Measurements of structures and concentrations of carbon particle species in premixed flames by the use of in-situ wide angle X-ray scattering
Introduction
Soot and nanoparticles from combustion are of considerable interest as commercial products and because of the impacts they can have on health [1], [2], [3] and the environment [4], [5], [6], [7], [8]. During the last decade much attention has been directed towards unraveling the mechanisms that control particle production [9], [10]. They span wide ranges of length and time scales [10], [11], [12], [13], and molecular dynamics simulations show that carbon cluster formation and structural changes are strongly dependent on temperature [14], [15], [16], [17]. To significantly advance models of particle generation and destruction, it is necessary to develop complementary theoretical and experimental tools that probe reactive structures on the atomic scale, and provide accurate size distributions. Many experimental techniques that are employed to study molecules and nanoparticles [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41] require physically extracting the samples from the reaction/flame region. This usually introduces local disturbances on the temperature and flow conditions, hence on the formation dynamics and concentrations. Consequently, non-perturbative in-situ measurements are preferable. Hard X-ray methods utilizing photons of wavelengths comparable to interatomic distances have considerable potential for interrogating morphology and chemical bonding. These techniques can overcome some of the limitations of laser techniques [42], [43] that operate in the infrared to ultraviolet regions and frequently suffer from interference by flame/reactor background radiation. In addition, the intensity and resolution of optical signals generally decrease with increasing size, temperature, and internal energy of the soot precursor molecules such as polycyclic aromatic hydrocarbons (PAHs) [42], [43], [44], [45], [46].
Three principle X-ray techniques have been applied to soot formation studies: X-ray absorption spectroscopy (XAS); small-angle X-ray scattering (SAXS); and wide-angle X-ray scattering (WAXS). XAS methods are most suitable for ex-situ studies, since they are element sensitive and can probe the local environment around the absorbing atom and thus deliver important chemical information on solid carbon [47]. For carbon species absorption occurs in the soft X-ray wavelength region and therefore requires relatively low pressure conditions. WAXS is primarily sensitive to two parameters: the size of the electron cloud of a free atom (such as that of argon) and the distance between atoms in a molecule. The technique is usually applied in the hard X-ray wavelength region technique and therefore can be employed for in-situ atmospheric- and high-pressure in-situ measurements to ‘sense’ the structure of small nanoparticles and soot [48], [49]. Among the three, SAXS is probably the most extensively used technique for in-situ studies of the size distributions and morphologies of nanoparticles and soot [50], [51], [52], [53], [54], [55], [56]. The X-ray techniques have also been combined to give a more complete picture on the properties of soot. XAS methods have been used by Braun et al. [57] in combination with ex-situ WAXS to obtain the size of graphitic crystallites, lattice spacing and relative proportions of aliphatic and aromatic species in soots. This detailed structural information was complemented by SAXS measurements that determined sizes and fractal dimensions of particles. SAXS and WAXS have also been applied in-situ in flames, supplying important information on the sizes, fractal properties, concentrations, and structure of soot and nanoparticles that has given more insight into the formation dynamics of soot and precursors [58], [59], [60].
Among the above mentioned techniques WAXS has a particular advantage in that it can provide information on the clustering and atomic level arrangement of atoms and thus deliver important information of the chemical character of carbon containing molecules, clusters and particles. In-situ studies on nanoparticles and soot were performed by Ossler and co-workers [58], [59], [60] on flames stabilized with a plate that created a high density of particles just below the plate surface that could be probed with a relatively low flux X-ray source (∼1010 photons/s) [49]. While valuable information is obtained from such measurements the presence of the plate perturbs the flow field in the flame. SAXS usually yields higher signals than WAXS and has been applied to flames without plates that are stabilized just by the burner surface [50], [51], [52], [53], [54], [55], [56]. In this manuscript these flames will be referred to as freely propagating flames and are preferred as the flow fields are not perturbed. However, the particle densities are much lower than just beneath a stabilization plate and much higher photon fluxes are required for adequate in-situ WAXS measurements.
In this work we present a novel methodology with results from an exploratory study on the use of intense X-ray radiation to obtain in-situ quantitative structural/chemical information and concentrations about carbon-based nanoparticles that were produced in fuel rich freely propagating atmospheric pressure flames. The work describes how particle species concentrations and structural information were obtained from the WAXS profiles and chemical calculations. From these data the amount of carbon bound in small molecular species and particles was also determined.
Section snippets
Definitions
The theoretical background on coherent and incoherent scattering required for interpreting the experimental data presented in this work has been described in detail in Ref. [49] and references therein. Thus only a brief overview of the essential equations is outlined below. Additional details and sources to atomic scattering data can be found in Ref. [61] and references therein. For the hydrogen atom the cross sections given by Bentley and Stewart were used. Whereas, the floated sphere approach
Experimental
The experiments were conducted at the BESSRC beam line 12-ID-C of the Advanced Photon Source (APS) at Argonne National Laboratory. The energy of the X-ray beam was 12.0 keV (1.03 Å). The spectral bandwidth was 2% or better, and the incident average power was ∼1 W in a rectangular cross-section of 600 μm × 600 μm (Flux ∼ 1016 photons s−1). A premixed atmospheric pressure ethylene/oxygen flame diluted with either Ar or N2 was generated above the surface of a porous bronze-plug, McKenna-type,
Experimental results and discussion
Scattering data were obtained for six flames and two cold reference flows. Fig. 8a and b presents scattering data related to the Ar-diluted, A1–A3, flames and the reference measurement, Aref. Similar data are displayed in Fig. 8c and d for the N2-diluted, N1–N3, flames and Nref, the nitrogen reference flow. In Fig. 8a and c the scattering intensities normalized by the intensity of the X-ray beam are given. The drop in intensity with decreasing q for q < 1.7 Å−1 is attributed to the partial
Conclusion
This work has presented an in-situ study and a method to characterize and measure carbon species in sooting atmospheric pressure premixed flames, taking into account effects from high temperature chemistry. The flame temperatures and the concentrations of nanometer sized structural carbon species such as graphene, graphitic, and amorphous, and major small molecular flame species were obtained by combining WAXS measurements and detailed chemical and scattering calculations. The technique is able
Acknowledgments
We gratefully acknowledge Raghu Sivaramakrishnan and Nicole J. Labbe for the chemical calculations and the group that installed the silicon mirror on BESSERC 12- ID-C: Mark A. Beno, Randall E. Winans, Mark S. Engbretson, Guy Jennings, Charles A. Kurtz, Nadia E. Leyarovska, Lynn W. Ribaud, and Soenke Seifert. We are grateful for the care taken by Traffic Analysis Group with regard to the transport of the equipment. The use of the Advanced Photon Source was supported by the U. S. Department of
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