Investigation of Reforming Catalyst Deactivation by Exposure to FlyAsh from Biomass Gasification in Laboratory Scale

Jessica Einvall,† Simone Albertazzi,‡ Christian Hulteberg,§ Azhar Malik,† Francesco Basile,‡

Ann-Charlotte Larsson,† Jan Brandin,§ and Mehri Sanati*,†,|

School of Technology and Design, Va¨xjo UniVersity, 351 95 Va¨xjo, Sweden, Dipartimento di ChimicaIndustriale e dei Materiali, Bologna UniVersity, 40136 Bologna, Italy, Catator AB, Ideon,

223 70 Lund, Sweden, and DiVision of Ergonomics and Aerosol Technology, Lund UniVersity,Box 118, SE-22100 Lund, Sweden

ReceiVed December 12, 2006. ReVised Manuscript ReceiVed May 11, 2007

Production of synthesis gas by catalytic reforming of product gas from biomass gasification can lead tocatalyst deactivation by the exposure to ash compounds present in the flue gas. The impact of fly ash frombiomass gasification on reforming catalysts was studied at the laboratory scale. The investigated catalyst wasPt/Rh based, and it was exposed to generated K2SO4 aerosol particles and to aerosol particles produced fromthe water-soluble part of biomass fly ash, originating from a commercial biomass combustion plant. The noblemetal catalyst was also compared with a commercial Ni-based catalyst, exposed to aerosol particles of thesame fashion. To investigate the deactivation by aerosol particles, a flow containing submicrometer-size selectedaerosol particles was led through the catalyst bed. The particle size of the poison was measured prior to thecatalytic reactor system. Fresh and aerosol particle exposed catalysts were characterized using BET surfacearea, XRPD (X-ray powder diffraction), and H2 chemisorption. The Pt/Rh catalyst was also investigated foractivity in the steam methane reforming reaction. It was found that the method to deposit generated aerosolparticles on reforming catalysts could be a useful procedure to investigate the impact of different compoundspossibly present in the product gas from the gasifier, acting as potential catalyst poisons. The catalyticdeactivation procedure by exposure to aerosol particles is somehow similar to what happens in a real plant,when a catalyst bed is located subsequent to a biomass gasifier or a combustion boiler. Using differentenvironments (oxidizing, reducing, steam present, etc.) in the aerosol generation adds further flexibility to thesuggested aerosol deactivation method. It is essential to investigate the deactivating effect at the laboratoryscale before a full-scale plant is taken into operation to avoid operational problems.

1. Introduction

The objective of having a 5.75% share of biofuels in thetransportation sector1 has led to a series of interesting projectinitiations in the field of renewable energy; one of them isCHRISGAS (Clean Hydrogen-Rich Synthesis Gas, IP-projectwithin FP6). The project focuses on modifying an existingintegrated gasification combined-cycle (IGCC) pilot plantsituated in Va¨rnamo, Sweden, in order to produce synthesis gasby catalytic upgrading of product gas from the biomass gasifier.A detailed description of the process, with emphasis on thedifficulties identified within the proposed catalytic treatment,has already been published.2 The produced synthesis gas is tobe used in the production of, e.g., Fischer-Tropsch diesel,dimethyl ether, methanol, or hydrogen.3

The plant modification of the Va¨rnamo gasifier includes areforming and water gas shift system. The reforming and watergas shift reactors are however preceded by a hot-gas filter forremoval of ashes. This filtration procedure is however notsufficient to totally eliminate impurities present in the flue gas,4

and as the impurities are potential poisons for most reformingand shift catalysts, they can cause deactivation of the catalyst.5

Inorganic particles can also be formed during cooling of theflue gas via condensation, via nucleation,6 and by chemicalsurface reactions. The steam reforming catalyst in biomassgasification systems will be affected by gaseous sulfur com-pounds as well as by the emitted inorganic submicrometercompounds present in the flue gas originating from the biomassfuel. The latter kind of compounds have shown a deactivatingeffect on selective catalytic reduction (SCR) catalysts forabatement of NOx in the flue gas from biomass combustionboilers.7* To whom correspondence should be addressed: Phone:+46-470-

