Environ. Sci. Techno/. 1995, 29, 181-186
Destruction of Fonnaldehvde with Dielectric Banier Discharge Plasmas MOO BEEN CHANG* AND CHIN CHING LEE Graduate Institute of Environmental Engineering, National Central University, Chungli, Taiwan 320, Republic of China
Destruction of formaldehyde (HCHO) molecules via dielectric barrier discharges (DBDs) is experimentally investigated in this study. HCHO is chosen due to its ubiquitous existence, potential health hazards, and chemical form. Destruction of HCHO molecules can be achieved by both "direct electron attack" and "in d irec t g a s- p h ase radic a I re a ction" mec hanism s. Experimental results also indicate competition between these two mechanisms. Operating parameters that affect HCHO destruction efficiency include gas composition, applied voltage, and gas residence time in the dielectric barrier discharge reactor. As high as 97% destruction efficiency is achieved in this study. Experimental results demonstrate the potential for applying DBDs as an alternate technologyfor destroying volatile organic compounds from gas streams.
Introduction Emissions of volatile organic compounds (VOCs) into the atmosphere have the potential to cause adverse effects on human health and the environment (1-3). As a result, more stringent standards have been mandated for VOCs emitted from industrial sources by the 1990 Clean Air Act Amendments of the United States. Depending on the source of contaminants, the concentrations ofVOCs in the resulting gas stream may vary from tens of ppbv to a few percent by volume. Conventional methods for removing VOCs from gas streams include absorption, adsorption, condensation, and incineration (including thermal and catalytic). Absorption and adsorption transfer VOCs from the gas phase to aliquid or solid phase, potentially resulting in other forms of pollution while resolving the air quality problem. Condensation is good for recovering valuable materials; nevertheless, it is efficient and cost-effective for gas streams with VOCs concentration typically greater than a few thousand ppmv. Incineration can be effective in controlling VOCs. However, the possibility of generating other air pollutants (such as CO, NO,, HC1, COZ, and particulate matter) and the potential poisoning of the catalyst have to be taken into account as well. In addition, applying these technologies to control VOCs at low concentrations (typically 5200 ppmv) could bevery costly. In view of the constraints of existing air pollution control technologies that remove VOCs from gas streams, it would be beneficial to develop a process that removes VOCs with relatively low concentrations from gas streams in a more cost-effective and environmentally-safemanner. Formaldehyde (HCHO), an important chemical intermediate, has been widely used in industry to manufacture various materials, such as adhesives, resins, paper products, and cosmetics (4). In addition, HCHO is also used as a preservative. Nonetheless, the widespread use of HCHO has caused much public concern in recent years mainly due to its high volatility and potential adverse health effects (5). HCHO has been classified as a suspected carcinogen. Moreover, HCHO also participates in complex photochemical reactions (either as a reactant or a product) resulting in the formation of photochemical smog, which has received much public attention in numerous urban areas (6). This paper describes and demonstrates a new concept of applying a gas-phase oxidation process that destroys VOCs, such as HCHO, from gas streams. This method generates electrons with sufficient energy to cause the formation of gas-phase radicals, thereby driving the reactions of decomposition and oxidation of HCHO to form end products including HzO, CO, and COZ. The removal of HCHO from gas streams depends on two removal mechanisms including (a) direct removal caused by the collision of electrons with HCHO (direct electron attack) and (b) reaction between HCHO molecules and gas-phase radicals (indirect gas-phase radical reaction). Gas-phase radicals may consist of OH (hydroxyl),HOz (hydroperoxyl), and 0 (oxygen atom),which are veryreactive and can react with HCHO to form other products with less environmental * Corresponding author; Fax: 011-886-3-422-1602; e-mail address: t52000l@spa~c20.ncu.edu.tw.
0013.936x/95/0929-0181$09.00/0
0 1994 American Chemical Sociew
VOL. 29, NO. 1,1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
1
181
HO2 OH
I
I * I
I
I
H
I
un
-I1
t
I
0 0
0
FIGURE 1. Dominant pathways leading to the destruction of HCHO molecules.
hazards. A suggested reaction scheme and relevant rate coefficients at 25 “C and 1 atm are shown with eqs 1-8 (7-9).
