生物质链条炉床层燃烧建模及一次风影响分析

基于多孔介质非热平衡的方法,考虑了床层高度的变化及颗粒内部温度梯度的影响,建立了一维非稳态燃烧模型来模拟炉排上移动床层的生物质燃烧。模拟计算结果与实验值对比分析表明,总体上数值计算结果与实验数据吻合较好。通过对不同一次风参数下床层燃烧的模拟结果分析得到,随着一次风风量的增加,床层剩余质量先减小后增大;在燃烧前期,床层出口气体温度上升速度减慢,挥发分析出速率降低,焦炭燃烧速率增大;在燃烧中期,床层出口气体温度先上升后下降,焦炭燃烧速率下降。一次风风温相比于一次风风量对床层燃烧影响较小,增大一次风风温可以提高挥发分析出速率,降低床层出口气体温度和床层剩余质量。     

一维炉中石油焦与煤燃烧特性的对比试验研究

在一维炉热态试验台上,采用双调风旋流燃烧器,对比研究了一次风率f1、外二次风当量旋流强度Ωdl和过量空气系数α对石油焦、河津贫煤和神木烟煤燃烧特性的影响.结果表明:随着一次风率f1的增大,河津贫煤和石油焦的NOx排放浓度提高,前期燃烧恶化,而神木烟煤的燃烧状况得到改善,燃尽率提高,NOx排放浓度先降低后升高;石油焦和神木烟煤的着火和燃尽率分别在Ωdl=0.69和Ωdl=0.87时最佳,而两者的NOx排放浓度均在Ωdl=1.08时最低;过量空气系数α的增大使石油焦着火推迟,燃烧状况更差,NOx排放浓度更高,但使神木烟煤的NOx排放浓度更低.     

大缸径点燃式天然气发动机爆震的影响因素

爆震被认为是缸内燃烧火焰传播过程中末端混合气局部热点自燃引起的压力振荡.通过抑制局部热点自着火的反应速率可以降低爆震强度.通过一台缸径为190,mm的天然气发动机,研究不同过量空气系数对降低爆震强度的影响.在过量空气系数较小的工况下,当自着火发生时可以观察到压力振荡;在过量空气系数较大的工况下,当自着火发生时爆震强度较微弱.爆震强度随着缸内最高温度和最大压力变化而变化.在过量空气系数大于1.45的工况下,爆震的发生由局部热点反应速率决定;而在过量空气系数小于1.45的工况下,爆震的发生受自着火出现时刻控制.     

煤水混烧层燃锅炉配风研究

介绍煤水混绕过程的主要特性及试验炉的测试结果。探讨层燃锅炉在采用煤水混烧技术时对送风系统的要求和对燃烧过程的影响,以及炉排面上配风速度的控制范围。提出并试验低风速运行的均匀配风方法。有效地改进送引风系统的运行参数,降低过量空气系数。     

优化配风对NO_x生成的影响

对一台1,000,MW超超临界锅炉中燃尽风(OFA)对NO_x生成的影响进行了数值模拟,分析了燃尽风配风比例以及燃尽风的SOFA喷口风门位置对出口NO_x的影响.结果表明:随着燃尽风配风比例的提高,由于主燃区过量空气系数减小导致还原性增强以及燃烬风补入空气对整体烟气的温度稀释和NO_x体积分数稀释的增强,使得出口NO_x体积分数降低.而上移SOFA喷口风门位置,由于拉长了还原区的长度导致出口NO_x降低.     

电厂多元优化动力配煤燃烧特性研究

针对线性规划动力配煤的缺点,研究了非线性规划实现多元优化动力配煤的新技术,利用开发的多元优化动力配煤专家系统计算得到最优化配煤方案。利用沉降炉对13种最优化配煤进行燃烧特性试验。实验结果表明:随着燃煤细度R90由15%减小到5%,着火温度由1 283℃降低到1 167℃,着火距离由400 mm缩短到250 mm;随着过量空气系数由1.1升高到1.3,配煤着火温度由1 170℃升高到1 233℃,着火距离由200 mm延长到300 mm。炉壁温度由1 150℃上升到1 350℃时,排烟中NO含量由63 mg/kg升高到229 mg/kg;煤粉细度R90由15%减小到5%时,烟气中O_2含量由11.16%降低到9.72%,同时CO_2含量由9.4%上升到10.0%,SO_2含量由556 mg/kg升高到711 mg/kg。13种配煤基本上没有结渣,说明优化动力配煤技术可以改善煤质结渣特性。     