708943. Fax:+46-470-708756. E-mail: mehri.sanati@vxu.se.† Vaxjo University.‡ Bologna University.§ Catator AB.| Lund University.(1) Directive 2003/30/EC of the European parliament and of the council,

May 8, 2003.(2) Albertazzi, S.; Basile, F.; Brandin, J.; Einvall, J.; Hulteberg, C.;

Fornasari, G.; Rosetti, V.; Sanati, M.; Trifiro`, F.; Vaccari, A.Catal. Today2005, 106, 297-300.

(3) Tijmensen, M. J. A.; Faaij, A. P. C.; Hamelinck, C. N.; van Hardeveld,M. R. M. Biomass Bioenergy2002, 23, 129-152.

(4) Strand, M.; Pagels, J.; Szpila, A.; Gudmundsson, A.; Swietlicki, E.;Bohgard, M.; Sanati, M.Energy Fuels2002, 16, 1499-1506.

(5) Forzatti, P.; Lietti, L.Catal. Today1999, 52, 165-181.(6) Hinds, W. C. Aerosol Technology. Properties, BehaVior, and

Measurement of Airborne Particles; John Wiley & Sons, Inc.: New York,1999; pp 283-292.

(7) Moradi, F.; Brandin, J.; Sohrabi, M.; Faghihi, M.; Sanati, M.Appl.Catal., B: EnViron. 2003, 46, 65-76.

(8) Rostrup-Nielsen, J. R.Steam reforming catalysts; Teknisk ForlagA/S: Copenhagen, 1975; pp 35, 45-46, 63-76.

2481Energy & Fuels2007,21, 2481-2488

10.1021/ef060633k CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 08/22/2007

An early study on reforming catalysts has shown that Nicatalysts are sensitive to sulfur,8 and that sulfur in the flue gascan poison the catalyst, leading to decreased catalyst lifetime.

H2S in the gas phase will react with Ni in the reformingcatalyst, and NiSO4 and NiS will be formed. Rostrup-Nielsenreported that H2S is dissociatively adsorbed on the surface andfound a correlation between H2 and S adsorption on Ni whereH and S competitive adsorb on the same sites.8 If the hydrogencontent is low in the gas, hydrogen desorbs into the gas phase,leaving NiS as product. The formation of H2S can be favoredif the hydrogen concentration is high, and sulfur will thenreadsorb on the nearest unoccupied nickel surface. Rostrup-Nielsen also reported the difficulty of regenerating sulfurdeactivated Ni catalyst in the presence of alkali.8 If a sulfur-containing catalyst is oxidized, sulfur dioxide is formed, whichreacts with alkali and forms sulfates. In reducing conditionssulfur will be reduced into H2S and readsorbed on the catalyst.The regeneration procedure for the Ni catalyst will thus be veryslow.

The sulfur effect on noble metal reforming catalysts is slightlydifferent compared to the effect on nickel-based catalysts, andit has been reported that Ni catalysts are more sulfur-sensitivethan Pt catalysts.9 Noble metal catalysts do not form nonregen-erable bulk sulfur; they are however affected by the presenceof sulfur, but the effect is reversible.9 The hydrocarbon speciesto be reacted in the reformer are of two general categories:short-chain alkanes and alkenes (C1-C4) and tar-type com-pounds (aromatic compounds).2 Rioche et al.10 reported thatsupported Rh and Pt catalysts were more active for steamreforming of acetic acid, phenol, acetone, and ethanol than other

catalysts tested in this study. This finding has further beenconfirmed by Kolb et al.11 in investigating Pt/Rh catalysts forthe steam reforming of propane. Rh has shown exceptionalability of aromatic degradation and is therefore a suitable choiceas an active phase for this type of gas composition.12,13 Thechoice of a Pt/Rh catalyst seems to be able to handle both typesof hydrocarbons that need to be reacted in this type of reformingsystem.

