Char Formation Mechanism in Supercritical Water Gasification

Ind. Eng. Chem. Res. 2010, 49, 4055–4062

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Char Formation Mechanism in Supercritical Water Gasification Process: A Study of Model Compounds Athika Chuntanapum and Yukihiko Matsumura* Department of Mechanical System Engineering, Hiroshima UniVersity, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan

Char is one of the undesirable products in the supercritical water gasification of biomass, causing a reduction in carbon gasification efficiency. 5-HMF (5-hydroxymethylfurfural) is believed to be an intermediate in the char formation pathway. In this Article, the rate of formation of char particles obtained from experiments using glucose (a biomass model compound) was compared to that from experiments using 5-HMF (Chuntanapum and Matsumura, 2009). The glucose experiments were conducted in a temperature range of 300-400 °C, at 25 MPa, and for a residence time up to 70 s. The initial concentration of glucose was varied from 1.5 to 3 wt % (i.e., 0.083 to 0.167 M). Glucose was found to produce char particles 2 orders of magnitude faster than 5-HMF feedstock. The copresence of the other glucose decomposition products is thought to cause the difference in the char formation mechanism (i.e., the side reaction between 5-HMF and the other decomposition products) from that of 5-HMF alone. FT-IR and Raman spectroscopy of the solid products was carried out. 1. Introduction Biomass gasification is one of the promising alternatives for producing renewable energy. Carbon and hydrogen in biomass are converted into useful gases, such as H2, CO, CH4, and other hydrocarbon gases. This process is accompanied by CO2 production, but as the same amount of CO2 has already been consumed in the photosynthesis of the plant growth, the process can be recognized as being “carbon-neutral”. Supercritical water gasification (SCWG) technology is suitable for wet biomasses and organic wastes.1 This technology takes advantage of the large amount of water in biomasses by using the water as a reaction medium, eliminating the costly feedstock drying step. Supercritical water has a low dielectric constant close to that of organic compounds. The organic reactions under supercritical water, therefore, become more homogeneous, resulting in a higher reaction rate. The free radical condition of supercritical water also enhances the gas formation, leading to the high gas yield. As compared to conventional dry gasification, SCWG produces a lower amount of tarry material and char as byproduct, due to the higher solubility and reactivity of the organic compounds in supercritical water.2 Nevertheless, because tar and char are difficult to gasify, they act as a barrier to achieving complete gasification. The formation of tar and char also causes a reduction in the energy efficiency of the process by means of reactor plugging,3,4 heat exchanger fouling, and catalyst deactivation.5,6 An understanding of the formation mechanism of these high-molecular-weight compounds, therefore, would lead to the possibility of preventing or minimizing their formation in the gasification process. A visual observation of the dissolution process of lignocellulosic biomass into hot compressed water carried out by Hashaikeh et al. provided an overview of how char is formed.7 The solid precipitation was found to occur only after the amorphous phase of biomass dissolved into water. This type of the carbon solid was the product of the polymerization8 and recondensation7 of the water-soluble products. The insoluble part of the biomass, on the other hand, was pyrolyzed to yield * To whom correspondence should be addressed. Fax: +81-82-4227193. E-mail: [email protected].

a blackened biomass solid, which retained the original shape of the biomass, similar to that obtained from dry gasification.8 Among the water-soluble products from biomass decomposition, while low-molecular-weight acids, aldehydes, and ketones are the intermediates in the formation of gases (the desirable product), the ring compounds are responsible for the polymerization forming solid particles (hereinafter referred to as char). 5-HMF (5-hydroxymethylfurfural) is believed to play a leading role in the char polymerization reaction. In our previous works, we exposed aqueous 5-HMF feedstock to subcritical and supercritical conditions.9,10 The reaction condition, therefore, was simplified, and the chemical mechanism of char formation from 5-HMF could be revealed (Figure 1). 5-HMF was found to be able to react in two pathways concurrently: gasification and polymerization. 5-HMF decomposed to liquid products (TOC), such as 1,2,4-benzenetriol, 1,4-benzenediol, 5-methyl2-furaldehyde, levulinic acid, and formic acid, which were further gasified to the small gas molecules (mainly CO, CO2, and H2). 5-HMF and its ring products could also polymerize to form char particles. While the gasification is independent of the initial 5-HMF concentration, the polymerization pathway is enhanced by a higher concentration of 5-HMF. The rate of char formation depends strongly on the 5-HMF concentration, with the order of reaction of 4.29. To generalize the system under investigation, the formation of char from glucose (a biomass model compound) was examined in the present work. It occurs to us that even though

Figure 1. Char formation pathway from 5-HMF (see the kinetic parameters in Table 2c).10

