ORIGINAL PAPER
Aromatic Production from Catalytic Fast Pyrolysis of Biomass-Derived Feedstocks
Torren R.Carlson ÆGeoffrey A.Tomptt ÆWilliam C.Conner ÆGeorge W.Huber
冬天的英语单词Published online:14January 2009
ÓSpringer Science+Business Media,LLC 2009
Abstract The conversion of biomass compounds to aromatics by thermal decomposition in the prence of catalysts was investigated using a pyroprobe analytical pyrolyzer.The first step in this process is the thermal decomposition of the biomass to smaller oxygenates that then enter the catalysts pores where they are converted to CO,CO 2,water,coke and volatile aromatics.The desired reaction is the conversion of biomass into aromatics,CO 2and water with the undesired products being coke and water.Both the reaction conditions and catalyst properties are critical in maximizing the desired product lectivity.High heating rates and high catalyst to feed ratio favor aromatic production over coke formation.Aromatics with carbon yields in excess of 30molar carbon%were obtained from gluco,xylitol,cellobio,and cellulo with ZSM-5(Si/Al =60)at the optimal reactor conditions.The aro-
贾宝玉的性格
matic yield for all the products was similar suggesting that all of the biomass-derived oxygenates go through a common intermediate.At lower catalyst to feed ratios volatile oxygenates are formed including furan type com-pounds,acetic acid and hydroxyacetaldehyde.The product lectivity is dependent on both the size of the catalyst pores and the nature of the active sites.Five catalysts were tested including ZSM-5,silicalite,beta,Y-zeolite and sil-ica–alumina.ZSM-5had the highest aromatic yields (30%carbon yield)and the least amount of coke.
Keywords Catalytic pyrolysis ÁAromatics ÁZeolite catalysts
1Introduction
Due to its low cost and large availability,lignocellulosic biomass is being studied worldwide as a feedstock for renewable liquid biofuels [1–4].Lignocellulosic biomass is not currently ud as a liquid fuel becau economical process for its conversion have not yet been developed [1].Currently there are veral routes being studied to convert solid biomass to a liquid fuel,which involve multiple steps thus greatly increasing the cost of biomass conversion [5].For example,ethanol production from lig-nocellulosic biomass,involves multiple steps including:pretreatment,enzymatic or acid hydrolysis,fermentation,and distillation [2].Dumesic and co-workers have dem-onstrated that diel r
ange alkanes can be produced by aqueous-pha processing (APP)of aqueous carbohydrate solutions at low temperatures (100–300°C)[6].APP first requires that solid lignocellulosic biomass be converted into aqueous carbohydrates which would require pretreat-ment and hydrolysis steps.At high temperatures (*800°C),Dauenhauer et al.have shown that solid bio-mass can be reformed to produce synthesis gas through partial oxidation in an auto thermal packed bed reactor over Rh catalysts [7].The ideal process for solid biomass conversion involves the production of liquid fuels directly from solid biomass in a single step at short residence times.The catalytic fast pyrolysis process discusd in this paper comes very clo to this ideal process since solid biomass is converted directly into liquid fuels (aromatics)in a single reactor at short residence times (\4min).Fast pyrolysis involves rapidly heating biomass ([500°C s -1)
Electronic supplementary material The online version of this article (doi:10.1007/s11244-008-9160-6)contains supplementary material,which is available to authorized urs.
T.R.Carlson ÁG.A.Tomptt ÁW.C.Conner ÁG.W.Huber (&)
Department of Chemical Engineering,University of
Massachutts,159Goessmann Lab,Amherst,MA 01003,USA e-mail:huber@ecs.umass.edu
电视多少瓦Top Catal (2009)52:241–252DOI 10.1007/s11244-008-9160-6
to intermediate temperatures(400–600°C)followed by rapid cooling(vapor residence times1–2s)[8].The importance of pyrolysis heating rate is well known[9,10]. One of the chief advantages of fast pyrolysis is that liquid fuels,called bio-oils or pyrolysis oils,are directly produced from solid biomass.This technology is economical on the smaller scale where smaller distributed plants can be built clo to the location of the biomass[11,12].However,the bio-oils are of poor quality.They are thermally unstable, degrade with time,acidic,have a low heating value,and are not compatible with existing petroleum-derived oils [13].Bio-oils must be catalytically upgraded if they are to be ud as a conventional liquid transportation fuel [14–16].As we have previously shown introduction of zeolite catalysts into the pyrolysis process can convert oxygenated compounds generated from pyrolysis into aromatics[17].The purpo of this paper is to discuss in more detail aromatic production by catalytic fast pyrolysis of biomass-derived feedstocks.
