【作者】 赵清森；

【导师】 向军； 徐明厚；

【作者基本信息】 华中科技大学 ， 热能工程， 2009， 博士

【摘要】 大气中的SO₂和NOx所引起的酸雨问题是现今世界普遍关注的三大环境保护课题之一，而燃煤电站是SO₂和NO_x排放的主要来源。研究和实际运行经验表明：烟气净化技术能最大程度上经济有效控制SO₂和NO_x排放。可再生干法烟气同时脱硫脱硝技术是燃煤污染物排放控制发展的重要方向。在目前已开发的干法同时脱硫脱硝剂中，CuO/γ-Al₂O₃催化剂既可催化氧化吸附烟气中的SO₂形成硫酸盐，又可在喷入氨气条件下将烟气中的NOx选择性催化还原为N₂，且吸附SO₂饱和的催化剂可于还原性气氛中还原再生，故而得到广泛关注，但因催化剂在耐磨性和硫容等方面存在的问题，至今未实现工业化。基于此，本论文工作从增强催化剂结构特征，提高催化剂脱硫脱硝性能的要求出发，通过催化剂制备工艺的改进与添加助催化剂，开发出一种高效稳定且可实现再生循环利用的干法同时脱硫脱硝技术。结合脱硝机理的研究，系统分析了SO₂和助催化剂对催化剂SCR活性的作用本质。论文主要从以下几个方面开展了研究：采用溶胶凝胶法制备CuO/γ-Al₂O₃颗粒，研究催化剂制备工艺中的各种影响因素，并对制备工艺进行了改进。简化后的制备工艺不仅缩短催化剂制备周期，也降低催化剂制备成本；溶胶凝胶法制备催化剂拥有较大的比表面积和孔容积，较高的活性组分负载量，从而有望拥有较高的脱硝反应活性和较大的脱硫硫容。以溶胶凝胶法制备CuO/γ-Al₂O₃催化剂，在固定床上开展烟气脱硝性能实验研究。溶胶凝胶法与浸渍法制备的CuO/γ-Al₂O₃催化剂相比，拥有较高的脱硝活性和较宽的温度适用窗口，在250-400℃内脱硝效率维持在85％以上；研究了NH₃/NO摩尔比、O₂浓度和不同活性组分负载量对脱硝性能的影响；分析了水蒸汽对催化剂脱硝性能的毒化作用；研究发现NH₃在高温区（400-500℃）过度氧化生成N₂、NO和N₂O，是脱硝效率下降的主要原因。选用不同类别的助催化剂，对CuO/γ-Al₂O₃催化剂进行改性。催化剂中添加不同助催化剂，脱硝反应温度窗口呈现明显差别。催化剂表面氧化性越强，催化剂在低温区（200-300℃）脱硝活性越高，但在高温区（400-500℃）脱硝活性却越低。制备的CuO-CeO₂-MnOx/γ-Al₂O₃催化剂在100-200℃内呈现了65％以上的脱硝活性，是一种有潜力的低温类型催化剂。随着温度变化SO₂对催化剂脱硝性能有双重影响。对比新鲜催化剂和硫化后催化剂，SO₂降低了硫化后催化剂低温区脱硝活性，但增强了在高温区脱硝活性。SO₂对催化剂脱硝性能的影响归功于NH₃在催化剂表面的吸附量和催化剂对NH₃氧化性能；硫化后催化剂表面氧化性能较低是其在低温区脱硝活性较差的主要原因。高的NH₃吸附量保证了催化剂在高温区拥有较高脱硝效率。CuO/γ-Al₂O₃再生后脱硫脱硝性能得到了恢复，但是其硫容有所降低。改性后催化剂的硫容得到一定程度提高，CuO-V₂O₅/γ-Al₂O₃催化剂可再生性能最好，在三个循环过程中硫容降低程度较小。采用在线原位红外漫反射技术，并应用程序升温和暂态响应等方法对NH₃和NO在CuO/γ-Al₂O₃催化剂表面的吸附性能、吸附态NH₃和NO的反应性以及SCR反应机理作了系统研究。CuO/γ-Al₂O₃催化剂上的SCR反应遵循Eley-Rideat机理，即SCR反应发生于吸附态NH₃与气相或弱吸附态的NO之间；CuO/γ-Al₂O₃催化剂的Lewis酸性位具有SCR活性；Brφnsted酸性位的NH₄⁺不参与SCR反应；吸附态NO及其氧化生成的亚硝基物种参与SCR反应，而硝酸盐物种很难参与SCR反应。采用密度泛函法研究NH₃、NO₂和NO分子在CuO/γ-Al₂O₃催化剂表面的吸附行为。这几种分子在CuAl₂O₄（100）表面Cu²⁺位上吸附作用的发生都比较容易进行，其中NO和吸附位的结合力最强，那里被吸附物和CuAl₂O₄（100）表面之间的电荷转移最强。催化剂和被吸附物种之间发生了电荷的转移，并且电荷的转移在吸附过程中起着非常重要的作用。吸附过程中相应几何和电子特性的变化也与被吸附物种NH₃、NO₂和NO的吸附能力相关。对此研究有助于我们了解脱硝过程机理，同时为进一步改善设计脱硝催化剂提供依据。更多还原

