Chemical Engineering Journal 228 (2013) 535–544

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Chemical Engineering Journal

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Biomass gasification in supercritical and subcritical water: The effectof the reactor material

1385-8947/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.04.119

⇑ Corresponding author. Tel.: +39 0461 282692.E-mail address: luca.fiori@unitn.it (L. Fiori).

Daniele Castello a, Andrea Kruse b,c, Luca Fiori a,⇑a University of Trento, Department of Civil, Environmental and Mechanical Engineering, Via Mesiano 77, 38123 Trento, Italyb Karlsruhe Institute of Technology, Institute of Catalysis Research and Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germanyc University of Hohenheim, Institute of Agricultural Engineering, Conversion Technology and Life Cycle Assessment of Renewable Resources, 70593 Stuttgart, Germany

h i g h l i g h t s

� Gasification in subcritical (350 �C) and supercritical (400 �C) water was performed.� Glucose and beech sawdust at 15 wt.% were gasified without a catalyst.� Micro-autoclaves made of stainless steel and Inconel� 625 were compared.� Stainless steel showed higher H2 productions.� Inconel� 625 enhances CO methanation.

a r t i c l e i n f o

Article history:Received 7 January 2013Received in revised form 24 April 2013Accepted 29 April 2013Available online 21 May 2013

Keywords:Supercritical water gasificationHydrothermal conversionReactor material

a b s t r a c t

Batch reactors made of stainless steel and Inconel� 625 were used for the hydrothermal gasification ofglucose and beech sawdust under both subcritical (350 �C) and supercritical (400 �C) conditions at a pres-sure of 30 MPa. These tests were executed over residence times ranging from 60 to 300 min. The amountsof solid, liquid and gas produced and the composition of the resulting gas phase were measured. Theresults showed that a higher H2 output was achieved in the stainless steel reactors, while the Inconel�

625 reactors were more effective for the synthesis of CH4 and light hydrocarbons. A visual observationof the metal surfaces was performed using scanning electron microscopy to explain the persistence ofcatalytic activity even after an aging treatment and many hours of operation.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Biomass is among the energy sources that have been mostwidely considered as possible substitutes for fossil fuels. Comparedto fossil resources, biomass is much better distributed throughoutthe world, and if grown and handled in a sustainable way, it canhave a reduced or even no impact on the environment and globalwarming. Traditional thermo-chemical processes, such as combus-tion, pyrolysis and gasification, usually require dry feedstock forproper performance. This is an important drawback to the use ofbiomass because much of the available biomass, especially thatresulting from agro-industrial processes, is wet, with moisturecontents greater than 80%. To overcome these issues, hydrothermalprocesses have been proposed. Among these processes, supercriti-cal water gasification (SCWG) has been the focus of significantattention by the scientific community.

SCWG is an innovative technology for converting biomassinto valuable combustible gas, possibly with a large content ofhydrogen [1]. Such a process is based on the use of water aboveits critical point (T > 374.15 �C; P > 22.1 MPa). At these condi-tions, water exhibits unique properties, behaving in an interme-diate manner between a liquid and a gas. As a liquid, it retains ahigh density, resulting in a high reactivity and solvent power. Asa gas, it shows high diffusivity and the ability to mix with mostother gases in any proportions. Moreover, in water at the super-critical state, a significant change in the dielectric constant is ob-served. At such conditions, water behaves like a non-polarsolvent, eventually becoming capable of dissolving most organiccompounds [2].

SCWG can overcome several problems typically associated withtraditional gasification technologies. Wet biomass can be treateddirectly, without prior drying, paving the way to a feasible energyvalorization of many organic wastes and by-products. Moreover,the unique reaction environment drastically reduces the formationof char and tar, which is a crucial issue for the development of bio-mass gasification technologies [3].

Table 1Average composition of the metal alloys used in this study.

Ni Cr Mo Nb Mn Fe C Minor

Stainless steel 1.4571 11.5 16.4 2.1 – 1.5 67.8 0.04 0.7Inconel� 625 61.0 21.5 9.0 3.6 – 2.5 0.06 2.3

536 D. Castello et al. / Chemical Engineering Journal 228 (2013) 535–544

In the literature, several papers address sub- and supercriticalwater gasification [4,5], most of which involve tests with differenttypes of biomass in tubular continuous reactors [6–8].

Catalysis is often used in combination with SCWG because thetechnique’s feasibility requires a reduction in the energy demandof the process. A number of papers in the literature deal with thistopic; interesting reviews can be found in [9,10]. Some previous re-search has focused on determining the intrinsic reaction rateswhile operating the reactor in a non-catalytic environment ableto exclude any catalytic wall effects. This approach was introducedby a group from the University of Twente who developed a meth-odology based on quartz capillaries [11]. Through this technique,glucose and wood were gasified; when the catalyst was insertedin the capillary, the catalytic influence of ruthenium was clearlyobserved and distinguished from the non-catalyzed behavior[12]. A similar approach was followed by DiLeo and Savage [13].In their work, the authors performed experiments using quartzbatch reactors filled with methanol, with or without a nickel wire.In this way, they concluded that nickel catalyzes H2 formation andis subject to deactivation with use. In a subsequent work, Resendeand Savage performed similar experiments using cellulose and lig-nin as the feedstock and adding different metals in the form ofpowders or wires [14]. They confirmed that a catalytic effect tookplace, and they described differences between the various metals.

