Partial Oxidation Reforming of Biomass Fuel Gas Over Nickel-based Monolithic Catalyst With Naphthalene as Model Compound

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Korean J. Chem. Eng., 25(4), 738-743 (2008) SHORT COMMUNICATION Partial oxidation reforming of biomass fuel gas over nickel-based monolithic catalyst with naphthalene as model compound Chen Guang Wang*,***, Tie Jun Wang*,**,†, Long Long Ma*,**, Yan Gao*,***, and Chuang Zhi Wu*,** *Guangzhou Institute of Energy Conversion, Chinese Academy Sciences, Guangzhou, 510640, China **Key Laboratory of Renewable Energy and Gas Hydrate, Chinese Academy Sciences, Guangzhou, 510640, China ***Graduate Univer
   Korean J. Chem. Eng  ., 25 (4), 738-743 (2008) SHORT COMMUNICATION 738 † To whom correspondence should be addressed.E-mail: Partial oxidation reforming of biomass fuel gas over nickel-based monolithic catalystwith naphthalene as model compound Chen Guang Wang * , *** , Tie Jun Wang * , ** ,† , Long Long Ma * , ** , Yan Gao * , *** , and Chuang Zhi Wu * , ** *Guangzhou Institute of Energy Conversion, Chinese Academy Sciences, Guangzhou, 510640, China**Key Laboratory of Renewable Energy and Gas Hydrate, Chinese Academy Sciences, Guangzhou, 510640, China***Graduate University of Chinese Academy Sciences, Beijing, 100039, China(  Received 20 March 2007 ã accepted 17 January 2008 ) Abstract − With naphthalene as biomass tar model compound, partial oxidation reforming (with addition of O 2 ) anddry reforming of biomass fuel gas were investigated over nickel-based monoliths at the same conditions. The resultsshowed that both processes had excellent performance in upgrading biomass raw fuel gas. Above 99 % of naphthalenewas converted into synthesis gases (H 2 +CO). About 2.8wt % of coke deposition was detected on the catalyst surfacefor dry reforming process at 750 o C during 108h lifetime test. However, no coke deposition was detected for partialoxidation reforming process, which indicated that addition of O 2 can effectively prohibit the coke formation. O 2 canalso increase the CH 4 conversion and H 2 /CO ratio of the producer gas. The average conversion of CH 4 in dry and partialoxidation reforming process was 92 % and 95 % , respectively. The average H 2 /CO ratio increased from 0.95 to 1.1 withthe addition   of O 2 , which was suitable to be used as synthesis gas for dimethyl ether (DME) synthesis.Key words:Biomass Fuel Gas, Monolithic Catalyst, Partial Oxidation Reforming, Tar Elimination INTRODUCTION The use of biomass fuel gas is attracting more and more atten-tion nowadays. The main components of biomass fuel gas producedfrom biomass gasification are H 2 , CO, CO 2 , CH 4 , tar and other lighthydrocarbons [1]. The upgrading of biomass fuel gas to syngas is avery important process to the utilization of biomass, because syn-gas is the building block for many chemicals just like methanol,dimethyl ether and higher hydrocarbons through the Fischer-Trop-sch process. Up to now, syngas has been mainly produced by steamreforming of natural gas in industry: CH 4 +H 2 O=3H 2 +CO, ∆ H r  =205.9kJ/mol(1) This reaction is endothermic, so it is necessary to supply heat. At a pressure between 1.5 and 3.0MPa over Ni/ α  -Al 2 O 3 catalysts, thereaction temperature has to be kept around 900 o C. In order to re-duce the energy consumption, partial oxidation of methane has beenextensively studied [2-4]. Researchers has investigated various reac-tion conditions, and high yields of syngas have been reported [5-11].Recently, more and more researchers have been paying atten-tion to the use of monolithic catalysts for conversion of methane.Different kinds of monoliths have been used and show good perfor-mance on methane conversion and natural gas utilization. Corella’sgroup made nickel-based monoliths and used on hot gas steam re-forming in biomass gasification in fluidized beds [12-14]. The useof monoliths has attracted more attention from VTT in Finland andthe Fraunhofer UMSICHT Institute in Oberhausen, Germany. Com- pared with a fix-bed reactor and slurry reactor, using monolithiccatalysts has several advantages: high catalytic performance per unitmass of active