Effects of CO_2 Dilution on Methane Ignition in Moderate or Intense Low-oxygen Dilution(MILD) Combustion:A Numerical Study

Homogeneous mixtures of CH4/air under moderate or intense low-oxygen dilution（MILD） combustion conditions were numerically studied to clarify the fundamental effects of exhaust gas recirculation（EGR）,espe-cially CO2 in EGR gases,on ignition characteristics.Specifically,effects of CO2 addition on autoignition delay time were emphasized at temperature between 1200 K and 1600 K for a wide range of the lean-to-rich equivalence ratio（0.2~2）.The results showed that the ignition delay time increased with equivalence ratio or CO2 dilution ratio.Fur-thermore,ignition delay time was seen to be exponentially related with the reciprocal of initial temperature.Special concern was given to the chemical effects of CO2 on the ignition delay time.The enhancement of ignition delay time with CO2 addition can be mainly ascribed to the decrease of H,O and OH radicals.The predictions of tem-perature profiles and mole fractions of CO and CO2 were strongly related to the chemical effects of CO2.A single ignition time correlation was obtained in form of Arrhenius-type for the entire range of conditions as a function of temperature,CH4 mole fraction and O2 mole fraction.This correlation could successfully capture the complex be-haviors of ignition of CH4/air/CO2 mixture.The results can be applied to MILD combustion as ＂reference time＂,for example,to predict ignition delay time in turbulent reacting flow.

Numerical Investigation of Fuel Dilution Effects on the Performance of the Conventional and the Highly Preheated and Diluted Air Combustion Furnaces

This numerical study investigates the effects of using a diluted fuel （50% natural gas and 50% N2） in an industrial furnace under several cases of conventional combustion （air with 21% O2 at 300 and 1273 K） and the highly preheated and diluted air （1273 K with 10% O2 and 90% N2） combustion （HPDAC） conditions using an in-house computer program. It was found that by applying a combined diluted fuel and oxidant instead of their uncombined and/or undiluted states, the best condition is obtained for the establishment of HPDAC＇s main unique features. These features are low mean and maximum gas temperature and high radiation/total heat transfer to gas and tubes; as well as more uniformity of theirs distributions which results in decrease in NOx pollutant formation and increase in furnace efficiency or energy saving. Moreover, a variety of chemical flame shape, the process fluid and tubes walls temperatures profiles, the required regenerator efficiency and finally the concentration and velocity patterns have been also qualitatively/quantitatively studied.

Flow characterization and dilution effects of N_2 and CO_2 on premixed CH_4/air flames in a swirl-stabilized combustor

Numerically-aided experimental studies are conducted on a swirl-stabilized combustor to investigate the dilution effects on flame stability, flame structure, and pollutant emissions of premixed CH4/air flames. Our goal is to provide a systematic assessment on combustion characteristics in diluted regimes for its application to environmentally-friendly approaches such as biogas combustion and exhanst-gas recirculation technology. Two main diluting species, N2 and CO2, are tested at various dilution rates. The results obtained by means of optical diagnostics show that five main flame regimes can be observed for Nz-diluted flames by changing excess air and dilution rate. CO2-diluted flames follow the same pattern evolution except that all the domains are shifted to lower excess air. Both N2 and CO2 dilution affect the lean blow- out （LBO） limits negatively. This behavior can be counter-balanced by reactant preheating which is able to broaden the flammability domain of the diluted flames. Flame reactivity is degraded by increasing dilution rate. Meanwhile, flames are thickened in the presence of both diluting species. NOx emissions are significantly reduced with dilution and proved to be relevant to flame stability diagrams： slight augmentation in NOx emission profiles is related to transitional flame states where instability occurs. Although dilution results in increase in CO emissions at certain levels, optimal dilution rates can still be proposed to achieve an ideal compromise.

