Environ. Sci. Technol. 2003, 37, 3985-3988
Studies of 2.45 GHz Microwave Induced Plasma Abatement of CF4 MARILENA T. RADOIU* BOC Edwards, Exhaust Gas Management, Kenn Business Park, Kenn Road, BS21 6TH Clevedon, U.K.
Microwave plasmas at 2.45 GHz frequency operated at atmospheric pressure in synthetic gas mixtures containing N2 and CF4 are investigated experimentally for various operating conditions, with respect to their ability to destroy perfluorocompounds. It was found that the destruction and removal efficiency of the process is highly dependent on the total gas flow and concentration of CF4. Destruction and removal efficiencies of CF4 up to 98% have been achieved using 1.9 kW of microwave power at 16 L/min total flow rate.
Introduction Authorized emission levels of a great number of atmospheric pollutants are being lowered for an increasing number of both large and small industrial companies. Thus the development of new pollution control devices and processes is necessary so that the efficiency, output, flow, rate, and costs of processing are not adversely affected. These needs are particularly acute for gases that influence the global climate. These gases are known as greenhouse gases. The greenhouse effect is primarily a function of the concentration of water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), ozone (O3), and other trace gases in the atmosphere that absorb the terestrial radiation. Changes in the atmospheric concentrations of these greenhouse gases can alter the balance of energy transfers between the atmosphere, space, land, and the oceans. Several classes of halogenated substances that contain fluorine, chlorine, or bromine are also greenhouse gases, but they are, for the most part, solely a product of industrial activities. Chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and bromofluorocarbons (halons) are stratospheric ozone depleting substances. Some other fluorine containing halogenated substancess hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF6)sdo not deplete stratospheric ozone but are potent greenhouse gases. PFCs and SF6 are predominantly emitted from various industrial processes including aluminum smelting, semiconductor manufacturing, electric power transmision and distribution, and magnesium casting. Currently, the radiative forcing impact of PFCs and SF6 is small; however, they have a significant growth rate and extremely long atmospheric lifetimes, are strong absorbers of infrared radiation, and therefore have the potential to influence climate in the future (1). To reduce the overall PFC emission to the lowest economically feasible limit, over the course of the past several years numerous studies of multiple PFC emission control technologies have been reported globally. * Corresponding author phone: (+44-1275)337173; fax: (+441275)337200; e-mail:
[email protected]. 10.1021/es0263846 CCC: $25.00 Published on Web 07/25/2003
2003 American Chemical Society
There are three main routes for PFC emissions reduction: abatement (combustion, plasma, and thermal-chemical) of the PFCs to nonhazardous materials (2-12), recycle and recovery of the unused PFCs (13, 14), and process optimization and/or replacement of the PFCs with other gases (1518). Nowadays, the usage of atmospheric plasmas is studied with growing interest, as it provides a more economic and convenient alternative to low-pressure plasma technology (19, 20). Depending on the condition in questions and requirements, a variety of discharges has been studied with respect to their ability to reduce pollutants (21, 22). All plasma techniques have to provide electrons, ions, free radicals, and other active species in the efficient gas volume. Nonthermal plasmas are formed by the use of electrical fields to break down the gas in which the plasmas are formed and where there is insufficient collisional distribution of the electron energy, while the plasma is on to thermalize all plasma components to the same temperature (23, 24). In microwave excited high pressure plasmas, nonequilibrium results from the fact that at a sufficiently high frequency (usually 2.45 GHz), only light electrons can follow the oscillations of the electric field of the applied electromagnetic field leading to homogeneous electron densities of 1012-1015 cm-3. Microwave plasmas can be excited inside resonant cavities, within waveguide microwave circuits or by means of surface wave field applicators (26, 27). Resonant cavities offer significant advantages, the most important being the design features ensuring both efficiency and ease of operation. The microwave resonant cavity used as a plasma source in this research had to be substantially improved with respect to a microwave “standard” resonant cavity in order to couple a very large amount of incident microwave power with efficiency close to 100% and ensure sufficient reliability of the plasma reactor exposed to the highly energetic and corrosive atmospheric plasma in fluorinated nitrogen and water vapor. The other components of the plasma reactor, the opposed field-enhancing electrodes, also exhibit a number of original features related to their shape and position in the plasma reactor. This configuration increases the intensity of the electric field in the space between them and, therefore, helps to ignite/maintain very stable plasmas from power levels as low as 300 W. This paper describes the development and application of a nonequilibrium plasma system sustained by 2.45 GHz frequency microwaves (MW) and operated at atmospheric pressure that can effectively remove PFCs from gas streams. The technology has been tested on gas flows containing CF4 to illustrate its effectiveness. As it will be presented in the following paper, successful abatement is dependent on the total gas flow, the total power level, and the concentration of CF4.
