Numerical evaluation of NOx mechanisms in methane-air counterflow premixed flames

The control of nitrogen oxides (NO_x) has been a major issue in designing combustion systems, since NO_x play a key role in ozone depletion and the generation of photochemical smog. The characteristics of NO_x emission can be essential information for the development of a clean combustor having suitable reduction methodologies. In the present study, NO_x emission characteristics were evaluated numerically, accounting for the effect of equivalence ratio, stretch rate, pressure, and initial temperature. In general, peak NO_x emission appeared near the equivalence ratio of unity case, and NO_x emission increased with pressure and initial temperature due to the temperature sensitivity in NO_x mechanism. NO_x decreased with stretch rate due to the decrease in residence time in high temperature region. Furthermore, the thermal and prompt mechanisms were evaluated with equivalence ratio for two calculation methods. The conventional methods ignore the interaction of coupled mechanism of thermal and prompt NO_x. The reaction path diagram was introduced to understand effective reaction pathways in various conditions. This paper was recommended for publication in revised form by Associate Editor Kyoung Doug Min Dr. Eun-Seong Cho received his B.S. and M.S. degrees in Mechanical Engineering from Hanyang University, Korea, in 1996 and 1998, respectively. He then received his Ph.D. degree from Seoul National University, Korea, in 2005. He was a principal engineer of KD Navien research center and currently a research associate at Delft University of Technology, The Netherlands. His research interests include eco-friendly clean combustion technology, new and renewable energy systems. Prof. Suk Ho Chung received his B.S. degree from Seoul National University, Korea, in 1976 and Ph.D. degree in Mechanical Engineering from Northwestern University, USA, in 1983. He is a Professor since 1984 in the School of Mechanical and Aerospace Engineering at Seoul National University in Seoul, Korea. His research interests cover combustion fundamentals, pollutant formation, laser diagnostics, and plasma-assisted combustion.     

Effects of dimethyl-ether (DME) spray behavior in the cylinder on the combustion and exhaust emissions characteristics of a high speed diesel engine

The purpose of this study was to analyze the exhaust emissions of DME fuel through experimental and numerical analyses of in-cylinder spray behavior. To investigate this behavior, spray characteristics such as the spray tip penetration, spray cone angle, and spray targeting point were studied in a re-entrant cylinder shape under real combustion chamber conditions. The combustion performance and exhaust emissions of the DME-fueled diesel engine were calculated using KIVA-3V. The numerical results were validated with experimental results from a DME direct injection compression ignition engine with a single cylinder.The combustion pressure and IMEP have their peak values at an injection timing of around BTDC 30°, and the peak combustion temperature, exhaust emissions (soot, NO_x), and ISFC had a lower value. The HC and CO emissions from DME fuel showed lower values and distributions in the range from BTDC 25° to BTDC 10° at which a major part of the injected DME spray was distributed into the piston bowl area. When the injection timing advanced to before BTDC 30°, the HC and CO emissions showed a rapid increase. When the equivalence ratio increased, the combustion pressure and peak combustion temperature decreased, and the peak IMEP was retarded from BTDC 25° to BTDC 20°. In addition, NO_x emissions were largely decreased by the low combustion temperature, but the soot emissions increased slightly.     

Numerical analysis on combustion process and sodium transformation behavior in a 660 MW supercritical face-fired boiler purely burning high sodium content Zhundong coal

