基于激光诱导炽光法的生物柴油碳烟测量
Biodiesel is a type of renewable fuel designed to mimic the properties and performance of conventional diesel. Thus, biodiesel can be used to partly replace conventional diesel without modification to the existing combustion devices. At present, biodiesel is widely used as a transportation fuel mostly by blending with fossil diesel. However, due to the diversity of feedstock used in biodiesel production, the physico-chemical properties of biodiesel may vary, which results in unexpected emissions and combustion performance. Driven by increasingly stringent environmental regulations, the research on particulate matter emissions from the combustion of biodiesel and its blends has attracted much attention. In the present work, the soot emission characteristics of different biodiesels produced from vegetable oils and animal fats are investigated. The chemical composition of the biodiesel is characterized before the biodiesel is burnt in a well-controlled flame environment, so as to examine the soot characteristics. In this study, we apply the laser-induced incandescence (LII) method calibrated by the extinction method to quantify the soot volume fraction produced by the neat oxygenated biodiesel and the blends and then assess the effect of the fuel chemistry on soot formation. Subsequently, the morphology and particle size of soot particulate matters produced from the fuels are compared.
An open pool flame combustion device is utilized to establish the laminar pool flame of the biodiesel and blends. The crucible used has a diameter and depth of 20 mm and a wall thickness of 2.5 mm. A co-flow of air is supplied at a constant speed of 18.2 cm/s to shroud the pool flame from air entrainment. At the bottom of the crucible, a ceramic heating plate is installed to maintain a constant heat supply to the liquid fuel and a constant evaporation rate. The fuel crucible is connected to a fuel tank to replenish the fuel, which thus enables the fuel to stay at a fixed level from the crucible rim and not be unaffected by the fuel consumption rate. In order to measure the soot volume fraction, the non-intrusive laser diagnostic method of planar two-dimensional (2D) LII is employed. The measured LII signal is quantitatively calibrated via absorption, and signal trapping is corrected. The dependence of the LII signal on the energy intensity per unit area of the laser sheet is also examined. The peak laser fluence (about 0.16 J/cm2) is used to conduct the LII measurement because the LII signal is less sensitive to the local laser energy fluctuations. The soot produced from the flames is collected using the thermophoretic deposition method. A quartz plate cooled to 0 ℃ is placed in the flames to collect the soot. The soot's morphology and size are examined via a scanning electron microscope. Five different types of biodiesel, produced from palm, waste cooking oil, duck fat, goose fat, and rice bran, respectively, are tested and compared against the baseline diesel.
Images of the pool flames show that the flame height decreases with the increase in biodiesel blends. The diesel pool flame appears to be the sootiest, but the tendency decreases with the increase in biodiesel fraction owing to the oxygen molecules assisting in soot oxidation. This implies that biodiesel, regardless of the feedstock type, is effective in suppressing the formation of soot. From the LII result, the peak value of the soot volume fraction of pure oxygenated biofuel is 7.1%-30.5% lower than that of conventional diesel. The soot formation decreases with the increase in the biodiesel blending ratio, which is similar to the trend exhibited by biodiesel/diesel blends. Oxygenated fuels with a high degree of unsaturation level tend to emit a higher amount of soot. Palm and rice bran biodiesels with the highest degree of unsaturation among all the biodiesels tend to emit a large amount of soot due to the presence of the double bond promoting the formation of soot. On the basis of Roper's model, the predicted diffusion flame height decreases with the diffusion flame temperature, with palm and duck biodiesel producing the tallest flames among all fuels. The soot particle morphology of the biodiesel and diesel is similar, which is spherical and clustered. Overall, the particle size of biodiesel is relatively 9.5%-41.3% smaller than that of traditional diesel. The soot particle size produced by highly unsaturated biodiesel is relatively larger in spite of lower particle number density.
