Physical and chemical comparison of soot in hydrocarbon and biodiesel fuel diffusion flames: A study of model and commercial fuels
Introduction
As worldwide energy demands increase, renewable fuels are attracting a great deal of attention as a pathway towards an environmentally sustainable future and improved energy independence. Biodiesel fuels offer a further benefit that they alleviate hydrocarbon and particulate matter (PM) emissions that detract from the efficiency advantage of diesel combustion [1], [2]. For engine applications, raw vegetable oils are converted to fatty acid methyl (or ethyl) esters [3] to avoid pour point, deposits, corrosion, and related problems. The resulting biodiesel fuels exhibit good cetane values and lubricity. But, there are a number of possibly advantageous or disadvantageous differences with petroleum diesel fuel, including presence of oxygen, low sulfur and aromatic content, and higher viscosity and bulk modulus [4].
Prior to US Environmental Protection Agency 2007 and European Union EU5 particulate matter (PM) emissions standards, interest in biodiesel fuel’s impact on soot formation was principally from the perspective of its ability to lower engine-out PM emissions. Because the stringencies of these new standards necessitate the use of diesel particulate filters (DPF), the question of fuel impact on PM has become more complex. DPFs are sufficiently efficient that they are used to indirectly help control NOx emissions via exhaust gas recirculation (EGR) [5]. Here, biodiesel fuel’s ability to lower engine-out PM emissions can extend the time interval between DPF regenerations and, thereby, increase fuel economy. But of concern are questions such as: “How do differences between petroleum and biodiesel fuel soot affect DPF loading and regeneration?” [6], [7].
There have been numerous engine studies of biodiesel fuel’s impact on PM emissions [1], [8], [9], [10], [11]. These directionally agree that biodiesel fuels, neat or in blends, provide a PM emissions benefit, but they vary in quantitative details. In engine studies there are a multitude of factors that can contribute to variability, such as fuel spray properties, ignition characteristics, exhaust aftertreatment, and so forth, which make it difficult to uncover the mechanism by which biodiesel fuels benefit PM emissions. Investigations in flames and high temperature reactors offer attractive complements to engine studies, since they focus on combustion generated soot, the generally dominant constituent of diesel exhaust PM.
Because fatty acid methyl esters (FAME) constitute complex mixtures of saturated and unsaturated long chain methyl esters [3], model compounds have been sought to help develop insight into the chemical kinetics of biodiesel fuel oxidation [12]. One choice has been methyl butanoate, which has served in jet-stirred reactor, counter-flow diffusion flame, and modeling studies [13], [14]. Recent work has found that methyl butanoate lacks sufficient low temperature reactivity to serve as an ideal surrogate for compression ignition studies, but suggests that it remains useful as a means to examine the influence of methyl esters on soot formation [15].
The goal of the present work is to compare soot formation along co-flow diffusion flames run with methyl butanoate, hexadecane, benzene, soy derived FAME, and ultra-low sulfur diesel (ULSD) fuels. The tools used to accomplish this are differential mobility analysis (DMA) and laser ablation particle mass spectrometry. DMA measurements recently demonstrated bimodal soot size distributions in rich premixed flames, where a persistent nucleation mode indicates sustained particle inception along the entire height of the flame [16], [17], [18], [19], [20] This nanometer size soot mode is identified with the precursor particles observed by Dobbins et al. [21], [22] in transmission electron micrographs, and by D’Alessio and coworkers [23], [24] in UV absorption, and which higher in the flame combine into the aggregates that comprise the accumulation mode.
Aerosol mass spectrometry is seeing widespread use in ambient measurements, and is also extending to combustion sources [25], [26], [27]. It has thus far seen less use for studying flames. Grotheer and coworkers have developed a mass spectrometer capable of recording ions of more than 1 million mass units, and used it to identify two classes of soot precursor particles: one consisting of PAH stacks and other of molecular clusters [28], [29]. Whereas this application was to whole particles, other studies by Öktem et al. [30], Bouvier et al. [31], and Maricq [32] have applied particle mass spectrometry to interrogate soot composition in premixed and diffusion flames. These all find that PAHs figure prominently, as previously observed in laser microprobe mass spectrometry by Dobbins et al. [33], [34] and Blevins et al. [35], but they differ in which species are dominant, and also on the question of to what extent aliphatic species are present. The discrepancies are minor compared to the benefits of the approach, and likely arise from differences between specific ionization schemes used to probe the soot material, as well as the inherent difficulties in quantifying the compositions.
The differential mobility and particle mass spectrometry techniques are described in the following section along with the burner and soot sampling method. Results are then presented on the size distributions, electrical charge, and composition of soot particles from the five fuels of this study, with more attention given to methyl butanoate and hexadecane flames. The size and electrical charge distributions include comparisons to coagulation model predictions and to transmission electron microscope images. The results are then discussed with respect to previous flame studies and the implications for particulate emissions from biodiesel fuels.
Section snippets
Burner and soot sampling
Figure 1 presents an overview of the experimental apparatus. A coflow burner is constructed from a 2.2 cm diameter, 15 cm long, quartz outer tube for air flow and an inner 1.0 cm tube to deliver the fuel. The inner tube is mounted with a sliding seal to permit height adjustment, and the last 1–2 cm contains a quartz wool plug to curb bubbles forming in the fuel. The air flow is set to 7.6 L/min with a flow controller; however, changing this flow by ±25% has little perceptible effect on the flame. A
Soot mobility size distributions
We examine in detail three aspects of soot formation: particle size, electrical charge, and composition. The data derive from a flame geometry dictated by the coaxial burner described above, run with an air flow of 7.6 L/min and a liquid fuel rate adjusted to produce a stable flame approximately 4.5 cm from top of burner to flame tip. Figure 3 illustrates the evolution in soot size distribution with increasing height in methyl butanoate and hexadecane flames. The particles evolve through three
Discussion
As the above results attest, differential mobility analysis and aerosol mass spectrometry offer a wealth of data on soot evolution in flames. The necessity to sample soot from the flame introduces a degree of intrusion, for example a drop in temperature in proximity to the probe [16]. But rapid dilution quenches flame chemistry, and on-line capability allows immediate interrogation of soot particles, avoiding potential interferences from sample collection and handling. The DMA provides detailed
Conclusion
The present work applied differential mobility analysis and laser ablation particle mass spectrometry to examine what influence methyl ester biodiesel fuels might have on the size distributions, electrical charge, and composition, of soot formed in diffusion flames. The chemical composition contains the same characteristic features for both biodiesel and hydrocarbon fuels: a series of Cx ions, groups of PAHs belonging to the stabilomer series observed by Dobbins et al. [34], [33], and a long
Acknowledgments
I am grateful to Jon Hangas for his help producing the soot particle TEM images and to Jim Anderson for providing the soy biodiesel fuel and describing its properties.
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2022, FuelCitation Excerpt :This assertion can be evidenced by Zhang and Boehman [43] who found that the soot generated from laminar co-flow diffusion flames of the oxygenated fuel (methyl 2-butenoate) and non-oxygenated fuels (n-pentane) possessed similar O/C ratio. Similarly, the results obtained by Matti Maricq [44] revealed that the soot particles generated from diffusion flames of non-oxygenated hydrocarbon fuels (hexadecane) and biodiesel exhibited similar chemical composition. Instead, the injection system response and combustion characteristics changed by the various fuel formulation may considerably affect the O/C ratio of the resulting soots.