Abstract
In this work, we present rigorous calibration and assessment of a surface ionization detector (SID) for alkali monitoring in industrial process gases and compare it to an in situ laser diagnostic method called collinear photofragmentation and atomic absorption spectroscopy (CPFAAS). The side-by-side comparison of the time-resolved alkali concentration was performed in a technical-scale gas burner seeded with selected alkali salts, corresponding to alkali molar fractions of 10–100 ppm in the flue gas. The SID operates at room temperature and relies on extraction, dilution, and conditioning of the sample gas, whereas CPFAAS provides in situ molecular data. During KCl addition, the instruments were in good agreement: 80.1 ppm (SID) and 88.5 ppm (CPFAAS). In addition to the field measurements, internal validation of SID performance parameters (flow, electric field strength, and filament temperature) and external parameters (particle size and salt composition) was performed. The difference in sensitivity toward different alkali salts was found to be considerable, which limits the quantitative assessment for a sample gas of unknown composition. The results demonstrate the capability and limitations of the SID and show that a SID can satisfactory monitor KCl levels in a process gas over several days of continuous measurements. However, for heterogeneous fuels with deficient characterization of the gas composition, the obtained SID signal is difficult to interpret without supportive diagnostics. The generic ability of the SID to detect Na and K in both gas and particle phases makes it a valuable complement to alkali diagnostics, such as spectroscopic techniques.
Highlights
The alkali metals sodium and potassium are among the most abundant elements on Earth and known to play an important role in thermal processing techniques as a result of their volatility and reactive nature
The applicability and limitations of a surface ionization (SI)-based alkali detector have been assessed by internal validation and comparative field measurements with collinear photofragmentation and atomic absorption spectroscopy (CPFAAS) in a simplified technical-scale combustion environment
The presented results demonstrate that a surface ionization detector (SID) is functional to provide quantitative data, as a standalone alkali detector, for process gas that contains a known alkali salt composition
Summary
The alkali metals sodium and potassium are among the most abundant elements on Earth and known to play an important role in thermal processing techniques as a result of their volatility and reactive nature. SI is commonly used to create a highly efficient, wellcharacterized ion source[11] for mass spectrometry applications.[12] The principle was applied by the University of Gothenburg to develop a surface ionization detector (SID) for quantitative alkali measurements used for atmospheric and fuel processing research.[13−16] The simplicity, high selectivity toward alkali atoms, and wide measurement range are the main advantages with SI techniques. SID detects a signal proportional to the molar concentration (ppm) of alkali in the extracted sample gas, while the CPFAAS captures in situ the molar fraction of KCl and KOH molecules in the gas phase This arrangement enables the evaluation of the SID extraction system and evaluates if a stand-alone SID can provide reliable alkali diagnostics under varying process conditions.
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