H2S and NH3 removal by silent discharge plasma and ozone combo-system

Plasma Chemistry and Plasma Processing, Vol. 21, No. 4, December 2001 ( 2001)

H2S and NH3 Removal by Silent Discharge Plasma and Ozone Combo-System Hongbin Ma,1 Paul Chen,1 and Roger Ruan1,2 Receiûed Noûember 11, 2000; accepted February 28, 2001

Treatment of H2S and NH3 using the non-thermal plasma (NTP) methods was inûestigated. Two NTP systems were used in this study, one consisting of a multicell plate-to-wire reactor (PTW), and the other consisting of an ozonization chamber and the multi-cell PTW reactor. Each cell of the PTW reactor had a sheet of copper foil embedded in dielectric layers as its high ûoltage electrode and a wired rack as its gounded electrode. Use of the wired rack type electrode allowed large flow throughput, and promoted intense local electric fields. The experiments showed that under constant energy input, the decomposition efficiency of H2S or NH3 decreased with increasing initial concentration of the gas, and increased with increasing injected ozone and relatiûe humidity. Injection of NH3 into H2S stream did not improûe the H2S decomposition efficiency but was necessary for remoûal of sulfite-containing compounds in the discharge air. KEY WORDS: Silent discharge plasma; ozone; H2S; NH3 .

1. INTRODUCTION The rapid development of animal industry has prompted serious concerns over odors emitted from the animal production sites and manure storage facilities. Though a 1999 survey(1) shows that the average H2S and NH3 levels from animal production sites in Minnesota, USA, are only 295 ppb and 7.4 ppm, respectively, the noxious odors are harmful to human and animal health, and have also resulted in numerous complaints from neighbors of animal farms, and therefore have negative impacts on the animal industry. Moreover, the gases produced by the animal industry have been implicated in degradation of the ozone layer as well as contributing to acid rain.(2) The economic return of some producers has been seriously reduced by public resistance to expansion of their production facilities. 1

Bio-Systems and Agricultural Engineering Department, University of Minnesota, 1390 Eckles Ave., St. Paul, MN 55108. 2 To whom all correspondence should be addressed. 611 0272-4324兾01兾1200-0611$19.50兾0  2001 Plenum Publishing Corporation

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Therefore, animal producers are seeking cost-effective techniques for odor control. Odors in animal production and manure storage facilities are products of anaerobic bacterial action. Researchers have identified more than seventy-five specific odorous components from animal manure odor, which are intermediates and end products of various biological reactions. Included in the list are a wide range of volatile organic acids, alcohols, carbonyls, esters, amines, sulfides, mercaptans, and nitrogen heterocycles.(3) Because of the complexity of the odor components, and strong unpleasant smell even at very low concentration, odor reduction has always been a challenging issue for both the animal producers and the researchers. Currently, most animal producers control animal building odor strength through ventilation. Adequate ventilation can effectively reduce animal building odor strength, and is necessary for the health of both the workers and animals. However, the out-going odors degrade the ambient air quality of animal farms, and are the cause of many complaints. The recent progress in applying non-thermal plasma (NTP) technology to the control of polluting gases such as in acidifying components SO2 , NOx ,(4–11) styrene,(12) natural gas(13–15) and other volatile organic compound (VOC)(16) is very encouraging. NTP is normally generated in air at ambient or slightly higher temperatures under high voltage electric fields. Highly reactive NTP species include energetic electrons, atoms, molecules, and radicals. The composition of background gases and electron energy level determine what type and concentration of NTP species will be generated, which in turn, control the chemical reactions within the NTP reaction volume. For example, ozone, one of the energetic NTP species, is produced favorably at 6–9 eV electron energy levels through the following possible reactions: e(6 eV)CO2 → eCO2(A 3Σ+u) → eCO(3P)CO(3P)

(1)

e(8.5 eV)CO2 → eCO2(B Σu) 3

→ eCO(3P)CO(1D) OCO2CM → O3CM,

MG[O2 ], [O], [O3 ]

(2) (3)

where M is the third collision partner. In addition, formation of ozone in air plasma state can involve nitrogen atom and its excited molecular state.(17) Nitrogen can be liberated through the following reactions: N2Ce(10 eV) → NCN NCO2 → NOCO

(4) (5)

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NCNO → N2CO

(6)

N2(A)CO2 → N2OCO

(7)

N2(A)CO2 → N2C2O

(8)

