A Critical View on Catalytic Pyrolysis of BiomassR. H. Venderbosch*[a]

1. Introduction

Fast pyrolysis is the rapid heating of biomass in an oxygen-freeenvironment to optimize the yield of liquids. It offers a rapidand efficient means to depolymerize lignocellulosic biomass.Among the primary thermochemical conversion routes, fast py-rolysis is one of the most economically feasible ways to con-vert biomass into liquids, and has, therefore, attracted a lot ofinterest in the past two decades. Unfortunately, the liquidproduct contains a significant amount of reactive, oxygenatedspecies and is a mix of organic acids, (dehydrated) carbohy-drates, aldehydes, ketones, (depolymerized) lignin fragments,aromatics, and alcohols. For that reason the use of these liq-uids is up to now limited to combustion, and in the futurelikely gasification.

If the liquid is to be used in other applications, specificallyto replace hydrocarbons, these reactive oxygenates—precur-sors for gas and coke—represent significant challenges. Re-moval of these oxygenates requires catalysts, and processes in-vestigated include the use of catalysts directly in the pyrolysisstep (in situ), either in-bed when catalysts are used in the fastpyrolysis process itself or ex-bed in case a reactor is connectedto the pyrolysis process, to convert the non-condensed pyroly-sis vapor. Herein, the in-bed and ex-bed options are referred toas indicated in Figure 1.

For more than three decades catalytic pyrolysis has beenseen as one of the most promising routes to yield liquids witha better quality than the “original” pyrolysis liquids, wherequality is usually related to a (substantially) lower oxygen con-tent. It seems that in particular Mobil’s MTG process—convert-ing methanol to gasoline over ZSM5-type catalysts—inspiredresearchers. Use of catalysts in fast pyrolysis reportedly hashigh potential, as it enables the production of hydrocarbonsdirectly from biomass or liquids with higher quality than the

conventional in relation to its use in subsequent (upgrading)processes. The clear advantage is an integrated process, inwhich re-evaporation of the liquids can be avoided. A purelytheoretical, but commonly referenced, equation for the oxygenremoval to yield (aromatic) liquids, CO2, and water, amongstothers, is provided by Bridgwater:[1]

C6H8O4 ! 4:6 CH1:2 þ 1:4 CO2 þ 1:2 H2O ð1Þ

Coke formation is neglected and all carbon loss is reflectedonly as CO2, yielding a theoretical yield of 42 wt % and an ener-getic yield (HHV) of near 100 % [see also Eq. (4)] .

The earliest reference may be the Occidental Research Cor-poration process, in which pyrolysis vapors are passed overZSM5.[2] In Europe, a first project started in 1995 with the ob-jective to produce higher quality oils from biomass throughcatalytic pyrolysis (JOR CT95-0081). This project was followedby “Biocat” (ENK6-CT-2001-00510) in the period of 2001–2005.In hindsight, measurable objectives were absent or unclear,and analysis protocols not standardized. Surprisingly, even ele-mental analysis of liquids (including oxygen) was not carriedout routinely, but instead the deoxygenation was related toCO2 production. In both these projects, the use of catalysts inthe pyrolysis process resulted in significant lower carbon

The rapid heating of biomass in an oxygen-free environmentoptimizes the yield of fast-pyrolysis liquids. This liquid compris-es a mix of acids, (dehydrated) carbohydrates, aldehydes, ke-tones, lignin fragments, aromatics, and alcohols, limiting itsuse. Deoxygenation of these liquids to replace hydrocarbonsrepresents significant challenges. Catalytic pyrolysis is seen asa promising route to yield liquids with a higher quality. In thispaper, literature data on catalytic fast pyrolysis of biomass are

reviewed and deoxygenation results correlated with the overallcarbon yield. Evidence is given that in an initial stage of thecatalytic process reactive components are converted to coke,gas, and water, and only to a limited extent to a liquid prod-uct. Catalysts are not yet good enough, and an appropriatecombination of pyrolysis conditions, reactive products formed,and different reactions to take place to yield improved qualityliquids may be practically impossible.

Figure 1. In situ and ex situ possibilities suggested for the use of catalysts toimprove pyrolysis liquids.

