Environ Sci Pollut Res (2014) 21:13127–13137 DOI 10.1007/s11356-014-3244-6
RESEARCH ARTICLE
Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, nonthermal plasma, and combined plasma and photocatalysis at pilot scale Guillerm Maxime & Assadi Aymen Amine & Bouzaza Abdelkrim & Wolbert Dominique
Received: 2 April 2014 / Accepted: 20 June 2014 / Published online: 6 July 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract This study focuses on the removal of gas-phase ammonia (NH3) and hydrogen sulfide (H2S) in a continuous reactor. Photocatalysis and surface dielectric barrier discharge (SDBD) plasma are studied separately and combined. Though the removal of volatile organic compounds by coupling plasma and photocatalysis has been reported on a number of studies in laboratory scale, this is as far as we know the first time that it is used to remove inorganic malodorous pollutants. While each separate process is able to degrade ammonia and hydrogen sulfide, a synergetic effect appears when they are combined at a pilot scale, leading to removal capacity higher than the sum of each separate process. The removal capacity is higher when the gas circulates at a higher flow rate and when pollutant concentration is higher. The presence of water vapor in the gas is detrimental to the efficiency of the process. Operating conditions also influence the production of nitrogen oxides and ozone. Keywords Synergetic effect . Nonthermal plasma . Photocatalysis . By-products . Pilot scale . Continuous process
Introduction Some industries emit waste gases containing pollutants that might be harmful for the environment, pose public health problems, or cause nuisances (ADEME 2005). This is the Responsible editor: Bingcai Pan G. Maxime : A. Aymen Amine : B. Abdelkrim : W. Dominique Laboratoire Sciences Chimiques de Rennes - équipe Chimie et Ingénierie des Procédés, UMR 6226 CNRS, ENSCR, 11 allée de Beaulieu, CS 50837, 35700 Rennes, France G. Maxime : A. Aymen Amine : B. Abdelkrim (*) : W. Dominique Université Européenne de Bretagne, Rennes, France e-mail: [email protected]
case of animal rendering plants, which generate a variety of highly malodorous pollutants including volatile organic compounds (VOCs) and inorganics such as ammonia and sulfur compounds (Le Cloirec 2002). Biofilters are the traditional way to deal with the removal of odor from such plants. This technique allows the degradation of pollutants at low costs, but is space consuming and adapts poorly to changes in the waste gas composition (Malhautier et al. 2014). Oxidative techniques such as nonthermal plasma (NTP) and photocatalysis could be used instead of biofilters, but the incomplete oxidation of pollutants generates an indefinite number of by-products that might be more harmful than the original pollutant (Van Durme et al. 2007). Thus, these processes are not widely used for industrial odor removal purposes. Several studies have shown, though, that the coupling of photocatalysis and NTP degrades VOC at a higher rate than any of the single processes, while emitting less oxidative byproducts (Guaitella et al. 2008; Thevenet et al. 2008). Moreover, to improve the process’s efficiency, plasma and photocatalysis combination has been investigated and developed (Maciuca et al. 2012). It is well established now that the performance of nonthermal plasma systems for the removal of low concentrations of pollutants can be improved particularly by the addition of photocatalyst material like TiO2 into the discharge zone of the reactor (Subrahmanyam et al. 2007). The performance of a plasma-photocatalytic reactor has been found to be superior to a plasma reactor for a wide range of VOCs. Moreover, a synergetic effect can be expected when combining volumic plasma with a photocatalyst (Guaitella et al. 2008; Huang et al. 2010). The goal of our study was to combine the advantages of plasma and photocatalysis by using the two technologies in the same reactor. A coupled system of surfacic dielectric barrier discharge (DBD) plasma/photocatalysis is established in a pilot reactor. Since all the data available in the literature concerns VOC like
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acetaldehyde (Sano et al. 2006), toluene (Chang and Lin 2005; Huang and Ye 2009; Huang et al. 2010; 2007; Li et al. 2002; Sun et al. 2007; Van Durme et al. 2007), acetylene (Guaitella et al. 2008; Rousseau et al. 2005; Thevenet et al. 2008; 2007), benzene (Futamura et al. 2004; Lee et al. 2004; Zhu et al. 2009), isovaleraldehyde, and trimethylamine (Assadi et al. 2014), it is necessary to determine if the same synergetic effect applies for inorganic pollutants. If this is the case, this coupling process will be very promising for this application. In this study, a reactor combining photocatalysis and surface dielectric barrier discharge (SDBD) plasma is used to carry out ammonia and hydrogen sulfide degradation performances of each process and to compare them with the coupling process in a pilot scale. The effects of varying gas flow rate, pollutant concentration, and gas humidity are also studied.
Experimental Reactor and experimental setup The reactor is 80-cm long. It has a rectangular cross-section, the walls being 13.5-cm high and 4-cm apart (Fig. 1). SDBD plasma The surface plasma is generated on two of the inside walls of the reactor by SDBD plasma. The walls consist of an inner copper mesh electrode and an outer stainless steel plate electrode separated by a glass plate that acts both as a dielectric barrier and reactor wall. Those electrodes are connected to an electric setup (Fig. 1). The electrodes are 80-cm long and 13.5-cm wide. The applied high voltage is about 30 kV/ 40 mA and is a sine waveform. The SDBD plasma is obtained by submitting the electrodes to a sinusoidal high voltage Fig. 1 Scheme (b) and sectional drawing (a) of SDBD plasma coupled with photocatalysis in a planar reactor
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ranging from 0 to 30 kV at a 50-Hz frequency. The outer electrode is connected to the ground through 2.5 nF in order to collect the charges transferred through the reactor. The applied voltage (Ua) and high-capacitance voltage (Um) are measured by LeCroy high-voltage probes and recorded by a digital oscilloscope (Lecroy Wave Surfer 24 Xs, 200 MHz). The design of this reactor (Fig. 1) is the subject of a Patent Application BFF11L1041/MFH (CIAT Pattent, 2013).
Photocatalysis The used material is a coated glass fiber tissue (GFT) with 6.5 g m−2 of colloidal silica to ensure the fixation of 6.5 g m−2 of P25-Degussa titanium dioxide nanoparticles. It is supplied by Ahlstrom Research and Services. P25-Degussa nanoparticles were made of two titanium dioxide allotropic forms, 80 % is anatase, and 20 % is rutile. The coating process consists of an impregnation of glass fibers by SiO2 and TiO2 nanoparticles suspension in pure water using Ahlstrom industrial size-press. Specific surface area was measured using BET method and was equal to 20.6 m2 g−1. The used fluorescent UV lamp (Philips under reference PL-S 9 W/10/4P, 0.012-m bulb diameter, 0.13-m bulb length) had a major wavelength peak emission at 355 nm. The centerlines of the lamps were separated by a distance of 0.01 m. UV lamps, situated in between the active walls of the reactors and dispersed along its entire length, are used to activate the photocatalyst. The average light intensity received by the photocatalyst is 42 W m−2, measured using a radiometer. The reactor can therefore be used to study photocatalysis, nonthermal plasma, or their combination depending on which system—UV lamps or generator and amplifier—is on during the experiments.
