# Feasibility Study of Using Carbide Slag as In-Bed Desulfurizer in Circulating Fluidized Bed Boiler
## 3 Results and Discussion
### Comparison of Reactivity between Limestone and Carbide Slag
The Brumauer-Emmett-Teller (BET) method [22] is the most commonly used method for determining the pore surface area and distribution of porous materials, while the specific pore volume of the desulfurizer is calculated by the Barrett-Joyner-Halenda (BJH) equation [23], which is based on the Kelvin and Halsey equation. Based on the data measured by the BELsorp Mini II analyzer (Microtrac-BEL Crop., Japan), the pore structure parameters were calculated. The comparison of specific surface area, specific pore volume, and average pore size of calcined limestone and carbide slag particle are shown as Figure 5. The larger specific surface area and specific pore volume indicate that the calcined carbide slag is more likely to react with SO\({}_{2}\) than the calcined limestone. Carbide slag has a lot of Ca(OH)\({}_{2}\) while limestone has a lot of CaCO\({}_{3}\). During decomposing, both will release CaO. However, CaO decomposed from Ca(OH)\({}_{2}\) has better porosity and specific area than that from CaCO\({}_{3}\). So carbide slag may have better desulfurization performance. Figure 4: Derivative Thermogravimetry (DTG) curves of limestone and carbide slag during calcination process. The sulfation tests of the calcined products were also carried out on the TGA to compare the desulfurization capacity of these two desulfurizers. The reactor temperature was 850 \({}^{\circ}\)C, and the simulated flue gas was used as the reaction gas with a flow rate at 0.5 L min\({}^{-1}\). The balanced gas (N\({}_{2}\)), 5% of O\({}_{2}\), and 3000 ppm SO\({}_{2}\), were monitored separately by the flow meters and well mixed in the gas tank before feeding into the reactor. Figure 6 presents the CaO conversion rate of these two desulfurizers. Compared with previous studies [24], the CaO conversion rate of both desulfurizers was relatively low. The particles in this study were a bit larger (0.25-0.425 mm) than those in previous studies, which may have caused the bad mass transfer between gas and solid to slow down the sulfation rate of CaO. It can be seen from Figure 7 that as the reaction proceeded, the curve for limestone flattened, which meant the reaction gradually slowed down. The CaSO\({}_{4}\) formed by the sulfation reaction covered the surface of particles, clogging the micro-pores, and hampering the further reactions of limestone. With the reaction proceeding, its mechanism switched from the initial kinetics-controlled mode to the diffusion-controlled mode. The sulfation rate mainly depended on the diffusion of reactants or solid ions in the produced CaSO\({}_{4}\) layer, which is consistent with the study of [PERSON] et al. [25] The CaO conversion curve of carbide slag was almost a straight line and it was higher than that of limestone at the reaction time because the reaction time of 60 min was much longer than that of desulfurization reaction in furnace. This indicates that the sulfation of carbide slag was always in the kinetics-controlled stage during the whole sulfation process. The well-developed pore structure of calcined carbide slag brings higher reactivity to its sulfation. [PERSON] et al. [26] found that the H\({}_{2}\)O released during the calcination created and enlarged pores in Ca(OH)\({}_{2}\), which exposed greater surface areas for subsequent sulfation reactions. [PERSON] [27] and [PERSON] et al. [28] found that the small-particle limestone calcination reaction produced a richer pore structure, a larger specific surface area, and a higher reaction rate in the sulfurization reaction. [PERSON] et al. [29; 30] found that the addition of alkali metal ions improved the sulfur capture capacity of limestone. Thus the higher content of K\({}_{2}\)O may also promote the sulfation of carbide slag. [PERSON] et al. [31] found that the thermal treatment of Ca(OH)\({}_{2}\) formed CaO sorbents with high activity to retain SO\({}_{2}\).