[a] Dr. R. H. VenderboschBTG Biomass Technology Group B.V.Josink Esweg 34, 7545 PN, Enschede (The Netherlands)E-mail : venderbosch@btgworld.com

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yields, with a sometimes substantial lower oxygen level in theliquid. However, no significant breakthrough was obtained inthe area of catalyst or process development.

In the period 2001–2007 (catalytic) pyrolysis was of less in-terest for industries, government, and researchers due to possi-bilities to use vegetable oils and sugars for the production offirst-generation transportation fuels. Since 2007, when petrole-um prices increased and the interest in second-generationfuels increased, a revival is noted in pyrolysis in general (andspecifically catalytic pyrolysis). Since then a growing number ofuniversities, research institutes, and organizations are againworking on the development of catalytic pyrolysis technology,justified by the possibility that the product is compatible withpetroleum feedstocks or that aromatics [benzene, toluene, andxylene (BTX)] can be produced in sufficient yields.[3] In Europe[4]

and the US,[5] various projects are ongoing, possibly inspiredby the claims made by the company KiOR.[6]

In 2010, ground breaking activities in catalytic pyrolysis wereannounced by KiOR, a joint venture of Khosla Ventures (KV)and the Dutch BIOeCON, reporting the construction ofa 500 BDT day�1 (BDT = bone-dry ton) commercial productionunit (approx. 40 000 t per year fuel). The technology was basedon the idea of oxygen removal by fluid catalytic cracking(FCC)-derived catalysts in a modified fluid catalytic cracker. TheR&D behind KiOR was initiated by BIOeCON in 2006–2007, sug-gesting the impregnation of biomass with salts to reduce thecracking temperature. Its activities included involvement of re-searchers from Spain, Greece, USA, and The Netherlands. In2012, the construction of the Columbus plant based on a newconcept was completed, but the excitement was only fora rather short moment: in May 2014, KiOR’s facility wasbrought to a safe, idle state[7] and in November 2014, the com-pany filed for bankruptcy.

These developments justify a critical review of literature dataon the yields obtained in the catalytic pyrolysis of biomass.Several reviews have been prepared in the last few years ontypes of catalysts and reactor systems applied, but none so faractually focused on the liquids yield in relation to the quality.The present paper aims to provide a first approach to analyzethe existing experimental literature data related to yield and

deoxygenation, and will provide recommendations for futurework to improve tools. This includes suggestion for a minimumof analysis data to be reported in scientific publications.

2. Review of Literature Data

During the last years, a proliferation can be noticed in articlesand papers published on catalytic pyrolysis of biomass. Scien-ceDirect, a leading scientific database of nearly 2500 journalsand 26 000 books, lists a total of over 11 000 papers related toa combination of key words ‘catalytic’, ‘pyrolysis’, and ‘bio-mass’. Over 1600 were published in 2014 compared to a totalof 736 for the total period before 1995, which shows the stillincreasing interest of researchers in catalytic pyrolysis. Thistrend is further illustrated in Figure 2, in which the number of

publications is presented as a function of the year of publica-tion. In the last year, a few reviews have been prepared on cat-alytic pyrolysis.[8]

The information provided in most of these papers is usuallyrestricted to the effects of the catalyst, the product yields,sometimes providing elemental contents, but, in general, limit-ed or no specific details are presented on the product charac-teristics (viscosity, molecular weights, its tendency to char, andso on) to justify the better quality. In most of these papersa common assumption is that the oxygen content of the liquidis directly related to its quality, thus justifying the use of cata-lyst in the fast pyrolysis, but, surprisingly, information on theoxygen content in the oil is not always provided.[8d] Besides,oxygen reduction in itself cannot be the only single criterion:methanol and ethanol contain 50 and 25 wt % oxygen, respec-tively, and both fuels are regarded as high value (and quality).

Clearly, the oxygen content in the oil provides some clues tothe quality of the liquid, but it should be looked at very care-fully. Partial polymerization of pyrolysis liquids effectivelylowers the oxygen content, but also yields a very viscous(sometimes even solid) product. As important, oxygen reduc-tion comes with a (high) price. Herein, it will be demonstratedthat the overall biomass-to-liquid carbon yield is directly relat-

R. H. (Robbie) Venderbosch carried out

his Ph.D. research at the University of

Twente on the role of clusters in gas–

solids reactors. He then carried out

a post-doc at the same university on

the fast pyrolysis of biomass, before

joining BTG Biomass Technology

Group B.V. He is working on a range of

topics such as fast pyrolysis, hydroge-

nation reactions, sugar-based chemis-

try, supercritical gasification in water,

production of chemicals from biomass

(derivatives) and more. His research interests concern bio-based

applications, with emphasis on heterogeneous catalysis in relation

to chemical reaction engineering.

