Productionofhydrogenandsulfurfromhydrogensulfideina …scinet.dost.gov.ph/union/Downloads/279641_279641.pdf · 2013. 10. 29. · ment for the decomposition of H2S is over three orders

Chemical Engineering Science 62 (2007) 2216–2227www.elsevier.com/locate/ces

Production of hydrogen and sulfur from hydrogen sulfide in anonthermal-plasma pulsed corona discharge reactor

Gui-Bing Zhaoa, Sanil Johna, Ji-Jun Zhanga, Jerry C. Hamannb, Suresh S. Muknahallipatnab,Stanislaw Legowskib, John F. Ackermana, Morris D. Argylea,∗

aDepartment of Chemical and Petroleum Engineering, University of Wyoming, 1000 E. University Avenue, Laramie, WY 82071, USAbDepartment of Electrical and Computer Engineering, University of Wyoming, 1000 E. University Avenue, Laramie, WY 82071, USA

Received 15 February 2006; received in revised form 13 December 2006; accepted 21 December 2006Available online 13 January 2007

Abstract

Hydrogen sulfide (H2S) dissociation into hydrogen and sulfur has been studied in a pulsed corona discharge reactor (PCDR). Due to the highdielectric strength of pure H2S (∼ 2.9 times higher than air), a nonthermal plasma could not be sustained in pure H2S at discharge voltages upto 30 kV with our reactor geometry. Therefore, H2S was diluted with another gas with lower dielectric strength to reduce the breakdown voltage.Breakdown voltages of H2S in four balance gases (Ar, He, N2, and H2) have been measured at different H2S concentrations and pressures.Breakdown voltages are proportional to the partial pressure of H2S and the balance gas. With increasing H2S concentrations, H2S conversioninitially increases, reaches a maximum, and then decreases. H2S conversion and the reaction energy efficiency depend on the balance gas andH2S inlet concentrations. H2S conversion in atomic balance gases, such as Ar and He, is more efficient than that in diatomic balance gases,such as N2 and H2. These observations can be explained by proposed reaction mechanisms of H2S dissociation in different balance gases. Theresults show that nonthermal plasmas are effective for dissociating H2S into hydrogen and sulfur.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Hydrogen sulfide dissociation; Nonthermal plasma; Breakdown voltage; Pulsed corona discharge; Energy efficiency

1. Introduction

Gas streams containing hydrogen sulfide (H2S) are encoun-tered in almost all fossil fuel energy extraction and process-ing systems. The conventional treatment method for H2S isthe Claus process, which produces sulfur and water by the netreaction: H2S + 1/2O2 → S + H2O. The reaction is ineffi-cient because the valuable potential product hydrogen (H2) isconverted into water. The transformation of hydrogen from aweakly bound state in H2S to a strongly bound state in H2Oresults in the loss of a potential source of H2. Hydrogen sulfidewould have a much higher economic value if both sulfur andchemically pure hydrogen could be recovered instead of merelysulfur. Therefore, processes for direct dissociation of H2S intoH2 and sulfur are desirable.

∗ Corresponding author. Tel.: +1 307 766 2973; fax: +1 307 766 6777.E-mail address: [email protected] (M.D. Argyle).

0009-2509/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.ces.2006.12.052

Many methods have been investigated to dissociate H2Sinto its constituent elements, including thermal decomposi-tion, both noncatalytic and catalytic, electrochemical meth-ods, photochemical methods, and plasma methods (Zaman andChakma, 1995). Compared to electrochemical and photochemi-cal methods, thermal decomposition and plasma decompositionare promising because of relatively low energy consumption(Cox et al., 1998). However, the thermal decomposition reac-tion of H2S is endothermic with low equilibrium conversionseven at high temperatures (Kaloidas and Papayannakos, 1987).For example, thermal decomposition of H2S has an equilib-rium conversion of 12% at 1000 ◦C and 1 atmosphere pressurethat decreases to less than 1% at temperatures below 550 ◦C.Therefore, two methods have been proposed to overcome thethermodynamic limitation of H2S conversion. One is productremoval by condensation of the sulfur and separation of the hy-drogen with membranes (Zaman and Chakma, 1995 and refer-ences therein). The other is creation of a nonthermal equilibrium

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G.-B. Zhao et al. / Chemical Engineering Science 62 (2007) 2216–2227 2217

environment for H2S conversion, as found in nonthermal plas-mas. Nonthermal plasmas are characterized by low gas tem-perature and high electron temperature wherein high energyelectrons are produced in the gas while the bulk temperature ofthe gas is unchanged. A nonthermal plasma is a partially ion-ized gas that provides a source of chemically active species,including radicals, excited neutrals, and ions, that can promotechemical reactions at ambient temperatures. Therefore, non-thermal plasmas overcome the disadvantage of the need forhigh temperatures because the majority of the electrical energygoes into the production of energetic electrons rather than intogas heating. For reactions that are thermodynamically unfavor-able and for which low equilibrium conversions are obtainedat high reaction temperatures, nonthermal plasmas have an ad-vantage over thermal processes because thermal equilibrium isnot required to be achieved.

