In-situ study of the effect of operating conditions and additives on alkali emissions in fluidised bed combustion

In this work, alkali concentrations were determined in a realistic industrial flue gas by an in-situ laser spectroscopic method, excimer laser induced fragmentation fluorescence (ELIF). An excitation wavelength of 193 nm (ArF-excimer laser) was used to photofragment the alkali species and fluorescence signals from the atomic metal fragments were detected at 589 (Na) and 766 nm (K), respectively. These signals provide a measure of the alkali compounds in the flue gas. In the actual flue gas measurements, which were performed at the atmospheric pressure fluidised bed combustor of DMT, the flue gas alkali concentrations were observed to vary, e.g., with bed temperature and with the rate of fuel supply. Data taken following a hot gas filter in the flue gas line gave significantly reduced levels of alkali species compared to those before the filter. The addition of sodium or potassium acetate to the coal leads to increases of up to a factor of two in flue gas alkali concentrations. In some experiments, small quantities of heavy metal chlorides were added to the coal fired and this resulted in very strong increases in flue gas alkali concentrations. A possible explanation for this effect may lie in reactions with the mineral matter to form heavy metal aluminosilicates and consequent release of alkali.