大气温度对某型燃气轮机燃烧稳定性和 NOx排放影响的数值研究

为了对比扩散和预混两种不同燃烧模式下大气温度对燃气轮机燃烧稳定性和NO_x排放的影响规律,针对某重型燃气轮机燃烧室,对多旋流喷嘴燃烧室的燃烧稳定性和NO_x排放进行了数值研究。结果表明:对于扩散燃烧,大气温度升高,燃烧室内高频脉动增强,燃烧稳定性变差;对于预混燃烧,大气温度升高,有利于提高燃烧的稳定性;在扩散燃烧模式下燃烧室燃料喷嘴下游回流区的温度最高,NO_x生成量最大;预混燃烧下燃烧室头部温度分布较均匀,燃烧室NO_x生成主要集中在驻涡回流区和燃烧室中下游位置,燃料喷嘴下游回流区NO_x生成量很小;随着大气温度的升高,扩散燃烧和预混燃烧下燃烧室内NO_x的生成量均增加。研究结果可为指导燃气轮机运行提供参考。     

燃气轮机分级燃烧室NOx排放动力学模拟研究

通过建立轴向分级燃烧室的化学反应器网络模型,在燃气轮机典型工况下研究第一级与第二级燃烧段之间的燃料分配、流量分配和停留时间分配等因素对NO_x排放的影响。结果表明:在燃烧室出口温度恒定的条件下,减少第一级的燃料分配比例使第二级出口烟气温度高于第一级,调节第二级喷嘴位置来缩短第二级燃烧段的停留时间,可以显著降低NO_x排放;但是当第一级燃烧产物与第二级燃料/空气混合物混合不均匀时,会严重影响分级燃烧的NO_x减排效果。     

天然气柔和燃烧锅炉结构参数影响模拟研究

在天然气锅炉中引入柔和燃烧技术将大大降低NOx排放,高速未燃气卷吸高温烟气回流并与之快速掺混再燃烧是柔和燃烧的重要特征,因此,开展天然气锅炉关键结构参数优化设计以组织流场形成柔和燃烧所需的高温低氧反应气氛非常必要。基于天然气锅炉的工况特征,设计了热负荷15kW的模型燃烧室,采用数值模拟手段详细研究了燃烧室高度、喷嘴孔径、喷嘴相对位置及烟气出口尺寸对燃烧室流场、组分场及关键参数——烟气回流比的影响规律,并最终确定了燃烧室结构优选方案,对天然气锅炉柔和燃烧机设计提供理论基础数据。     

燃料热值对燃气轮机燃烧特性的仿真分析

针对燃气轮机运行过程中出现的燃烧不稳定和NOx排放高的问题,开展了不同负荷下燃料热值对 燃气轮机燃烧特性$燃烧稳定性和NOx排放影响规律）的仿真研究。以某燃气轮机分管型燃烧室为研究对 象,在不同负荷下,保持燃料流量、空气流量、大气温度等参数不变,仅改变燃料热值,采用数值仿真方法对 燃气轮机设计压力监测点的压力、燃烧室出口温度及NOx排放数据进行分析。结果表明：负荷区间相同, 热值增加,高频段所对应的压力脉动幅值减小,热释放率脉动的高频频率增加,NOx排放增加;热值相同,负 荷增加,高频段所对应的压力脉动频率增加,高频段压力脉动幅值减小,热释放率脉动频率增加,NOx排放 增加。     

燃料热值对不同燃烧方式下燃气轮机燃烧特性的影响

针对燃气轮机运行过程中出现的燃烧不稳定和污染物排放高的问题,开展了燃料热值对不同燃烧方式下燃气轮机燃烧特性（燃烧稳定性和污染物排放影响规律）影响的研究。以某重型燃气轮机分管燃烧室为研究对象,在扩散燃烧和预混燃烧方式下,保持燃料流量、空气流量及大气温度等参数不变,仅改变燃料热值,采用数值仿真方法对燃烧室设计监测点处压力、燃烧室出口温度及污染物排放等数据进行分析。研究表明：在扩散燃烧方式下,热值较低时,燃烧室高频压力脉动较大,热值增加,燃烧室低频压力脉动先减小后增加;在预混燃烧方式下,热值增加,燃烧室高频压力脉动减小;在两种燃烧方式下,热值增加,燃烧室出口NO_x排放均增加,而热值变化对燃烧室出口CO的排放影响较小。     