Effect of Steam Dilution on the MILD Combustion Characteristics of Methane in a Model Combustor

Moderate or Intense Low-oxygen Dilution(MILD)combustion has low emission potential in gas turbines.The present work has investigated the performance of MILD combustion with parallel-jet burner arrangement in dry and steam-diluted conditions.The combustion tests were conducted in atmospheric pressure at various equivalence ratios from LBO(Lean Blow Out)to near-stoichiometric conditions and steam-to-air mass ratios from 0 to 0.2.A simplified chemical reactors network(CRN)model based on MILD combustion concept has been established to study the effect of steam dilution on different pathways of NO production.The experimental results show that under the same adiabatic flame temperature,the reaction zone gradually moves downstream with the increase of steam content.For the high steam content(0.2 kg/kg),the reaction zone is widely distributed,and the distribution of reaction intensity in the reaction zone is more uniform.The average lift-off height of reaction zone is proportional to the steam content.For the steam content of 0.2 kg/kg,the average lift-off height reaches 2.5 times that of the dry conditions,which brings the risk of blowout.For the adiabatic flame temperature of 1650–1900 K,the emissions of NOxare below 3×10–6(at 15%O2,dry)when the steam content varies from 0 to 0.2 kg/kg,which indicates the ultra-low emissions can be obtained under large changes in steam content.For the inlet temperature of 381 K,as the steam content increases,the Prompt NO is dominant in the total NO production.Steam dilution results in a smaller operating range with lower CO emissions.When the steam content reaches 0.2 kg/kg,compared to the dry condition,the carbon monoxide emission increases significantly.In addition,the LBO equivalence ratio of combustion with larger steam content is significantly higher.

Reaction zone characterization of counter-flow diffusion flame with diluted and preheated reactants

Reaction zone characteristics were studied using hydroxy radical planar laser-induced fluorescence （OH-PLIF） technique for a counter-flow preheated （CH4＋N2）/（Air＋N2） diluted diffusion flames. The effects of preheat temperature and dilute ratio on the reaction zone characteristics were investigated by demonstrating the OH intensity distribution and reaction zone thickness from OH-PLIF images. Under the experimental conditions of constant cold flow velocity, the results show that the OH intensity and reaction zone thickness decrease with the increase of dilute ratio at constant preheat temperature and increase with preheat temperature at fixed dilute ratio. The OH maximum intensity shifts towards the ＂lean＂ side of counter flow at constant preheat temperature, and it shifts towards the fuel side with the increase of dilute ratio of fuel stream and towards the oxidizer side with the increase of dilute ratio of oxidizer stream respectively. The feasibility of OH as a reaction zone marker in this diluted combustion is verified further. The variation of diffusion and chemical reaction rate of reactants due to preheat and dilution contributes to the reaction zone characteristics simultaneously. The effect of strain on the flame reaction zone should be included in the future work.

MILD粉体燃烧技术研究进展与关键问题分析

MILD燃烧是一种在中度或极度低氧环境下发生的温和燃烧模式,兼具高燃烧热利用率和极低NOx排放的优势,国际燃烧领域认为它是拥有巨大应用潜力的清洁燃烧技术之一。自1990年左右开展相关研究以来,对于实现气体燃料的MILD燃烧的相关技术已相对成熟,而关于煤粉和生物质等固体燃料的MILD燃烧机制和实现条件的研究仍相对缺乏。基于高动量氧化剂射流来实现内部再循环,不再需要外部高温预热来建立MILD燃烧,极大拓宽了MILD燃烧的应用范围。从颗粒弥散、受热、着火、燃烧、污染物等方面概述了MILD粉体燃料燃烧的基础特性,由于颗粒的不均匀弥散和反应,煤粉等碳基固体燃料的MILD燃烧进程较气体燃料更复杂。高速射流MILD燃烧在增加点火延迟的同时也扩展了点火区和反应区,需要对燃烧各阶段的特征和机理开展系统研究。介绍了固体燃料MILD燃烧理论设计和装备研发领域所取得的研究成果,建议通过高精度数值模拟,改进现有燃煤锅炉燃烧器、调节工艺参数以匹配MILD燃烧模式,增加焦炭颗粒的停留时间以提高燃烬率,提高燃烧稳定性并抑制包括细颗粒物在内的各类污染物排放。基于互联能源系统的整体方法,推进MILD燃烧与各类新型燃烧技术的耦合研究,尤其加强煤粉、生物质与氢、氨等可燃气体共燃特性研究,助力能源转型。探究粉体MILD燃烧中的湍流两相流特征、湍流相间传热作用以及湍流-化学耦合作用是加深对粉体MILD燃烧理解的关键,涉及多变量分析和高精度模拟,是未来研究的重点和难点。

