NAWTEC20-70 DEACTIVATION AND ENERGY ANALYSIS OF CHAR CATALYSTS IN BIOMASS GASIFICATION SYSTEMS

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1 Proceedigs of the 20th Aual North America Waste-to-Eergy Coferece NAWTEC20 April 23-25, 2012, Portlad, Maie, USA NAWTEC20-70 DEACTIVATION AND ENERGY ANALYSIS OF CHAR CATALYSTS IN BIOMASS GASIFICATION SYSTEMS Naomi Klighoffer Columbia Uiversity New York, NY, USA Marco J. Castaldi Columbia Uiversity New York, NY, USA ABSTRACT Oe of the major barriers to gasificatio is the eed for elimiatio of tars that are geerated i thermochemical coversio processes. While metal catalysts ca be used for tar decompositio, ad cheaper alterative is. Deactivatio of catalysts has bee studied ad these deactivatio rates have bee applied to a model for a gasificatio system. The calculatios ad experimetal data preseted here show that if the from a gasifier is recycled to a tar reformer the some deactivatio will take place, but the activity will ot fall below 40% of its iitial activity. The eergy pealty for divertig, a potetial heat source, to a catalytic reactor has bee accouted for. This was doe by comparig the heatig value of to the heatig value of sygas geerated from reformig tar usig the as a catalyst. At high gasificatio temperatures, whe tar productio i the gasifier was low, the had a higher heatig value tha the sygas that was produced from tar reformig. At low temperatures, the heatig value of the sygas exceeded that of the combustio, which implies a overall eergy beefit. INTRODUCTION Gasificatio is cosidered to be a viable optio for recovery of eergy from waste or biomass. This process ivolves heatig a solid fuel, such as coal, waste, or biomass, i the presece of a oxidat to geerate sythesis gas, which is primarily composed of CO ad H 2. The oxidats that are most commoly used are H 2 O, O 2 (air), ad CO 2. The mai beefit of gasificatio is the ability to produce fuels or chemicals from the sythesis gas via Fischer-Tropsch sythesis, or electricity ad/or heat. Therefore, gasificatio is a way to produce covetioal fuels from ucovetioal eergy sources. Oe of the major barriers to commercializatio of this techology is the eed for a effective method to decompose tars. Tars are liquid orgaic hydrocarbos that are formed durig the gasificatio process from thermal decompositio of the solid fuel uder oxyge-lea coditios. These compouds ca break dow dowstream ad form carbo deposits, or ca cause cloggig i equipmet. Tar ca be decomposed catalytically via steam reformig. However, tars cause sigificat catalyst deactivatio therefore catalysts used should be cheap ad replaceable. It has bee show i literature that from biomass gasificatio ca be used as a catalyst for tar decompositio [1,2]. Oe major issue with usig as a catalyst is that decomposes at lower temperatures tha covetioal metal catalysts. Therefore, i the operatig regime of tar reformers (~ o C) is more likely to decompose tha metal catalysts such as ickel or platium. This paper studies the decompositio of i this temperature regime as well as the deactivatio rate whe the is catalyzig hydrocarbo crackig reactios. This will help to uderstad its stability at high temperatures. Aother issue with usig as a catalyst is that is a combustible solid that could be used for process heatig (for example, for providig heat to the gasifier or dowstream gas cleaup). So, the should ot be cosidered to be a waste product. Therefore it is importat to uderstad the eergy pealty for divertig this from possible heatig applicatios i order to use it i catalytic processes. I this paper, we compare the eergy cotaied i the to the eergy of the sygas that ca be produced if the reforms steam ito sygas. This will help to uderstad if the use of as a catalyst for tar reformig is beeficial to the overall eergy balace of the process. EXPERIMENTAL Experimets preseted i this paper were doe usig that was geerated i a fluidized bed reactor. The details 1 Copyright 2012 by ASME