10.1021/ie901346h  2010 American Chemical Society Published on Web 03/31/2010

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Ind. Eng. Chem. Res., Vol. 49, No. 9, 2010

the production of byproduct char has been widely noticed in the hydrothermal process of biomass and its model compounds, an extensive study of the char formation has not yet been done. Girisuta et al.11,12 and Qi and Xiuyang,13 for instance, obtained the rate of char formation from 5-HMF/glucose by balance (i.e., the difference between the total decomposition rate of 5-HMF/ glucose and the production rate of the desirable product) because the char yield was not measured. There is only the recent work of Knezevic et al.14 in which the rate of char formation was derived on the basis of the experimental char yield. However, the key compounds for the char formation were not investigated. The present work, therefore, attempts to ascertain the role of 5-HMF, the expected main culprit, in the char formation from glucose. Series of glucose experiments were carried out under both subcritical (300 and 350 °C) and supercritical (400 °C) conditions. The pressure was set at 25 MPa, and the residence time was varied up to 70 s. An effect of the glucose concentration was also investigated by varying the initial glucose concentration from 1.5 to 3 wt % at room temperature. The rate of char formation and the yields of 5-HMF and other ring products obtained from the glucose experiments were measured and compared to those from the experiments with 5-HMF as the starting material.10 2. Experimental Section 2.1. Experimental Procedure. Details on the continuous flow reactor and the procedures used in this experiment have been described previously.10 A difference was in the pore size of inline filters used to trap the large solid particles that might cause the reactor plugging and unsmooth pressure control. Glucose experiments used 60 or 90 µm inline filters, instead of 15 µm ones, due to the bigger size of char particles obtained from the glucose experiments. In brief, three high pressure pumps were used in the experiment. Deionized water was fed into the apparatus by a first pump and preheated. A glucose aqueous solution kept at room temperature was pumped via a second pump to mix with the preheated water prior to entry into the reactor. The mixing of the hot water into the cold glucose solution, in the volumetric ratio of 4:1, increased the feedstock temperature to reaction temperature almost instantly. The reactor was made of SS316 steel tube with the inner and outer diameters of 1 and 1.59 mm, respectively. The reactor was immersed in a molten salt bath set at the reaction temperature, with continuous bubbling with air to keep the temperature uniform. To ensure the steady state condition was reached, at least 10 reactor volumes of the feedstocks were pumped before the sampling was conducted. At the exit of the reactor, the product effluent was quickly cooled by mixing it with cold deionized water via a third pump, and further by the heat exchanger. Solid particles with size larger than the pore size of the inline filters were trapped inside the inline filters. The liquid effluent (with dispersed char particles) was collected at the liquid sampling point and subject to further analyses. The gas production rate was measured by the change of pressure inside the known-volume and pre-evacuated gas sampling port. The gas product was also sampled for a further composition analysis. The system pressure was controlled by a back pressure regulator. The experiments were conducted in a temperature range of 300-400 °C, at 25 MPa, and up to a residence time of 70 s (by adjusting the feedstock flow rates). Experiments at higher residence times were attempted but not successful due to the reactor plugging. The initial concentration of glucose (after

Figure 2. The product analytical methods.

mixing with preheated water) was varied from 1.5 to 3 wt % (i.e., 0.083 to 0.167 M at room temperature). 2.2. Product Analyses. The reaction products were analyzed in ways similar to those from the experiments of 5-HMF (see Figure 2).10 The gas product was analyzed by gas chromatography (GC). CO2 and CO were detected by GC-TCD (GC with a thermal conductivity detector) with He as the carrier gas. CH4, C2H4, and C2H6 were detected by GC-FID (GC with a flame ionization detector) with He as the carrier gas. H2 was detected by GCTCD with N2 as the carrier gas. The liquid effluent was analyzed by a total organic carbon (TOC) analyzer to quantify the amounts of carbon in the liquid product (nonpurgeable organic carbon or NPOC) and in the dissolved gas product (inorganic carbon or IC). The pH was measured with a pH meter. 5-HMF, 1,2,4-benzenetriol (BTO), furfural, 1,4-benzenediol, and 5-methyl-2-furaldehyde were analyzed by HPLC (high-performance liquid chromatography) with an RSpak DE-413 L column (Shodex). The analytical conditions were as follows: flow rate 0.5 mL/min; eluent 0.005 M HClO4 aqueous solution/CH3CN ) 90/10; oven temperature 40 °C. Glucose, fructose, glycolaldehyde, anhydroglucose, and dihydroxyacetone were analyzed by HPLC using a sugar KS802 column (Shodex). The analytical conditions were as follows: flow rate 0.8 mL/min; eluent water; oven temperature 60 °C. Pyruvaldehyde and glyceraldehyde have been reported as the glucose decomposition products in the literature15,16 but were only found in small amounts under the experimental conditions investigated. The solid product was the particles suspended in the liquid effluent and those trapped in the inline filter. The former was obtained by filtering the liquid effluent through a mixed cellulose ester membrane (0.1 µm pore size; Millipore) using vacuum suction. For the latter, the solid particles trapped in the inline filter were taken out by an ultrasonic cleaning device. The functional groups of the particles were analyzed by FT-IR (Fourier transform-infrared spectroscopy) with an IR Prestige21 spectrometer (Shimadzu) in an ATR (attenuated total reflectance) mode. The scanning was from the wavenumbers of 4700 to 400 cm-1 at 2 cm-1 resolution. Raman spectroscopy was conducted in a range of 500-2500 cm-1 using an Ar ion laser at 514.5 nm as the light source. No elementary analysis of the char particles has yet been done for this study. Many papers11,17 have reported carbon content in char particles similar to that in 5-HMF (57.14 wt %); therefore, we used this value for the calculation.


