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Fuel 86 (2007) 2410–2415

Biomass gasification in supercritical water: Part 1. Effect ofthe nature of biomass

Jale Yanik *, Steve Ebale, Andrea Kruse *, Mehmet Saglam, Mithat Yuksel

Institue for Technical Chemistry, Division of Chemical-Physical Processing, Forschungszentrum Karlsruhe,

Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

Received 20 June 2006; received in revised form 22 January 2007; accepted 30 January 2007Available online 27 February 2007

Abstract

In this study, biomass feedstocks, including lignocellulosic materials and the tannery wastes, were gasified in supercritical water. Gas-ification experiments were performed in a batch autoclave at 500 �C. The amount of gases, the gas compositions and the amount of watersoluble compounds from gasification were determined. The hydrogen yields ranging between 4.05 and 4.65 mol H2/kg biomass have beenobtained. The results showed that the yields and composition of gases depend also on the organic materials other than cellulose andlignin in lignocellulosic material. In addition to this, it was concluded that the kind of lignin may also have an effect on gasification prod-ucts. In the case of tannery wastes, the type of tannen agent used in leather production considerably effected the gasification results.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Biomass; Supercritical water gasification; Hydrogen

1. Introduction

Hydrogen is an important raw material in petrochemicaland chemical industry. Today there is a growing interest inthe use of hydrogen as fuel. In most industrial hydrogenproduction processes, the hydrocarbons derived from fossiloil are used as raw material. However, hydrogen produc-tion by these ways cannot be considered a highly efficient‘‘clean’’ and sustainable way because it is contributing tothe increase of CO2 emissions.

From environmental and economic viewpoints, newways of hydrogen production, using a renewable and non-pollutant source, have being studied in the last years. Oneof the processes proposed is biomass gasification. Biomassis renewable and abundantly available in the world. Sinceplants fix the CO2 from the atmosphere during theirgrowth, the concentration of carbondioxide in the atmo-

0016-2361/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.fuel.2007.01.025

* Corresponding authors. Permanent address: Department of Chemis-try, Faculty of Science, Ege University, 35100 Izmir, Turkey (J. Yanik).Tel./fax: +90 232 3888264.

E-mail address: jale.yanik@ege.edu.tr (J. Yanik).

sphere would remain constant if biomass is used. In addi-tion, biomass based technologies reduce the dependenceof foreign petroleum.

One method for hydrogen production is the steamreforming of biomass. The major problem in gasificationby steam reforming is the formation of tars and char asthe biomass does not react directly with steam at atmo-sphere pressure [1]. Recently, biomass gasification in near-and supercritical water has been received much attention inconversion of biomass into combustible gas. For wet bio-mass containing large amounts of water up to 90%, super-critical water gasification (SCWG) appears as a usefultechnology. The main advantage of gasification in super-critical water is the high solid conversion, i.e. low levelsof char and tars. Although, conventional gasification yieldsin synthesis gas, SCWG produces the H2 and CO2 at tem-peratures of 500 �C or higher. Because of the high waterexcess, up to half of the H2 formed originates from water,the conversion of CO with water into H2 and CO2.

In order to evaluate hydrogen production by SCWG ofvarious types of biomass, extensive experimental investiga-tions have been conducted in recent years. Model

Table 2Composition of lignocellulosic biomass (wt%)

B1 B2 B3 B4 B5 B6

Cellulose 48.5 27.0 47.1 43.1 31.7 33.8Lignin 11.1 3.0 11.0 9.7 3.4 9.3Hemicellulose 16.2 27.1 13.1 7.4 31.7 10.9

J. Yanik et al. / Fuel 86 (2007) 2410–2415 2411

compounds such as cellulose, lignin, glucose and glycerol[2–11] have been widely tested to get information on thechemistry of biomass gasification in supercritical water.On the other hand, gasification of the real biomass suchas sawdust and different starches [12], clover grass and cornsilage [13], baby food and zoo mass [14] in supercriticalwater was also investigated. A recent review on biomassgasification in near- and supercritical water was given byMatsumura et al. [15].

