15 MW生物质循环流化床NOx和温室气体排放特性

生物质是零碳可再生能源，对我国实现碳达峰、碳中和目标具有重要意义。虽然被视为清洁能源，但生物质燃烧过程仍会排放NO_x(NO、N₂O)和温室气体(CH₄、N₂O、CO₂),有必要对生物质直燃的NO_x和温室气体排放特性进行研究。测量某15 MW生物质循环流化床的NO_x和温室气体排放，并探究了改变床压、一二次风比、前后墙二次风比、废木料掺烧比例等因素对NO_x和温室气体排放特性的影响。燃烧调整试验表明：升高床压有利于降低NO排放，但降幅很小，且会造成CO和CH₄体积分数上升，CO₂体积分数降低；随一二次风比增大，NO排放略降低，这意味着可适当降低二次风以降低NO排放量，CO和CH₄体积分数降低，CO₂体积分数升高；当前墙二次风开度/后墙二次风开度较小或较大时，均有利于降低NO,CO和CH₄排放量也较低；高含氮废木...     

新型双流化床炉内NOx生成特性数值模拟

运用煤燃烧及NO_x生成的详细化学反应机理,通过搭建一维化学反应器网络（1D-CRN）,对一个新型双流化床（DCFB）内燃料型N转化为NO_x的基元化学反应进行了敏感性分析并讨论了反应温度、过量空气系数以及一、二次风配比对燃料型NO_x生成的影响。研究发现,在相同条件下,循环流化床炉膛出口的NO_x排放值为224.48 mg·m^-3,而双流化床炉膛出口的NO_x排放值为97.29 mg·m^-3,双流化床对于燃料型NO_x的减排幅度达到了56.66%。此外,促进NO_x生成的基元反应主要有R398（NH₂+O?HNO+H）、R1-N-1（N-Vol?NH₃+HCN）、R569（NCO+O₂?NO+CO₂）、R17（H+O₂?O+OH）等反应,而抑制NO_x生成的反应包括R411（NH₂+NO?N₂+H₂O）、R412（NH₂+NO?NNH+OH）、R570（NCO+NO?N₂O+CO）、R571（NCO+NO?N₂+CO₂）以及R5（Char+NO?Char+N₂+O₂）和R6（Soot+NO?n Soot+N₂+CO）等反应。这说明反应区域氧气浓度是影响NO_x生成的关键,低氧浓度可抑制燃料N向NO_x转化。另外,NO_x生成值随着反应温度的升高而降低,但随着过量空气系数和一次风所占比例的增大而增加。     

低温燃烧下NH_3的氧化特性

NH3的气相氧化是低温燃烧过程中NOx(NO和NO2)与N2O的重要来源,为了深入认识其反应规律,在管式流动反应器系统中进行了实验研究。重点考察了挥发分中的可燃气(CO、CH4或H2)和NO对NH3氧化及氮氧化物排放的影响规律,并根据化学反应机理对实验结果进行了分析。研究结果表明,低温氧化性气氛下微量的可燃气就能够显著促进NH3的氧化,并使NOx和N2O的生成量大幅度升高。当可燃气体浓度相同时,H2对NH3氧化的影响最大,CO的影响最小,CH4对NH3氧化的影响略大于CO。随着可燃气体浓度的升高,其对NH3氧化与氮氧化物生成的影响先逐渐增加,然后趋于稳定。反应初始气体中存在NO时,也会加速NH3的氧化。     

选择催化还原(SCR)反应机理研究进展

简述了NH₃和NO在催化剂表面吸附、转化活化和反应历程及H₂O和SO₂对以上反应行为的影响。分析表明,NH₃氧化脱氢进而与NO反应是决定NH₃反应性和最终产物的关键。NO以气态（Eley-Rideal机理）或硝基类物质等吸附态（Langmuir-Hinshelwood机理）形式参与选择催化还原（SCR）反应。提高催化剂酸性和氧化还原循环性能,利于NH₃和NO吸附和转化及相互间反应。高温时,H₂O影响轻微,而SO₂增强催化剂酸性,提高脱硝活性。低温时,H₂O和SO₂抑制NO吸附和转化活化,导致硫铵盐累积和活性位转变为硫酸盐使催化剂失活。因此,提高抗H₂O、抗SO₂性能是低温脱硝催化剂研发的重要方向。而发展在线升温等再生工艺以解决硝酸盐或含硫化合物导致的失活问题,对保障低温脱硝系统长期稳定运行具有重要意义。     