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Table 1. Liquid, Gas, and Solid Yields for the Glucose Experiments at 25 MPa with Varying Temperatures, Residence Times, and Concentrations

T (°C)

glucose concentration (wt %)

300

1.5

350

1.5

350

3

400

1.5

product yield (-) residence time (s)

liquid

gas

solid

carbon balance

9.9 19.8 29.9 49.6 60.6 1.0 2.0 5.0 9.9 19.9 49.2 5.0 10.0 0.5 1.0 2.0 4.9 9.8 19.8

1.047 1.031 0.985 0.986 0.850 1.070 1.020 0.942 0.947 0.865 0.826 0.967 0.910 0.999 0.974 0.985 0.994 0.954 0.884

0.008 0.017 0.021 0.033 0.045 0.009 0.015 0.034 0.033 0.054 0.054 0.023 0.034 0.018 0.026 0.036 0.055 0.048 0.061

0.000 0.010 0.017 0.029 0.045 0.000 0.000 0.020 0.018 0.045 0.090 0.014 0.070 0.000 0.000 0.000 0.000 0.000 0.000

1.055 1.057 1.023 1.048 0.940 1.079 1.035 0.995 0.998 0.964 0.970 1.003 1.013 1.017 0.999 1.021 1.049 1.001 0.945

3. Results and Discussion 3.1. Product Yields. An overall rate of reaction could be observed from the yields of liquid, gas, and solid at different temperatures and residence times shown in Table 1. The yield was calculated on the basis of the carbon content in the glucose feedstock: product yield )

carbon content in product (mol-C/L) (1) 0.083 or 0.167 (mol/L) × 6 (mol-C/L)

The rate increased with increasing temperature. Higher temperature resulted in higher solid yield only in the subcritical region (300 and 350 °C). The reaction in supercritical water (400 °C), however, gave a negligible amount of char. This could be attributed to a change of water properties into a more organiclike solvent in the supercritical region. An enhanced dissolution of organic compounds in supercritical water accelerated their homogeneous reactions (e.g., decomposition and gas formation), and this led to a subsequent reduction in the rate of the polymerization to form char particles. An increase in the initial concentration of glucose resulted in the higher char yield and unchanged gas yield. Decomposition behavior of glucose was usually described by the first-order reaction.14,18-20 This assumption gave us a pre-exponential factor of 3.80 × 1010 s-1 and activation energy of 127 kJ/mol in the present work. The parameters are in agreement with those of the previous studies (Figure 3). The activation energies of 114, 121, and 88 kJ/mol were obtained from the works of Knezevic et al.,14 Matsumura et al.,18 and Kabyemela et al.,19 respectively. However, the first-order model has limitations, which have been stated in many publications14,18,20 and are discussed in section 3.2. Main liquid compounds from glucose decomposition were 5-HMF, 1,2,4-benzenetriol (BTO), furfural, fructose, glycolaldehyde, anhydroglucose, and dihydroxyacetone. Their yields were plotted in Figure 4, separated into two groups: ring compounds (1) and sugars, ketone, and aldehydes (2). The ring compounds are expected to be the key compounds for the char formation, whereas the ketone and aldehydes are those for the gas production. 1,4-Benzenediol and 5-methyl-2-furaldehyde were detected in the relatively much smaller amount and hence were not included in Figure 4. A difference in the behaviors

Figure 3. Literature comparative Arrhenius plot of the rate constant for overall glucose decomposition (kg).