Figure 2. Number of publications in the last 20 years (‘catalytic’ + ‘pyroly-sis’ + ‘biomass’).

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Minireviews

ed to the oxygen reduction. Thecarbon yield hC is defined by themol of carbon in the feed that isretained in the preferred (organ-ic) product. The actual oxygenreduction xo is based on theoxygen content compared withthat obtained by a thermal reac-tion only. These two parametersare defined as follows:

xO ¼ 1� Ooil;daf

Othermal oil;daf

� �� 100%

hC ¼nC;oil;daf

nC;feed;daf

� �� 100%

ð2Þ

where nC,i,daf represents themoles of carbon in oil and feedrespectively, Ooil,daf is the oxygencontent of the liquids producedover a catalyst, and Othermal oil, daf

the oxygen content reported bythe authors for the liquids de-rived in an uncatalyzed fast py-rolysis run. All data are providedon a dry-ash-free basis (daf). Clearly, the carbon yield can alsobe calculated on basis of the uncatalyzed oil as well or theoxygen reduction on basis of the feed; however, similar con-clusions to the ones presented below will be obtained. It mustbe noted that the definition for the deoxygenation rate here isdifferent from the one recently proposed, in which it is relatedto the absolute number of oxygen mol retained in the liquid.[9]

This latter definition is rather trivial as very low bio-oil yieldswith high oxygen content in bio-oil also lead to a highoxygen-removal degree number.[8a]

In addition to liquids (potentially valuable), by-productsfrom catalytic pyrolysis include gaseous hydrocarbons, some-times referred to as olefins and representing ethylene, propyl-ene, and butylene. The yield of such olefins can be significant(hC up to 20 wt %[10]), but in this Minireview, this gas-phasecontribution is not taken into account in the carbon balance.

2.1. Biomass catalytic pyrolysis

From the wealth of publications provided by ScienceDirect,only a few provide sufficient information on the residualoxygen content and related overall carbon yield. In Table 1these references, further restricted to the use of woody bio-mass as a feed, are summarized, from which the respective fig-ures have been constructed. A typical plot that effectivelyshows the relation between xo and hC is presented in Figure 3.For a large variety of catalysts, temperatures, processes, andprocess conditions, it suggests that there are two distinct re-gions: a region I, where the carbon yield is drastically reducedwhile the oxygen content remains relatively high (mild deoxy-

genation), and a region II where substantial deoxygenation ispossible at the expense of carbon (severe deoxygenation). Theresults are rather consistent, but some remarks can be made:

· The data presented are rather conservative as in mostcases only the yields and oxygen contents of the organic prod-ucts have been reported and the (usually oxygenated) organicspresent in the aqueous phase are neglected. Addition of theseoxygenates to the liquids will increase the carbon yield butalso reduces the overall deoxygenation rate.

· Data considered are taken irrespective of catalyst type, op-erating conditions, catalyst-to-oil ratios, biomass feed type, re-actor type (ex-bed or in situ), and so on. A further division inthese individual parameters results in an even more distinctpattern, but, for the sake of simplicity, will not be taken intoaccount here.

To estimate the carbon loss in terms of energy, Figure 3 alsoshows the so-called ‘isoenergetic’ curves. These dashed curvesrepresent the theoretically energetic yields (hE) of 10 %, 25 %,50 %, 75 %, and 100 % efficiency, defined by the energeticvalue of the liquid product divided by the energetic value ofthe feed material, both heating values arbitrarily defined bythe higher heating value:

hE ¼ 1� HHVoil;daf

HHVfeed;daf

� �� 100% ð3Þ

For simplicity, the HHV value here is calculated by the Milneequation [Eq. (4)][11] neglecting ash, sulfur, and nitrogen con-tent:

Figure 3. Compilation of literature data for the deoxygenation of biomass-derived liquids in catalytic pyrolysis pro-cesses, with the oxygen reduction related to the oxygen content of uncatalyzed oil and the carbon yield, repre-sented by the ratio between the amount of carbon in the liquid products and the carbon in the biomass, seeEquation (2): the solid lines given are calculated on basis of Equations (1), (5), and (6) whereas the broken linesrepresent isoenergetic lines calculated from Equation (3).