This paper aims to give new results for the gasificationof eight different types of real biomass in supercriticalwater.

2. Materials and methods

2.1. Materials

In this study, eight different types of biomass were used.Six of them were agricultural wastes (lignocellulosic mate-rials), the others were leather wastes (vegetable and chro-mium tanned). These were tobacco stalk (B1), corn stalk(B2), cotton stalk (B3), sunflower stalk (B4), corncob(B5), oreganum stalk (B6), chromium-tanned (B7) wasteand vegetable-tanned waste (B8) were used. The biomasseswere crushed until a particle size less than 200 lm in a cryo-genic mill was reached. The composition of biomasses isgiven in Tables 1 and 2.

2.2. Experimental setup

Experiments were carried out in an Inconel 625-lined,tumbling batch autoclave (volume of 1 l), using three ther-mocouples that were inserted internally into the reactor.

Table 1Properties of biomasses

B1 B2 B3

Proximate analysis (wt%)

Moisture 3.2 2.9 3.6Ash 7.3 6.7 4.5

Ultimate analysis (dry, wt%)

C 40.60 39.70 42.20H 5.70 5.90 6.00N 0.80 1.50 0.70S 0.49 0.29 0.39O (from difference) 39.68 44.86 45.71Na – – –K 7.480 3.480 2.900Ca 3.370 0.949 0.738Mg 0.0840 0.109 0.143Al 0.121 – –Fe 0.142 0.014 0.007P 0.129 0.372 0.091Si 0.264 1.180 0.030W 0.213 0.584 0.470Cl 0.924 1.060 0.617Cr

Details about the reactor were presented in a previousstudy [16].

For each experiment, 8.3 g of biomass (as received) and140 ml of water were loaded into the reactor. This amountcorresponds approximately to 5 wt% of biomass in the sus-pension. After the loading of suspension, the autoclave wasswept by N2 to remove the air in reactor. Then the reactorwas heated to 500 �C at a heating rate of 3 �C/min and washeld at this temperature for hour. At the end of the reac-tion, the reactor was cooled to room temperature underatmospheric conditions. After cooling, the amount of gasesformed was measured by a gasometer. The aqueous solu-tion was filtrated and samples of the gaseous, liquid andsolid products were analyzed.

2.3. Analysis: gaseous products

Gas analysis was carried out by two gas chromato-graphic procedures. An HP 5889A gas chromatograph[GC; with nitrogen as the carrier gas and equipped with aPorapak Q column (80/100 Porapak Q by Supelco,1.83 m length) and a thermal conductivity detector] wasused to determine the hydrogen content. For analysis ofall other gases, an HP 6890 GC with column switchingand helium as the carrier gas was used. Here the first col-umn was 80/100 Hayesep Q (2 m length, by Resteck),and the second column was a 60/80 Molesieve 5 A (4 m

B4 B5 B6 B7 B8

4.9 6.3 9.0 9.0 5.311.2 2.1 4.0 9.0 4.1

36.10 42.90 42.50 37.2 46.205.30 6.40 6.00 6.40 5.501.30 0.60 0.70 13.60 6.600.59 0.29 0.29 1.79 1.29

38.95 45.46 42.16 40.84 36.87– – – 0.958 0.5037.910 2.400 5.240 0.047 0.9814.910 0.117 1.360 0.156 0.5920.236 – 0.166 0.133 –0.111 – – – –0.196 – 0.018 0.114 0.1320.225 0.388 0.191 0.025 0.0311.150 0.234 0.225 0.054 0.0790.143 0.617 – 0.214 0.6202.880 0.594 1.150 4.110 0.575

– 4.350 0.021

2412 J. Yanik et al. / Fuel 86 (2007) 2410–2415

length, by Resteck). The two columns as well as thermalconductivity and flame ionization detectors were connectedin series. The second column was bypassed by a six-portvalve for analysis of CO2 and hydrocarbons.

2.4. Analysis: liquid products

A commercial TOC analyzer (Rosemount DohrmannDC-190) is used to determine the residual total organic car-bon (TOC) content in the aqueous phase. The amounts ofdifferent phenols are determined colorimetrically (reactionof phenols with 4-nitroaniline to a yellow complex) withthe photometer Cadras 200 by Lange-Hach.

