THE STUDY OF INFLUENCE OF ELECTRIC FIELD ON SOOT FORMATION AT LOW PRESSURE Z.A.Mansurov, N.G.Prikhodko, T.T.Mashan, B.T.Lesbaev Al-Farabi Kazakh National University, Institute of Combustion Problems, Bogenbay-Batyrst., 172, 480012, Almaty, Kazakhstan, E-mail:
[email protected] Abstract Influence of a constant electric field of different polarity on sootformation process, yield and structure of soot particles, on yield of fullerenes in a range of pressure U=0.5 … 20 kV at burning of benzene vapours in the environment of oxygen at atomic ratio С/О=1.0 with addition of 10 % of argon on volume, at pressure in system Р=40 Torr is investigated. The analysis of obtained results has shown, that influence of external electric field under certain conditions can increase yield of soot (up to 10 %) in comparison with yield without applying electric field. It is found that for soot particles most typical the normal law of distribution with dislocation in the zone of equilibrium for negative polarity at pressure U≥20 кV. The electric field, as a whole, promotes growth of the average size soot particles in comparison with the average size of particles received without field influence. It is established, that the length of diameter soot package La is subject to greater change aside increases, than its height Lс, at voltage change. Fullerenes C60, С70 and PAH are identified in extracts of soot by the method of IR-spectroscopy. It is established, that yield of fullerenes significantly starts to grow (α≥13%) at applying on flame of electric field in the abnormal glow discharge area. The analysis of Xray photogram of soot samples dry extracts showed presence of peaks, characteristic for fullerenes C60 and C70. Peaks correspond to following crystal phases of fullerenes: orthorhombic, cubic and hexagonal (C70 only hexagonal).
Introduction Disperse carbon in the form of soot usually is a product of incomplete combustion of hydrocarbon substances. Such soot material consists of separate closed particles where primary particles are spherical globules in diameter from 9 up to 600 nm, [1,2] that are capable to combined chemically with each other and to form secondary structure, being united in aggregates of type linear dendrite chains, spirals, clusters, so-called soot structures. The form of particles of soot majority is close to spherical. Despite of a number of hypotheses and assumptions available to the present time, the standard concept of sootformation and structure of soot particles do not exist yet. It is known, that particles of soot (globules) represent a chaotic set of separate microcrystallines with the characteristic sizes on thickness Lc=12÷17 Å and distance Lа=15÷30 Å, [3]. However, depending on soot obtainment process, characteristic sizes can change and in wider range of values: Lc=10÷80 Å и Lа=15÷150 Å, [4, 5]. Microcrystallines represent packs that are parallel embedded hexagonal grids; random focused concerning the general normal. Distance between planes of atoms parallel layers d002=3.45÷3.7 Å, i.e. microcrystallines are formed by 3-5 layers, [3-5.] There is soot, for example acethylene for which is available 7 and more flat layers of carbon atoms [1]. Plane grids in microcrystalline are displaced one concerning another that increases disorder of soot particle structure even more. All variety of disperse solid carbon can to reduced to the narrow circle of forms, including soot particles in the form of fullerenes and fullerene-like nanoparticles, multiplayered globules and aggregates, and also loose amorphous carbon, [1]. The latest is thermodynamically is unstable system that at annealing will be transformed in closed soot particles. At present time investigations on fullerenes obtainment during process of hydrocarbons combustion (benzene, acetylene, naphthalene, etc.) in the environment of inert gas (argon, helium) proceed are continued. Thus fullerenes yield under optimum conditions varies from 13 up to 22 % from received weight of soot. Nowadays process of fullerenes obtainment in a flame at use of the combined (hybrid) methods has a great interest. One of such methods is the method of influence on a flame of the discharge under electric field action. It is necessary to note, that any discharge increases not only temperature, but also concentration of charged particles in flame that can render the big influence on fullerenes formation.
As analysis shows, the effect of electric field action on flame consists in additional ionization (to already taking place thermal and chemical ionization of combustion products) neutral molecules and occurrence of the charged ions, or, on the contrary, in their recombination. The greatest ionization of combustion products occurs under condition when voltage between electrodes does not reach the value leading spark breakdown or to the arc discharge. It was experimentally revealed that fullerenes are formed effectively at presence in plasma spontaneous or compelled ionization instability. Spherically closed fullerene molecule С60 keeps its structure without dependence from its charge. Plane cluster С60 behaves differently depending on its charge. The purpose of given research work was detection of electric field influence (in the range from dark up to the abnormal glow discharge) on yield and structural characteristics of formed soot, fullerenes and PAH. Experimental Investigations were carried out in combustion of benzene vapours with oxygen at atomic ratio С/О=1.0 with addition 10 % argon on volume, at pressure Р=40 Torr, at rate of yield of a gas mixture from torch Vх.см.=18.4 sm/s. In experiments was used the system of electrodes: "needle-plane" (interelectrode distance L=18 sm) and "plane-plane" (L=21.4 sm), Fig.1, pos. 4.