+ + + + + + +
--
rate coefficient (cd/s)
+ +
HCHO OH CHO H20 HCHO 0 CHO OH CHO+ H - C O + Hz CHO 02-CO OH CHO OH-CO H20 CHO 0 - COz H CHO HOz- OH H CO2 CO OH-COz H
+ + + + + +
1.0 x 10-11 1.6 10-13 6.6 x IO-” 5.6 x
8.3x 1.7 x IO-” 5.0 x IO-” 1.5 10-13
It can be seen from these reactions and rate constants that generation of the formyl radical (CHO) is a very important step resulting in further oxidation and removal of HCHO molecules. The magnitudes of relevant reaction rate coefficientsindicate that the reaction of OHwith HCHO is the primary route for formyl radical formation. As a result, gas-phase HCHO removal efficiency (~HCHO)may be enhanced by increasing OH generation. In addition, the formyl radicals can also be generatedvia the direct electron collision with HCHO molecules and break the H-CHO bond. Since the energy for H-C bonding is 4.3 eV, the electron with energy greater than this value may break the bond leading to the formation of a CHO radical once the collision takes place (IO). The dominant pathways for removing HCHO from gas streams are described in Figure 1. Provided that HCHO molecules are completely oxidized, the final products will be innocuous COz and H20. Generation of electrons with sufficient energy to collide directly with HCHO molecules and to form gas-phase radicals can be achieved with various methods, such as e-beam, corona discharge, microwave discharge, and dielectric barrier discharge (DBD). DBD is a gas discharge process used to generate 0 3 for industrial application (11, 12). DBDs utilize a dielectricmaterial between the discharge gap and one of the two discharge electrodes. Typically, a material with high dielectric strength Wlmm) and a high dielectric constant, such as glass or ceramics, is used as the dielectric. When the potential across the gap reaches breakdown voltage, the dielectric acts as a stabilizing material leading to the formation of a large number of microdischarges of short pulses which are statistically spread spatially and temporally over the discharge gap (11). The advantages of applying DBD technology to generate plasmas for controlling VOCs include (a) use of a simple and readily available ac power supply for easier operation and (b)DBD can be generated at typical ambient conditions (e.g., 1 atm and 298 K). Such characteristics allow ready application of DBDs as an air pollution control device. DBD technology has been studied as a possible technology to remove SO:! and/or NO from gas streams (13-13 and 182 0 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 1, 1995
organic compounds such as benzene, trichloroethylene, and toluene (10, 16-18) from gas streams. Storch and Kushner (19) theoretically investigated the feasibility of applying DBD plasmas to remove HCHO from gas streams. The modeling results indicated that the destruction and removal of HCHO was primarily accomplished with chemical attack by OH and 0 radicals and the primary end products included CO and H20. In this study, DBD is experimentally evaluated as an alternative technology to destroy and remove HCHO from gas streams via generation of energetic electrons and gas-phase radicals.