稀燃模式下直喷氢气发动机性能试验

为研究氢气直喷发动机在稀薄燃烧下的燃油经济性、环保性和动力性,进行不同混合气浓度和不同负荷下的氢气直喷发动机燃烧特性研究。结果显示稀薄燃烧可以降低燃烧率和缸内压力升高率,并通过提高工质多变系数降低传热损失,从而显著提高热效率。混合气变稀对火焰传播的影响较大,负荷对燃烧持续期的影响较小。过量空气系数λ在1.1~1.6范围时,NO_(X)排放最高,但加浓和稀释混合气均可降低排放。负荷增加也会显著增加NO_(X)排放。在小负荷下(平均有效制动压力BMEP=0.2 MPa),当λ>1.8时,NO_(X)排放低于10-5;而在负荷较大(BMEP=1.2 MPa)时,即使λ=2.0,NO_(X)排放仍超过0.002。

本质安全型可燃气体稀释装置的研制

本文依据JJG 693-2011《可燃气体检测报警器》,针对玻璃浮子流量计存在较大的测量误差,提出选择适宜流量计量限、定点计量及气路切换的设计方案,并研发具有本质安全型可燃气体稀释装置,解决了在复杂现场校准可燃气体检测报警器的难题。

进气温度和含湿量对湿空气透平循环燃烧室实现柔和燃烧的影响研究

本文基于甲烷柔和燃烧的简化化学反应器网络模型,研究了空气进气温度和含湿量对湿空气透平(HAT)循环燃烧室内实现柔和燃烧的影响。针对HAT循环三种典型工况的研究结果表明:空气进口温度和燃烧室头部温度越高越容易实现柔和燃烧。空气含湿量对实现柔和燃烧的临界回流比的影响可以忽略,但空气含湿量的增加会引起点火延迟时间的延长,该影响持续至蒸汽含量达到其临界值,这有利于未燃反应物与烟气的充分掺混。当空气进口温度从381 K上升到881 K时,实现柔和燃烧的临界回流比下降了70%。空气加湿抑制NOx生成效果随着空气进口温度升高变得更加显著。随着空气进口温度的升高,由于热力型NO占主导地位,增加含湿量使得热力型NO迅速下降,而加湿引起快速型NO增加的效果不显著,从而使得整体NOx下降。针对模型柔和燃烧室的实验结果表明:绝热火焰温度在1 650~2 050 K内,热负荷36~59 k W条件下,不同含湿量(Ω=0~0.2 kg/kg)和不同空气进口温度(381,503,640 K)的NOx排放均低于1×10^(-5)@15%O2,而CO排放在含湿量0~0.1 kg/kg范围内同样能保持1×10^(-5)@15%O2以下。上述结果表明,该模型燃烧器能够在不同空气含湿量和空气进口温度下均保持较低的污染物排放。

掺氢对天然气燃烧室燃烧及排放特性影响数值模拟

针对以天然气为燃料的燃气轮机使用掺氢燃料后的燃烧及其排放特性开展研究,基于DLN1.0燃烧室火焰筒,采用数值模拟的方法研究不同掺氢比(H_(2)体积分数0~30%一共7种工况)在恒定热功率和相同当量比条件下对天然气混合气燃烧过程的影响,获得火焰筒内流场、温度场和燃烧产物的分布参数以及对应关系。结果表明:随着掺氢比的增加火焰温度上升,燃烧反应区扩大,火焰筒出口处的温度分布均匀性变差;掺氢造成局部释热量的改变,会导致NO_(x)的排放随着掺氢比的增加而呈现增加趋势;CO和CO_(2)的排放量都有显著的减少,H_(2)O的生成量显著增加。同时,采用所提出稀释燃烧的方法解决由掺氢造成的一系列问题,研究不同当量比工况下掺氢燃烧后的特性变化,并计算得到各个掺氢比工况下最佳的当量比值。研究成果为后续工业燃气轮机天然气掺氢燃烧技术的应用提供了理论指导。