All of these studies reproduce the presence of the metal by add-ing a wire or a powder inside the reactor. Although this approachallows one to clearly understand the influence of a specific metal,it does not reflect the real-world situation in which the metal sur-face is comprised by the entire inner part of the reactor. Impor-tantly, some authors have reported that the reactor wall effectsare non-negligible in methanol SCWG [15,16] and biomass gasifi-cation [17]. Moreover, in industrial operations the reactor surfaceis exposed to the reaction conditions for several hours. It would,therefore, be interesting to observe if the reactor material contin-ues to play a catalytic role even after several runs. Previous studiesin the literature do not clarify this aspect in a direct way becausethey compare data obtained in different reactor types and underdifferent operating conditions [12]. Catalyst deactivation has beenshown to occur after a few cycles of several minutes each [13], butthe persistence of catalytic activity over a longer time period hasnot been reported.

Furthermore, all of the data available in the literature refer onlyto catalytic effects at supercritical conditions over residence timesranging from a few seconds to 2 h [18–20]. Longer residence timeshave been documented only at subcritical conditions, in which res-idence times of up to 10 days were adopted [21], but the evolutionof the gaseous products was not investigated. It would be useful toadopt much longer residence times to understand if such catalyticeffects only occur in the first minutes of the reaction, or if they alsoplay a role on a longer time-scale.

In this work, batch reactors made of stainless steel and Inconel�

625 were compared to see if catalytic wall effects significantly af-fect the gasification output. Two substrates were tested: glucose,which has been used in many SCWG studies as a model compoundfor cellulose, and beech sawdust, a common biomass, which is of-ten produced as a by-product of the furniture industry. Real bio-mass, such as beech sawdust, comes with ashes and salts, whichcan have catalytic effects of their own [22]. The combination ofsuch effects with those of the catalytic walls has not been investi-gated yet. Both sub-critical (350 �C) and supercritical (400 �C) con-ditions were considered. The tests were conducted at differentresidence times, ranging from 60 min to 300 min, to evaluate theeffect on those reactions and the processes taking place at lowrates.

The scientific novelty and advances of this work can be summa-rized as follows:

� Straight-forward comparison of hydrothermal experimentalresults obtained in stainless steel and Inconel� 625 reactors;� Long-term testing with residence times of up to 5 h and with a

focus on the reaction kinetics;� Study of the catalytic activity of the reactor walls even after

hundreds of hours of operation;� Critical comparison of results obtained with a model compound

(glucose) and with real biomass (beech sawdust);� Analysis through scanning electron microscopy (SEM) of the

reactor surfaces after hours of exposure to SCWG conditionsin the presence of common char- and ash-forming substrates;� Comparison of the catalytic reactor wall effects between sub-

and supercritical conditions.

2. Materials and methods

The work performed in this study involves a number of experi-mental tests conducted in small batch reactors. These devices wereused to perform more than 80 experimental tests, in which solid,liquid and gaseous products were sampled, quantified and ana-lyzed. To observe the behavior of the metallic surfaces, tests wereexecuted in which metallic burrs were added to the reacting mix-ture and observed using SEM.

2.1. Micro-autoclave preparation and reaction

The tests were executed in small-scale batch reactors, alsoknown as micro-autoclaves. These devices are small metallic ves-sels with a volume of 5 ml that are able to withstand high pres-sures. Because of their small volume, the reactors can be heatedquickly. The micro-autoclaves consisted of two hollow cylindricalelements to be screwed together. The reaction volume was theinternal cavity obtained by screwing together the two parts.Micro-autoclaves have been successfully used in previous studiesin the literature [18,19].

Two different micro-autoclaves were used, one made of stain-less steel and one made of Inconel� 625. This use of two materialsallowed a comparison to be made between the two materials andallowed the significance of wall effects due to the specific materialsto be assessed. Stainless steel was chosen because it is one of themost popular manufacturing materials in chemical plants. It isinexpensive, and it can handle the most severe reaction conditionsoccurring in the present study (400 �C and 30 MPa). The type ofstainless steel that was used was 1.4571, the main constituentsof which are iron, chromium and nickel in the amounts shown inTable 1. Inconel� 625 is a non-iron alloy made of nickel, chromiumand molybdenum (see Table 1 for further details of its composi-tion). Nickel-alloys are generally used in hydrothermal processesbecause they show great mechanical resistance to harsh reactionconditions, i.e., very high temperature and pressure. Furthermore,they have sufficient chemical resistance to corrosion [23]. How-ever, Inconel� 625 is much more expensive than stainless steel,making its usage particularly burdensome. For each material, twoidentical micro-autoclaves were adopted to ensure that the resultswere not affected by any peculiarities of the specific device.

