煤燃烧中影响NOx生成因素的分析

概述了煤燃烧中空气分级燃烧技术,分析了NOx生成的影响因素。影响NOx形成的主要锅炉操作参数为:初级燃烧区化学计量比为0.7～0.9,停留时间较长时,NOx的生成量较少;温度的影响与环境的氧化/还原气氛有关;增加过量空气,NOx生成量增加。影响NOx生成的最主要的煤性质是氮含量和挥发分,氮含量的影响与燃烧所采用的控制技术有关,随挥发分含量增加,NOx的生成量减少。     

燃烧条件对燃煤过程多环芳烃生成特征的影响

采用小型管式实验炉,通过控制燃烧温度、过剩空气量研究了不同燃烧条件对燃煤过程16种多环芳烃(PAHs)生成与分布的影响。实验结果表明:燃煤烟气中PAHs生成量随燃烧温度的升高而增加,灰渣中PAHs生成量随燃烧温度的升高呈先增加后减少的规律;随着过剩空气系数(α)的增大,烟气中PAHs生成量逐渐减少,灰渣中PAHs生成量呈先明显减少而后基本不变的趋势;PAHs组成的分布表现出三、四环含量突出的特征;烟气中PAHs毒性当量(TEQ)远大于灰渣;在保证煤完全燃烧的情况下,综合考虑对烟气和灰渣中PAHs生成总量以及毒性当量TEQ的控制,燃烧条件为温度为850℃,α=1.57时,PAHs对环境的影响较小。     

发生炉造气过程中NO_x及前驱体生成的探讨

结合煤气发生炉的造气原理和过程,对煤氮在发生炉热解、还原、燃烧过程中的转化以及NOx与前驱体的生成进行了定性的分析。指出发生炉热解、气化过程中,煤氮一部分转化为焦油;一部分以NH3、HCN、N2形式转化为煤气;另外一部分残存于灰渣中。通过分析,说明一段式发生炉、两段式发生炉和干馏式发生炉三种炉型在气化过程中,NH3、HCN和N2的生成量基本没有差异;NH3和HCN主要来源于气化过程;而热解过程次之,但干馏式发生炉在煤的热解过程中NH3和HCN的生成量最少。     

掺配BDO焦油水煤浆燃烧性能及动力学分析

利用热重分析法研究了掺配BDO焦油水煤浆的燃烧性能及燃烧反应动力学,并结合XRD和SEM-EDX分析了掺配BDO焦油对水煤浆燃烧性能的影响。结果表明:浆体失重过程主要分为三个阶段,在第二阶段由于BDO焦油中大部分组分失重,随着掺配BDO焦油质量分数的增加,失重速率增大;在第三阶段TG曲线向低温区偏移,说明掺配BDO焦油促进了浆体燃烧。由于BDO焦油中Na~+在煤颗粒的表面、边缘和煤颗粒之间形成燃烧的活化中心,使得掺配BDO焦油浆体的(dw/dt)_(mean)和R_w等燃烧性能参数均优于原煤水煤浆的相应燃烧性能参数;在选取Coats-Redfern近似时,水煤浆燃烧在选取反应级数n=1时线性相关性最高,掺配BDO焦油的浆体表观活化能均低于原煤浆体的表观活化能,频率因子均大于原煤浆体的频率因子。     

固体热载体煤热解过程中氮的迁移

在固体热载体煤热解实验装置上考察了石英砂、燃烧灰和气化半焦为热载体对煤热解过程中氮迁移的影响。固体热载体煤热解过程中,热解温度升高有利于煤中挥发分析出,可以促进煤中含氮官能团发生断键,利于氮的脱除。快速热解可促进煤中氮脱除生成HCN和NH_3。分别以石英砂、燃烧灰和气化半焦为热载体,研究表明:热载体中矿物质可促进焦油氮分解;通过气化半焦和脱矿物质气化半焦为热载体研究发现,气化半焦为热载体时,高比表面积和孔结构可延长在其表面停留时间,对焦油氮分解起主要作用。     

循环流化床中谷壳与煤共燃SO2生成特性研究

在循环流化床实验台上,对谷壳与煤共燃的SO2生成特性进行了实验研究,着重研究了床温、过量空气系数对SO2生成特性的影响.实验表明,谷壳与煤共燃能够使煤燃烧的SO2生成量降低50%以上;加入谷壳的比例存在一个最佳范围,不超过30%;共燃时SO2的生成量随床温的升高而增加,SO2的减排率随床温的升高呈起伏变化,温度为850℃时减排率最大;共燃时过量空气系数变化对SO2的生成量无显著影响.     