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Experimental setup Figure 2 represents the experimental setup, which is identical to the one used in another study (Assadi et al. 2012) except for the reactor. Compressed gas flows through a packed column working in countercurrent with water to reach the desired humidity. Ammonia from a 20 % aqueous solution is injected continuously using an automatic syringe, and a static mixer is used to homogenize the gas ahead of the reactor. Hydrogen sulfide is supplied from a cylinder containing 5 % molar hydrogen sulfide (H2S) in nitrogen; the flow rate of which is controlled by the Bronkhorst flow meter coupled to an accurate valve. Samples are taken upstream and downstream of the reactor for analysis. A standard iodometric titration method was used to estimate the formation of the downstream ozone. Thus, at the outlet of the plasma reactor, a constant flow rate of 200 L h−1 was bubbled on iodine solution. Experimental procedure Once the desired flow rate, humidity, and inlet concentration are set, the air circulates in the setup for an hour with the UV lamps and plasma generating system off, in order to obtain a gas-solid equilibrium at the surface of the photocatalyst. The UV lamps and/or plasma generating system are then turned on, and the system is allowed to reach a steady state for an hour before doing any outlet measurement. Analysis Ammonia bubbles through an HCl solution, where it is absorbed as ammonium ion NH4+. The resulting NH4+ Fig. 2 Experimental setup
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solution is analyzed with a spectrophotometric method using Nessler reagent (Rodier 1996). In fact, the Nessler reagent is employed to obtain colored solutions as follows: NH4þ þ 2½HgI4 2− þ 4HO− →HgO⋅ HgðNH2 ÞI þ 7 I− þ 3H2 O Yellow Brown
The sulfur compounds concentrations were measured by a TRS MEDOR® analyzer (Chromatotec, France). The 400-mL sample loop was continuously swept by the sample under a 100-mL min−1 flow rate. The separation was performed on a capillary column swept by reconstituted air under 230 mbar, followed by an electrochemical detection in a cell filled with CrO3 at 10 g L−1. The retention times were 110, 180, and 420 s, respectively, for hydrogen sulfide, sulfur dioxide (SO2), and sulfur trioxide (SO3). Temperature and relative humidity (RH) are determined using a specific sensor (Testo 445). Moreover, we use the Lissajous plot method (Manley 1943) for calculating mathematically the power input (P) (Eq. 1). PðWÞ ¼ E ð JÞ F ðHzÞ
ð1Þ
P is the input power (W), E is the injected energy (J), and F is the frequency (Hz). The value of input power is varied by changing the applied voltage Ua. In fact, the injected energy E per cycle dissipated in the DBD plasma reactor is equal to the area of Lissajous curve (Fig. 3). A Lissajous curve was obtained by plotting the charges transferred by the plasma versus the voltage applied to the reactor, as shown in Fig. 3. The quantity of charges Q (μC) transferred by the discharge was measured by multiplying the capacitance Cm (nF) by the applied voltage Ua (kV) in the plasma reactor.
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Charge (pC)
Fig. 3 Lissajous curve obtained at 50 Hz
4000
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5000
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U DBD (V)
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In the case of Fig. 3, the discharge power was estimated to be 6 W. After, the specific energy (SE) is then calculated as: SEð J=LÞ ¼ PðWÞ= 1; 000Q m3 h−1 =3; 600
ð2Þ
Results and discussion The removal capacity of pollutant “R” is calculated as: Q R ¼ ðC in −C out Þ⋅ 3; 600
ð3Þ
where Cin and Cout are, respectively, the inlet and outlet pollutant concentration (mol m−3) and Q is the volumetric flow rate (m3 h−1). Photocatalysis In a first series of experiments, the UV lamps are used while the plasma generating system is off, so that photocatalysis was the only active process. Influence of gas flow rate and inlet concentration Ammonia at concentrations between 0.3 and 6.5 10−3 mol m−3 was degraded by photocatalysis for flow rates of 4, 8, and 10 m3 h−1 (Fig. 4). The removal capacity of ammonia increases with the inlet concentration. At the lowest flow rate, it seems to stabilize for higher concentrations. For a given concentration, it increases with the gas flow rate.
In photocatalysis, the pollutant degradation is a reaction happening at the surface of the material. It happens once the pollutant is adsorbed on TiO2. Since adsorption is ruled by a gas-solid equilibrium, the increase of the gas-phase ammonia concentration provokes an increase of the ammonia density at the surface of the photocatalyst. This leads to a higher removal capacity, up to the saturation point of TiO2. When the gas-phase concentration is high enough, all the adsorption sites are occupied and the removal capacity no longer depends on the gas-phase concentration. This is usually described by the Langmuir-Hinshelwood model. The flow rate increase has two adverse effects on the ammonia removal capacity. First, because of added turbulences, it enhances the mass transfer from the bulk of the gas phase to the photocatalyst surface, reducing concentration gradient. This enhances the pollutant degradation. But it also decreases the residence time in the reactor, hindering the degradation. Since the observed removal capacities at a given concentration are proportional to the gas flow rate, the degradation in this reactor is limited by the ammonia mass transfer (Assadi et al. 2012). Ammonia degradation is also auto-catalyzed as it reacts, on the TiO2 surface, with its degradation product NO to generate N2 (Busca et al. 1998). Influence of gas humidity The influence of gas humidity was studied for a flow rate of 4 m3 h−1 and for ammonia concentrations from 0.2 to 7 10−3 mol m−3 (Fig. 5).
Fig. 4 Influence of inlet concentration and gas flow rate on the removal capacity of ammonia by photocatalysis. Humidity 10 gH2O m−3
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Removal capacity (mol s-1)
Environ Sci Pollut Res (2014) 21:13127–13137 1.E-06
8.E-07
6.E-07
4.E-07
44Nm3/h m3 h-1 2.E-07
88Nm3/h m3 h-1 10 Nm3/h m3 h-1 10
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Inlet concentration (mol m-3)
From 2 to 16 gH2O m−3, the removal capacity decreases with increasing humidity. No optimal humidity is observed. At 16 gH2O m−3, the removal capacity of ammonia is two to three times lower than at 2 gH2O m−3. This result is similar to one obtained by Vallet (Vallet 2006). When there is water in a gas mixture, it can react on the photocatalyst to produce HO radicals that can readily oxidize ammonia. An increase in the gas humidity results in an increase in the HO• generation (Assadi et al. 2012), which enhances the degradation of ammonia. This degradation enhancement is counterbalanced by the competitive effect of water that adsorbs on the same sites than ammonia. Since the water concentration is much higher than ammonia
Plasma For those experiments where the plasma process was studied, the UV lamps were not turned on. Influence of gas flow rate and inlet concentration When the inlet concentration is increased for a given flow rate and when the flow rate is increased, the removal capacity of
0.2E-06
Removal capacity (mol s-1)
Fig. 5 Influence of gas humidity and inlet concentration on the removal capacity of ammonia by photocatalysis. Flow rate 4 m3 h−1
concentration, it occupies most of the adsorption sites, and the removal capacity of ammonia is reduced (Boulinguiez et al. 2008).
