# Feasibility Study of Using Carbide Slag as In-Bed Desulfurizer in Circulating Fluidized Bed Boiler
## 2 Experimental Design  
### Limestone, Carbide Slag and Coal  
In order to make the investigation more practical and applicable, the coal, limestone, and carbide slag were all from an industrial power plant. Table 1 shows the proximate and ultimate analyses of the coal used in this study.  
As the waste residue produced by the hydrolysis of calcium carbide, the main component in carbide slag is Ca(OH)\({}_{2}\). During the static dehydration process in the open air, a part of Ca(OH)\({}_{2}\) reacts with CO\({}_{2}\) to form CaCO\({}_{3}\). Table 2 shows the composition analysis of limestone and carbide slag. It is obvious that the CaO content in the carbide slag is higher than that in the limestone. The bulk density of carbide slag is only 0.55 g cm\({}^{-3}\), which is much lower than that of limestone.  
[PERSON] et al. (1977) found that particle size distribution (PSD) is one of the key factors that determines the sulphur capture performance. Due to the limitation of separation efficiency, the fine-grained desulfurizer cannot be captured effectively by the cyclones, which shortens its residence time in the furnace. The coarse particles have sufficient residence time, but their specific surface areas are small, which is not conducive to the desulfurization reaction in the furnace. Therefore, the desulfurizer needs to have a suitable PSD. Its optimum PSD is generally similar to that of the circulating ash. The PSD of limestone and carbide slag is shown in Figure 1. The mean size of limestone is 486.8 \(\upmu\)m, whereas for carbide slag it is only 52.5 \(\upmu\)m. The carbide slag is significantly finer, which has a certain impact on its desulfurization reaction in the furnace. Because there was no other limestone with more satisfactory particle size that could be purchased around the power plant of the 690 t-h\({}^{-1}\) CFB boiler, this limestone with coarser particle size was selected to be fed into the furnace. In the 1 MW\({}_{\text{th}}\) pilot CFB boiler and 690 t-h\({}^{-1}\) CFB boiler tests of this paper, this same kind of coarse limestone was also used. This kind of limestone is coarser than the one usually used in CFB boiler. Besides, the diameter of the return leg of the 1 MW\({}_{\text{th}}\) pilot CFB boiler was smaller, so the sorbent could only be added directly into the fuel.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{3}{c}{**Ultimate Analysis**} & \multicolumn{5}{c}{**Proximate Analysis**} & \multicolumn{2}{c}{**Lower Heating**} \\ \hline
**C\({}_{\text{ar}}\)** & **H\({}_{\text{ar}}\)** & **O\({}_{\text{ar}}\)** & **N\({}_{\text{ar}}\)** & **S\({}_{\text{ar}}\)** & **A\({}_{\text{ar}}\)** & **V\({}_{\text{ar}}\)** & **FC\({}_{\text{ar}}\)** & **M\({}_{\text{ar}}\)** & **Value (kJ kg\({}^{-1}\))** \\ \hline
49.05 & 3.02 & 9.35 & 0.44 & 1.19 & 21.35 & 25.90 & 37.15 & 15.60 & 18170 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Proximate and ultimate analyses of coal (wt, %).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Item** & **Unit** & **Limestone** & **Carbide Slag** \\ \hline SiO\({}_{2}\) & wt, \% & 3.72 & 8.77 \\ Al\({}_{2}\)O\({}_{3}\) & wt, \% & 0.61 & 3.08 \\ Fe\({}_{2}\)O\({}_{5}\) & wt, \% & 0.31 & 1.20 \\ CaO & wt, \% & 49.92 & 57.28 \\ MgO & wt, \% & 2.75 & 0.65 \\ Na\({}_{2}\)O & wt, \% & 0.08 & 0.07 \\ K\({}_{2}\)O & wt, \% & 0.11 & 0.52 \\ TiO\({}_{2}\) & wt, \% & \textless{}0.01 & 0.16 \\ SO\({}_{3}\) & wt, \% & \textless{}0.01 & 0.49 \\ MnO\({}_{2}\) & wt, \% & \textless{}0.001 & / \\ P\({}_{2}\)O\({}_{5}\) & wt, \% & / & 0.10 \\ True density & kg m\({}^{-3}\) & 2860 & 2720 \\ Bulk density & kg m\({}^{-3}\) & 1380 & 550 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Compositions and densities of limestone and carbide slag.