Multi-point LIBS measurement and kinetics modeling of sodium release from a burning Zhundong coal particle

A multi-point Laser-Induced Breakdown Spectroscopy (LIBS) method for quantitative measurement of sodium concentrations in the gas phase, the surface temperature and the particle diameter during the combustion of a Zhundong coal particle is presented. To obtain multi-point LIBS data, the laser focusing and signal collection optics are mounted on a translational platform which is able to traverse cyclically. With this setup multi-point LIBS measurements above a burning particle can be performed and the time-resolved sodium release process can be obtained. The results show that 42.2% of the total sodium mass is released during the burning of the Zhundong coal sample. For a 4 mm particle, in the char burnout stage sodium is released most strongly, i.e., 87% of the total released sodium mass, while in the de-volatilization and ash reaction stages the percentages are 5% and 8%, respectively. The atomic sodium and NaOH are the most favored species at chemical equilibrium in the plume according to CHEMKIN. The sodium release is found to be closely related to the particle burning stages by analyzing the sodium release, particle surface temperature and its diameter. A linear relationship is found between the residual sodium mass in the particle and the volume of the particle. The volatile sodium release rate obeys a two-step Arrhenius expression. Predictions by the developed two-step kinetics model agree well with the measured sodium release profiles in all the three coal-burning stages.