Direct dissociation of H2S has been investigated using vari-ous plasma processing technologies, including arc discharge orthermal plasmas, microwave plasma, glow discharge, silent dis-charge, and pulsed corona discharge. Dalaine et al. (1998a,b)investigated H2S conversion in gas systems with 0–100 ppmH2S in air using gliding arc discharges. This type of reactor israther inefficient, with an energy consumption of 500 eV/H2Smolecule dissociated. The theoretical minimum energy require-ment for the decomposition of H2S is over three orders of mag-nitude less than this. For the reaction: H2S(g) → H2(g)+S(s),�H298 = 0.21 eV/H2S = 20.3 kJ/mol. A large amount of workon microwave decomposition of H2S has been carried out inthe former Soviet Union (Asisov et al., 1985; Bagautdinovet al., 1992, 1993a,b, 1995, 1998), where both laboratory andpilot units were reportedly used for the decomposition of pureH2S or mixtures with CO2 with a very low energy consump-tions of ∼ 0.76 eV/H2S. Encouraged by these reports of highconversions and low energy requirements, a joint project forH2S conversion using microwave plasmas was undertaken bythe Alberta Hydrogen Research Program, Atomic Energy ofCanada, and Shell Canada Limited. Unfortunately, this groupreported the energy consumption for H2S conversion to beabout 4.5 eV/H2S (Cox et al., 1998) and thus was unable toreproduce the low energy consumption reported by the Rus-sian researchers. All microwave plasma experiments for H2Sconversion were performed at pressures below 1 atmosphere,which requires additional energy consumption for compressionand vacuum costs. Traus and Suhr (1992) and Traus et al. (1993)investigated conversion of H2S at 10–100% concentrations inAr, N2, and H2 in a silent discharge reactor and a rotating glowdischarge reactor. They concluded that the energy consump-tion for H2S conversion in a rotating glow discharge reactor(∼ 27 eV/H2S) is less than that in a silent discharge reactor(∼ 81 eV/H2S). In addition, Abolentsev et al. (1995) and Maet al. (2001) investigated decomposition of low (ppm) concen-trations of H2S in different balance gases including air, N2, H2,He, and CH4 using a silent discharge reactor. H2S conversion inpulsed corona discharge reactors was also investigated by sev-eral investigators (Helfritch, 1993; Ruan et al., 1999; Wisemanand Douglas, 1972; Averin et al., 1996). These investigationswere conducted at low H2S concentrations (< 2%) with high

(> 100 eV/H2S) energy consumption, which are not practicalconditions for commercial application.

Despite this extensive research on H2S conversion, manyquestions remain unanswered. First, all of the researchdescribed above has been performed either below atmosphericpressure or at low H2S concentrations (< 2%). H2S conversionat pressures above atmospheric and at high H2S concentrationsis desirable to determine if nonthermal plasmas have potentialfor industrial application.

Second, there are no reports on the breakdown voltage ofH2S at pressures higher than atmospheric and H2S concentra-tions > 2%. Gases at normal temperatures and pressures con-tain very low concentrations of current carriers (free electronsand ions) and therefore behave as insulators. In an electric field,any electrons or ions present are accelerated over a distance cor-responding to their mean free path between collisions. If theygain enough kinetic energy to ionize gas molecules, they createnew current carriers which in turn ionize more molecules. Thisavalanche-like process forms channels of conducting plasmacalled streamers. The electrical resistance of the gas betweenthe electrodes becomes nearly zero. This transition of a gas be-tween the insulating and conducting states is known as break-down. The voltage at which it occurs is called the breakdownvoltage. The specific breakdown voltage depends on the gas,as well as on the electrode geometry, the electrode composi-tion, and the gas pressure (Lide, 2003). Breakdown voltage dataare important because they define the operating limits for thereaction.

H2S is an electronegative gas with a high dielectric strengthof about 2.9 (Christophorou et al., 1987). Common gases likeair, N2, H2, He, and Ar have very low dielectric strengths of 1,1, 0.5, 0.15, 0.18, respectively (Lide, 2003). Therefore, muchhigher applied voltages are required for electrical breakdownof H2S compared to these gases in the same reactor geometry.In addition, electrons are accelerated over the mean free path ofgas molecules during the process of electrical breakdown (Zhaoet al., 2005a). As the mean free path of gas molecules increaseswith decreasing gas pressure, individual electrons gain morekinetic energy in low pressure plasmas than in high pressureplasmas under otherwise similar operating conditions (Zhaoet al., 2005a), which causes the breakdown voltage of a gasto decrease with decreasing gas pressure. Therefore, the elec-trical breakdown of H2S at either low pressure or low H2Sconcentration in a balance gas with a low dielectric strength iscomparatively easy, whereas the electrical breakdown of H2Sat pressures above atmospheric and at high H2S concentrationsis more difficult.

Third, the mechanism of H2S conversion in the plasmais not clear. Since the ionization potential of H2S (10.4 eV)is considerably lower than He (24.6 eV), Ar (15.8 eV),N2 (15.6 eV), H2 (15.4 eV), CH4 (12.6 eV), O2 (12.1 eV),and H2O (12.6 eV) (Lide, 2003), Ma et al. (2001) andHelfritch (1993) proposed that the H2S conversion mechanismin any of these gases involves ionization of H2S (e + H2S →H2S+ + 2e) and subsequent charge neutralization withdissociation (H2S+ + e → HS + H). Abolentsev et al.(1995) proposed an alternate three step mechanism for H2S

2218 G.-B. Zhao et al. / Chemical Engineering Science 62 (2007) 2216–2227

conversion: (1) the balance gas (M) is ionized to M+, (2)H2S+ is formed by charge transfer reaction (M+ + H2S →M+H2S+), and (3) H2S is dissociated by reaction with an ion-ized H2S molecule (H2S+ + H2S → H3S+ + HS). However,neither of these mechanisms may appropriately represent theactual process because the ionization degree in nonthermal plas-mas is quite low. A recent investigation by Zhao et al. (2006a)showed that ionization reactions in nonthermal plasmas arenegligible. Alternately, Traus and Suhr (1992) and Traus et al.(1993) proposed that radicals, such as H and HS, formed inthe plasma are responsible for H2S conversion.