In the present work, the soot volume fraction produced from five types of biodiesel, biodiesel blends, and conventional diesel is measured by using the LII technique calibrated by the extinction method. The pool flame height is not visibly different among the tested neat biodiesels, but the flame appearance varies with different biodiesel blend fractions in the diesel. The flame height reduces with the increase in biodiesel fraction, and the soot emission is reduced. The LII measurement shows that biodiesel with a higher degree of unsaturation is more prone to emit a large amount of soot. The emission of soot decreases linearly with the increase in biodiesel fraction in the diesel. The peak value of the soot volume fraction of the neat oxygenated biodiesel is 7.1-30.5% lower than that of the conventional diesel. Oxygenated fuels with a higher degree of unsaturation are inclined to emit more soot, which can be explained by the fact that unsaturated C-C double bond is more prone to generate acetylene or benzene during the oxidation process and thus provides precursors for the formation of soot. In general, biodiesel produces soot size that is about 9.5-41.3% smaller than that of diesel. The generated soot is clustered and spherical. Biodiesel with a higher degree of unsaturation tends to produce more fuels in spite of a lower particle number density.
1 引言
生物柴油作为一种可再生燃料,其理化性质与传统柴油相似,并且可在不改变传统柴油机结构的前提下,利用生物柴油部分替代传统柴油来减少对传统柴油的消耗。从经济角度而言,生物柴油生产原材料广泛、生产工艺成熟,其大规模生产制造基础已基本具备且制造经济性良好[1]。因此,生物柴油具有发展成大规模应用替代燃料的潜力。与传统柴油相比,生物柴油掺混到柴油里能减少颗粒物、一氧化碳和碳氢化合物的排放。因此,在排放要求日益严格的环境法规促使下,对实验室规模火焰和柴油发动机中生物柴油及其混合物燃烧产生的颗粒物排放进行的研究日益增加[2-3]。
与长链碳氢化合物燃料相比,添加生物柴油显著降低了颗粒物排放,而后者的含氧分子对氧化碳烟具有良好效果。Kholghy等[4]利用消光法与热电偶采样法研究了生物柴油替代品(辛酸甲酯/正癸烷混合物,摩尔比为50%/50%)在层流共流扩散火焰中的碳烟形成低于正癸烷、1-癸烯和5-癸烯,并进一步分析了酯键对碳烟演变的影响和化学反应机理,指出了位于中央位置的非饱和键显著增加了碳烟的形成。同样是利用消光法测量技术,Abboud等[2]研究得出,当使用含氧燃料,碳烟排放的减少与燃料的氧含量密切相关,而碳烟的生成与燃料中的官能团有关。这些基础含氧燃料的研究表明了含氧燃料的化学成分对碳烟形成有很大的影响,但由于消光法是基于点测量的光学手段,火焰里的二维碳烟分布较难获取,而一些难以测量的区域(如燃烧室出口)往往会导致信息流失。