2Experimental
Fast pyrolysis experiments were conducted using a model 2000pyroprobe analytical pyrolizer(CDS Analytical Inc.).The probe is a computer controlled resistively heated element which holds an open e
nded quartz tube (pictured in Fig.1).Powdered samples are held in the tube with loo quartz wool packing;during pyrolysis vaporsflow from the open ends of the quartz tube into a larger cavity(the pyrolysis interface)with a helium car-rier gas stream.
The carrier gas stream is routed to a model5890gas chromatograph(GC)interfaced with a Hewlett Packard model5972A mass spectrometer(MS).The pyrolysis interface was held at100°C and the GC injector temperature ud was275°C.Helium was ud as the inert pyrolysis gas as well as the carrier gas for the GCMS system.A0.5mL min-1constantflow program was ud for the GC capillary column(Restek Rtx-5sil MS).The GC oven was programmed with the following temperature regime:hold at50°C for1min,ramp to200°C at10°C min-1,hold at200°C for15min.Products were quantified by injecting calibration standards into the GC/MS system. All yields are reported in terms of molar carbon yield where the moles of carbon in the product are divided by the moles of carbon in the reactant.The aromatic lectivity reported is defined as the moles of carbon in an aromatic species divided by the total moles aromatic species carbon. Similarly,the oxygenate lectivity is defined as the moles of carbon in an oxygenated species divided by the total moles oxygenated species carbon.Carbon on the spent catalyst was quantified by elemental analysis(performed by Schwarzkopf Microanalytical Lab,INC).The missing carbon can be attributed to:non quantified therm
ally unstable oxygenated species(which cannot be detected in our experimental tup),and coking of the pyrolysis interface or transfer lines.
Powdered reactants were prepared by physically mixing the carbohydrate feed and the catalyst.For a typical run 8–15mg of reactant–catalyst mixture was ud.Both the feed and the catalyst were sifted to\140mesh before mixing.The physical mixtures of gluco were prepared with a ZSM-5(Si/Al=60,WR Grace)to D-gluco (Fisher)ratio of19,9,4,2.3,and1.5.Xyliol(Fisher)/ZSM-5.Cellobio(Acros)/ZSM-5,and cellulo(Whatman)/ ZSM-5with a catalyst to feed ratio of19were also tested. ZSM-5was calcined at500°C in air for5h prior to reaction.Samples with a catalyst:gluco ratio of19were also prepared with the following catalysts:Silicalite (Grace),b-zeolite,Y-zeolite(Si/Al=50,Degussa),and mesoporous SiO2-Al2O3(Si/Al=8,Davison).
3Results
3.1Chemistry of Catalytic Fast Pyrolysis
As shown in Fig.2catalytic fast pyrolysis first involves pyrolysis of solid biomass (e.g.,cellulo)into volatile organics,gas,and solid coke.The volatile organics undergo dehydration reactions to produce water and the dehydrated species.The reactions can occur in either the heterogeneous catalyst or in the homogeneous gas pha.The dehydrated species then enter into the zeolite cata-lyst where they are converted into aromatics,carbon monoxide,carbon dioxide,water,and coke.Inside the zeolite catalyst,the volatile species undergo a ries of dehydration,decarbonylation,decarboxylation,isomeriza-tion,oligomerization,and dehydrogenation reactions that lead to aromatics,CO,CO 2and water.The challenge with lectively producing aromatics is minimizing undesired coke formation.The coke formation comes from homo-geneous gas pha thermal decomposition reactions and from heterogeneous reactions on the catalyst.The coke can form from the biomass feedstock,the volatile oxygenates,the dehydrated species or the aromatics.As will be shown in this paper,high heating rates and high catalyst to feed ratio can minimize homogeneous coke formation.
Ligno-cellulosic biomass is compod of three compo-nents:cellulo,hemicellulo,and lignin [13].For this study we ud the compounds gluco,cellobio (dimer of gluco),cellulo and xylitol.The overall stoichiometry for conversion of xylitol and gluco to toluene,CO and H 2O is shown in Eqs.1and 2,respectively.Oxygen must be removed from the biomass as a combination of CO (or CO 2),and H 2O when aromatics are produced.The maximum theoretical molar carbon yield of toluene from xylitol and gluco is 76%and 63%,respectively,when CO and H 2O are produced as by-products.
