Nonpremixed Catalytic Combustion of Methane in a Fluidized Bed Reactor

Mario Iamarino,* ,† Paola Ammendola,‡ Riccardo Chirone,§ Raffaele Pirone,§Giovanna Ruoppolo,§ and Gennaro Russo§

Dipartimento di Ingegneria e Fisica dell’Ambiente, UniVersita degli Studi della Basilicata, Via dell’AteneoLucano 10, 85100 Potenza, Italy, Dipartimento di Ingegneria Chimica, UniVersita Federico II, PiazzaleTecchio, 80125 Napoli, Italy, Istituto Ricerche sulla CombustionesCNR, Piazzale Tecchio, 80125 Napoli, Italy

The catalytic combustion of methane has been investigated in a 0.10 m bubbling fluidized bed reactor withnonpremixed feedings of reactants. Copper dispersed on porousγ-Al2O3 spheres (1 mm diameter) characterizedby high mechanical strength has been used as catalyst. The effect of design variables such as methane inletconcentration, bed temperature, and superficial gas velocity on methane conversion has been quantified inthe ranges 4-10 vol %, 650-750 °C, and 0.40-1.30 m s-1, respectively. The propensity to attrition of thecatalyst has been separately investigated under the experimental conditions tested. Results have been interpretedin the light of a simple reactor model which assumes a plug-flow pattern for the gas through the bed andmth-order catalytic kinetics with respect to fuel concentration.

1. Introduction

Fluidized bed catalytic converters represent a viable alterna-tive to packed beds and monoliths for catalytic combustionapplications, mainly due to improved heat transfer properties,enabling, on one hand, an efficient heat recovery by means ofexternal or submerged heat transfer surfaces and preventing,on the other, undesired catalyst overheating1 which could resultin its thermal deactivation. In addition, catalytic fluidized bedfed with gaseous, liquid, or solid fuel has revealed very attractivefeatures when compared with traditional oxidation systems,mainly due to the lower content of pollutants in the exhaustgas and the higher volumetric density of heat produced.2

Despite these favorable considerations, in the past fluidizedbed catalytic converters have not received enough attention andopen issues, such as the loss of expensive catalyst due toattrition4 and the criticality of fuel/air mixing phenomena influidized beds operated in the bubbling regime,3 still posetechnical barriers to the full exploitation of the fluidized bedcatalytic conversion technology. Actually, the possibility ofoperating the reactor in the turbulent fluidization regime,5

advocated as a way to overcome the onset of bubble-to-emulsionphase mass transfer resistances, results in larger losses of catalystas attritted material due to the higher gas superficial velocities.It follows that additional research efforts are required to findan optimal tradeoff between these conflicting requirements:setup of a catalyst with low attrition propensity and carefuloptimization of hydrodynamics and gas mixing regimes offluidized catalytic converters.

According to previous studies,6-8 a catalytic system basedon copper supported on porousγ-alumina spheres has beenproven to display good activity, low cost, and excellent thermaland mechanical stability for use in fluidized bed catalyticcombustion of light hydrocarbons. The papers refer to a studyon catalytic combustion of methane and propane in a premixedbubbling fluidized bed under fuel-lean conditions and highlighthow the use of coarse catalyst pellets could favorably affect

the effectiveness of bubble-to-emulsion phase mass transfer, withcomplete and stable hydrocarbon conversion obtained at tem-peratures below 700°C; propensity to attrition and thermaldeactivation of the catalyst turned out to be fairly low.Experimental results have been interpreted on the basis ofcompeting phenomena between intraparticle and interphasediffusional processes and intrinsic kinetics of heterogeneousreaction. With reference to the premixed option, the boundarylayer diffusion around the catalyst particle turns out to be fasterthan the other processes, while the competition between intrinsicreaction kinetics, intraparticle diffusion, and bubble-to-emulsionphase mass transfer controls the converter performance underthe range of temperatures investigated.

The present work moves further to address the performanceof catalytic converters characterized by nonpremixed feedingsof methane and air, relevant to the combustion of mixtureswithin the flammability limits. This extension is of increasinginterest with reference to practical applications of gas fluidizedbed boilers when considering the high potentiality required andthe safety problems related to a flammable feeding. Effects ofmethane inlet concentration, gas superficial velocity, andtemperature on reactor performance have been investigated interms of fuel conversion, unburned fuel emissions, and particu-late elutriation rate. In addition, the effect of methane inletconcentration on conversion has been interpreted in the lightof a simple reactor model that assumes a plug-flow pattern forthe gas through the bed andmth-order catalytic kinetics withrespect to fuel concentration.

