# Feasibility Study of Using Carbide Slag as In-Bed Desulfurizer in Circulating Fluidized Bed Boiler
## 3 Results and Discussion
### The 690 t-h\({}^{-1}\) CFB Boiler Field Tests  
Saastamoinen [35] found that small particles were reactive but they had a short residence time depending on the separation efficiency of the cyclone. With large particles, the residence time was longer, but the rate and degree of sulfur capture were lower. Therefore, the structure of the cyclone vortex finder of the 690 t-h\({}^{-1}\) CFB boiler is retrofitted, which is shown in Figure 9. Because the three separators are of exactly the same size, only one of them is drawn. As can be seen from Figure 10, PSD of fly ashes in the 690 t-h\({}^{-1}\) CFB boiler had been greatly improved. The particle size of limestone and carbide slag during the 690 t-h\({}^{-1}\) CFB boiler test was close to that of the 1 MW\({}_{\rm th}\) pilot CFB boiler. Figure 8: Effect of bed temperature and Ca/S mole ratio on SO\({}_{2}\) emissions in 1 MW\({}_{\rm th}\) pilot CFB boiler; (**a**) limestone, (**b**) carbide slag. Because the desulfurizer supply point was different, the results of the 690 t-h\({}^{-1}\) CFB boiler test and the 1 MW\({}_{\text{th}}\) pilot CFB boiler test were different. Overall, the limestone and the carbide slag had worse desulfurization performance in the 690 t-h\({}^{-1}\) CFB boiler than in the 1 MW\({}_{\text{th}}\) pilot CFB boiler. Figure 9: Structure of cyclone inlet duct and vortex finder; (**a**) original design, (**b**) retrofit design. Table 3 is the matrix of field tests on the 690 t-h\({}^{-1}\) CFB boiler. In all the tests, the bed temperature was maintained around 915 \({}^{\circ}\)C for at least 4 h. Figure 11 shows the bed temperature curve and the original emission of SO\({}_{2}\) without any desulfurizer addition. The average SO\({}_{2}\) concentration was 2515 mg\(\cdot\)m\({}^{-3}\). In the test, it was found that even temperature increases of 10-15 \({}^{\circ}\)C brought about a large increase in the SO\({}_{2}\) emission. Figure 12 shows the average SO\({}_{2}\) emission for all the field tests. When Ca/S = 2, SO\({}_{2}\) concentration with limestone addition was 949 mg\(\cdot\)m\({}^{-3}\) (shown in Table 3, test 2-1), whereas for carbide slag this concentration increased to 1044 mg\(\cdot\)m\({}^{-3}\) (test 3-1, original design of cyclone vortex finder) and decreased to 743 mg\(\cdot\)m\({}^{-3}\) (test 3-3, improved design of cyclone vortex finder). For Ca/S = 3, the three corresponding values for SO\({}_{2}\) concentration were 586 mg\(\cdot\)m\({}^{-3}\) (test 2-2), 884 mg\(\cdot\)m\({}^{-3}\) (test 3-2, original design of cyclone vortex finder), and 342 mg\(\cdot\)m\({}^{-3}\) (test 3-4, improved design of cyclone vortex finder). That means in the 690 t-h\({}^{-1}\) CFB boiler, the desulfurization efficiency of carbide slag was higher than that of limestone after the cyclone modification. Besides the bed temperature and desulfurizer type, the most important factor for the desulfurization efficiency is the effective residence time of desulfurizer in the furnace [36; 37], which largely depends on the particle size and separation efficiency of cyclones. From Figure 10, it is obvious that the mean particle size of fly ash in the 1 MW\({}_{\text{th}}\) pilot CFB boiler was smaller than that in the 690 t-h\({}^{-1}\) CFB boiler, which meant that the separation efficiency of cyclone in  
Figure 10: Particle size distribution (PSD) of fly ashes obtained from 1 MW\({}_{\text{th}}\) pilot CFB boiler and 690 t-h\({}^{-1}\) CFB boilers. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**No.**} & **Bed Temperature/\({}^{\circ}\)C** & **Desulfurizer** & **Ca/S** & **Remarks** \\ \hline Test 1-1 & & / & / & without desulfurizer \\ \hline Test 2-1 & & & 2 & cyclone original design \\ Test 2-2 & & & 3 & cyclone original design \\ \hline Test 3-1 & & & 2 & cyclone original design \\ Test 3-2 & & & 3 & cyclone original design \\ Test 3-3 & & & 2 & cyclone modification \\ Test 3-4 & & & 3 & cyclone modification \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Matrix of field tests on the 690 t-h\({}^{-1}\) CFB boiler.