Earlier studies of particles emitted from biomass combustionin small grate type boilers4,14 showed a fine particle mode ofaround 100 nm and the main elements in the fine mode to beK, S, and Cl with the main components being K2SO4 and KCl.

In the present work, the effect of generated submicrometeraerosol particles of K2SO4 salts and salt derived from fly ashon reforming catalysts was investigated. In the latter case, thefly ash from a commercial biomass combustion plant wasdissolved in boiling water, and the catalyst was exposed to thegenerated particles from the dissolved fly ash solution. This flyash sample contains a realistic mixture of compounds, whichthe reforming catalyst is assumed to be, at least to some extent,exposed to in a commercial plant. A new approach fordeactivating reforming catalysts in the laboratory scale byaerosol particle deposition is proposed in order to simulate thedecay processes, prior to initial operation in full scale. Theproposed catalyst deactivation experiment by deposition ofaerosol particles at the laboratory scale has previously beenverified for deactivation of commercial SCR catalysts in earlier

(9) Petersen, E. E.; Bell, T. A.Catalyst deactiVation; Marcel Dekker:New York, 1987; pp 83, 151-152.

(10) Rioche, C.; Kulkarni, S.; Meunier, F. C.; Breen, J. P.; Burch, R.Appl. Catal., B: EnViron. 2005, 61, 130-139.

(11) Kolb, G.; Zapf, R.; Hessel, V.; Lo¨we, H. Appl. Catal., A: Gen.2004, 277, 155-166.

(12) Duprez, D.Appl. Catal., A: Gen.1992, 82, 111-157.(13) Duprez, D.; Miloudi, A.; Delahay, G.; Maurel, R.J. Catal.1986,

101, 56-66.(14) Pagels, J.; Strand, M.; Rissler, J.; Szpila, A.; Gudmundsson, A.;

Bohgard, M.; Lillieblad, L.; Sanati, M.; Swietlicki, E.J. Aerosol Sci.2003,34, 1043-1059.

Figure 1. Experimental setup for catalyst exposure to the aerosol particles at 800°C.

Figure 2. Experimental setup for aerosol particle characterization by the SMPS at 150°C.

2482 Energy & Fuels, Vol. 21, No. 5, 2007 EinVall et al.

work.15 A bimetallic catalyst of Pt/Rh has been evaluated as itcan provide qualities suitable for steam reforming, and acommercial Ni catalyst was used as comparison. The fresh andaerosol particle exposed catalyst samples were characterizedusing different techniques, such as BET surface area, XRPD(X-ray powder diffraction), and H2 chemisorption, in order tocomprehend the induced catalytic deactivation in biomassgasification system. The effect of the deactivation was alsoinvestigated by measurement of the catalyst activity in the steammethane reforming reaction of the Pt/Rh catalyst, before andafter K2SO4 aerosol particle exposure.

2. Experimental Section

2.1. Catalyst.A catalyst with a Pt/Rh active phase dispersed ona heat-treated Al/Mg spinel was prepared. A slurry comprised ofthe sintered material, a binder (Boehmit), and water was dried at150°C for 24 h. The dried mixture was calcined for 2 h at 800°C,and the residue oxide was crushed. The crushed catalyst wasfractioned, and the fraction of about 0.1 mm was used for catalystpreparation. The chosen fraction was impregnated with a Pt(NO3)2

solution, dried at 150°C, and calcined at 800°C for 2 h. Theplatinated catalyst was impregnated by RhCl3 and again calcinedas previously. The resulting catalyst contained 4.5% noble metalby weight, and had an atomic ratio Pt/Rh equal to 4. The preparedcatalyst was compared to a commercial, Ni-MgAl2O4, steamreforming catalyst, having a nominal Ni content of 13 wt %.

(15) Larsson A.-C.; Einvall, J.; Andersson, A.; Sanati, M.Energy Fuels2006, 20, 1398-1405.