The amounts of organic acids and furfurals were mea-sured by an ion chromatograph equipped with a high-per-formance liquid chromatograph pump, an AminexTMHPX-87 H column by Biorad for organic acids, anRP-18 column by Merck for furfurals, and an L-4250UV–Vis detector by Merck.

3. Results and discussion

In the gasification of biomass, the yield and compositionof formed gases and other products depend on the gasifica-tion conditions and the composition of biomass. In thisstudy, the reaction conditions such as the ratio of biomassto water, heating rate, reaction temperature and periodwere chosen as based on previously studies [16–18].

Biogas gasification in SCW involves a series of complexreactions which effects each other. Many studies in litera-ture provide the information on the reaction mechanismsfor model compounds such as glucose, cellulose and ligninin SCW. But, for real biomass, little is known on the reac-tion mechanism, especially regarding the interaction ofeach component of biomass. This paper mainly focusedon the investigation of hydrogen production from somewaste materials by SCWG.

3.1. Products from SCWE

Table 3 gives the gasification results obtained with dif-ferent biomasses. In the case of lignocellulosic biomass,highest amount cokes were obtained from B3 and B6, eventhough SCW gasification deals with fewer amount ofchars/coke and tars compared with ‘‘dry processes’’.Chars/coke may originate from not decomposed biomassby a solid–solid conversion. On the other hand some partsof aromatic compounds such as lignin or other unsaturated

Table 3Product distribution from SCWE of different biomass feedstocks (T = 500 �C

Materials B1 B2 B3Maximum pressure (bar) 278.5 335.8 275.8

Products

Gas (g gas/kg biomass) 247 534 418Coke (g coke/kg biomass) 55 66 104

species, are eventually polymerized to tar and char materi-als. Saisu et al. [10] reported that, in conversion of lignin inSCW, at first decomposition of the lignin by hydrolysis anddealkylation yielded low-molecular-weight fragments hav-ing reactive functional groups and compounds. Then, across-linking reaction among these fragments and residuallignin gave higher-molecular-weight fragments. In other-words, SCW promoted the conversion of lignin not onlyto lower molecular weights but also to higher molecularweights. As expected, fragment species depends on the lig-nin structure as well as gasification condition. In addition,in the SCW conditions, the resistance to degradation of lig-nin is linked to its cross-linked structure [19]. Takuya Yos-hida [2] compared the gasification characteristics amongfive different lignin species. Experimental findings showedthat each lignin had different gasification characteristics.Similarly, Ando et al. [20] decomposed three kind of plantbiomass (bamboo, chinquapin, Japan ceder) in hot com-pressed water and found that the structure differencescould affect the gasification characteristics of biomass.Although lignin content of Japan ceder was similar to thatof chinquapin, the residue yield of Japan ceder was signif-icantly higher than that of chinquapin.

In this study, although B6 and B4 have similar lignincontent, the amount of coke obtained from B6 was fivetimes more than that of from B4. Similarly, B3 gave twotimes more coke than that of from B4 even their lignin con-tents were identical. This results show that coke formationin biomass gasification due to the lignin not only dependson the lignin amount and also strongly depends on thestructure of lignin and interactions between other compo-nents in biomass.

3.2. Products in gas phase

In gasification of lignocellulosic biomass, B2 and B4 gavethe highest amount gas yields. The gas products composedof hydrogen, carbondioxide and methane as major compo-nents and carbonmonoxide, methane, ethane, ethene, pro-pane and propene as minors. It must be noted that, verylow amounts of carbonmonoxide are obtained from all bio-mass. Volume fraction of carbonmonoxide in gas productsranged between 0.23 and 2.56 v/v%. Most of the carbon-monoxide produced was expected to come from water solu-ble organic compounds that were the first products fromlignocellulosic material gasification. And it was consumedthrough water–gas shift reaction ( CO + H2O = CO2 + H2)and methanation reaction (CO + 3H2 = CH4 + H2O). It

; sample amount, 8.3 g; water, 140 ml; * sample amount, 1.6 g)

B4 B5 B6 B7* B8*

308 287.6 241.2 249.3 314.5

592 340 390 165 51919 21 113 175 237

J. Yanik et al. / Fuel 86 (2007) 2410–2415 2413

should be noted that methanation reaction is only possiblein the presence as a suitable catalyst [17].