碳烟的测量方法主要分为光学诊断方法和化学取样分析两大类。其中,光学诊断方法具有非介入性,对火焰及碳烟干扰少,可得到瞬时碳烟数据。对于碳烟测量的光学诊断方法目前主要有自发光技术、散射法、消光法[5]、双色法和激光诱导炽光法(LII)[6-7]。消光法、散射法都是对火焰区域内的单点测量,获得的测量结果是平均值,并且很难直接获得测量区域的二维分布图像。为了分析碳烟在火焰空间的分布,双色法和激光诱导炽光法被引用到碳烟测量研究中,两者均可以获得测量范围内碳烟体积分数的二维分布[8]。LII技术引入了激光片光源技术,能够获取激光片光照射薄层内瞬时碳烟浓度的二维空间分布,结合消光法标定可以实现对碳烟体积分数的定量测量[6]。吴建等[9]利用LII和激光诱导荧光技术测试了不同碳氢燃料的碳烟生成特性。王孟等[8]发现,双色LII的碳烟测量与光腔衰荡光谱法(CRDS)所获取的路径积分衰减系数得到很好的拟合。Lemaire等[10]运用LII技术广泛研究了柴油、柴油替代品、油菜籽甲酯(RME)和不同RME替代品的湍流喷射火焰中的碳烟和多环芳香烃(PAHs)的形成,并得出结论:在纯RME火焰中测量的碳烟体积分数约为纯柴油的16%。同样是LII技术,Das等[11]将其运用于研究双键对酯类碳烟生成的影响,并深入了解了酯类碳烟生成趋势对其化学结构和非饱和度的强烈依赖性。由此可见,LII技术已在碳烟测量方面获得广泛的应用。
针对生物柴油的原料多样性,所生产的燃油理化特性不同,导致碳烟生成产率不同[12]。然而,生物柴油的碳烟生成机理研究相对匮乏。因此,本文针对生物柴油的理化特性差异,选取不同饱和程度的动植物油脂制备的生物柴油,通过可控的池火焰燃烧装置,利用消光法校准激光诱导白炽光技术,探讨碳烟生成的基础特性,揭示含氧生物燃料的碳烟生成机理。激光诱导白炽光技术的运用不仅能定量测量碳烟体积分布,同时也能揭示碳烟在火焰里的二维空间分布,进而对比不同生物柴油及掺混燃料的碳烟分布结构。通过无扰式光学测量手段,研究了生物燃料里的化学成分对碳烟形成的影响。
2 实验装置及研究方法
2.1 池火焰燃烧器与实验工况
本研究所采用的燃烧设备是一种层流并以空气为协流的液体燃料池火燃烧器(
生物燃油在池火焰燃烧情况下不受外界控制,因此其燃油消耗率会因燃油而异。影响池火焰的燃油消耗率主要因素包括燃油的理化特性、火焰的传热特性以及火焰池的结构。如
图 2. 测试燃油在池火焰工况下的平均消耗率
Fig. 2. Average fuel consumption rate tested under pool flame condition
2.2 激光诊断碳烟系统
本研究的碳烟测量采用二维激光诱导炽光技术。激光源为10~25 Hz频率发射的532 nm Nd∶YAG激光器(Litron nanoPIV)。激光光束通过两片柱面透镜(焦距分别为-25 mm和100 mm,在532 nm的波长透光率大于95%),形成一个宽度约为80 mm的平行光片。光片经过一个宽度为0.5 mm、高度为33 mm的狭缝,其上下和左右光强较弱的部分被裁剪掉,形成了一个高度为33 mm的矩形光片。激光面穿过火焰中心,由激光激发的炽光LII信号被装有紫外镜头(Nikon AF Micro Nikkor 60 mm,f/5.6)和带通滤镜(Thorlabs FB400-40,中心带通式,中心波长为(400±8)nm,半峰全宽为40 nm,400 nm中心透光率为48%)的ICCD相机(LaVision Nanostar,1024 pixel×1280 pixel)成像采集。此光路的搭建设计能将PAH荧光、C2辐射和火焰自然光辐射的干扰降至最低[13]。ICCD相机拍摄时相对激光脉冲有20 ns的延迟,以避免多环芳烃荧光信号(PAH LIF)和残留激光散射的干扰。ICCD相机的快门时间较短,为30 ns,可避免测量结果偏向于较大的颗粒物[14]。
为了在炽光信号峰值区域内进行实验,实验前将各燃料激发的激光诱导炽光信号对激光光片能量密度的响应做了标定。本研究在不同激光能量密度下,对六种燃料池火焰在高度33~66 mm的平均LII信号强度进行了测量,并且对于每种燃料的LII响应曲线,均由其对应的最大值进行统一归化。
图 4. 归一化LII信号强度与激光能量密度的关系
Fig. 4. Relationship between normalized LII signal intensity and laser energy density
