Continuous Monitoring of Toxic Metals in Gas Flows Using Direct-Current Plasma Excited Atomic Absorption Spectroscopy

Abstract

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.