Therefore, the goals of this work are to investigate break-down voltage and conversion mechanism of H2S in four balancegases (Ar, He, N2, and H2) in a pulsed corona discharge reac-tor (PCDR) at higher pressures (above atmospheric) and H2Sconcentrations (�4%) than previously reported. A PCDR waschosen to investigate H2S conversion because (1) PCD plas-mas have been extensively investigated and used in methane(Yao et al., 2001) and NOx conversion (Zhao et al., 2004a,b,2005b,c) and (2) comparison of energy efficiency of methaneconversion among three types of nonthermal plasma reactors(PCD, microwave, and silent discharge) shows that PCD reac-tors are one to two orders of magnitude more energy efficientthan the other two (Zhao et al., 2006b).

2. Experimental section

Fig. 1 shows a diagram of the experimental system. The sys-tem consists of a reactor with an electrical system built arounda thyratron switch, a flow control and distribution system, and agas sampling system. The reactor was oriented vertically, withthe gas flow from bottom to top. The electrical system can de-liver charge voltages from 6.9 to 30 kV at pulse frequenciesfrom 0 to 1000 Hz. The capacitor bank provides space for fourdoorknob capacitors in increments of 640 pF. The capacitorswere charged to the desired voltage using a 40 kV oil-cooled

AC power

vent120V/60Hz11

12

3

3 5

2

4

4

6

7

78 9

P

P

1

10

Fig. 1. Experimental setup: 1. H2S gas cylinder; 2. balance gas cylinder (Ar,He, N2, H2); 3. mass flow controller; 4. pressure gauge; 5. pulsed coronadischarge reactor; 6. sulfur condenser; 7. valve; 8. RGA; 9. data collectioncomputer; 10. thyratron switch; 11. HV power supply and control circuit; 12.discharge waveform recorder.

high voltage power supply. On triggering the thyratron, thestored energy in the capacitors is discharged in a few nanosec-onds to the anode, giving rise to a high rate of change of voltage(dv/dt) on the anode. This process of charging and discharg-ing the capacitors is repeated based on the thyratron triggerfrequency, leading to sustained current streamers or plasma.Electrical breakdown during corona discharge can be detectedby a discharge waveform recorder. The cathode was a stainlesssteel tube 0.024 m in diameter and 0.914 m in length, while theanode was a stainless steel wire 0.001 m in diameter passingaxially through the center of the tube. The wire was positivelycharged, while the tube was grounded. The gas flowing throughthe reactor tube was converted to plasma by the high voltagedischarge from the reactor anode. A sulfur trap immersed inice water at the reactor discharge was filled with stainless steelwool to enhance heat transfer and surface area for sulfur vaporremoval from the exit gas.

The four gas mixtures of H2S in Ar, H2S in He, H2S inN2, and H2S in H2 were prepared by mixing ultra high purity(UHP) H2S with the UHP balance gas. Gas mixtures flowedthrough the PCDR at entrance conditions of ambient tempera-ture (∼ 300 K) and a controlled pressure. The highest pressureused in this work was 5.0 bar. The desired entrance mole frac-tion of H2S was achieved by setting flowrates of H2S and thebalance gas using two well-calibrated mass flow controllers.The energy released by the capacitors per pulse was calculatedfrom 12CV

2c , where C is the pulse forming capacitance, fixed

at 1920 PF in this work, and Vc is the constant charge voltagebefore discharge. The power consumed, W (J s−1), was calcu-lated as the product of the input energy per pulse and the pulsefrequency, 12f CV

2c , where f is the pulse frequency in Hz. The

gas leaving the sulfur condenser was analyzed using an onlineResidual Gas Analyzer (RGA, Stanford Research Systems, Inc.QMS100), which is a quadrupole mass spectrometer. To per-form quantitative measurements, an internal standard method(Watson, 1997) was used to calibrate the ion signal responseat an m/z ratio of 34 with the H2S mole fraction, in which thebalance gas was used as an internal standard. The calibrationresults are shown in Fig. 2.

Fig. 2 shows the ratio of the H2S and balance gas molefractions as a function of the measured H2S and balance gasintensities, which show a linear relationship:

yH2S

yB= a · IH2S

IB+ b, (1)

where y is the mole fraction of gas, I is the ion current fromRGA, and the subscript B represents the balance gas of Ar, He,N2, and H2. Therefore, the measured ion current ratio of H2Sand the balance gas can be used to determine the mole fractionratio, K , of H2S and the balance gas from Fig. 2. For a binarygas mixture at the reactor entrance, the mole fraction of H2Sand the balance gas can be calculated from

yi,H2S =Ki

Ki + 1 , (2)

yi,B = 1Ki + 1 , (3)

