Effect of Alkali Vapor Exposure on Ni-MgO/γ-Al2O3/Cordierite MonolithicCatalyst for Biomass Fuel Gas Reforming

Y. P. Li, T. J. Wang, C. Z. Wu,* Y. Gao, X. H. Zhang, C. G. Wang, M. Y. Ding, and L. L. Ma

Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou Institute of Energy ConVersion, ChineseAcademy of Sciences, Guangzhou, Guangdong 510640, P.R .China

Fly ash compounds, such as alkali salts, in the raw biomass fuel gas can contaminate and deposit ontraditional granular Ni-based catalysts, which resulted in catalyst deactivation and pressure increase ofthe downstream reformer. The impact of alkali salt exposure (KCl, K2SO4, K2CO3, by evaporation atabout 7.8 mg/L for 6 h) on dry CH4/CO2 reforming of model biomass fuel gas (H2/CO/C2H4/CH4/CO2/N2 ) 15.8/12.1/2.51/ 15.0/22.1/32.6 vol %) over Ni-MgO/γ-Al2O3/cordierite monolithic catalyst (MC)was investigated and studied. The results showed that CH4 and CO2 conversions and CO and H2 yieldsincreased at 700-850 °C for undeposited and deposited MC. Compared with undeposited MC, thedeposited catalysts show lower CH4 conversion but higher CO2 conversion and CO yield at 750-830°C. The stability tests also show that CH4 conversion and H2 content in the tail gas decreased dramaticallyfrom 87.2% to 32.0% and from 35.1% to 26.7%, respectively, after 17 h time on stream (TOS) for thedeposited MC, while CH4 conversion kept steady of above 90% after 60 h TOS for undeposited MC at750 °C. Characterization by N2-physisorption, XRD, ICP-AES, SEM-EDS, and XPS of MC indicatethat alkali salt aerosol covering the catalyst surface or blocking mesopore channels was the main reasonfor the decreased reforming performance and MC deactivation, which occurred mainly at the top part ofmonolithic catalyst (K ) 1.39 wt % by EDS), vicinal to the alkali source. The reforming of real biomassfuel gas (H2/CO/C2H4/CH4/CO2/N2 ) 10.2/16.8/0.5/6.4/15.2/51.0 vol %) from air gasification of pinesawdust in the pilot plant (200-250 kg/h) by the reformer packed with MCP, larger in size than MC,exhibits pressure drop of less than 700 Pa, CH4 conversion of about 84%, and tar content from 4.8-5.3g/m3 to 0.12-0.14 g/m3 during 60 h TOS at 600 °C. The porosity structure of MCP catalytic bed andrelatively low alkali (K, Na ) 0.03-0.07 wt %) deposition by fly ash from real biomass fuel gas werethe main reasons for the excellent reformer performance.

1. Introduction

As one of the most promising technology to substitute fossilfuels, second-generation biofuel production has been studiedwidely,1 such as the Hynol project in the United States, Bio-Fuels projects in Sweden, Sun-diesel in Choren Company,Germany.2 Yet thermochemical gasification of biomass isinevitably accompanied by the formation of tar and inorganicimpurities(Si, Ca,K, Na, S, Cl, Mg, Zn, Fe, Mn) and the obtainedraw fuel gas is usually CO2-rich with low H2/CO ratio.3 To fulfillthe stoichiometric requirement for syngas conversion to liquidfuels, reforming of CH4 and tar to increase H2/CO ratio hasbeen carried out extensively in the past decades.4 Although Ni-based catalysts showed high efficiency in reforming, theirdeactivation was obvious due to carbon deposition during shortoperation period.5,6 And the deterioration of inorganic impuritieslike alkali compounds in fly ash is another adverse factor onreforming, which has not received considerable attention untilrecently.7,8 During biomass gasification, some alkali and alkaliearth compounds, abundant in the feedstocks, are apt to formsulphates and chlorides and cause heterogeneous nucleation andagglomeration in the gasifier.9,10 And the alkali particles mayexist in the fly ash or form aerosol particles in fuel gas, whichoften foul the downstream gas turbine and even inactivatereforming catalyst ultimately.11,12 Several techniques have beenapplied to reduce the alkali vapor emissions, such as alkaliabsorbent in the gasifier, hot-gas filter and gasification temper-ature decrease, et al.9,13 These procedures, however, are

insufficient to drastically eliminate alkali content in the raw fuelgas. It was reported that CH4 conversion of Pt-Rh/MgAl(O)catalyst was decreased to below 25% after exposed to thegenerated K2SO4 aerosol particles and to the alkali aerosolparticles from the water-soluble part of biomass fly ash duringsteam reforming in lab-scale.14 Few concrete experiments havebeen focused on other catalysts, especially for monolithiccatalysts,15 which have advantages as high catalytic performanceper unit mass of active metal, high operating stability and lowpressure drop of catalyst bed over fix-bed reactor or slurryreactor.16

