Catalytic combustion of methane in the presence of organic and inorganic compounds over...

Applied Catalysis B: Environmental 39 (2002) 311–318

Catalytic combustion of methane in the presence of organic andinorganic compounds over La0.9Ce0.1CoO3 catalyst

Róbert Auera, Mihai Alifanti b, Bernard Delmonb, Fernand C. Thyriona,∗a Chemical Engineering Institute, Université Catholique de Louvain 1, Voie Minckelers, 1348 Louvain la Neuve, Belgium

b Unité de Catalyse et de Chimie des Matériaux Divisés, Université Catholique de Louvain,Place Croix du Sud 2/17, 1348 Louvain la Neuve, Belgium

Received 27 December 2001; received in revised form 25 February 2002; accepted 9 May 2002

Abstract

The catalytic combustion of the emission from coke ovens containing various volatile organic compounds (VOCs) andinorganic species over a La0.9Ce0.1CoO3 catalyst is investigated in an integral fixed reactor through several steps. (1) Com-bustion of a mixture of VOC reveals that the kinetics of total oxidation of methane determines the total VOC conversion. (2)The conversion of methane, in the case of sulfur-free feed is inhibited by H2O and CO2.© 2002 Elsevier Science B.V. All rights reserved.

Keywords:Coke oven emission; Perovskite catalyst; Methane combustion; Mixture effect

1. Introduction

Low temperature catalytic combustion gains a lotof applications in the environmental techniques, i.e.in the field of gasoline vapor removal, odor controland volatile organic compounds (VOCs) removal frommany industries.[1]. The complete oxidation of awide range of hydrocarbons, O-, S-, and Cl-containingVOCs along with different inorganic species such asH2, CO, NH3, NO as well as CO was extensively re-viewed elsewhere[1]. Rules for relative reactivity be-tween different VOCs were established and mixtureeffects were identified[2–7]. The combustion of vari-ous VOCs inhibited by the reaction products H2O andCO2 were reported in several studies[2,4,8–13].

∗ Corresponding author. Tel.:+32-10-472327;fax: +32-10-472321.E-mail address:[email protected] (F.C. Thyrion).

Perovskite-type oxide catalysts, having a generalformula A1−xA ′

xB1−yB′yO3 have received a lot of

attention for hydrocarbon combustion[14] sinceVoorhoeve et al.[15] reported their high catalyticactivity. Various compositions were developed andtested for activity[16–18] as well as for sulfur resis-tance[19–21]using combustion of methane or naturalgas as test reaction.

Preliminary results[22,23]showed that among a se-ries of perovskite catalysts described by the formulaLa1−xCexCoyMn1−yO3, the species La0.9Ce0.1CoO3exhibits a fairly good activity and SO2-tolerance inthe course of methane combustion. In this paper, themain results of the catalytic abatement of fugitiveemissions from the coke ovens are presented. Thecomplexity of gas composition, with nitrogen- andsulfur-containing molecules, COx , H2, and H2O, inaddition to methane and a few other VOCs (see a typ-ical composition inTable 1), was expected to compli-cate the reaction. Methane was considered to be the

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312 R. Auer et al. / Applied Catalysis B: Environmental 39 (2002) 311–318

Nomenclature

RLA relative loss of activity,(X(0)−X(60 h))/X(0)

X conversion of methane,(yCH4

in − yCH4out)/yCH4

in (%)X(0) initial conversion (deactivation

experiments)X0 methane conversion in the absence

of H2O and CO2yCH4 volumetric ratio of methane

(vol.%)T10, T50, T90 temperatures of 10, 50, and 90%

conversion, respectively

key compound in the emission since it is present atrelatively high concentration among them and it is themost difficult hydrocarbon to be ignited. Therefore,the overall VOC combustion was supposed to be deter-mined by the kinetics of methane combustion. Sulfur-and nitrogen-containing molecules were expected toact as catalyst poisons. Besides, inhibition and syner-getic effects due to the presence of VOCs and inor-ganic molecules had to be identified.