Figure 3. Schematic view of experimental setup for activity measurement.

Figure 4. Number size distribution of generated submicron aerosolparticles from K2SO4 and salt leached from fly ash measured by theSMPS.

Figure 5. Mass size distribution of generated submicron aerosolparticles from K2SO4 and salt leached from fly ash calculated fromSMPS measurement using the salt bulk density of potassium sulfate.

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2.2. DeactiVation Species.As K2SO4 is one of the most abundantcompounds present in biomass combustion gases, this compoundwas used to study the effect of emitted biomass associatedsubmicrometer aerosol particles on reforming catalysts. To inves-tigate the influence of other impurities present in fly ash, anothercase was investigated. The fly ash from a commercial biomasscombustion plant was dissolved in boiling water, and the catalystwas exposed to the generated aerosol particles. This fly ash samplecontains a realistic mixture of compounds, which the reformingcatalyst might be exposed to in a commercial full-scale plant. Thecollected fly ash from a commercial boiler (circulating fluidizedbed, 104 MW) is assumed to contain the most abundant elementsemitted from biomass burning. The ash salt solution was prepared

by boiling fly ash with deionized water, which was then filtered.The main compounds, oxygen aside, in the ash salt were potassium(38%), sulfur (16%), sodium (5.6%), and calcium (1.9%) analyzedusing an inductively coupled plasma atomic emission spectrometer(ICP-AES) and high-resolution inductively coupled plasma massspectrometry (HR-ICP-MS).

One of the major components in the ash salt is K2SO4. The sulfateis stable in an oxidizig environment, i.e., combustion with sulfurpresent in the gas phase, and it has a relatively high melting point(1067°C). The stability of K2SO4 was investigated by thermody-namic equilibrium calculations with HSC 5 (manufactured byOutokumpu Research Oy) in N2/H2O (aerosol particle experiment)and in reducing (synthesis gas) environments, respectively.

Figure 6. Calculation of the stability of potassium at aerosol particle generation. (A) Solid phase in N2/H2O environment; (B) potassium in gasphase in N2/H2O environment.

Table 1. Gas Flow during Activity Measurement Experiment

GHSV 50 000 h-1 50 000 h-1 100 000 h-1 100 000 h-1

S:C ratio 1.7 2.5 1.7 2.5

CH4 92.6 Ndm3/h 74.6 Ndm3/h 185.2 Ndm3/h 149.2 Ndm3/hH2O 157.4 Ndm3/h 178.6 Ndm3/h 314.8 Ndm3/h 357.1 Ndm3/hN2 250 Ndm3/h 250 Ndm3/h 500 Ndm3/h 500 Ndm3/htotal 500 Ndm3/h 500 Ndm3/h 1000 Ndm3/h 1000 Ndm3/h

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2.3. DeactiVation by Exposure of the Reforming Catalysts toGenerated Potassium and Ash Salt Particles in Laboratory-ScaleExperiments.The generation and deposition of aerosol particles wascarried out in a setup shown in Figure 1.

The aerosol particles were generated by a pneumatic atomizer(Palas GmbH, AGK-2000). The atomizer was connected to pres-surized nitrogen with a gauge pressure of 1 bar, and the flow ratefrom the atomizer was 4 Ndm3/min. The atomizer reservoircontained the salt solution. The concentration of the K2SO4 solutionused for aerosol generation was 50 g of salt/dm3 of deionized water,and the mass of salt in the ash solution was 16.5 g/dm3 (originatingfrom 665 g of fly ash). The flow of nitrogen and generated aerosolparticles passed through an inertial impactor separating large liquiddroplets before entering an electrically resistance heated oven. Acatalyst bed consisting of 5.5 g of approximately 0.1 mm diametercatalyst pellets was placed in the center of a tubular reactor, whichwas heated to 800°C at atmospheric pressure. A continuous flowof 0.5 Ndm3/min containing the generated aerosol particles wasforced through the catalyst bed for 2 h. The relative humidity atdeposition was less than 1% measured with a TH-Calc 8720 (TSIInc.).

