Spontaneous Emission Measurements of Selected Alkali Radicals during the Combustion of a Single Biomass Pellet

Spontaneous emission intensities of Na*, Ca*, and K* during the combustion of single wood and straw pellets doped with known concentrations of Na, Ca, and K were measured using optical bandpass filters mounted on an intensified charge-coupled device camera. The impregnated biomass pellets were suspended in a natural gas flat flame at 750 and 1000 degrees C. Before the pellets were made, the biomass samples were washed and soaked to demineralize and doped with different concentrations of Na, Ca, and K (0.5, 2, and 5 wt %). During the experiments, the temperature at the center of the pellets was measured with a thermocouple and the combustion stages were identified with the help of the temperature derivative. The results reveal that, at the lower gas temperature, the emission of the selected alkalis is marginal, in agreement with previous studies. At the higher gas temperature, the emission profiles reveal that K and Na are released distinctively in the volatile combustion and char combustion stages. The presence of large amounts of silica and alumina may trap the alkalis in the solid phase, leading to a reduced emission of these species during the char combustion. Calcium does not evaporate at the tested temperature conditions, but the temperature is high enough to promote the decomposition of calcium oxalate in the outer layer of the pellet, leading to a flat emission profile during the combustion stages. Finally, the total integrated emission increased proportionally with the increase of the doping concentration for all species.