Quantitative in-situ measurements of sodium release during the combustion of single coal particles using planar laser induced fluorescence.

Abstract

The release of sodium from low rank coal during combustion is known to be an important factor in the phenomena of fouling and corrosion in industrial boilers. Although much is known about the gas phase chemistry of sodium compounds, and the likely sequence of events that lead to fouling and corrosion, very little fundamental work has been undertaken on the release of sodium from the coal particle as it is combusted. The principal objective of this study was to perform detailed quantitative measurements and mathematical modelling of sodium release during combustion of single brown coal particles. Planar Laser Induced Fluorescence (PLIF) was applied for the in-situ measurement of the atomic sodium concentration field in the plume of single Loy Yang brown coal particles. Laser absorption measurements at the sodium D1 line (589.59 nm) were utilised to calibrate PLIF measurements of atomic sodium in a purpose designed flat flame environment. Detailed measurements of atomic sodium in the plume of single combusting brown coal particles of varying amounts and forms of sodium and of varying particle sizes were then undertaken. A run-of-mine Loy Yang brown coal sample and two samples that were processed using Mechanical/Thermal Expression (MTE), which removed a fraction of the inherent moisture and concomitant dissolved salts, were investigated. An experiment was also performed to simultaneously measure the particle temperature, particle size, and the atomic sodium concentration in the plume of a single burning Loy Yang brown coal particle. From the experimental results, the proportions of sodium released during the stages of coal devolatilisation, char combustion and from the remaining ash after combustion were determined for the three coals used at various particle sizes. The relative differences between the sodium release behaviour of water-bound and organically bound sodium were also inferred. During char combustion, the release of sodium was determined to be dependent on both the particle temperature and particle size. In order to decouple these parameters and determine the true controlling parameter(s) for sodium release, a model was established for the release of sodium, the char burnout behaviour of the particles, and the particle surface temperature. By combining the modelling with further analysis of the experimental data, the temperature dependent kinetics of sodium release during brown coal char combustion were established. A full mechanism was also proposed for sodium release during the various stages of coal combustion, which suggested that the rate determining step for sodium release during char combustion is the formation of a reduced form of sodium in the char, which subsequently leads to the rapid loss of sodium from the particle. The results of this study advance the knowledge of the release of sodium from brown coal combustion. Two major contributions are the development of a methodology that enables the direct in-situ measurements of the concentration of atomic sodium in the plume of individual burning coal particles, and the establishment of the kinetics and mechanism for the release of sodium during Loy Yang brown coal combustion. These results provide, for the first time, essential data for the development of sodium release sub-models within large scale brown coal boiler Computational Fluid Dynamic (CFD) models. Such models will help in the development of improved measures to mitigate fouling and corrosion problems in brown coal fired combustion and gasification systems.