天然气燃料轴向分级预混燃烧特性研究

低NOx排放是燃气轮机燃烧室的重要性能指标,面对燃烧室出口温度不断增加的趋势,新型燃烧技术探索应用成为必然。燃料轴向分级(Axial Fuel Staging,AFS)燃烧作为一项可行技术方案已在各燃机厂商的最先进燃气轮机燃烧室上获得应用,其主要通过降低高温烟气有效停留时间和低氧浓度燃烧来实现低NOx排放。基于Chemkin平台建立轴向分级预混燃烧室化学网络模型,针对1 973 K燃烧室,研究二级负荷比例、当量比和停留时间对NOx/CO排放的影响规律,对比分析主燃区高温烟气与二级未燃预混气掺混特征的影响,获得AFS燃烧的污染物排放特性和关键影响因素。同时,在贫预混燃烧器上,设计二级喷射段,实验研究二级火焰结构、污染物排放等燃烧特性。结果表明,相比于常规贫预混燃烧,AFS燃烧在高温区体现出很好的低NOx优势,且能拓宽低NOx工况范围,其中主燃区温度、二级当量比和停留时间匹配特征、主燃区高温烟气与二级预混气掺混性能等是关键。     

射流喷嘴旋流强度对燃烧室回流和燃烧特性的影响研究

随着燃气轮机参数的提高和稳定低排放运行工况的拓宽,对燃烧的要求也越来越高。柔和燃烧作为一种有潜力的燃烧技术,具有温度均匀、燃烧稳定和污染物排放低等优点,而如何在燃烧室内组织流动是实现柔和燃烧的关键。采用高速射流引射掺混的方式可以较好的满足柔和燃烧产生所需的条件。预混射流喷嘴结构和布置对流场和燃烧特性有重要影响,如何选择射流喷嘴结构值得进一步研究。本文通过实验和数值模拟相结合的方式,研究了柔和燃烧器中预混射流喷嘴的旋流强度对燃烧器流动结构和燃烧排放的影响。结果表明,旋流能增强燃料/空气的掺混,低旋流作用下能使喷嘴出口掺混不均匀度ISMD下降0. 15左右;但是喷嘴旋流对燃烧室的烟气回流有减弱的作用,使回流区向喷嘴和中轴线靠近;同时,旋流会造成温度场和火焰面不均匀分布,略微拓宽燃烧工况范围并略微增加火焰的稳定性。实验结果表明喷嘴旋流进气角从0°变化到45°时,NOx排放随旋流角的增大而增加。     

燃气轮机无焰燃烧技术的研究进展

燃气轮机无焰燃烧具有分布式火焰、低压力波动、低污染排放等特性,总结了氧化剂温度、氧浓度和烟气循环率对无焰燃烧效果的影响,以及无焰燃烧的多燃料适应性,给出了适合无焰燃烧数值模拟的燃烧模型,归纳了产物的停留时间和燃烧室尺寸对污染物排放的影响;对国内外出现的燃气轮机无焰燃烧室进行了总结和可行性分析,指出了下一步的研究重点是液体燃料无焰燃烧的基础研究和应用研究。     

二次燃料喷射对燃气轮机中低热值燃烧室性能的影响

针对燃气轮机天然气燃烧室改烧中低热值气所导致的头部流速过快、回流区减小、壁面温度过高等问题,提出从火焰筒横侧实施二次燃料喷射的方案。建立了简化的模型燃烧室,利用计算流体动力学(CFD)软件和Chemkin软件进行了计算和分析。结果表明,实施二次燃料喷射,能够增大回流区,有效加强空燃掺混。二次燃料喷射对燃烧室总压损失和燃烧效率影响不大,合理分配两级燃料喷射比例,能够有效降低燃烧室壁面温度,同时也能达到较低污染排放。     