重型柴油机主要含氮化合物的排放特性

采用傅里叶变换红外光谱(FTIR)技术,研究了加装选择性催化还原SCR装置的重型柴油机主要含氮化合物排放,重点探索了不同工况下主要含氮化合物NO、NO₂和N₂O的排放特性。结果表明:未加装SCR的原机,随负荷的增加,柴油机NO排放持续上升,NO₂排放先升后降,N₂O排放很少。加装SCR后该柴油机NO与NO₂排放均明显下降,标定转速下NO₂排放降幅较大,主要是其NO₂/NO值稍高导致快速SCR反应较多的原因。由于存在SCR副反应,与原机相比,柴油机N₂O排放比原机平均增加2倍以上,最大转矩转速下N₂O排放升幅更高。N₂O排放随负荷的增加而上升,主要是排温升高导致NH₃氧化生成N₂O反应速率增加的原因。加装SCR后,该机排气中的NO/NO_x值要明显低于原机状态,而外特性的NO₂/NO_x值和N₂O/NO_x值高达12.8%和20.7%,均远高于原机的3.0%和0.5%。     

单空缺石墨烯负载的Pd单原子催化剂上NO还原的密度泛函理论研究

采用密度泛函理论（DFT）方法对单空缺石墨烯负载的Pd单原子（Pd/SVG）催化剂上H₂还原NO的反应进行了研究,探究了Pd/SVG上NO还原生成N₂和NH₃的路径。在Pd/SVG上NO容易加氢形成HNO,需要的活化能为67.0 kJ·mol^-1,显示了极高的催化活性。N₂生成的有利路径为NO活化生成HNO后,HNO继续加氢生成中间体NH₂O和NH₂OH,然后NH₂OH解离生成NH₂和OH,生成的NH₂中间体结合NO形成NH₂NO,然后NH₂NO异构化形成的NHNOH再经解离生成N₂与H₂O,这个过程中的决速步骤为NH₂NO分子内氢转移生成NHNOH,能垒为144.3 kJ·mol^-1。对于NH₃的生成,从NO的活化到中间体NH₂的形成与N₂的形成过程相同,最后NH₂加氢即可形成NH₃,这个过程中的决速步骤为NH₂O加氢生成NH₂OH,能垒为86.4 kJ·mol^-1。比较生成N₂和NH₃的决速步能垒可见,Pd/SVG催化剂上NO经H₂还原更容易形成NH₃。本研究为石墨烯负载型Pd基催化剂上H₂还原NO的实验及工业应用提供理论参考。     

汽车尾气净化三效催化剂中N2O和NH3的生成及控制研究进展

N₂O和NH₃的排放主要来自于机动车尾气排放。本文总结了近十几年来轻型汽油车N₂O和NH₃排放的研究进展,阐述了两种气态污染副产物在三效催化剂中的形成机理,通过对影响N₂O和NH₃生成的贵金属种类和含量、载体材料、不同气体组成和浓度、老化条件、不同车辆及测试工况、反应温度等主要影响因素的综述,总结了各要素对N₂O和NH₃形成的影响,得出N₂O和NH₃主要在富燃条件下冷启动阶段生成,NO的解离在N₂O和NH₃的生成中起关键作用;影响N₂O和NH₃生成的各因素之间相互关联,相互影响;催化剂的老化增加N₂O和NH₃的排放;贵金属Rh比Pd和Pt更有利于N₂O和NH₃的分解等结论。发动机、后处理策略系统的升级、更合适测试循环的开发以及催化剂的优化可以进一步降低N₂O和NH₃的排放。     

NTP转化C3H6/NO/N2气氛中NO及发射光谱分析

利用介质阻挡放电产生低温等离子体转化C₃H₆/NO/N₂气氛中NO,结合发射光谱诊断法研究了碳氢化合物C₃H₆对NO转化的影响。研究结果表明,随着放电功率的升高,NO转化率先升高后逐渐趋于平缓,NO₂浓度持续降低,N₂O浓度呈先升高后降低趋势,NO主要被还原为N₂;相同放电功率下,随着C₃H₆初始浓度升高,NO_x转化率和N₂O浓度升高、NO₂浓度降低;添加C₃H₆会降低N₂第二正带系和NO-γ带的发射光谱强度,产生CN自由基的激发跃迁谱线,影响NO的化学反应机制,同时生成了棕黄色的聚合物。     