metal, high operating stability and low pressure dropof catalyst bed [15-19].We prepared high stable nickel catalysts and tested their charac-terization and performance of reforming biomass fuel gas [21,22].The catalyst exhibited excellent performance. However, due to theatmospheric biomass gasifier, the high temperature raw fuel gasfrom gasifier cannot flow through a reforming catalyst bed that hasa high pressure drop.In this paper, nickel monolithic catalysts were prepared by wetimpregnation of NiO on the surface of cordierite. The performanceof dry reforming and partial oxidation reforming of biomass fuel gaswas investigated over nickel monolithic catalysts. The catalyst char-acterization was also made by method of thermogravimetry. EXPERIMENTAL1.Preparation of the Catalysts The nickel monolithic catalysts were prepared by wet impregna-tion of NiO on cordierite support whose surface area was increased by wet impregnation of  γ   -Al 2 O 3 superfine powder. The preparedmethod was as follows. The cordierite was first etched in 30wt % oxalic acid solution under vacuum for 30minutes and then the water evaporated. After being washed by distilled water, the support wasdried overnight at 120 o C. The surface area of the cordierite supportwas increased by wet impregnation with superfine γ   -Al 2 O 3 powder ethanol solution. After drying overnight at 120 o C, the support wascalcined at 750 o C for 10h. The process was repeated for three times,then the support was impregnated with nickel nitrate solution (72.38g Ni(NO 3 ) 2 and 200ml distilled water) for 24hours. The catalystwas dried at 110 o C overnight and calcined at 800 o C. The catalystwas designated as MC-3. Physical characteristics and chemical com- position of catalyst MC-3 are listed in Table1. 2.Characterization The crystal structure of the catalysts before and after reaction was  Partial oxidation reforming of biomass fuel gas over nickel-based monolithic catalyst with naphthalene as model compound739 Korean J. Chem. Eng.(Vol. 25, No. 4) determined by X-ray powder diffraction (XRD) in a Rigaku D/max-IIIB apparatus by using CuK  α  radiation, at 40kv and 30mA. Dif-fraction peaks recorded in a 2 θ  range between 5 o and 80 o wereused to identify the structure of the samples.TG-DTG studies of used catalysts powder ware carried out un-der an oxidative atmosphere with analyzers by using 10-15mg of sample and a 10 o C/min temperature increasing rate. 3.Catalytic Reforming Reaction A schematic diagram of the reforming system is shown in Fig.1.The reactor was composed of an electric furnace and a quartz tube(outer diameter: 45mm, wall thickness: 2mm). The gap betweenthe monolithic catalyst and reactor wall was sealed with quartz fiber.The composition of the biomass fuel gas was H 2 :16.04vol % ,CO:12.10vol % , C 2 H 4 :2.50vol % , CO 2 :21.95vol % , CH 4 :15.08vol % , N 2 :32.33vol % . The reforming reactor was atmospheric andtemperature was kept at 750 o C. Naphthalene was added in the bio-mass fuel gas as the tar model compound to study the performanceof tar elimination. For all runs, the flow rate of biomass fuel gas was300sccm. In the partial oxidation reforming runs, additional 60sccmof mixed gas (O 2 /N 2 =95/5, mol/mol) was added into the reactor toinvestigate the promotion effect of oxygen addition.First, the catalysts were reduced by mixed gas (H 2 /N 2 =5/95, mol/mol) with flow rate of 300sccm from room temperature to 750 o Cwithin 2h. Then reactant gases were fed into the reactor. After reforming runs were over, the pure N 2 was fed into the reactor tocool the reactor temperature to room temperature and the catalystwas taken out for coke deposition analysis. The composition of dry producer gases was analyzed by gas chromatograph (GC) and gaschromatogram-mass spectrum (GC-MS) after sampling. 