms by metastable nitrogen molecules was roughly the same for all the studied metals. When larger amounts of O2 or H2O were introduced into the plasma jet, the emission intensity of molecular lines and atomic lines of metals decreased drastically. We showed that radiative emissions do not interfere with absorption measurements provided that the percentage of O2 in the plasma jet is above 5 vol% or of water vapour above 2 vol%.

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A measuring instrument employing direct current (dc) plasma excited atomic line spectroscopy was developed for continuous measurement of alkali in combustion flue gases. Alkali compounds are dissociated by mixing sampled flue gas with a nitrogen plasma jet generated with a non-transferred dc plasma torch. The instrument can be used in two operating modes. The molar fraction is determined either by measuring the transmittance of the gas jet or by monitoring the emission. A tungsten halogen lamp and scanning monochromator are used for the measurement of the optical signals. Measurement of sodium and potassium has been demonstrated. The detection limits of the instrument are 50-70 ppb in the absorption mode. The detection limits are 2-3 ppb at 0.1 MPa pressure and 0.1-0.2 ppb at 1.0 MPa in the emission mode. The instrument is designed to withstand corrosive, particle laden, and pressurized flue gases at temperatures up to 1373 K.

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