低温等离子体用于柴油机尾气脱硝的实验

柴油机作为卡车、重型机械以及船舶的主动力装置仍被广泛采用,其尾气中氮氧化物的脱除技术也是目前的研究热点。本文搭建了模拟柴油机尾气的配气系统,采用介质阻挡放电产生低温等离子体（non-thermal plasma,NTP）的方法对模拟柴油机尾气进行了脱硝的实验研究。实验结果表明：针对本系统,电源效率和能量密度随着输入电压的增大而升高,当输入电压高于60V时,电源效率在90%以上;在O₂/N₂条件下,随着O₂浓度以及能量密度的增加,NO生成量逐渐增加,NO₂生成量先增加后降低最终趋于稳定;在NO/N₂条件下,低温等离子体对NO的脱除率接近100%;在NO/O₂/N₂条件下,随着NO浓度的增加,临界O₂浓度升高,O₂体积分数为1%时脱硝效率在90%以上,O₂体积分数高于14%时低温等离子体的脱硝率为负值,且随着能量密度的增加,生成的NO _x 浓度也更高,O₂浓度对低温等离子体的脱硝性能起决定性作用;在低能量密度时,加入NH₃会提高脱硝性能,高能量密度时NH₃会略微降低NTP的脱硝性能,当加入H₂O模拟真实柴油机尾气成分且喷氨时,获得的脱硝率最高为40.6%。     

添加剂CH3COONH4对介质阻挡-电晕放电耦合法脱除NO、SO2的影响

采用自行研究设计的介质阻挡-电晕放电等离子体反应装置在模拟烟气中进行NO、SO₂的脱除研究。考察了O₂、CO₂、水蒸气等气体组分对脱除NO、SO₂的影响,并进一步探讨了添加剂CH₃COONH₄对脱除NO、SO₂的影响及作用机理。实验结果表明：O₂、CO₂和水蒸气浓度的增加对NO脱除有抑制作用,而引入CH₃COONH₄后,这些抑制作用会被减弱,使NO的脱除率得到大幅度提升,但这些抑制作用不会完全消除。在引入CH₃COONH₄后,气体组分和输入电流的变化对脱除SO₂的影响不明显,SO₂脱除率可达到94%左右。在N₂/O₂/CO₂/H₂O/NO/SO₂体系中加入0.27%的CH₃COONH₄后,NO初始浓度不变的条件下,SO₂含量较少时,对NO的脱除影响不明显,随着SO₂浓度的增加,NO的脱除率不断下降,增加CH₃COONH₄的添加量可消除SO₂的影响;另一方面,在SO₂初始浓度恒定的条件下,随着NO含量的增加,SO₂的脱除率保持在94%左右。在N₂/O₂/CO₂/H₂O/NO/SO₂体系中加入0.51%的CH₃COONH₄后,输入电流2.5A时,NO的脱除率达到72%。     

Role of active oxygen transients in selective catalytic reduction of N2O with CH4 over Fe-zeolite catalysts

Sharp NO and O₂ desorption peaks, which were caused by the decomposition of nitro and nitrate species over Fe species, were observed in the range of 520–673 K in temperature-programmed desorption (TPD) from Fe-MFI after H₂ treatment at 773 K or high-temperature (HT) treatment at 1073 K followed by N₂O treatment. The amounts of O₂ and NO desorption were dependent on the pretreatment pressure of N₂O in the H₂ and N₂O treatment. The adsorbed species could be regenerated by the H₂ and N₂O treatment after TPD, and might be considered to be active oxygen species in selective catalytic reduction (SCR) of N₂O with CH₄. However, the reaction rate of CH₄ activation by the adsorbed species formed after the H₂ and N₂O or the HT and N₂O treatment was not so high as that of the CH₄ + N₂O reaction over the catalyst after O₂ treatment. The simultaneous presence of CH₄ and N₂O is essential for the high activity of the reaction, which suggests that nascent oxygen species formed by N₂O dissociation can activate CH₄ in the SCR of N₂O with CH₄.     

