Experimental investigation of ash deposits characteristics of co-combustion of coal and rice hull using a digital image technique

•Digital image technique was used to monitor deposits growth process.•A type of four stages mode of ash deposit growth was presented.•The heat flux of ash deposits fit a three-stage mode.•The addition of rice hull increased the porosity of deposits.

This paper investigated the ash deposit characteristics during the co-firing Da Tong (DA) coal with different proportions of rice hull (0%, 5%, 10%, and 20%, based on weight) in a pilot-scale furnace. The growth of ash deposit with a four-stage mode was presented. The stable thickness values of DA coal, 5% rice hull, 10% rice hull, and 20% rice hull were 0.5, 1.4, 2.9, 5.7 cm, with stable heat flux values of 230, 200, 175, and 125 kW/m2, respectively. According to the results of scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDX), the amount of Si in the deposits increased with the increasing proportion of rice hull rich in SiO2. The X-ray diffraction (XRD) analysis results indicated that most elements except Si were in the amorphous state because of the formation of eutectics. The stable thicknesses of deposits increased exponentially with the proportion of rice hull. The deposit was loose, easy removable but it reduced the heat transfer significantly. Consequently, sootblowing timely was necessary when co-firing DA coal with rice hull.