The generated aerosol particles were physically characterizedaccording to their electrical mobility using a scanning mobilityparticle sizer (SMPS; TSI Inc.) consisting of a differential mobilityanalyzer (DMA; TSI Inc. Model 3081) and a condensation particlecounter (CPC; TSI Inc. Model 3010), shown in Figure 2.

The SMPS measures the particle number size distribution, andfrom the bulk density, the mass size distribution can be calculated.The number size distribution for a size range of 20-478 nm of theaerosol particle flow was measured, and the flow led through the

instrument was 0.3 Ndm3/min. Particle characterization was carriedout at 150°C in order to avoid losses of particles during sampling.Particle sampling was carried out isokinetically using a glass-sampling probe. To decrease the temperature, the particle concen-tration, and the relative humidity prior to analysis, the sample wasdiluted with particle-free, dry pressurized air (20°C) to give a ratioof 1/27. The relative humidity after dilution was measured with aTH-Calc 8720 (TSI Inc.) to 4%.

2.4. Chemical Characterization of Fresh and Aerosol ParticleExposed Catalyst Samples.A Perkin-Elmer 4100 atomic absorptionspectrometer (AA) was used to determine the potassium contentof the catalyst. For this purpose, the catalyst was partially dissolvedin boiling HNO3 and water, and the sample was diluted with waterbefore the measurement.

Adsorption of N2 at the temperature of liquid nitrogen (the BETmethod) was measured on a Sorpty 1750 instrument, to give thesurface area of fresh and particle exposed catalyst samples. A freshcatalyst was also exposed to the steam and nitrogen without thepresence of poison, in order to investigate the stability of thecatalyst. In the remainder of this paper this catalyst is designated“reference”.

The metallic surface area, dispersion, and metal particle size weremeasured by H2 chemisorption (Micrometrics, ASAP 2020). Beforeadsorption of H2 the Ni catalyst was preheated at 750°C in Heand then in H2. The Pt/Rh catalyst was preheated at 350°C in Heand then in H2. The measurement of H2 adsorption for both catalystswas made at 35°C.

The metal crystallite size was determined by XRPD by a PhilipsAnalytical X-Pert Instrument with Cu KR radiation at 35 kV and35 mA using the peak broadening in conjunction with the Debye-Scherrer equation.

2.5. ActiVity Measurements of the Pt/Rh Catalyst.The activitywas measured for fresh and K2SO4 exposed Pt/Rh catalyst in thesteam reforming reactions of methane, eqs 1 and 2.

The main parts of the catalytic system consist of an oven, massflow controllers, and gas mixing equipment (Figure 3).

All flows are presented at normal conditions: 0°C and 1.01×105 Pa. The reaction gases are mixed and preheated in a vesseland then led into a tubular reactor, where the reaction takes place.The gas composition at different gas hourly space velocities(GHSVs) is reported in Table 1.

Ten milliliters of catalyst pellets was used in the experiments,and the temperature was varied between 600 and 800°C. Theconversion has been calculated asX ) (CO + CO2)out/CH4,in.

Figure 7. Possible mechanism for clogging of a porous catalyst surfaceat K2SO4 aerosol particle exposure.

Table 2. Characteristics of Generated K2SO4 and Ash-Salt AerosolParticles

K2SO4 ash salt

bulk densitya (g/cm3) 2.66 2.66concn in reactor (mg/m3N) 37 18mean particle mass size (nm) 231 187concn in reactor (number/cm3N) 7.4× 106 6.5× 106

mean particle number size (nm) 119 98

a The bulk density of potassium sulfate was used in the mass concentra-tion calculation.

Table 3. BET Surface Areas of Fresh and Aerosol Particle ExposedSamplesa

catalyst samples surf. area (m2/g)

Ni catalystfresh 28reference 10K2SO4 exposed 11ash salt exposed 10

Pt/Rh catalystfresh 16reference 12K2SO4 exposed 12ash salt exposed 18

a The reference samples have been exposed to the same conditions asthe aerosol particle exposed samples but with no salt present.

