Advanced Optical Diagnostic Techniques for Detection of Alkali Vapors in High-Temperature Gases

Two advanced optical techniques for the detection of alkali vapors in high-temperature gases are introduced in this Thesis. Alkali vapors are released in solid fuel combustion and increase the maintenance costs of the power plant boilers. Especially, biomass and waste are challenging fuels for contemporary technology. Alkali related problems are controlled with proper fuel mixture ratios and additives that are mixed to fuels or sprayed into the boiler. However, the controlling increases the cost of the produced energy and the additives increase SO2 concentration. In order to study the alkali formation mechanisms and to control alkali concentrations in power plant boilers, fast, accurate and highly sensitive detection techniques are needed. This Thesis presents the fundamental absorption properties of the K, KCl and KOH vapors, reviews previously applied techniques for the detection of alkalis, and introduces two novel detection methods. The first method is based on the Photoacoustic Spectroscopy (PAS), that utilizes two branches of physics, namely optics and acoustics. A high-temperature PAS cell is introduced and applied to detect KCl and NaCl. Moreover, the possibility to apply the PAS cell to high-temperature salt-induced metal oxidation is discussed. The second method, which was fully developed during this work, is called Collinear Photofragmentation and Atomic Absorption Spectroscopy (CPFAAS). CPFAAS provides the limit of detection below parts per billion and the dynamic range of eight orders of magnitude in the detection of alkali chlorides. Moreover, the detection time of the CPFAAS measurement is in the order of micro second, which enables the diagnostics of the fast changing processes, such as combustion. The technique is calibration free and is integrable to "see-through" applications. In this Thesis, CPFAAS is applied to determine potassium vapor concentrations from a laboratory cell, flue gases emanated from 10 mg fuel samples, and flue gases in intermediate and large scale combustion boilers. The presented applications, modeling and comparison with previously applied detection methods show that CPFAAS enables new detection and monitoring capabilities from corrosive high temperature gases

Transversely Excited Multipass Photoacoustic Cell Using Electromechanical Film as Microphone

A novel multipass photoacoustic cell with five stacked electromechanical films as a microphone has been constructed, tested and characterized. The photoacoustic cell is an open rectangular structure with two steel plates facing each other. The longitudinal acoustic resonances are excited transversely in an optical multipass configuration. A detection limit of 22 ppb (10−9) was achieved for flowing NO2 in N2 at normal pressure by using the maximum of 70 laser beams between the resonator plates. The corresponding minimum detectable absorption and the normalized noise-equivalent absorption coefficients were 2.2 × 10−7 cm−1 and 3.2 × 10−9 cm−1WHz−1/2, respectively

Advanced Optical Diagnostic Techniques for Detection of Alkali Vapors in High-Temperature Gases

Two advanced optical techniques for the detection of alkali vapors in high-temperature gases are introduced in this Thesis. Alkali vapors are released in solid fuel combustion and increase the maintenance costs of the power plant boilers. Especially, biomass and waste are challenging fuels for contemporary technology. Alkali related problems are controlled with proper fuel mixture ratios and additives that are mixed to fuels or sprayed into the boiler. However, the controlling increases the cost of the produced energy and the additives increase SO2 concentration. In order to study the alkali formation mechanisms and to control alkali concentrations in power plant boilers, fast, accurate and highly sensitive detection techniques are needed. This Thesis presents the fundamental absorption properties of the K, KCl and KOH vapors, reviews previously applied techniques for the detection of alkalis, and introduces two novel detection methods. The first method is based on the Photoacoustic Spectroscopy (PAS), that utilizes two branches of physics, namely optics and acoustics. A high-temperature PAS cell is introduced and applied to detect KCl and NaCl. Moreover, the possibility to apply the PAS cell to high-temperature salt-induced metal oxidation is discussed. The second method, which was fully developed during this work, is called Collinear Photofragmentation and Atomic Absorption Spectroscopy (CPFAAS). CPFAAS provides the limit of detection below parts per billion and the dynamic range of eight orders of magnitude in the detection of alkali chlorides. Moreover, the detection time of the CPFAAS measurement is in the order of micro second, which enables the diagnostics of the fast changing processes, such as combustion. The technique is calibration free and is integrable to "see-through" applications. In this Thesis, CPFAAS is applied to determine potassium vapor concentrations from a laboratory cell, flue gases emanated from 10 mg fuel samples, and flue gases in intermediate and large scale combustion boilers. The presented applications, modeling and comparison with previously applied detection methods show that CPFAAS enables new detection and monitoring capabilities from corrosive high temperature gases

Phase-sensitive method for background-compensated photoacoustic detection of NO2 using high-power LEDs

A photoacoustic (PA) sensor has been developed for the detection of nitrogen dioxide (NO2). Ten amplitude-modulated high-power light emitting diodes (LEDs), emitting a total optical power of 9W at 453 nm, are used to excite the photoacoustic signal in NO2. The LEDs are attached to the circumference of a cylindrical PA cell. The induced longitudinal acoustics waves are detected using two electromechanical film stacks, located at the ends of the cell. Background signal cancelation is achieved by using phase-sensitive detection of the difference signal of the two pressure transducers. The phase-sensitive approach allows for improved dynamic range and sensitivity. A detection limit of 10 parts per billion by volume was achieved for flowing NO2 gas sample in an acquisition time of 2.1 s, corresponding to a minimum detectable absorption coefficient of 1.6 x 10−7 cm−1Hz−1/2. The developed sensor has potential for compact, light-weight, and low-cost measurement of NO2.Peer reviewe

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.publishedVersionPeer reviewe

An analysis of factors affecting the market price of electricity: the case of Phelix index

The addition reaction of potassium atoms with oxygen has been studied using the collinear photofragmentation and atomic absorption spectroscopy (CPFAAS) method. KCl vapor was photolyzed with 266 nm pulses and the absorbance by K atoms at 766.5 nm was measured at various delay times with a narrow line width diode laser. Experiments were carried out with O₂/N₂ mixtures at a total pressure of 1 bar, over 748–1323 K. At the lower temperatures single exponential decays of [K] yielded the third-order rate constant for addition, <i>k</i>_R1, whereas at higher temperatures equilibration was observed in the form of double exponential decays of [K], which yielded both <i>k</i>_R1 and the equilibrium constant for KO₂ formation. <i>k</i>_R1 can be summarized as 1.07 × 10^–30(<i>T</i>/1000 K)^−0.733 cm⁶ molecule^–2 s^–1. Combination with literature values leads to a recommended <i>k</i>_R1 of 5.5 × 10^–26<i>T</i>^–1.55 exp­(−10/<i>T</i>) cm⁶ molecule^–2 s^–1 over 250–1320 K, with an error limit of a factor of 1.5. A van’t Hoff analysis constrained to fit the computed Δ<i>S</i>₂₉₈ yields a K–O₂ bond dissociation enthalpy of 184.2 ± 4.0 kJ mol^–1 at 298 K and Δ_f<i>H</i>₂₉₈(KO₂) = −95.2 ± 4.1 kJ mol^–1. The corresponding <i>D</i>₀ is 181.5 ± 4.0 kJ mol^–1. This value compares well with a CCSD­(T) extrapolation to the complete basis set limit, with all electrons correlated, of 177.9 kJ mol^–1