G.-B. Zhao et al. / Chemical Engineering Science 62 (2007) 2216–2227 2219

0.30

0.25

0.20

0.15

0.10

0.05

0.000.300.250.20

IH2S/IAr

0.150.100.050.00

y H2S

/yA

r

0.30

0.25

0.20

0.15

0.10

0.05

0.000.300.250.20

IH2S/IHe

0.150.100.050.00

y H2S

/yH

e

IH2S/IN2

y H2S

/yN

2

0.5

0.4

0.3

0.2

0.1

0.00.50.40.30.20.10.0

IH2S/IN2

y H2S

/yN

2

0.5

0.4

0.3

0.2

0.1

0.00.50.40.30.20.10.0

H2S in Ar

a = 1.1941

a = -0.03008

H2S in N2a = 0.9986

a = -0.03656

H2S in H2a = 23.1479a = -0.04828

H2S in He

a = 1.1317

a = -0.03292

Fig. 2. Calibration plot for H2S relative to the four balance gases used as an internal standard: (a) Ar; (b) He; (c) N2; (d) H2.

where the subscript i represents the inlet gas. When the coronadischarge is on, H2S dissociates into H2 and sulfur. For thebalance gases Ar, He, and N2, the effluent gas mixture is theternary system including H2 because sulfur is captured by thesulfur condenser. However, the mole fraction of balance gas atthe reactor outlet is the same as that at the reactor inlet becauseH2S dissociation is an equimolar gas phase reaction when thesulfur product is condensed. The outlet H2S mole fraction canbe determined from

yo,H2S = Ko · yi,B , (4)where the subscript o represents the outlet gas. For the balancegas H2, the outlet H2S mole fraction is

yo,H2S =Ko

Ko + 1 . (5)

Therefore, the conversion of H2S in the PCDR is calculatedfrom

XH2S =yi,H2S − yo,H2S

yi,H2S. (6)

Conversion rate and energy consumption of H2S conversion arecalculated from

r = PFRT

· yi,H2S · XH2S (mol s−1), (7)

En = Wr

· 1.0364 × 10−5 (eV molecule−1), (8)where P is the gas pressure, F is the gas flowrate, T is thetemperature, and R is the gas constant.

For each parameter set, at least two experiments wereperformed to assure that the results are repeatable. All exper-imental data were reproducible within a ±10% error limit,including the RGA and flow measurement uncertainties. Con-versions based on either H2S consumption or H2 productionwere similar, but the H2S data provided higher accuracy. Massbalances could not be accurately calculated because not allsulfur was trapped in the condenser and some deposited on thereactor wall and the reactor outlet tube that could not be fullyrecovered to be weighed. However, the observed amounts ofsulfur recovered were consistent with the reported conversions.Energy dispersive spectroscopy and X-ray diffraction wereused to analyze the sulfur product.

3. Results and discussion

3.1. Breakdown voltage of H2S in the various balance gases

Gas breakdown voltage depends on the specific reactor con-figuration, especially the electrode configuration and structure.Breakdown voltages of many pure gases have been investigated

2220 G.-B. Zhao et al. / Chemical Engineering Science 62 (2007) 2216–2227

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20

15

10

5

00.5 1.0 1.5 2.0 2.5

Pressure, bar

3.0 3.5 4.0 4.5

Bre

akdo

wn

Vol

tage

, kV

Vb = 2.34 + 1.49 P

Vb = 4.95 + 3.43 P

Vb = 3.74 + 7.94 P

Fig. 3. Breakdown voltage of pure Ar, H2 and N2 as a function ofpressure. (�): Ar at 1.18 × 10−4 SCM s−1 and 400 Hz; (�): H2 at1.18 × 10−4 SCM s−1 and 400 Hz; (�): N2 at 1.18 × 10−4 SCM s−1 and400 Hz; (�): N2 at 7.87 × 10−6 SCM s−1 and 400 Hz.

in both uniform and nonuniform fields (Blair, 1978). For uni-form fields, the breakdown voltage usually follows Paschen’slaw, which states that breakdown voltage, Vb, is a function ofnd only, where n is the gas number density (molecules cm−3)and d is the distance between the electrodes. For non-uniformfields, the breakdown voltage is a function of nr, where r isthe radius of curvature of the electrode surface at the pointwhere the highest value of the electric field strength occurs(Blair, 1978). For the PCDR used in this work, r is the radiusof the wire anode. For many pure gases in nonuniform fields,the breakdown voltage is proportional to nr at pressures higherthan 0.5 bar (Blair, 1978).

Gas breakdown can be detected by the discharge waveformrecorder, shown in Fig. 1. In addition, the audible dischargenoise from PCDR can also be clearly heard when the coronadischarge occurs. The breakdown voltage was determined byincreasing the charge voltage in increments of 0.1 kV from alow value at which no discharge occurs until the discharge isdetected by both the discharge waveform recorder and the au-dible noise from the reactor. The measured breakdown volt-ages at different pressures for pure Ar, H2, and N2 are shownin Fig. 3. For this reactor, the anode radius, r , is 0.0005 m andthe inlet temperature is 300 K. At these conditions, nr is pro-portional to gas pressure. The results presented in Fig. 3 showthat breakdown voltage is proportional to gas pressure, whichis consistent with previous reports (Blair, 1978). Breakdownvoltages of pure N2 measured at flowrates of 1.18 × 10−4 and7.87 × 10−6 SCM s−1 are almost the same, which indicates noeffect of gas flowrate on breakdown voltage. In addition, pulsefrequencies above 300 Hz do not affect breakdown voltage.