4.Outlet Gas Analysis System The contents of N 2 , CO, CO 2 , CH 4 , H 2 in outlet gas were analyzedwith a gas chromatograph (model GC-9800, Shanghai KechuangCorp., China) equipped with a thermal conductivity detector (TCD).The column was TDX-01 and carrier gas was He. The contents of CH 4 , C 2 H 4 were analyzed with another GC-9800 gas chromatographequipped with a flame ionization detector. The column was Pora- pak Q and carrier gas was N 2 .The products of tar cracking were analyzed by GC/MS (Agilent,Hp-1). The preconcentrator, GC and MS are Entech 7100, Agilent6890, Agilent 6973, respectively. RESULTS AND DISCUSSION1.Reforming Reaction In the partial oxidation reforming runs, the oxidation of biomassfuel gas occurred first when reactants were fed into the reactor. Themain oxidation reactions are shown as below [23]: CH 4 +1/2O 2 =CO+H 2 (2)2H 2 +O 2 =2H 2 O(3)2CO+O 2 =2CO 2 (4) Then reforming reactions occurred over monolithic nickel cata-lyst. The main reforming reactions are shown as below: CH 4 +CO 2 =2 H 2 +2 CO(5)CnHm+nCO 2 =m/2 H 2 +2nCO(6)CO 2 +H 2 =CO+H 2 O(7)CnHm=nC+m/2 H 2 (8)CH 4 +H 2 O=3H 2 +CO(9)CO+H 2 O=CO 2 +H 2 (10)2CO=C+CO 2 (11)CH 4 =C+2H 2 (12) At the desired reforming temperature, all of reactions mentionedabove reached chemical equilibrium and determined the compositionof outlet gas. Because reforming reactions are endothermic, high Table1.Physical characteristic and chemical composition of cat-alyst MC-3 catalyst dimension/mm NiO/wt % Al 2 O 3 /wt % LengthOuter diameter Holediameter Wallthickness5040113.743.81 Fig.1.Schematic diagram of the reforming experiment. 1.Gas4.Water-bath2.Mass flow controller5.Catalyst3.Tar pot6.Furnace Fig.2.CO 2 conversion vs. time on stream in partial oxidation anddry reformings.  740C.G. Wang et al. July, 2008 reforming temperature is suitable for the tar conversion.The conversion of CO 2 in dry reforming runs and partial oxida-tion reforming runs is shown in Fig.2. The average conversion of CO 2 was 80 % in the dry reforming runs. However, the addition of O 2 decreased the conversion of CO 2 sharply due to the oxidation of CO and partial oxidation of CH 4 .The H 2 /CO ratio of outlet gas is shown in Fig.3. In dry reform-ing runs, the H 2 /CO ratio varied between 0.95 and 1.15. In partialoxidation reforming runs, the average H 2 /CO ratio varied between1.09 and 1.35. It indicated that addition of O 2 could increase theH 2 /CO ratio of outlet gas. The reasons may be that O 2 reacted withCH 4 and H 2 in biomass fuel gas to produce H 2 O, which promotedthe water gas shift reaction to increase the H 2 /CO ratio. So the com- position of outlet gas can meet the requirements of the liquid fuelssynthesis process.In both dry reforming runs and partial oxidation reforming runs,C 2 H 4 was not detected in the outlet gas, which means it was con-verted completely. 2.The Effect of GHSV (Gas Hourly Space Velocity)  Nowadays, it is still difficult to test the tar content in syngas rap-idly. However, CH 4 conversion has the same trend with the con-version of tar, so we can use the online test of CH 4 conversion toindicate the effect of GHSV on the tar elimination. With the GHSVincrease, not only the conversion of CH 4 decreased, but also the con-version of naphthalene decreased. A tiny amount of smoke appearedat the reactor outlet when GHSV increased to 11,894h − 1 . With theGHSV increase further, the smoke became thicker and thicker. CH 4 is more stable than other hydrocarbons, so its conversion is moredifficult than that of naphthalene. When the conversion of CH 4 de-creased, the conversion of naphthalene decreased too.The effect of GHSV on the conversion of CH 4 is shown in Fig.4. The conversion of CH 4 decreased slowly from 56.24 % to 52.9 % with the GHSV increase from 4,250h − 1 to 8,495h − 1 . Then it de-creased sharply from 47 % to 25 % with the GHSV increase from10,088h − 1 to 25,487h − 1 . It indicated that molecular diffusion con-trolled the reforming reaction.The H 2 /CO ratio variation with the GHSV is shown in Fig.5.With the GHSV increase from 4,250h − 1 to 25,487h − 1 , the H 2 /COratio increased from 0.91 to 0.97. The water gas shift reaction is afast reaction. So with the GHSV increase, the reforming reactioncould not keep equilibrium, but the water gas shift reaction was fastenough to keep equilibrium, which resulted in the H 2 /CO ratio in-crease. Especially when GHSV raised to 8,495h − 1 , the two reactionsreached the balance point, and the H 2 /CO ratio reached its highest point. 