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.

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