Flow Characteristics of High Temperature Eutectic Alkali SulphateMixtures Containing Inert Silica Inclusions

摘要

The issues of agglomeration and defluidisation in fluid bed gasifiers andcombustors can be related to the mineral composition of the feed coals and thetransformations undergone by the minerals during gasification and combustion.Research has indicated that the alkali sulphates, primarily sodium, calcium andmagnesium, are the key mineral constituents in coal that contribute to ash depositionand hence agglomeration and defluidisation in fluid beds. The combinations of thesethree constituents readily form low melting point eutectic mixtures, which can form aliquid phase deposit on solid ash particles. This molten eutectic liquid willsubsequently bind bed material to form agglomerates at temperatures well below theoperating temperature of fluid bed gasifiers and combustors. Thus, in order to fullyunderstand the mechanism involved in ash deposition and agglomeration, it isimportant that the rheological behavior of mixtures of these key constituents becharacterised as a function of their composition and operating temperature. Using arecently developed high temperature ash rheometer; the rheological properties ofsynthetic eutectic mixtures containing Na2SO4, CaSO4 and MgSO4 at differingproportions have been measured. The data obtained has been used to develop astatistical-mixture model that satisfactorily describes the rheological properties ofseveral Australian brown coal and lignite ashes. However, in the recent work, we havefound that the introduction of an inert solid phase such as silica (SiO2), into the alkalisulphate mixture substantially affects the rheological behavior of the eutectic mixture.The effects of silica inclusions on the rheological characteristics of alkali sulphatemixtures have been studied at a constant temperature of 1000°C under an oxidizingatmosphere. The experimental results obtained have been used to develop arelationship between the chemical compositions and the rheological observations todevelop a- statistical rheological model.