Investigation of the Initial Stage of Ash Deposition during Oxy-Fuel Combustion in a Bench-Scale Fluidized Bed Combustor with Limestone Addition

Abstract

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 CO 2 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 O 2 concentration, and limestone addition.

A review of oxy-fuel combustion in fluidized bed reactors

“…However, there were no significant differences observed in the chemical compositions of the fly ash, and ash deposits between studied combustion atmospheres. Furthermore, Wang et al used the same bench scale FBC facility to assess ash deposition characteristics with limestone under oxy‐fuel combustion conditions, where it was observed that the deposition propensity was higher for oxy‐combustion than for air combustion. This was attributed to physical changes in the particle size distributions rather than in the chemical composition of the fly ash.…”

A novel method used to study growth of ash deposition and in situ measurement of effective thermal conductivity of ash deposit

“…The details about controlling probe surface temperature can be found in Refs. and . The temperature controlling system is the same as that of the previous ash deposit probe.…”

A Comprehensive Review of Ash Issues in Oxyfuel Combustion of Coal and Biomass: Mineral Matter Transformation, Ash Formation, and Deposition

“…However, no remarkable differences were observed in the chemical compositions of the ash deposits and fly ash between different combustion modes. By using the same anthracite, the effects of limestone addition on ash deposition behavior were explored. , The results showed that, in the absence of limestone, the ash deposition rate did not change significantly when switching from air to 30% O 2 /70% CO 2 combustion. It was different from the observation in the previous work, where 30% O 2 /70% CO 2 combustion led to a significantly higher ash deposition rate.…”