激光诱导炽光的信号随着激光能量密度的增加而迅速上升,随着碳烟颗粒温度的升高,达到升华点(约3500 K),炽光信号达到一个峰值,并且此时响应曲线较为平缓,表明炽光信号已经达到饱和,并且对于激光能量密度不敏感。经过此饱和区间之后,由于碳烟颗粒升华愈发显著,消耗大量能量,LII信号随着激光能量的增强不升反降。选取饱和区间的能量值进行实验能确保较强的信噪比,同时也可以减少由于激光能量波动而引起的误差[13]。本研究最终选择的激光能量密度约为0.16 J/cm2,如
激光诱导炽光技术测量中所使用的激光能量密度设置需足以激发出强炽光信号。每个能量密度下的激光诱导炽光信号代表在20 Hz的拍摄频率下减去背景基础值后200幅图像的平均值。该平均值是根据燃烧器杯口(HAB)出口0~32 mm高度之间的平均信号强度获得的。测量之前,燃料池的表面均保持在杯口下方1 mm处,以保持火焰高度的一致性。由于使用了燃油箱并连接了燃烧池,燃料在杯里的水平位并未因燃烧而下降。
在研究生物柴油与柴油的掺混时,本文假设激光诱导炽光信号对于激光能量密度的依赖性与纯燃料情况相似,主要原因是由于所有响应曲线是介于纯生物柴油及柴油之间。激光片的能量分布图也在前期的工作中被仔细表征过[17-18]。结果表明,激光片的局部强度波动小于5%,而由空间通量波动引起的响应误差小于6%。本研究的32 mm×32 mm的成像区域里,空间分辨率为40 μm/pixel。然而,在高碳烟含量的情况下,激光诱导炽光存在着信号在火焰中被屏蔽的现象(signal trapping),导致相机捕获到的炽光信号比实际信号弱[13,18]。为了解决该问题,利用了一种基于迭代的反卷积方法来校正二维激光诱导炽光技术图像的方法,以提供更为准确的定量测量结果[13]。通过消光法对激光诱导炽光技术进行标定[13],结合处理信号屏蔽的现象校对方法,不同生物燃料的二维碳烟浓度分布得以测量。消光法是目前被广泛使用的激光诱导炽光技术的定量标定方法,其原理是基于瑞利近似[14],碳烟颗粒体积分数与LII信号强度成正比,比例系数称之为LII标定系数,记为KLII,碳烟颗粒体积分数为fv,LII信号强度为SLII,则根据定义有
所以,在LII信号可通过实验测得的情况下,只要得到标定系数KLII,即可通过线性标定的方法得到碳烟体积分数分布。对于非均匀碳烟分布场,对上式两边分别求积分,可得:
式中:It和I0分别为光线的透射强度和入射强度;dx为光线在介质中传播的距离,如
图 5. 消光系数和透射/入射强度比的关系
Fig. 5. Relationship between extinction coefficient and transmission/incident intensity ratio
根据瑞利近似[14],消光系数Kext定义为
在非均匀介质中,Kext不是常数,此时为了求得光线在一段介质中的总消光量P0,需要求得消光系数沿着激光传播路径的积分:
整理
由于通过ICCD相机得到的LII信号为每个像素的离散值,故可将
需要指出的是,由于本研究产生的碳烟颗粒具有类似的光学性质[14],故仅对柴油(D0)火焰的HAB为25 mm这一高度进行标定,获得的标定系数可用于其他所有LII图像。
本研究中LII测量的误差分为两部分:随机误差和系统误差。1)随机误差主要由火焰的随机微小抖动、激光的能量波动、相机CCD的扰动等随机测量因素引起,这部分误差可以由每个工况200张的LII图像在每个测量点位上的信号标准差计算出来,计算可得小于6%;2)系统误差主要由LII的标定过程引起。因为LII的标定过程要用到碳烟颗粒对于激光的吸收函数E(m),如
2.3 实验原料
本文所测试的生物柴油来源包括棕榈油、鸭脂、鹅脂、餐厨废油及米糠。这些油脂一般是由几种不同的脂肪酸甲酯构成,可以通过酯交换过程炼制成生物柴油。酯交换的典型反应如
表 1. 生物柴油的组成(体积分数)
Table 1. Composition of biodiesel (volume fraction)
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图 7. 废弃餐饮油生物柴油的制备过程——预处理及脂交换过程
Fig. 7. Preparation process of biodiesel from waste cooking oil—pretreatment and transesterification processes
生物柴油和甲酯的理化特性及元素质量分数显示在
表 2. 生物柴油的物理性质[20]