The tests were performed with two substrates: glucose andbeech sawdust. Glucose can be considered a model biomass com-pound because it is the monomer of cellulose, one of the main con-

Table 2Elemental compositions (wt.%) of the substrates tested in this study.

C H O N S

Glucose 40.0 6.7 53.3 – –Beech sawdust 48.9 6.2 40.6 1.0 0.1

D. Castello et al. / Chemical Engineering Journal 228 (2013) 535–544 537

stituents of vegetal tissues. Moreover, many studies consider bio-mass to be a compound of carbon, hydrogen and oxygen (CxHyOz)[24,25], and this approach is consistent with the actual composi-tion of glucose. Furthermore, glucose has the advantage of a defi-nite composition, which helps in the understanding andinterpretation of the results. For this study, glucose monohydrate(Merck KGaA, Darmstadt, Germany) was used; to obtain the properamount of glucose, the hydration water of the glucose monohy-drate molecule was considered.

Beech sawdust is a ligno-cellulosic waste biomass produced inthe wood processing industry. In addition to cellulose and lignin,beech sawdust contains several other minor components, includ-ing Ca, Mg and K. Such components, usually simply referred to asashes, may play a significant role in the hydrothermal reactions.The beech sawdust used in this study consisted of particles witha mean linear dimension of 0.7 mm; the elemental composition,obtained by means of inductively coupled plasma (ICP) analysison an Agilent 7025 ICP-OES instrument, is reported in Table 2.

Before performing each series of experiments, the micro-auto-claves first underwent an ‘‘aging’’ treatment. This was performedto avoid the possible catalytic effect of clean, non-utilized reactorwalls, which is expected to be greatest when the reactor wallsare completely new. The aging treatment consisted of conductingreactions in the micro-autoclaves for a long time with a source ofcarbon at the same temperature and pressure as the subsequentseries of experiments in order to reproduce the conditions of areal-life reactor. The influence of such an aging treatment has al-ready been reported, e.g., in the work by Yu et al. [26].

In all of the experiments, both substrates were used at 15 wt.%,with the remaining part constituted by Milli-Q water. This is a highconcentration compared to other studies in the literature, whichtypically use greater dilutions [27–29]. Nevertheless, this high con-centration is more technically interesting because processes withhigh water/biomass ratios appear not to be energetically self-sustainable [30,31].

The amount of the mixture to be loaded in the reactor wasdetermined according to the steam tables [32] to ensure that after

Fig. 1. Schematic of the gas

the reactor was heated to the desired temperature, the systemcould reach the desired pressure. For this purpose, it was assumedthat the reacting mixture had approximately the same density aspure water. At 400 �C and 30 MPa, water has a density of357.6 g/l. Thus, because the internal volume of the reactor was5 ml, 1.78 g of the mixture was loaded in the autoclave. In the sameway, when the reaction temperature was 350 �C, the water densityof 644.0 g/l leads to a mixture amount of 3.22 g.

After being loaded with the desired reacting mixture, the micro-autoclaves were placed in a closed container and purged with N2 toremove oxygen and other atmospheric gases. By using a dynamo-metric key, the two parts of the reactor were fastened with a spec-ified torque (100 Nm) to ensure perfect sealing. The reactors werethen placed in a hot fixed-temperature oven. For this purpose, a gaschromatograph oven (HP series) was used because it allows goodtemperature control. The oven was pre-heated to the reaction tem-perature, and the micro-autoclaves were then placed inside it; thisapproach was used to achieve the fastest reactor heating rate andthereby minimize the effect of thermal transients. After the reac-tion, the reactors were quenched in ice to instantaneously stopany reactions.

2.2. Analytic procedure

For sampling, a device (Fig. 1) was adopted that consisted of asealed cell connected to a pipe that was closed at the oppositeend by a valve; a sampling volume with a septum was placed inthe middle of the tube. Once room temperature was reached, thereactor was placed in the container, and the device was purgedwith N2. The reactor was then opened inside the container, allow-ing the syngas to flow into the pipe. There, gas samples were takenfrom the septum using a 100 ll syringe. The samples were then in-jected into the gas chromatographs. After the gas sampling wasperformed, the gas volume was measured with a water gasometer.

The sample was first injected into a gas chromatograph Agilent�

6890A with thermal conductivity detector (TCD) and flame ioniza-tion detector (FID) using helium as the carrier gas and using Molsi-eve� 5A 80/100 and Porapak� Q columns. This apparatus enabledthe measurement of CO, CO2, CH4, C2H4, C2H6, C3H6, C3H8 andC4H10. The analytical method applied in this device, however, didnot allow the detection of small amounts of H2 (<20%) with ade-quate accuracy. For this reason, the sample was injected into an-other gas chromatograph, an Agilent� 5890 model with TCD,using nitrogen as the carrier gas and using a packed ShinCarbon�

ST 80/100 column.

sampling equipment.

