Tunable Diode Laser Atomic Absorption Spectroscopy for Detection of Potassium under Optically Thick Conditions

Abstract

Potassium (K) is an important element related to ash and fine-particle formation in biomass combustion processes. In situ measurements of gaseous atomic potassium, K(g), using robust optical absorption techniques can provide valuable insight into the K chemistry. However, for typical parts per billion K(g) concentrations in biomass flames and reactor gases, the product of atomic line strength and absorption path length can give rise to such high absorbance that the sample becomes opaque around the transition line center. We present a tunable diode laser atomic absorption spectroscopy (TDLAAS) methodology that enables accurate, calibration-free species quantification even under optically thick conditions, given that Beer-Lambert's law is valid. Analyte concentration and collisional line shape broadening are simultaneously determined by a least-squares fit of simulated to measured absorption profiles. Method validation measurements of K(g) concentrations in saturated potassium hydroxide vapor in the temperature range 950-1200 K showed excellent agreement with equilibrium calculations, and a dynamic range from 40 pptv cm to 40 ppmv cm. The applicability of the compact TDLAAS sensor is demonstrated by real-time detection of K(g) concentrations close to biomass pellets during atmospheric combustion in a laboratory reactor.

Distribution of temperature, H2O and atomic potassium during entrained flow biomass combustion – Coupling in situ TDLAS with modeling approaches and ash chemistry

“…Detailed knowledge of the alkali release forms and ash transformation reactions is needed to improve numerical models, identify suitable parameters for process optimization and control, and mitigate operational and environmental issues. During devolatilization, high amounts of atomic potassium, K(g), may be present due to direct release or decomposition from larger molecules [7,[17][18][19]. The problematic K-containing species in the flue gas, KOH(g) and KCl(g,s,l), are likely formed through secondary ash transformation reactions consuming K(g) [3].…”

“…Simultaneous real-time detection of H2O, temperature and K(g) in one beam path was achieved by combining two existing TDLAS instruments previously applied and validated in flat-flames [35], single pellet conversion experiments [19,36] and pilot-scale entrained flow biomass gasification [27]. Calibration-free scanned wavelength modulation spectroscopy (CF-WMS) at 1398 nm provided path-averaged H2O concentrations and H2O density-weighted, path-averaged gas temperature [35,36],…”

“…whereas direct absorption spectroscopy (DAS) at 769.9 nm was used to obtain pathaveraged K(g) concentrations [19]. The laser beams were combined with the help of an optical fiber coupler (Newport, F-CPL-B12355), and, after passing the EFR, separated again employing a beam splitter for subsequent detection with suitable photodetectors (Fig.…”

A novel multi-jet burner for hot flue gases of wide range of temperatures and compositions for optical diagnostics of solid fuels gasification/combustion

“…This test indicates that the burner can provide various hot gas environments and it is convenient to apply optical techniques to the study of heterogeneous reactions, such as biomass combustion/gasification. To have more information about the release of potassium, several other methods can be employed, such as laser-induced breakdown spectroscopy (LIBS) 21 , laser-induced photo-fragment 32 , and absorption spectroscopy 33 .…”

Real-time in situ multi-parameter TDLAS sensing in the reactor core of an entrained-flow biomass gasifier

“…A second TDLAS system (Sensor 2), based on distributed feedback lasers, was used to simultaneously measure H2O and T at 1398 nm with calibration-free scanned wavelength modulation spectroscopy (CF-WMS) [20,21], and K(g) at 769.9 nm with direct TDLAS [22]. Sensors 1 and 2 were also used to detect soot via transmittance variations in the baseline.…”