秸秆涡旋燃烧颗粒排放特性影响研究

利用颗粒物分级采样及烟气实时监测技术,研究稻草在涡旋燃烧炉燃烧时燃烧温度、一二次风配比、过量空气系数等参数对飞灰颗粒排放特性的影响。实验表明稻草燃烧产生的亚微米颗粒PM1及微细颗粒PM1-2.5主要由KCl和K2SO4组成。提高燃烧温度、增大一二次风配比将增加PM1排放浓度。试验中随着一二次风配比的增大或燃烧过量空气系数的增大,不同粒径颗粒的S/Cl摩尔比将增大,而适当的燃烧过量空气系数有利于减少PM和CO排放。     

过量空气系数对HCCI汽油机燃烧特性的影响

在一台Ricardo Hydra单缸四气门汽油机上,利用气门重叠负角方法实现了均质充量压缩着火(HCCI)燃烧,并通过试验研究了过量空气系数对HCCI汽油机燃烧特性的影响.研究结果表明,在相同的转速和气门相位角下,随着过量空气系数的增加,平均指示压力减小,缸内残余废气率也减小,但燃油消耗率的变化趋势与转速有关.在大多数工况下,过量空气系数为1.05时,HCCI发动机的着火时刻最早,燃烧持续期最短.过量空气系数对循环波动的影响与转速和气门相位角有关.随着转速的增加,循环波动增大.     

1000MW机组燃烧调整对NO_x排放影响

在1 000MW机组锅炉上进行了燃烧调整试验,通过改变过量空气系数、机组负荷、燃尽风率和配风方式,对烟气NO_x的排放规律进行了研究。结果表明:随着过量空气系数的增大,NO_x排放浓度显著增大,锅炉排烟热损失呈上升趋势,飞灰含碳量呈下降趋势。锅炉负荷对NO_x排放的影响主要来自燃料量、炉膛温度、氧浓度等多方面因素的综合影响,随着锅炉负荷下降,过量空气系数增大,烟气NO_x排放浓度呈缓慢下降趋势,单位质量燃料的NO_x转化率有所升高。增大炉膛的燃尽风率可显著降低烟气NO_x排放浓度。在燃尽风率较低的燃烧工况下,NO_x排放浓度对燃尽风率的变化尤为敏感。与均等配风方式相比,束腰配风方式可降低炉膛主燃料区的氧浓度,使烟气NO_x排放浓度下降。     

大型褐煤锅炉煤粉再燃技术的数值模拟

考虑煤焦还原NO的反应动力学模型，利用Fluent软件对元宝山电厂3号锅炉超细煤粉再燃的不同配风方式进行了炉膛整体的燃烧数值模拟。模拟结果表明，再燃燃料比例、再燃风中的风煤比、再燃区的大小等因素对燃烧效率和NOx排放具有重要影响。通过优化计算得到，当主燃区空气过量系数控制在1．1时，再燃燃料占总燃料的15％，再燃风中的风煤比为2，烟气在再燃区的停留时间为0．5S左右的方案是一种较好的再燃组织方式。     

Investigation of factors affecting channelling in fixed-bed solid fuel combustion using CFD