2. Experimental Section

The experimental apparatus used in this work is reported inFigure 1. The catalytic converter consisted of a 0.10 m stainlesssteel fluidized bed reactor, equipped with a porous plate as airdistributor. Methane was injected in the bed 2 cm above theporous plate through a distributor consisting of a cross-shapedmanifold, with four branches 4 cm long. Four holes with 0.8mm diameter were drilled along each branch, equally spacedand perpendicularly oriented with respect to the reactor axis.On the basis of literature studies,9 the discharge mode ofmethane at the distributor level is expected, at the experimentalconditions adopted, to give rise to stationary horizontal jetspenetrating into the bed rather than to trains of single ascending

* To whom correspondence should be addressed. E-mail: iamarino@unibas.it. Tel.: +39 0971205208. Fax:+39 0971205160.

† Universitadegli Studi della Basilicata.‡ UniversitaFederico II.§ Istituto Ricerche sulla Combustione.

1009Ind. Eng. Chem. Res.2006,45, 1009-1013

10.1021/ie051015e CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 12/27/2005

bubbles, but the setup did not allow a direct visual observationof the bed to verify the discharge regime. Below the porousplate, a packed bed (0.60 m height) consisting of Rasching ringsof different sizes was used to level off the inlet air velocityprofile. An electrical oven surrounding the packed bed wasemployed during startup to initially heat the reactor to about400 °C. At this temperature, bed activity was high enough tostart the reaction and ensure further temperature increase. Duringsteady-state operation, the oven was switched off and thetemperature was stabilized by removing the excess of heatgenerated by means of a cooling system consisting of a coppercoil wound up around the external reactor surface for an heightof 0.15 m. Liquid water was used as exchange medium,subtracting heat by complete vaporization; vapor was thencondensed and recirculated to the reactor. Temperature profilesalong the reactor axis were measured by means of five equallyspaced thermocouples (T1-T5 in Figure 1) vertically insertedinto the bed and located at fixed positions (0.06, 0.145, 0.23,0.315 and 0.4 m, respectively, above to the air distributor).Continuous monitoring of gas composition at the exhaust linewas accomplished by on-line nD-IR analyzers (for CH4, CO,CO2, O2, and NOx) after water removal. A data acquisition unitwas used to log on signals from the experimental facility, andconcentration measurements were properly corrected to take intoaccount the removal of water.

The bed consisted of 2100 g of catalyst with correspondingunexpanded bed height of 0.30 m. The catalyst was obtainedby dispersing copper on porousγ-Al2O3 spheres supplied bySasol, characterized by high mechanical strength and lying atthe borderline between groups B and D of the Geldart clas-sification of powders.10 Catalyst was thermally pretreated at 800°C to yield copper mainly as superficial spinel phase CuAl2O4,stable upon repeated thermal treatments. Preparation techniquesand physicochemical properties of catalyst are detailed in aprevious work.6

Methane conversion experiments were carried out at methaneinlet concentrations mostly ranging from 4 to 9 vol %, while afew tests were performed up to 10 vol %, hence above the

stoichiometric value (9.5 vol %) for methane in air. Thetemperature range investigated was between 650 and 750°C,where the upper limit for temperature is consistent with thecatalyst thermal deactivation threshold (800°C).

Catalyst mechanical stability was assessed at different tem-peratures, methane inlet concentrations, and superficial velocitiesby collection of solid particles in the exhaust gas by means ofan isokinetic withdrawing system, since high pressure dropswould be reached by filtering of the total flow. This systemconsists of a 1 mmstainless steel probe, coaxially inserted withrespect to the fluidized bed reactor and located 0.2 m below itsupper end, a thermocouple (T6 in Figure 1) for temperaturemeasurement of the exhaust gas at the collecting point, a ceramicfilter able to collect fines larger than 300 nm, and a pump ableto extract a desired gas flow.

3. Results and Discussion

Figure 2 reports typical temperature profiles measured at fixedaxial positions in experiments at 700°C and 6 vol % methaneinlet concentration for different values of the fluidization velocity

Figure 1. Experimental setup.

Figure 2. Axial temperature profiles measured atCin ) 6 vol % andu0 )0.40 (b), 0.60 (9), 0.80 (2), and 1.0 (1) m s-1. Dotted line indicatesunexpanded bed height.