燃气轮机燃油燃烧室改用双燃料流场数值分析

针对燃气轮机燃油燃烧室改成双燃料燃烧室对燃料喷嘴进行一体化概念设计,并采用CFD技术对其双燃料燃烧流场进行数值模拟。针对燃烧室燃用C7H16和裂解气燃料的不同情况,采用标准κ-ε湍流模型、化学平衡条件下的快速化学反应系统和简单概率密度函数（PDF）燃烧模型、液体燃料的喷雾模型以及SIMPLE算法。模拟并对比分析了两种燃料燃烧时的燃烧效率、出口温度均匀性、壁面最高温度以及速度分布等参数随工况变化的趋势,并得出结论：1）不同燃料燃烧时的流场特征基本保持一致;2）裂解气燃料燃烧时,其燃烧效率较高,但出口温度均匀性较差;3）在加入相同焓值的燃料进入燃烧室时,裂解气燃料燃烧得到的出口温度低于燃油的燃烧状态。     

MILD combustion for hydrogen and syngas at elevated pressures

As gas recirculation constitutes a fundamental condition for the realization of MILD combustion, it is necessary to determine gas recirculation ratio before designing MILD combustor. MILD combustion model with gas recir- culation was used in this simulation work to evaluate the effect of fuel type and pressure on threshold gas recir- culation ratio of MILD mode. Ignition delay time is also an important design parameter for gas turbine combustor, this parameter is kinetically studied to analyze the effect of pressure on MILD mixture ignition. Threshold gas re- circulation ratio of hydrogen MILD combustion changes slightly and is nearly equal to that of 10 MJ/Nm3 syngas in the pressure range of 1-19 atm, under the conditions of 298 K fresh reactant temperature and 1373 K exhaust gas temperature, indicating that MILD regime is fuel flexible. Ignition delay calculation results show that pres- sure has a negative effect on ignition delay time of 10 MJ/Nm3 syngas MILD mixture, because OH mole fraction in MILD mixture drops down as pressure increases, resulting in the delay of the oxidation process.     

预热室结构对多段式自预热燃烧器内燃烧及NOx排放特性的影响

提出了一种多段式自预热燃烧器及其4种典型的预热室结构,通过计算流体力学（CFD）方法研究了燃烧室内流场、烟气卷吸率、温度场、燃气燃尽率以及NOx体积分数,并与传统燃烧器的情况进行了对比．结果表明：与传统燃烧器相比,多段式自预热燃烧器改变了燃烧室内流场,对低热值燃料适应性强,其预热室结构同时影响烟气卷吸率和预热效果,并最终影响燃尽率与NOx体积分数;此外,燃烧器负荷对燃尽率影响甚微,但对NOx体积分数影响较大．     

双燃料燃气轮机燃料快速切换技术分析与试验研究

通过分析国外机组资料和实际燃料切换运行曲线,逆推双燃料切换控制策略,并利用现有燃烧试验台,在保证燃烧室出口温度稳定波动的前提下,对规定时间内完成燃料快速切换的燃烧室内火焰稳定性加以验证。试验结果表明:某型机组试验可依据理论切换时间30 s换算燃机最大负荷时燃料的变化量,保证机组在60 s内完成燃料的快速切换,在此切换过程中,燃烧室内火焰稳定,动态压力最大值为4.55 k Pa。由于燃烧试验台条件与整机试验环境存在较大的差异性,试验过程得出燃料变化量仅可作为参考,主要用于减少整机试验次数并规避试验风险,最终值仍应以整机试验结果为准。     

Characteristics of oxy-fuel combustion in gas turbines

This paper reports on a numerical study of the thermodynamic and basic combustion characteristics of oxy-fuel combustion in gas turbine related conditions using detailed chemical kinetic and thermodynamic calculations. The oxy-fuels considered are mixtures of CH₄, O₂, CO₂ and H₂O, representing natural gas combustion under nitrogen free gas turbine conditions. The GRI Mech 3.0 chemical kinetic mechanism, consisting of 53 species and 325 reactions, is used in the chemical kinetic calculations. Two mixing conditions in the combustion chambers are considered; a high intensity turbulence mixing condition where the combustion chamber is assumed to be a well-stirred reactor, and a typical non-premixed flame condition where chemical reactions occur in thin flamelets. The required residence time in the well-stirred reactor for the oxidation of fuels is simulated and compared with typical gas turbine operation. The flame temperature and extinction conditions are determined for non-premixed flames under various oxidizer inlet temperature and oxidizer compositions. It is shown that most oxy-fuel combustion conditions may not be feasible if the fuel, oxygen and diluent are not supplied properly to the combustors. The numerical calculations suggest that for oxy-fuel combustion there is a range of oxygen/diluent ratio within which the flames can be not only stable, but also with low remaining oxygen and low emission of unburned intermediates in the flue gas.     