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Minireviews

HHVi ¼ ð0:341 wt %C þ 1:322 wt %H�0:12 wt %OÞ � 100 ð4Þ

In these isoenergetic calculations, the carbon content of theliquid produced is taken as the balance, as the oxygen is givenby the deoxygenation rate and the hydrogen content is as-sumed. As a first simplification, the hydrogen content is takenlinearly with the conversion of the biomass, from 6.2 wt % forzero conversion to 11.5 wt % in a fully deoxygenated product.This level is rather arbitrary, but simple calculations show thatother values (from 6 to 10 wt %) or a hydrogen content in-creasing linearly with a decreasing oxygen content do nothave major effects on the overall trends of the curves.

In Figure 3, three additional curves are presented. The firstone is the theoretical curve provided by Equation (1), wheredeoxygenation is presumably taking place linearly, rangingfrom 0 to 100 % conversion. The theoretical curve closely fol-lows the curve for hE = 100 %. Clearly, the thus calculatedcarbon yields do not come close to any of the experimentaldata points. The experimental yields are much lower, specifical-ly as carbon and gas yields are much higher: an optimisticcase is presented by Equation (5), constructed by addressingthe experimental yields for the carbon and the ratio betweenCO2 and H2O:

C6H8O4 ! ð1�xÞC6H8O4

þx ðCH1:8 þ 0:45 CO2 þ 3:1 H2Oþ 4:55 CÞð5Þ

Here, x represents the biomass conversion, ranging from 0 to1.

Clearly, also Equation (5) overestimates the carbon yield. Alower trend line can be predicted only if it is assumed thata significant part (here 65 %) of the biomass is instantaneouslyconverted to carbon and water (and does not take place inany further reaction), while the remaining 35 % is converted ac-cording to Equation (5). Overall, Equation (6) can be construct-ed in Figure 3 for 0<x<1:

C6H8O4 ! ð3:9 Cþ 2:6 H2OÞ þ 0:35 C6H8O4

! ð3:9 Cþ 2:6 H2OÞ þ 0:35ð1�xÞC6H8O4

þx ð0:14 CH1:8 þ 0:07 CO2 þ 1:3 H2Oþ 1:9 CÞð6Þ

In the ideal case, deoxygenation in the catalytic pyrolysisoccurs though removal of CO2 and H2O, as a result of whichmost of the hydrogen and carbon is retained in the liquid. InFigure 3, this case would have been reflected by a low carbonloss at a high deoxygenation rate [see Eq. (1) for oxygen re-moval by CO2 and H2O], and a steep slope in the deoxygena-tion line. The reality, however, is a worst-case scenario, namelyan initially high carbon loss associated with low deoxygenationrates. The two boundaries in this plot are (i) uncatalyzed pyrol-ysis (hc; xO) = (65 %; 0 %), and (ii) full deoxygenation (hc; xO) =

(15 %; 100 %). Depending on the process severity (defined bythe deoxygenation rate), carbon yields for liquids with oxygencontents <10 wt % (or 75 wt % reduction) are well below25 wt %. To retain higher carbon yields in the liquids, less deox-ygenation is thus allowed, the latter usually associated with

similar or worse product properties compared to the originalpyrolysis liquids (such as high viscosity, residual water, highacid number, and alike).

This general conclusion from the data points in Figure 3 isnot new or very surprising: already in the nineties, an exten-sive set of data was provided for a two-staged catalytic pyroly-sis process of biomass. In papers by Williams and Horne (1994–1996), pyrolysis vapors were produced in fluid beds withoutcatalyst, and further conversion of the vapors took place overa packed bed catalyst.[12] Data for the catalytic conversion ofpyrolysis liquids over FCC or zeolite catalysts were publishedaround the same time by Adjaye et al. (1995).[13] The organicyields for the catalytic pyrolysis of biomass over acidic conven-tional FCC catalysts are far less than 20 wt %,[14] whereas someminor optimization could be achieved by varying the actualprocess conditions. The patterns shown in Figure 3 have notbeen presented elsewhere yet, but trends can be concludedfrom the early experiments of Williams and Horne. For thesake of clarity, these are separately presented in Figure 4.Again, the oxygen reduction and carbon yield are based onthe ratio between product and feed obtained by thermal proc-essing. The curve trend is convincing: an initially high carbonloss is associated with low deoxygenation rates, while the pre-ferred deoxygenation step occurs when most of the carbon isalready converted into less valuable products, gas, and char.Trends are indicated by solid lines similar to Figure 3, and ap-parently the low efficiency trend line is followed, with resultingbiomass-to-liquid carbon yields even less optimistic than thatprovided for the data in Figure 3.