Experimental Design The experimental system designed for evaluating the effectiveness of DBD plasmas to remove HCHO from simulated gas streams is shown in Figure 2. The system consisted of a continuous flow gas generation system, a laboratory-scale DBD reactor, and a gas sampling and detection system. Gas streamswith specificmass flow rates of O2and N2were generated from compressed gas cylinders and mass flow controllers (Teledyne Hastings-Raydist HFC202). H20(g)was incorporated into the gas stream by passing part of the gas stream through a controlled temperature water bath. Gaseous HCHO was generated by heating a solution of paraformaldehyde ( T z 50 “C) with a concentration of 0.045 g of dissolved paraformaldehyde/ mL of H20 and passing N2 through the container of paraformaldehyde solution. Gaseous HCHO was then mixed with the humidified gas stream resulting in gas streams with specific composition and mass flow rate. All the tests were completed at atmospheric pressure and ambient temperature (24 2 “C). The laboratory-scale DBD reactor was made of a Pyrex glass tube with an inner diameter of 3.2 cm and a wall thickness of 0.2 cm. The inner electrode, made of a molybdenum rod with diameter of 0.24 cm, was aligned vertically along the centerline of the reactor. The tip of the inner electrode was covered with a glass ball to prevent the generation of corona discharge. The outer electrode was made of stainless steel wire mesh and was wrapped around the outside of the Pyrex glass tube. Plasma was sustained within a control volume of 240 cm3. A 60-Hz power supply set with a nominal capacity of 3 kVA was used to power the system for plasma generation. HCHO concentration ([HCHOI) in the resulting gas stream was determined by first passing the gas stream into an absorber containing a chronotropic acid-sulfuric acid solution which would then react with absorbed HCHO to form a purple monocationic chromogen. [HCHO]in the resulting gas stream was then determined by measuring light absorbance at 580 nmwith a spectrophotometer (GBC, UV/vis Model 91 1) (20). A typical standard curve relating absorbance with the mass of HCHO in solution is shown in Figure 3. A linear relationship between absorbance and mass of HCHO in solution was observed with a correlation coefficient of 0.994. Before applying power to the DBD reactor, the gas stream with known composition and mass flow rate was generated and passed through the reactor for 5 min to allow the system to reach steady-state conditions. Gas mass flow rates and [HCHO]were then monitored, and the power was applied to generate plasma. Depending on the gas composition, the electrical discharge glow could generally be seen in the dark when the applied voltage was greater than 15 kV (rms value), This discharge process continued for 5 min to make
*
Discharge MFC: Mass Flow Controller FIGURE 2. Schematic of the gas generation system. dielectric barrier discharge reactor, and gas sampling and detection syslem.
"' 1.2
-
ABS. =
W,,,,, x 0.059
R' = 0.994 1 -
0
0
0,
-e 0, 2
0.8
-
06
O'/.
-
02
-
_I
0 0
I
IO
stream was conducted. After the resulting [HCHO] was determined, DBDvoltagewasthenshut off, and thesystem was monitored to make sure the system returned to its original conditions. vHcHowas then determined by measuring the initial [HCHOI and the resulting [HCHO] of the gas stream when voltage was applied to the DBD reactor
I5
07.
where the subscripts indicate if the power supply for the DBD reactor was on or off. VOL. 29. NO. 1.1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY.
183
1uu
,
I
Gas Flow Rate
0
0
I 20
0-0
1 . 0 slpm
0-0
1.S s l p m 2.0 s l p m
A-A
016
L re
17
20
19
Appl'ed Voltage (kV)
100,
I
-
1
1 7 kV ~ - ~ 1 8 k V 0-0
1I
0
6.5
1 .o
1.5
04 4
e
12
I
16
20
Gas Residence Time (sec)
FIGURE 4. Dependence of ~ H C H Oon applied voltage for select gas flow rates. The gas streams contain 100 ppmv HCHO, 5% by volume O b 1.0% by volume HZO(g), and N2 as the carrier gas.
-
I
Q 16 kV
2.5
2.0
Inlet H 2 0 ( s ) C o n c e n t r a t i o n (% by volume)
FIGURE 5. Dependence of ~ H C H Oon inlet [HzO(g)]for select applied voltages. Gas streams contain 110 ppmv HCHO, 20% by volume O b and N2 as the carrier gas. The gas flow rates are kept at 2.0 slpm.
Results and Discussion Experimental tests were first conducted to determine the dependence of ~ H C H Oon applied voltage ranging from 16 to 19 kV rms and gas flow rate (Figure 4). Experiments were carried out with the gas streams containing 100 ppmv HCHO, 5% by volume 02,1.0%by volume H20(g),and N2 as the carrier gas. With an applied voltage of 16 kV, ~ H C H O is 25-30% and increases to 75% as the applied voltage increases to 17 kV at a gas flow rate of 1slpm. It is assumed that as the applied voltage reaches 17 kV, the number concentration of energetic electrons generated is much higher than that at 16 kV. This leads to a higher concentration of radicals and free electrons with sufficient energy to react with HCHO resulting in a higher ~ H C H Ocompared with that achieved at 16kV. As the applied voltage is further increased to 18 and 19 kV, ~ H C H Oincreases further, and it reaches 97% at 19 kV. When the applied voltage is kept constant, VHCHO increases slightly ( ~ 8 %as ) the gas flow rate decreases from 2 to 1slpm for all four levels of applied voltage . Dependence of ~ H C on H ~inlet [H,O(g)]of the gas stream and applied voltage is shown in Figure 5. Gas streams contain0.5-2.0% byvolumeH*O(g),llOppmvHCHO, 20% by volume 0 2 , and N2 as the carrier gas with a total gas flow rate of 2 slpm. VHCHO decreases with increasing inlet [HzO(g)] from 0.5% to 2.0% by volume. With sufficient applied voltage, higher [H20(g)]of the gas stream would lead to more efficient generation of OH radicals via the reaction between O('D) and HzO(g) (Le., O('D) + HZO(g) 20H).
