【作者】 唐念；

【导师】 吴忠标；

【作者基本信息】 浙江大学 ， 环境工程， 2011， 博士

【摘要】 氮氧化物是烟气中的主要污染物之一,氧化吸收脱硝技术是脱除固定源烟气氮氧化物的重要方法。本文围绕Mn/TiO2催化氧化NO联合CaSO3浆液吸收的脱硝新工艺展开了系统的研究。文中通过制备方法的改进和掺杂改性等手段提高了Mn/TiO2对NO氧化的催化活性,获得了较高的NO转化率,并通过原位红外等手段揭示了NO催化氧化的反应机理,为催化剂性能的进一步改善提供了指导。在NO催化氧化的基础上,本文还对脱硝吸收剂进行了筛选比较,并通过加入添加剂等手段改善吸收剂的性能,为该工艺的应用奠定了基础。首先,本文比较了制备方法对Mn/TiO2催化氧化NO活性的影响,发现沉淀沉积法相比于浸渍法可以大大提高催化剂的活性(250℃时的NO转化率从40%提高到89%)。表征结果表明,沉淀沉积法能显著提高活性相的分散性,制得的MnOx以高活性的Mn(3+)为主,有利于NO2的生成和释放。工艺操作参数研究结果表明,Mn/TiO2对NO氧化具备持久稳定的高催化活性。动力学研究表明,该NO催化氧化反应对NO和02均为0.5级反应,催化剂的表观反应活化能约为20 kJ/mol。其次,本文深入研究了Mn/TiO2对NO氧化的催化反应过程和机理。研究发现,亚硝酰和硝酸盐(尤其单齿型硝酸盐)是NO催化氧化中两类重要的中间产物。NO催化氧化遵循Mars-van Krevelen机理,即NO先吸附在Mn活性位上,形成亚硝酰,之后亚硝酰被催化剂表面的晶格氧氧化成硝酸盐,硝酸盐随之分解而释放出N02,消耗的晶格氧则通过气相O2的吸附得到补充。Fe掺杂改性的相关研究也部分印证了该反应机理。测试分析结果表明,Fe的加入能进一步提高Mn/TiO2对NO氧化的催化活性,主要原因在于Fe掺杂既改善了催化剂对NO的吸附性能,又提高了其持续氧化还原能力。最后,本文针对氮氧化物吸收过程进行了研究。通过比较不同吸收剂对氧化后氮氧化物的吸收特性,发现CaSO3是一种合适的脱硝吸收剂,其脱硝效率高于水和碱性吸收剂,同时消耗速率远低于Na2SO3,且能在脱硝的同时被氧化成石膏,为实现联合脱硫脱硝提供了可能。在此基础上,进一步考察了MgSO4、Na2SO4和MgCl2等添加剂对CaSO3浆液吸收NO2过程的影响,发现MgSO4对NO2吸收的促进作用最为明显,能使脱硝效率从71%显著提高至86%。同时,MgSO4的加入还会加速CaSO3的氧化,并改变NO2液相吸收产物的组成。实验分析结果表明,MgSO4添加剂发生作用的原因在于,Mg2+和SO42-都能提高CaSO3浆液中溶解态的亚硫酸盐的含量(CDSS),且CDSS中的MgSO30也是一种有效的还原性脱硝吸收剂,在NO2吸收过程中能发挥与SO32-类似的作用。更多还原

【Abstract】 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-.更多还原