538 D. Castello et al. / Chemical Engineering Journal 228 (2013) 535–544

After the gas sampling, the reactor was removed from the con-tainer and opened to collect liquid samples. The liquid inside thereactor was sampled with a 2 ml syringe, filtered using a0.45 lm round filter and collected. The total organic carbon(TOC) of the liquid sample was measured to estimate the amountof organic compounds dissolved in the water after the reaction. ADimatec� 2000 (Dimatec, Germany) analytical device, based onthe principle of thermo-catalytic oxidation with subsequent infra-red detection, was used for this purpose.

The solid, which had remained inside the reactor, was removedby washing the micro-autoclave three times with acetone andcarefully stirring with a glass stick. The liquid–solid mixture waspoured into a beaker along with the syringe and filter used for filtra-tion. All of these tools had been previously weighed on a PL 203-SMettler Toledo� balance (Mettler Toledo, Germany). The beaker,syringe and filter were then put in a dryer at 105 �C overnight,cooled with hygroscopic salts and weighed again. The solid contentwas determined as the difference between the two weights. Thesolid sample also underwent elemental analysis, performed usingICP.

2.3. Surface analysis

To explain why the inner surface of the reactors can producerelevant differences in the syngas composition (see Sections 3.1.1and 3.1.2) after the aging treatment and many experimental runs,direct observation of the reactor surface was performed. To ourknowledge, no image of the metallic surfaces undergoing SCWGwith char-forming substrates has previously been published inthe literature. However, it was not possible to directly collectimages of the surface of the reactor without destroying it. There-fore, small burrs were used that were made of the reactor material

Fig. 2. Mass balance for glucose gasification. (a) Mass yields of solid, (b) mass yie

and which were provided by the workshop that manufactured themicro-autoclaves.

The metallic burrs, two of stainless steel and two of Inconel�

625, were first washed carefully with acetone to remove the man-ufacturing oils. Next, they were reacted with a solution of 15% glu-cose during 7 runs of 16 h each, for a total duration of 112 h. Aftereach run, the reactor and the metallic piece were treated in exactlythe same way, namely, they were washed three times with acetoneand dried before the next run.

The four samples were then observed with a scanning electronmicroscope (FE-SEM DSM 982 Gemini; Carl Zeiss Ltd., Oberkochen,Germany) equipped with an annular high brightness in-lens sec-ondary electron (SE) detector for high resolution and true surfaceimaging. A laterally mounted SE detector (Everhart–Thornley-type)provided topographical contrast. Additionally, a highly sensitive4-quadrant solid state back-scattered electron (BSE) detector formaterial contrast and a diode-type transmitted electron (TE) detec-tor for thin specimens (STEM-in-SEM) were used (K.E. Develop-ments Ltd., Cambridge, UK).

The same samples were also observed before the test to providea point of comparison.

3. Results and discussion

For each biomass, balances are presented, showing the massyields of the gaseous and solid phases, as well as the distributionof carbon between the two phases; moreover, the masses of thegaseous products are shown. Subsequently, the liquid phase wastaken into account to highlight the molar content of C, H and O;this analysis contributes to understanding the nature of the possi-ble liquid products arising from gasification. Finally, the SEMimages of the metallic burrs used to simulate the inner reactor sur-faces are shown.

lds of gas, (c) carbon balance in the solid, and (d) carbon balance in the gas.

D. Castello et al. / Chemical Engineering Journal 228 (2013) 535–544 539

3.1. Gaseous and solid products

3.1.1. GlucoseFigs. 2a and b show the mass yields of the solid and gaseous

products, respectively. The yield is defined as the mass of the con-sidered phase divided by the mass of biomass fed. Generally, moresolid is obtained than gas under the experimental conditions. Moresolid is obtained at subcritical conditions (approximately 35%) thanat supercritical ones (approximately 25%). Conversely, more gas isproduced at supercritical conditions (approximately 20%) than atsubcritical conditions (<15%). This behavior demonstrates that anincreased temperature promotes the selectivity towards the gas-eous products. As is clear from the figures, most of the organicswere present in the liquid phase. Moreover, the amount of liquidwas not greatly affected by the reaction conditions and was alwaysapproximately 50%. Further details on the liquid phase are pro-vided in Section 3.2. The trend with time shows that the solidyields decreased slightly as the reaction proceeded, while the gasyields increased slightly. This behavior demonstrates that thereare slow reactions that occur on long time scales. This is surprisingbecause previous studies in the literature report that most reac-tions take place in the first few minutes of the residence time[21,29].

Inconel� 625 promoted gas production instead of solid forma-tion at both sub- and supercritical conditions. In our opinion,this is caused by the ability of the material to promote reactionsthat lead to the production of higher weight molecules, such asCH4.