Breakdown of pure He occurred at any pressure from 0.8to 5.0 bar at the lowest charge voltage of 6.9 kV used in this

work. However, breakdown of pure H2S did not occur overthe entire operation range for our reactor, which included pres-sures from 0.8 to 5.0 bar and charge voltages from 6.9 to 30 kV.These results combined with the results in Fig. 3 indicate thatthe order of increasing breakdown voltage at constant pressureis: He < Ar < H2 < N2 < H2S, which is consistent with the or-der of increasing dielectric strength of these gases (dielectricstrength of He: 0.15, Ar: 0.18, H2: 0.50, N2: 1.0, H2S: 2.9)(Christophorou et al., 1987; Lide, 2003).

Because no corona was formed in pure H2S at the maximumcharge voltage (30 kV) with this reactor geometry, H2S wasmixed with another gas with lower dielectric strength to initiateelectrical discharge. He, Ar, N2, and H2 were used as balancegases in this work because they do not produce byproducts inthe corona.

As neither gas flowrate nor pulse frequency (> 300 Hz) af-fect breakdown voltage, gas breakdown experiments were per-formed at a fixed gas flowrate of 1.18 × 10−4 SCM s−1 and apulse frequency of 400 Hz. Fig. 4 shows the breakdown volt-age of H2S in H2. At each fixed H2S concentration, the break-down voltage is proportional to total gas pressure, as shown inFig. 4(a), according to

Vb = mi · Pt + ni , (9)where Pt is the total gas pressure in bar and mi and ni arethe slope and the intercept at a specific H2S mole fraction,respectively. Fig. 4(b) shows the slope mi and the intercept nias a function of H2S mole fraction. These results show that theslope mi is proportional to H2S mole fraction and the interceptni is essentially constant. Therefore Eq. (9) can be rewritten as

Vb = (a1 · yH2S + b1) · Pt + n= a1 · PH2S + b1 · (PH2S + PH2) + n, (10)

where a1 and b1 are the slope and the intercept for the linear re-lationship of mi and H2S mole fraction, respectively, and PH2Sand PH2 are the partial pressures of H2S and H2, respectively.Eq. (10) can be further simplified as

Vb = a2 · PH2S + b2 · PH2 + c (0.8 bar < Pt < 3.6 bar), (11)where a2 = a1 + b1, b2 = b1, and c = n. Eq. (11) indicates thatbreakdown voltage is proportional to the partial pressures ofthe components in binary gas mixtures. Parameters a2, b2, andc were obtained through a least-square regression analysis byapplication of Eq. (11) to mixtures of H2S in Ar, H2S in He,H2S in N2, and H2S in H2,. The breakdown voltages (Vb) are,

H2S in Ar : Vb (kV) = 22.2 × PH2S(bar)+ 2.52 × PAr(bar) + 6.48, (12a)

H2S in He : Vb (kV) = 16.2 × PH2S(bar)+ 2.42 × PHe(bar) + 3.35, (12b)

H2S in N2 : Vb (kV) = 16.1 × PH2S(bar)+ 6.44 × PN2(bar) + 4.00, (12c)

H2S in H2 : Vb (kV) = 15.2 × PH2S(bar)+ 4.74 × PH2(bar) + 2.70. (12d)

G.-B. Zhao et al. / Chemical Engineering Science 62 (2007) 2216–2227 2221

20

16

12

8

4

Vb,

kV

0.8 1.2 1.6 2.0 2.4 2.8

Gas pressure, bar H2S molfraction

6

4

2

0

solp

e &

inte

rcep

t

0.00 0.05 0.10 0.15 0.20 0.25 0.30

Fig. 4. Breakdown voltage of H2S in H2. (a) Breakdown voltage as a function of total gas pressure. Experimental data: (�): 4% H2S; (©): 8% H2S; (�):12% H2S; (�): 16% H2S; (+): 25% H2S. Linear regression: (—): 4% H2S; (- -): 8% H2S; (· · ·): 12% H2S; (-·-): 16% H2S; (- - -): 25% H2S. (b) Slopeand intercept from linear regression in (a) as a function of H2S mole fraction. (�): slope mi ; (�): intercept ni .

These correlations are valid for total absolute pressures between0.8 and 3.6 bar and geometrically similar coaxial cylinder reac-tor systems. Fig. 5 shows the experimental results and the fitteddata using Eqs. (12a)–(12d). Most experimental data matchedthe fitted data, except for low concentrations (< 4%) of H2S inAr. In this exceptional case, an increase in gas pressure causesthe breakdown voltage to deviate from linearity at intermedi-ate pressures before returning to linearity with a similar slopewith a new intercept. Similar experimental results are obtainedfor 2% H2S in Ar, but the reason for this exception is notyet clear.

3.2. H2S conversion in various balance gases

Experiments on H2S conversion in Ar, He, N2, and H2 werecarried out at a fixed pulse frequency of 400 Hz, charge volt-age of 17 kV (corresponding to power input of 110 W), reactorpressure of 1.34 bar, and gas flowrate of 1.18×10−4 SCM s−1,corresponding to a gas residence time of 4.25 s in the reactor.As shown in Fig. 5, the charge voltage of 17 kV is higher thanall breakdown voltages for gas mixtures of H2S in Ar, H2S inHe, H2S in N2, and H2S in H2 at the total pressure of 1.34 bar,which confirmed that electrical discharges occur. Sulfur de-posits in the sulfur condenser, as well as the reactor tube andoutlet, further confirmed the active discharge. The presence ofsulfur was confirmed by energy dispersive spectroscopy. Thefirst two principal peaks for orthorhombic �-sulfur were ob-served in the X-ray diffraction data.