In addition to the negative trend of deoxygenation versuscarbon yield, it is also apparent that most literature data showa relatively high residual oxygen content, on the order of 10–20 wt %. Only in a few cases complete deoxygenation isindeed achieved. Supposedly, only if fresh and very reactivecatalysts are applied, and at relatively severe operating condi-tions, oxygen contents <1 wt % are reached. In practical appli-cations, a so-called ‘spent’ catalyst will be used, which is a cata-lyst regenerated by burning off the coke. This spent catalyst isusually less active than the fresh one, even more so as the ashfrom biomass can quickly affect the catalyst’s structure andcomposition. Full deoxygenation in the catalytic pyrolysisduring continuous operation is not likely.[8d]

Values obtained by KiOR in large-scale processing are notpublished in the open literature, but a recent patent applica-tion by KiOR aims at hydrotreating processes using feedstockswith a residual oxygen content of up to 18 wt %,[16] suggestingthat the residual oxygen contents in the commercial BFCC isaround that value. These oxygenates probably include residualphenols (typically 10–30 wt % oxygen), which are much morerecalcitrant to deoxygenation and require severe conditions.

It is postulated that in an initial stage of the process themost reactive components are converted to coke (with associ-ated gas and water) and only to a limited extent to the pre-ferred liquid products. It is likely that these highly reactivecomponents are or at least include the (dehydrated) carbohy-drates in the case of a pyrolysis liquid and (holo-)cellulose-de-rived fragments when solid biomass is fed as these carbohy-

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drates are the least stable. At the increased reaction severities(reflected by the higher oxygen reduction levels) cracking of(some of) the more stable components, and likely the ligninfraction, is taking place, eventually yielding hydrocarbons. Thelatter usually contains a larger BTX fraction.

Liquids from thermal pyrolysis already experienced a crackingstep but also contain significant amounts of (dehydrated) car-bohydrates. It is interesting to note what occurs if these liquidsare further cracked over zeolite, and data for the conversion ofpyrolysis liquids over a variety of catalysts in packed bed weretabulated.[13b] Interesting in these experiments is that the con-version of the liquids could be carried out at rather low tem-peratures (290–410 8C). The dataare summarized in Figure 5, fol-lowing the same methodologyas in Figures 3 and 4. Althoughthese researchers did not pro-vide information on the elemen-tal composition of the producedliquids, rough but rather reliableestimates can be estimated fromthe overall product distribution(assuming that acids are aceticacid, aromatics have a carbon-to-hydrogen ratio of 1, and soon). For the overall carbon yield(biomass-to-liquid), a carbonyield of 70 % has been assumedin the prior pyrolysis step(weight yields from biomass toliquids were 74 %).

Again, similar trends as forbiomass are observed, with over-all biomass-to-liquid carbonyields well below 30 wt % at

rather low deoxygenation rates.Clearly, molecular structures arepresent in the liquid similar tothose originally in the biomass,providing indirect proof thatduring the fast pyrolysis theproducts are trapped beforethermodynamic equilibria can bereached.

Figures 3–5 show that zeolitecatalysts (and specifically thefresh ZSM5 ones) are too reac-tive to selectively convert suchoxygenated materials, and thealternative is to apply lower tem-peratures and/or (much) loweramounts of reactive catalyst ma-terial. In case of biomass, tem-peratures over 400 8C are essen-tial to break down the solidstructure, and lowering the tem-perature is not an option. In

a quest for a better catalytic process, steam pyrolysis in thepresence of catalyst is applied, suggesting that steam affectsthe yield and composition and promotes the oxygen removalfrom the liquid.[18] These data are not included in Figures 3–5,but for a better comparison, are separately plotted in Figure 6.The solid symbols represent catalytic pyrolysis of biomassunder steam pyrolysis conditions whereas open symbols areused for the uncatalyzed runs.