Numerical study on combustion of diluted methanol-air premixed mixtures

The effect of nitrogen dilution on the premixed combustion characteristics and flame structure of laminar premixed methanol-air-nitrogen mixtures are analyzed numerically based on an extended methanol oxidation mechanism. The laminar burning velocities, the mass burning fluxes, the adiabatic flame temperature, the global activation temperature, the Zeldovich number, the effective Lewis number and the laminar flame structure of the methanol-air-nitrogen mixtures are obtained under different nitrogen dilution ratios. Comparison between experiments and numerical simulations show that the extended methanol oxidation mechanism can well reproduce the laminar burning velocities for lean and near stoichiometric methanol-air-nitrogen mixtures. The laminar burning velocities and the mass burning fluxes decrease with the increase of nitrogen dilution ratio and the effect is more obvious for the lean mixture. The effective Lewis number of the mixture increases with the increase of nitrogen dilution ratio, and the diffusive-thermal instability of the flame front is decreased by the nitrogen addition. Nitrogen addition can suppress the hydrodynamic instability of methanol-air-nitrogen flames. The decrease of the mole fraction of OH and H is mainly responsible for the suppressed effect of nitrogen diluent on the chemical reaction in the methanol-air-nitrogen laminar premixed flames, and the NOx and formaldehyde emissions are decreased by the nitrogen addition.

白银熔池熔炼炉稀氧燃烧技术工业实践

借鉴阳极炉使用稀氧燃烧技术的成功应用经验,在白银炉沉淀区采用稀氧燃烧,解决了富氧燃烧时粉煤消耗较多、烟气量大、炉膛温度分布不均匀等问题,而且延长了耐火材料的寿命,改善了作业环境。

烟气对采空区遗煤自燃特性抑制作用研究

为研究烟气对采空区遗煤自燃的抑制作用,通过分析烟气中各组分对国家能源集团国神公司三道沟煤矿采空区遗煤在低温氧化过程中的气体释放特性及对煤炭的吸附量和竞争吸附选择性等特性参数的影响规律,探究了低温氧化过程中烟气组分抑制煤自燃特性的竞争吸附与稀释作用,揭示了烟气对煤自燃特性的抑制作用机制。结果表明,烟气可以通过关键组分CO_(2)的竞争吸附作用优先占据吸附位点,同时通过烟气的稀释作用降低环境中的O_(2)浓度,进而减少煤氧接触,抑制煤氧复合反应,减缓遗煤低温氧化进程。利用数值模拟采空区不同注入条件下氧化带的变化规律,注气后氧化带宽度变小,且向工作面推移,最终确定最佳注气位置为进风侧距工作面156m处,最佳注气流量为1 205m^(3)/h。

EGR的热效应和稀释效应对柴油机燃烧和排放的影响

以改进的KIVA-3V程序为计算平台,在转速为1,600,r/min,每循环喷油量为52.8,mg工况下,分别进行了EGR的热效应和稀释效应对柴油机燃烧和排放影响的数值模拟.结果表明,EGR的热效应对滞燃期影响较大,而稀释效应成为制约最高爆发压力和缸内温度的主要因素,且随着氧气体积分数的降低,热效应对最高爆发压力的影响增强;EGR导致碳烟排放增加是由于EGR的稀释效应,低温燃烧时碳烟排放的降低是EGR的稀释效应和热效应相互作用的结果;相比稀释效应,EGR的热效应是实现低温燃烧的主要因素,高进气热容更易改善NOx和碳烟排放的折衷关系.

固定燃烧源颗粒物稀释采样系统的研制与应用

为研究固定燃烧源颗粒物排放特征,研制开发了稀释烟道采样系统,该系统能模拟高温烟气排放到大气中的冷却、稀释、凝结等物理化学过程,捕集燃烧源排放的一次颗粒物.详细阐述了采样系统的结构并对系统进行了检验.整个采样系统稳定度高,测量结果可靠,操作简便.采样系统小型化设计,适合于现场应用.该系统已成功应用于我国典型燃烧源颗粒物排放特征研究.

