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• The effects of particle size on sulfation was studied under various conditions. • Fine limestones are less sensitive to the variation in SO 2 concentrations. • A model was established to predict the effects of particle size on sulfation. • The effects of particle size mainly depend on the reaction on external surface. Limestone particle size has a significant influence on the desulfurization effect in CFB boilers. However, few studies have focused on the effects of particle sizes under different conditions. As a result, many existing sulfation models have deficiencies in predicting the sulfation process of limestones with a broad range of particle sizes. In this paper, the sulfation process of limestone with a broad range of particle sizes was investigated under different SO 2 concentrations, CO 2 concentrations, and temperatures by using a large-capacity TGA. A semi-empirical model was established based on the experimental results. It is found that the fine limestone exhibited the superior desulfurization performance compared with the coarse limestone, and it is less sensitive to the variation in SO 2 concentrations. The highest ultimate conversion achieved at 850 °C–900 °C, and showed little difference between different particle sizes. Besides, the conversion and initial reaction rate increase slightly with the CO 2 concentrations. The established model divided the sulfation process of limestone into the direct contact reaction on the external surface and the diffusion-controlled reaction on the inner pore surface, which can well predict the particle sizes under different SO 2 concentrations. [ABSTRACT FROM AUTHOR]

Limestone particle size has a crucial influence on SO2 capture efficiency, however there are few studies on the sulfation reactivity, which covers a broad range of particle sizes at low SO2 concentrations. In this paper, a large-capacity thermogravimetric analyzer (LC-TGA) was developed to obtain the sulfur removal reaction rate under a wide range of particle sizes (3 μm–600 μm) and SO2 concentrations (250 ppm–2000 ppm), and then compared with the results of a traditional fixed bed reactor and a commercial TGA. The experimental results showed that the LC-TGA can well eliminate the external mass transfer and obtain a better measurement performance. Both the final conversion and the reaction rate reduced with the decreasing of SO2 concentration, but ultrafine limestone particles still showed the good sulfation reactivity even at 250 ppm SO2. An empirical sulfation model was established based on the experimental results, which can well predict the sulfation process of different limestone particle sizes at low SO2 concentrations. The model parameters have a strong negative correlation against the particle size, and the fit of the reaction order of SO2 was found to be about 0.6. The model form is very simple to incorporate it into available fluidized bed combustion models to predict SO2 emission. [ABSTRACT FROM AUTHOR]

• Sulfation was inhibited under simultaneous calcination and sulfation condition. • Calcination was impeded due to product formation of CaSO 4. • Product layer thickness during regime change was found to be approximately 0.7 μm. • Outward development but non-uniform distribution of product layer was observed. • Mercury porosimeter is more appropriate than BET for pore structure measurement. This study investigated the physical and chemical characteristics of a Korean limestone used in a large-scale CFB boiler under simultaneous calcination and sulfation conditions in a laboratory-scale fluidized bed reactor. The conversion rate and maximum conversion degree of sulfation under simultaneous conditions were found to be lower than those under subsequent calcination and sulfation conditions. The development of the product layer during sulfation impedes the calcination reaction and results in a lower pore growth rate, which leads to a reduction in the available reaction surface area for the sulfation reaction. The simultaneous pore growth of calcination and product layer development of sulfation eventually lead to a lower rate and reduction in the maximum conversion degrees of both calcination and sulfation. Under simultaneous conditions, the elutriation rate is found to be higher than that under subsequent conditions; this is attributed to the inhibited calcination and sulfation under simultaneous conditions. To investigate the development of the product layer, the microstructure of limestone particles was studied via a scanning electron microscope coupled with an energy-dispersive X-ray analyzer. This paper reports the characteristics of the product layer development during the different sulfation reaction regimes that were found to be significant for understanding the characteristics of both the conversion degree and elutriation rate. The findings of this work can provide an insight into the physical and chemical behaviors of limestone under simultaneous conditions in a fluidized bed reactor to better comprehend the mechanism of desulfurization in CFB boilers. [ABSTRACT FROM AUTHOR]