1010 Ind. Eng. Chem. Res., Vol. 45, No. 3, 2006

u0. For the lowest value ofu0 (0.40 m s-1), the highestthermocouple (at 0.40 m above air distributor) hangs outsidethe bed due to the limited bed expansion and the correspondingtemperature is remarkably lower. Increasingu0, all thermo-couples became immersed in the bed and temperature differ-ences were smaller, according to the improved solid mixing. Inmore detail, while temperature differences up to 10°C havebeen observed atu0 ) 0.40 and 0.60 m s-1, at 1.0 m s-1 theydo not exceed 1-2 °C. In any case the higher temperatures werealways measured in the lower section of the bed, where mostof the heat release takes place. Due to small but possibletemperature differences, data reported hereafter have beenalways conventionally referred to the temperature measured bythe lowest thermocouple in the bed, indicated as T1 in Figure1 and located 0.06 m above the air distributor. It can be alsoobserved that, since thermocouples are directly in contact withthe solid phase, the values measured are mostly indicative ofthe temperature of the catalyst external surface.

During combustion tests, the reactor was characterized byvery high methane conversion degrees (mostly above 0.99) andsteady-state operation in the autothermal regime. For every testat inlet fuel concentration below the stoichiometric value (9.5vol %), analysis of the exhaust gas only revealed products ofcomplete methane oxidation without any detection of CO orNOx. This evidence indicates that oxidation takes place entirelyon the catalyst surface and that homogeneous reaction, whichis a well-known source for the formation of CO and NOx inhot fluidized bed of inert11 and catalytic12 particles (where thereactive mixture can ignite in bubble phase or even in thefreeboard region), is not relevant under the experimentalconditions investigated. Moreover, O2, CO2, and CH4 amountsmeasured at the outlet were always consistent with the corre-sponding mass balances.

A few tests performed at a fuel concentration of 10 vol %,hence above the stoichiometric value, also revealed the forma-tion of very small amounts of CO (20 and 50 ppm, respectively,at 700 and 750°C, with u0 ) 0.60 m s-1), while fuel conversionaccordingly decreased due to oxygen shortage.

Figure 3 reports methane concentration measured at the outletnormalized with respect to inlet concentration,Cout/Cin, as afunction of the ratio between gas superficial velocity and theminimum fluidization velocity,u0/umf. According to previousfindings,8 values of 0.21, 0.20, and 0.19 m s-1 have beenconsidered forumf at temperatures of 650, 700, and 750°C,respectively. Experimental data corresponding to the same bed

temperature have been reported with the same symbol whateverthe methane inlet concentration (in the range 4-9 vol %).Analysis of the figure shows that there is a range of gasvelocities, u0/umf < 2.5, where full conversion of methane(conversion degrees higher than 0.999) has been achievedregardless of reactor temperature. Atu0/umf > 2.5, unburnedmethane increases and this effect is larger when the operativetemperature is decreased.

It is worthwhile to observe that, in any case, a very smallamount of methane were detected in the exhaust gas. As shownin Figure 4, the methane outlet concentration remains belowapproximately 100 ppm foru0/umf < 3.0 at 700°C and foru0/umf < 3.4 at 750°C in accordance with the highest methaneinlet concentration (9 vol %). On this basis, a maximum thermalpower of about 4.5 kW can be produced by the converter at750 °C with respect to constraints limiting fuel emissions to100 ppm.

In Figure 5, experimental results are reported in terms ofnormalized outlet methane concentration with respect to reactortemperature for three different gas superficial velocities (0.40,0.60, and 0.80 m s-1) and constant methane inlet concentration(6 vol %). At 0.40 m s-1, almost complete methane conversionis reached in the whole temperature range investigated and anegligible effect of reactor temperature is recognized. Theincrease of gas velocity decreases methane conversion (increas-ing Cout/Cin), according to a decrease of the residence time ofthe gas in the reactor. This effect can be partially compensated

Figure 3. Cout/Cin as a function ofu0/umf at temperatures of 650 (b), 700(9), and 750°C (2). For each temperature, data correspond to inlet methaneconcentrations in the range 4-9 vol %.

Figure 4. Outlet methane concentration [ppm] at high conversion degrees(Cin ) 9 vol %, T ) 700 (9) and 750°C (2)).

Figure 5. Normalized outlet methane concentration atCin ) 6 vol % andu0 ) 0.40 (b), 0.60 (9), and 0.80 (2) m s-1.