Table 4. Deposited Amount of Potassium after Aerosol ParticleExposure Analyzed by AA

catalyst samples K concn (wt %)

Ni catalystK2SO4 exposed 0.0088ash salt exposed 0.0023

Pt/Rh catalystK2SO4 exposed 0.0036ash salt exposed not detected

Table 5. H2 Chemisorption of Fresh and Particle Exposed Catalysts

catalyst samples dispersion (%) metallic surf. area (m2/gsample)

Ni catalystfresh 5.1 3.07K2SO4 exposed 2.4 1.44ash salt exposed 3.0 1.81

Pt/Rh catalystfresh 3.6 0.48K2SO4 exposed 0.5 0.06ash salt exposed 0.5 0.07

CH4 + H2O T CO + 3H2 (1)

CH4 + 2H2O T CO2 + 4H2 (2)

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3. Results and Discussion

3.1. Salt Exposed Samples at Laboratory Scale.The numberconcentration of the generated particles was measured by theSMPS, and the mass concentration was calculated. The numberand mass size distributions are presented in Figures 4 and 5.All obtained results are presented at normal conditions: 0°Cand 1.01× 105 Pa. The mean number particle size was 0.1µmand the calculated mass concentration was 37 mg/m3 for theK2SO4 aerosol particles. The mass concentration of the aerosolparticles generated from the ash salt was lower due to the lowersalt concentration in the solution. The mean number particlesize was also 0.1µm, and the calculated mass concentrationwas approximately 18 mg/m3. The bulk density for the ash salt

was estimated to be 2.66 g/cm3 (the bulk density of K2SO4) inthe calculation of mass concentration. The results are presentedin Table 2.

In the thermodynamic calculations an amount of 1 kmol ofK2SO4 was calculated to be in contact with a total gas flow of1 × 106 kmol. The sulfur content (SO2 respectively H2S) in thegas phase in the reducing environment was calculated to 100ppm, while in the N2/H2O calculation no sulfur in the gas phasewas present. In the N2/H2O environment, potassium sulfate isthe only stable solid phase up to 750°C, and above thistemperature the solid K2SO4 is in equilibrium with a melt KOHphase and gas phase containing SO2 and KOH(g). The resultsare presented in Figure 6.

In the reducing environment, as during biomass gasification,the calculations were made on synthesis gas: H2O (33 vol %),CO2 (20 vol %), CO (24 vol %), and H2 (23 vol %). There isno solid phase present, only a melt KOH phase which is inequilibrium with H2S and KOH(g) in the gas phase.

From the equilibrium calculations it can be concluded that,in the model deactivation experiments with K2SO4 aerosolparticles in N2/H2O atmosphere, potassium sulfate will bepresent and deposited on the catalyst mainly as a solid phase,which differs from the synthesis gas environment where theequilibrium calculation reveals a melted phase KOH(l), or atleast a state where the aerosol particles move toward formationof a melted KOH(l) phase. A proposed mechanism is thatsolid K2SO4 in equilibrium with liquid KOH deposits on thecatalyst surface, while KOH(g) and H2S(g) are released; seeFigure 7.

Figure 8. XRPD of fresh Pt/Rh catalysts and catalyst exposed to K2SO4 and ash-salt aerosol particles.

Table 6. Average Metal Crystallite Size of Fresh and ExposedCatalyst Samplesa

catalyst samples

relative metalcrystallite size(exposed/fresh)

metalcrystallitesize (nm)

Ni catalystfresh 1K2SO4 exposed 1.05ash salt exposed 1.00

Pt/Rh catalystfresh 61K2SO4 exposed 61ash salt exposed 64

a The results for the Ni catalyst are reported as relative values comparedto the fresh sample.