Figs. 6(a)–(d) show H2S conversion and rate data as a func-tion of initial H2S mole fraction. Similar trends of conversionand rate for gas mixtures of H2S in Ar, H2S in He, H2S in N2,and H2S in H2 were found. H2S conversion decreases with in-creasing H2S mole fraction, while the rate initially increases,reaches a maximum, and then decreases with increasing H2Smole fraction.

There are four proposed mechanisms for H2S conversion innonthermal plasmas.

(I) Direct ionization of H2S followed by dissociative neu-tralization (Ma et al., 2001; Helfritch, 1993):

e + H2S → H2S+ + 2e, (R1)H2S

+ + e → HS + H. (R2)(II) Ionization of the balance gas (M), leading to charge trans-

fer reaction, and subsequent dissociative neutralization(Abolentsev et al., 1995):

e + M → M+ + 2e, (R3)M+ + H2S → H2S+ + M, (R4)H2S

+ + e → HS + H. (R2)(III) Direct electron collision dissociation of H2S:

e + H2S → HS + H + e. (R5)(IV) Electron collision dissociation or excitation of the bal-

ance gas, which produces active species that contributeto H2S dissociation:

e + M → M∗ + e, (R6)M∗ + H2S → H + HS + M. (R7)

Pathways (I) and (II) are unlikely for H2S conversion for thefollowing reasons:

(1) If pathway (I) is responsible for H2S conversion, an in-creasing number of H2S molecules should be ionized withincreasing H2S concentration, which should lead to in-creasing H2S conversion rate with increasing H2S concen-tration. This effect is not observed, as shown in Fig. 6.

2222 G.-B. Zhao et al. / Chemical Engineering Science 62 (2007) 2216–2227

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Vb,

kV

0.8 1.2 1.6 2 2.4 2.8 3.2 3.6

Gas pressure, bar

Fig. 5. Breakdown voltage of H2S in different balance gases as a function of total gas pressure. (a) H2S in Ar; (b) H2S in He; (c) H2S in N2; (d) H2S inH2. Experimental data: (�): 4% H2S; (©): 8% H2S; (�): 12% H2S; (�): 16% H2S; (�): 20% H2S; (+): 25% H2S; (×): 30% H2S. Calculated data: (—):4% H2S; (- -): 8% H2S; (· · ·): 12% H2S; (− · −): 16% H2S; (- · ·-): 20% H2S; (- - -): 25% H2S; (- · -): 30% H2S.

(2) If pathway (II) is responsible for H2S conversion, then theionization energies of the balance gases must be reasonablyachieved within the reactor. However, this is not the case,as shown by the following example using He, which hasan ionization energy of 24.6 eV/He or 2370 kJ/mol He. At110 W power input, if the whole energy input is assumedto be absorbed by He to form He+, the limiting conversionrate of H2S is 46.3 �mol/s. However, the results presentedin Fig. 6(b) show that most H2S conversion rates are largerthan 46.3 �mol/s, which leaves pathway II unable to ex-plain all of the observed H2S conversion.

(3) As shown in our recent investigation (Zhao et al., 2006a),the degree of ionization in the pulsed corona dischargeis low. The major active species are produced throughelectron collision in the streamers, whose total volume is10−4.10−3 of the reactor volume (van Veldhuizen et al.,1996). In the streamer head, the concentration of ions (cor-responding to the concentration of electrons) is around15 ppm (Zhao et al., 2006a). If pathways (I) and (II) areresponsible for H2S conversion and all cations formedfrom reactions (R1) and (R3) contribute to H2S conver-

sion, the conversion of H2S for initial H2S mole fractionsof 0.04 is 400 Hz×4.25 s×15 ppm×(10−4.10−3)/0.04=0.064.0.0064%, which is at least two orders of magnitudelower than the H2S conversions observed during the exper-iments, as shown in Fig. 6. Therefore, the observed H2Sconversion solely through ionic reactions is not possible.

Conversion of H2S through pathways (III) and (IV) is sup-ported by the following points:

(1) As demonstrated by Eliasson and Kogelschatz (1986,1991), the concentration of radicals and excited statesformed from electron collision reactions in the streamerhead is at least two orders of magnitude higher than thatof ions. In the streamer channel, the concentration ofradicals and excited states formed from electron colli-sion reactions is at least four orders of magnitude higherthan that of ions. Most reactions are known to occur inthe streamer channel (Zhao et al., 2006a). Therefore, ifreactions (R5)–(R7) contribute to H2S conversion, theconversion of H2S for initial mole fractions of 0.04 is

G.-B. Zhao et al. / Chemical Engineering Science 62 (2007) 2216–2227 2223

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Fig. 6. H2S conversion and H2S reaction rate as a function of H2S mole fraction in different balance gases. (�): conversion; (�): reaction rate.

400 Hz×4.25 s×(104×15 ppm)×(10−4.10−3)/0.04= ∼64%, which exceeds all the experimental results shownin Fig. 6. However, this result is reasonable because theefficiency of such plasma reactions is less than 100%.

(2) The occurrence of H2S conversion through direct elec-tron collision reaction (R5) is suggested by the experimen-tal data on H2S conversion in He. Our previous study ofAr plasma in PCDR’s (Zhao et al., 2006c) showed thatthe main active species formed during electron collisionreactions with Ar are excited states and not cations. Byanalogy, the main active species contributing to H2S con-version formed from electron collision reaction with Heare assumed to be excited states of He and the contributionof ions to H2S conversion in He is excluded from consid-eration. The first electronic excited state of He, He(23S1),has an excitation energy of 19.82 eV (Prestage et al., 1985).If the excited states of He were the only active speciescontributing to H2S (R6) and (R7), the highest conver-sion rate of H2S in He would be 110 W/(19.82 eV ×96.5 kJ/eVmol) = 57.5 �mol/s. However, for concentra-

tions of H2S in He less than 12%, the conversion rates ofH2S are all higher than 57.5 �mol/s, which indicates thatdirect electron collision reaction of H2S (R5) must con-tribute to H2S conversion in addition to the He excitedstates.