Preliminary studies from our group have achieved remarkableCH4 and tar reforming performance from biomass fuel gas overmonolithic Ni-based catalyst.17 In the present work, the effectof alkali salt vapor exposure (KCl, K2SO4, and K2CO3) on Ni-MgO/γ-Al2O3/cordierite monolithic catalyst(MC) for modelbiomass fuel gas reforming, as well as the stability of unde-posited and deposited catalyst were investigated to evaluate theimpact of inorganic salts (expected to be present in the biomassfuel gas) on a catalytic reformer unit. And the typical reformingresults of real biomass fuel gas from fluidized bed gasificationof pine sawdust (200-250 kg/h) in the pilot plant were alsodiscussed.

2. Experimental Section

2.1. Catalyst Preparation. Two types of ceramic cordieritemonolithic substrates (Yixing Nonmetallic Chemical Factory,Jiangsu Province, China) with the density, porosity, BET, celldensity, specification of 1.71 g/cm3, 60%, 10.9 m2/g, 60 cell

* To whom correspondence should be addressed. E-mail: wucz@ms.giec.ac.cn. Tel.: +86-20-87057688. Fax: +86-20-87057737.

Ind. Eng. Chem. Res. 2010, 49, 3176–31833176

10.1021/ie901370w 2010 American Chemical SocietyPublished on Web 02/25/2010

s/cm2, �4.0*5.0 cm and 1.03 g/cm3, 90%, 10.4 m2/g, 1.5cells/cm2, �9.0*10.0 cm were used to prepare catalysts for reformingmodel biomass fuel gas and real biomass fuel gas respectively.At first, the bare monoliths were immersed into 30 wt % oxalicacid solution for 0.5 h, and then dipped into the ethanolsuspension of ultrafine γ-Al2O3 powder under vacuum conditionto increase pore volume and surface area. Then γ-Al2O3-coatedmonoliths were impregnated by 1N aqueous magnesium nitrate(Mg(NO3)2.6H2O) solution, dried overnight at 110 °C andcalcined at 900 °C. After that, MgO-loaded monolith wasimpregnated again by 1 N aqueous nickel nitrate(Ni(NO3)2.6H2O) solution, dried overnight at 110 °C, andcalcined at 850 °C. The two types of well-prepared catalystswere denoted as MC and MCP, respectively.

2.2. Alkali Salt Vapor Deposition on MC and ModelBiomass Fuel Gas Reforming Procedure. The deposition ofalkali salt vapor on MC was carried out in a horizontal reactorwith two thermocouples, one was for alkali salt vaporization at750 °C and the other was for reforming temperature controllingin Figure 1. In each run, MC was reduced in situ in 300 mL/min of 5% H2/N2(vol) flow at 750 °C for 2 h, which was veryuniformly black, and then exposed to the 300 mL/min N2 flowcontaining alkali salt vapor(KCl, K2SO4 and K2CO3), settled ina porcelain boat before MC bed. The alkali content in the carriergas was determined by the weight loss of the boat after 6 hdeposition and the effluent gas was introduced into water toabsorb surplus alkali salts.

The dry CH4/CO2 reforming activity of model biomass fuelgas (H2/CO/C2H4/CH4/CO2/N2) 15.8/12.1/2.51/15.0/22.1/32.6vol %) over undeposited and deposited MC was carried out atatmospheric pressure, 300 mL/min of gas flow rate, and gashour space velocity (GHSV) of 541.8 h-1. The general reactiontime for the reaction was kept constant of 3-5 h to make thereforming results comparable. The tail gas during this periodpassed through water absorber, and then was sampled into gasbag every 15 min for off-line gas chromatogram analysis. Andthe derivations were usually below 5%.