The plot indeed shows a high carbon yield related to a signif-icant deoxygenation rate, and one might conclude that itoffers serious potential. Remarkably, however, the results of theuncatalyzed experiments almost perfectly coincide with those

Figure 4. Representation of data from Williams and Horne.[15] See Figure 3 for the explanation of the legends.

Figure 5. Representation of the oxygen reduction as a function of overall (biomass-to-product) carbon yield incase of feeding pure pyrolysis liquid over a catalyst bed.[13, 17]

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of the catalytic experiment, suggesting that catalysis does notplay a significant role. The catalyst seems essentially inactive.Apparently, a different mechanism plays a role compared tothat of the zeolite catalysts. This mechanism is dehydration in-stead of a preferred decarboxylation. Generally, dehydrationleads to products with high viscosity, large molecular weight,and an oil quality even less than that of the uncatalyzed pyrol-ysis liquids. Unfortunately, not enough information is providedby the researchers to justify the approach as a lower-viscosityoil is obtained; average molecular weights presented, however,suggest that severe polymeri-zation occurred.

3. Discussion and Fur-ther Outlook

The results presented in the liter-ature do not provide ample evi-dence that on a micro, mesoand macro level, catalytic pyroly-sis provides sufficient benefitscompared with uncatalyzed fastpyrolysis.

On a catalyst level, the mostlikely primary mechanism takingplace is the formation of cokeon the very active catalyst sur-face before any other reactionleading to hydrocarbons cantake place. Coke defined here isthe carbon deposited on the cat-alysts and is different from thecarbon-containing solid char par-ticles derived after pyrolysis ofbiomass. The gases evolved from

these coking reactions are fur-ther reacted with the catalystsurface to potentially form (par-tially) deoxygenated com-pounds. As an overall reaction,dehydration is observed, a reac-tion promoted by the high tem-peratures and acidic catalyst sur-face. This can be further eluci-dated by comparing the O/Cand the H/C ratios of the catalyt-ic pyrolysis liquids.

To take into account the varie-ty of feeds used, the ratios of O/H and H/C for the catalytic pyrol-ysis liquid and the uncatalyzedexperiment, respectively, areused here (see Figure 7).Although the data are ratherscattered in most of the publica-tions, a common trend is noted,namely that the decrease in the

O/C versus the H/C ratios follows the dehydration slope. This ismore clearly noted for the data generated using one catalystunder different conditions and less for the papers where differ-ent catalyst are applied (for example Stefanidis et al. , 2011[19]).For comparison, the results in Figure 7 for the individualpapers are plotted in Figure 8.

In most cases, the H/C ratio for the catalytic oil is substan-tially higher than expected on the basis of the dehydrationline, which, as no hydrogen is fed to the system, which meansthat carbon rejection is significant as no hydrogen is fed to the

Figure 6. Representation of data for steam pyrolysis in catalyzed (open symbol) and in uncatalyzed processing(closed symbols).[18]

Figure 7. Comparison of O/C and H/C ratios for catalytically derived pyrolysis liquids. The solid line represents thedehydration line.

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system. Together with the low overall carbon yield at low de-oxygenation rates (thus limited COx formation), this clearlypoints to carbon rejection by formation of coke—char. Consid-ering the many reactions that can take place at different tem-perature levels for the various oxygenates, it is fair to postulatethat the catalysts so far tested in catalytic pyrolysis (apart fromcatalyst stability and effects of biomass contaminants) are notselective enough to steer towards either the preferred reac-tions (the latter include decarboxylation, cracking, aromatiza-tion, and alike).

Removal of carbon through decarboxylation might indeedtake place (as is noted by high CO2 yields reported in all litera-ture references), but, (i) not to the extent one wishes to obtain,(ii) probably only at the initial stage of the pyrolysis process,(iii) not at the temperature range the catalysts are supposedlyactive, and (iv) at a rate lower than removal of carbon as coke.