The observed maximum in H2S conversion rate in Ar, He,N2, and H2 with increasing mole fraction of H2S can be ex-plained through pathways (III) and (IV). For H2S in Ar, previ-ous investigation (Zhao et al., 2006c) has shown that the majorproduct for direct electron collisions with Ar is the lowest ex-cited state of Ar, Ar(3P2), which has an excitation energy of11.55 eV,

e + Ar → Ar(3P2) + e. (R8)Ar(3P2) contributes to H2S dissociation and H2 dissociation asfollows (Velazco et al., 1978; Gundel et al., 1976):

Ar(3P2) + H2S → Ar + H + HS,k = 5.18 × 1014 cm3 mol−1 s−1, (R9)

2224 G.-B. Zhao et al. / Chemical Engineering Science 62 (2007) 2216–2227

Ar(3P2) + H2 → Ar + H + H,k = 3.97 × 1013 cm3 mol−1 s−1. (R10)Similarly, the following reactions contribute to H2S conver-

sion for H2S in He (Bevsek et al., 1995; Someda et al., 1988;Yencha and Wu, 1978):

e + He → He(23S1) + e, (R11)He(23S1) + H2S → He + H + HS, (R12)He(23S1) + H2 → He + H + H. (R13)There are no reports of measured or calculated rate constantsfor reactions (R12) and (R13).

For H2S in N2, the major products of electron collision re-actions with N2 are N radicals and N2(A), the first electronicexcited state of N2 (Zhao et al., 2004c).

e + N2 → N + N + e, (R14)e + N2 → N2(A) + e. (R15)Previous investigation (Zhao et al., 2005a) has shown that therate of electron collision reaction (R15) is about seven timeshigher than that of (R14). These active species react with N2,H2S, and H2 (Herron, 1999; Aleksandrov et al., 1994; Kossyiet al., 1992) as follows:

N + H2 → NH2, k = 1.14 × 104 cm3 mol−1 s−1, (R16)N + N → N2, k = 8.54 × 1010 cm3 mol−1 s−1, (R17)N2(A) + H2 → N2 + 2H,

k = 2.11 × 109 cm3 mol−1 s−1, (R18)N2(A) + H2S → N2 + H + HS,

k = 1.81 × 1014 cm3 mol−1 s−1, (R19)There are no reports of reaction of H2S and N. However, byanalogy with the extremely low rate constant for the reactionof N with H2O (4×103 cm3 mol−1 s−1 at 1073 K) (Cohen andWestberg, 1991), we presume that N does not contribute sig-nificantly to H2S conversion and that N radicals predominantlyrecombine to form N2 because the rate constant for this recom-bination reaction (R17) is about 8 × 106 higher than that of(R16). In addition, no nitrogen containing byproducts, such asammonia, were detected, which confirms that the only productsof H2S conversion in N2 are H2 and S.

For H2S in H2, the major product of electron collision withH2 is atomic H because the dissociation energy of H2 (4.4 eV)is far less than the excitation energy of the first excited state ofH2 (11 eV) (Sharp, 1971), which results in all electronic excitedstates of H2 preferentially dissociating to H radicals:

e + H2 → H + H + e. (R20)Atomic H further contributes to H2S conversion and forma-

tion in an autocatalytic manner through the following sequenceof reactions (Peng et al., 1999; Stachnik and Molina, 1987;

Schofield, 1973):

H + H2S → H2 + HS, k = 4.46 × 1011 cm3 mol−1 s−1,(R21)

HS + HS → H2S + S, k = 2.41 × 1013 cm3 mol−1 s−1,(R22)

S + HS → S2 + H, k = 2.41 × 1013 cm3 mol−1 s−1.(R23)

At low H2S concentrations, most electrons collide with the bal-ance gas, which suggests that pathway (IV) through reactions(R6) and (R7) is the major pathway for H2S conversion. (R8)and (R9) are responsible for initiating H2S conversion in Ar;(R11) and (R12) are responsible for initiating H2S conversionin He; (R15) and (R19) are responsible for initiating H2S con-version in N2; and (R20) and (R21) are responsible for initiatingH2S conversion in H2. With increasing H2S concentration, theH2S conversion rate increases because the rate of direct electroncollision dissociation of H2S (R5) increases. Moreover, the in-creasing rate of H2S conversion through (R5) is expected to belarger than the decreasing rate of M* formation through (R6)(which further contributes to H2S dissociation through (R7))with increasing H2S concentration because the dissociation en-ergy of H2S (3.4 eV) is far less than the excitation energy ofAr (11.55 eV for Ar(3P2)), He (19.82 eV for He(23S1)), or N2(6.1 eV for N2(A)), and the dissociation energy of H2 (4.4 eV).This explains the initial increase in H2S conversion rate withincreasing H2S concentration, as shown in Fig. 6. However,H2S is electronegative (Christophorou et al., 1987). The pres-ence of an electronegative gas as a reactant reduces the dis-charge current in the reactor by capturing electrons. Thus, theelectron concentration during discharge is reduced due to thehigh electron affinity of H2S, which results in a decreasing rateof electron collision reactions, as observed previously (Zhaoet al., 2005d,e). With increasing H2S concentration, the elec-tronegative effect of H2S becomes more prominent and finallyresults in decreasing rates of electron collision reactions ((R5)and (R6)). These effects explain the maxima and subsequentdecrease of H2S conversion rates with increasing H2S concen-tration shown in Fig. 6.