2 o this setup are described i a previous publicatio [3]. The stability testig of the was doe i a thermo gravimetric aalyzer (TGA, Netzsch model # STA449). The reactivity of the was tested by exposig the to CO 2 at 800 o C to eable the Boudouard reactio to take place. The mass loss ad temperature were measured cotiuously. The TGA was coupled to a Agilet 3000A micro gas chromatograph (microgc) ad separatio was doe usig a molecular sieve 5A colum. The colum was coupled to a thermal coductivity detector (TCD), which measured CO productio. The TGA was also used to measure the deactivatio of the. This was doe by itroducig CH 4 ito the TGA at 750 o C for 3 hours. Mass gai represets reactio rate, as discussed i the discussio sectio of this paper. The compositio of the was aalyzed usig eergy-dispersive x-ray spectrometry (EDX) coupled with a Evirometal Scaig Electro Microscope (ESEM, FEI XL30). Details of the calculatios preseted i this paper are outlied i the discussio sectio. The reactio rate over time is show i Figure 1. The reactio rate was calculated accordig to Equatio 2, assumig zero order kietics. The reactio rate decreases over time, which reflects catalyst deactivatio. Two clear regios of deactivatio are observed ad iitial rapid deactivatio followed by a secod regime of more gradual deactivatio. Deactivatio rate is calculated to be the slope of the lie, which is the rate at which the reactio rate is decreasig. d m m dt t MW carbo m iitail, Equatio 2 DISCUSSION Char catalyst deactivatio Char deactivatio was studied for the methae crackig reactio. Deactivatio is a commo problem with catalysts ad must be cosidered whe desigig catalytic processes. It is particularly relevat for the system described here, sice our data has show that the cotais both catalytic metals ad reactive carbo. The metals are most likely the primary catalytic compoet i the. This was observed durig a experimet where the was exposed to propae at 900 o C i a TGA. Propae was broke dow ito solid carbo, H 2, ad possibly other hydrocarbos. The mass gai suggests that solid carbo deposited o the surface. After the experimet, the was viewed i a ESEM coupled with EDX. I the raw that had ot bee used as a catalyst, iro sites were detected o the surface of the. After the had bee used to catalyze propae decompositio, carbo depositio was detected o the iro, idicatig that this is a primary catalyst site. El-rub et al. compared the catalytic activity of ash to ad foud the activity of the to be higher tha that of the ash [1]. This suggests that the carbo support ehaces the activity of the catalyst while the metals are the primary catalytic sites. The active lifetime of the will be highly depedet o the rate of deactivatio of the metal sites ad the possible loss of carbo support durig reactio. I this work, the deactivatio rate of the catalyst was measured i a TGA. The was heated to 750 o C i the presece of methae ad the catalyzed the decompositio of methae accordig to Equatio 1. The mass gai is proportioal to the reactio rate, sice the reactio forms carbo o the catalyst surface. CH C Equatio 1 4 2H 2 Figure 1. Deactivatio rate of whe catalyzig decompositio of methae at 750 o C. The deactivatio rate for regimes 1 ad 2 were -2.2x10-3 mmol (g -1 )(h -2 ) ad -1.1x10-3 mmol (g -1 )(h -2 ), respectively. I order to uderstad how these deactivatio rates relate to a gasificatio process, a gasifier is modeled where all of the is recycled to be used for tar decompositio. The system described by Carpeter et al. is used to obtai relative amouts of tar ad [4]. The yield was measured to be 22% ad the tar yield was 10%. I that process, mixed wood was gasified with steam at 650 o C ad a thermal tar cracker dowstream was used at 875 o C. After the thermal cracker, some tars remai ad the catalytic decompositio of these tars is studied here. I this calculatio, a fixed bed of is placed dowstream of the gasifier. The coversio rates of were calculated based o the coversio rates that were measured for methae. A paper by Wag et al. measured the kietics of catalytic steam reformig of methae ad toluee ad reported the rate of methae reformig to be approximately 1.6 times the rate of toluee reformig [5]. Toluee is a major compoet of tar, so it is reasoable to assume that toluee reactio rates are similar to that of tar. The kietics that were measured for methae crackig, as show i Figure 1, were divided by 1.6 i order to achieve reasoable kietics for toluee decompositio. This calculatio gives a iitial reactio rate of 2 Copyright 2012 by ASME