NO removal by CH4 on Co-NaX-CO and Ag-NaX catalysts in a dual-bed system

NO removal using CH₄ as a reductant in a dual-bed system has been investigated with Co-NaX and Ag-NaX catalysts, which were prepared by Co²⁺-, Ag⁺-ion exchange into zeolite NaX, respectively, and activation for 5 h at 500 °C. The experimental result has been compared with that of a Co-NaX-CO catalyst, additionally pre-treated under CO flow for the Co-NaX catalyst. The cobalt crystal structure of a Co-NaX-CO catalyst is Co₃O₄, which promotes NO oxidation to NO₂ by excess O₂ at a low temperature (523 K). The mechanical mixture of Co-NaX-CO and Ag-NaX catalysts shows a synergy effect on NO reduction to N₂ by CH₄ in the presence of excess O₂ and H₂O, but the NO reduction decreases quickly as time passes. However, the NO reduction to N₂ in a deNO bed at 523 K and a deNO₂ bed at 423 K, which are relatively lower than the reaction temperatures for common SCR systems, still remained at 67% even in a H₂O 10% gas mixture after 160 min.     

NO decomposition and reduction by CH4 over Sr/La2O3

Both NO decomposition and NO reduction by CH₄ over 4%Sr/La₂O₃ in the absence and presence of O₂ were examined between 773 and 973 K, and N₂O decomposition was also studied. The presence of CH₄ greatly increased the conversion of NO to N₂ and this activity was further enhanced by co-fed O₂. For example, at 773 K and 15 Torr NO the specific activities of NO decomposition, reduction by CH₄ in the absence of O₂, and reduction with 1% O₂ in the feed were 8.3·10⁻⁴, 4.6·10⁻³, and 1.3·10⁻² μmol N₂/s m², respectively. This oxygen-enhanced activity for NO reduction is attributed to the formation of methyl (and/or methylene) species on the oxide surface. NO decomposition on this catalyst occurred with an activation energy of 28 kcal/mol and the reaction order at 923 K with respect to NO was 1.1. The rate of N₂ formation by decomposition was inhibited by O₂ in the feed even though the reaction order in NO remained the same. The rate of NO reduction by CH₄ continuously increased with temperature to 973 K with no bend-over in either the absence or the presence of O₂ with equal activation energies of 26 kcal/mol. The addition of O₂ increased the reaction order in CH₄ at 923 K from 0.19 to 0.87, while it decreased the reaction order in NO from 0.73 to 0.55. The reaction order in O₂ was 0.26 up to 0.5% O₂ during which time the CH₄ concentration was not decreased significantly. N₂O decomposition occurs rapidly on this catalyst with a specific activity of 1.6·10⁻⁴ μmol N₂/s m² at 623 K and 1220 ppm N₂O and an activation energy of 24 kcal/mol. The addition of CH₄ inhibits this decomposition reaction. Finally, the use of either CO or H₂ as the reductant (no O₂) produced specific activities at 773 K that were almost 5 times greater than that with CH₄ and gave activation energies of 21–26 kcal/mol, thus demonstrating the potential of using CO/H₂ to reduce NO to N₂ over these REO catalysts.     

含氨燃料预混火焰的层流火焰速度及NO排放特性

将甲烷或氢气与氨气共燃可以克服NH₃火焰的点火能量高、燃烧速度慢的缺点。为了解NH₃作为燃料的燃烧特性,对含NH₃燃料进行一维层流预混火焰数值模拟,研究其层流火焰速度及NO排放特性。采用文献中5个简化反应机理进行数值计算,发现Okafor机理模拟NH₃/CH₄/air火焰精度更高;Xiao机理模拟NH₃/H₂/air、NH₃/air精度适中,计算时间较短。此外,开展了当量比、燃料混合物组分比例、压力等参数对含NH₃燃料燃烧时烟气中NO浓度影响的研究。研究发现：含NH₃燃料燃烧时NO主要通过OH、H、O自由基和O₂分子的消耗而生成,主要通过与NH_i（i=0, 1, 2）自由基反应消耗;含NH₃燃料在富燃状态下燃烧可有效减少NO排放,但富燃燃烧效率低,可采用富燃-贫燃分级燃烧技术来提高燃烧效率,同时保持NO的低排放;掺有较多NH₃的含NH₃燃料在中高压下燃烧时可有效减少NO排放。     