C 5O 5H 12!12=22C 7H 8ð76%carbon yield)
þ26=22CO ð24%carbonyield)þ84=22H 2O
ð1Þ
C 6O 6H 12!12=22C 7H 8ð63%carbon yield)
þ48=22CO(36%carbonyield)þ84=22H 2O
ð2Þ
The hydrogen-to-carbon effective ratio (H/C eff )as defined in Eq.3is a way of comparing the relative amounts of hydrogen in different feeds [18,19].This metric can be ud to classify biomass feedstocks.Feedstocks with the same H/C eff ratio will have similar theoretical yields of aromatics.For example,cellulo,gluco and cellobio all have a H/C eff ratio of 0.All feedstocks with a H/C eff ratio of 0will have a molar carbon toluene yield of 63%if CO and water are the byproducts.The H/C eff ratio of biomass-derived feedstocks is significantly lower than petroleum feedstocks.For example,gluco,sorbitol and glycerol (all biomass-derived compounds)have H/C eff ratios of 0,1/3and 2/3,respectively.The H/C eff ratio of petroleum-derived feeds ranges from slightly larger than 2(for liquid alkanes)to 1(for benzene).H/C eff ¼渡的禅意
H À2O C
ð3Þ
Figure 3shows the carbon yields for catalytic fast pyrolysis of xylitol,gluco,cellobio and cellulo with ZSM-5.As can be en from Fig.3,the major products include aromatics,CO,CO 2and coke.No olefins were detected during catalytic fast pyrolysis in our reactor system.Olefins have been obrved when glycerol and sugars where pasd over ZSM-5catalysts in previous studies [20,21].X
ylitol has a higher yield of aromatics than the other feeds.The aromatic yields of the reactions are about half the theo-retical yield given by Eqs.2and 3.The yield of coke
is
Fig.2Reaction chemistry for the catalytic fast pyrolysis of cellulo on solid acid catalyst
有关雨的诗over30%for all of the catalysts.The coke can be burned to provide process heat for the pyrolysis reactor.
The aromatic distribution from catalytic fast pyrolysis of biomass-derived oxygenates with ZSM-5is shown in Fig.4.The feedstocks had a similar aromatic product distribution when tested under the same reaction condi-tions.The similarity of the aromatic distributions for the various feeds suggests that a common intermediate forms from all of the products.The aromatic lectivity decreas as naphthalene[[toluene[xylenes[ben-zene[substituted benzene*indane.
Naphthalene is the aromatic that is made in the highest yield.It is known that this larger poly-aromatic has very slow diffusion in ZSM-5[22]and therefore,it might be speculated that naphthalene is not formed within the pores. However,naphthalene has a sufficiently small kinetic diameter(*6.2A˚)tofit within the ZSM-5pore(*6.2A˚with Norman radii adjustment[23]),and furthermore at the elevated reaction temperature(600°C),the energetic bar-rier to diffusion will be decread.Hence,naphthalene is believed to be formed within the pores rather than on the surface.
3.2Heating Rate and Reaction Time
We ud gluco to determine how the reaction parameters affect the product lectivity.As shown i
n Fig.4the aro-matic distribution is similar for the all feeds suggesting that the other feeds will be similar to gluco.High heating rates are needed to avoid coke formation by homogeneous thermal decomposition reactions as shown in Fig.5.This figure shows product yields as a function of nominal heating rate with ZSM-5as the catalyst and gluco as the feed.As can be obrved from Fig.5,the maximum aro-matic yield and the lowest coke yield are obtained at the highest heating rate(1000°C s-1).The aromatic yield decreas by half and the coke yield increas from35%to 40%when the heating rate decreas from1000°C s-1to 1°C s-1.The aromatic lectivity is not a function of heating rate,for heating rates greater than50°C/s as shown in Fig.6.However,for lower heating rates the aromatic lectivity is a function of heating rate.The naphthalene lectivity decreas from57%to44%when the heating rate increas from1°C s-1to50°C s-1.At high heating rates the biomass spends a maximum amount of time at the reaction temperature thus maximizing the liquid yield. The results show the importance of the heating rate in obtaining high yields of aromatics.The heating rate
in
continuous catalytic fast pyrolysis reactors can be con-trolled by proper reaction engineering.
Figures7and8show the yields and aromatic lectiv-ities as a function of reaction time with gluco and ZSM-5 at600°C and the highest heating rate(1000°C/s).The time on both of thefigures is shown on a logarithmic scale,and varies from1s to240s.As can be obrved from Fig.7the rates of product formation are a function of time.The rate of aromatic production changes significantly as the reaction time increas from1to240s.In com-parison the rate of CO and CO2production does not change as much over this same time period as the rate of aromatic production.Thus,the gas leave the reactor faster than the aromatics.This is probably due to transport restrictions of the aromatics versus the CO and CO2since the aromatics absorb more strongly onto the zeolites than the CO and CO2.
After3s of time on stream the aromatic lectivity does not change with time as shown in Fig.8.However in the initial stages of reaction(1s of reaction time)the lighter aromatics(toluene,xylenes)are higher in lectivity than after3s.Again this could be to a combination of both transport and kinetic reasons.
3.3Catalyst-to-Feed Ratio
薪酬核算
森下仁丹
In addition to high heating rates,the product yields are also a function of the catalyst to biomass ratio.Figure9shows the product lectivity for catalytic fast pyrolysis of glu-co with ZSM5as a function of the catalyst-to-gluco
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