gH2O m-3 22gH2O/Nm3 gH2O m-3 6 gH2O/Nm3 0.2E-06
10 10 gH2O/Nm3 gH2O m-3
16gH2O/Nm3 gH2O m-3 16 0.1E-06
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ammonia increases at inlet concentration equal to 1.1 10−3 mol m−3 (Fig. 6). This result, similar to what is obtained with photocatalysis, is explained by an enhancement of the mass transfer. This leads to a higher ammonia concentration near the walls of the reactor, which are the active plasma zones. Influence of gas humidity
Fig. 6 Influence of inlet concentration and gas flow rate on the removal capacity of ammonia by plasma. Humidity 2 gH2O m−3. Specific energy 10.3 J L−1
Removal capacity [mol.s-1]
Ammonia at a flow rate of 4 m3 h−1 and a concentration of 1.1 10−3 mol m−3 is degraded by plasma at various energy densities and humidities (Fig. 7). An increase of the specific energy enhances the degradation of ammonia. When increasing the energy density, the discharges snatch more electrons from the gas-phase components, inducing a higher concentration of reactive species in the plasma. Since more reactive species are available for the pollutant to react with, the removal capacity gets higher (De Visscher et al., 2008). This effect of specific energy has been observed in other studies (Park et al. 2011; Xia et al. 2008). Gas humidity has a negative influence on the removal capacity of ammonia (Fig. 7). As in photocatalysis, gas humidity may have two effects. Water decomposition by plasma produces reactive radicals enhancing ammonia degradation, but its presence in the gas weakens the discharge, reducing the number and energy of electrons. It can also scavenge the reactive species, preventing them from reacting with ammonia (Guo et al. 2006; Vandenbroucke et al. 2011). It appears that, in the conditions of these experiments, the adverse effects of the presence of water prevail. Ammonia elimination is higher for drier waste gases.
Combining plasma and photocatalysis It was observed that in this reactor, the presence of the plasma alone did not activate the photocatalyst: the plasma removal performances were identical with or without photocatalyst in the absence of other UV light sources. Thus, experiments combining plasma and photocatalysis were done using both the UV lamps and the plasma generating system. The influences of gas flow rate, ammonia inlet concentration, humidity, and specific energy are identical to those observed during the plasma experiments and thus will not be detailed. Processes synergy For each set of experimental conditions where photocatalysis, plasma, and their combination were all studied, the removal capacity of ammonia is compared to the sum of the removal capacities of photocatalysis and plasma. The removal capacity with both processes active is always higher than the sum of each separate process, on average 7 % higher (Fig. 8). So as to better understand the presence of synergetic effect with plasma and photocatalysis, other experiments with hydrogen sulfide have been carried out. Figure 9 shows the variation of the H2S removal capacity with specific energy using the three configurations at a flow rate of 2 m3 h−1. On the other hand, Fig. 9 illustrates the results of H2S removal capacity for the photocatalysis alone (TiO2 +UV), plasma alone (without TiO2) and for the coupling of plasma and photocatalysis. By photocatalysis alone (irradiation of TiO2 by external UV light), the removal capacity of H2S is
0.1E-06
44 Nm3.h-1 m3 h-1 m3 h-1 88Nm3.h-1
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2.6E-04
[mol.Nm-3]
Environ Sci Pollut Res (2014) 21:13127–13137 8.0E-08
Removal capacity [mol s-1]
Fig. 7 Influence of gas humidity and energy density on the removal capacity of ammonia by SDBD plasma. Inlet concentration 1.1 10−3 mol m−3, flow rate 4 m3 h−1
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22gH2O/Nm3 gH2O m-3
7 gH2O/Nm3 gH2O m-3 18 gH2O/Nm3 gH2O m-3 18
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1.7510−5 mol s−1. When plasma is used alone and at SE=16 J L−1, the removal capacity reaches 1.8610−5 mol s−1. By coupling plasma and photocatalysis, the removal capacity increases to 3.8010−5 mol s−1. Thus, the removal capacity of H2S by coupling plasma and photocatalysis was 2 10−6 mol s−1 higher than the sum of the removal capacities recorded at the same conditions for plasma alone and photocatalysis alone. We note that the synergistic effect was higher for H2S than that for ammonia. Thus, whatever the pollutant used, experimental results when plasma and photocatalysis were combined show a better
&
The desorption of by-products attached at TiO2 surface by plasma. This leads to renewal of catalytic surface and so improves conversion and mineralization processes (Allegraud 2007; Maciuca et al. 2012) The contribution of reactive species, formed by NTP, in photocatalytic mechanisms (Thevenet et al. 2008)
&
1.2E-04
Removal capacity (mol s-1)
Fig. 8 Variation of removal capacity of ammonia with specific energy using three configurations (inlet concentration 1.1 10−3 m−3, flow rate 4 m3 h−1)
removal capacity than the simple addition of the two processes. This synergy appears at all the values of tested specific energy. This synergy may be assigned to:
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0.8E-04
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4
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10 Specific energy (J L-1)
Plasma
15
Photocatalysis
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Removal capacity (mol s-1)
Fig. 9 Variation of H2S removal capacity with specific energy using three configurations (H2S inlet concentration 10−3 mol m−3, 2 m3 h−1, humidity 2 gH2O m−3)
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Specific energy (J L-1) Coupling
This synergy, already observed for the degradation of VOC, might originate in reactions between plasmagenerated ozone and UV light or between ozone and activated TiO2. These ozone decomposition reactions produce highly reactive radicals that can easily degrade ammonia (Guaitella et al. 2008; Huang and Ye 2009; Rousseau et al. 2005; Taranto et al. 2007; Van Durme et al. 2008; 2007). It is interesting to see that by comparing the removal capacity of each pollutant, Figs. 8 and 9 show that plasma is more effective than photocatalysis in ammonia removal, while on contrary, plasma is less effective than photocatalysis in hydrogen sulfide removal. This behavior can be due to the fact that the chemical bond strength and molecule stability are the main factors that can affect the removal capacity of pollutant in the NTP process (Assadi et al. 2014). In the case of coupling process, particular importance is given to degradation mechanisms by identifying major decomposition reactions of hydrogen sulfide. The evolution of the formed amount of SOx with the energy density is shown in Fig. 10. Moreover, we note the formation of ozone and its behavior is similar to that seen with ammonia. The result of amount ozone is not represented. We note that the amount of formed SOx increases with the energy density. Moreover, it is interesting to note that at high energies the sulfur balance is close to 80 %. Literature shows that the mechanism of decomposition of hydrogen sulfide is initiated by the attack of reactive species (such as atomic oxygen O, radical hydroxyl OH•), which leads to the dehydrogenation of the molecule (Jarrige 2008):