可控烟气回流量型低NOx燃烧器流场特性的试验研究

设计了一种预混式可控烟气回流量型低NOx燃烧器,以适应双气头多联产系统中燃料组分、成分变化时燃气轮机发电系统稳定工作的需要.在常压条件下,利用TSI热线风速仪对燃烧室内的速度分布特性进行了直接测量,并利用温度场比拟浓度场的方法,对燃烧室内气流混合特性进行了间接测量.结果表明:燃烧室内的速度分布及回流等特性可满足设计要求,气流之间的混合效果则需作进一步增强.同时,对燃烧器二次风分配器的结构提出了改进方案.     

Experimental investigation on the kinetics of biomass combustion in vertical tube reactor

The paper presents results of experimental investigation performed in order to examine kinetics of loose biomass combustion in vertical tube reactor. The investigation conducted included continuous measurement of the fuel mass loss rate, with two biomass combustion models (piston and batch model) proposed, each relying on appropriate theoretical postulates. Results obtained indicated that piston combustion model had shown better agreement between theoretical and experimental data and was therefore used to further analyse effects of excess-air on the combustion kinetics, as well as associated effects of flue gas recirculation. Recirculation of cold flue gases is used to lower peak temperature inside the furnace, as well as to reduce a zone where ash melting problems may potentially occur. During the investigation performed, effects of flue gas recirculation on the combustion process were simulated by simultaneously injecting nitrogen and air flows into the furnace. This was deemed appropriate to simulate real-life conditions prevailing in the furnace with gas recirculation. Experiments were conducted on specially designed and constructed apparatus that enabled kinetic parameters to be determined for the combustion of different types of biomass. Results obtained have indicated that quantity of air affects kinetics of biomass combustion and that increased recirculation leads to reduced biomass reaction rate. The same conclusion was reached based on the results of experiments conducted with two different types of agro-biomass, namely wheat straw and corn stalks, which are most commonly used for energy generation. Results achieved are deemed particularly important when it comes to design of new plants that utilize cigarette type combustion system, but also for development of numerical models used to simulate combustion of biomass bales, with special emphasis placed on the impact of recirculation gases on the combustion kinetics.     

Development of high intensity CDC combustor for gas turbine engines

Colorless distributed combustion (CDC) has been demonstrated to provide ultra-low emission of NOx and CO, improved pattern factor and reduced combustion noise in high intensity gas turbine combustors. The key feature to achieve CDC is the controlled flow distribution, reduce ignition delay, and high speed injection of air and fuel jets and their controlled mixing to promote distributed reaction zone in the entire combustion volume without any flame stabilizer. Large gas recirculation and high turbulent mixing rates are desirable to achieve distributed reactions thus avoiding hot spot zones in the flame. The high temperature air combustion (HiTAC) technology has been successfully demonstrated in industrial furnaces which inherently possess low heat release intensity. However, gas turbine combustors operate at high heat release intensity and this result in many challenges for combustor design, which include lower residence time, high flow velocity and difficulty to contain the flame within a given volume. The focus here is on colorless distributed combustion for stationary gas turbine applications. In the first part of investigation effect of fuel injection diameter and air injection diameter is investigated in detail to elucidate the effect fuel/air mixing and gas recirculation on characteristics of CDC at relatively lower heat release intensity of 5 MW/m³ atm. Based on favorable conditions at lower heat release intensity the effect of confinement size (reduction in combustor volume at same heat load) is investigated to examine heat release intensity up to 40 MW/m³ atm. Three confinement sizes with same length and different diameters resulting in heat release intensity of 20 MW/m³ atm, 30 MW/m³ atm and 40 MW/m³ atm have been investigated. Both non-premixed and premixed modes were examined for the range of heat release intensities. The heat load for the combustor was 25 kW with methane fuel. The air and fuel injection temperature was at normal 300 K. The combustor was operated at 1 atm pressure. The results were evaluated for flow field, fuel/air mixing and gas recirculation from numerical simulations and global flame images, and emissions of NO, CO from experiments. It was observed that the larger air injection diameter resulted in significantly higher levels of NO and CO whereas increase in fuel injection diameter had minimal effect on the NO and resulted in small increase of CO emissions. Increase in heat release intensity had minimal effect on NO emissions, however it resulted in significantly higher CO emissions. The premixed combustion mode resulted in ultra-low NO levels (<1 ppm) and NO emission as low as 5 ppm was obtained with the non-premixed flame mode.     