Ind. Eng. Chem. Res., Vol. 45, No. 3, 20061011

by an increase in reactor temperature: with respect to 0.40 ms-1, comparable reactor performances can be reached at 0.60m s-1 for an operative temperature of 750°C. The favorableeffect of temperature is less effective at 0.80 m s-1, where themethane outlet concentration curve does not level off to zeroin the range of temperature of practical interest for the catalystused.

Figure 6 reportsCout/Cin as a function of the inlet fuelconcentration in experiments performed at 750°C andu0 )0.60, 0.70, and 0.80 m s-1. The analysis has been restricted togas velocities larger than 0.6 m s-1 according to the evidencethat, resulting from Figure 3, the effect ofCin on methaneconversion is not appreciable in the low velocity range. Analysisof data shows that, for all values ofu0, unconverted methaneincreases when the inlet fuel concentration is increased.

The effect ofCin has been analyzed in the light of a simplemathematical model of the reactor. A primary approximationof the catalytic converter is represented by the assumption of aplug-flow pattern for the gas through the bed (as suggested forfluidized beds of large particles13) and a reaction rate ofmthorder with respect to fuel concentration. Under these hypotheses,a mass balance on methane yields

The Damko¨hler number,Da, is defined by

whereVcat is the volume of catalyst,Q is the total gas flowrate, andk is an apparent kinetic constant. This model does nottake into account the detailed mechanisms ruling the effectivefuel conversion, and it lumps into both parametersk andm theeffects of the intrinsic kinetics and of transport phenomena.

Equation 1 was used to fit experimental data in Figure 6 withDa andmconsidered as unknown parameters. Good agreementwith model prediction was found withm ) 0.94 at everyu0,while best-fit Damko¨hler numbers showed an inverse propor-tionality with respect to the three gas superficial velocities,consistent with eq 2. The estimated value of the reaction order,0.94, is intermediate betweenm ) 0.7, which corresponds tothe intrinsic reaction order of methane oxidation on the catalyst

used,6 andm ) 1, corresponding to first-order phenomena withrespect to methane concentration, as transport phenomena. Thisfinding indicates that, in the experimental conditions investi-gated, the converter is not operating under purely kinetic controland that transport phenomena strongly affect reactor perfor-mance. To reinforce this conclusion, an apparent activationenergy has been calculated on the basis of experimental dataalready reported in Figure 5 and obtained atu0 ) 0.60 and 0.80m s-1 and different temperatures (data atu0 ) 0.40 have beenexcluded since the temperature effect is here extremely weak).By fitting of these data using eq 1 andm ) 0.94, apparentactivation energies of 3900 and 4900 cal mol-1 were calculatedrespectively atu0 ) 0.60 and 0.80 m s-1. These values areconsiderably lower than the value of 29 800 cal mol-1 of theintrinsic catalytic kinetics7 and support the evidence of importantmass transfer limitations on methane conversion. This conclu-sion is in good agreement with what was found in the premixedmode,7 but in this case the higher complexity of the mixingphenomena at the methane distributor level and the lack of adetailed reactor model do not allow any further conclusion onthe specific nature of the controlling mechanisms.

A first assessment of catalyst mechanical stability was carriedout in ad hoc tests. The elutriation rates ware calculated byweighting the solids collected, and the resulting values showeda relatively strong dependency on the fluidization velocity butno relationship with respect to fuel inlet concentration andtemperature. As reported in Table 1, an undetectable amountof fines was collected atu0 ) 0.40 m s-1, while atu0 ) 0.60and 0.80 m s-1 elutriation rates of 0.25 and 1.6 g h-1,respectively, were measured. A further increase ofu0 up to 1.0m s-1 led to a more significant presence of solid in the exhaustgas with a corresponding elutriation rate of 4.50 g h-1, warningabout the risk of not negligible catalyst loss when operating atvery large gas velocities.

4. Conclusions

Catalytic combustion of methane in a fluidized bed of acopper-based catalyst has been investigated in a bench-scalereactor with nonpremixed feeding of reactants. The reactor hasbeen operated in the bubbling regime of fluidization, and verylimited temperature differences inside the bed have beenmeasured. Moreover, stable reactor operation has been observedduring the entire experimental campaign, and bed activityreduction due to catalyst thermal deactivation or to catalyst lossby attrition did not occur to any significant extent.