CO, N2, CH4, and CO2 were analyzed by carbon-sieve columnwith TCD detector; gaseous hydrocarbons (CH4, C2H4) wereanalyzed by Porapak Q column with FID detector. N2 in thegas was used as internal standard to calculate CH4 and CO2

conversion. CO2 or CH4 conversion and CO or H2 yield werecalculated as the following equations:

where Xin,p is inlet gas composition (vol%), Xout,p is outlet gascomposition (%), Fin is total inlet gas flow rate(mL/min), Fout

is total outlet gas flow rate (mL/min), X is conversion (%), andY is yield(%).

2.3. Biomass gasification-reforming procedure. As shownin Figure 2, the system was made up by biomass feeder, gasifier,downer reactor, reformer, water bath, gas tank. Pine sawdustof 200-250 kg/h was air-gasified. The raw fuel gas passedthrough a purification unit to separate most fly ash particles andthen through a multitube(49 columns) reformer ((cube-shaped,1 m*1 m*1 m) heated with a burner to supply reforming heatfrom diesel combustion. The 49 tubes (ID: 93 mm, Length:1.2 m) was arranged to be 7 columns and 7 lines in the reformer.Ten pieces of MCP were packed in one tubular tube and eachpiece of MCP was separated by 1 cm of inert Rashig rings.After that, the gas was washed by water to further eliminateparticles and other impurities and pumped by roots blower togas tank for dimethyl ether synthesis.

2.4. Catalyst Characterization. The elemental compositionsof the catalysts were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) on a PEPLASMA-2000 instrument. The textural properties of undeposited anddeposited MC were measured by N2 physisorption at -196 °Con Micromeritics SORBFLOWIII-3210 apparatus and poredistribution was described by BJH model. X-ray diffraction(XRD) patterns were obtained on a BDX3300 using CuKaradiation at 40 kV and 30 mA. Ni Metal crystallite size wasdetermined from the peak at 2θ ) 44.3°, which is assigned tothe diffraction of Ni(1 1 1) plane in XRD patterns [30], usingScherrer equation.18 The morphology of MC was obtained byscanning electron microscopy(SEM) on a JSM-6330F equipmentwith energy diffusive spectroscopy detector (EDS). The el-emental compositions of the catalyst surface were identified byNP-1 X-ray Photoelectron Spectrometer (XPS) equipped withMg-KR radiation. The binding energies were referenced to theC1s band at 284.6 eV.

3. Results and Discussion

3.1. Textural Properties of MC. The textural properties ofcordierite substrate and MC were summarized in Table 1.

It was commonly accepted that alkali metal elements couldbe converted from organic form into inorganic form as sulphatesand chlorides after gasification or combustion and the principalalkali metal was K, followed by Na.19,20 In this paper, KCl wasused as model alkali salt in fly ash deposited on MC. The Kvapor content in the carrier gas was 7.8 mg/L and K content inMC was 0.85 wt % after 6 h exposure by ICP-AES, which washigher than that from fly ash deposition during practical biomassgasification.21 It confirmed that the alkali salt vaporizationoperation was applicable for simulating alkali salt depositionin a catalytic reforming bed. While K content after reformingreaction decreased to 0.57 wt %, which mainly resulted frommetal loss during high temperature reforming reaction.

It is clear that BET and BJH surface area increase remarkablyfrom 10.9 m2/g to 40.0 m2/g and from 7.75-9.52 m2/g to47.0-48.1 m2/g, respectively, after oxalic acid immersion and

Figure 1. Schematic diagram of the experimental apparatus.

XCH4) (FinXin,CH4

- FoutXout,CH4)/(FinXin,CH4

)

XCO2) (FinXin,CO2

- FoutXout,CO2)/(FinXin,CO2

)

YH2) (FoutXout,H2

- FinXin,H2)/(2FinXin,C2H4

+ 2FinXin,CH4)

YCO ) (FoutXout,CO - FinXin,CO)/(2FinXin,C2H4+ FinXin,CO2

+FinXin,CH4

)

Ind. Eng. Chem. Res., Vol. 49, No. 7, 2010 3177

superfine γ-Al2O3 deposition (BET surface area ) 350 m2/g)in Table 1. The pore volume and the average pore diameteralso increased to 0.094 cm3/g and 8.14 nm, respectively, whichattributed to mesopore amount increase by pore expansiontreatment on cordierite substrate. Reduction in H2/N2 flow didnot change textural properties of the catalyst, because relativelylow Ni and Mg impregnation amounts of 5.5 wt % and 3.3 wt% in Table 1. While BET area of KCl deposited MC decreasedevidently to 15.1 m2/g, similarly with 10.9 m2/g of the originalcordierite substrate. Pore volume decreased nearly by half to0.05 cm3/g, but pore diameter kept constant of about 10 nm.11

The decrease of mesopore volume, BET might be caused byblockage of some mesopore channels and coverage of surfacearea by alkali salt aerosol deposition, discussed in section 3.3.