Table 2. Physical properties of biodiesel[20]
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3 分析与讨论
3.1 二维碳烟分布及体积分数
图 8. 火焰实际图(左侧)与相应的碳烟体积分数分布图(右侧)。火焰的二维碳烟体积分数范围介于HAB 为 0~32 mm。不同棕榈油与柴油混合比例:(a)20%;(b)40%;(c)60%;(d)80%;(e)100%,以及(f)传统柴油;(g)废弃餐饮油;(h)米糠;(i)鸭脂;(j)鹅脂生物柴油的火焰及相应的碳烟体积分数分布图
Fig. 8. Actual flame images (left) and corresponding soot volume fraction (SVF) distribution images (right). 2D SVF of flame is derived from HAB of 0-32 mm. Flame appearance and soot volume fraction distribution of different blend fraction of palm biodiesel at (a) 20%, (b) 40%, (c) 60%, (d) 80%, and (e) 100% with diesel, and neat biodiesel from (f) conventional diesel, (g) waste cooking oil, (h) rice bran, (i) duck, and (j) goose
对于碳烟体积分数的分布,生物燃油火焰系列均可观察到随含氧燃料比例的增加,碳烟体积分数随之而降,与其他含氧燃料的碳烟排放趋势一致[13]。对于小于40%含氧燃油比例的火焰,碳烟明显主要分布于火焰翼侧,而60%以上的碳烟主要生成于火焰中心区域。传统柴油则呈现出较高的碳烟体积分数,尤其当碳烟在接近HAB 为15~20 mm时已达到峰值,而含氧燃料的掺混不仅降低了火焰的高度,其碳烟生成也被抑制。
图 9. 碳烟体积分数峰值与生物柴油掺混比例的关系
Fig. 9. Relationship between soot volume fraction peak value and biodiesel blending fraction
对于柴油与生物柴油的混合,当生物柴油的掺混比从0增加至80%时,P的碳烟体积峰值比纯柴油降低了52.8%。D、G、R、W的碳烟体积峰值则分别降低了58.7%,59.0%,47.1%,67.2%。表明了生物柴油的掺混能抑制碳烟的生成。在某些混合工况下,会出现一些峰值线的交叉,不排除是测量时不确定性的影响,但总体的趋势对于所有含氧燃料都是一致的。
生物燃料生成的总碳烟体积显示于
图 10. 碳烟总体积与生物柴油掺混比例的关系
Fig. 10. Relationship between total soot volume and biodiesel blending fraction
3.2 池火焰结构与火焰温度关系
为了研究池火焰燃料蒸发对火焰温度和结构的影响,本研究采用扩散火焰混合率关系以及火焰绝热温度的方法[15]计算了所有测试火焰的理论火焰温度,如
图 11. 预测扩散火焰温度与生物柴油混合体积分数的关系
Fig. 11. Relationship between predicted diffusion flame temperature and biodiesel blending volume fraction
可见火焰高度与计算的火焰温度的关系展示于
图 12. 可见火焰高度与计算的火焰温度的关系
Fig. 12. Relationship between visible flame height and calculated flame temperature
在Roper扩散火焰模型理论研究中[21],圆形射流浮力主导层流扩散火焰Hf可通过下式表示:
式中:T∞、Tf和TF分别为环境温度、火焰温度和燃料温度;S为化学计量摩尔氧化物-燃料比;D∞为氧化物在T∞处的平均扩散系数。在Roper后续的研究[22]中,将实验测得的Hf/QF与1/ln(1+1/S)进行线性拟合,得到
根据1330 m-2.s得到了1500 K时被测火焰的平均温度。此外,对于实验火焰,将
然而,在本研究中,燃料在进入火焰前是经过加热的,因此预计Tf将高于1500 K。如