稀释气对高甲烷含量天然气燃烧特性的影响

针对高甲烷含量天然气在实际发动机中燃烧温度过高、NOx排放过高的问题,利用定容燃烧弹实验和Chemkin软件模拟计算相结合的方法,对其预混层流燃烧特性进行研究,分析了不同稀释比和稀释气种类(N2和CO2)对混合气的层流火焰速度、NOx摩尔分数、燃烧压力和燃烧期等燃烧特性参数的影响。研究表明,层流火焰速度、质量燃烧率和热释放率均随稀释比的增加而减小,稀释气添加导致火焰温度下降,从而降低了NOx摩尔分数。Markstein长度和火焰厚度都随稀释比的增加而增加,火焰流动不稳定性得到抑制。添加稀释气导致燃烧压力峰值和压力升高率降低、燃烧期延长,与N2相比,CO2对混合气燃烧特性的稀释效果更加显著,从而为通过废气再循环技术路径降低高甲烷含量天然气发动机燃烧温度,控制NOx排放提供了理论指导。

稀释合成气微混合燃料喷射燃烧火焰特性研究

研究了耦合CO2稀释和微混合燃料喷射燃烧的火焰特性.结果表明:耦合CO2稀释和微混合燃料喷射燃烧是一种降低合成气扩散火焰NOx生成量的有效途径;实验范围内排放的NOx质量浓度一般低于2mg/m3,排放的CO质量浓度低于10mg/m3,排放的CO质量浓度随火焰热功率的增大而降低;燃烧器出口温度、壁面温度和喷嘴出口温度等均随火焰热功率的增大而升高.

CO_2稀释燃料对富氧扩散燃烧中NO_x生成的抑制作用

降低高温火焰中的氮氧化物排放是富氧燃烧作为新型节能燃烧技术得以推广应用的关键之一。本文针对烟气再循环等方法的氮氧化物抑制原理,以对向流扩散火焰为对象,利用详细的基元反应动力学模型研究了在不同富氧浓度条件下CO2稀释燃料对火焰特性和NOx生成的影响。结果表明,用CO2稀释燃料对燃烧特性的影响随着氧化剂中氧浓度的增大趋于显著,而且在较高氧浓度时,CO2稀释燃料可以在维持较高火焰温度的同时有效地降低NO生成及NO排放指数EINO。

二甲醚—空气—N_2/CO_2混合气燃烧特性研究

在定容燃烧弹中系统地研究了常温常压下,不同稀释气、不同稀释系数、不同燃空当量比时二甲醚—空气—稀释气混合气的燃烧特性。基于容弹计算的准维双区模型和实测的燃烧压力数据,计算了燃烧温度及规范化质量燃烧速率。研究结果表明:随着稀释系数的增加,最大燃烧压力值、最高燃烧温度值以及规范化质量燃烧速率均降低,而火焰发展期和快速燃烧期随稀释系数的增加而增加。研究发现,CO2作为稀释气体时对燃烧温度降低的效果较N2作为稀释气体时更加明显。

交叉射流分级燃烧器中CH_4柔和燃烧特性分析

柔和燃烧具备燃烧稳定、烟气出口温度均匀、NO和CO排放低的特点,有望成为新一代燃气轮机燃烧技术。因此,开展柔和燃烧技术应用于燃气轮机燃烧室的探索研究非常必要。烟气温度、氧浓度及其与新鲜空气的掺混对柔和燃烧的发生及燃烧性能有重要影响。为此,基于轴向分级概念建立燃烧器,通过精确控制烟气回流比例和当量比使得烟气的流量、温度、氧浓度连续可调,设计合适的燃烧器结构型式使得烟气在掺混区和燃料、空气以交叉射流方式掺混。通过数值模拟结合实验的方法,从流动、热力学角度研究分析了回流比例、当量比对OH﹡分布、火焰稳定性、NO/CO排放等燃烧性能的影响规律和机制。实验以甲烷为燃料,在回流比例r 0.5、当量比0.6工况下获得最佳的污染物排放性能:NO为6 mg/m3,CO为5 mg/m3。研究结果将为柔和燃烧在燃气轮机燃烧室上的应用提供理论依据和基础数据。