Selective catalytic reduction of nitric oxide with ammonia at low temperatures

Selective catalytic reduction of nitric oxide with ammonia in synthetic low temperature flue gases has been investigated on a commercially available precious metal catalyst, NO_xCAT 920 LT^TM. It has been found that this catalyst is capable of achieving up to 90% conversion at temperatures below 300°C and low space velocities (12 000 h⁻¹), even in the presence of 20 ppm sulfur dioxide. The ideal ammonia concentration to reduce slip and achieve maximum conversion seems to be a stoichiometric match between ammonia concentration and nitric oxide concentration. A dual site model is proposed to explain the selectivity dependence on the presence of water vapor or sulfur dioxide.     

Reduction of nitric oxide with ammonia on silica-supported vanadium oxide catalysts

Highly active catalysts for the reduction of nitric oxide with ammonia can be obtained by supporting vanadium oxide, more than 15% by weight, on a silica gel with micropores of a mean diameter larger than 10 nm, followed by calcining it in the temperature range from 250 to 350°C. Both pre-impregnation with a small amount of titania and addition of ammonium bromide increased the activity markedly. The catalyst gave an NO conversion level of 100% at 150°C. A series of life tests of the catalysts, which was performed at 230°C using a simulated flue gas containing 700 parts/10⁶ of sulphur dioxide, demonstrated that their activities were stable for more than 300 h. Little irreversible change in the catalyst properties was observed after the test.     

Activity enhancement of copper-containing oxide catalysts by addition of cesium in the reduction of nitric oxide

Catalytic reduction of nitric oxide in the presence of propylene and oxygen over alumina and copper-containing oxide catalysts has been studied. The optimum temperature for this reaction is dependent upon the composition of the catalysts: 640 K on Cu-Cs/Al₂O₃, 680 K on Cu/Al₂O₃, and 780 K on Al₂O₃. IR spectroscopic measurements show that an isocyanate (–NCO) intermediate formed on Cu-Cs/Al₂O₃ is more reactive with NO to give N₂ than the intermediate produced on Al₂O₃ and Cu/Al₂O₃. Electron donation from Cs to Cu may activate the intermediate.     

Effect of gas composition on nitric oxide removal from simulated flue gas with DBD-NPC method

A new method for nitric oxide (NO) removal was developed by combining dielectric barrier discharge (DBD) and negative pulse corona (NPC). The effects of gas composition (O₂, CO₂, and H₂O) on NO removal were investigated with this method, and the effect of alcohols (methanol and ethanol) addition on NO removal was also investigated as well as the reaction mechanisms to enhance the NO removal efficiency. The experimental results showed that O₂, CO₂, and H₂O had obvious inhibition effects on NO removal, and the negative effects were in the following order:O₂ > CO₂ > H₂O. The addition of methanol or ethanol in the reaction system could mitigate the negative effects of O₂, CO₂, and H₂O on NO removal, and also eliminated the production of NO₂. The positive effect of alcohols addition with DBD-NPC denitration method was also validated in the simulated flue gas, in which the NO_x (NO, NO₂) was mainly converted into N².     

应用催化氧化催化剂脱除烟气中NO的研究进展

如何控制减少氮氧化物的产生排放已成为当今中国空气污染治理的主要内容。本文简要分析了燃烧过程控制技术,针对目前国内外较为关注的烟气NOx脱除工艺进行了优缺点两方面的分析比较。综述了催化氧化NO催化剂的研究进展：分子筛及其负载型催化剂由于其低温活性较低,目前研究较少;活性炭类的研究主要集中在改性以及新型炭材料的制备;贵金属类催化活性较好,但是成本过高限制了其工业化;金属氧化物类由于具有较高的催化活性,明显的价格优势,已经成为目前国内外催化剂研究的重点;将新材料以及废物应用于NO的氧化近几年吸引了越来越多的目光。     

SCR烟气脱硝催化剂再生研究进展

介绍了SCR烟气脱硝催化剂的水洗再生、热再生、热还原再生、酸液再生、SO2酸化热再生、活性盐溶液活化再生等再生方法的研究进展。再生技术应用时,应根据SCR催化剂的配方、构型、应用场合等因素,分析SCR催化剂失活的主要原因,选择适当的一种或者几种再生方法,才能达到提高或者恢复催化剂脱硝活性的目的。     