Channelling is an undesirable phenomenon in fixed-bed combustion. It is characterised by an uneven air distribution, and thus fuel conversion, throughout the fuel bed. To investigate factors that influence channelling, an unsteady, two-dimensional numerical model capable of predicting solid fuel combustion under fixed-bed conditions is presented. Biomass is the focus of this study, but the model can be readily applied to other solid fuels, such as coal and municipal solid waste. The model includes drying, pyrolysis, and heterogeneous char reactions, and incorporates bed shrinkage processes comprised of both continuous shrinkage and abrupt collapses. It is also capable of representing spatial non-uniformities which may occur throughout a bed, arising from irregular packing and non-homogeneous fuel composition. The overall model is validated by means of two different data sources: the first for ignition rates and the second for species profiles through a biomass fuel bed. The validated model is then applied to investigate factors affecting channelling in a randomly packed bed containing a high-porosity passage. The influence of flow resistance through the grate and bed height are compared with previous observations. Additional factors investigated include flue gas recirculation and initial moisture content of the fuel. Predictions show that increasing the flow resistance of the grate improves the gas distribution and reduces channelling because it inhibits flow from deviating towards the relatively porous passage. For deeper beds, however, the effectiveness of grate resistance is diminished because the gas then has more residence time within the bed to track towards the passage. Increasing the initial moisture content from 0% to 30% has a weak influence; nonetheless, wetter fuels show a propensity to amplify channelling. The impact of flue gas recirculation on channelling appears to be insignificant, although its benefits, such as reduced peak temperature, are apparent.     

典型生物质燃料层燃燃烧特性的试验研究

在小型单元体炉中进行了不同形状尺寸及种类的生物质燃料的层燃燃烧试验.采用着火锋面传播速率及着火锋面温度研究了生物质燃料在同一给风量条件下的层燃燃烧特性,并分析了不同给风量对层燃燃烧的影响.结果表明:尺寸较小的燃料颗粒,完全燃尽需要的时间较长,燃烧过程中床层温度较高,而经过压缩的成型生物质燃料,燃烧稳定性好,适合层燃燃烧;对于不同种类的生物质,挥发分含量越高,其燃尽时间越短,灰分含量越高,燃烧稳定性越差;着火锋面传播速率与着火锋面温度都随着给风量的增加而提高.     

Direct numerical simulations of ignition of a lean n-heptane/air mixture with temperature inhomogeneities at constant volume: Parametric study

The effect of thermal stratification on the ignition of a lean homogeneous n-heptane/air mixture at constant volume and high pressure is investigated by direct numerical simulations (DNS) with a new 58-species reduced kinetic mechanism developed for very lean mixtures from the detailed LLNL mechanism (H.J. Curran et al., Combust. Flame 129 (2002) 253–280). Two-dimensional DNS are performed in a fixed volume with a two-dimensional isotropic velocity spectrum and temperature fluctuations superimposed on the initial scalar fields. The influence of variations in the initial temperature field, imposed by changing the mean and variance of temperature, and the ratio of turbulence to ignition delay timescale on multi-stage ignition of a lean n-heptane/air mixture is studied. In general, the mean heat release rate increases more slowly with increasing thermal stratification regardless of the mean initial temperature. Ignition delay decreases with increasing thermal stratification for high mean initial temperature relative to the negative temperature coefficient (NTC) regime. It is, however, increased with increasing thermal fluctuations for relatively low mean initial temperature resulting from a longer overall ignition delay of the mixture. Displacement speed and Damköhler number analyses reveal that the high degree of thermal stratification induces deflagration rather than spontaneous ignition at the reaction fronts, and hence, the mean heat release rate is smoother subsequent to thermal runaway occurring at the highest temperature regions in the domain. Chemical explosive mode analysis (CEMA) also verifies that mixing counterbalances chemical explosion at the reaction fronts for cases with large temperature fluctuation. It is also found that if the ratio of turbulence to ignition delay timescale is short, resultant diminished scalar fluctuations cause the overall ignition to occur by spontaneous ignition. However, the overall effect of turbulence is small compared to the effect of thermal stratification. These results suggest that the critical degree of thermal stratification for smooth operation of homogeneous charge compression-ignition (HCCI) engines depends on both the mean and fluctuations in initial temperature which should be considered in controlling ignition timing and preventing an overly rapid increase in pressure in HCCI combustion.     