Experimental Section Experimental System. A schematic diagram of the experimental system is shown in Figure 1. Carbon tetrafluoride (CF4, > 99.7%) was obtained from BOC Edwards. Mass flow controllers were used to achieve the desired concentration of CF4 in nitrogen used as carrier gas. The flow was passed through a mixing reactor where water was added in stoichiometric amount or slight excess. The conditioned gas was then introduced into the plasma reactor, neutralized in a wet scrubber, and evacuated into the acid exhaust. Analysis. The analyses of the gas mixtures before and after reaction were monitored online by mass spectrometry. VOL. 37, NO. 17, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Fragmentation Patterns and Ion Percent Abundance in the Mass Spectra CF4 (Mwa ) 88)
CF2O (Mwa ) 66)
ion
m/eb
RAc
CF3+ CF2+ CF+ F+
69 50 31 19
100 11.6 4.9 7.1
a
Molecular weight.
b
OF2 (Mwa ) 54)
ion
m/eb
RAc
CF2O+ CF2+ COF+ CF+
66 50 47 31
54.9 1.9 100 3.7
The quadrupole mass spectrometer (QMS) used was a Fisons Sensorlab system with the following characteristics: triple filter, 0-300 amu, resolution +/-0.3 amu, Faraday cup and electron multiplier detectors, sensitivity 5 × 10-14 [ion] Torr atmospheric sampling via fused silica heated stainless steel capillary operated with head pressure of 2 × 10-6 mbar (Penning). Peak heights were quoted in nominal ion partial pressures, uncorrected for relative ionization efficiency. Baseline conditions were established prior to taking each measurement, and the destruction and removal efficiency (DRE) was expressed as a ratio of the measured ion partial pressure to the baseline condition. Fragmentation patterns of CF4 and the most likely to be formed byproducts are shown in Table 1. CF4 destruction and byproduct formation were monitored using the peak intensitites of fragment ion highlighted (28). The destruction and removal efficiency (DRE) was calculated
DRE(%) )
C i - Cf × 100 Ci
(1)
where Ci is the initial concentration of the recorded species and Cf is its final concentration after the plasma abatement, estimated from the relative ion intensities. Microwave System. The microwave system consists of a 2.45 GHz frequency microwave generator GMP 20 K/SM (SAIREM, France) with variable power output from 0 to 1950 W, a 20 dB isolator (circulator + dummy load) for magnetron protection against the reflected power, a three-stub tuner and sliding short circuit for impedance matching, and a microwave resonant cavity (plasma reactor). The isolator is provided with a crystal detector for reflected power measurements. Accentus’ patented microwave plasma reactor consists mainly of a resonant microwave cavity which has been described in detail elsewhere (29, 30). It is basically a stainless steel enclosure (V ) 120 cm3) within which there is a pair of opposed field-enhancing electrodessFigure 2. Gas flows into the plasma reactor, passes the gas region where the plasma is formed, and flows out of the system. The resonant microwave cavity is provided with an efficient water9
ion
RAc
ion
m/eb
RAc
OF2+ OF+ F+ O+
54 35 19 16
63.1 100 9.7 44
C2F5+ CF3+ CF2+ CF+
119 69 50 31
42.2 100 10 18.7
Mass to charge (amu). cRelative abundance (%).
FIGURE 1. Schematic diagram of the experimental system. 1 carbon tetrafluoride; 1′ - mass flow controller; 2 - nitrogen; 2′ mass flow controller; 3 - gas mixing chamber; 4 - gas-liquid mixing chamber; 5, 7 - pressure gauge; 6 - plasma reactor/ microwave resonant cavity; 8 - wet scrubber; MS - mass spectrometer; T - thermocouple.
3986
C2F6 (Mwa )138)
m/eb
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 17, 2003
FIGURE 2. Schematic of the microwave/plasma system. cooling circuit. In addition, a microwave window is used to prevent gas flow up the waveguide.