3.Biomass Tar Cracking 8.28g naphthalene as tar model compound was filled in the tar  pot (shown in Fig.1) to investigate the mechanism of the tar crack-ing on MC-3 monolithic catalyst. The resultants produced by tar cracking were analyzed by GC/MS.The content of naphthalene in biomass fuel gas was 4.26g/m 3 .After partial oxidation reforming over MC-3 catalyst, above 99 % of naphthalene was converted to permanent gases such as H 2 /COand lighter components. Corella reported that the tar conversionrate of his monoliths in real fuel gas cleaning ranged from 21 % to96 % depending on different operation conditions [10,16]. Our oper-ation condition is cleaner than real fuel gas condition, so the con-version rate is higher. Fig.3.H 2 /CO ratio vs. time on stream in partial oxidation and dryreforming.Fig.4.GHSV vs. conversion of CH 4 in dry reforming.Fig.5.GHSV vs. H 2 /CO of outlet gas in dry reforming.  Partial oxidation reforming of biomass fuel gas over nickel-based monolithic catalyst with naphthalene as model compound741 Korean J. Chem. Eng.(Vol. 25, No. 4) As shown in Table2, the concentration of lighter components inoutlet gas stream is below 500 µ g/m 3 , which indicates that partialoxidation reforming over MC-3 catalyst shows good performanceon tar elimination.The mechanism of tar cracking may be as follows. The tar mole-cular absorbed on the surface of the catalyst, then formed a seriesof media compounds. Some are active species, and others are inertspecies. CO 2 in biomass fuel gas absorbed on the surface of catalyst,and divided to form CO and O free radicals. CO desorbed from thecatalyst surface, and O free radical attacked the media compounds.As shown in Table2, butylene, benzene and styrene are the maincomponents. The phenomenon may be explained as follows. Naph-thalene molecule has different C-C bonds. When O free radical at-tacks the molecule, the longer bonds like II, IV bonds are easier torupture and form species like butylene, benzene and styrene. Theinert species formed coke via dehydrogenation. I, III bonds are shorter and more difficult to rupture, so the species just like hendecane anddecane are less in the outlet gas. 3.Characterization of Catalyst Stability A large amount of CH 4 and CO in biomass fuel gas can result incarbon deposition on the surface of MC-3 catalyst by methane de-composition and CO disproportionation side reactions. The tar in the biomass fuel gas would make the carbon deposition more serious.Carbon deposition can cover the surface of the catalysts, reduce thecatalysts’ activity and even block the reactor. The pressure variationof reactor in partial oxidation reforming runs is shown in Fig.6. No evident pressure variation was observed, which indicated themerits of the monoliths catalysts.The results of a lifetime test are shown in Fig7. The average con-version of CH 4 was 92 % in dry reforming process. However, theaverage conversion of CH 4 was 95.4 % in the partial oxidation re-forming process. This indicated that the partial oxidation reforming process had higher performance of upgrading biomass fuel gas thandry reforming process. For both processes, no drop of the conver-sion of CH 4 was observed, which indicated that the MC-3 catalysthad excellent reforming activity and stability. The curve of partialoxidation changes sharply was caused by the asymmetry of the mix-ture of O 2 and fuel gas.The amount of carbon deposited on the MC-3 catalysts was in-vestigated by measuring the catalyst weight variation after the usedcatalysts were calcined from room temperature to 900 o C and kept Fig.8.TG curves of the coke sample on the used catalysts.Table2.The content of tar species in the outlet fuel gas over MC-3 catalyst Matter Residue time(min)Concentration( µ g/m 3 )Butylene04.76151.37Benzene08.87276.19Toluene11.78035.33Ethylbenzene14.74008.90m-Xylene14.99009.20 p-Xylene15.72005.044,7-Dimethyl hendecane20.79010.043,6-Dimethyl decane20.95007.563,7-Dimethyl hendecane21.96006.265-Methyl hendecane22.10008.63Tridecane26.54005.35 Fig.6.Pressure as functions of time on stream.Fig.7.CH 4 conversion vs. time on stream in both partial oxida-tion and dry reforming.
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