# 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  
In order to study the desulfurization activity of limestone and carbide slag, the calcination and sulfation tests of these two desulfurizer particles were carried out using a PRT-1 thermogravimetric analyzer (TGA, Beijing Optical Instrument Factory, China). In the calcination test, 100 mg of limestone or carbide slag particles with a given size (0.25-0.425 mm) were calcined under an air atmosphere, respectively. The reactor temperature was raised from room temperature to 1100 \({}^{\circ}\)C at a rate of 20 \({}^{\circ}\)C min\({}^{-1}\). The flow rate of reaction gas was kept constant at 0.5L min\({}^{-1}\). Figure 4 shows the differential thermogravimetry (DTG) curves of limestone and carbide slag during the calcination process. The two curves are quite different. Only one peak can be found from the limestone curves, which was caused by the thermal decomposition of CaCO\({}_{3}\). It decomposed from 637 to 851 \({}^{\circ}\)C and reached its maximum decomposition rate at 827 \({}^{\circ}\)C. Weight loss of carbide slag calcination was divided into two parts. In the first stage, weight loss of Ca(OH)\({}_{2}\) decomposition took 6 min. The initial and final decomposition temperatures were 406 and 515 \({}^{\circ}\)C, respectively. The maximum reaction rate was 4.00 mg\(\cdot\)min\({}^{-1}\) and the corresponding temperature was 479 \({}^{\circ}\)C. In the second stage, weight loss of CaCO\({}_{3}\) decomposition lasts for 11 min. The initial and final decomposition temperatures were 604 and 782 \({}^{\circ}\)C, respectively. The maximum reaction rate was 3.90 mg\(\cdot\)min\({}^{-1}\) and the relevant temperature was 750 \({}^{\circ}\)C. Carbide slag exceeded limestone in desulfurization performance. Carbide slag was decomposed to CaO above 400 \({}^{\circ}\)C which matches the reaction of SO\({}_{2}\) generation form coal, whereas limestone is decomposed to CaO at higher temperature and does not react quickly. Figure 3: Schematic diagram of 690 t-h\({}^{-1}\) CFB boiler.1. Furnace, 2. cyclone, 3. loop seal, 4. drum, 5. reheater, 6. superheater, 7. high temperature economizer, 8. flue gas damper, 9. low temperature economizer, and 10. air heater. During the calcination process, the release of CO\({}_{2}\) will form some pores on the surface and inside of particles [18]. These pores create the pathways for the diffusion of SO\({}_{2}\) into the interior of particles and the occurrence of desulfurization reactions. Different pore structure will significantly affect the reactivity of limestone or carbide slag. SO\({}_{2}\) and O\({}_{2}\) molecules diffuse from the CaO surface into the micropores. Then these molecules are absorbed and gradually diffused through the product layer to reach the unreacted CaO layer [19]. Therefore, the specific surface area, specific pore volume, and pore size distribution will directly affect the sulfation process and the reaction rate of CaO particles. The specific surface area reflects the size of the gas-solid reaction interface, whereas the specific pore volume reflects the size of the diffusion space for the gas molecule. Since CaO\({}_{4}\) has a larger molar volume than CaCO\({}_{3}\) and CaO (52.2, 36.9, and 16.8 cm\({}^{3}\) mol\({}^{-1}\), respectively) [20], the produced CaCO\({}_{4}\) blocks the pores, leaving a large amount of unreacted CaO inside the particles, and reducing the utilization of the desulfurizer [21]. For the CaO with large pore size, the blockage can be delayed and the effective reaction time will be prolonged. The pore properties of calcined limestone and carbide slag samples are determined by a low-temperature N\({}_{2}\) adsorption method. The pore structure parameters and pore size distributions of different desulfurizers could be obtained by using the appropriate calculation model.