On the product side, catalytic pyrolysis can clearly yield liq-uids with oxygen contents lower than those of the convention-al uncatalyzed pyrolysis liquids. However, and contrary to whatis expected from general literature reviews, only in a few casesis full deoxygenation of the liquid indeed experimentally ob-tained, and the general view on the use of catalytic pyrolysis

for the production of oxygen-free liquids may be overly opti-mistic. The data summarized in Figures 3 and 4 suggest a maxi-mum value for xO of around 75 %, with only two publicationsreferring to full deoxygenated liquids. This deoxygenation ratecorresponds with a residual oxygen content in the liquid ofnear 10 wt %, a value that might allow further refining withoutsubstantial problems. This unfortunately comes at the expenseof carbon, and carbon yields (from biomass to liquid) betweenonly 10 and 20 wt % are realistic. More than 80 % of the carbonfed is thus converted in a less valuable by-product (gas, char,coke), in practice probably to be combusted (or gasified). Incase of a less active catalyst (and in the presence of steam),higher carbon yields are obtained, and the products havea lower oxygen content. In the latter case, it is not clear wheth-er the products obtained are of better quality than the ther-mally produced oils or are similar to oils derived from pyrolysisby dehydration (for example at high temperatures and pres-sures[20]).

However, the oxygen content of the liquid is not per sea measure for the quality of the liquids. Upon deoxygenationof liquids through dehydration, substantial amounts of oxygencan be removed, but the viscosity of the liquids increases dra-

Figure 8. Comparison of O/C and H/C ratios for catalytically derived pyrolysis liquids. For the data see the legend in Figure 7.

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matically as in case of such a thermal treatment. This is almostcertainly the case in the steam pyrolysis experiments listed inFigure 3 and this might in general be true for any liquid prod-uct prepared through catalytic pyrolysis. The oxygen content,if not removed below 10–15 wt %, might not be the key per-formance indicator to look for in the development of catalysts.

Although carbohydrates can be converted into hydrocar-bons,[21] a certain extent of oxygen defunctionalization shouldbe aimed at, specifically in the case of carbohydrate(s). At thesame time, this defunctionalization should occur while retain-ing the potentially high overall carbon yield of uncatalyzed py-rolysis. These objectives pose sufficient challenges in itself, asvery reactive carbohydrate fractions formed in the pyrolysis re-action (and present in the produced pyrolysis liquids) must beselectively converted at the high temperatures required tocrack the biomass. The following additional objectives are con-sidered important but less so as they could also be obtainedin subsequent processing:

· Reducing the (average) molecular weight (to an estimated200–500 Da region);

· Reducing the water content (<10 wt %, by reducing thepolarity of the liquids), and

· Increasing the high H/C ratio.It is likely that less active catalysts in combination with less

severe process conditions are required that reduce the carbonloss while allowing a large enough oxygen reduction to bebeneficial in further processing. The catalysts studied in litera-ture, including solid acids (FCC, zeolites, silica–alumina, alumi-na, etc.), metal oxides (such as zinc oxide, zirconia, ceria, andcopper chromite), and other inorganic materials (alkali, metalchlorides/phosphates/sulfates) are not good enough. Althoughcatalysis may yield a pyrolysis liquid with less increase in vis-cosity upon standing, no clear oxygen reduction or defunction-alization is noted.[22] Similarly, impregnation of thebiomass with potassium or magnesium salts canreduce the decomposition temperature of the bio-mass, but does not reduce the oxygen content or itsacidity.[23] On the contrary, magnesium promotes de-hydration reactions, leading to a more viscous prod-uct.

On an overall process level, the typical carbonyields at high severity are close to the original lignincontent of the wood. It can be suggested and justi-fied that catalytic pyrolysis appears to favor the pro-duction of deoxygenated products from the ligninfraction only, whereas the carbohydrates are convert-ed to coke, gas, and water. This hypothesis requiresfurther proof, as there are also indications that aro-matics can be produced from cellulose.[24] Experi-ments might be done by taking the pyrolysis ligninseparated from the pyrolysis liquid as a feed materialto show elevated yields of aromatics. Figures 3 and 4show that carbon yields of near 50 wt % (correspond-ing to the 92 gallons per BDT referred to by KiOR) areunrealistic and far too optimistic on basis of the liter-ature data provided here. Carbon yields at completedeoxygenation levels will probably be lower than

20 wt %, values close to the 30 gallons per BDT published in2014. Although no detailed information is provided by twoother companies claiming that they will commercialize catalyt-ic fast pyrolysis within the next few years, Annelotech andBioBTX, it seems likely that these figures will be realistic. In theex-bed option, the temperatures of the fast pyrolysis and thefurther catalytic deoxygenation of the vapors can be individu-ally controlled, but the results shown in Figures 3 and 4 forthese options are remarkably similar.

