| NOx is one of the key air pollutants from flue gas, and the combined oxidation-absorption process is an important method for denitrification of flue gas from stationary sources. In this paper, a new combined denitrification process of NO catalytic oxidation over Mn/TiO2 catalysts and absorption with CaSO3 slurry was investigated systematically. The activity of Mn/TiO2 for NO oxidation was promoted by improving the preparation method and modification with doped elements, and a high NO conversion was obtained. The mechanism of NO catalytic oxidation over the specific catalyst was subsequently revealed by DRIFTS study, which would be able to provide some directions for further improving the catalyst’s performance. On the basis of NO catalytic oxidation, NOx absorption was also studied and absorbents were screened, and then different additives were used to improve the performance of the chosen absorbent, which could lay a foundation for the application of the combined process.Firstly, the effect of preparation method on the activity of Mn/TiO2 was investigated, and deposition-precipitation (DP) method seemed to be much more favorable than the conventional wet impregnation (WI) method (NO conversion was promoted from 40% to 89% at 250℃by DP). From the results of characterizations, it could be seen that MnOx was better dispersed in the catalyst prepared by DP, and the active phase of Mn(3+) was the main component. Thus, the generation and release of NO2 were significantly promoted. Further investigation on the influence of operating parameters indicated the high activity of Mn/TiO2 was persistent. Additionally, the results of dynamic study showed that the reaction of NO catalytic oxidation was 0.5 order with respect to both NO and O2, and the apparent activation energy was calculated to be about 20 kJ/mol.Secondly, the reaction mechanism of NO catalytic oxidation over Mn/TiO2 was studied thoroughly. Nitrosyls and nitrates (especially monodentate nitrate) were found to be the key intermediates for NO oxidation, and the reaction followed a Mars-van Krevelen mechanism as follows:NO was first adsorbed on Mn sites to form nitrosyls, and then nitrosyls were readily oxidized to nitrates by active lattice oxygen. Subsequently, nitrates were decomposed to the final product NO2, and the consumed lattice oxygen was supplemented by gaseous O2. Such a mechanism was partly verified by the study of the effect of Fe modification. Experimental results showed that the doped Fe improved the activity of Mn/TiO2 obviously, because it could not only favor the adsorption of NO but also promote the persistent reoxidation capability of the catalyst.Finally, the absorption of NOx into liquid phase was studied. By comparing the absorption characteristics of different absorbents towards the pre-oxidized NOx, it was found that CaSO3 was the most appropriate absorbent. It was more efficient than H2O and alkali, and its consumption rate was much lower than that of Na2SO3. Besides, CaSO3 could be oxidized to gypsum in the process of denitrification, which provided a possibility for achieving the combined desulfurization and denitrification. On the basis of absorbent screening, the effect of MgSO4, Na2SO4 and MgCl2 additives on the NO2 absorption into CaSO3 slurry was investigated further. MgSO4 seemed to be the most effective additive, and the absorption efficiency increased significantly from 71% to 86%. At the same time, the addition of MgSO4 would also accelerate the CaSO3 oxidation and influence the liquid phase composition. Experimental results revealed that the enhancement of NO2 absorption by MgSO4 could be ascribed to the promotion of the capacity of dissolved sulfite species (CDSS). Both SO42- and Mg2+ contributed to the CDSS promotion. As one of the main components of CDSS, MgSO30 ion pair was also an effective absobent, which played the same role in the NO2 absorption as dissociated SO32-. |
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