-
184 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29. NO. 1,1995
FIGURE 6. Dependence of ~ H C H Oon gas residence time for select applied voltages. Gas streams contain 100 ppmv HCHO, 1.09/0 by volume HZO(g), and Nz as the carrier gas.
More OH radicals should then result in higher ~ H C H Oas described in eq 1. However, the experimental results are in contradiction with the concept of increasing ~ H C H Owith increasing [HzO(g)]for more OH production. The apparent contradiction is caused by the electronegative properties of HzO(g),which tends to increase the minimum reduced electric field, (EIN),, needed to generate the plasma (with E and N denoting electric field strength and total gas molecule number density, respectively). At constant applied voltage, (EIN), increases with increasing [H20(g)l; the power deposited into the plasma actually decreases as [HzO(g)]increases (15). Consequently, both average electron energy and the number of electrons with high energy decrease. As stated previously, electrons with sufficient energy can collide directlywith HCHO molecules and break the C-H or C-0 bonds, resulting in the remova! of HCHO from gas streams. As a result, increasing [H20(g)]may enhance the generation of OH radicals and lead to a higher ~ H C H Oat a given [ 0 2 1 . On the other hand, if the applied voltage is not sufficient,increasing [H20(g)]of the gas stream can reduce the concentration of electrons with sufficient energy, resulting in a reduction of ~ H C H O . Competition between direct electron-attacking and indirect gas-phase radical attacking mechanisms determines the overall performance of the DBD reactor to remove HCHO from gas streams. Decrease in ~ H C H Owith increasing [H20(g)l up to 2% by volume indicates that direct electron-attacking mechanism plays a more important role than indirect gasphase radical attacking mechanism under the previously described operating conditions. The dependence of ~ H C H Oon gas residence time and applied voltage is presented in Figure 6. [HzO(g)]of the gas streams was controlled at 1.0% by volume, [HCHO] was 100 ppmv, [ 0 2 ] was either 0% or 20% by volume, and N2was the carrier gas. ~ H C H Oapproaches a constant value for gas residence times between 8.8 and 13.2s, and decreases slightlyfor gas residence times smaller than 8.8 s. With the higher operating voltage (18kV), VHCHO is higher for the gas stream containing 20% by volume [02] when compared with the gas stream containing no 0 2 . On the contrary, VHCHO is slightly lower for the gas stream containing 20% O2 compared with the gas stream without 0 2 at 16 kV. This observation is attributed to the dependence of plasma formation with electrons of sufficient energy on applied voltage, concentration of electronegativegases (Le.,0 2 , H20(g)), and the generation of OH radicals. As discussed previously for HzO(g), the electronegative property of 0 2
100
110
80
88
I 0 I 0
60
66
5XI 0
40
44
20
22
0 '
0
1
< m n -0 -0
3
5
1
120
40 80 Inlet HCHO Concentration
FIGURE 7. Dependence of ~ H C M Oon inlet [HCHO] for select applied voltages. Gas streams contain 0.5% by volume HzO(g), 5% by volume O t and NZas the carrier gas. The gas flow rates are kept at 2.0 slpm. Solid lines are for percent removal, dashed lines are for absolute
