The Experimental Study On Desulfurization And Denitrification By CuO/γ-Al₂O₃ And Modified Catalysts

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.