between the two product groups could be observed. Sugars, ketone, and aldehydes were formed instantly after glucose was exposed to subcritical and supercritical conditions and decomposed in an exponential manner similar to glucose (Figure 4a2, b-2, c-2). This indicated that the sugars, ketone, and aldehydes were the primary products of glucose and, therefore, emerged very early in the reaction and were subject to further decomposition. The yield of ring compounds, on the other hand, initially increased gradually, reached a peak, and then decreased gradually (Figure 4a-1, b-1, c-1). Their production rate was slower than that of sugars, ketone, and aldehydes. For 5-HMF and furfural, even if they can be produced directly from glucose by dehydration reaction, the previous finding showed that they are produced at a higher rate from fructose.15,21-23 5-HMF was also subject to further reactions, such as a ring hydrolytic reaction to BTO. Hence, once the consumption rate exceeded the production rate, 5-HMF yield started to decrease, as can be seen in Figure 4a-1 and b-1 after around 50 and 5 s in the experiments at 300 and 350 °C, respectively. The reaction of glucose of 1.5 wt % (0.083 M) gave the concentrations of 5-HMF varying from 0 to 0.02 M at room temperature (Figure 4a-1, b-1, c-1). From our previous study,9 at these concentrations, 5-HMF did not produce any char particles. The rate of char formation depends strongly on the 5-HMF concentration with the order of reaction of 4.29 and not less than 0.05 M 5-HMF being needed to produce a measurable amount of char.10 However, a decrease in the 5-HMF yield with longer residence times could be observed and was sped up at higher temperature in subcritical condition where the char solid was formed (Figure 4a-1, b-1). Therefore, it is possible that 5-HMF was consumed in the char formation reaction but by a different mechanism than that of 5-HMF alone. In addition to the polymerization among the 5-HMF molecules as when 5-HMF was the starting material (i.e., reaction 5c in Figure 1), 5-HMF produced in the glucose experiment may polymerize with the other glucose decomposition products as a side reaction. An attempt to fit the experimental data to the kinetic model based on this assumption is shown in section 3.2. 3.2. Kinetic Models. Only the limited number of the kinetic models describing the formation of char in subcritical and supercritical conditions is found in the literature. Girisuta et al.11,12 studied the production of levulinic acid from glucose and 5-HMF catalyzed by HCl and found humic solid as the main byproduct. The reactions were modeled by using a powerlaw approach; therefore, the concentration effect of glucose, 5-HMF, and acid could be clearly expressed. However, because the yield of char particles was not measured, the rate of the

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Figure 4. Yields of the main liquid products obtained from the experiments of 1.5 wt % glucose at (a-1, a-2) 300 °C, (b-1, b-2) 350 °C, and (c-1, c-2) 400 °C and at 25 MPa.

char formation was obtained from the difference between the overall decomposition rate of 5-HMF/glucose and the production rate of levulinic acid. Recently, a char formation mechanism has been proposed on the basis of the systematically measured yield of char.14 A separate experiment for each primary decomposition product of glucose was also conducted so that the compounds responsible for the char production could be identified. They were fructose, levoglucosan, dihydroxyacetone, erythrose, 5-HMF, furfural, glyceraldehyde, and glycolaldehyde. However, because the char yield from each product was not measured, the main contributor to char formation was still unknown. The chemical reaction pathway, therefore, was based on the three groups of the water-soluble compounds: those that form initial gas, those that quickly transform to char, and those that slowly transform to char. A second-order reaction was used to describe the polymerization reaction to form char. In the present work, two kinetic models (first-order and nthorder models) were constructed for explaining the glucose char formation, based on the experimentally measured char yield. The first-order kinetic model assumes that all reactions are firstorder, while the nth-order model takes into account the

Figure 5. Proposed formation pathways of glucose char particles.


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Figure 6. The comparison between the product yields obtained from the experiment (symbol) and the first-order model (line) at the temperatures of (a) 300 °C, (b) 350 °C, and (c) 400 °C [experimental conditions: 1.5 wt % glucose, 25 MPa].

concentration effect on the polymerization pathway. The proposed reaction pathway is shown in Figure 5. In the models, the contributions to char particles from 5-HMF and furfural, the expected chief components for char formation, were distinguished from the other water-soluble products (TOC). 5-HMF and furfural were produced from glucose and fructose, and 5-HMF was not the intermediate of furfural.10,22 All products could decompose to give TOC, which could be further gasified or polymerized to form char. The side reaction of 5-HMF with furfural and TOC to produce char (i.e., reaction 5ftc) was applied in the nth-order model. On the other hand, it was not considered in the first-order model (i.e., k5ftc was set as zero) because the reaction 5ftc had a higher reaction order. The corresponding rate equations based on carbon content are as follows: d[glucose] ) -(kgf + kgfu + kgt + kg5)[glucose] dt