It is interesting to compare the results of the mass balance(Figs. 2a and b) to those of the sole carbon balance, both in the solidand in the gas phases, shown in Figs. 2c and d. The first noticeabledifference is that the fraction of fed carbon found in the solid was

Fig. 3. Gas production for glucose gasification. (a) Hydrogen, (b) methane, (c) carbon diox

approximately 70% for sub-critical experiments and 55–60% forsupercritical experiments. A comparison with Fig. 2a highlightsthat carbon was largely concentrated in the solid phase. The carbonfractions in the gas (Fig. 2d) showed the same tendencies as the gasyields (Fig. 2b). As a consequence, the carbon fraction in the liquidranged from 20% to 30%, while the mass fraction, as previouslymentioned, was approximately 50%. More carbon was found inthe gas phase in the Inconel� 625 reactor (Fig. 2d).

This framework can be better explained if the focus is on thepermanent gases (H2, CO, CO2, CH4, C2H4, etc.) considered one-by-one. In Fig. 3a, H2 yields are shown; clearly, some reactions oc-curred during the hours-long residence time. Subcritical conditionsgenerally lead to lower H2 productions than do supercritical condi-tions. However, here, the reactor material plays a key role. Up to120 min of residence time, the Inconel� 625 reactor producedmore H2 than did the stainless steel reactor at both reaction tem-peratures. However, in the stainless steel reactor, the yield contin-uously increased with the reaction time, while in the Inconel� 625reactor, the yield was approximately constant. Starting at a resi-dence time of 180 min, the H2 production was much higher inthe stainless steel reactor. After 300 min. of reaction, the stainlesssteel reactor at 350 �C produced more H2 than did the Inconel� 625reactor at 400 �C.

The behavior of CH4 was completely different than that of H2.Fig. 3b shows that for both reactor materials, the CH4 yield alwaysincreased with time. However, although the two metals producedapproximately the same yields of CH4 at subcritical conditions,Inconel� 625 produced more CH4 than did stainless steel at super-critical conditions. At 400 �C and for a reaction time of 300 min, thedifference in CH4 yield between the two metals was smaller thanfor shorter reaction times as the CH4 production in the Inconel�

625 reactor stabilized.

ide, (d) carbon monoxide, (e) C2+ hydrocarbons, and (f) hydrogen balance in the gas.

540 D. Castello et al. / Chemical Engineering Journal 228 (2013) 535–544

A similar trend can be found in Fig. 3e, which shows the hydro-carbons with more than one carbon atom, the so-called C2+ species.The Inconel� 625 reactor produced more hydrocarbons than didthe stainless steel reactor, both at 350 �C and 400 �C.

Based on these results, the stainless steel reactor promotedthe production of H2, while the Inconel� 625 reactor promotedthe formation of hydrocarbons. It would be therefore interestingto assess which metal is able to transfer more hydrogen to thegas phase. This information can be obtained through a balanceon hydrogen, which is presented in Fig. 3f. Here, the amountof elemental hydrogen present in the gas, divided by the totalamount of hydrogen fed with the glucose, is reported. It canbe observed that at supercritical conditions, the Inconel� 625reactor converted more hydrogen into the gas phase than didthe stainless steel reactor despite its lower H2 production. Inother words, the Inconel� 625 reactor yielded more overallhydrogen in the gas, but predominantly in the form of hydrocar-bons (CH4 and other light C2+ compounds). The stainless steelreactor enhanced selectivity towards H2.

Useful information is provided by the CO yields (Fig. 3d). Here, anoticeable difference between the two reactors is shown. Thestainless steel reactor produced much higher amounts of CO atboth reaction temperatures than did the Inconel� 625 reactor; at400 �C, the CO production was higher than the production at350 �C, and at both reaction temperature the CO production alwaysdecreased with increasing reaction time. In the Inconel� 625 reac-tor, significant CO production only occurred at a 60 min residencetime. After that, the measured amounts often decreased below thedetection threshold of the gas-chromatograph.

These reported trends, especially those of CO, are crucial tounderstanding the different behavior of the two reactors. In

Fig. 4. Mass balance for beech sawdust gasification. (a) Mass yields of solid, (b) mass

hydrothermal processes, two main gas-phase reactions are pres-ent: the water–gas shift (WGS) reaction and CO methanation.

The WGS reaction occurs in the following manner:

COþH2O! CO2 þH2 ð1Þ

The WGS is a key-reaction under hydrothermal conditions, as ithas been reported that H2 production from hydrothermal biomassgasification occurs mainly due to the WGS reaction [33]. Under theconsidered reaction conditions, ‘‘ferrochrome’’ catalysts, i.e., cata-lysts mainly constituted by iron (Fe2O3) and chromium (Cr2O3) oxi-des [34], are effective in promoting the WGS reaction. Stainlesssteel, which is mainly an iron-chromium alloy, acts as a good cat-alyst for the WGS reaction. Nonetheless, the reaction can also beeffectively catalyzed by nickel to some extent [35].

The other key-reaction is CO methanation, which belongs to thefamily of hydrogenation reactions. Through this reaction, CO reactswith H2 to produce CH4:

COþ 3H2 ! CH4 þH2O ð2Þ

Similar reactions could be envisioned for the formation of higherhydrocarbons.