Results and Discussion Microwave induced atmospheric plasma destruction of CF4 in the presence of water (source of hydrogen and oxygen) was tested over a range of flow rates, power levels, and CF4 concentrations. The dilution gas was nitrogen. It is noteworthy that the perfluorocarbon compounds often produce CF4 as an ultimate byproduct. In many respects the abatement of CF4 provides the highest challenge due to its stability [D(CF3-F) ∼ 130 kcal/mol] (31), its large infrared absorption cross-section and corresponding large global warming potential (GWP ) 5700) (1) which makes it particularly difficult to destroy effectively. As CF4 is the most stable of all PFCs, its destruction was intensively investigated, assuming that if it could be destroyed, all the other fluorocompounds should also decompose under the same conditions. According to the reaction shown below (2), the ideal chemistry for CF4 abatement would yield only CO2 and HF, stable final byproducts that could be effectively converted to nonhazardous salts in a caustic water scrubber. Note that the product CO2 is a global warming potential compound itself, but its global warming potential is significantly less than CF4.
CF4 + 2H2O f CO2 + 4HF
(2)
Our experiments were carefully optimized for maximum destruction and removal efficiencies (DRE) of the tested gas mixtures. Total Flow Level. The destruction of CF4 was first tested at 16, 20, 24, 28, and 30 L/min total flow rate in the presence of water vapor as reactant. CF4 is readily decomposed in the plasma reactor. Mixtures of water and CF4 at 2.5 molar ratio (H2O:CF4) were fed into the reactor, and the decomposition was characterized as a function of flow rate and microwave power level. It can be seen from Figure 3 that the destruction achieved was highly dependent on the total flow rate. For a given amount of input power, the extent of CF4 decomposition increases with decreasing flow rate. This implies that the extent of CF4 decomposition is determined by the residence time of the reactant in the plasma.
FIGURE 3. Removal and destruction efficiencies of CF4 vs total flow rate. Reaction conditions: CF4 ) 0.8 L/min; molar ratio H2O:CF4 ) 2.5; Pf ) 1.9 kW.
FIGURE 5. DRE of CF4 vs microwave power. Reaction conditions: Total flow rate 21 L/min; molar ratio H2O:CF4 ) 2.5; CF4 0.8 L/min.
FIGURE 4. DRE of CF4 vs initial CF4 concentration. Reaction conditions: Total flow rate 21 L/min; molar ratio H2O:CF4 ) 2.5; Pf ) 1900 W.
FIGURE 6. DRE of CF4 in the presence of water. Reaction conditions: Total flow rate 21 L/min; CF4 0.8 L/min; Pf ) 1900 W.
To identify the decomposition products of the treated gas, a 1 L/min sample from the reactor exhaust was passed through a water scrubber and to the QMS afterward. Reaction products such as F2 and COF2 will dissolve in the water and form HF. Indeed, pH measurements of the scrubbing water indicated acidic pH after a short run time. Therefore, the only products which pass on to the QMS are nonwater soluble species such as CF4 and other perfluorocarbons. However, after abatement, the mass spectrum showed just two lines above m/e ) 30, at m/e ) 69 and 50 corresponding to CF3+ and CF2+, respectively. Their intensities were approximately 9.75:1, a value which is indistinguishable from the literature value of the relative abundance ratio CF3+:CF2+ ) 8.54:1 in the mass spectrum of CF4 (28). The experimental observations suggest that the reaction of CF4 with H2O vapor proceeds via reaction 2, the main fluorinated byproduct being HF. The linearity of the plots in Figure 3 indicates that the decomposition rate of CF4 follows a first-order dependence on total flow rate, although clearly the rate depends on the starting concentration of CF4 and energy. Concentration of CF4. The dependence of the destruction and removal efficiencies of CF4 versus concentration of CF4 was tested using 1900 W of microwave power in the presence of water at 21 L/min total flow ratesFigure 4. The decomposition of carbontetrafluoride in nitrogen and water was found to decrease as a function of CF4 concentration. The highest rate of destruction was observed as 94% at a microwave forward power (Pf) level of 1900 W for ∼1% (vol) CF4 in nitrogen. Microwave Forward Power Level. A set of experiments was performed to evaluate the efficiency of the process as a function of the microwave forward power level. A microwave discharge appears and remains stable while especially low values of the reflected microwave power rate (Pr