(2)

d[fructose] ) kgf[glucose] - (kf5 + kffu + kft)[fructose] dt

(3) d[5-HMF] ) kg5[glucose] + kf5[fructose] - k5t[5-HMF] dt - k5c[5-HMF]R5c - k5ftc[5-HMF][furfural][TOC]

(4) d[furfural] ) kgfu[glucose] + kffu[fructose] - (kfut + kfuc)[furfural] dt - k5ftc[5-HMF][furfural][TOC]

(5)

d[TOC] ) kgt[glucose] + kft[fructose] + k5t[5-HMF] + kfut[furfural] dt - ktc[TOC]Rtc - ktg[TOC] - k5ftc[5-HMF][furfural][TOC]

(6) d[char] ) kfuc[furfural] + k5c[5-HMF]R5c + ktc[TOC]Rtc dt (7) + 3k5ftc[5-HMF][furfural][TOC] d[gas] ) ktg[TOC] dt

(8)

where [glucose] ) glucose concentration (mol-C/L), [fructose] ) fructose concentration (mol-C/L), [5-HMF] ) 5-HMF concentration (mol-C/L), [furfural] ) furfural concentration (mol-C/L), [TOC] ) lumped carbon concentration of the other liquid products (mol-C/L), [char] ) char concentration (molC/L), [gas] ) carbon concentration in the gas product (mol-C/ L), ki ) rate constant ((mol-C/L)1-Ri · s-1), Ri ) reaction order, and t ) residence time (s). When all of the reactions are assumed to be first-order, the fit between the model and experiment is reasonable as shown in Figure 6. The fitting was carried out by using the least-squares of error (i.e., the difference between the experimental and calculated values) as the criterion to find all of the rate constants. The rate constants obtained are shown in Table 2a. At 350 °C, the rate of char formation from 5-HMF produced from glucose, k5c (1.28 × 10-2 s-1), is 1 order of magnitude higher than that from 5-HMF feedstock (4.01 × 10-3 (mol-C/L)-3.29 · s-1). The char contribution from 5-HMF is just above that from TOC (Figure 7a). The first-order assumption, however, is hypothetical. The decomposition of glucose involves the reactions of many compounds, and not all of them can be properly described as

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Table 2. Kinetic Parameters feedstock T (°C)

glucose 300

350

model: kinetic parameter gf gfu gt g5 f5 ffu ft 5t 5c fut fuc tc tg g 5tfc a

400

5-HMF 300-400

(a) first-order model k, s-1

k, s-1 -2

2.09 × 10 0 5.13 × 10-2 6.00 × 10-3 1.62 × 10-1 5.94 × 10-2 0 0 1.59 × 10-3 0 0 9.35 × 10-4 2.69 × 10-3 7.81 × 10-2

k, s-1 -1

2.21 × 10 0 9.30 × 10-1 3.27 × 10-2 3.30 × 10-1 2.74 × 10-1 0 2.45 × 10-5 1.28 × 10-2 9.94 × 10-3 0 1.29 × 10-3 2.35 × 10-3 1.18

Ra -1

2.86 × 10 1.60 × 10-1 3.60 2.15 × 10-1 3.82 × 10-1 1.91 × 10-1 0 0 0 0 0 1.60 × 10-3 6.64 × 10-3 4.26

1 1 1 1 1 1 1 1 1 1 1 1

350

350

(b) nth-order model

(c) 5-HMF model (Chuntanapum and Matsumura)10

k (mol-C/L)1-R · s-1 -1

2.35 × 10 0 9.27 × 10-1 8.24 × 10-3 5.13 × 10-1 4.05 × 10-1 0 1.70 × 10-3 a 4.01 × 10-3 a 1.23 × 10-3 0 1.90 × 10-4 2.30 × 10-3 1.30 5.20 × 10-1

k (mol-C/L)1-R · s-1

R

1 4.29 1

1.70 × 10-3 4.01 × 10-3

1 4.29

1.21 1 1 3

1.52 × 10-4 1.45 × 10-4

1.21 1

Ra 1 1 1 1 1 1

Fixed values in the regression.

Figure 7. Contributions to the char formation from 5-HMF, TOC, and side reaction from (a) the first-order model, and (b) the nth-order model (lines), as compared to the experimental char yields (symbol) [experimental conditions: 350 °C, 25 MPa, 1.5 wt % glucose].