Conversion of methane was complete at temperatures rangingfrom 650 to 750°C depending on the operative conditions inthe reactor (inlet fuel concentration and fluidization velocity).As a consequence, under the conditions investigated, a maxi-mum thermal power of about 4.5 kW could be produced by thereactor with fuel emissions lower than 100 ppm and withoutany formation of CO or NOx.

Nomenclature

Cin ) inlet methane concentrationCout ) outlet methane concentrationDa ) Damkohler number

Figure 6. Effect of methane inlet concentration onCout/Cin at T ) 750°Candu0 ) 0.60 (b), 0.70 (9), and 0.80 (2) m s-1. Continuous lines havebeen obtained by the best fit of eq 1.

Cout

Cin) [1 + Da(m - 1)]1/(1-m) (1)

Da )kVcat

QCin

m-1 (2)

Table 1. Fines Elutriation Rate at Different Superficial Velocities

u0 [m s-1] elutriation rate [g h-1]

0.4 not detectable0.6 0.30.8 1.61.0 4.5

1012 Ind. Eng. Chem. Res., Vol. 45, No. 3, 2006

k ) apparent kinetic constantm ) reaction order with respect to fuel concentrationQ ) total gas flow rateT ) temperatureumf ) minimum fluidization velocityu0 ) superficial velocityVcat ) volume of catalyst

Literature Cited

(1) Hayhurst, A. N.; John, J. J.; Wazacz, R. J. The Combustion ofPropane and Air as Catalyzed by Platinum in a Fluidized Bed of Hot Sand.Proc. Combust. Inst.1998, 27, 3111.

(2) Simonov, A. D.; Yazykov, N. A.; Vedyakin, P. I.; Lavrov, G. A.;Parmon, V. N. Industrial Experience of Heat Supply by Catalytic Instal-lations.Catal. Today2000, 60, 139.

(3) Mulder, A.; der Kinderen, J.; Riphagen, G. J. Catalytic Combus-tion: from Catalyst to Reactor.Proc. World Gas Conf.1997, 20th, 485.

(4) Marshall, K. J.; Mleczko, L. Catalytic Combustion of Hydrocarbonsin an Internally Circulating Fluidized-Bed Reactor.Proc. Int. Conf. Circ.Fluid. Beds1999, 6th, 539.

(5) Foka, M.; Chaouki, J.; Guy, C.; Klvana, D. Natural Gas Combustionin a Catalytic Turbulent Fluidized Bed.Chem. Eng. Sci.1994, 49, 4269.

(6) Iamarino, M.; Chirone, R.; Lisi, L.; Pirone, R.; Salatino, P.; Russo,G. Cu/γ-Al2O3 Catalyst for the Combustion of Methane in a Fluidized BedReactor.Catal. Today2002, 75, 317.

(7) Iamarino, M.; Chirone, R.; Pirone, R.; Salatino, P.; Russo, G.Catalytic Combustion of Methane and Propane in a Fluidized Bed Reactor.Proc. Combust. Inst.2002, 29, 827.

(8) Iamarino, M.; Chirone, R.; Pirone, R.; Salatino, P.; Russo, G.Catalytic Combustion of Methane in a Fluidized Bed Reactor under Fuel-Lean Conditions.Combust. Sci. Technol.2002, 174, 361.

(9) Chyang, C. S.; Chang, C. H.; Chang, J. H. Gas Discharge Modes ata Single Horizontal Nozzle in a Two-Dimensional Fluidized Bed.PowderTechnol.1997, 90, 71.

(10) Geldart, D. Types of Gas Fluidization.Powder Technol.1973, 7,285.

(11) Sotudeh-Gharebaagh, R.; Chaouki, J.; Legros, R. Natural gascombustion in a turbulent fluidized bed of inert particles.Chem. Eng. Sci.1999, 54, 2029.

(12) Zukowski, W. The Role of Two-Stage Combustion in the Develop-ment of Oscillations during Fluidized Bed Combustion of Gases.Fuel2000,79, 1757.

(13) Kunii, D.; Levenspiel, O. Fluidized Bed Reactor Models. 1. ForBubbling Beds of Fine, Intermediate and Large Particles. 2. For the LeanPhase: Freeboard and Fast Fluidization.Ind. Eng. Chem. Res.1990, 29,1226.

ReceiVed for reView September 10, 2005ReVised manuscript receiVed November 29, 2005

AcceptedNovember 30, 2005

IE051015E

Ind. Eng. Chem. Res., Vol. 45, No. 3, 20061013