After reforming reaction, BET area and pore volume of KCldeposited MC decrease further to 11.2 m2/g and 0.03 cm3/g,respectively. According to BJH pore distribution results, me-sopore fraction of the used deposited MC decreased withincreased macropore fraction and increased pore diameter of12.2 nm.

3.2. Structure Properties of MC. Figure 3 illustrates theXRD patterns of undeposited, reduced, and KCl deposited MC.

No obvious nickel diffraction peaks were detected forundeposited MC, but it exhibited obvious presence of NiO

(JCPDS 34-0396, 37.1°, 43.3°, 62.8°). According to the litera-ture,22 three major peaks at 44.3°, 51.8°, 76.5° and at 36.8°,44.9°, 65.3° of the reduced MC were identified as Ni0 andNiAl2O4 (or NiMgO2), respectively. Because of the overlappingof main diffraction peaks of NiAl2O4 and NiMgO2 phase, it wasdifficult to identify the two compositions by XRD solely. Theweak diffraction intensity corresponding to Ni0 for the reducedMC and KCl deposited MC indicates small Ni crystallite sizeand high dispersion of Ni particles on the catalyst surface. Itmight also be attributed to the formation of NiAl2O4 (orNiMgO2) spinel and the interaction between active Ni metaland high surface area of γ-Al2O3 support, which is difficult tobe reduced. And the existence of NiO diffraction lines for thereduced MC and KCl deposited MC was the result of thereoxidation of metal Ni under atmosphere condition during XRDanalysis. The similar XRD patterns also indicated that no newphase was formed during KCl deposition on MC or K contentwas too low to be identified by XRD or K existed as a noncrystalform.23

Calculated by Scherrer equation, Ni(111) crystal size forreduced MC and KCl deposited MC were 42.2 and 42.3 nm,respectively, in Table 1, which illustrated that no obvioussintering happened during KCl deposition process. It could alsobe concluded preliminarily that the decreased catalyst surfacearea should be attributed to KCl exposure. Such results are indisagreement with the results from Moradi,24 who found BETarea slightly increased over KCl deposited Pt-based catalyst at300 °C, but are correspondent with the work of Einvall,21 whichalso showed the BET decrease over Ni-based catalyst. Thedifference may be resulted from the various deposition processand distinctly different deactivation mechanism of these catalysts.

3.3. SEM and XPS Analysis. To evaluate the change ofsurface morphology after KCl exposure, MC samples, perpen-dicular to biomass fuel gas flow were obtained from differentpositions of MC bed. The photos of sampling positions wereshown in Figure 4 and the corresponding SEM images wereshown in Figure 5.Figure 3. XRD patterns of MC catalyst samples.

Figure 2. Schematic diagram of biomass gasification-reforming system (1, pine sawdust feeder; 2, fluidized bed gasifier; 3, Downer reactor; 4, powergeneration; 5, low BTU gas for direct combustion; 6, gas purification and upgrading; 7, burner; 8, reformer; 9, chimney; 10, MCP packed in the reformer;11, water bath; 12, roots blower; 13, gas tank; 14, cross-section image of the reformer and analysis positions).

Table 1. Textural Properties of MCa

items cordierite undeposited MC reduced MCKCl

deposited MCKCl deposited

MC after reaction

BET specific surface area (m2/g) 10.9 40.0 39.6 15.1 11.2BJH cumulative surface area (1.7-300 nm) (m2/g) 7.75-9.52 47.0-48.1 49.2-49.3 16.9-17.5 9.77-12.0BJH cumulative volume (1.7-300 nm) (cm3/g) 0.01 0.094 0.09 0.05 0.03average pore diameter (nm) 4.10 8.14 10.11 10.3 12.2K concentration (wt %) 0.85 0.57Ni particle size (nm) 42.2 42.3

a Chemical composition of untreated undeposited MC: Al2O3/MgO/NiO/Cordierite ) 6.6/3.3/5.5/84.6 wt %.