Figure 8. The effect of concentration on the char product yield for the experiments at 350 °C and 25 MPa: the experiment (symbol), the nth-order model (line).

first-order reactions. The polymerization to form char particles, for example, usually has an order of reaction higher than unity. Moreover, the enhancing effect of the initial glucose concentration on the char formation (Figure 8) cannot be reconciled with the first-order model. To explain the effect of the concentration, an nth-order reaction model was developed. The model development started from setting k5c, R5c, k5t, R5t, and Rtc equal to those of the 5-HMF model (Table 2c), setting the orders of the reactions other than the polymerization to char equal to unity, and then iterating the other rate constants until the criterion of the least-squares of error was satisfied.

By an application of the 5-HMF model to the nth-order model, we assumed the polymerization rate of 5-HMF and its concentration effect to be the same as those when 5-HMF was the feedstock. The decomposition rate of 5-HMF was also set to be the same. The effect of the copresence of the other glucose decomposition products was taken into account by the side reaction of 5-HMF with TOC and furfural (k5ftc). Obtained rate constants and an experiment-model comparison are shown in Table 2b and Figure 8, respectively. It can be seen that the proposed nth-order model could capture the concentration effect on the char yield reasonably well. The contributors to the char formation in this model are comparatively shown in Figure 7b. The contributions from the polymerization of 5-HMF and furfural (k5c and kfuc) were negligible. The char particles derived mostly from the polymerization among 5-HMF, furfural, and TOC (k5ftc), and only a minute portion came from the polymerization of TOC (ktc). The polymerization among 5-HMF, furfural, and TOC, therefore, might represent a different char formation mechanism from glucose to that from 5-HMF alone. The rate of char formation from glucose governed by the side reaction of 5-HMF (k5tfc ) 5.20 × 10-1 (mol-C/L)-2 · s-1) was 2 orders of magnitude higher than that of the polymerization of 5-HMF molecules (k5c ) 4.01 × 10-3 (mol-C/L)-3.29 · s-1). Because the pH of the liquid effluent was measured to be in the range of 2.3-2.7 at room temperature, showing acidity, one might consider the possibility of the polymerization reaction of 5-HMF forming char particles being autocatalyzed by the


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Figure 9. The comparison of FTIR spectra of 5-HMF and the char particles from 5-HMF and glucose.

acidic products of glucose. The main acidic products consist of formic acid, acetic acid, lactic acid, and levulinic acid.21,23,24 Girisuta et al.11 studied the kinetics of the 5-HMF decomposition with various acid concentrations. They found that acid catalyzes both the decomposition of 5-HMF and the polymerization of 5-HMF to form humic solids. A catalytic effect of the copresent, weak acids, therefore, might be possible. However, at the moment, we still cannot take this into consideration in the model. A further study on the acid dissociation in subcritical and supercritical water is needed and is our current investigation topic. The model proposed here is only for the char formation mechanism from cellulose portion in lignocellulosic biomass. For real biomasses, one would expect lignin, the strongest component, to play a role in the char formation as well. The decomposition rate of cellulose and lignin is totally different in magnitude. For example, at 400 °C with no catalyst, the complete decomposition of glucose (cellulose monomer) was within 10 s (Figure 4c-2), while that of guaiacol (lignin model compound) was around 200 min.25 This is due to the polyaromatic structure of lignin. Therefore, the main char production from lignin portion of lignocellulosic biomass may be from the direct conversion from the unreacted lignin into the solid product. This is called the solid-solid conversion in the work of Karayildirim et al.8 On the other hand, cellulose is rapidly hydrolyzed into glucose in the hydrothermal conditions. Glucose readily reacts to give many other water-soluble compounds, some of which can be polymerized to give char particles. Therefore, glucose-derived compounds are present in much higher concentration in the aqueous phase. Because the char polymerization is strongly dependent on concentration, the glucose-derived compounds are relatively more important than the lignin-derived compounds in the char formation mechanism via polymerization reaction. In terms of compound types, because lignin-derived compounds are mainly aromatics, such as guaiacol, phenol, catechol, and cresol,25 their participation in the char polymerization is possible. However, no work has explored this aspect in detail. One of the examples is the work of Yoshida et al.6 who found the retarding effect on the cellulose gasification when lignin was added into the feedstock. 3.3. Char Product Characterization. The visual appearance of the char particles obtained from glucose experiments was similar to that from the 5-HMF feedstock. In fact, char particles