Vanadium loaded carbon-based catalysts for the reduction of nitric oxide

Carbon-based SCR catalysts for the reduction of NO with NH₃ at low temperatures have been prepared using activated carbons obtained from a local Spanish coal, doped with several vanadium compounds. Among them, the ashes of a petroleum coke (PCA) were also employed. Both the catalysts and the carbon supports have been characterized by means of N₂ and CO₂ physisorption, NH₃ and O₂ chemisorption and temperature programmed desorption (TPD). The activity of the catalysts has been tested in a laboratory-scale unit, measuring significant conversions of NO (above 50%) with almost 100% selectivity toward N₂ at 150 °C. The feasibility of using the petroleum coke ashes as the active phase was confirmed comparing the activity of the catalysts doped with these residues, with the one measured for the catalysts prepared using model vanadium compounds. The physical–chemical features of the carbon support resulted of key importance for achieving a considerable catalytic activity. The values of apparent energy of activation calculated for the catalysts presented in this paper were very similar to other carbon-based catalysts and smaller than the ones corresponding to TiO₂-supported systems. The gas residence time on the catalytic bed influences the catalytic activity to a great extent, thus being a determinant parameter for designing the SCR de-NO_x unit. To avoid ammonia slip, inlet concentrations of NH₃ has to be little under the stoichometric NH₃/NO ratio (0.7). The catalysts stability was tested in terms of carbon support gasification followed by termogravimetric analysis and gas chromatography. The activity of the catalysts was maintained at least over 24 h of reaction.     

烟气还原脱硫催化剂的反应机理研究进展

综述了烟气直接还原至单质硫催化剂的研究进展,主要介绍了负载型催化剂、金属氧化物催化剂、钙钛矿催化剂以及萤石型催化剂在不同种类还原剂条件下的脱硫效果,并分析了二氧化硫转化率及生成单质硫选择性受温度、氧气和水蒸汽等因素的影响。总结各种催化剂性能和脱硫机理,并从脱硫机理角度指出了各种催化剂存在的问题及研发方向。     

煤焦直接还原脱除烟道气氮氧化物

选用3种不同煤焦,采用程序升温和恒温实验方法,在固定床上考察了不同实验条件下的NO转化率和C对NO选择性,分析了煤焦脱硝的机理和影响因素。实验结果表明：NO和O₂在煤焦表面发生化学吸附所形成的络合物在脱硝过程中起着关键作用,影响C-NO反应和C-O₂反应的竞争与协作关系。在所考察的煤焦中锡林浩特褐煤焦脱硝效果最好,当温度为723 K时烟道气中NO还原率可达99%;在温度623～923 K、O₂浓度0～5%范围内,提高温度和O₂浓度均有利于提高NO转化率,而降低O₂浓度有利于提高C对NO选择性;烟道气中NO浓度越高,其转化率越低,但C对NO选择性越高。     

On the mechanism of selective catalytic reduction of NO by propylene over Cu-Al-MCM-41

The selective catalytic reduction of NO by propene in the presence of excess oxygen over Cu-Al-MCM-41 catalyst has been studied by a number of catalytic techniques to characterize the structural, chemisorptive and reactive properties. The characterization using XRD and NMR revealed that the structure of Cu-Al-MCM-41 remained unchanged after the reaction. The active sites related to the reduction of NO and the reaction mechanism were explored based on the data of H₂ temperature programmed reduction (TPR), NO temperature programmed desorption (TPD), X-ray spectrometry (XPS) and in situ FT-IR. The results showed that a redox of Cu ions in Cu-Al-MCM-41 between monovalent and divalent states happened during the selective catalytic reduction of NO, and the reduction seemed to proceed via the intermediate of organic nitro compounds produced by the reaction of propylene adspecies and NO₂. In addition, the presence of oxygen is essential to the formation of NO₂ intermediate and to the cycle of active center between Cu²⁺ and Cu⁺. However, it also caused the deep oxidation of propylene, leading to the depletion in reducing agent at higher temperature.     

Selective catalytic reduction of nitric oxide with ammonia using MoO3/TiO2: Catalyst structure and activity

A series of titania supported MoO₃ catalysts (0–20 wt.-% MoO₃) were prepared by dry impregnation. The influence of the MoO₃ content on their catalytic performance for the selective catalytic reduction (SCR) of nitric oxide by ammonia in the presence of oxygen, as well as on their textural and structural properties has been studied. The samples were characterized by XRD, XPS, IR, and BET and porosimetry measurements. The coverage of the TiO₂ support by surface polymeric molybdenum species (where molybdenum is octahedrally coordinated) increases with the molybdenum loading. The formation of a layer of these interacting species on top of the titania surface is complete in the range 15–20 wt.-% MoO₃. The formation of crystallites of bulk MoO₃ starts before the completion of this surface layer (at around 10 wt.-% MoO₃) and increases progressively as the molybdenum loading increases from 10 to 20 wt.-% MoO₃. The SCR activity of the MoO₃/TiO₂ catalysts increases as the MoO₃ content increases to 15 wt.-% and then, for a further increase of the molybdenum loading, it slightly decreases. No specific influence of the molybdenum content on the resistance of catalysts towards SO₂ was observed; the same slight deactivation took place, when the SCR activity was measured in the presence of SO₂ in the feed, for all samples. Our results indicate that the octahedrally coordinated polymeric molybdenum surface species are mainly responsible for the exhibited SCR activity of the MoO₃/TiO₂ catalysts.     