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The surface micrograph of the reduced MC revealed that themonolithic surface was covered by amorphous massive materi-als, mainly NiAl2O4 spinel structures, which are adhered bypolydispersed spherical particles of Ni phase according to theEDS analysis. This is in good accordance to XRD characteriza-tion in Figure 3, where there are existences of NiAl2O4 and Niphase. The particle size measured by SEM dates was larger thanthat by XRD patterns because the particle observed by SEMwas the agglomeration result of numerous supported Ni crystal-line by XRD.

As shown in Figure 5B, some catalyst surface of the top partof KCl deposited MC(position 1 in Figure 4B) is covered byirregularly shaped sheet microstructure with round edges, mainlyK aerosol with the content of 1.39 wt % by EDS. In the sametime, the K/Ni ratio at this position was much higher than thatat the middle part of MC(position 2 in Figure 4B) in Table 2,about 17-fold by EDS. And the SEM image at the middle partof KCl deposited MC exhibits that small spherical particles areattached to the surface of the amorphous surface, similar to thefreshly reduced MC in Figure 5A. It could be concluded thatalkali salt deposition mainly happened on the positions vicinalto the alkali salt source. It thus appears that the majority of thealkali aerosols were retained at the top part of MC. So theapplication of monolithic catalyst would reduce the alkali saltdeposition tendency down the length of catalytic bed.

The K/Ni atom ratio difference by EDS and XPS for the toppart of KCl deposited MC in Table 2 was due to the variationof layer thickness analyzed by EDS and XPS, about 5 µm byEDS and 5 nm by XPS which means that XPS analyses onlyfew layer of the samples. About 52-fold of K/Ni atom ratio byEDS and XPS, which was 5.74 and 0.11 respectively, indicatedthat K not only deposited on the MC surface, but permeated inthe mesopores of MC and decreased active metal surface insection 3.1.

According to the literature,25 binding energy at 294.7 and292.7 eV was assigned to K2p and K2p+, respectively. The XPSresults of binding energy at 293.0 eV for KCl deposited MCmight be Kσ+ with σ varying from 0.5 to 0, which means anindirect electrostatic interaction of K with the active Ni metal.26

For the top part of reduced MC, XPS pattern shows two peaksat ∼855.8 and 862.0 eV, respectively, indicated that Ni was intwo different conditions and corresponded to NiAl2O4 and NiOrespectively with relative intensities of 70.6% and 29.4%. Theseresults also suggest the strong interaction between Ni andγ-Al2O3. And the oxidation state of Ni for the reduced MC byXPS was due to the reoxidation of Ni by O2 in air conditionbefore analysis, similar with the oxide phase of NiO by XRDin Figure 3.

As shown in Figure 5D, the filamentous carbon was foundon the surface of KCl deposited MC after dry CH4/CO2

reforming reaction. Closer inspection from inset of Figure 5Dshows that these filaments were entangled with catalyst particles.EDS spectra of the filamentous domain also shows peakscorresponding to catalytic Ni element, cordierite elements andcarbon. The spent KCl deposited MC was also examined forthe amount of carbon formed by combustion with air flow overthe sample at 800 °C for 2 h in a furnace. The weight changeof 1.94 wt % was primarily caused by carbon deposition onthe sample since the weight change due to Ni phase wasrelatively low.

Nevertheless Ni-based catalysts are more prone to carbondeposition than precious metal catalysts during reformingreaction, particularly due to the formation of filamentous carbon,which was found by many other researchers.5 While the carbondeposition degree in this paper was relatively low, there stillexists small domains of spherical particles of active phase onMC surface. This would be the reason why the catalyst couldmaintain its activity after few hours’ reaction in section 3.5.

3.4. CH4/CO2 Reforming Activities for Model BiomassFuel Gas over MC. Both reduced MC and KCl, K2SO4, andK2CO3 deposited MC were evaluated for dry CH4/CO2 reform-ing of model biomass fuel gas mentioned in section 2.2 at700-850 °C. The results are listed in Table 3. C2H4 in the feedgas was converted completely. For undeposited MC and

Figure 4. Axial photos of MC samples: (A) reduced, (B) KCl deposited, and (C) KCl deposited after reaction (1 or 2 refers to SEM-EDS and XPS analysisposition in Figure 5 and Table 2).

Figure 5. SEM images of MC samples: (A) reduced, (B) KCl deposited atposition 1 in Figure 4B, (C) KCl deposited at position 2 in Figure 4B, and(D) KCl deposited after reaction at position 2 in Figure 4C (Inset is theclose-up micrograph of filament domain morphology).