In the tests in the Inconel� 625 reactor, lower CO and H2 andhigher CH4 concentrations were measured after the reaction com-pared to the reaction in the stainless steel vessel, and this is consis-tent with the hypothesis that Inconel� 625 enhances the COmethanation reaction. Nickel is the most widely used CO methana-tion catalyst [36–38]. Because of its high nickel content, Inconel�

625 can effectively serve as a CO methanation catalyst.Finally, Fig. 3c shows the CO2 production. In this case, the Inco-

nel� 625 reactor produced higher yields than the stainless steelreactor. CO2 yields generally increased with the residence time.

yields of gas, (c) carbon balance in the solid, and (d) carbon balance in the gas.

D. Castello et al. / Chemical Engineering Journal 228 (2013) 535–544 541

At supercritical conditions, more CO2 was produced than at sub-critical conditions, for both reactors. However, the increase in theamount produced under supercritical conditions is essentiallydue to the larger quantity of gas produced. If the volumetric com-position of the gas is taken into account, the Inconel� 625 reactorat 350 �C yielded a gas with an average CO2 content of 86.0%, whileat 400 �C the CO2 content was 73.5%. This analysis also shows thatthe stainless steel reactor produced gases with lower CO2 concen-trations than did the Inconel� 625 reactor, namely, 74.7% at 350 �Cand 63.2% at 400 �C. A possible explanation is that Inconel� 625 ismore active in promoting the early decarboxylation reactionsthrough which organic molecules are converted into smaller inter-mediates by the loss of one molecule of CO2. This difference ispartly counterbalanced by the behavior towards the WGS reaction,which is favored by stainless steel and produces CO2. As a conse-quence, the difference between the two reactors decreased as theresidence times increased.

3.1.2. Beech sawdustThe analyses that were performed for glucose were repeated for

the beech sawdust. In this case, the biomass was initially present inthe reacting mixture as a solid. Thus, a preliminary step of dissolu-tion into liquid was required before gasification. Moreover, in thiscase heteroatoms, such as N and S, as well as potential catalysts,such as alkali ions, were present.

Figs. 4a and b report the mass balance, which highlights howthe biomass was distributed between the solid and gas phases.Similarly to the glucose tests, the Inconel� 625 reactor producedmore gas than did the stainless steel reactor, although the differ-ence between the two was very low in this case. The solid yieldsdecreased slightly with time but were essentially constant.

Fig. 5. Gas production for beech sawdust gasification. (a) Hydrogen, (b) methane, (c) carbthe gas.

In addition, no difference between the two reactor materialswas found in the carbon balance (Figs. 4c and d). The carbon frac-tion found in the gas (Fig. 4d) was approximately the same as whatwas found in the tests with glucose (Fig. 2d). In contrast, the car-bon fraction found in the solid (Fig. 4c) was generally lower thanwhat was found in the tests with glucose (Fig. 2c). This implies thatthe carbon present in the liquid phase when beech sawdust wasgasified was larger than what was found when glucose wasgasified.

The gas composition obtained with the beech sawdust is shownin detail in Fig. 5. As described in Section 3.1.1, the H2 yield mea-sured in the stainless steel micro-autoclaves was higher than thatmeasured in the Iconel� 625 reactors, especially at 400 �C (Fig. 5a).At 400 �C, after an initial equivalence at a residence time of 60 min,the H2 production in the stainless steel reactor increased from 1.7to 2.5 mg/g biomass, and it remained approximately constant withincreasing residence time. In contrast, the H2 produced by the Inco-nel� 625 reactor decreased with time (except at 300 min, where itincreased again). Fig. 4b shows that the total gas production in theInconel� 625 and stainless steel reactors was similar (although theformer yielded somewhat more), thus demonstrating that H2 wasconverted into hydrocarbons through hydrogenation reactions.This effect was also evident at subcritical conditions.

Higher CH4 amounts were produced from the beech sawdustcompared to glucose (Fig. 5b); at 400 �C, an average productionof approximately 15 mg/g was measured. The differences betweenthe two reactor materials were not so apparent in this case. Thesame reasoning can be applied to the C2+ hydrocarbons (Fig. 5e)and the hydrogen fraction in the gas (Fig. 5f).

These results reveal a strange phenomenon; while the H2 pro-duction reflects the catalytic effect of the reactor material, the

on dioxide, (d) carbon monoxide, (e) C2+ hydrocarbons, and (f) hydrogen balance in

542 D. Castello et al. / Chemical Engineering Journal 228 (2013) 535–544

production of other gaseous species does not. One possible expla-nation may be that, unlike glucose, beech sawdust contains a smallash content, which includes many salts. It is known that alkali saltsare effective in catalyzing the WGS reaction. This catalytic effect issuperimposed over that of the metallic vessel, and it may dominatethe reaction. The methanation reaction (Eq. (2)) is thus discouragedin favor of the WGS reaction. However, in the case of H2, the twoeffects act synergistically, and the differences are retained.