The effect of alkali salts on gas yields is well known frompervious studies carried out with model compounds.Accordingly, alkali salts could increase the H2 and CO2

yields and decrease the CO yields, catalyzing the water–gas shift reaction [21]. But, in this study, although B1and B4 have similar potassium content, the yields and com-position of gases obtained from B1 and B4 are significantlydifferent from each other. In addition to the similarity ofpotassium content, cellulose, hemicellulose and lignin con-tent of two biomasses are also similar.

These findings indicate that organic materials other thancellulose, hemicellulose and lignin may have also effect theyield and composition of gas products, as in coke yield.Concerning the hydrogen yield, gasification of B5 resultedin lowest amount of hydrogen yield (2.09 mol H2/kg bio-mass) (Fig. 1), even it has high amount of cellulose plushemicellulose.

In the case of tannery wastes, both the amounts of gasand coke yields were significantly changed with wastetypes. Vegetable tanned waste produced a higher amountof gas and coke than chromium tanned waste. We maymention that the gasification yields varied depending onthe type of tannin reagent. Vegetable tannin agent consistsof phenolic compounds. As seen from their contents (Table1), B8 contained more amount of carbon, because of thestructure of tannin agent used. On the other hand, B7 con-tained considerable amount of Cr.

Although, to our knowledge, no study on the gasificationof leather in SCW has been published so far, the studiesrelating to hydrolysis of proteins to amino acids and the deg-radation of amino acid showed that these reactions are rel-atively fast reactions in sub- and supercritical water [22–24].

0

2

4

6

8

10

12

14

16

18

20

B1 B2 B3 B4

Yie

ld, m

ol g

as/k

g bi

omas

s

Fig. 1. Gas yields from diffe

For hydrothermal degradation of proteinaceous wastes,the suggested simplified reaction pathway [22] is

Proteins! Amino acids! Low-molecular-weight

carboxylic acid! End products ðformic acid; acetic

acid; etc:Þ

The general reaction network of amino acids takes twomain paths, namely decarboxylation to produce carbonicacid and amines, and deamination to produce ammoniaand organic acids. The ratio of deamination/decarboxyl-ation differed depending on the type of amino acid. In addi-tion, the pH of reaction media has an effect on the reactionpathway of amino acid decomposition [23]. The degrada-tion of organic acid to produce mainly CO, CO2, H2 andwater.

Taking into account of the complex reactions in degra-dation of proteins, the fact that the chromium led to theformation of different degradation products may beexpected.

3.3. Products in the liquid phase

The aqueous liquid phase obtained from gasification ofbiomass contains a huge number of compounds. Accordingto purposed mechanism of cellulose decomposition in SCWby Kruse et al. [17], cellulose is hydrolyzed to sugar units(e.g., glucose and fructose) further decomposed to acidsand alcohols of 1–3 carbons. Part of glucose degraded tofurfurals, which are condensed to phenols and dehydratedto acids. All of these compounds are highly reactive andreadily cracked to gases. In this study, some of the organiccompounds which are the key compounds in degradationof cellulose were determined in aqueous liquids (Table 4).

B5 B6 B7 B8

CH4

CO2

H2

rent biomass feedstocks.