Ind. Eng. Chem. Res., Vol. 49, No. 7, 2010 3179

deposited MC, both CH4 and CO2 conversion increased withthe increased temperature. This can be explained by thefollowing chemical equations, mainly occurring in the catalyticbed:27,28

The endothermic dry reforming reaction between CH4 andCO2 was enhanced by temperature increase to favor CH4 andCO2 conversions, which also resulted in the increased H2 andCO yields. While high reaction temperature also promoted theendothermic reverse water gas shift reaction, which consumedH2 and produced CO. So CO yield increase is higher than thatof H2, which resulted in decreased H2/CO ratio in tail gas,accordingly from 700 to 850 °C for undeposited and depositedMC in Table 3.

Compared with undeposited MC, most potassium salt vapordeposited MC exhibited lower CH4 conversion, but a little higherCO2 conversion and CO yield under same reaction condition at750-830 °C. It could be attributed to the presence of alkaliaerosol deposition on MC surface, which caused the decreaseof BET area, pore volume, Ni metal surface, and CH4 adsorptionamount. Praliaud described that alkali metal acted as a promoter,existing in the form of alkali silicate or nitrate, and mainlyassociated with an anion.26 However, K vapor deposited onNi(111) were in the form of Kσ+ with σ varying from 0.5 to 0as the alkali aerosol coverage increase by XPS results in this

paper. It could be assumed that an indirect electrostaticinteraction occurred between the deposited alkali metal and theactive Ni metal. Alkali metal would migrate from the top partof the monolithic catalyst (vicinal to the alkali source) to thewhole reforming bed.24 Besides, the deposition of alkali saltvapor may decrease catalyst surface acidity and change theadsorption characteristics of MC, which promoted the carbonelimination reaction, shown in eq 4. And the electron enrichmentof metallic phase by alkali metal might increase the back-donation of metal electron to the 2π* antibonding of CO2,

29 soCO2 molecules were adsorbed preferentially on MC surface.That promoted the reversed reaction of eq 4, which increasedCO2 conversion and CO yield, but decreased the H2/CO ratioin the tail gas.

Although K2SO4 has the highest melt temperature of 1067°C, K2SO4 deposited MC showed most serious decrease of CH4

conversion among three potassium salts of KCl, K2SO4 andK2CO3. And the deactivation results in Table 3 were similar tothat of powdered Pt/Rh catalysts, where BET order after saltdeposition was: NiKCl > NiZnCl2 > NiK2SO4 and BET decreasedegree was in linear relationship with molecular weight of thedeposited salt.11 Besides, KCl was in the melting state, whileK2SO4 and K2CO3 still kept in solid state under reformingreaction of 750 °C in this paper, so it might result in differentcatalytic reforming mechanism on these three potassium saltvapor deposited MC. In the same time, the deactivation ofK2SO4 deposited MC may also be caused by sulfide poisoningbesides the active Ni phase decrease mentioned above.14

It also should be pointed that the irregular CO2 conversionresults at high temperature of 850 °C and H2 yield might alsoresult from the different reforming mechanism and interactionsof the complex reforming system,30 where the main reactionswere accelerated at different temperature respectively accordingto their thermodynamic properties.

Table 3. Effect of Potassium Salt Vapor Deposition on MC Reforming Activitiesa

X (%) Y (%) Xout,p (%)

catalysttemperature

(°C) Fout/Fin CH4 CO2 H2 CO CH4 CO2 H2 CO nH2/nCO

undeposited catalyst

850 1.21 95.45 90.03 74.59 75.82 0.56 1.74 34.47 36.20 0.95830 1.16 96.37 75.03 66.54 63.32 0.46 4.64 33.44 33.14 1.00780 1.08 91.20 68.78 56.84 46.31 1.19 8.26 33.21 27.51 1.13750 1.08 88.86 64.18 58.53 43.34 1.66 9.28 33.46 25.31 1.32700 1.09 78.99 50.78 58.97 33.93 3.01 11.59 33.26 22.78 1.38

KCl deposited catalyst

850 1.22 92.19 88.81 71.37 78.13 0.96 2.02 33.96 36.76 0.91830 1.21 90.23 79.76 72.28 70.58 1.21 3.67 34.03 34.31 0.99780 1.11 88.26 75.43 60.79 53.68 1.04 4.85 33.78 30.99 1.07750 1.07 89.29 67.62 60.25 45.22 1.46 6.52 33.93 28.25 1.19700 1.08 76.43 56.26 57.45 36.08 3.11 8.47 33.18 23.57 1.39