The behavior of CO is also interesting (Fig. 5d). The trends thatwere reported for glucose (Fig. 3d) were similar in this case; thestainless steel reactor produced more CO, especially at higher tem-peratures. However, the amount produced was much lower (max-imum of 5.5 mg/g, compared to over 30 mg/g for glucose).Nonetheless, CO was only found in significant amounts at a resi-dence time of 60 min. After that, it decreased (i.e., it reacted withother molecules) to a concentration below the detection thresholdof the measuring device. Therefore, virtually no CO was detectedafter a residence time of 120 min.

The disappearance of CO can be attributed again to a moreaccentuated WGS reaction, which occurs in the first reaction stepsand is catalyzed by the salts present in the biomass. Moreover, thelower CO amounts may be caused by a lower tendency of the feed-stock to lose oxygen through decarbonylation reactions.

The differences between the two reactor materials were alsonegligible in the case of CO2 (Fig. 5c). The trends were the sameas those observed with glucose, but the amounts of CO2 were high-er. This is consistent with the hypothesis of an enhanced WGSreaction. However, an increased decarboxylation, which producesCO2 from the conversion of organic molecules in the liquid phase,should also be taken into account.

3.2. Liquid phase

An interesting result is represented by the amounts of carbon,hydrogen and oxygen that originated from the feedstock and werefound in the liquid phase. This information can be acquiredthrough the elemental balances because the elemental composi-tion of the feedstock (Table 2), both gas and solid, and the carboncontent in the liquid are known. The amounts of oxygen andhydrogen transferred to the liquid phase were calculated by thedifference. Through analysis of the elemental molar ratios, thecompounds present in the liquid phase were estimated.

Table 3 reports the amount of C, H and O, originating from glu-cose and beech sawdust, found in the liquid phase. For glucose, C, Hand O were calculated in the approximate molar ratio of 1:12:6,while the feedstock had a ratio of 1:2:1. For beech sawdust, the ra-tio was approximately 1:5:1.5, and the initial biomass ratio was1:1.5:0.6. In both cases, a greater number of hydrogen and oxygenatoms for each atom of carbon were found after the reaction. It isunlikely that these molar ratios can be entirely imputed to organiccompounds. Small organic molecules, which are often found in theliquid, have different molar ratios. For example, the C:H:O ratio is1:2:1 in formaldehyde, 1:2:2 in formic acid, 1:2.4:0 in pentane, and1:1:0.5 in 5-HMF. As a matter of fact, the observed C:H:O ratios

Table 3Amount (mmol) of C, H and O, derived from glucose and beech sawdust, present inthe liquid phase (average values of five different residence times: 60, 120, 180, 240,300 min).

Reactor material and conditions Glucose Beech sawdust

C H O C H O

Inconel� 625 350 �C 2.1 25.5 11.8 3.6 16.1 5.1Stainless steel 350 �C 2.0 25.4 12.1 3.6 16.2 5.3Inconel� 625 400 �C 1.0 12.7 5.6 1.6 9.8 2.6Stainless steel 400 �C 0.8 13.5 6.1 1.7 9.6 2.8

cannot be achieved by combining these possible organic products;the numbers of H and O atoms are much greater than the numberof C atoms. These results suggest that under the experimental con-ditions used in our study, water is formed among the liquid prod-ucts; thus, H2O is a product rather than a reactant, as would beexpected under conditions of excess water.

A possible source for water formation may be glucose degrada-tion. As reported in [39], glucose results in different products whenit is degraded under hydrothermal conditions. A number of thesereaction steps take place through dehydration with the productionof water [40].

Table 3 shows that the water production was greater when glu-cose was used as the feed than when beech sawdust was used. Be-cause beech sawdust is a biomass composed of cellulose and lignin,it requires a prior dissolution step, which occurs with the hydroly-sis process [41]. For example, hydrolysis of cellulose occurs in thefollowing manner:

½C6H10O5�nþ1 þH2O! ½C6H10O5�n þ C6H12O6 ð3Þ

Thus, water is a product in the beech sawdust gasification, but alower quantity is formed because part of it was previously con-sumed during hydrolysis.

Water formation was previously reported to occur under sub-critical conditions [21,42], but no evidence of this process wasfound at supercritical conditions where, according to thermody-namics, water should be a reactant. A previous study by our groupconcerning the modeling of methanol SCWG [43] has postulatedsuch a phenomenon on a theoretical basis. In that work, the reac-tions leading to the ‘‘direct’’ formation of permanent gases (CO,CO2, H2, CH4) from the feedstock occurred with a net water produc-tion, which was later consumed by the WGS reaction.

The results presented in Table 3 clearly show that the mecha-nism of water formation was only slightly determined by the typeof metal; the outputs obtained with the two reactors were almostidentical. This information indicates that the catalytic activitylikely influenced the reactions of small molecules primarily, andit affected only the composition of the gaseous products that werefound after the reaction. This suggests that the catalyzed reactionsoccurred as a second step, after the first permanent gases wereformed. This is consistent with the aforementioned hypothesis ofWGS and methanation as the two main consecutive reactions.