O þ H2 S→SH þ OH
ð1Þ
plasma
OH þ H2 S→SH þ H2 O
Photocatalysis
ð2Þ
The formed radical mercaptan °SH is unstable in the presence of atomic or molecular oxygen. Thus, its consumption can be due to these reactions:
O þ SH → SO þ H
ð3Þ
O2 þ SH → SO þ OH
ð4Þ
Then, sulfur monoxide (SO) is converted to sulfur dioxide SO2 by the following reactions: O þ SO → SO2
ð5Þ
OH þ SO → SO2 þ H
ð6Þ
O2 þ SO→SO2 þ O
ð7Þ
Similar to the process of conversion of NO to NO2 in air plasma, a balance between SO2 and SO3 compounds in the presence of atomic oxygen can be established (Jarrige 2008): O þ SO2 →SO3
ð8Þ
O þ SO3 →SO2 þ O2
ð9Þ
Environ Sci Pollut Res (2014) 21:13127–13137 Amount (pm)
Fig. 10 Variation of H2S degraded and formed by-products vs. specific energy using three configurations (H2S inlet concentration 10−3 mol m−3, flow rate 2 m3 h−1, humidity 2 gH2O m−3)
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SO3 SO3 formé formed SO2formé SO2 formed
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Finally, the sulfur dioxide may react with H2S to form deposit sulfur (Sn) according to the following reaction (ZareNezhad and Hosseinpour 2008). SO2 þ H2 S→Sn þ 2 H2 O
ð10Þ
not desired, as ozone emissions provoke health and environmental issues. The ozone formation is due to a two-chemical-step process. First, atomic oxygen are generated by O2 dissociation due to impact with high-energy electrons (reaction 11). e− þ O2 →e− þ O þ O
Ozone is generated by plasma. While it helps degrading ammonia in the reactor, its presence at the reactor output is
Atomic oxygen act as a strong oxidizer, but its stability is very limited. Due to fast recombination processes, the
Fig. 11 Variation of amount of ozone with specific energy at different values of relative humidity using plasma alone and plasma coupled with photocatalysis (ammonia inlet concentration 1.1 10−3 mol m−3, flow rate 4 m3 h−1)
Amount of ozone (mol m-3)
Ozone formation
6.0E-04
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2 gH2O.Nm-3, plasma gH2O m-3 , plasma
-3 2 ggH2O.Nm-3, coupling H2O m , coupling
gH2O m-3 , plasma 88 gH2O.Nm-3, plasma
88gH2O.Nm-3, coupling gH2O m-3 , coupling
18 gH2O.Nm-3, gH2O m-3 , plasma plasma
18 coupling 18 gH2O.Nm-3, gH2O m-3 , coupling
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lifetime is only a few microseconds at atmospheric pressure. On a second step, atomic oxygen reacts successively with O2 in three-body collisions, forming ozone by the following reaction: O þ O2 þ M→O3 þ M
ð12Þ
where M can be either molecular oxygen or molecular nitrogen (Atkinson et al. 2003). The output ozone concentration is monitored while varying specific energy and gas humidity. It is found to be proportional to specific energy and to decrease strongly when water is present (Fig. 11). The proportionality between ozone concentration and specific energy is expected, as ozone generation arises from reactions between oxygen and electrons (Futamura et al. 2004). When energy density increases, the number of free electrons in the plasma increases and more ozone is produced. Water, and the radicals created by its decomposition in the reactor, may react with ozone, eliminating it, and scavenge atomic oxygen (O) that might otherwise react with oxygen to produce ozone. Thus, humidity both degrades ozone and prevents its formation, leading to the very low ozone concentration observed at high humidity. Certainly, water vapor content enhances the formation of reactive species as (Thevenet et al. 2008): H2 O þ e− →H þ HO
ð13Þ
The consumption of ozone in presence of radical (H• +HO•) can be explained by two reactions (Futamura et al. 2004):
O3 þ OH →O2 þ HO2
O3 þ H →O2 þ HO
ð14Þ ð15Þ
Similar trends is observed for coupled SDBD plasma/ photocatalyis.
Conclusion The decomposition of ammonia and hydrogen sulfide by the combination of plasma and photocatalysis was studied and compared to each process considered separately. The removal capacity when the processes are combined in the same reactor is higher than the sum of its removal capacities with each individual process in identical conditions. This result is due to a synergy between plasma and photocatalysis with inorganic pollutants at a pilot scale.
Moreover, the amount of formed SOx increases with the specific energy. Therefore, it is interesting to note that at high energies, the sulfur balance is close to 80 %. Water vapor plays a very important role on the elimination of the ammonia. Relative humidity inhibits the removal capacity of ammonia for all processes tested. On the other hand, humidity reduces ozone formation. Acknowledgments The authors gratefully acknowledge the financial support provided by the French National Research Agency (ANR) for this research work. They thank also the Ahlstrom Company which provided them with the photocatalytic material.
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Environ Sci Pollut Res (2014) 21:13127–13137 Huang H, Ye D (2009) Combination of photocatalysis downstream the non-thermal plasma reactor for oxidation of gas-phase toluene. J Hazard Mater 171:535–541 Huang HB, Ye DQ, Fu ML, Feng FD (2007) Contribution of UV light to the decomposition of toluene in dielectric barrier discharge plasma/ photocatalysis system. Plasma Chem Plasma Process 27:577–588 Huang H, Ye D, Leung D (2010) Removal of toluene using UV-irradiated and nonthermal plasma-driven photocatalyst system. J Environ Eng 136:1231–1236 Jarrige J., (2008) Etude expérimentale des décharges électriques impulsionnelles nanoseconde à pression atmosphérique: application au traitement des effluents gazeux et à la décontamination de surface, Université Rouen Le Cloirec P. (2002) Introduction au traitement de l’air, Les techniques de l’ingénieur Traité environnement (G 1700) 1-8. Lee B-Y, Park S-H, Lee S-C, Kang M, Choung S-J (2004) Decomposition of benzene by using a discharge plasma–photocatalyst hybrid system. Catal Today 93–95:769–776 Li D, Yakushiji D, Kanazawa S, Ohkubo T, Nomoto Y (2002) Decomposition of toluene by streamer corona discharge with catalyst. J Electrost 55:311–319 Maciuca A, Batiot-Dupeyrat C, Tatibouët J-M (2012) Synergetic effect by coupling photocatalysis with plasma for low VOCs concentration removal from air. Appl Catal B Environ 125:432–438 Malhautier L, Quijano G, Avezac M, Rocher J, Fanlo JL (2014) Kinetic characterization of toluene biodegradation by rhodococcus erythropolis: towards a rationale for microflora enhancement in bioreactors devoted to air treatment. Chem Eng J 247:199–204 Manley T.C., 1943. Proceedings of the 84th General Meeting, New York Park CW, Byeon JH, Yoon KY, Park JH, Hwang J (2011) Simultaneous removal of odors, airborne particles, and bioaerosols in a municipal composting facility by dielectric barrier discharge. Sep Purif Technol 77:87–93 Rodier, J., 1996. L’analyse de l’eau, 8th ed. Dunod Rousseau, A., Guaitella, O., Gatilova, L., Thevenet, F., Guillard, C., Ropcke, J., Stancu, G.D. (2005) Photocatalyst activation in a pulsed low pressure discharge. Applied Physics Letters 87:221501–221501–3. Sano T, Negishi N, Sakai E, Matsuzawa S (2006) Contributions of photocatalytic/catalytic activities of TiO2 and gamma-Al2O3 in