K2SO4 deposited catalyst

850 1.29 90.39 91.37 78.54 90.07 1.12 1.46 33.67 38.58 0.87830 1.19 78.65 90.07 60.53 77.47 2.67 1.83 31.09 37.23 0.83780 1.17 50.65 80.74 49.70 66.38 6.30 3.61 38.44 33.97 0.84750 1.06 32.95 65.40 24.90 45.81 9.48 7.17 23.29 29.44 0.79700 1.01 14.84 61.17 15.10 33.70 12.61 8.43 21.02 25.83 0.81

K2CO3 deposited catalyst

850 1.24 91.24 81.57 72.23 80.17 1.01 2.06 33.47 37.78 0.89830 1.18 88.65 80.14 70.32 70.47 1.26 3.63 32.32 36.53 0.88780 1.15 85.35 75.23 59.97 56.38 2.30 4.96 30.01 32.79 0.92750 1.06 82.36 65.54 54.01 45.13 2.78 6.17 30.15 28.34 1.06700 1.03 74.68 60.38 55.10 34.70 3.81 8.53 29.78 24.29 1.23

a Note: nH2 is outlet H2 content (%, vol), and nCO is outlet CO content (%, vol).

Table 2. SEM-EDS and XPS Analysis Results of MC

binding energy (eV)

catalystposition

(Figure 4)

K/Ni atomicratio by EDSanalysis (%)

K/Ni atomicratio by XPSanalysis (%) Ni2p3/2 K2p

reduced catalyst 1 855.8 (70.648%)862.0 (29.352%)

deposited catalyst 1 5.74 0.11 856.0 293.02 0.33

CH4 + CO2 ) 2CO + 2H2 (1)

C2H4 + 2CO2 ) 4CO + 2H2 (2)

CO2 + H2 ) CO + H2O (3)

2CO ) C + CO2 (4)

CH4 ) C + 2H2 (5)

3180 Ind. Eng. Chem. Res., Vol. 49, No. 7, 2010

3.5. Stability of KCl Deposited MC. It is known that Ni-based catalyst deactivation due to carbon deposition via Bou-douard and methane decomposition reactions was serious duringCH4 or liquid fuel reforming.31 In this paper, it was mainlyfocused on the effect of alkali impurities on biomass fuel gasreforming system, which also contributed to catalyst. Thestability tests were carried out at 750 °C.

For undeposited MC, CH4 conversion maintained about 90%after 60 h time on stream (TOS) and tail gas compositionschanged slightly in Figure 6: CH4 content increased from 1.03to 1.38 vol % and H2 content was almost the same of 32.0%.While after KCl vapor deposition, CH4 conversion decreasedfrom 87.2% to 32.0% and H2 content in tail gas decreased from35.1 to 26.7 vol % after 17 h TOS. According to SEM-EDSresults, it could be conferred that the deactivation by KCldeposition was probably caused by a weak interaction betweenalkali and Ni and MgO, resulting in rapid migration of alkalimetal through the whole catalyst bed in 3.3. In the beginning,the alkali was primarily retained at the top part of MC bed(position 1 in Figure 5B). Subsequently, extra alkali salt vaporspread the bed and poisoned the whole catalyst, which mightbe the main reason for the decreased CH4 conversion due tothe coverage of Ni phases by alkali vapor deposition.

3.6. Reforming of Real Practical Biomass Fuel Gasover MCP. The quick deactivation of KCl deposited MC fordry CH4/CO2 reforming of model biomass fuel gas in section3.5 was due to the low Ni loading amount(5.5 wt %, NiO) andhigh KCl deposition amount(7.8 mg/L KCl in carrier gas for6 h, K content of 0.85 wt % and 1.39 wt % at the top part ofdeposited MC by ICP-AES and EDS respectively), which isfar higher than K amount by emission of atomized alkali saltsolution for 2 h(37 mg/m3)11 and real K content in fly ash duringbiomass gasification.24 To investigate the real performance ofcordierite monolithic catalyst of MCP, larger in size than MC,

the reforming procedure of our pilot-scale integrated dimethylether synthesis system from air gasification of pine sawdust(200-250 kg/h) was carried out and the results are shown inFigure 7 and Table 4.