Fig. 7 shows energy consumption during H2S conversionas a function of H2S mole fraction in the four balance gases.The energy consumption of H2S conversion initially decreases,reaches a minimum, and increases with increasing H2S molefraction, which is consistent with the trend of H2S conversionrate shown in Fig. 6. Energy consumption during H2S con-version in H2 is higher than in N2 because the cross-sectionalarea of molecular H2 is 1.86 times smaller than that of N2 (asshown by the respective effective molecular radius of 1.35 ver-sus 1.84 Å) (Daubert and Danner, 1997), which causes a lowerrate of electron collision reactions with H2 compared to N2 andresults in more energy dissipation in H2 compared to N2. En-ergy consumption during H2S conversion in Ar and He are thelowest of the tested gases and similar in magnitude.

G.-B. Zhao et al. / Chemical Engineering Science 62 (2007) 2216–2227 2225

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Fig. 7. Energy consumption of H2S conversion as a function of H2S molefraction in different balance gases. (�): H2S in Ar; (�): H2S in He; (�):H2S in N2; (�): H2S in H2.

Energy consumption during H2S conversion in monatomicbalance gases is far lower than in diatomic balances gas, whichcan be explained through analysis of electron collision pro-cesses for H2S in the monatomic and diatomic balance gases.When an energetic electron collides with an atomic molecule,the electron predominantly experiences elastic collision with-out energy loss if the electron energy is less than the excitationenergy of target atom. The electron is then further acceleratedin the electric field and hence gains more energy. If the elec-tron collides with H2S in the next collision, H2S can be disso-ciated easily because the electron has already experienced twoaccelerations over approximately two mean free path lengthsof the gas molecules. When an energetic electron collides witha diatomic molecule, the electron can lose energy through themany energy levels available to diatomic molecules, includingexcitation, rotation, vibration, and dissociation, depending onthe electron energy. For example, an energetic electron wouldbe deactivated by contributing its energy to rotation and vi-bration of the diatomic molecule if the electron energy is lessthan excitation energy or dissociation energy. This implies thatelectrons cannot gain energy as efficiently in a diatomic bal-ance gas compared to monatomic gases. The electron energyin atomic gases can be used more efficiently because there areno paths for energy loss to rotation and vibration. Therefore,energy efficiency of H2S conversion in atomic balance gases isexpected to be higher than that in diatomic balance gases, asobserved in Fig. 7.

The results in Fig. 7 show that the lowest energy consumption(highest efficiency) of H2S conversion is 17 eV/H2S molecule.This value is lower than the energy consumption reported inall previous investigations (Helfritch, 1993; Ruan et al., 1999;Abolentsev et al., 1995; Ma et al., 2001; Traus and Suhr, 1992;Traus et al., 1993; Dalaine et al., 1998a,b), except in microwave

discharges at sub-atmospheric pressures (∼ 4.5 eV/H2Smolecule) (Cox et al., 1998). This result confirms that pulsedcorona discharges are more efficient than other types and thatrelatively low energy consumption can be obtained at highpressures and H2S concentrations. However, most hydrogenproduced industrially by steam reforming of methane andother light alkanes has an energy consumption of 3.92 eV/H2molecule (Cox et al., 1998), which is a factor of 4 less thanthe best (lowest) experimental values for energy consumptionduring H2S conversion found during the present investiga-tion. Thus, further improvements in plasma efficiency must beachieved before plasma processes can compete with currenthydrogen production methods. H2S decomposition energy ef-ficiency in nonthermal plasma reactors would be improved byremoving either of the products: hydrogen or sulfur. Develop-ment of efficient hydrogen membranes to separate hydrogenfrom the plasma is an ongoing part of this study.

4. Conclusions

Breakdown voltages of H2S in four balance gases (Ar, He,N2, and H2) measured at different H2S concentrations and pres-sures are proportional to the partial pressures of H2S and therespective balance gas. H2S conversion rates and energy ef-ficiencies depend on the balance gas and H2S inlet concen-trations. With increasing H2S concentrations, H2S conversionrates initially increase, reach a maximum, and then decrease.H2S conversion in atomic balance gases, such as Ar and He, ismore efficient than that in diatomic balance gases, such as N2and H2.These observations can be explained by reaction mech-anisms that involve electron collision reactions either with H2Sthat cause direct dissociation or with the balance gas to pro-duce active species in electronic excited states that then relaxby dissociating H2S. The results show that nonthermal plasmasare effective for dissociating H2S into hydrogen and sulfur, butfurther increases in energy efficiency are necessary.

Acknowledgments

This work was supported by the Department of Energy (DE-FC26-03NT41963) and the University of Wyoming ResearchOffice. The authors gratefully acknowledge experimental assis-tance provided by Mr. R. Borgialli.

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Production of hydrogen and sulfur from hydrogen sulfide in a nonthermal-plasma pulsed corona discharge reactorIntroductionExperimental sectionResults and discussionBreakdown voltage of H2S in the various balance gasesH2S conversion in various balance gases

ConclusionsAcknowledgmentsReferences