2486 Energy & Fuels, Vol. 21, No. 5, 2007 EinVall et al.

The effect of aerosol particles on reforming catalysts is ofinterest, and thus K2SO4 serves as an aerosol particle modelcompound in the total evaluation of the impact of aerosolparticles on deactivation of reforming catalysts.

3.2. BET Measurement.The surface area measured by BETfor the fresh, aerosol particle exposed, and reference catalystsamples are shown in Table 3. The BET surface area decreasedfor both the Pt/Rh catalyst and the Ni catalyst because of theconditions during exposure. The decrease in surface area was64% for the Ni catalyst and 25% for the Pt/Rh catalyst. ThePt/Rh is less affected by the environment probably because ofthe calcination procedure when manufactured.

For both catalysts the surface area after K2SO4 aerosol particleexposure decreases to the same magnitude as the referencesample. For the Pt/Rh catalyst, the surface area was slightly

increased after ash salt particle exposure, which can be due toformation of needle-shaped crystals. On the other hand, nopotassium was detected by AA for the Pt/Rh catalyst exposedto ash salt. The increase in surface area may be contributedfrom other compounds present in the ash salt. It has previouslybeen reported that the surface area can increase after aerosolparticle exposure, at least for solid particles.7

For the Ni catalyst the surface area decreased to a magnitudesimilar to the reference sample after exposure to ash salt. Adecrease in surface area after steam and hydrogen exposure hasbeen reported for a MgAl2O4-based Ni catalyst due to sintering,8

but in the current measurement, no sintering of the Ni or Ptcrystallites was observed due to the aerosol particle exposureaccording to the XRPD measurement.

Figure 9. Conversion as a function of S:C ratio, 1.7 ([) and 2.5 (9), for fresh Pt/Rh catalyst at GHSV 50 000.

Figure 10. Conversion of fresh (9) and K2SO4 ([) particle exposed Pt/Rh catalyst at GHSV 100 000, S:C) 1.7.

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3.3. Chemical Characterization of the Samples.The bulkconcentration of potassium measured by AA is presented inTable 4. The concentration of accumulated potassium in theexposed Ni catalysts was 0.0088 wt % after K2SO4 aerosolparticle exposure and 0.0023 wt % after ash-salt aerosol particleexposure. The accumulation of potassium in the Pt/Rh catalystwas 0.0036 wt % potassium after K2SO4 aerosol particleexposure, while in the case of ash-salt aerosol particle exposure,no potassium deposition was detected. The ratio of the depositedpotassium concentration to the active metal concentration is alittle higher for the Ni catalyst (7× 10-4) compared to Pt/Rhcatalyst (5× 10-4). The measured deposition of potassium inthe catalysts exposed to K2SO4 aerosol particles compared tothe amount forced through the catalyst bed is 45% and 18% inthe Ni and Pt/Rh catalyst, respectively. The deposited amountof potassium from the ash-salt aerosol particles is 30% in theNi catalyst, while no deposition of potassium in the Pt/Rhcatalyst was detected. The lower mass deposited of the ash saltin both catalysts may be attributed to the lower mass concentra-tion of the salt solution used for aerosol particle generation.

The reason for the lower deposition in the Pt/Rh catalyst couldbe a slightly larger size of the Pt/Rh catalyst particles. Thedeposition of aerosol particles is affected by the catalyst particlesize, and the difference in deposition could also be attributedto different catalyst particle shapes, particle densities, and surfaceareas.

The mass deposition is lower than compared to previouslyreported aerosol deposition experiments by an electrostatic fieldon the SCR catalyst,15 which were in compliance with a full-scale exposure. No data for a full-scale steam reforming plantto upgrade gas after biomass gasification are available, but thelow weight percent of deposited material indicates that a longerdeposition time is required in order to achieve results close toconditions in a full-scale reforming plant.