Taking into account the complexity of the problem,a specific research strategy, comprising several steps,is devised. The major interactions among VOCs whencombusting them in mixture are identified by factorialexperimental design. The influence of the molecules

Table 1Typical composition of the fugitive emission from coke ovens(volumetric units)

Gas Concentration

N2 80–95%O2 5–20%CH4 0.2–2.2%C2H4 2000 ppmC2H2 1600 ppmBenzene 78 ppmToluene 16 ppmH2O 0.1–2.2%H2 <3.3%CO 0.15–0.3%CO2 <3%NH3 170–550 ppmNOx 20–30 ppmH2S 15–60 ppmSO2 20–40 ppm

containing heteroatoms on the combustion of methaneis investigated through two steps. First, the effect ofsingle inorganic compounds on the catalytic activityand on aging, then their additive inhibition on the com-bustion is examined.

2. Experimental

2.1. Preparation and characterizationof the catalyst

The La0.9Ce0.1CoO3 catalyst was prepared accord-ing to the citrate method[24]. The amorphous citrateprecursors were calcined during 5 h in air at 580◦C forthe study of VOCs mixture combustion and at 700◦Cfor the other experiments.

BET specific surface areas (SSAs) were deter-mined by adsorption of nitrogen at 77 K using aMicromeritics Flowsorb II 2300 instrument, operat-ing in a single-point mode. Prior to analysis, sampleswere degassed 2 h at 150◦C. The catalysts had aBET surface of 19.5 and 10.2 m2/g at the calcinationtemperature of 580 and 700◦C, respectively.

XRD patterns were collected by means of aKristalloflex Siemens D5000 diffractometer using theCu K� radiation atλ = 1.5418 Å. Data acquisitionwas realized in the 2θ range 2–65◦ with a scan stepsize of 0.03◦.

XPS spectra were recorded at room temperatureand under a vacuum of 10−7 Pa on a SSX-100 Model206 Surface Science Instrument spectrometer withmonochromatized Al K� radiation (hν = 1486.6 eV).The charge neutralization was achieved using an elec-tron flood-gun adjusted at 10 eV and placing a Ni grid3 mm above the sample. The baseline was consideredlinear and tangent to the peak wings. The chargecorrection was made considering the C 1s signal ofcontaminating carbon (C–C or C–H bonds) centeredat 284.8 eV.

2.2. The experimental set-up

The experimental installation is depicted inFig. 1.Brooks mass-flow controllers equipped with in-linefilters and check valves regulated the gas flows. Watervapor was introduced by passing a flow of nitrogenthrough a saturator that was maintained under isother-mal conditions.

R. Auer et al. / Applied Catalysis B: Environmental 39 (2002) 311–318 313

Fig. 1. Schematic diagram of the experimental set-up: (1) quartz reactor in electrical oven; (2) gas cylinder; (3) filter; (4) mass-flowcontroller; (5) check valve; (6) three- and four-way valves; (7) syringe pump; (8) water saturator; (9) cross-flow section; (10) needle valve;(11) mass-flow meter; (12) temperature controller; (13) temperature indicator. M1 and M2—gas mixtures.

The “W” shaped quartz reactor (i.d. 10 mm), placedin a cylindrical ceramic oven, was fed down-flow. Thecatalyst was pressed into binderless wafers, crushedand sieved; the 40–90�m fraction was used in allexperiments. Before the catalytic tests, the catalystswere pretreated by flowing air for 2 h at a temperature50◦C below the calcination temperature.

Omron E5CK-T temperature controllers regulatedthe temperature of the catalyst bed. One thermocouplewas placed at the bottom of the catalyst bed, whilstanother one allowed the measurement of the temper-ature 5–10 mm above the bed.

The so-called “light-off curve” of the compoundswas obtained by measuring methane and VOCsconversions using 0.12 g catalyst and a total flow100 cm3/min (STP). The temperature was increasedat a rate of 2.5◦C/min. The highest reaction tempera-ture was limited by the calcination temperature of thecatalyst (580◦C). After each experiment, the catalystwas reactivated under air flow at 570◦C for 2 h.