13137 nonthermal plasma on oxidation of acetaldehyde and CO. J Mol Catal A Chem 245:235–241 Subrahmanyam C, Magureanu M, Laub D, Renken A, Kiwi-Minsker L (2007) Nonthermal plasma abatement of trichloroethylene enhanced by photocatalysis. J Phys Chem C 111:4315–4318 Sun R-B, Xi Z-G, Chao F-H, Zhang W, Zhang H-S, Yang D-F (2007) Decomposition of low-concentration gas-phase toluene using plasma-driven photocatalyst reactor. Atmos Environ 41:6853–6859 Taranto J, Frochot D, Pichat P (2007) Combining cold plasma and TiO2 photocatalysis To purify gaseous effluents: a preliminary study using methanol-contaminated air. Ind Eng Chem Res 46:7611–7614 Thevenet F, Guaitella O, Puzenat E, Herrmann J-M, Rousseau A, Guillard C (2007) Oxidation of acetylene by photocatalysis coupled with dielectric barrier discharge. Catal Today 122:186–194 Thevenet F, Guaitella O, Puzenat E, Guillard C, Rousseau A (2008) Influence of water vapour on plasma/photocatalytic oxidation efficiency of acetylene. Appl Catal B Environ 84:813–820 Vallet, C., 2006. Dégradation photocatalytique de composés odorants en phase gazeuse: application aux effluents d’élevage porcin. Université de Rennes 1 Van Durme J, Dewulf J, Sysmans W, Leys C, Van Langenhove H (2007) Efficient toluene abatement in indoor air by a plasma catalytic hybrid system. Appl Catal B Environ 74:161–169 Van Durme J, Dewulf J, Leys C, Van Langenhove H (2008) Combining non-thermal plasma with heterogeneous catalysis in waste gas treatment: a review. Appl Catal B Environ 78:324–333 Vandenbroucke AM, Morent R, De Geyter N, Leys C (2011) Nonthermal plasmas for non-catalytic and catalytic VOC abatement. J Hazard Mater 195:30–54 Xia L, Huang L, Shu X, Zhang R, Dong W, Hou H (2008) Removal of ammonia from gas streams with dielectric barrier discharge plasmas. J Hazard Mater 152:113–119 ZareNezhad B, Hosseinpour N (2008) Evaluation of different alternatives for increasing the reaction furnace temperature of Claus SRU by chemical equilibrium calculations. Appl Therm Eng 28:738–744 Zhu T, Li J, Jin YQ, Liang YH, Ma GD (2009) Gaseous phase benzene decomposition by non-thermal plasma coupled with nano titania catalyst. Int J Environ Sci Technol 6:141–148
RESEARCH ARTICLE
Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, nonthermal plasma, and combined plasma and photocatalysis at pilot scale Guillerm Maxime & Assadi Aymen Amine & Bouzaza Abdelkrim & Wolbert Dominique
Received: 2 April 2014 / Accepted: 20 June 2014 / Published online: 6 July 2014 # Springer-Verlag Berlin Heidelberg 2014
Abstract This study focuses on the removal of gas-phase ammonia (NH3) and hydrogen sulfide (H2S) in a continuous reactor. Photocatalysis and surface dielectric barrier discharge (SDBD) plasma are studied separately and combined. Though the removal of volatile organic compounds by coupling plasma and photocatalysis has been reported on a number of studies in laboratory scale, this is as far as we know the first time that it is used to remove inorganic malodorous pollutants. While each separate process is able to degrade ammonia and hydrogen sulfide, a synergetic effect appears when they are combined at a pilot scale, leading to removal capacity higher than the sum of each separate process. The removal capacity is higher when the gas circulates at a higher flow rate and when pollutant concentration is higher. The presence of water vapor in the gas is detrimental to the efficiency of the process. Operating conditions also influence the production of nitrogen oxides and ozone. Keywords Synergetic effect . Nonthermal plasma . Photocatalysis . By-products . Pilot scale . Continuous process
Introduction Some industries emit waste gases containing pollutants that might be harmful for the environment, pose public health problems, or cause nuisances (ADEME 2005). This is the Responsible editor: Bingcai Pan G. Maxime : A. Aymen Amine : B. Abdelkrim : W. Dominique Laboratoire Sciences Chimiques de Rennes - équipe Chimie et Ingénierie des Procédés, UMR 6226 CNRS, ENSCR, 11 allée de Beaulieu, CS 50837, 35700 Rennes, France G. Maxime : A. Aymen Amine : B. Abdelkrim (*) : W. Dominique Université Européenne de Bretagne, Rennes, France e-mail: [email protected]
case of animal rendering plants, which generate a variety of highly malodorous pollutants including volatile organic compounds (VOCs) and inorganics such as ammonia and sulfur compounds (Le Cloirec 2002). Biofilters are the traditional way to deal with the removal of odor from such plants. This technique allows the degradation of pollutants at low costs, but is space consuming and adapts poorly to changes in the waste gas composition (Malhautier et al. 2014). Oxidative techniques such as nonthermal plasma (NTP) and photocatalysis could be used instead of biofilters, but the incomplete oxidation of pollutants generates an indefinite number of by-products that might be more harmful than the original pollutant (Van Durme et al. 2007). Thus, these processes are not widely used for industrial odor removal purposes. Several studies have shown, though, that the coupling of photocatalysis and NTP degrades VOC at a higher rate than any of the single processes, while emitting less oxidative byproducts (Guaitella et al. 2008; Thevenet et al. 2008). Moreover, to improve the process’s efficiency, plasma and photocatalysis combination has been investigated and developed (Maciuca et al. 2012). It is well established now that the performance of nonthermal plasma systems for the removal of low concentrations of pollutants can be improved particularly by the addition of photocatalyst material like TiO2 into the discharge zone of the reactor (Subrahmanyam et al. 2007). The performance of a plasma-photocatalytic reactor has been found to be superior to a plasma reactor for a wide range of VOCs. Moreover, a synergetic effect can be expected when combining volumic plasma with a photocatalyst (Guaitella et al. 2008; Huang et al. 2010). The goal of our study was to combine the advantages of plasma and photocatalysis by using the two technologies in the same reactor. A coupled system of surfacic dielectric barrier discharge (DBD) plasma/photocatalysis is established in a pilot reactor. Since all the data available in the literature concerns VOC like
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acetaldehyde (Sano et al. 2006), toluene (Chang and Lin 2005; Huang and Ye 2009; Huang et al. 2010; 2007; Li et al. 2002; Sun et al. 2007; Van Durme et al. 2007), acetylene (Guaitella et al. 2008; Rousseau et al. 2005; Thevenet et al. 2008; 2007), benzene (Futamura et al. 2004; Lee et al. 2004; Zhu et al. 2009), isovaleraldehyde, and trimethylamine (Assadi et al. 2014), it is necessary to determine if the same synergetic effect applies for inorganic pollutants. If this is the case, this coupling process will be very promising for this application. In this study, a reactor combining photocatalysis and surface dielectric barrier discharge (SDBD) plasma is used to carry out ammonia and hydrogen sulfide degradation performances of each process and to compare them with the coupling process in a pilot scale. The effects of varying gas flow rate, pollutant concentration, and gas humidity are also studied.