Investigation of CO oxidation and NO reduction on three-way monolith catalysts with transient response techniques

The kinetics of CO oxidation and NO reduction reactions over alumina and alumina-ceria supported Pt, Rh and bimetallic Pt/Rh catalysts coated on metallic monoliths were investigated using the step response technique at atmospheric pressure and at temperatures 30–350°C. The feed step change experiments from an inert flow to a flow of a reagent (O₂, CO, NO and H₂) showed that the ceria promoted catalysts had higher adsorption capacities, higher reaction rates and promoting effects by preventing the inhibitory effects of reactants, than the alumina supported noble metal catalysts. The effect of ceria was explained with adsorbate spillover from the noble metal sites to ceria. The step change experiments CO/O₂ and O₂/CO also revealed the enhancing effect of ceria. The step change experiments NO/H₂ and H₂/NO gave nitrogen as a main reduction product and N₂O as a by-product. Preadsorption of NO on the catalyst surface decreased the catalyst activity in the reduction of NO with H₂. The CO oxidation transients were modeled with a mechanism which consistent of CO and O₂ adsorption and a surface reaction step. The NO reduction experiments with H₂ revealed the role of N₂O as a surface intermediate in the formation of N₂. The formation of NN bonding was assumed to take place prior to, partly prior to or totally following to the NO bond breakage. High NO coverage favors N₂O formation. Pt was shown to be more efficient than Rh for NO reduction by H₂.     

Experimental and numerical study of applicability of porous combustors for HCl synthesis

Porous combustors have been studied intensively concerning the combustion of natural gas. The advantages of combustion in porous inert media, such as low emissions, high power turndown ratio of typically 10:1 and compactness, can also be used for different chemical gas phase reactions, e.g. the HCl synthesis from H₂ and Cl₂. The advantages of porous reactors result from the heat transport properties of the porous medium, i.e. emissivity and conductivity. Heat transport mechanisms and chemical reactions were implemented in a numerical code in order to investigate the H₂/Cl₂ system. Important parameters of the reaction, e.g. the laminar flame speed and the adiabatic flame temperature, are higher for the H₂/Cl₂ reaction compared to the CH₄/air combustion. By studying the influence of H₂O and HCl as inert components it was shown by numerical investigations that the maximum temperature could be decreased to a level, which makes the usage of a porous reactor feasible. A porous reactor for laboratory use was tested with O₂/CH₄/N₂ combustion, which delivers even higher adiabatic temperatures and flame speeds than the H₂/Cl₂ reaction. Finally, experiments with H₂/Cl₂/HCl reaction were carried out and first results are presented.     

Circumventing fuel-NOx formation in catalytic combustion of gasified biomass

It has been shown that it is possible to decrease fuel-NO_x produced from NH₃ using catalytic combustion of a synthetic gasified biomass at fuel-lean conditions. In a certain temperature regime where the conversion of fuel components, such as CO, H₂ and CH₄, is low and conversion of NH₃ is high, it is suggested that the formed NO_x is reduced by the remaining fuel components, mainly hydrocarbons. With oxide catalysts only ca. 10% of the NH₃ was converted to NO_x, the rest to N₂. It has also been shown that the ignition sequence of CO, H₂ and CH₄ varied for different catalysts and different experimental conditions, and that methane coupling and methanation reactions occurred before ignition of CH₄.     

Carbon oxidation with platinum supported catalysts

The effect of the support oxide, Pt precursor and reactant gas composition on the catalysis of soot oxidation was investigated using carbon black as a model soot and simulated exhaust gases. The Pt precursors used were Pt(NH₃)₄(OH)₂, H₂PtCl₆·6H₂O, Pt(NH₃)₄(NO₃)₂, and Pt(NH₃)₄Cl₂. The support metal oxides used were SiO₂, Al₂O₃, and ZrO₂. Pt/SiO₂ prepared from Pt(NH₃)₄(OH)₂ showed the highest carbon oxidation activity. It had much higher activity in the condition of N₂+O₂+H₂O+NO+SO₂ than without NO and SO₂.