It can be seen that the pressure drop of the reformer fluctuatedover the first 2 h TOS, and the highest pressure drop was up to1200 Pa (Figure 7A) because of the unstable gasification process.But after about 2 h TOS, the pressure drop kept stable of about700 Pa and that would decrease the power requirement for rootsblower, which ventilated biomass fuel gas to the gas tank. Thereforming effect of real raw biomass fuel gas over MCP wasobvious: CH4 conversion of raw biomass fuel gas (initialcompositions, H2/CO/C2H4/CH4/CO2/N2 ) 10.2/16.8/0.5/6.4/15.2/51.0 vol % in Figure 7C) was above 84% during 60 h TOSat 600 °C in Figure 7B. CO2 conversion was relatively low tobelow 40% because of the low reforming temperature in thereformer, resulting from inadequate external heat transfer of theburner by combusting diesel. And tar content decreased from4.8-5.3 to 0.12-0.14 g/m3 after catalytic reforming, whichresulted in more than 40 vol % content of H2 and CO with H2/CO ratio of around 1, higher than the original content of 27 vol%.

The particle size of fly ash was 0.15-0.85 mm in thisgasification process, confirmed by SEM. After 60 h operation,the K and Na content in MCP at different positions of reformer(as shown in Figure 2) was similar of 0.03-0.07 wt % exceptat the reformer inlet (0.26 and 0.11 wt %, respectively, in theintermediate tube) in Table 4. It was because of the relativelyslow fuel gas flow in the up-draft reformer and vicinity to thealkali source, where K and Na in the fly ash were inclined todeposit. And this was accordance with high K content at thetop part of MC during model experiments in section 3.3, whichapproved the application possibility of model alkali saltevaporation method to investigate fly ash impurities on reform-

Figure 6. CH4 conversion (A) and tail gas composition (B) with time on stream over undeposited MC(close symbols) and KCl deposited MC(open symbols).

Figure 7. Typical reforming results of real biomass fuel gas over MCP with time on stream: (A) pressure drop of the reformer, (B) CH4, CO2, and tarconversion, and (C) tail gas composition.

Ind. Eng. Chem. Res., Vol. 49, No. 7, 2010 3181

ing performance. Because of the low alkali content in practicalreforming process, CH4 conversion, H2, and CO yields kept highafter 60 h operation in Figure 7, which is different from thequick deactivation of deposited MC by high K amount in Figure6.

Conclusion

The deposition of alkali salt vapor (KCl, K2SO4 and K2CO3)on the Ni-MgO/γ-Al2O3/cordierite monolithic catalyst showedthat the high content of alkali was mainly existence at the toppart of monolithic catalyst(vicinal to alkali source), which causedquick catalyst deactivation for dry CH4/CO2 reforming perfor-mance of model biomass fuel gas(H2/CO/C2H4/CH4/CO2/N2)15.8/12.1/2.51/ 15.0/22.1/32.6 vol %). The extent of catalystdeactivation increased with alkali salt agglomeration on thecatalyst surface and the potassium compound, in the form ofKσ+ (Naσ+) with σ varying from 0.5 to 0, which resulted in anindirect electrostatic interaction between the oxygen atom ofadsorbed CO2 and the active Ni metal.

Compared with the high conversion of CH4 (above 80%) andtar (above 90%) and high H2, CO content (> 40 vol %) in tailgas after 60 h real biomass fuel gas reforming operation in thepilot-scale integrated gasification/reforming system, it could beconcluded that far lower K and Na content in monolithic catalystin the reformer, caused by real fly ash, was the main reason.And the reforming process in the pilot-scale plant was testedto be feasible for produce biomass synthesis gas for thefollowing liquid fuel synthesis in pilot-scale or large-scale andthe utilization of monolithic catalyst was applicable because ofits low pressure drop of the reformer and excellent performance.In the same time, a longer reforming operation and directdimethyl ether synthesis process are ongoing by our group. Thesynthesis reaction heat and tail gas combustion will be appliedto generate heat for the reformer as much as possible.

Acknowledgment

This work was supported by the National Key Basic ResearchProgram 973 Project founded by MOST of China (Project No.2007CB210207) and the National High Technology Researchand Development Program of China via 863 plan (Project No.2007AA05Z416) and the Natural Science Foundation of Guang-dong Province China (Project No. 9451007006004086).

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ReceiVed for reView August 31, 2009ReVised manuscript receiVed January 22, 2010

Accepted February 3, 2010

IE901370W

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