Investigation of the initial stage of ash deposition during oxy-fuel combustion in a bench-scale fluidized bed combustor with limestone addition

The technology of oxy-fuel combustion in a circulating fluidized bed boiler is one of the advanced technologies for carbon capture and storage; however, operating problems related to ash deposition are worth investigating. When limestone is added as a sorbent during oxy-fuel combustion, excess CaO particles in the fly ash deposit on heating surfaces and react with high concentrations of CO₂ in the flue gas. These carbonation reactions lead to structural change and formation of bonded deposits. Therefore, deposition experiments with limestone were carried out in a bench-scale fluidized bed under oxy-fuel combustion conditions to evaluate deposition propensity and compositions of ash deposits formed under different experimental conditions. Effects of the molar ratio Ca/S, probe surface temperature, and combustion atmosphere on deposition behavior were evaluated. Experimental results showed that during the initial stage of deposition, the degree of carbonation increases, with increasing molar ratios of Ca/S; although this is not significant for the total deposition process. Deposits were loose for 1 h, following deposition. Raising the surface temperature of the probe reduced the deposition rate of the fly ash, since this was strongly affected by thermophoresis. Except for elements K, Na, and S, there were no significant changes for other chemical components in the ash deposit under the varied conditions. There were clear differences in the deposition rates of fly ash for oxy-fuel and air combustion cases, which were probably caused by differences in the ash formation mechanism for the case of high O₂ concentration, and limestone addition.