3.2 Design of the green S doping process
SO2 is a typical acidic gas, which arouses serious environmental issues. Therefore, a green S doping process with recycling of S species is highly desirable, with the purpose of environmental protection and cost reduction. As shown in Figure 4a, a SO2-containing gas is generated by pyrolysis of CaSO3 in a N2 flow, and the tail gas discharged by the S doping reactor is introduced into an absorption column filled by CaO particles for SO2 removal. The CaSO3 obtained in the absorption column after SO2 absorption can be reused as the S source in the SO2 generator, and the CaO produced by the pyrolysis of CaSO3 can be transferred into the flue gas absorption column as absorbent, thus realizing the recycling of S species. As the result, no S-containing tail gas is ejected by the whole SCNT-S synthesis process. Compared to the S doping process using MgSO4 as S dopant, purification steps that might result in further solid wastes are not needed for the gaseous S doping process, thus realizing a simple and green S doping process. Furthermore, considering CaSO3 is the main component of flue gas desulfurization ash, it put forward a new direction to help realize the utilization of flue gas desulfurization ash.
The CaSO3 calcination temperature for SO2 generation has been optimized to match with the S doping process. CaSO3 starts to decompose slightly to release SO2 at 715 ºC. As shown in Figure 4b, when CaSO3 was calcined at 715℃, the SO2concentration is lower but long-lasting. When the calcination temperature rise to 735 ℃ or 755 ℃, a large amount of SO2 is released in a short time. It indicates that the pyrolysis activity of CaSO3 is significantly enhanced by a higher calcination temperature. The S content in SNCNTs increases from 1.02 wt% to 1.95 wt% as the S doping reaction time increased from 20 to 35 min, when the CaSO3 calcination temperature is fixed at 715 ℃. The S content of SNCNTs remains almost unchanged when the S doping time is extended to 45 min. It indicates that the reaction time of 35 min is suitable for the S-doping reaction, even at a low S concentration. As shown in Figure 4c, although higher CaSO3 decomposition ratio is observed at higher temperature, the S content in SNCNTs slightly decreases as the CaSO3 calcination temperature rises from 800 to 1000 ºC. It might be attributed to the different reaction rates for the CaSO3 decomposition and the S doping of NCNTs. At the temperature higher than 900 ºC, the pyrolysis rate of CaSO3 is quite high, and all SO2 is released in a short period. However, the gaseous S doping of NCNTs might not complete in a short time. Therefore, a relatively delayed SO2 release at a temperature slop from 715 to 800 ºC is determined as the optimized parameter. Under the same S doping condition, the S content in SNCNTs is higher than that in SCNT-S (Figure 4d). It indicates that NCNTs have exhibited higher S doping reactivity as compared to the undoped CNTs, due to the existence of more disorders and defects in NCNTs. As shown in Figure 4e, with 20 g of CaO filled in the absorption column, the SO2 concentration is 0.08 vol% after 30 minutes, showing much better SO2 removal effect than that with 10 g of CaO. It indicates that CaO can effectively absorb SO2 in the tail gas from the S-doping reactor.