also tends to increase the minimum reduced electric field needed to generate the plasma. With the lower applied voltage (16 kv), the slightly lower VHCHO for the gas streams containing 20% by volume Ozpossibly results from the less power deposited into the gas stream due to the electronegative property of Oz. On the other hand, generation of 0 and OH needed for HCHO removalwould be more efficient and resulted in a higher VHCHO for the gas stream containing 20% by volume O2 than the gas stream containing no Oz with the higher appliedvoltage (18kv). Since the discharge volume is constant, gas residence time varies inverselywith gas flow rate. The magnitude of the needed gas residence time and relative insensitivity of changing gas flow rate on VHCHO implies that the prospect of scalingup this apparatus for treating large volumes of gases generated from industrial sources is quite promising. To evaluate how inlet [HCHOI affects VHCHO, gas streams containing [HCHOI ranging from 40 to 110 ppmv were examined experimentally (Figure 7). The gas residencetime was controlled at 6.6 s. Gas composition was 0.5% by volume [HzO(g)land 5% by volume [OZ],and NZwas the carrier gas. Experimental results indicate that while ~;IHCHO decreases with increasing inlet [HCHO], the absolute removal of HCHO moleculesincreases with increasinginlet [HCHOI. Since the lifetime of gas-phase radicals and energetic electrons is short compared to typical gas residence times in the DBD reactor, higher inlet [HCHOI favors the better use of these radicals and electrons, resulting in ahigher absolute removal of HCHO. The trends observed from experimental results for the dependence of both fractional and absolute removal efficiencies on inlet [HCHOI are consistent with the theoretical results of Storch and Kushner's work (19). The results also demonstrate that DBD technology is very effective in destroying HCHO even when the concentration is relatively low (down to tens of ppmv). One of the reaction products of treating HCHO with DBDs was investigated by measuring [CO]of the gas stream at the outlet of the DBD reactor (Figure 8). The gas stream contained 1.0% by volume HzO(g) and 150 ppmv HCHO while [ 0 2 ] varied from 0 to 20%, with NZ as the carrier gas. The applied voltage was 19 kV, and the gas flow rate was
150
E
2
v
125
C
e
'zj
100
c
s
75
0
0
0 0 +-
3 0
50 25
-
nl 5
0
10
15
20
25
Inlet 02 Concentration (% by volume)
FIGURE 8. Dependence of outlet [COI on inlet [Od of the gas stream. The applied voltage is 19 kV. Gas streams contain 150 ppmv HCHO, 1.0% by volume HzO(g), and Nt as the carrier gas. The gas flow rates are controlled at 1.0 slpm.
1.5 slpm. [COI decreases with increasing [021of the gas stream. The reduction in [COI at higher [02] is possibly attributed to its oxidation to COZmainly via the reactions with OH (eq 8) and the direct oxidation of CHO radicals into COz (eq 6) with the generation of more OH and 0. Experimentalresults indicate that the concentration of CO, one of the end products, varies with the 1 0 2 1 of the gas stream. High [OZ](=20% by volume) tends to reduce the formation of CO molecules. Simultaneous measurements of CO and COz would provide a better insight in identifying the end products. Nevertheless, a good COn analyzer that could accurately measure the [COZIdown to tens of ppmv was not available in this laboratory to perform such measurements. Accurate measurement of power deposited into the gas stream was difficult due to the rapid change of current within the discharge volume. A digital power meter (Chen Hwa Model 2100)was connected to the variac for measuring the power consumed in the DBD reactor for removing HCHO from gas streams. At a typical operating condition, the power consumption (including the power deposited into the gas stream and the power that was wasted within the transformer) was about 0.6 J/cm3. This value was relatively large in terms of energy efficiency: however, the VOL. 29, NO. 1,1995
I ENVIRONMENTALSCIENCE &TECHNOLOGY1 186
power supply used in this study had not been optimized for power utilization. Further research is needed to better characterize the power deposited into the gas stream to enhance the energy efficiency with the discharge system for removing HCHO and possibly other VOCs.
