| Dielectric barrier discharge (DBD) technology is widely used in removing volatile organic compounds (VOCs). Previous studies focused on using DBD with or without catalysts for the decomposition of VOCs, such as benzene, toluene, xylene, halo-hydrocarbons, and formaldehyde. However, it has been reported that alkanes provide the largest percentage of total volatile organic compounds (TVOCs) to the atmosphere, while the removal of alkane VOCs are rarely studied. Moreover, the existing DBD technologies normally produce a large number of particles when VOCs are decomposed, resulting in secondary pollutions and seriously harmful to human health. Therefore, a DBD technology removing gaseous contaminants without emitting secondary pollutants, such as particles, is urgently required. The aim of this study is to develop a DBD technology that can effectively remove alkane VOCs without particle formation after characterization of alkane decompostion. In this study, decomposition of alkane (n-hexane, n-heptane, n-octane, n-nonane, and n-decane) in simulated air in a tubular dielectric barrier discharge (DBD) reactor was firstly carried out. Influences of specific energy density (SED) and initial concentration on their decomposition were investigated, and the kinetic model of n-alkane decomposition was established. Products of alkane decomposition were analyzed using a gas chromatograph (GC), gas chromatography-mass spectrometry (GC-MS), and ozone analysis meter. The mechanism of n-alkane decomposition was proposed. Secondarly, the decomposition of n-hexane in 15% O2 (N2 balance) were also investigated in planar DBD reactors filled with three different kinds of dielectric balls, viz. quartz(Q), less porous (LP), and activated alumina (PA) balls. Products and particle emission characteristics of n-hexane decomposition were analyzed by using a GC, GC-MS, Fourier-transform infrared (FTIR) spectroscopy, scanning mobility particle sizer (SMPS), and ozone analysis meter. The composition of particles and the mechanism of particle emission reduction were also discussed. The main results are summarized as follows:(1) The conversions of n-alkane decomposition increase with increasing SED, but decrease with increasing initial concentration. When SED was 211.2 J/L and n-octane initial concentration was 80.4 ppmv, the conversion reached the highest value (96.5%). The energy efficiencies decrease with increasing SED, but increase with increasing initial concentration. The highest energy efficiency was obtained at 62.5 J/L SED and 343.4 ppmv initial concentration of n-octane.(2) A kinetic model of n-alkane decomposition was established. It was found that decomposition rates of alkane decomposition are proportional to the exponent a of alkane concentration and the exponent β of energy density. The values of a for n-alkanes with carbon number of 6,7,8,9, and 10 in their molecules were 0.495,0.89, 0.659,0.73, and 0.52; and the β values were -0.264,-0.232,-0.198,-0.339, and -0.503, respectively. The values of β have a linear realation with carbon numbers. The values of rate constants k are almost the same even calculated by different ways when carbon number m in the alkane molecule CmH2m+2 is fixed, indicating that the present kinetic model for alkane decomposition is acceptable.(3) From the GC and GC-MS analysis results, n-alkane are mainly decomposed to CO2, CO, and hydrocarbons containing oxygen, such as ketones which O atoms are located on the second, third, or fourth carbon. The hydrocarbons containing oxygen can presence as liquid particles since they have boiling points higher than n-alkane and melting points lower than room temperature.(4) The mechanism of n-alkane decomposition is as follow: n-alkane decomposition is initiated by dehydrogenated to form alkane free radicals. Alkane free radicals are oxidized to ketones and other hydrocarbons containing oxygen, or further dehydrogenation to form alkene and alkane free radicals containing less carbon. Alkane free radicals containing less carbon can further react to form small alkane free radicals, such as ·CH3 and ·C2H5. Alkene and small alkane free radicals can be finally oxidized to CO and CO2 by the reactive species.(5) The analysis results of particles decomposed in four kinds of different DBD reactors using SMPS, FTIR, and GC-MS showed that the number concentration of the particles from the DBD reactor without balls has three peaks at 22.7,51.4, and 100.9 nm. The total particle number concentration was around 106 orders of magnitude. The number concentration of the particles from the DBD reactor with Q balls has two peaks at 20.8 and 56.5 nm, and the total particle number concentration was similar to that of None. The number concentration of the particles from the DBD reactor with LP balls also had two peaks at 68.24 and 213.77 nm, and the total particle number concentration was around 104 orders of magnitude. However, the total particle number concentration using the DBD reactor with PA balls was only 33～83 #/cm3, and the particle number concentration for each diameter was less than 15 #/cm3. The particles are composed of 2-hexanone,3-hexanone, and little amount of other hydrocarbons containing oxygen.(6) The DBD reactor filled with PA balls is the DBD reactor which can effectively decompose hexane as well as reduce particle emission. n-Hexane conversion and energy efficiency using the DBD reactor filled with PA balls were both higher than those of other three reactors, and the highest values of conversion and energy efficiency were 78.6% and 0.15 mol/kWh, respectively. Furthermore, this kind of DBD reactor could completely reduce the emission of the liquid particles for a wide range of 10-1000 nm and the total particle number concentration in decomposition products was 2 to 5 orders of magnitude lower than that of other reactors.(7) The emission results of the particles from the additional DBD reactor filled with PA balls but without supplying pulse voltage that was installed downstream the DBD reactor without balls were combined with the results of BET test and SEM photos, in order to summarized the mechanism of n-hexane decomposition together with reduction of particle emission in the DBD reactor filled with PA balls as follows:PA balls can effectively adsorb the intermediate products of n-hexane decomposition due to their large specific surface area and large amount of macropores. Meanwhile, electrostatic precipitation promotes the reduction of particle emission within the DBD reactor. |
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