3 3.82x10-3 mmol (h -1 ) (g -1 ). The objective is to determie if the amout of geerated will covert all of the tar that is geerated, or if the deactivatio rate is so rapid that all of the tar caot be coverted with the amout of produced i the gasificatio process. I this calculatio a time frame of oe day ad a flow rate of 1kg/h of biomass was selected (i other words, the that is produced i oe day is placed i a dowstream tar reformer). This system produces 5.28 kg of per day ad a fixed bed reactor with 5.28kg of ad 0.1 kg/h of tar is modeled. The amout of tar that ca be coverted before the reaches regime 2 of deactivatio was calculated accordig to Equatio 3. t phase 1 d rtoluee phase m dt m dt r 0 methae 1 Equatio molch4 1mol phase1 ( 2.2t 6) 5280g g h 1.6mol toluee 0 ch4 The result of the calculatio outlied i Equatio 3 shows that durig regime 1 deactivatio, the geerated by the reactor modeled here ca reform 29 kmol of tar. The total tar geerated over oe day was calculated to be 26kmol. This idicates that for the to tar ratios of this reactor, the will ot reach regime 2 deactivatio. Therefore, recyclig the has the capacity to reform tar ad the activity of the will ot fall below 40% of its iitial activity. While the ratios of tar to will vary based o gasificatio coditios, this calculatio suggests that the reactivity of is o a reasoable scale for coversio of tar. Eergy Balace The eergy pealty for divertig from possible heat recovery is cosidered. The eergy available by combustig the is compared to the eergy of sythesis gas that would be geerated from tar reformig. This is doe by comparig the heat of combustio of to the heatig value of the sygas that would be obtaied from tar coversio. The two systems cosidered are show i Figure 2. Both literature data ad our ow data are used to complete this aalysis. Relative ad tar productio rates are take from a paper by Gomez-Barea, where wood waste was pyrolyzed at differet temperatures ( o C) ad the amout of tar,, ad gas was measured [6]. The coversio of tar is obtaied from a paper by Abu Elrub et al. where was used as a catalyst to reform pheol, a model tar compoud, ad 81.6% coversio was achieved at 700 o C i the presece of steam ad CO 2 [1]. I our eergy balace eergy iputs to the steam reformer should also be accouted for. If the is placed dowstream of the biomass i the gasifier, the the catalytic reformer would ot require additioal heatig, sice gasifiers typically operate aroud 700 o C or higher. However, the heat of reactio must be accouted for, therefore the ethalpy for steam reformig of dt toluee has bee icluded, accordig to Equatio 4. Steam reformig is typically used to decompose tars, ad, sice biomass cotais water, the steam will be available i the reactor. The eergy recovery from reformig the tar is calculated accordig to Equatio 5, where the ethalpy of the reformig reactio is subtracted from the heatig value (heat of combustio) of the gasificatio products. A. B. Figure 2 A. Gasificatio system where heat is recovered from combustio. B. Gasificatio system where is recycled to covert tar to sygas. Equatio 4 C7H8( g ) 7 H 2O( g) 7CO( g) 11H 2( g) H ref 869kJ / mol Equatio 5 CO H 2 Q sygas coversio tar HVCO HVH2 tar tar H The heatig value of the was calculated accordig to Equatio 6, where the heat of combustio of was kj/g [7]. This value is for cotaiig approximately 90% carbo. The carbo cotet of the geerated from gasificatio experimets was measured to be approximately 90%, therefore this heatig value is appropriate for this system. Q HV Equatio 6 The pyrolysis system that is used for this example produces ~20%, ad this value did ot vary sigificatly with temperature. The cocetratios of tar were highly depedet o temperature. Gomez-Barea et al. report the percetage of ref 3 Copyright 2012 by ASME