图 13. 根据Roper模型预测的可见火焰高度与计算的火焰温度的关系
Fig. 13. Predicted visible flame height based on Roper's model as a function of calculated flame temperature
由
如
3.3 碳烟形貌及粒径分布特性
当含氧燃料在池火焰工况下燃烧时,在火焰高度HAB为20 mm处收集碳烟颗粒样品,并使用扫描电子显微镜(LEO GEMINI 1530VP FEG-SEM)系统进行分析。
图 14. 碳烟颗粒物的SEM图像及平均粒径分布。(a)~(c),(g)~(i)碳烟颗粒的SEM图像;(d)~(f),(j)~(l)颗粒物的平均粒径分布。D0、P、D、G、W和R分别代表柴油、棕榈、鸭脂、鹅脂、废弃餐饮油及米糠生物柴油。颗粒物直径分布的最佳对数正态拟合以曲线显示
Fig. 14. SEM images of soot particles and mean particle diameter distributions.(a)-(c), (g)-(i) SEM images of soot particles; (d)-(f), (j)-(l) mean particle diameter distributions. D0, P, D, G, W, and R represent diesel, palm, duck, goose, waste cooking oil, and rice bran biodiesels, respectively. Best lognormal fitting of particle diameter distribution is shown as a curve
碳烟颗粒物的粒径符合正态分布,因此将直径分布的最佳对数正态拟合应用于每种情况,以得出几何平均直径Dm。结果表明,柴油的Dm为63 nm,相对于其他含氧燃料粒径较大。P、D、G、R、W的Dm分别为57、46、41、53、37 nm。由此可见,碳烟体积分数越高的情况下,产生的Dm也会较大。通过
基于用
表 3. 燃油在池火焰工况下的碳烟颗粒的粒径和数量密度
Table 3. Soot particle diameter and density of different fuels under pool flame condition
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4 结论
本文采用消光法校准的激光诱导白炽光技术测量了五种生物柴油、柴油及含氧燃料与柴油掺混的层流池火焰所生成的碳烟体积分数。尽管生物柴油的甲酯组分不同,导致燃烧时的火焰温度不同,但与碳烟排放的关联性不强。在宏观的模型预测中,生物柴油之间的火焰高度区别不大,与实验结果一致。从理化特性角度而言,虽然生物柴油之间的氧质量分数相似,但饱和程度因不同甲酯组分而大不相同。生物柴油的饱和度与碳烟形成具有关联性,不饱和度较高的生物柴油更倾向于生成较多的碳烟颗粒物。对于柴油与生物柴油的混合燃料,随着生物燃料组分的增加,碳烟生成也随之下降,呈现出近线性的关系。纯含氧燃料的碳烟体积分数峰值为传统柴油的7.1%~30.5%之间,表明含氧燃料能有效抑制碳烟的生成。通过电子显微透镜分析,相对于不饱和度较低的燃料,高不饱和度燃料更倾向于生成较大的碳烟颗粒,但颗粒物数量密度相对较低。后者的不饱和C-C双碳键在氧化过程当中更倾向于生成乙炔或苯,为火焰提供碳烟生成的初始物质及表面增长物质。较长碳链的甲酯更倾向于生成更多的碳烟,而碳烟颗粒物的粒径也会更大。本文证实了生物柴油的碳烟生成会因甲酯组分而异。为了更好地研究并展现生物柴油的碳烟排放情况,所采用的反应机理模型需根据原物料的特点,考虑不同甲酯组分的配比及理化特性,以构建合适的反应子机理模型。
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Article Outline
曹铭锟, 张振东, 田波. 基于激光诱导炽光法的生物柴油碳烟测量[J]. 光学学报, 2023, 43(10): 1012003. Mingkun Cao, Cheng Tung Chong, Bo Tian. Measurement of Soot Generated by Biodiesels Using Laser-Induced Incandescence Method[J]. Acta Optica Sinica, 2023, 43(10): 1012003.