Free radicals and energetic oxygen atoms can also be generated through the following reactions: H2OCe(5.1 eV) → HCOH

(9)

NOCe(6.5 eV) → NCO

(10)

O( D)CH2O → 2OH

(11)

1

The reactive NTP species can attack pollutants through many possible reactions. For example, H2S can be directly oxidized by ozone: H2SCO3 → H2OCSO2

(12)

Some other reactions will influence the decomposition of H2S in the plasma state of mixture gas. Since the ionization potential of H2S (10.4 eV) is considerably lower than O2 (12.2 eV), H2O (12.7 eV), N2 (15.5 eV), and H2 (15.4 eV), the decomposition of H2S by discharge plasma is considered easy. By pulse corona discharge, H2S decomposes to H2 and S directly.(18) In silent discharge, the decomposition reaction is different for the lower electron energy (1–10 eV) than that in pulse corona discharge (1–20 eV).(19) Depending on different electron energy, the decomposition reactions are different as shown below: H2SCe → H2S +C2e +

H2S Ce → SHCH

(13) (14)

H2SCe → H −CSH

(15)

HCSH → H2CS

(16)

In plasma state of ammonia mixture, the possible reactions involving ammonia decomposition are NH3Ce(4.8 eV) → NH2CH

(17)

NH2Ce(12 eV) → NHCH2

(18)

NH3 can also be directly oxidized by O3 . 2NH3C4O3 → NH4NO3C4O2CH2O

(19)

The reaction (16) only needs 4.8 eV. It can be very effective in the silent discharge plasma.

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One of the major concerns in NTP application to decomposition of H2S and NH3 is the secondary hazards of the products of the reactions such as SOx and NOx . Fortunately, SOx and NOx may be further decomposed through the following reactions involving free radicals: OHCSO2 → HSO3

(20)

OHCHSO3 → H2SO4

(21)

OHCNO2 → HNO3

(22)

OHCNO → HNO2

(23)

2NH3CH2SO4 → 2(NH4)2SO4

(24)

NH3CHNO3 → NH4NO3

(25)

From above reactions, we see that the oxides react with radical OH to form acid such as HSO3 , H2SO4 , HNO3 , and HNO2 . The formed acids can combine with NH3 to form ammoniated compounds, which can be removed easily through physical means. Studies have shown that NTP is especially capable of cleaning dilute polluting gases with high energy efficiency, and is able to remove different polluting gases and VOCs simultaneously(20,21) NTP could be advantageous for odor reduction because animal facility odors are composed of many dilute odorous gases, such as ammonia, hydrogen sulfide, sulfur dioxide, and various VOCs. Zhang(22) reported 95% reduction using the packed-bed NTP. Ruan et al.(23) achieved 100% reduction for 100 ppm ammonia and 60 ppm hydrogen sulfide using a pulsed corona plasma reactor. For treatment of dilute, medium flow rate odor gases emitted from animal farms, the non-thermal plasma equipment should be a simple and economical solution. There are mainly two types of NTP reactors, namely pulsed corona streamer and silent (barrier) discharge reactors. It is typical that electron energy in silent discharge NTP ranges from 1 to 10 eV, which is an ideal energy range for the excitation of atomic and molecular species and breaking of chemical bonds.(24) The non-thermal plasma system induced by pulse corona discharge for flue gas(4–12) are not suitable because of the expensive and complicated pulse source system and its control system. The other methods such as ferroelectric packed-bed plasma(24) is also not suitable since plasma reaction volume is filled with ferroelectric beads, which limit flow rate and may trap dusts. In this study, treatment of H2S and NH3 using a silent discharge plasma reactor with new type of electrodes, and the plasma reactor combined with a pre-ionization treatment chamber was investigated. Variables such as initial concentration of H2S or NH3 gas, concentration of injected ozone, mixture of H2S and NH3 and relative humidity were studied.

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Fig. 1. Schematic of a single cell planar reactor.

2. EXPERIMENTS 2.1. Single Cell Silent Discharge Reactors Two types of single cell planar reactors were constructed. Both reactors had two high voltage (HV) electrodes with a grounded electrode in between (Fig. 1). The HV electrodes were embedded between two dielectric layers (epoxy resin, 1.56 mm thick). The difference between the two reactors was the grounded electrodes. One reactor had a metal plate grounded electrode (thus termed plate-to-plate (PTP) reactor) and the other had a wired rack electrode (thus termed plate-to-wire (PTW) reactor) as shown in Fig. 2. The wired electrode consisted of many fine stainless steel wires (0.23 mm in diameter) spaced 10 mm apart and wound on an epoxy resin rack. The gaps between the HV electrode and grounded electrode were 10 mm. All electrodes had an effective area of 1000 mmB200 mm.

