DEVELOPMENT OF TECHNIQUES FOR MONITORING GAS-PHASE SODIUM SPECIES FORMED DURING COAL COMBUSTION AND GASIFICATION

The development of techniques suitable for on-line analysis of gas-pha se molecular sodium species, resulting from the combustion and gasific ation of low-rank coals is described. In particular two techniques hav e been demonstrated, these are: (a) laser-induced photofragment fluore scence and (b) microwave spectroscopy. The photofragment technique can be applied in situ and yields excellent detection limits (< 0.1 ppb o f NaCl and 1 ppb NaOH) with good discrimination between the two major gas-phase species - NaCl and NaOH. Limitations of this technique inclu de absorption of the ultraviolet (UV) photolysis excitation by other c hemical species and particulates. Microwave spectroscopy shows promise in determining the sodium species distribution in the gas-phase. A pu rpose built analytical instrument (known as MILLIJET HT) is being deve loped for application in high-temperature environments. Early results indicate detection limits of < 100 ppb NaCl. Lack of sensitivity for N aOH is being attributed to the formation of NaOH dimers and larger clu sters. Detailed calculations of the dimer structure and the dimer-mono mer reaction have been undertaken in order to maximise the performance of the instrument.