烧结烟气活性炭脱硝机制

烧结烟气氮氧化物(NOx)排放占钢铁行业NOx排放总量的50％以上,随着环保法规的日益严格,现有及新建烧结机只有装设烟气脱硝装置才能满足排放法规的NOx排放要求.而活性炭微孔丰富、比表面积大、吸附能力强,低温时即可同时脱除烟气中的SO2、NOx、粉尘及其他有害气体.因此,低温烧结烟气活性炭脱硝具有显著的特点及技术优势,...     

溴元素改性椰壳活性炭对实际燃煤烟气的脱汞性能

以椰壳活性炭（YAC）为原料,通过NH₄Br溶液浸渍改性,制备了溴素改性椰壳活性炭脱汞吸附剂（YAC-Br）。在固定床实验台上开展了YAC和YAC-Br的汞脱除实验,主要研究了入口汞（Hg⁰）浓度对YAC-Br脱汞性能的影响,并结合BET、SEM、XRF等表征手段分析了YAC-Br的脱汞原理。在0.3MW燃煤循环流化床锅炉上对YAC-Br进行了烟气管道喷射吸附剂脱汞（ACI）实验,验证了其在实际燃煤烟气中对汞的脱除效果。结果表明：改性过程不会破坏椰壳活性炭原有的孔隙结构和微孔容积,而会使活性炭表面更加平整;化学改性后活性炭表面Br负载量提高,成为Hg⁰的主要活性吸附位。固定床实验结果说明：改性后椰壳活性炭的初始汞吸附效率和单位累积汞吸附量分别提高了6.02倍和21.8倍,吸附效率随汞浓度增大而降低。0.3MW燃煤循环流化床实验结果表明：改性后椰壳活性炭对元素汞和氧化汞均有很好的脱除作用,脱汞效率随着吸附剂喷射量的增加而增加,当喷射量为0.7kg/h时,脱汞效率可达到76.38%。     

Active sites of H-ZSM5 catalysts for the oxidation of nitric oxide by oxygen

The catalytic activities of H-ZSM5-18, H-ZSM5-150, Li-ZSM5-18, and H-Mag (numerical suffixes mark the Si/Al ratios of zeolites, H-Mag is the proton exchanged form of the layered sodium silicate, magadiite) were compared for the oxidation of NO by O₂ at different ratios of reactants at reaction temperatures from 25 to 600°C. H-ZSM5-18 typically has activity maxima near 25 and 400°C at most O₂/NO reactant ratios. Regardless of the partial pressures of reactants, NO±1/2O₂ NO₂ equilibria are attained at 400°C and above. The H-ZSM5-150 and Li-ZSM5-18 zeolites are only active at temperatures near 25°C. H-Mag is practically inactive at the reaction conditions used. Results indicate that Lewis acidic lattice aluminium ions and silanol hydroxyls are not active in the oxidation of NO to NO₂ over H-ZSM5 zeolites. Brønsted acidic bridging hydroxyls are probably active sites for this reaction at temperatures above 200°C.     

Nitric oxide reduction and carbon monoxide oxidation over carbon-supported copper-chromium catalysts

Carbon supported copper-chromium catalysts are shown to be very active for both the reduction of nitric oxide with carbon monoxide and the oxidation of carbon monoxide with oxygen. Mixed copper-chromium oxide active phases have good activity in the simultaneous removal of nitric oxide and carbon monoxide from exhaust gases. The influence of several catalyst variables has been investigated. The activity per volume of catalyst increases with increasing loading, while the intrinsic activity shows a maximum around C/M=100−50. An optimum catalyst for nitric oxide reduction and carbon monoxide oxidation has a copper/chromium ratio of 2/1. The apparent activation energy for the carbon monoxide oxidation over carbon supported copper-chromium catalysts is 77 kJ/mol, suggesting that the Cu---O bond rupture is the rate-limiting process. The reduction of nitric oxide takes place at higher temperatures. Since all catalysts have a low selectivity for molecular nitrogen formation at lower temperatures, the dissociation of nitric oxide is probably rate determining, resulting in a slightly reduced catalyst system. In an excess of carbon monoxide the reaction is first-order in nitric oxide and zero-order in carbon monoxide. Moisture inhibits the reaction by reversible competitive adsorption, whereas carbon dioxide does not. Oxygen completely inhibits the reduction of nitric oxide due to the more rapid reoxidation of the catalytic sites compared to nitric oxide. Therefore, the reduction of nitric oxide takes place only when all oxygen has been converted and, hence, is shifted to higher temperatures. As a possible consequence, the production of nitrous oxide is reduced. Nitric oxide and molecular oxygen react preferentially with carbon monoxide, so, in an excess of oxidizing component, gasification of the carbon support occurs at higher temperatures after carbon monoxide has been completely consumed.     