Experimental Reactor and experimental setup The reactor is 80-cm long. It has a rectangular cross-section, the walls being 13.5-cm high and 4-cm apart (Fig. 1). SDBD plasma The surface plasma is generated on two of the inside walls of the reactor by SDBD plasma. The walls consist of an inner copper mesh electrode and an outer stainless steel plate electrode separated by a glass plate that acts both as a dielectric barrier and reactor wall. Those electrodes are connected to an electric setup (Fig. 1). The electrodes are 80-cm long and 13.5-cm wide. The applied high voltage is about 30 kV/ 40 mA and is a sine waveform. The SDBD plasma is obtained by submitting the electrodes to a sinusoidal high voltage Fig. 1 Scheme (b) and sectional drawing (a) of SDBD plasma coupled with photocatalysis in a planar reactor
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ranging from 0 to 30 kV at a 50-Hz frequency. The outer electrode is connected to the ground through 2.5 nF in order to collect the charges transferred through the reactor. The applied voltage (Ua) and high-capacitance voltage (Um) are measured by LeCroy high-voltage probes and recorded by a digital oscilloscope (Lecroy Wave Surfer 24 Xs, 200 MHz). The design of this reactor (Fig. 1) is the subject of a Patent Application BFF11L1041/MFH (CIAT Pattent, 2013).
Photocatalysis The used material is a coated glass fiber tissue (GFT) with 6.5 g m−2 of colloidal silica to ensure the fixation of 6.5 g m−2 of P25-Degussa titanium dioxide nanoparticles. It is supplied by Ahlstrom Research and Services. P25-Degussa nanoparticles were made of two titanium dioxide allotropic forms, 80 % is anatase, and 20 % is rutile. The coating process consists of an impregnation of glass fibers by SiO2 and TiO2 nanoparticles suspension in pure water using Ahlstrom industrial size-press. Specific surface area was measured using BET method and was equal to 20.6 m2 g−1. The used fluorescent UV lamp (Philips under reference PL-S 9 W/10/4P, 0.012-m bulb diameter, 0.13-m bulb length) had a major wavelength peak emission at 355 nm. The centerlines of the lamps were separated by a distance of 0.01 m. UV lamps, situated in between the active walls of the reactors and dispersed along its entire length, are used to activate the photocatalyst. The average light intensity received by the photocatalyst is 42 W m−2, measured using a radiometer. The reactor can therefore be used to study photocatalysis, nonthermal plasma, or their combination depending on which system—UV lamps or generator and amplifier—is on during the experiments.
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Experimental setup Figure 2 represents the experimental setup, which is identical to the one used in another study (Assadi et al. 2012) except for the reactor. Compressed gas flows through a packed column working in countercurrent with water to reach the desired humidity. Ammonia from a 20 % aqueous solution is injected continuously using an automatic syringe, and a static mixer is used to homogenize the gas ahead of the reactor. Hydrogen sulfide is supplied from a cylinder containing 5 % molar hydrogen sulfide (H2S) in nitrogen; the flow rate of which is controlled by the Bronkhorst flow meter coupled to an accurate valve. Samples are taken upstream and downstream of the reactor for analysis. A standard iodometric titration method was used to estimate the formation of the downstream ozone. Thus, at the outlet of the plasma reactor, a constant flow rate of 200 L h−1 was bubbled on iodine solution. Experimental procedure Once the desired flow rate, humidity, and inlet concentration are set, the air circulates in the setup for an hour with the UV lamps and plasma generating system off, in order to obtain a gas-solid equilibrium at the surface of the photocatalyst. The UV lamps and/or plasma generating system are then turned on, and the system is allowed to reach a steady state for an hour before doing any outlet measurement. Analysis Ammonia bubbles through an HCl solution, where it is absorbed as ammonium ion NH4+. The resulting NH4+ Fig. 2 Experimental setup
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solution is analyzed with a spectrophotometric method using Nessler reagent (Rodier 1996). In fact, the Nessler reagent is employed to obtain colored solutions as follows: NH4þ þ 2½HgI4 2− þ 4HO− →HgO⋅ HgðNH2 ÞI þ 7 I− þ 3H2 O Yellow Brown
The sulfur compounds concentrations were measured by a TRS MEDOR® analyzer (Chromatotec, France). The 400-mL sample loop was continuously swept by the sample under a 100-mL min−1 flow rate. The separation was performed on a capillary column swept by reconstituted air under 230 mbar, followed by an electrochemical detection in a cell filled with CrO3 at 10 g L−1. The retention times were 110, 180, and 420 s, respectively, for hydrogen sulfide, sulfur dioxide (SO2), and sulfur trioxide (SO3). Temperature and relative humidity (RH) are determined using a specific sensor (Testo 445). Moreover, we use the Lissajous plot method (Manley 1943) for calculating mathematically the power input (P) (Eq. 1). PðWÞ ¼ E ð JÞ F ðHzÞ
ð1Þ
P is the input power (W), E is the injected energy (J), and F is the frequency (Hz). The value of input power is varied by changing the applied voltage Ua. In fact, the injected energy E per cycle dissipated in the DBD plasma reactor is equal to the area of Lissajous curve (Fig. 3). A Lissajous curve was obtained by plotting the charges transferred by the plasma versus the voltage applied to the reactor, as shown in Fig. 3. The quantity of charges Q (μC) transferred by the discharge was measured by multiplying the capacitance Cm (nF) by the applied voltage Ua (kV) in the plasma reactor.
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Charge (pC)
Fig. 3 Lissajous curve obtained at 50 Hz
4000
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In the case of Fig. 3, the discharge power was estimated to be 6 W. After, the specific energy (SE) is then calculated as: SEð J=LÞ ¼ PðWÞ= 1; 000Q m3 h−1 =3; 600
ð2Þ
Results and discussion The removal capacity of pollutant “R” is calculated as: Q R ¼ ðC in −C out Þ⋅ 3; 600
ð3Þ
where Cin and Cout are, respectively, the inlet and outlet pollutant concentration (mol m−3) and Q is the volumetric flow rate (m3 h−1). Photocatalysis In a first series of experiments, the UV lamps are used while the plasma generating system is off, so that photocatalysis was the only active process. Influence of gas flow rate and inlet concentration Ammonia at concentrations between 0.3 and 6.5 10−3 mol m−3 was degraded by photocatalysis for flow rates of 4, 8, and 10 m3 h−1 (Fig. 4). The removal capacity of ammonia increases with the inlet concentration. At the lowest flow rate, it seems to stabilize for higher concentrations. For a given concentration, it increases with the gas flow rate.
