Soot formation and growth with palladium acetylacetonate-toluene injection in ethylene base flames investigated by in situ synchrotron small-angle X-ray scattering
Introduction
Soot emission caused by incomplete combustion of hydrocarbon fuels impacts negatively upon environment and human health [1]. For combustors and engines themselves, soot formation is of particular concern since it leads to carbon deposition on engine's inner wall, fuel nozzle and other places, and this deposition decreases combustion efficiency and engine performance [2]. Thus, there is an urgent need to suppress the soot pollution in combustion systems. To date, several strategies on soot reduction have been proposed, including engine operation condition controlling, catalytic combustion and so on [3,4]. Catalysis is an effective approach to improve combustion efficiency and reduce soot emission, which is mainly used in gas turbines such as power plants and ships. Thus, investigations in the field of soot suppression by adding catalysts in combustion process attract scholars increasingly.
Transition metals as additives in combustion play a vital role in inhibiting soot yield [2]. Recently, GE used organo-Ce and organo-Fe compounds as catalyst additives in heavy-duty gas turbines and the results showed that the additives reduced the soot emission significantly [5]. Iron-based and manganese-based additives are the widely used catalysts in laboratory studies for soot removal. Kim and Hahn [6] demonstrated that the catalytic effect of iron atoms and iron oxides decreased soot emission by promoting soot oxidation. Marsh et al. [7] pointed out that methylcyclopentadienyl manganese tricarbonyl (MMT) could inhibit the soot formation in JP-8 flame as oxidative catalysts. Besides, an in situ study by Tang et al. [8] identified the soot reduction in an ethylene flame associated with nickel acetylacetonate addition. In contrast to the above research, Hirasawa et al. [9] argued that ferrocene additive enhanced soot emissions because iron oxide nanoparticles could induce soot particle nucleation. In addition, Feitelberg et al. [10] described that the catalysts increased the soot volume fraction by accelerating the rate of soot growth reaction. Though many studies have been carried out on transition metal additives, the mechanisms for their effects on soot formation remain unclear.
In addition to the transition metal additives mentioned above, there is an evidence suggesting that organo-Pd compounds are effective catalysts for rapid ignition and ignition temperature reduction [11]. However, effects of Pd-based organometallic compounds on soot formation were not reported previously. In this article, we will study the effect of Pd(acac)2 additive on the formation and growth of the soot particles with the additive injection in ethylene flames. Synchrotron small-angle X-ray scattering (SAXS) was used to in situ probe the soot particles in the flames. In recent years, there has been an increasing amount of SAXS investigations on the soot particles [8,12,13]. As an in situ measurement, SAXS can probe the size, distribution, morphology information and volume of soot particles. Besides, transmission electron microscopy (TEM) was used as a complementary method to ex situ characterize the size, morphology and nanostructure of the soot particles.
Section snippets
Experimental setup
The SAXS experiments were conducted at the beamline 12-ID-C at the Advanced Photon Source in Argonne National Laboratory. The parameters of the X-ray configuration were as follows: the incident X-ray energy E = 8 keV, wavelength λ = 0.155 nm, and beam size 0.6 × 0.1 mm (width × height). The exposure time was 0.5 s for each measurement. The SAXS data were recorded on a homebuilt 1024 × 1024 pixels 2D platinum CCD detector. The experimental apparatus was depicted in detail in a previous study [8]
Flame structure
Flame temperature is an important property of the flame structure, and plays a key role in soot formation and particle size [20]. A previous study has established that flame temperature affects the rate of PAH formation [21]. In order to assess the influences of Pd(acac)2 additive on the flame temperature, the temperatures for both undoped and doped flames along the centerline of the flames were measured by using a Pt-10%Rh/Pt thermocouple (Inconel 600 coated with 1.57 mm sheath diameter). Due
Conclusions
The effects of the Pd(acac)2 addition on the soot formation and growth with the additive-toluene injection in the ethylene flame have been studied by using synchrotron SAXS and TEM analysis. For the primary soot particles, the mean particle size increases with increasing HAB for both undoped and Pd(acac)2 doped flames. The volume fraction of the soot particles is also larger at higher flame region. Comparing the undoped flame with the doped flame, the additives affect the particle size, volume,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors gratefully acknowledge the National Natural Science Foundation of China (the grant number 91641125 and 71690245) for their financial support. We thank for the “IRENA” micro package provided by Dr. Jan Ilavsky for data fitting. This research was performed, in part, at the APS, a U.S. Department of Energy (DOE), Office of Science User Facility under Contract No. DE-AC02-06CH11357.
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