4 biomass that is i the, gas, ad total codesate, where total codesate icludes water. Typically wood cotais about 5% water, so 5% of the codesate was cosidered as water ad the remaiig codesate was said to be hydrocarbos. The amout of eergy that could be recovered from the ad the sygas (from tar coversio) is show i Figure 3. At 750 o C, codesate was reported to be ~23% whereas at 900 o C, codesate was reported to be oly ~10% so sigificatly less sygas would be produced by reformig tar from high temperature pyrolysis systems. Low temperature systems geerate more tars, ad therefore more sygas could be recovered from tar reformig. Therefore, at 750 o C the eergy value of the sygas from tar reformig is higher tha that of the combustio. At higher temperatures, the heat of combustio of is higher tha that of the sygas produced. Although the objective of this study is to develop a uderstadig of the eergy pealty for usig as a catalyst for tar decompositio, it does ot absolutely determie if the use of i this applicatio is beeficial. For example, there is a ecoomic beefit to usig i place of metal catalysts, or high temperature thermal coversio systems. This is ot reflected i the eergy balace. CONCLUSIONS Char is beig cosidered as a catalyst for decompositio of tar i biomass gasificatio systems. This paper studies the deactivatio of durig catalytic applicatios ad two deactivatio regimes have bee idetified. The catalyst activity data was used to model a system where from the gasifier is recycled to reform the tar. The catalytic activity was sufficiet to reform the tar before regime 2 deactivatio occured. The eergy pealty for divertig the from possible process heatig applicatios to sygas coversio was also studied. The eergy value of the sygas that would be geerated is greater tha the heatig value of at 750 o C. At higher temperatures, whe less tar is formed, the heatig value of the sygas is lower tha that of the. Figure 3. Heatig value of compared to heatig value of sythesis gas geerated from tar reformig with a catalyst. Our research has show that the is ot completely stable ad will decompose at high temperatures, specifically i the presece of steam or CO 2. Therefore, usig to catalyze the decompositio of tar ca also result i additioal eergy recovery by covertig some of the carbo i the ito CO. This ca be see i Figure 4, where was exposed to CO 2 at 800 o C i a thermo gravimetric aalyzer. Mass loss of the was measured, which is attributed to coversio of carbo to CO via the Boudouard reactio. The productio of CO was measured with a micro-gc which cofirmed that CO was produced, as show i Figure 4B. Figure 4. Reactio of with CO 2 i a TGA at 800 o C. A. Mass loss ad temperature B. CO geerated via boudouard reactio 4 Copyright 2012 by ASME

5 NOMENCLATURE phase 1 molesof tolueereformed durig phase1catalyst deactivatio t phase 1 time whe catalyst deactivatio trasitios from phase1 tophase 2 rtoluee relative reactio rates of tolueead methae from [5] rmethae Q total eergy value of x i thesystem modeled x HV x molar ethalpy of combustio of x ACKNOWLEDGMENTS The authors thak the Waste to Eergy Research ad Techology Coucil (WTERT) at Columbia Uiversity for supportig this work. REFERENCES [1] Abu El-Rub, Z.; Bramer, E.A.; Brem, G., Experimetal compariso of biomass s with other catalyst for tar reductio, Fuel 87 (2008) [2] Wag, D.; Yua, W.; Wei, J., Char ad -supported ickel catalysts for secodary sygas cleaup ad coditioig, Appl Eergy (2011) [3] Klighoffer, N,; Castaldi, M.J.; Nzihou, A., Beeficial use of ash ad from biomass gasificatio, Proceedigs of the 19 th Aual North America Waste-to-Eergy Coferece (2011) [4] Carpeter, D.L.; Bai, R.L., et al., Pilot-Scale Gasificatio of Cor Stover, Switchgrass, Wheat Straw, ad Wood: 1. Parametric Study ad Compariso with Literature, Id. Eg. Chem. Res. 49 (2010) [5] Wag, X.; Gorte, R.J., A study of steam reformig of hydrocarbo fuels o Pd/ceria, Appl Catal A 224 (2002) [6] Gomez-Barea, A.; Nilsso, S, et al., Devolatilizatoi of wood ad wastes i fluidized bed, Fuel Proc Tech 91 (2010) [7] Perry, R.H.; Gree, D.W., Perry s Chemical Egieer s Hadbook, 7 th ed., McGraw-Hill, 1997 USA. 5 Copyright 2012 by ASME