Assuming the low carbon yields to liquids, well below20 wt %, ‘severe’ catalytic pyrolysis of biomass to liquid fuels isprobably barely economically feasible. It might be reasonableonly if the focus is redirected towards high-value-added chemi-cals (of which BTX might be an example, but also the olefinswill have a certain value if they can be properly handled at thepyrolysis site) or when the conversion of the initially morestable lignitic fraction is aimed at.

A better option is the more controllable way to catalyticallydeoxygenate (or to defunctionalize) such liquids at tempera-tures at which the thermal pathways are mostly absent. So far,hydrotreatment of the liquids, or an integrated approach sug-gested by GTI (the so-called integrated hydropyrolysis and hy-droconversion (IH2) seem better alternatives. The latter (IH2)includes the in-bed catalysts and hydrogen in a high pressurefast pyrolysis process, combined with ex-bed hydrotreatingover hydrogenation catalysts. Figure 9 presents an overview ofyields and deoxygenation rates for these processes:

· Fast pyrolysis followed by a hydrotreatment (carbon yieldson the order of 35–45 wt % at 100 % deoxygenation rates).[25]

· The IH2 process yielding values between 43 and 47 wt %for wood and seaweed.[26]

· KiOR, reporting 30 gallons per dry ton biomass (near17 wt % carbon yield).

Figure 9. Comparison of yields for fast pyrolysis, catalytic pyrolysis, conventional fast py-rolysis followed by hydrotreatment, KiOR’s published yield, and yields reported for theIH2 process.

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Finally, in literature quite some efforts have been devoted todetailed analysis of pyrolysis liquids; uncatalyzed as well as cat-alytic. It can be stated that the chemical products should beanalyzed to the full extent possible in an effort to movebeyond existing insights. Some of these analysis tools may beuseful but sometimes too complex for the current state of de-velopment. As a recommendation, and to allow a much bettercomparison of data from literature, it is suggested that at leastthe following, rather straightforward, analysis on the liquids isprovided:

· Elemental analysis (at least carbon, hydrogen, and oxygen)of feed and all products obtained from catalytic pyrolysis anda clear representation of yields and elemental composition interms of as received or dry ash free. One simple option is todewater the liquids before further analysis (for example byvacuum distillation, allowing a minor error in the overallcarbon balance due to the loss of small organic components);preferably a complete elemental balance for the completeproduct slate is provided.

· Gel permeation chromatography (GPC) analysis to provideinformation on the molecular weight distribution of the oils :Complete GPC plots are relevant as only weight or number-averaged molecular weights do not provide sufficient informa-tion on the sample. In addition, care should be taken in theactual analysis method while using liquids derived from thefast pyrolysis of biomass.[27]

· Data on the charring behavior of the liquid product; a rep-resentative parameter in this area is the micro-carbon residuetest unit, a standardized and easy tool to evaluate the charringtendency by micro carbon residue testing (MCRT) or by ther-mogravimetric analysis (TGA).

Acknowledgements

The author specifically would like to thank Erik Heeres (Universityof Groningen, The Netherlands) and Wolter Prins (Ghent Universi-ty, Belgium) for their input. In addition, to get additional data oncarbon and oxygen yields, some of the researchers referred to inthis paper were contacted, and they all were very willing toassist. The author is specifically grateful to Nii Ofei Mante (Virgin-ia Polytechnic Institute and State University, US), Angelos Lappas(CERTH, Greece), and Efthymios Kantarelis (KTH, Sweden) for theirvaluable input.

Keywords: biomass · carbon yield · catalytic pyrolysis ·review · transportation fuel

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Received: January 24, 2015Revised: February 24, 2015Published online on && &&, 0000

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R. H. Venderbosch*

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A Critical View on Catalytic Pyrolysisof Biomass

The pitfalls of pyrolysis: Catalytic py-rolysis is seen as a promising route toyield liquids with a higher quality. How-ever, in an initial stage of the catalyticprocess, reactive components aremainly converted to coke, gas, andwater, and to a limited extent toa liquid product. Catalysts are not goodenough, and finding ones to yield im-proved quality liquids may be practicallyimpossible.

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