The fact that water is a product instead of a reactant and thatboth solid and liquid compounds are found indicates that thermo-dynamic equilibrium is not achieved under these experimentalconditions. At thermodynamic equilibrium, only gaseous productsshould be present after conversion [24]. The catalytic action of thereactor walls is thus insufficient to overcome the strong kineticconstraints present under the reaction conditions, which likely re-quire a catalytic action not only for permanent gases but also in theprior steps.

3.3. Metal surface observation

Especially in the case of glucose gasification, the results demon-strate that large differences exist between the two reactor materi-als. Such differences are significant because they are present evenafter the aging treatment undergone by the reactors and manyexperimental runs. Therefore, the catalytic activity was furtherexamined.

Fig. 6 shows the SEM images of the metallic pieces that were re-acted with glucose (see Section 2.3). Figs. 6a and b show the metal-lic pieces before reaction. The two types of metal appeared quitesimilar. Both surfaces were characterized by very thin stripes,which were caused by the cutting machinery. Figs. 6c and d showthe surfaces after they were reacted at 350 �C for more than 100 h.

Fig. 6. SEM images of the reacted metallic surfaces with 15 wt.% glucose after 7 cycles of 16 h reaction each. (a) Inconel� 625 blank, (b) stainless steel blank, (c) Inconel� 625at 350 �C, (d) stainless steel at 350 �C, (e) inconel� 625 at 400 �C, and (f) stainless steel at 400 �C.

D. Castello et al. / Chemical Engineering Journal 228 (2013) 535–544 543

In both cases, a black layer was deposited on the metal, with smallcarbon spheres on top. A more detailed observation revealed thatthese spheres had a diameter of approximately 1 lm or less. Thesurface coverage by the carbon was not uniform. In the upper-leftpart of Fig. 6c, two metal spots can be noticed. In Fig. 6d, it isclearly visible that the carbon layer has detached from the bottommetal, revealing a shiny metal region.

Figs. 6e and f show the surfaces after reaction at 400 �C. At thispoint, the carbon layer was less uniform than the layer observed at350 �C. The carbon formed more aggregates of smaller spheres,leaving more exposed metal zones. This tendency was more evi-dent for stainless steel than for Inconel� 625.

Despite the differences, the two metal surfaces were not cov-ered uniformly with the carbon material, and some metallic spotswere still exposed to the reactive environment, thus exerting a cat-alytic activity that persists over a long-time.

4. Conclusions

In this study, several tests of supercritical water gasificationwere performed with the model compound, glucose, as well aswith beech sawdust. The experimental methodology, based on

metallic micro-autoclaves, included the conduction of tests undera variety of operating conditions, thus enabling an understandingof the influence of the reactor material on the reaction. The reactorwall catalytic effects turned out to be significant even after hun-dreds of hours of operation. This finding indicates that the indus-trial implementation of SCWG cannot neglect a carefulconsideration of such an effect.

Under the experimental conditions considered, some reactionstook place over long time scales. These reactions included gasphase reactions, which were mainly represented by the WGS reac-tion and CO methanation.

The use of a nickel alloy resulted in slightly higher gas produc-tions. Inconel� 625 promoted hydrogenation reactions, such as COmethanation (which consumes CO and H2 and produces CH4 andother light hydrocarbons). However, the use of a stainless steelreactor resulted in higher H2 yields, which can be attributed tothe ability of this material to catalyze the WGS reaction. Thesebehaviors only occur at supercritical conditions; under subcriticalconditions, the two materials gave similar results. The catalyticactivity affected primarily the permanent gases, whose composi-tion is affected by the action of the catalytic reactor surface. Con-versely, the solid and dissolved compounds (i.e., those found inthe liquid phase after conversion) were not influenced by the reac-

544 D. Castello et al. / Chemical Engineering Journal 228 (2013) 535–544

tor material. However, future work should investigate the actualcomposition of these condensed phases to determine if further cat-alytic effects occur.

Glucose and beech sawdust behaved differently; when beechsawdust was gasified, the differences between the two reactorswere less apparent than when glucose was gasified. This differencewas attributed to the role of salts, which are naturally present inreal biomass and affect the gasification outputs via their own cat-alytic action. This influence is combined with that of the reactormaterial. Further work is needed to understand the extent of suchactions and the reaction mechanisms involved.

Acknowledgments

The authors would like to thank the technical staff at the Insti-tute of Catalysis Research and Technology in Karlsruhe for theirvaluable contributions, especially Mrs. Birgit Rolli, Mr. ArminLautenbach, Mr.Wilhelm Habicht, Mr. Matthias Pagel and Mrs.Alexandra Boehm. Special thanks are also due to the EuropeanCommission for funding Mr. Castello’s mobility in the LLP ErasmusPlacement program.

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