CO2气氛对伊宁煤热解过程中酚分布的影响

以伊宁煤为原料,将其粉碎至5 mm～8 mm粒径置入固定床热解炉中,分别在N2和CO2两种气氛下程序升温至终温为500℃,600℃和700℃,收集热解产物并用GC-MS联用仪定量分析了焦油中的7种酚类化合物,对比分析了两种气氛下热解产物的分布,考察了CO2热解气氛对焦油及酚类化合物生成的影响.结果表明,CO2气氛能促进焦油的生成.700℃时,CO2气氛下煤热解生成总酚量为N2气氛下生成总酚量的1.3倍.两种气氛下总酚的生成量均随热解终温的升高而减少.CO2气氛还影响了酚类化合物中不同种类酚所占的比例,特别是二甲基苯酚在总酚中的比例不断增大,700℃达到最大,为7.66％.     

煤热解过程分析与工艺调控方法

通过煤热解技术获取紧缺的油气资源是低阶煤清洁利用的有效途径之一。针对煤热解工艺存在焦油产率与品质难以控制以及焦油中粉尘含量高等关键技术问题,从煤的热解反应机理出发,详细探讨了热解挥发分二次反应的种类和发生条件以及影响热解过程的主要因素,结合煤热解技术应用,总结了逆向传热与传质所导致的挥发分气相二次反应是焦油产率下降的主要原因;同时,分析了热解过程中煤颗粒破碎机理以及煤热解过程中粉尘的主要来源。在前人研究结果的基础上,提出控制热解挥发分的流动方向从高温区向低温区流动、热解耦合气化以及耦合原位的焦油提质与除尘等方法可以调控煤热解过程,抑制重质焦油生成、提高焦油中轻质组分含量以及减少焦油中的含尘量,从而实现煤的定向热解。     

超细煤粉燃烧过程中污染物生成特性的研究

利用国产wRT-3P型微热量天平与英国产KM91006烟气分析仪联用对不同变质程度的超细煤粉进行了超细煤粉低温(小于800℃)燃烧实验,并应用QuinTox—Firework软件对燃烧过程中NO。和SO2的生成量进行跟踪检测、数据采集和整理。实验表明煤经超细化处理后,煤粉燃烧过程中NOx和SO2的生成特性发生了改变。同时,超细煤粉在燃烧过程中产生的CO延缓了煤表面发生的强氧化反应,有利于H2S与煤中其他矿物质的反应生成硫化物如CaS。     

煤热解过程分析与工艺调控方法

通过煤热解技术获取紧缺的油气资源是低阶煤清洁利用的有效途径之一。针对煤热解工艺存在焦油产率与品质难以控制以及焦油中粉尘含量高等关键技术问题,从煤的热解反应机理出发,详细探讨了热解挥发分二次反应的种类和发生条件以及影响热解过程的主要因素,结合煤热解技术应用,总结了逆向传热与传质所导致的挥发分气相二次反应是焦油产率下降的主要原因;同时,分析了热解过程中煤颗粒破碎机理以及煤热解过程中粉尘的主要来源。在前人研究结果的基础上,提出控制热解挥发分的流动方向从高温区向低温区流动、热解耦合气化以及耦合原位的焦油提质与除尘等方法可以调控煤热解过程,抑制重质焦油生成、提高焦油中轻质组分含量以及减少焦油中的含尘量,从而实现煤的定向热解。     

Soot generation of diesel fuels with substantial amounts of oxygen-bearing compounds added

Two-colour pyrometry, thermodynamic analysis, and exhaust emissions analysis have been used to improve understanding of the formation of soot during combustion in a high speed direct-injection automotive diesel engine. Three fuel blends were used: a Base Fuel commercially available in Northern Europe; a blend of the Base Fuel (70%) and esterified rape-seed oil (RME) (30%) and a blend of the Base Fuel (90%) and an ether compound (diglyme) (10%). While the Base Fuel contained no oxygen, both the other two fuels contained equal amounts of oxygen of 3% by mass. The principal results show significant differences in soot generation during combustion between the two oxygenated fuel blends, despite both having the same amount of oxygen.     

Oxidative Reactivity of Particles Emitted from a Diesel Engine Operating at Light Load with EGR

The impact of exhaust gas recirculation (including three levels: 0, 10%, and 30%) on engine combustion characteristics, gaseous emissions, and particulate properties (i.e., oxidative reactivity, carbonaceous compositions, size distribution, and nanostructure) was studied on a common rail diesel engine operating at low engine load. The results showed that the lack of oxygen with EGR prolongated ignition delay and the premixed portion of combustion started to rise significantly. Higher EC (accumulation mode) with larger particle size could be observed with increasing EGR from 0 to 30%, which is attributed to the promotion of soot formation with less available oxygen and the inhibition of soot oxidation with low in-cylinder temperature with increasing EGR. The soot nanostructure observation showed that soot changed from smooth surface under 0 EGR to rugose surface under 10% EGR. Moreover, the amorphous core turned larger with increasing EGR. With increasing EGR to 30%, the amorphous core appeared to include the whole primary soot particle. The increase of accessible carbons on the edge sites correlates with the high reactivity with increasing EGR. Through the quantitative analysis of the correlation between the combustion parameters and particle properties, we speculated that in this work, the engine at low load producing very little to no conventional soot or soot-EC coupling with low combustion temperature and short residence time with increasing EGR lead to the soot exhibiting less carbonization level (short fringe length and large fringe curvature) and result in higher reactivity.