Fig. 1. Scheme of burning equipment Burner (plane electrode) has been chosen as one of electrodes, fig. 1, pos. 2. To the electrode ("needle" or "plane"), placed in the top part of burning equipment constant was applied high voltage U negative ("minus" on the top electrode - « m » or positive ("plus" on the top electrode - « m ») polarity in a range from 0.5 up to 20 kV. With applying electric field it was created negative or positive corona discharge, that at U≥10 кV, (irrespective of polarity) transfer to glow, accompanied by occurrence of a shone twisting thin cord (dc=3÷5 mm). Herein magnitude of a current of gas interval conductivity in process of growth of size of the applied voltage, changed within limits I=5×10-7÷0.3 А, at a corresponding voltage loss. Influence on structure and yield of soot particles As it is known soot is polydisperse substance: in the sample of the same kinds of soot there are particles of various sizes, fig. 2.
Fig. 2. Microphotograph of soot sample: Е=1кV ( m ).
Usually degree of soot dispersion is characterized by average arithmetic value of soot particles diameter. However this parameter does not reflect character of particles disorder on sizes, does not allow making a conclusion about degree of their homogeneity and about the law of particles distribution on sizes. The degree of soot dispersion directly defines its specific surface that is important physical property of soot. Herein, the smaller size of particles, the bigger theirs specific surface. With the purpose of revealing of soot particles distribution law on soot samples sizes were investigated on microphotos, received on electronic microscope Jem-100CX at the accelerating voltage 100 kV. Results of these investigations are presented on fig. 3, and numerical characteristics of distributions of on sizes are resulted in table 1.
Fig. 3. Distribution of soot samples particles on sizes in dependence on voltage and polarity: 1 – U=1 кV ( m ); 2 - U=5 кV ( m ); 3 - U=5 кV (±); 4 - U=10 кV ( m ); Table 1. Parameters of soot particles distribution on sizes U, Polarity Sizes of soot RootCoefficient of кV particles, d, nm meanvariation, V dav dmax dmin square deviation, δ, nm 0 17.2 22.5 11.2 3.36 0.195 1 18.2 31.4 7.84 6.38 0.351 m 5 17.5 26.2 9.84 4.29 0.245 m 10 18.9 26.7 11.1 4.01 0.216 m 20 18.8 30.6 14.1 4.19 0.223 m 1 22.8 33.3 11.5 4.76 0.209 ± 5 19.1 34.5 6.9 5.12 0.268 ± 10 20.9 36.4 12.1 4.68 0.224 ± 20 16.4 28.1 10.8 4.64 0.283 ±
Normalized value asymmetry, β1
excess, β2
0.019 0.28 0.053 0.0004 2.36 0.016 0.95 0.55 0.93
1.9 2.3 2.42 2.37 3.5 2.86 3.81 3.54 3.16
As have shown results of researches, distribution of soot particles on sizes are characterized by obvious absence of the expressed asymmetry without dislocation of a mode of distribution concerning them median values, (see fig. 3). It is established also that with increase of applied voltage value, irrespective of polarity, the coefficient of distributions variation practically is in one range of values, (see table 1). At that the least factor of a variation is observed without applying electric field. The estimation of computation results by means of Pearson curves [6] on sizes normalized parameters of asymmetry β1=µ32/µ23 and excess β2=µ4/µ22 (µi - central moments of 1-st order of distribution) has shown (see tab. 1), that for soot particles formed at combustion of premixed
benzene-oxygen mixture at pressure 40 Torr, not only without applying field, but also irrespective of applied voltage value and polarity, most typical normal law of distribution. However for negative polarity at U≥20 кV voltage there is a tendency of dislocation of the law of soot particles distribution in area of homogeneity distribution that confirms earlier conclusion in work [7], about ordering structure soot particles with voltage increase. Influence of applied voltage on soot yield has been estimated, and obtained samples have been subjected to X-ray diffraction method. The radiographic analysis spent on diffractometer DRON-3M (CuKα-radiation, l=1.54051) at intensity 1000 imp/s, U=30 кV and I=20 мA. Results of studies are given in tab. 2. Table 2. X-ray structural characteristics of soot with applying electric field U, Polarity La, Å Lc, Å кV 0 62.98 10.43 0.5 87.2 11.1 m 1 56.88 10.85 m 2 54.16 11.1 m 5 81.04 10.65 m 10 86.95 10.44 m 20 45.64 11.1 m 0.5 62.93 11.34 ± 1 56.58 11.35 ± 2 51.71 10.87 ± 5 51.6 10.86 ± 10 75.57 10.87 ±
d002, Å 3.68 3.71 3.80 3.71 3.68 3.65 3.71 3.68 3.63 3.71 3.74 3.71
Range of voltage (1.0