have a dark brown to black color. The char particles formed are well suspended in the liquid product. Char particles from glucose have a bigger size than those from 5-HMF feedstock (15 µm inline filters were used in the experiment of 5-HMF, while 60 or 90 µm inline filters were used in the experiment of glucose.) The FT-IR and Raman spectroscopy of the char particles obtained from glucose was carried out and compared to those obtained from 5-HMF experiment.10 FT-IR spectra of the solid particles obtained from glucose at the different residence times were carried out. Similar spectra were obtained; therefore, only one spectrum at 50 s is shown in comparison with those of 5-HMF and the solid char from the experiment of 5-HMF (Figure 9). This means that while the char yield was increasing with residence time, the same reaction took place, resulting in the same char structure for any residence time under investigation. From Figure 9, FT-IR spectra of char particles from 5-HMF and that from glucose look similar. Both appear to have the same functional groups but at a different relative intensity. Three peaks typifying furan ring exist in both cases, as indicated by the three dashed lines. Thus, the char structure from both experiments is similar. One of the obvious differences is the wavenumber shift of the peak representing CdO stretching in the region of 1700-1675 cm-1. This might be caused by more compounds participating in the formation process of char particles in the glucose experiment than those in the 5-HMF experiment. A clear example is furfural, which was not observed as a product in the 5-HMF experiment under subcritical condition.10 Furthermore, fructose, levoglucosan, dihydroxyacetone, erythrose, glyceraldehyde, and glycolaldehyde could also be sources of char formation.14 The comparison between the Raman spectra of the char particles obtained from 5-HMF and glucose is shown in Figure 10. Both G and D bands, respectively designating the crystalline and amorphous structures of the carbon ring compounds, were observed in all cases. The presence of these two bands indicated the possibility of the carbonization occurring after the polymerization in both experiments. Usually, the G/D ratio is used to compare the characteristics of the carbon materials. The G/D ratio of the 5-HMF char particles is 2.1, while those of glucose are 1.8 and 1.4 (see Figure 10 for the experimental conditions). Even if we do not have sufficient data to observe the trend of

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Figure 10. The comparison of Raman spectra of the char particles from 5-HMF and from glucose.

the G/D ratio with temperature and residence time, it could be stated roughly that G/D ratios in both cases were not completely different. The similarity of the 5-HMF and glucose char solids in terms of the functional groups and structure could be identified. Therefore, it is possible that 5-HMF is one of the contributors to the formation of char particles. 4. Conclusions The char formation mechanism in the SCWG process of biomass was strategically investigated by using glucose as a model compound to simplify the system under investigation. Experiments on glucose were carried out at 25 MPa, temperature ranging from 300 to 400 °C, and for residence times up to 70 s. Two concentrations of glucose (1.5 and 3 wt % at room temperature) were used to examine the concentration effect on the char formation rate. The char product was detectable only in the subcritical conditions (300 and 350 °C) and increased with temperature and residence time. It was known from our previous findings that the polymerization of 5-HMF was strongly dependent on the concentration.10 The rate of char formation in the glucose experiment, however, was found to be faster than that in the 5-HMF experiment even if a low concentration of 5-HMF was produced. Therefore, it may be summarized that, for glucose, the char produced from 5-HMF polymerization is negligible and the interaction of 5-HMF with the other glucose decomposition products plays the more important role in the production of char. An nth-order model for the glucose decomposition was formulated and could be used to explain reasonably well the effect of glucose concentration on the formation of char. The characteristics of 5-HMF and glucose char solids show some similarities, indicating 5-HMF as one of the possible intermediates for char production. Literature Cited (1) Matsumura, Y.; Minowa, T.; Potic, B.; Kersten, S. R. A.; Prins, W.; Van Swaaij, W. P. M.; Van De Beld, B.; Elliott, D. C.; Neuenschwander, G. G.; Kruse, A.; Antal, M. J. Biomass gasification in near- and supercritical water: Status and prospects. Biomass Bioenergy 2005, 29, 269. (2) Kruse, A.; Dinjus, E. Hot compressed water as reaction medium and reactant properties and synthesis reactions. J. Supercrit. Fluids 2007, 39, 362–380. (3) Byrd, A. J.; Pant, K. K.; Gupta, R. B. Hydrogen production from glucose using Ru/Al2O3 catalyst in supercritical water. Ind. Eng. Chem. Res. 2007, 46, 3574–3579.