Progress and recent trends in homogeneous charge compression ignition (HCCI) engines

HCCI combustion has been drawing the considerable attention due to high efficiency and lower nitrogen oxide (NO_x) and particulate matter (PM) emissions. However, there are still tough challenges in the successful operation of HCCI engines, such as controlling the combustion phasing, extending the operating range, and high unburned hydrocarbon and CO emissions. Massive research throughout the world has led to great progress in the control of HCCI combustion. The first thing paid attention to is that a great deal of fundamental theoretical research has been carried out. First, numerical simulation has become a good observation and a powerful tool to investigate HCCI and to develop control strategies for HCCI because of its greater flexibility and lower cost compared with engine experiments. Five types of models applied to HCCI engine modelling are discussed in the present paper. Second, HCCI can be applied to a variety of fuel types. Combustion phasing and operation range can be controlled by the modification of fuel characteristics. Third, it has been realized that advanced control strategies of fuel/air mixture are more important than simple homogeneous charge in the process of the controlling of HCCI combustion processes. The stratification strategy has the potential to extend the HCCI operation range to higher loads, and low temperature combustion (LTC) diluted by exhaust gas recirculation (EGR) has the potential to extend the operation range to high loads; even to full loads, for diesel engines. Fourth, optical diagnostics has been applied widely to reveal in-cylinder combustion processes. In addition, the key to diesel-fuelled HCCI combustion control is mixture preparation, while EGR is the main path to achieve gasoline-fuelled HCCI combustion. Specific strategies for diesel-fuelled, gasoline-fuelled and other alternative fuelled HCCI combustion are also discussed in the present paper.     

Investigation of forward flow distributed combustion for gas turbine application

New innovative advanced combustion design methodology for gas turbine applications is presented that is focused on the quest towards zero emissions. The new design methodology is called colorless distributed combustion (CDC) and is significantly different from the currently used methodology. In this paper forward flow modes of CDC have been investigated for application to gas turbine combustors. The CDC provides significant improvement in pattern factor, reduced NO_x emission and uniform thermal field in the entire combustion zone for it to be called as an isothermal reactor. Basic requirement for CDC is carefully tailored mixture preparation through good mixing between the combustion air and product gases prior to rapid mixing with fuel so that the reactants are at much higher temperature to result in hot and diluted oxidant stream at temperatures that are high enough to autoignite the fuel and oxidant mixture. With desirable conditions one can achieve spontaneous ignition of the fuel with distributed combustion reactions. Distributed reactions can also be achieved in premixed mode of operation with sufficient entrainment of burned gases and faster turbulent mixing between the reactants. In the present investigation forward flow modes consisting of two non-premixed combustion modes and one premixed combustion mode have been examined that provide potential for CDC. In all the configurations the air injection port is positioned at the opposite side of the combustor exit, whereas the location of fuel injection ports is changed to give different configurations. Two combustion geometries resulting in thermal intensity of 5 MW/m³-atm and 28 MW/m³-atm are investigated. Increase in thermal intensity (lower combustion volume) presents many challenges, such as, lower residence time, lower recirculation of gases and effect of confinement on jet characteristics. The results are presented on the global flame signatures, exhaust emissions, and radical emissions using experiments and flowfield using numerical simulations. Ultra-low NO_x emissions are found for both the premixed and non-premixed combustion modes at the two thermal intensities investigated here. Almost colorless flames (no visible flame signatures) have been observed for the premixed combustion mode. The reaction zone is observed to be significantly different in the two non-premixed modes. Higher thermal intensity case resulted in lower recirculation of gases within the combustion chamber and higher CO levels, possibly due to lower associated residence time. The characteristics at the two thermal intensity combustors investigated here were found to be similar.