3.4. H2 Chemisorption.The metallic surface area and disper-sion were measured with H2 chemisorption for fresh and aerosolparticle exposed catalysts. The results from the H2 chemisorptionare presented in Table 5. The metallic surface area measuredby H2 chemisorption for both the Ni and Pt/Rh catalysts exposedto K2SO4 and fly ash salts decreased compared to fresh catalyst.The decrease can be attributed to poisoning by particle coverageof the catalyst surface. Rostrup-Nielsen has previously reporteda decrease in Ni surface area measured by H2S chemisorptionafter steam and hydrogen streaming of a MgAl2O4-based Nicatalyst,8 while Pt and Rh catalysts are stable in inert andreducing environments.9

3.5. XRPD of the Catalyst Samples.The XRPD spectra ofthe Pt/Rh catalysts are presented in Figure 8, the Ni catalyst(commercial) is not presented due to the secrecy agreement withthe catalyst supplier. The average metal crystallite size calculatedby the Scherrer equation from the XRPD patterns is presentedin Table 6. Only relative values are shown for the Ni catalyst.

Neither the Pt/Rh nor the Ni catalyst crystallite size wasaffected by the aerosol particle exposure as measured usingXRPD. This proves that the reduction of metallic surface areain Table 5 is due to poisoning by pore blockage by the aerosolparticles and not due to active phase crystallite sintering duringthe experiment.

3.6 ActiVity Measurements of the Pt/Rh Catalyst.The activitymeasurement of the fresh Pt/Rh at different steam to carbon(S:C) ratios indicated an increased conversion at increased S:Cratio. The results at different S:C ratios are presented in Figure9. The deactivation with aerosol particles of K2SO4 is demon-strated in Figure 10. The results show that exposing the catalystto the aerosol particles decreases the activity of the catalyst.The conversion of the catalyst decreased from around 8% to5% at 700°C after particle exposure, thus showing that theparticle exposure had a negative effect on the catalyst activity.The decrease in metallic surface area measured by H2 chemi-sorption after K2SO4 aerosol particle exposure is 88%, whilethe decrease in activity is about 50%. This might be explainedby an oversizing of the initial noble metal concentration andthus the initial decrease in metal surface area is not proportionalto the loss of activity.

4. Conclusion

Catalyst deactivation after exposure to aerosol particles onreforming catalysts has been investigated. The AA analysisshowed potassium accumulation caused by particle depositionof 0.0088 wt % in the Ni catalyst after exposure to generatedK2SO4 aerosol particles, thus verifying the deposition of aerosolparticles. The deposition of potassium on the Ni catalyst is 45%of the exposed amount, which confirms that the method isapplicable for aerosol particle deposition in a catalyst bed.

The conversion of methane in the steam reforming reactiondecreased for the Pt/Rh catalyst exposed to K2SO4 aerosolparticles compared to fresh catalyst, showing that aerosol particleexposure had a negative effect on the catalyst activity. Thisdecrease is probably due to coverage of the catalyst active sitesby the deposited aerosol particles. This is further emphasizedby the decrease in metal surface area measured by H2 chemi-sorption after the aerosol particle deposition despite the factthat the active phase crystallite size is unaltered as measuredby XRPD.

The deactivation method can be applied to further evaluatethe impact of inorganic salts and carbon compounds expectedto be present in the product gas fed to a catalytic steam reformerunit following a biomass gasifier, as well as for the investigationof other parameters that influence process conditions. Furtherinvestigations can lead to a better understanding of the mech-anism of deactivation related to steam reforming catalysts whenexposed to a combination of soot and ash compounds frombiomass gasification. This developed decay study of catalyticprocesses has been shown, in conformity with previous stud-ies,7,15,16to assist in predicting the influence of emitted aerosolparticles on catalytic deactivation in a full-scale experiment inthermochemical conversion of biomass.

Acknowledgment. Financial support through the EC 6thFramework Program (CHRISGAS Project Contract No. SES6-CT2004-502587), the Swedish Energy Agency, and the SwedishResearch Council is gratefully acknowledged.

EF060633K

(16) Larsson, A.-C.; Einvall, J.; Sanati, M.Aerosol Sci. Technol.2007,41, 369-379.

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