Fig. 2. Schematic of wired rack as ground electrode for PTW reactor.

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Fig. 3. Schematic of electrode arrangement of the multi-cell PTW reactor system.

2.2. Multi-Cell PTW Reactor Seven single PTW cells were put together to form the multi-cell PTW reactor as shown schematically in Fig. 3. The system was driven by a 10 kV AC transformer, and consumed about 700 W power. The flow rate of the system was regulated by adjusting the power input of the blower. A flow rate of 360 N m3兾hr was used for most of experiments unless stated. The specific energy consumption was therefore about 2 W hr兾m3. 2.3. Combination of Ozonization Chamber and Multi-Cell PTW Reactor (The Combo System) The combo system consisted of an ozonization chamber and the multicell PTW reactor (Fig. 4). An ozone generator was used to supply the ozonization chamber with gaseous ozone. The ozonization chamber furnished the first stage reactions, followed by the second stage reactions in the multicell PTW reactor. 2.4. Procedures 2.4.1. Analysis of Characteristics of Electric Discharge of the Single Cell Reactors The single cell reactors were driven by a 1.2 kVA transformer. The current waveforms of the working reactors were measured using a 100 MHz

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Fig. 4. Schematic diagram of ozone-silent discharge combo system.

digital oscilloscope (FLUKE 105, FLUKE, Eindhoven, Netherlands). The data were transferred to a PC through a IEEE 488 GPIB interface. 2.4.2. Treatment of H2S and NH3 H2S and NH3 gases in cylinders were purchased from (Oxygen Senze, Co., St. Paul, MN), and used in the experiments throughout. H2S or NH3 , mixed with air, was pumped through the multi-cell PTW system or the combo system at a fixed flow rate of 360 m3兾hr, 20°C and relative humidity (RH) of 26% or as stated. Variables studied included (i) initial concentration of H2S or NH3 ranging from 2.5 to 30 ppm, (ii) RH ranging from 26 to 70%, (iii) concentration of injected ozone (introduced to the ozonization chamber) ranging from 0 to 26 ppm, and (iv) concentration of injected NH3 to the H2S-air stream, ranging from 0 to 80 ppm. H2S, NH3 , and O3 were tested before and after treatment. H2S was measured using the SO2 UV monitor equipped with SO2 scrubber and heater oxidizer (43C SO2 Analyzer, Thermo Environmental Instruments, Inc., Franklin, MA). NH3 was measured using the NH3 detector tube (SENSIDYNE, Clearwater, FL), O3 was tested using a UV analyzer (Model IN-2000, IN-USA, Needham, MA). 3. RESULTS AND DISCUSSION 3.1. The Characteristics of Electric Discharge in Single Cell Reactors The current waveforms of silent discharge that occurred in the single PTP and PTW reactors are shown in Fig. 5 (a and b). It can be seen that the PTW reactor is characterized by a more sharply rising and higher magnitude current waveform compared with the PTP reactor. The difference in the current waveforms could be attributed to the difference in uniformity of the electric fields between the two reactors, i.e., the PTW had very strong local fields along the wire electrodes because of the use of wire electrode as its grounded electrode compared with the uniform electric field of the PTP reactor that

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Fig. 5. Current waveforms of electric discharge in air. (a) Discharge current of plate-to-wire electrode reactor (one cell) at 6.4 kV; (b) discharge current of plate-to-plate electrode reactor (one cell) at 6.4 kV.

employed sheet electrodes. Usually, the geometrical nonuniformity of the arrangement of two electrodes favors the generation of intensive local fields, and requires low inception voltage of discharge. Figure 6 demonstrates the linear relationship between voltage and current for the PTP and PTW reactors. The data shows that with increasing voltage, current in the PTW reactor rose much faster than that in the PTP reactor. The fast rising, narrow current peaks for the PTW reactor suggest that the discharge in intense local fields of the reactor may be very strong so that the propagation of discharge could proceed much farther, which could potentially induce the plasma more effectively with less energy input. Because the local electric field in PTW reactor was so intense, it is possible to increase the discharge gap from 1–3 mm (in the case of PTP reactors) to 10 mm while maintaining effective silent discharge at relatively low voltage (only 10 kV in this case).(22) An increased discharge gap is certainly desirable

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Fig. 6. Characteristic of current vs. voltage (single discharge cell).

for high flow rate. Based on the electrical discharge characteristics observed with the single cell reactors, PTW was chosen for construction of the multicell system for further study.