CHEMKIN software was used to optimize the reaction mechanism of sodium in flue gas to study the influence of targeted design for purely burning Zhundong (ZD) coal on boiler characteristics. Then, the optimized 32-step elemental reaction was combined with CFD software. An eddy-dissipation concept model considering detailed chemical reactions was used to simulate the transformation behavior of sodium-containing substances. The combustion characteristics of the 660 MW face-fired boiler under various loads were also simulated. The field distribution in the furnace and the migration path of sodium along the track of pulverized coal particles were obtained. The results show that the interference between each burner in the furnace is small at the BMCR load, and the phenomenon of “wind wrapping fire” is distinctly clear. The temperature at furnace outlet is approximately 970.98 °C. At a low load, the combustion in the furnace is stable, and the temperature at the furnace outlet reaches the design value. The sodium present in ZD coal is involved in the reaction after it is released in the form of Na and NaCl. Sodium is present in different forms in the main burner zone, mainly NaCl (67%), NaOH (12%), Na (9%), and Na₂SO₄ (7%). The forms of sodium at the furnace outlet are NaCl (50%), Na₂SO₄ (37%), Na₂Cl₂ (9%) and NaHSO₄ (4%). A small amount of Na₂SO₄ is formed by NaHSO₄ reaction in the main burner zone. It then reacts to form NaSO₄, wherein NaHSO₄ is formed by path 2. Na₂SO₄ is mainly generated in the burnout zone through path 1, and paths 2, 3, and 4 are hardly observed. The findings of this research can provide reference for the design of a purely fired ZD coal boiler and further studies on slagging observed on the heating surface.     

The mechanism characterizations of methane steam reforming under coupling condition of temperature and ratio of steam to carbon

To elucidate the coupling effects of temperature and ratio of steam to carbon on the methane steam reforming process, the characterizations of methane steam reforming at different temperature and ratio of steam to carbon in term of distribution of H₂ and CO, and the elementary reaction rate were investigated. Meanwhile, the formation mechanisms of H₂ and CO via sensitivity analysis and reaction path analysis were obtained. The results showed that the coupling effects of temperature and ratio of steam to carbon on the methane steam reforming were higher than that of individual factor. The effects of temperature on the methane steam reforming were higher than that of the ratio of steam to carbon. The adsorption and desorption reaction of CH₄ on the surface of Ni-based catalyst had the most obvious effect on the sensitivity of H₂, CH₄ and CO. Besides, the effects of adsorption and desorption reaction of H₂O on the sensitivity of H₂ were higher than that of CH₄ and CO. Hydrogen was generated by the desorption reaction of H(s) in the adsorbed state and from three generating paths: a) CH₄(s) dissociated directly or reacted with O(s) to form H(s); b) The dissociation reaction of H₂O(s) produced H(s); c) OH(s) dissociated directly or reacted with C(s) to form H(s). Carbon monoxide was generated from single path: CH₄(s)→CH₃(s)→CH₂(s)→CH(s)→C(s)→CO(s)→CO(g).     

Reduction of a detailed kinetic model for the ignition of methane/propane mixtures at gas turbine conditions using simulation error minimization methods

Natural gas is the primary fuel for industrial gas turbines, which provide about one quarter of the world’s primary energy supply. Beside methane it also contains larger hydrocarbons in small, varying ratios. This variation is expected to rise due to the increasing usage of non-traditional gas sources. Fuel composition has a large impact on auto-ignition delay time, which is a fundamental parameter for the optimal design and operation of gas turbines. For the oxidation of such mixtures, Curran, Petersen and co-workers recently developed a detailed reaction mechanism (NUIG NGM), which reproduces the ignition delays over a wide range of conditions. However, due to its large size: 229 species and 1359 reactions, it cannot be used in computational fluid dynamics simulations, which is an important fundamental tool in the development of gas turbines. A mechanism reduction case study of the NUIG NGM is presented using the recently developed simulation error minimization methods (SEM). A new version of the SEM program package is also proposed, which allows the reduction of mechanisms for a wider range of combustion phenomena. Combinational strategies have been introduced in the SEM connectivity method to enhance the reduction procedure and a hierarchical reduction procedure is proposed for multi-scenario problems. Ignition of lean and stoichiometric mixtures containing 90% methane and 10% propane as fuel were investigated for 22 conditions relevant to gas turbines, covering temperature and pressure ranges of 877–1465 K and 7–40 atm, respectively. The smallest reduced mechanism developed contains 50 species and 186 reactions. It can reproduce ignition delays with 3.1% maximum error and reproduces pressure rise precisely (error∼10⁻³%). The mechanism can be simulated 62 times faster than the full mechanism. Robustness analysis showed that it is reliably applicable over a much wider range of conditions compared to that for which it was developed.     