【Abstract】 Sulfur dioxide （SO₂） and nitrogen oxides （NOx） are mainly produced during thecombustion of fuels from Coal-fired power plant, such combustion results in serious airpollution by the formation of acid rain, photochemical smog, and green house effect. It hasbeen reported that flue gas cleaning techniques provided one of the most effective ways toremove SO₂ and NOx. Numerous research works on simultaneous removal of SO₂ and NOxhave been carried out to find cost effective catalysts. In simultaneous removal of SO₂ andNO_X, the CuO/γ-Al₂O₃ catalysts have distinct advantages to remove SO₂ and NO_Xbecause of the high desulfurization and denitrification efficiency at the same temperaturerange （300-400℃） and easy regeneration under the reductant gas atmosphere. However, thelow sulfur capacity and poor mechanical strength of the catalyst available at the timepresent a major problem for the practical application. Therefore, the attentions of this workare focused on increasing mechanical properties, improving desulfurization anddenitrification activity. The modified sol gel method was used and the promoters wereadded into the CuO/γ-Al₂O₃ catalysts. The effect of SO₂ and promoters on selectivecatalytic reduction （SCR） activity and the SCR reaction mechanism were also studiedsystematically; the main and important results are described as follows:Granular CuO/γ-Al₂O₃ catalysts were synthesized by the sol gel method. Variousparameters affecting the granulation process were studied, and several improvements weredone to the synthesis procedure for sol gel derived copper doped Al₂O₃ granules. Theseimprovements simplify the catalysts synthesis process and reduce the preparationtime and catalysts cost. The sol gel derived CuO/γ-Al₂O₃ granules have a large surfacearea, large pore volume, uniform pore size distribution, high CuO content. It can be inferredthat CuO/γ-Al₂O₃ catalyst may have high desulfurization and denitrification activity.Performance of the CuO/γ-Al₂O₃ catalysts synthesized by sol gel method was studiedin a fixed bed system. The CuO/γ-Al₂O₃ catalysts prepared by sol gel method had higherdenitrification activity and wider temperature widows than that prepared by wetimpregnation method. The optimum temperature range for NO reduction over the CuO /γ-Al₂O₃ catalyst was 200-450℃. In this temperature window, the catalysts maintained atmore than 85% NO conversion efficiency. Comprehensive tests including NH₃/NO ratio,O₂ concentration and CuO content were carried out to study the SCR activity over thecatalyst, and the poisonous effect of water vapor on SCR activity was also analyzed. It wasfound that the over-oxidation of NH₃ to N₂, NO and N₂O at high temperature range was themain reason causing the decrease of NO conversion.Several promoters such as Mn, Ce, Na, V and Ni were added into the CuO/γ-Al₂O₃catalyst, and the modified Catalysts were tested in fixed bed system. It was found that thecatalysts with different promoters had various temperature windows. The strongeroxidation of NH₃ the catalysts had, the higher denitrification efficiency at low temperaturerange （200-300℃） the catalysts got, but correspondingly they had worse activity at hightemperature range （400-500℃）. The CuO-CeO₂-MnOx/γ-Al₂O₃ catalysts maintained atmore than 65% NO conversion efficiency at 100-200℃. It could be inferred thatCuO-CeO₂-MnOx/γ-Al₂O₃ catalysts had a potential ability to remove NO at lowtemperature.SO₂ showed dual effect on SCR activity of CuO/γ-Al₂O₃ catalysts according thetemperature. Compared with the fresh and sulfated CuO/γ-Al₂O₃ catalysts, SO₂ haddeactivating effect on NO conversion efficiency at 200-300℃, but they had promotingeffect at 400-500℃. It was found that the effects of SO₂ on SCR activity of CuO/γ-Al₂O₃catalyst are contributed to changes in NH₃ adsorption capacity and NH₃ oxidation property.The sulfated catalysts had worse oxidation degree of NH₃, so the activity was lower. Thehigh adsorption amount of NH₃ ensured that sulfated catalysts had higher activity than freshcatalyst. SO₂ showed no deactivating effect on NO conversion efficiency at 300-400℃. Thesulfated catalysts could be effectively regenerated at 450℃in NH₃/N₂, but the sulfurcapacity decreased. Compared with the CuO/γ-Al₂O₃ catalysts, the sulfur capacity ofmodified catalysts was improved. CuO-V₂O₅/γ-Al₂O₃ catalysts showed the bestregenerated property among the modified catalysts.NH₃/O₂ and NO/O₂ adsorption processes on the catalyst and the transient response ofNH₃ and NO were investigated by in situ diffuse reflectance infrared transformspectroscopy （DRIFTS） to study the SCR mechanism. It was found that the SCR reaction on the CuO/γ-Al₂O₃ catalysts occured between adsorbed NH₃ on Lewis acid site and gasousNO, accorded the Eley-Rideal mechanism. The NH₄⁺ adsorbed on Brφnsted acid site didnot take part in the SCR reaction. NO could be adsorbed on the fresh and sulfatedCuO/γ-Al₂O₃ catalysts. The adsorbed NO was oxidized to nitrate and nitrite species, and thenitrate was difficult to participate in the SCR response.Adsorption properties of NH₃, NO₂ and NO molecules on CuO/γ-Al₂O₃ catalystssurface had been investigated by using a density functional approach. The results showedthat the adsorptions of all the molecules on the Cu²⁺ site of CuAl₂O₄（100）were energeticallyfavourable, whereas NO was the most strongly bound with the adsorption site, where thecharge transfer between the adsorbates and the CuAl₂O₄ （100） surface always took placeand played an important role during the adsorption processes. The change in the geometricand electronic properties induced by the adsorption of NH₃, NO₂ and NO were correlatedwell with the adsorption ability. This study might be of help to understand the de-NOxmechanism and further to design de-NOx catalysts with excellent performance.更多还原