In photocatalysis, the pollutant degradation is a reaction happening at the surface of the material. It happens once the pollutant is adsorbed on TiO2. Since adsorption is ruled by a gas-solid equilibrium, the increase of the gas-phase ammonia concentration provokes an increase of the ammonia density at the surface of the photocatalyst. This leads to a higher removal capacity, up to the saturation point of TiO2. When the gas-phase concentration is high enough, all the adsorption sites are occupied and the removal capacity no longer depends on the gas-phase concentration. This is usually described by the Langmuir-Hinshelwood model. The flow rate increase has two adverse effects on the ammonia removal capacity. First, because of added turbulences, it enhances the mass transfer from the bulk of the gas phase to the photocatalyst surface, reducing concentration gradient. This enhances the pollutant degradation. But it also decreases the residence time in the reactor, hindering the degradation. Since the observed removal capacities at a given concentration are proportional to the gas flow rate, the degradation in this reactor is limited by the ammonia mass transfer (Assadi et al. 2012). Ammonia degradation is also auto-catalyzed as it reacts, on the TiO2 surface, with its degradation product NO to generate N2 (Busca et al. 1998). Influence of gas humidity The influence of gas humidity was studied for a flow rate of 4 m3 h−1 and for ammonia concentrations from 0.2 to 7 10−3 mol m−3 (Fig. 5).
Fig. 4 Influence of inlet concentration and gas flow rate on the removal capacity of ammonia by photocatalysis. Humidity 10 gH2O m−3
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Removal capacity (mol s-1)
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8.E-07
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From 2 to 16 gH2O m−3, the removal capacity decreases with increasing humidity. No optimal humidity is observed. At 16 gH2O m−3, the removal capacity of ammonia is two to three times lower than at 2 gH2O m−3. This result is similar to one obtained by Vallet (Vallet 2006). When there is water in a gas mixture, it can react on the photocatalyst to produce HO radicals that can readily oxidize ammonia. An increase in the gas humidity results in an increase in the HO• generation (Assadi et al. 2012), which enhances the degradation of ammonia. This degradation enhancement is counterbalanced by the competitive effect of water that adsorbs on the same sites than ammonia. Since the water concentration is much higher than ammonia
Plasma For those experiments where the plasma process was studied, the UV lamps were not turned on. Influence of gas flow rate and inlet concentration When the inlet concentration is increased for a given flow rate and when the flow rate is increased, the removal capacity of
0.2E-06
Removal capacity (mol s-1)
Fig. 5 Influence of gas humidity and inlet concentration on the removal capacity of ammonia by photocatalysis. Flow rate 4 m3 h−1
concentration, it occupies most of the adsorption sites, and the removal capacity of ammonia is reduced (Boulinguiez et al. 2008).
gH2O m-3 22gH2O/Nm3 gH2O m-3 6 gH2O/Nm3 0.2E-06
10 10 gH2O/Nm3 gH2O m-3
16gH2O/Nm3 gH2O m-3 16 0.1E-06
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ammonia increases at inlet concentration equal to 1.1 10−3 mol m−3 (Fig. 6). This result, similar to what is obtained with photocatalysis, is explained by an enhancement of the mass transfer. This leads to a higher ammonia concentration near the walls of the reactor, which are the active plasma zones. Influence of gas humidity
Fig. 6 Influence of inlet concentration and gas flow rate on the removal capacity of ammonia by plasma. Humidity 2 gH2O m−3. Specific energy 10.3 J L−1
Removal capacity [mol.s-1]
Ammonia at a flow rate of 4 m3 h−1 and a concentration of 1.1 10−3 mol m−3 is degraded by plasma at various energy densities and humidities (Fig. 7). An increase of the specific energy enhances the degradation of ammonia. When increasing the energy density, the discharges snatch more electrons from the gas-phase components, inducing a higher concentration of reactive species in the plasma. Since more reactive species are available for the pollutant to react with, the removal capacity gets higher (De Visscher et al., 2008). This effect of specific energy has been observed in other studies (Park et al. 2011; Xia et al. 2008). Gas humidity has a negative influence on the removal capacity of ammonia (Fig. 7). As in photocatalysis, gas humidity may have two effects. Water decomposition by plasma produces reactive radicals enhancing ammonia degradation, but its presence in the gas weakens the discharge, reducing the number and energy of electrons. It can also scavenge the reactive species, preventing them from reacting with ammonia (Guo et al. 2006; Vandenbroucke et al. 2011). It appears that, in the conditions of these experiments, the adverse effects of the presence of water prevail. Ammonia elimination is higher for drier waste gases.
Combining plasma and photocatalysis It was observed that in this reactor, the presence of the plasma alone did not activate the photocatalyst: the plasma removal performances were identical with or without photocatalyst in the absence of other UV light sources. Thus, experiments combining plasma and photocatalysis were done using both the UV lamps and the plasma generating system. The influences of gas flow rate, ammonia inlet concentration, humidity, and specific energy are identical to those observed during the plasma experiments and thus will not be detailed. Processes synergy For each set of experimental conditions where photocatalysis, plasma, and their combination were all studied, the removal capacity of ammonia is compared to the sum of the removal capacities of photocatalysis and plasma. The removal capacity with both processes active is always higher than the sum of each separate process, on average 7 % higher (Fig. 8). So as to better understand the presence of synergetic effect with plasma and photocatalysis, other experiments with hydrogen sulfide have been carried out. Figure 9 shows the variation of the H2S removal capacity with specific energy using the three configurations at a flow rate of 2 m3 h−1. On the other hand, Fig. 9 illustrates the results of H2S removal capacity for the photocatalysis alone (TiO2 +UV), plasma alone (without TiO2) and for the coupling of plasma and photocatalysis. By photocatalysis alone (irradiation of TiO2 by external UV light), the removal capacity of H2S is
0.1E-06
44 Nm3.h-1 m3 h-1 m3 h-1 88Nm3.h-1
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Environ Sci Pollut Res (2014) 21:13127–13137 8.0E-08
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Fig. 7 Influence of gas humidity and energy density on the removal capacity of ammonia by SDBD plasma. Inlet concentration 1.1 10−3 mol m−3, flow rate 4 m3 h−1
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22gH2O/Nm3 gH2O m-3
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1.7510−5 mol s−1. When plasma is used alone and at SE=16 J L−1, the removal capacity reaches 1.8610−5 mol s−1. By coupling plasma and photocatalysis, the removal capacity increases to 3.8010−5 mol s−1. Thus, the removal capacity of H2S by coupling plasma and photocatalysis was 2 10−6 mol s−1 higher than the sum of the removal capacities recorded at the same conditions for plasma alone and photocatalysis alone. We note that the synergistic effect was higher for H2S than that for ammonia. Thus, whatever the pollutant used, experimental results when plasma and photocatalysis were combined show a better
