Elsevier

Combustion and Flame

Volume 158, Issue 1, January 2011, Pages 105-116
Combustion and Flame

Physical and chemical comparison of soot in hydrocarbon and biodiesel fuel diffusion flames: A study of model and commercial fuels

https://doi.org/10.1016/j.combustflame.2010.07.022Get rights and content

Abstract

Data are presented to compare soot formation in both surrogate and practical fatty acid methyl ester biodiesel and petroleum fuel diffusion flames. The approach here uses differential mobility analysis to follow the size distributions and electrical charge of soot particles as they evolve in the flame, and laser ablation particle mass spectrometry to elucidate their composition. Qualitatively, these soot properties exhibit a remarkably similar development along the flames. The size distributions begin as a single mode of precursor nanoparticles, evolve through a bimodal phase marking the onset of aggregate formation, and end in a self preserving mode of fractal-like particles. Both biodiesel and hydrocarbon fuels yield a common soot composition dominated by CxHy+ ions, stabilomer PAHs, and fullerenes in the positive ion mass spectrum, and Cx- and C2xH in the negative ion spectrum. These ion intensities initially grow with height in the diffusion flames, but then decline during later stages, consistent with soot carbonization. There are important quantitative differences between fuels. The surrogate biodiesel fuel methyl butanoate substantially reduces soot levels, but soot formation and evolution in this flame are delayed relative to both soy and petroleum fuels. In contrast, soots from soy and hexadecane flames exhibit nearly quantitative agreement in their size distribution and composition profiles with height, suggesting similar soot precursor chemistry.

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 CxHy+ 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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