煤和生物质床层燃烧氮氧化物生成规律的对比研究

为揭示和评估燃料特性对床层燃烧过程中氮氧化物生成规律的影响,选取了燃煤与生物质成型颗粒这两种代表性燃料,在单元体炉试验台上模拟层燃反应过程,并测定了床层表面和内部的温度及气氛。结果表明:无论燃煤还是生物质,床层内部的氮氧化物浓度峰值都显著高于床层表面,证实了炭层对氮氧化物优良的还原作用。生物质与燃煤相比火焰锋面处的氮氧化物浓度更高,且火焰传播过程床层表面氮氧化物浓度下降更为平缓。     

The black smoke control in a chain‐grate stoker‐fired boiler at the time of ignition and restart of the combustion

The black smoke is always emitted from the chimney in the chain‐grate stoker‐fired boiler at the time of ignition at ambient temperature and restart of the combustion after temporary flameout in China. The purpose of the work is to reduce the black smoke emission. A laboratory fixed‐grate model has been used to simulate the combustion of coal in chain‐grate stoker‐fired boiler. The CO and O₂ concentration in the flue gas have been measured with a flue gas analyser, and the black smoke emitted from the chimney has been screened with Charge Couple Device (CCD) video camera. Power 2^# coal, sized at 5–25 mm, has been fired in the fixed‐grate model. The secondary air has been used to enhance the turbulence in the furnace after the numerical simulation. The results of experiments show that the emission of the black smoke at the time of ignition of the coal at ambient temperature is more serious than that of restart of the combustion after the temporary flameout for the case of the temperature; the secondary air is helpful for reducing the black smoke emission for enhancing the intensity of the turbulence in the furnace; selection of coal particle size is necessary, the smaller the size of the coal is, the more serious the black smoke emission is, and the effect of reducing the black smoke emission with the secondary air is more evident with smaller‐size coal. The industrial test has been employed to study the effect of the air demand. It indicates that rational stoichiometric air/fuel ratio is helpful for reducing the black smoke emission in the restart of the combustion after the temporary flameout. Copyright © 2006 John Wiley & Sons, Ltd.     

链条炉中推迟配风方式的特性研究

采用气相色谱法研究了某6.5 t/h链条炉在3种不同配风方式(尽早配风、均匀配风和推迟配风)下煤层表面各气体组分沿链条长度的分布规律。结果显示:尽早配风和推迟配风相对均匀,配风整体氧量水平较低,推迟配风在煤燃烧的中后期促进焦碳燃烧,集中释放出大量CO和CO2,而且固体碳转化为气态碳(CH4,CO,CO2)的量最大,这说明其煤层燃尽率最高。对炉膛内气相燃烧的数值模拟进一步表明,推迟配风还有利于新型双人字型炉拱在前拱下部形成火焰漩涡,直接冲刷新煤,保证和加速新煤的引燃。     

链条炉区段烟气再循环对锅炉运行及NOx排放特性影响的工业试验研究

针对一台采用尽早配风方式的29MW链条炉进行分区段烟气再循环对锅炉运行及NOx排放特性影响的工业试验。在挥发分析出及燃烧区段煤层下的一次风室混入再循环烟气将有效强化该区段煤层燃烧，降低该区段煤层以上燃烧空间的氧浓度，控制及消减挥发分N向NOx的转化，同时降低了穿过该区段煤层一次风的氧浓度，抑制焦炭N向NOx转化，NO消减效果最高达到25%。在焦炭燃烧区段煤层下的一次风室混入再循环烟气，能够降低穿过床层气流的氧浓度，抑制焦炭氮向NO的转化过程，该区段烟气再循环低氮效果有限，最大降幅9%。再循环烟气可以替代部分一次风，以维持足够的风室风压，进而降低穿过煤层气流的O2浓度，从而强化链条炉区段燃烧特性的低氮特征，实现链条炉的NOx减排。随着工业锅炉NOx排放指标的不断提高，烟气再循环作为一项有效的前置低氮环节，能有效降低整个低氮系统的投资，进而取得较好的经济性。     

链条炉内型煤着火过程的试验研究

在工业锅炉特别是链条炉上推广燃用型煤,是当今我国防治煤烟型大气污染和节约能源的重要措施。本文通过仿照链条炉内煤层的燃烧过程及特点,建立了实验研究炉内型煤燃烧过程的实验台,进行了型煤和散煤着火延迟时间及煤层中着火线向下扩展速度等有关试验。并强调了煤层中着火线向下扩展速度对型煤稳定着火的重要性。最后,根据实验研究结果和工业链条炉试烧的实践,提出了解决链条炉内型煤稳定着火的有关措施。