&
The desorption of by-products attached at TiO2 surface by plasma. This leads to renewal of catalytic surface and so improves conversion and mineralization processes (Allegraud 2007; Maciuca et al. 2012) The contribution of reactive species, formed by NTP, in photocatalytic mechanisms (Thevenet et al. 2008)
&
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Fig. 8 Variation of removal capacity of ammonia with specific energy using three configurations (inlet concentration 1.1 10−3 m−3, flow rate 4 m3 h−1)
removal capacity than the simple addition of the two processes. This synergy appears at all the values of tested specific energy. This synergy may be assigned to:
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Fig. 9 Variation of H2S removal capacity with specific energy using three configurations (H2S inlet concentration 10−3 mol m−3, 2 m3 h−1, humidity 2 gH2O m−3)
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This synergy, already observed for the degradation of VOC, might originate in reactions between plasmagenerated ozone and UV light or between ozone and activated TiO2. These ozone decomposition reactions produce highly reactive radicals that can easily degrade ammonia (Guaitella et al. 2008; Huang and Ye 2009; Rousseau et al. 2005; Taranto et al. 2007; Van Durme et al. 2008; 2007). It is interesting to see that by comparing the removal capacity of each pollutant, Figs. 8 and 9 show that plasma is more effective than photocatalysis in ammonia removal, while on contrary, plasma is less effective than photocatalysis in hydrogen sulfide removal. This behavior can be due to the fact that the chemical bond strength and molecule stability are the main factors that can affect the removal capacity of pollutant in the NTP process (Assadi et al. 2014). In the case of coupling process, particular importance is given to degradation mechanisms by identifying major decomposition reactions of hydrogen sulfide. The evolution of the formed amount of SOx with the energy density is shown in Fig. 10. Moreover, we note the formation of ozone and its behavior is similar to that seen with ammonia. The result of amount ozone is not represented. We note that the amount of formed SOx increases with the energy density. Moreover, it is interesting to note that at high energies the sulfur balance is close to 80 %. Literature shows that the mechanism of decomposition of hydrogen sulfide is initiated by the attack of reactive species (such as atomic oxygen O, radical hydroxyl OH•), which leads to the dehydrogenation of the molecule (Jarrige 2008):
O þ H2 S→SH þ OH
ð1Þ
plasma
OH þ H2 S→SH þ H2 O
Photocatalysis
ð2Þ
The formed radical mercaptan °SH is unstable in the presence of atomic or molecular oxygen. Thus, its consumption can be due to these reactions:
O þ SH → SO þ H
ð3Þ
O2 þ SH → SO þ OH
ð4Þ
Then, sulfur monoxide (SO) is converted to sulfur dioxide SO2 by the following reactions: O þ SO → SO2
ð5Þ
OH þ SO → SO2 þ H
ð6Þ
O2 þ SO→SO2 þ O
ð7Þ
Similar to the process of conversion of NO to NO2 in air plasma, a balance between SO2 and SO3 compounds in the presence of atomic oxygen can be established (Jarrige 2008): O þ SO2 →SO3
ð8Þ
O þ SO3 →SO2 þ O2
ð9Þ
Environ Sci Pollut Res (2014) 21:13127–13137 Amount (pm)
Fig. 10 Variation of H2S degraded and formed by-products vs. specific energy using three configurations (H2S inlet concentration 10−3 mol m−3, flow rate 2 m3 h−1, humidity 2 gH2O m−3)
13135 16
SO3 SO3 formé formed SO2formé SO2 formed
14
H2S H2S degradé degraded 12
10
8
6
4
2
0 05
09
11
13
17
Specific energy (J L-1)
Finally, the sulfur dioxide may react with H2S to form deposit sulfur (Sn) according to the following reaction (ZareNezhad and Hosseinpour 2008). SO2 þ H2 S→Sn þ 2 H2 O
ð10Þ
not desired, as ozone emissions provoke health and environmental issues. The ozone formation is due to a two-chemical-step process. First, atomic oxygen are generated by O2 dissociation due to impact with high-energy electrons (reaction 11). e− þ O2 →e− þ O þ O
Ozone is generated by plasma. While it helps degrading ammonia in the reactor, its presence at the reactor output is
Atomic oxygen act as a strong oxidizer, but its stability is very limited. Due to fast recombination processes, the
Fig. 11 Variation of amount of ozone with specific energy at different values of relative humidity using plasma alone and plasma coupled with photocatalysis (ammonia inlet concentration 1.1 10−3 mol m−3, flow rate 4 m3 h−1)
Amount of ozone (mol m-3)
Ozone formation
6.0E-04
5.0E-04
2 gH2O.Nm-3, plasma gH2O m-3 , plasma
-3 2 ggH2O.Nm-3, coupling H2O m , coupling
gH2O m-3 , plasma 88 gH2O.Nm-3, plasma
88gH2O.Nm-3, coupling gH2O m-3 , coupling
18 gH2O.Nm-3, gH2O m-3 , plasma plasma
18 coupling 18 gH2O.Nm-3, gH2O m-3 , coupling
ð11Þ
4.0E-04
3.0E-04
2.0E-04
1.0E-04
0.0E+00 04
10
Specific energy (J L-1)
15
13136
Environ Sci Pollut Res (2014) 21:13127–13137
lifetime is only a few microseconds at atmospheric pressure. On a second step, atomic oxygen reacts successively with O2 in three-body collisions, forming ozone by the following reaction: O þ O2 þ M→O3 þ M
ð12Þ
where M can be either molecular oxygen or molecular nitrogen (Atkinson et al. 2003). The output ozone concentration is monitored while varying specific energy and gas humidity. It is found to be proportional to specific energy and to decrease strongly when water is present (Fig. 11). The proportionality between ozone concentration and specific energy is expected, as ozone generation arises from reactions between oxygen and electrons (Futamura et al. 2004). When energy density increases, the number of free electrons in the plasma increases and more ozone is produced. Water, and the radicals created by its decomposition in the reactor, may react with ozone, eliminating it, and scavenge atomic oxygen (O) that might otherwise react with oxygen to produce ozone. Thus, humidity both degrades ozone and prevents its formation, leading to the very low ozone concentration observed at high humidity. Certainly, water vapor content enhances the formation of reactive species as (Thevenet et al. 2008): H2 O þ e− →H þ HO
ð13Þ
The consumption of ozone in presence of radical (H• +HO•) can be explained by two reactions (Futamura et al. 2004):
O3 þ OH →O2 þ HO2
O3 þ H →O2 þ HO
ð14Þ ð15Þ
Similar trends is observed for coupled SDBD plasma/ photocatalyis.
Conclusion The decomposition of ammonia and hydrogen sulfide by the combination of plasma and photocatalysis was studied and compared to each process considered separately. The removal capacity when the processes are combined in the same reactor is higher than the sum of its removal capacities with each individual process in identical conditions. This result is due to a synergy between plasma and photocatalysis with inorganic pollutants at a pilot scale.
Moreover, the amount of formed SOx increases with the specific energy. Therefore, it is interesting to note that at high energies, the sulfur balance is close to 80 %. Water vapor plays a very important role on the elimination of the ammonia. Relative humidity inhibits the removal capacity of ammonia for all processes tested. On the other hand, humidity reduces ozone formation. Acknowledgments The authors gratefully acknowledge the financial support provided by the French National Research Agency (ANR) for this research work. They thank also the Ahlstrom Company which provided them with the photocatalytic material.
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