The deactivation tests were carried out at 550◦Cfor 60 h under the same inlet conditions as mentioned.The poisoning effect was quantified by means of therelative loss of activity, defined as:

RLA = X(0) − X(60 h)

X(0)

where X(0) is the initial fractional conversion ofmethane andX(60 h) represents the conversion after60 h time on stream.

The concentration of the organic compounds wasanalyzed by a FID gas chromatograph using a Chro-mosorb 101 (0.75 m; 80/100 mesh) packed column. ASpectraphysics Chromjet integrator made the analysisand controlled the automatic injection.

2.2.1. VOC mixture experimentsA syringe pump (BAS Bee MF9090) injected hex-

ane and toluene into a flow of nitrogen. Acetylene and


ethylene (2.5 vol.% diluted in N2) were fed separately(gas cylinders M1 and M2 inFig. 1).

2.2.2. Experiments with inorganic moleculesMass-flow controllers regulated most of the inor-

ganic gases (CO, CO2, H2, NO, and NH3 diluted byN2) either alone or in mixture (indicated by M1 inFig. 1). However, the flow of H2S and SO2 diluted inN2 (M2) was controlled by a needle valve and mea-sured by a digital mass-flow meter.

All gases (99.8%) supplied by Air Liquide wereused without any further purification. The gas mixtureswere prepared by adjustment of the partial pressure ofthe components in the cylinder. The precision of thepreparation amounted to±5%.

3. Results and discussion

3.1. Combustion of VOC in mixtures

The mutual influence of the deep oxidation ofmethane along with four VOCs, i.e. ethylene, acety-lene, n-hexane, and toluene was investigated usingfactorial experimental design[25]. The concentrationof the organic compounds was varied at three levelsaccording toTable 2. Measurement at all combina-tions of low and high concentration levels (25 = 32)and of the central point repeated three times gave atotal number of 35 experimental points.

The results of typical experiments with singlehydrocarbons are summarized inFig. 2. The orderof light-off curves versus temperature remained un-changed independently from the inlet composition:the reactivity of the molecules over the perovskitecatalyst decreased as follows: acetylene> toluene>

hexane> ethylene> methane.Temperatures of 10, 50, and 90% conversion (T10,

T50, andT90) were used as a measure of catalytic ac-

Table 2Concentration levels in the VOC combustion experiments(ppm vol.)

Level CH4 C2H4 C2H2 Hexane Toluene

Low (−1) 2000 250 250 50 50Center 8000 1000 1000 200 200High (+1) 20000 2500 2500 500 500

Fig. 2. Light-off curves of methane and four VOCs in mixtureover La0.9Ce0.1CoO3 catalyst. Catalyst weight, 0.12 g; total flow,100 cm3/min (standard temperature and pressure); rate of temper-ature increase, 2.5◦C/min; inlet concentration of oxygen, 1 vol.%methane, acetylene, and hexane are at the high concentration levelwhile the others are at the low level (Table 2).

tivity. In the case of methane,T90 was replaced by theconversion at the final temperature (X570) since 90%conversion was not reached at this temperature. Theresults were treated by Yates—ANOVA analysis[25].The combustion of methane was only slightly inhibitedby the presence of ethylene. The other compounds didnot influence methane combustion, probably becausethe temperature range of their conversion was signifi-cantly lower than that of methane (Fig. 2). Hence, as afirst approximation, the kinetics of methane combus-tion determines the temperature of complete combus-tion of the VOCs mixture. The mutual influence of theother compounds was more important. For the sakeof illustration, the effects of various concentrations ofethylene, acetylene, and hexane as well as the tolueneconcentration itself on the combustion of toluene aredepicted inFig. 3. It appears that an increase of con-centration increasesT90 in all the cases except withhexane where a large effect occurs at a low concen-tration level. At the intermediate level, the effect ofthe additives vanishes. On the same graph, the pres-ence of acetylene and ethylene inhibits the reaction ina similar manner although the first compound is moreeasily oxidized than toluene while the second is less.The graph ofT50 for toluene (not presented) obtainedunder the same conditions displays a similar behaviorwith the exception that the slope of the toluene curve ishigher than the other ones meaning a higher inhibition


