Role of condensed phases in the agglomeration of low rank coal ash in fluidized beds

Fuel diversity is critical in the utilization of fluidized bed combustors. To support the use of diverse fuels including low rank coals and biomass, interactions of chemical components that lead to operational issues such as agglomeration need to be studied. Low rank coals and biomass are generally rich in alkali and alkaline earth metals and their role in the initiation of agglomeration is investigated in this work. A novel modeling methodology to quantify the contribution of these components to the initiation of agglomeration via the condensation of gaseous species was developed. This condensation modeling methodology complements thermodynamic simulations such as those using FactSage and agglomeration models used for high rank coals. The study shows that at temperatures below 800 °C condensed phases play a critical role in the initiation of agglomerate growth. The condensation temperature is computed to be about 30-50 °C lower than the predicted molten slag-liquid formation temperature (740 °C for the lignite and 810 °C for the subbituminous coals studied). The extent of condensation correlates to the amount of alkali metals in the ash. Sodium sulfate forms a major component of condensates for low rank coals with condensation occurring over a larger temperature range for lignite than the sub-bituminous coals studied.