A comprehensive evaluation of slagging and fouling indicators for solid fuel combustion

Slagging and fouling risks from solid fuel combustion in a boiler can be determined using an experimental investigation, ash observation, or prediction indices based on theoretical calculation. The latter is widely developed and preferred due to its convenience, practicality, and time-saving. However, these indices are commonly based on ash composition alone and may have low accuracy. This study aims to overview and evaluate the prediction indices by employing experimental combustion tests and ash observation methods. An experimental test using a drop tube furnace (DTF) is conducted to represent the actual combustion. In addition, probe observation, scanning electron microscopy - energy dispersive X-ray spectroscopy (SEM-EDS), and X-ray diffraction (XRD) analysis are carried out to determine the propensity of slagging and fouling. Sizeable samples, 59 slagging samples, and 45 fouling samples have been used and analyzed, consisting of Indonesian-Indonesian coal blends and Indonesian-Australian coal blends. The results show that the silica ratio, base-to-acid ratio, simplified base-to-acid ratio, and composite index are proper slagging prediction methods. New slagging criteria are also proposed for better prediction accuracy. In addition, sodium content in the ash can effectively evaluate fouling propensity. Some fouling indices will also have better accuracy if the proposed fouling criteria can be applied. Moreover, the developed approach can be adopted for evaluating other solid fuels, including biomass.