# Feasibility Study of Using Carbide Slag as In-Bed Desulfurizer in Circulating Fluidized Bed Boiler
## 3 Results and Discussion
### The 1 MW\({}_{\text{th}}\) Pilot CFB boiler Tests  
Because of the similar heat and mass transfer characteristics to the industrial furnace and the flexibility of boiler operation, the 1 MW\({}_{\text{th}}\) pilot CFB boiler gives us a tool to explore the effects of different operating parameters on the performance of desulfurizers. [PERSON] et al. [32] found that the factors affecting the desulfurization in the limestone furnace mainly included the molar ratio of calcium to sulfur, temperature, reaction atmosphere, limestone reactivity, and limestone particle size. The Ca/S mole ratio of carbide slag and limestone can be calculated as  
\[m_{\text{lim}}=\frac{B_{\text{lim}}\times\frac{\
icefrac{{T_{\text{CaCO}_{3}}}}{{1 00}}}{B_{\text{coal}}\times\frac{\text{S}_{\text{ar}}}{32}}}{B_{\text{coal}} \times\frac{\text{S}_{\text{ar}}}{32}} \tag{1}\]  
where \(m_{\text{lim}}\) is the Ca/S mole ratio of limestone; \(B_{\text{lim}}\) is the limestone consumption, t-h\({}^{-1}\); \(\eta_{\text{CaCO}_{3}}\) is the CaCO\({}_{3}\) content in limestone, %; \(B_{\text{coal}}\) is the coal consumption, t-h\({}^{-1}\); and S\({}_{\text{ar}}\) is the sulfur content in coal, %. \[m_{\text{cs}}=\frac{B_{\text{cs}}\times\frac{\
icefrac{{T_{\text{CaCO}}}}{{74} }}{B_{\text{coal}}\times\frac{\text{S}_{\text{ar}}}{32}}}{B_{\text{coal}} \times\frac{\text{S}_{\text{ar}}}{32}} \tag{2}\]  
Figure 6: CaO conversion rate of limestone and carbide slag during the sulfation process in the thermogravimetric analyzer (TGA). Figure 7: Effect of bed temperature on original SO\({}_{2}\) emissions in 1 MW\({}_{\text{th}}\) pilot circulating fluidized bed (CFB) boiler (Ca/S = 0). where \(m_{\rm cs}\) is the Ca/S mole ratio of carbide slag; \(B_{\rm cs}\) is the carbide slag consumption, t-h\({}^{-1}\); and \(\eta_{\rm Ca(OH)2}\) is the Ca(OH)\({}_{2}\) content in carbide slag, %. Figure 7 presents the effect of bed temperature on the original SO\({}_{2}\) emission. The SO\({}_{2}\) concentration increased with increasing bed temperature in the range of 845-945 \({}^{\circ}\)C, which is also the typical combustion temperature of CFB boilers. Since CaCO\({}_{3}\) or CaO are one of the main components in coal ash, it has self-desulfurization ability during the combustion process [33]. When the temperature increases over 900 \({}^{\circ}\)C, the high temperature may cause the sintering of coal ash and the re-decomposition of the produced CaSO\({}_{4}\), resulting in a rise in SO\({}_{2}\) emission. Therefore, from the perspective of emission control, the bed temperature must be controlled within a reasonable range. Figure 8 shows the SO\({}_{2}\) emission concentrations at different Ca/S mole ratios. It can be seen that the desulfurization efficiency can be improved to a certain extent with the increase of Ca/S mole ratio. In Figure 8a, with the increase of Ca/S molar ratio of limestone, SO\({}_{2}\) emission could be reduced but SO\({}_{2}\) emission increased rapidly with the increase of bed temperature. At the bed temperature of 940 \({}^{\circ}\)C, the desulfurization effect of limestone with low Ca/S molar ratio was limited, and the SO\({}_{2}\) concentration was more than 2000 mg\(\cdot\)m\({}^{-3}\). However, for Figure 8b, increasing the Ca/S mole ratio of carbide slag had a more obvious impact on improving desulfurization efficiency. A low SO\({}_{2}\) emission could be reached by adding more desulfurizer into the furnace even at a relatively high temperature. But for limestone or carbide slag, there is an optimum temperature range. When the temperature grows over 910 \({}^{\circ}\)C, the desulfurization efficiency decreased rapidly. Compared with the limestone, the carbide slag had higher content of Fe\({}_{2}\)O\({}_{3}\) and SiO\({}_{2}\) (seen from Table 2), which can form the glassy ferric oxide and silicate with high-temperature resistance [34] and prevent the further decomposition of CaS and CaSO\({}_{4}\). General speaking, the carbide slag was less sensitive to the bed temperature than the limestone.