Conclusions The effectiveness of applying dielectric barrier discharge (DBD) plasmas to remove formaldehyde (HCHO) from simulated gas streams was experimentally evaluated with a laboratory-scale apparatus. Results obtained at room temperature (24 f 2 "C) and 1 atm indicate that DBD can effectively destroy and remove HCHO from gas streams. With an appliedvoltage of 19kV, as high as 97%destruction efficiency is achieved for the gas stream containing 100 ppmv HCHO. Destruction of HCHO molecules can be achieved via direct electron attack or indirect gas-phase radical reaction. HCHO removal efficienciesachieved with DBDs depends on the applied voltage, gas composition, and to a second order effect, gas residence time for the evaluated conditions. The great potential of applying DBD as an alternative technology for removing VOCs with relatively low concentration (down to tens of ppmv) from gas streams has been demonstrated in this study.
Acknowledgments The authors acknowledge funding from the National Science Council of the Republic of China under Project Number NSC 82-0410-E-008-119.
Abbreviations and Symbols atm DBD E
atmosphere dielectric barrier discharge electric field intensity reduced electric field El N electronvolt eV [HCHO] concentration of HCHO joule 7 kilovolt kV gas density N parts per billion by volume PPbV
186 1 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 1, 1995
ppmv rms slpm T VOCs ~HCHO
parts per million by volume root mean square L/minute at standard conditions temperature volatile organic compounds removal efficiency of HCHO
Literature Cited (1) O'Brien, R. J.; Crabtree, J. H.; Holmes, J. R.; Hoggan, M. C.; Bockian, A. H. Enuiron. Sci. Technol. 1975, 9, 577. (2) Blackwell, M.; Kang, H.; Thomas, A.; Infante, P. Am. Ind. Hyg. Assoc. J. 1981, 42, A34. (3) Hall, H. I.; Dhara, V. R.; Kaye, W. E.; Price-Green, P.Arch. Enuiron. Health 1994, 49, 45. (4) Hileman, B. Enuiron. Sci. Technol. 1982, 15, 543A. (5) Howard, P. H. In HandbookofEnvironmental FateandExposure
Data for Organic Chemicals; Lewis Publishers, Inc.: Chelsea, MI, 1989; pp 342-350. (6) Grosjean, D. Enuiron. Sci. Technol. 1982, 16, 254. (7) Morris, E. D., Jr.; Niki, H. J. Chem. Phys. 1971, 55, 1991. (8) Atkinson, R.; Pitts, J. N., Jr. J. Chem. Phys. 1978, 68, 3581. (9) Tsang, W.; Hampson, R. F. 1.Phys. Chem. Ref Data 1986, 15, 1087. (10) Nunez, C. M.; Ramsey, G. H.; Ponder, W. H.;Abbott, J. H.; Hamel, L. E.; Kariher, P. H. J. Air Waste Manage. Assoc. 1993, 43, 242. (11) Eliasson, B.; Hirth, M.; Kogelschatz, U. J. Phys. D: Appl. Phys. 1987, 20,1421. (12) Horvath, M.; Litzky, L. B.; Huttner, J. In Ozone; Elsevier Science Publishing Co., Inc.: Budapest, Hungary, 1985; pp 149-158. (13) Sardja, I.; Dhali, S. K. Appl. Phys. Lett. 1990, 56, 21. (14) Chang, M. B.; Balbach, J. H.; Rood, M. J.; Kushner, M. J. J. Appl. Phys. 1991, 69, 4409.
(15) Chang, M. B.; Kushner, M. J.; Rood, M. J. Environ. Sci. Technol. 1992, 26, 777. (16) Neely, W. C.; Newhouse, E. I.; Pathirana, S. Chem. Phys. Lett. 1989, 155, 381. (17) Bozzelli,J. W.; Barat, R. B. Plasma Chem. Plasma Process. 1988, 8, 293. (18) Evans, D.; Rosocha, L.A.;Anderson, G. K.; Coogan,J. C.; Kushner, M. J. J. Appl. Phys. 1993, 74, 5378. (19) Storch, G. D.; Kushner, M. J. J. Appl. Phys. 1993, 73, 51. (20) Methods of Air Sampling and Analysis, 3rd ed.; Lodge, J. P., Jr., Ed.; Lewis Publishers, Inc.: Chelsea, MI, 1989; pp 274-278.
Received for review May 9,1994. Revised manuscript received September 30, 1994. Accepted October 5, 1994."
ES940278P @
Abstract published in AdvanceACSAbsnacts, November 1,1994.