炭基材料低温SCR烟气脱硝催化剂的研究进展

燃煤电站机组低温SCR技术研究是近年来烟气脱硝技术研究的主要方向,低温SCR催化剂是该技术的核心,炭基材料因其比表面积大和孔隙结构发达在低温SCR催化剂研发中占重要地位。综述炭基材料包括活性炭、碳纳米管、活性炭纤维、碳包覆材料和活性焦在低温SCR催化剂的研究进展,对多种炭基材料的催化反应机理进行阐述,总结水和二氧化硫对炭基材料催化剂性能的影响,为炭基材料在低温SCR催化剂进一步研究提供参考。     

Effects of the pretreatment of Cu-Y zeolite catalysts on the reduction of nitric oxide with ammonia

Experiments were conducted to study the effects of pretreating Cu-Y zeolite catalysts on the reduction of nitric oxide with ammonia. Changing the oxidation state and environment of the copper in Cu-Y changed the optimal reaction temperature of the NO---NH₃ reaction and the activity of this catalyst. At the optimal temperature, the activity of Cu-Y after dehydration was higher than that of hydrated Cu-Y. In addition, at the optimal temperature, the activity of Cu-Y after the NO---NH₃ reaction followed by oxidation or ammonia pretreatment followed by oxidation was higher than that of dehydrated Cu-Y. The activity of Cu-Y at the optimal temperature was decreased by reduction at higher temperature but increased by reduction at lower temperature. Furthermore, the activity of Cu-Y increased as the copper loading was increased. The optimal reaction temperatures of the NO---NH₃ reaction over various pretreated Cu-Y catalysts were 106, 126, 220 and 300°C, respectively.     

Selective reduction of nitric oxide by methane on H-form zeolite catalysts in oxygen-rich atmosphere

Selective reduction of NO by CH₄ in the presence of excess oxygen was investigated using H-form zeolite catalysts. H-ZSM-5, H-ferrierite, and H-mordenite showed high catalytic activity and selectivity. On the contrary, H-USY and Al₂O₃ were not effective for this reaction. Both NO-CH4 and O₂-CH₄ reaction hardly proceeded on H-ZSM-5. Higher NO_x conversion was obtained in the NO₂-O₂-CH₄ and NO₂-CH₄ systems than in the NO-O₂-CH₄ system under high GHSV condition. It seemed that NO₂ plays an important role for selective reduction of NO by CH₄ on H-form zeolites.     

Simultaneous removal of VOC and NO by activated carbon impregnated with transition metal catalysts in combustion flue gas

Activated carbon (AC) was conventionally sprayed into the incineration flue gas for controlling the emissions of volatile organic compounds (VOC) by adsorption at low temperature (200–250 °C). However, the spent carbons required to be further disposed with more energy. In the present study, the inertness and hydrophobic AC was used as support for transition metal catalysts (Cu, Co, Fe and Ni) to remove VOC by deep oxidation into CO₂ and H₂O, and also to converse NO to N₂ with reduction at the temperature of 200–250 °C. The properties of the catalysts were analyzed by ICP-MS, XPS, FTIR, BET and SEM. The chemical state of transition metals supported on AC playing a significant role in improving the ability of VOC adsorption or catalytic oxidation at low temperature. The XPS analysis proved that the oxidized and reduced metals were coexisted on the fresh catalysts, but there was only the oxidized metal on the spent catalysts. These investigations indicated that AC impregnated with transition metal catalysts had the potential to simultaneously remove VOC and NO at 250 °C, with Co/AC being found to have higher activity.     

Development of gas phase bioreactors for the removal of nitrogen oxides from synthetic flue gas streams

Thermophilic denitrifying bacteria indigenous to composts and soils are capable of converting NO_x to environmentally benign nitrogen via a dissimilatory reductive pathway. The present study compares the performance of three bioreactor packing materials (compost, perlite, and biofoam) for the removal of nitric oxide (NO) from a simulated wet-scrubbed combustion gas. Although all three materials performed well (>85% NO removal) at residence times of 70-80 s, the compost performed better than the other materials at shorter residence times (13-44 s). The feasibility of biological NO_x conversion processes will depend on the combined factors of NO_x removal ability and pressure drop. The results presented here suggest that the compost, perlite and biofoam systems, subject to further optimization, offer potential for the biological removal of NO_x from gas streams.