Analysis of the kinetics for the H2 + - \frac{1}{2}O2 ⇌ H2O reaction on a hot Pt surface in the pressure range 0.10–10 Torr

The H₂ + O₂ ⇌ H₂O reaction on platinum at 700 and 1300 K has been studied. A stagnation flow geometry was used with a gas mixture of H₂ and O₂ at pressures between 0.10 and 10 Torr. Comparing SHG results with simulations using different reaction parameters, it was concluded that , and . LIF measurements showed an ambiguity in the choice of main water-producing channel. Both hydrogen addition with low sticking coefficients and hydroxyl disproportionation with high sticking coefficients are plausible. This revised version was published online in July 2006 with corrections to the Cover Date.     

介质阻挡放电低温等离子体转化NO的数值模拟

为探索介质阻挡放电低温等离子体反应器转化NO的反应规律,利用CHEMKIN软件的PLASMA PFR模型,编制气相动力学文件及热动力学文件,进行数值模拟.研究了O2初始浓度、气体温度、气体流速等反应条件对反应过程和反应最终产物的影响,探讨了C2H4添加物对NO转化效率的影响.反应最终产物及敏感性分析表明,C2H4能较为有效地提高NO转化为NO2的效率.通过模拟结果和实验结果的对比分析,论证了PLASMA PFR模型模拟介质阻挡放电低温等离子体反应器转化NO的可行性.     

水蒸汽对焦炉气火焰燃烧速度及NO生成特性影响研究

以山西华鑫肥业有限公司富氧空气转化炉为研究对象,大型化学动力学软件CHEMKIN为计算平台,采用包含53种组分、325个基元反应的甲烷燃烧详细反应机理(GRI-Mech 3.0),研究了水蒸汽对富氧空气转化炉烧嘴火焰燃烧速度及NO生成特性的影响。计算结果表明:H2O对燃烧速度的影响主要是通过解离出的H自由基参与燃烧反应引起的;H2O含量的提高可以有效抑制NO生成速率。     

燃煤烟气NO/SO_2对Cl/Cl_2形成过程的影响机制

燃煤烟气中Cl/Cl2的形成过程对Hg0的转化有重要影响。应用CHEMKIN软件包,在典型燃煤烟气温降速率下,通过化学动力学模拟的方法,研究了NO/SO2对Cl/Cl2形成过程的影响,主要得出以下结论:Cl原子形成过程中OH是重要反应物,NO抑制Cl原子形成的原因是由于NO会与Cl原子生成反应竞争反应物OH。另外,NO与OH反应后的产物HONO会与Cl原子反应生成HCl和NO2,从而进一步降低烟气中的Cl原子浓度。SO2会大量消耗反应体系中的O原子,而O原子主要通过OH转化生成,O原子的大量消耗促进了OH转化为O原子,使得OH在Cl原子形成过程中的作用被削弱,从而使SO2抑制了Cl原子形成。Cl2主要通过Cl原子转化形成,因此Cl原子的形成受到抑制将同时导致Cl2形成过程受到抑制。     

An elementary reaction-kinetic model for the gas-phase formation of 1,3,6,8- and 1,3,7,9-tetrachlorinated dibenzo-p-dioxins from 2,4,6-trichlorophenol

A 71-step reaction-kinetic model for the formation of 1,3,6,8- and 1,3,7,9-tetrachlorodibenzo-p-dioxins (TCDDs) from the oxidation of 2,4,6-trichlorophenol in the presence of hexane is developed based on experimental data and a simpler model that was previously published in the literature. The rate of reaction of phenoxyl-radicals with molecular oxygen has recently been experimentally demonstrated to be at least five orders of magnitude slower than the rate used in the previous model. With this correction, inclusion of radical-radical recombination reactions of 2,4,6-trichlorophenoxyl-radicals, and other minor modifications, the revised model yields satisfactory agreement between experimental and predicted concentrations of TCDDs above 900 K. The results of this study indicate that high-temperature, gas-phase reactions of chlorinated phenols that contain an ortho-chlorine will form poly-chlorinated dibenzo-p-dioxins (PCDDs) in yields approximately four to five orders of magnitude greater than believed, based on the previous modeling results.