3.2. H2S and NH3 Decomposition H2S or NH3 , or mixture of H2S and NH3 , when passing through the multi-cell PTW system, or ozone-multi-cell PTW combo system, was decomposed to different degrees depending on the initial concentration of the gases and humidity as shown in the following figures. Figure 7 shows the result obtained with the multi-cell PTW system. At an initial concentration of 2.5 ppm, H2S was almost completely decomposed. For NH3 , the decomposition rate was lower at low initial concentration (3.5 ppm) than for H2S under the same discharge conditions. With increasing initial concentration, both H2S and NH3 decomposition declined in a similar fashion. This could be attributed to the decreased energy depleted to unit gas under constant discharge conditions (constant energy input) when the initial concentration was increased. SO2 was detected in the ventilating gas, suggesting that ozone participated in the decomposition of H2S following reaction (11).

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Fig. 7. Removal of H2S, NH3 by silent discharge reactor at 20°C, relative humidity 26%.

Figures 8 and 9 show the results with the combo system. With increasing ozone injected in the ozonization chamber, the removal efficiency for both H2S and NH3 increased substantially. For example, the removal efficiency for H2S with initial concentration 15 ppm with the silent discharge reactor alone was 40%, but 87% with the combo system when 17 ppm of ozone was injected into the ozonization chamber. Ozone might have participated in the reactions outlined in reactions (12) and (19). NH3 often co-exists with H2S in farm odors, and thus it is worthwhile to investigate the decomposition behaviors of these gaseous compounds when they are mixed together. NH3 has been found to improve NOx and SO2 removal due to the formation of stable compounds such as NH4NO3 and (NH4)2SO4 as by-products in the reactions.(4) Figure 10 shows the results from experiments designed to determine the synergetic effect of NH3 on H2S removal assuming that reactions between NH3 and H2S in air discharge are expected to form stable (NH4)2SO4 . The data shown in Fig. 10 suggest that injection of NH3 up to 70 ppm did not influence the removal of H2S. However, NH3 should help control the side products exhausted from the silent discharge plasma reactor as suggested by the reactions (24– 25) involving NH3 in formation of ammoniated compounds.

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Fig. 8. Removal of H2S by silent discharge reactor and ozone combo system at 20°C, relative humidity 26%. The legends indicate the initial concentration of H2S.

Figure 11 shows that relative humidity had a significant influence on H2S removal. Within the initial concentration range of H2S from 8 to 20 ppm, the removal efficiency of silent discharge plasma reactor increased by about 25% when the relative humidity increased from 26 to 62%. The improved efficiency may be related to the increased formation of OH radicals because OH radicals improve the formation of ammoniated compounds as outlined in reactions (24–25).

4. CONCLUSIONS In this study, two non-thermal plasma systems were developed and used for treatment of H2S and NH3 under laboratory conditions. One system consisted of a multi-cell plate-to-wire reactor (PTW), and the other was a combination of an ozonization chamber and the multi-cell PTW reactor. Use of the wired rack type electrode was found to promote intense local electric fields, which furnished effective discharge, and to allow large discharge volume, which makes treating high flowrate air stream possible. The experiments showed that under constant energy input, the decomposition efficiency of H2S or NH3 decreased with increasing initial concentration of

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Fig. 9. Removal of NH3 by silent discharge reactor and ozone combo system at 20°C, relative humidity 26%. The legends indicate the initial concentration of NH3 .

Fig. 10. Influence of NH3 injection on H2S removal at 20°C, relative humidity 26%. The legends indicate the initial concentration of H2S.

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Fig. 11. Influence of humidity on H2S removal at 20°C by silent discharge reactor.

the gas being treated, and increased with increasing injected ozone and relative humidity. Injection of NH3 into H2S stream did not improve the H2S decomposition efficiency but was necessary for removal of sulfite-containing compounds in the discharge air. Considering the average concentration of H2S on midwest animal production sites, which is only a few hundred ppb, the non-thermal plasma system developed in this study has a great potential of becoming a practical technology for treatment of large volume dilute odorous airs.

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