Table 4Content of aqueous liquid phase

Material B1 B2 B3 B4 B5 B6 B7 B8

pH 8.21 6.96 7.27 6.82 7.14 7.49 8.52 7.88TOC (ppm) 1174 1354 1932 1182 1661 1947 531 589Phenol (mg/l) 29.6 540 468 520 49.7 50.6 10.2 46.7HMF (mg/l) 3.56 8.01 8.03 3.13 5.56 6.43 <1 (0.5) <1.00FU (mg/l) 2.70 3.40 3.12 3.58 2.03 <1.00 <1 (0.5) NilMF (mg/l) <0.1 (0.5) <1.00 1.04 3.58 <0.10 <1.00 <1 (0.6) <1.00Formic acid (mg/l) 54.0 68.8 48.3 82.0 62.4 135.9 22.1 8.1Acetic acid (mg/l) 107.0 167.2 324.5 54.8 323.7 393.8 52.9 44.6MeOH (%) 0.03 0.02 0.05 0.02 0.04 0.06 Nil <0.03Dioxan (ml/l) 4.28 6.76 4.93 17.50 3.20 11.40 8.01 5.91HAc (mg/l) 5.8 1.5 56.6 5.1 2.2 55.0 5.1 –

HMF, hydroxymethyl furfural; FU, furfural; MF, methyl furfural; HAc, hydroxyacedic acid.

Cellulose

Glucose/fructose

Furfurals

PhenolsAcids/aldehydes

Gases Higher molecular weight products

III

III

Fig. 3. Presentation of the decomposition of cellulose in supercriticalwater [17].

2414 J. Yanik et al. / Fuel 86 (2007) 2410–2415

For lignocellulosic biomasses, the highest phenol yieldswere obtained from B2 and B4 (8.5 and 7.5 g phenol/kgbiomass, respectively) (Fig. 2). By considering that phenolsare decomposed more slowly than other small aliphaticcompounds from degradation of cellulose [17], we can con-cluded that route III (Fig. 3) also occurred considerably inthe case of B2 and B6. However, in the gasification of B1,B5 and B6, degradation pathway was mainly processedthrough route I and II. It should be noted that the totalyields of carboxylic acids, phenols and furfurals from B1gasification was extremely low, although TOC content ofaqueous phase was not much low. This indicates that someother products, which were not determined, have beenformed rather than major products.

In the case of leather wastes, the yields of acetic acidwere more than that of other key compounds. Althoughvegetable tannen agent mainly consists of phenolic com-pounds, the yield of phenols is not much higher after gas-

0

2

4

6

8

10

12

14

B1 B2 B3 B4

g li

quid

pro

duct

/kg

biom

ass

Fig. 2. Yields of major products in aqueous p

ification of B8 than that of B7. The organic carboncontents in the aqueous phases were much more than that

B5 B6 B7 B8

acetic acid

formic acid

furfurals

phenols

hase from SCWE of different biomasses.

J. Yanik et al. / Fuel 86 (2007) 2410–2415 2415

of in the case of cellulosic biomass. Organic carbon yieldswere 64.18 and 46.02 g organic C/kg biomass for B7 andB8, respectively, whereas 14.14–25.60 g organic C/kg bio-mass for lignocellulosic biomass. As in the gasification ofB1, although the organic carbon yields in aqueous phasewere high, the sum amount of determined organic com-pounds was not high.

It seems that chromium present in B7 led to an increasein acetic acid content and decreased the formation of for-mic acid or increased the degradation of formic acid. Over-all, it increased the organic carbon content in aqueousphase.

4. Conclusion

The gasification of lignocellulosic and tannery wastesleads to formation of gases consisting of mainly hydrogen,carbondioxide and methane. The gas obtained fromtobacco stalks contained the highest amount hydrogen(39.47%), although tobacco stalks produced relatively lowgas yield. In addition to this, the amount of the key com-pounds in aqueous phase obtained from tobacco stalkswas the lowest. The lowest total gas yield was obtainedfrom chromium tanned waste. We may conclude that thereason is only the effect of chromium, because the othertannery waste containing no chromium gave the similargas yields and composition with the lignocellulosic materi-als. Although, chromium decreased the gasification of pro-teins, it did not effect the gas composition. In addition, thepresence of chromium led to an increase in the amount ofwater soluble organics in supercritical water pyrolysis ofproteins. Moreover, we mentioned that in the case of realbiomass, the gasification reactions were more complexand the whole biomass contents may be expected toevidence.

Acknowledgements

We thank DAAD (German Academic Exchange Ser-vice) for financial support to offer us opportunity to per-form this study in Forschungszentrum Karlsruhe.

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