Fig. 3. Influence of ethylene, acetylene, and hexane on the com-bustion of toluene. Low, center, and high concentration levels ofadditives or toluene are represented by−1, 0, and 1, respectively(Table 2).

by itself than by the other additives. Among the otherVOCs, the following influences were observed:

• the conversion of acetylene, which is oxidized atthe lowest temperature, is influenced neither bymethane nor by the other VOCs;

• the ethylene combustion is retarded by the presenceof acetylene;

• the hexaneT50 and T90 are the only ones whichdecreases with an increase of the concentration ofthe hydrocarbon itself: nevertheless, they increasesin the presence of ethylene and acetylene.

So for components reacting in a completely dif-ferent temperature range as mixtures of methane andother compounds, it is still possible to reproduce theindividual conversion profiles. If the reactions takeplace in the same temperature range, there is a mutualinfluence of the compounds. Depending on the natureof the components, the individual reactions are inhib-ited, remain unchanged or in some cases are even en-hanced. Several interpretations have been given in theliterature and among them, the competitive adsorptionand the occurrence of different types of activated oxy-gen as a function of the temperature.

The fact that the combustion of methane was onlyslightly inhibited by the presence of ethylene is prob-ably due to some inhibition by H2O and CO2 issuedfrom the combustion of ethylene (see later). For in-stance, the inhibition of propane combustion by ethy-lene was also explained by water production[13].

3.1.1. Catalyst characterizationThe catalyst did not loose any activity during the

experiments. This was verified from time to time byre-measuringT50 of methane combustion. The sam-ples tested under clean conditions, namely combustionof the mixtures methane–hydrocarbon–air, showed nosignificant variations of SSA nor structural modifica-tions observable in the XRD pattern. The same wasobserved in the case of the other studied hydrocarbons.Table 3shows some physico-chemical properties de-duced from XPS investigations for the fresh and used(for combustion of methane alone) catalyst. There isno major change of cation superficial concentration orin the binding energies, a fact that suggests that nochange in surface composition occurs due to methanecombustion. However, a slight increase of the adsorbedoxygen at the surface is observed in the XPS signal.Taking into account the surface enrichment in La andCe cations and the basic character of their oxides, thediminished Olatt/Oads ratio can be easily associated tothe adsorption of water molecules formed during thereaction or to surface hydroxylation. No formation ofcarbonates was evidenced.

3.2. Influence of some inorganic gases on methanecombustion

3.2.1. Single compoundsThe light-off curves for 0.5 vol.% CH4 (W/FCH4

in:0.350 s gcat/�mol CH4) in the presence of individualinorganic compounds (Table 4) in air were recordedin the range of 300–650◦C using a heating rate of2◦C/min. The reference temperature of 50% conver-sion of CH4 (T50 ref) in the absence of the inorganic

Table 3XPS investigations for the La0.9Ce0.1CoO3 fresh catalysts and aftertest in the presence of methane alone

Element/level Binding energy Superficial abundancea

Fresh Used Fresh Used

La 3d5/2 833.7 833.85 0.51 0.53Ce 3d5/2 882.28 882.35 0.13 0.11Co 2p3/2 779.86 780.11 0.35 0.36O 1s (Olatt/Oads)b 528.8 529.0 1.42 0.98

a The ratio between the considered cation and the sum of allcations.

b The ratio between the lattice and the adsorbed oxygen.


Table 4Inhibition and deactivation caused by the single inorganic molecules

SO2 H2S H2O H2 CO2 CO NO NH3

C (ppm vol.) 40 25 2 vol.% 7000 8000 3000 65 180�T50 (◦C)a 70 50 20 15 5 0 0 0RLA 0.32 0.25 0.06 0 0 0 0 0

a Temperature increase of 50% conversion of methane; reference temperature (in the absence of the inorganic molecules):T50 ref =500◦C; RLA = [X(0) − X(60 h)]/X(0).

molecules was 500◦C. The temperature increase(�T50) with respect to a reference value,T50 ref,(Table 4) shows that the sulfur-containing molecules,H2O, H2, and CO2 inhibited the reaction, while theinfluence of NO, NH3, and CO was negligible.