Copyright 2015 American Association for Aerosol Research     

Rb_2O掺杂对MnCe/ZrO_2催化燃烧碳烟的性能影响

采用柠檬酸溶胶-凝胶法制备掺杂不同Rb2O含量的MnCe/ZrO2复合氧化物催化剂,采用热重法考察其催化燃烧碳烟的活性,并借助XRD和H2-TPR手段探讨掺杂不同Rb2O含量对MnCe/ZrO2催化燃烧特性的影响。结果表明,适量Rb2O可以使催化剂与碳烟在高温下接触更为紧密,吸附空气中CO2,形成碳酸盐物质,提高催化剂的氧含量,并作为活性氧传输中心,提升MnCe/ZrO2催化燃烧性能。与未掺杂MnCe/ZrO2催化剂相比,完全燃烧温度下降31℃,碳烟在394℃内完全氧化。     

A novel catalyst for diesel soot oxidation

In this study, cobalt and lead based mixed oxide catalysts were tested for their soot oxidation ability. In addition to a mixed oxide formerly marketed as ceramic paint, a home made set was also prepared by incipient wetness impregnation of a cobalt oxide powder with a lead acetate solution and subsequent calcination. The materials investigated in this study were shown to decrease the peak combustion temperature of home made soot from 500 to 385 °C in air. Soot oxidation tests under inert (N₂) atmospheres revealed that the oxidation took place by using the lattice oxygen of the catalyst. Reaction temperature could be further decreased when these mixed oxide catalysts were impregnated with platinum. An optimum platinum loading was determined as 0.5 wt% based on the peak combustion temperature of the soot. The role of Pt was to assist the oxygen transfer from the gas phase to the lattice. It was observed that NO₂ is a better oxidizing agent as compared to air whereas NO had hardly any activity against soot oxidation reaction. When the mixed oxide catalyst was impregnated with platinum, the peak combustion temperature was measured as 310 °C in the presence of NO_x and air. The catalyst's unique performance was in terms of the rate of soot oxidation. Under the experimental conditions studied here, the soot oxidation was so facile that the oxygen in the gas phase was completely depleted. This stream of oxygen depleted and CO enriched gas phase can be used to reduce NO_x in the presence of a downstream or a co-catalyst.     

Uncertainty analysis of heat release rate measurement from oxygen consumption calorimetry

Oxygen consumption calorimetry remains the most widespread method for the measurement of the heat release rate from experimental fire tests. In a first step, this paper examines by theoretical analysis the uncertainty associated with this measurement, especially when CO and soot corrections are applied. Application of theoretical equations is presented for chlorobenzene which leads to high values of CO and soot yields. It appears that the uncertainty of CO and soot corrections are high when the fuel composition is unknown. In a second step, a theoretical analysis is provided when the simplest measurement procedure is used for oxygen consumption calorimetry. The overall uncertainty can be dominated either by the uncertainty associated with the oxygen concentration, the assumed heat of combustion, the fumes mass flow rate or the assumed combustion expansion factor depending on the oxygen depletion. Copyright © 2005 John Wiley & Sons, Ltd.     

水蒸气纯氧条件下合成气燃烧特性

水蒸气纯氧燃烧技术因具有高效零污染物排放的特点而备受关注,对合成气在水蒸气纯氧条件下的燃烧特性进行了实验研究。在扩散燃烧实验台上测量了H₂O/O₂为2.0时,燃烧室中心气体成分和火焰温度随停留时间的变化规律,分析了过量氧气系数对合成气水蒸气纯氧燃烧过程的影响。研究结果表明:过量氧气系数为0时,H₂和CO的燃烧主要在前28 ms内,H₂的燃烧速率较快,能够快速燃尽;CO燃烧较慢,燃烧室出口含量依然很高。过量氧气系数从0增大到10%时,CO的浓度整体降低,燃烧速率提高,燃烧前期火焰温度提高。燃烧室出口CO浓度随过量氧气系数的增加逐渐降低,氧气微过量时CO浓度迅速下降,继续增大时,燃烧室出口CO的浓度下降缓慢。     