(4) Lu, Y. J.; Guo, L. J.; Ji, C. M.; Zhang, X. M.; Hao, X. H.; Yan, Q. H. Hydrogen production by biomass gasification in supercritical water: A parametric study. Int. J. Hydrogen Energy 2006, 31, 822–831. (5) Yoshida, T.; Oshima, Y. Partial oxidative and catalytic biomass gasification in supercritical water: A promising flow reactor system. Ind. Eng. Chem. Res. 2004, 43, 4097–4104. (6) Yoshida, T.; Oshima, Y.; Matsumura, Y. Gasification of biomass model compounds and real biomass in supercritical water. Biomass Bioenergy 2004, 26, 71–78. (7) Hashaikeh, R.; Fang, Z.; Butler, I. S.; Hawari, J.; Kozinski, J. A. Hydrothermal dissolution of willow in hot compressed water as a model for biomass conversion. Fuel 2007, 86, 1614–1622. (8) Karayildirim, T.; Sinag, A.; Kruse, A. Char and coke formation as unwanted side reaction of the hydrothermal biomass gasification. Chem. Eng. Technol. 2008, 31, 1561–1568. (9) Chuntanapum, A.; Yong, K. T. L.; Miyake, S.; Matsumura, Y. Behavior of 5-HMF in subcritical and supercritical water. Ind. Eng. Chem. Res. 2008, 47, 2956–2962. (10) Chuntanapum, A.; Matsumura, Y. The formation of tarry material from 5-HMF in subcritical and supercritical water. Ind. Eng. Chem. Res. 2009, 48, 9837–9846. (11) Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J. A kinetic study on the decomposition of 5-hydroxymethylfurfural into levulinic acid. Green Chem. 2006, 8, 701–709. (12) Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J. A kinetic study on the conversion of glucose to levulinic acid. Chem. Eng. Res. Des. 2006, 84, 339–349. (13) Qi, J.; Xiuyang, L. Kinetics of non-catalyzed decomposition of glucose in high-temperature liquid water. Chin. J. Chem. Eng. 2008, 16, 890–894. (14) Knezevic, D.; van Swaaij, W. P. M.; Kersten, S. R. A. Hydrothermal conversion of biomass: I, Glucose conversion in hot compressed water. Ind. Eng. Chem. Res. 2009, 48, 4731–4743. (15) Kabyemela, B. M.; Adschiri, T.; Malaluan, R. M.; Arai, K. Glucose and fructose decomposition in subcritical and supercritical water: detailed reaction pathway, mechanisms, and kinetics. Ind. Eng. Chem. Res. 1999, 38, 2888–2895. (16) Watanabe, M.; Aizawa, Y.; Iida, T.; Levy, C.; Aida, T. M.; Inomata, H. Glucose reactions within the heating period and the effect of heating rate on the reactions in hot compressed water. Carbohydr. Res. 2005, 340, 1931–1939. (17) Luijkx, G. C. A.; Vanrantwijk, F.; Vanbekkum, H. Hydrothermal formation of 1,2,4-benzenetriol from 5-hydroxymethyl-2-furaldehyde and D-fructose. Carbohydr. Res. 1993, 242, 131. (18) Matsumura, Y.; Yanachi, S.; Yoshida, T. Glucose decomposition kinetics in water at 25 MPa in the temperature range of 448-673 K. Ind. Eng. Chem. Res. 2006, 45, 1875–1879. (19) Kabayemela, B. M.; Adshiri, T.; Malaluan, R. M.; Arai, K. Kinetics of glucose epimerization and decomposition in subcritical and supercritical water. Ind. Eng. Chem. Res. 1997, 36, 1552–1558. (20) Yoshida, T.; Yanachi, S.; Matsumura, Y. Glucose decomposition in water under supercritical pressure at 448-498 K. J. Jpn. Inst. Energy 2007, 86, 700–706. (21) Qi, X.; Watanabe, M.; Aida, T. M.; Smith, R. L., Jr. Catalytical conversion of fructose and glucose into 5-hydroxymethylfurfural in hot compressed water by microwave heating. Catal. Commun. 2008, 9, 2244– 2249. (22) Aida, T. M.; Tajima, K.; Watanabe, M.; Saito, Y.; Kuroda, K.; Nonaka, T.; Hattori, H.; Smith, R. L.; Arai, K. Reaction of D-fructose in water at temperatures up to 400 °C and pressure up to 100 MPa. J. Supercrit. Fluids 2007, 42, 110–119. (23) Aida, T. M.; Sato, Y.; Watanabe, M.; Tajima, K.; Nonaka, T.; Hattori, H.; Arai, K. Dehydration of D-glucose in high temperature water at pressures up to 80 MPa. J. Supercrit. Fluids 2007, 40, 381–388. (24) Kruse, A.; Gawlik, A. Biomass conversion in water at 330-410 °C and 30-50 MPa. Identification of key compounds for indicating different chemical reaction pathways. Ind. Eng. Chem. Res. 2003, 42, 267–279. (25) Wahyudiono; Kanetake, T.; Sasaki, M.; Goto, M. Decomposition of a lignin model compound under hydrothermal conditions. Chem. Eng. Technol. 2007, 30, 1113–1122.

ReceiVed for reView August 27, 2009 ReVised manuscript receiVed January 28, 2010 Accepted March 13, 2010 IE901346H

Char Formation Mechanism in Supercritical Water Gasification

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