The RLA values inTable 4indicate that H2S andSO2 caused significant deactivation and H2O a slightone. The other molecules did not deactivate the cata-lyst.

H2O and H2 have a similar effect, a result that canbe attributed to the fact that hydrogen is completelyconverted to water at the reaction temperature.Fig. 4shows the impact of hydrogen in the feed mixtureupon the methane conversion. A perfect reversibilityof the phenomenon and a complete recovery of theinitial catalytic activity are observed for two reaction

Fig. 5. Influence of inorganic molecules upon the conversion of methane: (yCH4in, 1.05 vol.%; yO2

in, 10.8 vol.%; andW/FCH4in,

0.774 gcat s/�mol. Concentration of other compounds: NO, 111 ppm vol.; NH3, 800 ppm vol.; CO, 5655 ppm vol.; CO2, 1.697 vol.%).

Fig. 4. The effect of hydrogen on methane combustion at 550◦C;inlet composition, 0.5 vol.% CH4 (W/FCH4

in: 0.350 s gcat/�molCH4); 7000 ppm vol. H2.


cycles. A similar effect of water has been observed forpropane combustion over a perovskite catalyst[9]. Nostructural and chemical modifications were observedby XRD and XPS measurements after tests in the pres-ence of sulfur-free inorganic molecules. On the otherhand, deactivation by the sulfur-containing moleculeswas found to be irreversible. The poisoning caused bythe sulfur compounds, investigated by XRD and XPS,seems to be related to the formation of La2(SO4)3 thatdestroys the perovskite structure[23,26]. The elimi-nation of these catalyst poisons from the upstream isabsolutely necessary in order to ensure a reasonablecatalyst lifetime.

3.2.2. Impact of mixtures of inorganic gasesThis was tested at five temperature levels: 360,

390, 420, 460, and 500◦C. Higher space–time value(W/FCH4

in: 0.774 gcats/�mol) in comparison withthe other experiments has been used since one ofthe objectives of the research was to obtain a totalVOC conversion superior to 95% at 500◦C. The inletconcentration of methane and oxygen were 1.05 and10.8 vol.%, respectively. The results are summarizedin Fig. 5. The inhibition caused by H2O, SO2, and H2is additive. Addition of the other compounds, i.e. NO,NH3, CO, and CO2 to the feed leads only to a slightsupplementary inhibition, which might be essentiallydue to the influence of CO2.

It can be concluded that only CO2 and H2O in-hibit the combustion of methane in the case whensulfur-containing molecules are removed from thefeed stream. These two molecules can be present inthe feed stream as well as produced by the combus-tion of H2, CO, CH4, and the other VOCs, since ata temperature higher than 390◦C, H2 and CO werecompletely oxidized to H2O and CO2.

4. Conclusions

The combustion of methane alone or in the pres-ence of other compounds as found in coke oven gasemission, over La0.9Ce0.1CoO3, leads to the followingconclusions:

• The total VOC conversion is determined by the ki-netics of methane oxidation since (a) among theother organic compounds, methane has the lowest

combustion reactivity; and (b) the temperature rangeof its conversion is not affected by the other VOCs.

• Methane conversion is inhibited by SO2, H2S, CO2,H2O, and H2. The catalyst is irreversibly poisonedby H2S and SO2, thus they should be removed fromthe feed stream in order to ensure a reasonablecatalyst lifetime. The other heteroatoms-containingmolecules do not influence the combustion ofmethane.

Acknowledgements

This work was supported by the European Commu-nity (ENV4-CT97-0599). We thank J.R. Paredes forthe analysis of the emissions in Aceralia CorporacionSiderurgica, Spain, and I. Mastroianni for her contri-bution in the experiments with VOC mixtures.

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