A novel catalyst for diesel soot oxidation

In this study, cobalt and lead based mixed oxide catalysts were tested for their soot oxidation ability. In addition to a mixed oxide formerly marketed as ceramic paint, a home made set was also prepared by incipient wetness impregnation of a cobalt oxide powder with a lead acetate solution and subsequent calcination. The materials investigated in this study were shown to decrease the peak combustion temperature of home made soot from 500 to 385 °C in air. Soot oxidation tests under inert (N₂) atmospheres revealed that the oxidation took place by using the lattice oxygen of the catalyst. Reaction temperature could be further decreased when these mixed oxide catalysts were impregnated with platinum. An optimum platinum loading was determined as 0.5 wt% based on the peak combustion temperature of the soot. The role of Pt was to assist the oxygen transfer from the gas phase to the lattice. It was observed that NO₂ is a better oxidizing agent as compared to air whereas NO had hardly any activity against soot oxidation reaction. When the mixed oxide catalyst was impregnated with platinum, the peak combustion temperature was measured as 310 °C in the presence of NO_x and air. The catalyst's unique performance was in terms of the rate of soot oxidation. Under the experimental conditions studied here, the soot oxidation was so facile that the oxygen in the gas phase was completely depleted. This stream of oxygen depleted and CO enriched gas phase can be used to reduce NO_x in the presence of a downstream or a co-catalyst.     

The effect of oxygen and carbon dioxide concentration on soot formation in non-premixed flames

The influence of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation was studied by Time Resolved Laser Induced Incandescence (TIRE-LII) and TEM photography in non-premixed co-flowing flames. TIRE-LII method was used to measure the distribution of two-dimensional soot volume fraction and primary particle size. The soot was directly sampled by the thermophoretic method, and its diameter was examined by TEM photography. Two suitable delay times of the TIRE-LII method affecting measurable range and sensitivity were determined by comparing TEM photographs with the TIRE-LII signal. The effects of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation were investigated with these calibrated techniques. An O₂+(CO₂, N₂, and [Ar+CO₂]) mixtures in co-flow were used to isolate carbon dioxide effects systematically. The primary particle number concentration and soot volume fraction were abruptly decreased by the addition of carbon dioxide to co-flow. This suppression was resulted from the short residence time in inception region because of the late nucleation and the decrease of surface growth distance by the low flame temperature due to the higher thermal capacity and the chemical change of carbon dioxide. The increase of oxygen concentration in the co-flow caused an enhancement of soot nucleation and thus the residence time increase, but the specific growth rate showed almost the same value regardless of the co-flow mixture in the growth region. This result suggests that the specific growth rate has a weak dependence on the relative change of co-flow conditions in non-premixed co-flowing flames.     

High-throughput approach to the catalytic combustion of diesel soot

A methodology for the evaluation of diesel soot oxidation catalysts by high-throughput (HT) screening was developed. The optimal experimental conditions (soot amount, catalyst/soot ratio, type of contact, composition and flow rate of gas reactants) ensuring a reliable and reproducible detection of light-off temperatures in a 16 parallel channels reactor were set up. The temperature profile measured in the catalyst/soot bed under TPO conditions when the exothermic combustion of soot takes place was shown to provide an accurate measurement of the ignition. Its reproducibility and relevance were checked. The results obtained with a reference noble metal free catalyst (La_0.8Cr_0.8Li_0.2O₃ perovskite) agree very well with literature data. Qualitative mechanistic features could be derived from these experiments, stressing the likely limiting step of oxygen transfer from catalyst surface to soot particulates to ignite the soot combustion. Ceria material was shown to be more appropriate than perovskite one. From an HT screening of a large diverse library (over 100 mixed oxides catalysts) under optimized conditions, about 10 new formulations were found to perform better than selected noble metal free reference materials.     

Coal char combustion under a CO2-rich atmosphere: Implications for pulverized coal injection in a blast furnace

Pulverized coal injection (PCI) is employed in blast furnace tuyeres attempting to maximize the injection rate without increasing the amount of unburned char inside the stack of the blast furnace. When coal is injected with air through the injection lance, the resolidified char will burn in an atmosphere with a progressively lower oxygen content and higher CO₂ concentration. In this study an experimental approach was followed to separate the combustion process into two distinct devolatilization and combustion steps. Initially coal was injected into a drop tube furnace (DTF) operating at 1300 °C in an atmosphere with a low oxygen concentration to ensure the combustion of volatiles and prevent the formation of soot. Then the char was refired into the DTF at the same temperature under two different atmospheres O₂/N₂ (typical combustion) and O₂/CO₂ (oxy-combustion) with the same oxygen concentration. Coal injection was also performed under a higher oxygen concentration in atmospheres typical for both combustion and oxy-combustion. The fuels tested comprised a petroleum coke and coals currently used for PCI injection ranging from high volatile to low volatile bituminous rank. Thermogravimetric analyses and microscopy techniques were used to establish the reactivity and appearance of the chars.