Sequential Collinear Photofragmentation and Atomic Absorption Spectroscopy for Online Laser Monitoring of Triatomic Metal Species.

Industrial chemical processes are struggling with adverse effects, such as corrosion and deposition, caused by gaseous alkali and heavy metal species. Mitigation of these problems requires novel monitoring concepts that provide information on gas-phase chemistry. However, selective optical online monitoring of the most problematic diatomic and triatomic species is challenging due to overlapping spectral features. In this work, a selective, all-optical, in situ gas-phase monitoring technique for triatomic molecules containing metallic atoms was developed and demonstrated with detection of PbCl2. Sequential collinear photofragmentation and atomic absorption spectroscopy (CPFAAS) enables determination of the triatomic PbCl2 concentration through detection of released Pb atoms after two consecutive photofragmentation processes. Absorption cross-sections of PbCl2, PbCl, and Pb were determined experimentally in a laboratory-scale reactor to enable calibration-free quantitative determination of the precursor molecule concentration in an arbitrary environment. Limit of detection for PbCl2 in the laboratory reactor was determined to be 0.25 ppm. Furthermore, the method was introduced for in situ monitoring of PbCl2 concentration in a 120 MWth power plant using demolition wood as its main fuel. In addition to industrial applications, the method can provide information on chemical reaction kinetics of the intermediate species that can be utilized in reaction simulations.

Interactions of PbCl2 with Alkali Salts in Ash Deposits and Effects on Boiler Corrosion.

A novel temperature gradient laboratory-scale corrosion test method was used to study PbCl2 migration, interactions with SiO2, NaCl, Na2SO4, KCl, K2SO4, or NaCl-KCl (50:50 wt %) and corrosion of carbon steel in waste-fired boilers. Two different steel temperatures (200 and 400 °C) were tested. The temperature in the furnace above the deposits was 700-800 °C. Exposure times of 4 and 24 h were used. The deposit cross sections were analyzed using SEM/EDXA. The results show that PbCl2 vaporized and condensed in the adjacent deposits. PbCl2 did not interact with SiO2 but caused severe corrosion. Deposits containing Na2SO4, K2SO4, and/or KCl reacted with the PbCl2, forming various new compounds (Na3Pb2(SO4)3Cl, K3Pb2(SO4)3Cl, and/or K2PbCl4). In addition, melt formation was observed with all alkali salt deposits. Visibly more Pb was found in deposits where reactions between PbCl2 and alkali salts were possible, i.e., Pb was observed to be bound to the reaction products. No measurable corrosion was observed with steel temperature at 200 °C, while steel temperature of 400 °C resulted in catastrophic corrosion. PbCl2 in contact with the steel surface lead to faster corrosion than K2PbCl4.