Influence of Educt Preparation on Gasification of Corn Silage in

Oct 29, 2005 - Supercritical water gasification of corn silage was investigated in a continuous down-flow reactor. The influence of potassium addition...
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Ind. Eng. Chem. Res. 2005, 44, 9071-9077

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Influence of Educt Preparation on Gasification of Corn Silage in Supercritical Water Pedro D’Jesu ´ s,*,† Cristian Artiel,† Nikolaos Boukis,† Bettina Kraushaar-Czarnetzki,‡ and Eckhard Dinjus† Institut fu¨ r Technische Chemie, Chemisch-Physikalische Verfahren (ITC-CPV), Forschungszentrum Karlsruhe, Karlsruhe, Germany, and Institut fu¨ r Chemische Verfahrenstechnik, Universita¨ t Karlsruhe, Karlsruhe, Germany

Supercritical water gasification of corn silage was investigated in a continuous down-flow reactor. The influence of potassium addition, particle diameter, maize sort, and dry matter content on the gasification efficiency and gas composition was discussed. The effect of potassium addition on the gasification of corn starch was compared with the effect of potassium addition in case of corn silage. Four different crushing units were used. The results obtained using each unit were discussed. Three corn silage with different maturity grades and two corn silage with different growing conditions were investigated. The dry organic matter was varied from 2.5 to 20 wt %. The gasification efficiency and gas composition depends on the biomass concentration. Plugging was observed at higher concentrations. Increase of the pressure inhibited and reduced plugging levels. The residence time was varied for a kinetic study. The gasification efficiency can be modeled with zero-order kinetics, but it is influenced by the concentration. Introduction The increase in the global energy demand, the greenhouse effect, and the political disturbances in the crude oil producing countries led to a steadily increasing interest in biomass energy. The energy contended in the biomass can be obtained through many types of technologies and is CO2-neutral. Almost half of the available biomass has high water content. The efficiency of actual gasification technologies decreases by the use of high moisture biomass. Compared to other biomass gasification processes, supercritical water gasification is the most efficient technology in case of using wet biomass (>40 wt % moisture).1 Numerous experiments with model substances and limited experiments with real biomasses are found in the literature. Kruse et al.2 studied the gasification of pyrocatechol in supercritical water. They found that an increase in the temperature and a decrease in the pressure improve the hydrogen formation. The addition of potassium salts in the reaction of biomass model compounds with supercritical water favored the CO conversion through the water-gas shift reaction. Experiments with variation of the particle size in the supercritical water gasification of biomass cannot be found in the literature. In the steam gasification of biomass, the particle size influence the gasification yield. The use of smaller particles improved the gasification yield of almond shells.3 Influence of the composition on the gasification of mixtures of model compounds in supercritical water have been published. A decrease of gas production was observed for mixtures containing lignin.4 This decrease was found to be dependent on the lignin type.5 The influence of the dry matter content was investigated in the literature. Boukis6 found that lowering the * To whom correspondence should be addressed. E-mail: [email protected]. † Forschungszentrum Karlsruhe. ‡ Universita ¨ t Karlsruhe.

methanol concentration at the same reaction conditions led to higher conversion. Yu et al.7 found that the hydrogen yield drops and the methane yield rises as glucose concentration increases. To model the process of supercritical water gasification of biomass, the influence of the process variables on the gasification of real biomass feedstock like corn was investigated.8 The preparation method of the educt could influence the gasification efficiency of this process. To optimize the generation of hydrogen from corn silage, the preparation methods have to be investigated. If there is an effect of potassium in corn silage, which is potassium-containing natural product, or if the size of the particle could influence the feed and reaction system are questions that must be answered. And what happened if the corn silage is obtained from another plant or another place? The process has to be flexible. Higher concentrations are more profitable,7 but how high could be the concentration to secure the constant production of hydrogen? Experimental Section Preparation of Educts. In this project corn starch and corn silage were used as educts for the investigation of the potassium effect. A water-starch gel was prepared at 97 °C. Potassium (in the form of KHCO3) was added to the mixture of starch and water before the formation of the gel. To prepare pumpable mixtures of water and biomass, xanthan was used as a thickening agent. Its optimal proportion in the reaction mixture was about 0.25 wt %. The amount of water added depended on the desired concentration of the mixture and the dry matter content of the biomass (DOM). A sample was dried during 24 h at 105 °C. With the quantification of the weights before and after drying, the dry matter of the educt was determined. Particle Diameter. To investigate the influence of the particle diameter on the gasification of biomass,

10.1021/ie0508637 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/29/2005

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helium as carrier gas. The total organic carbon (TOC) and total carbon (TC) contents in the reactor liquid effluent were determined with a TOC analyzer (Rosemount Dohrmann DC-190). One gas flow meter (Ritter) and two balances (Sartorius) were used to estimate the educt, gas product, and aqueous effluent flows. The composition of the corn silage probes was analyzed with a element analyzer (Vario). For the measure of the protein, fibers, and ash, the Weender analysis was used. In Tables 1 and 2, the results of these analysis are presented. Data Interpretation. Gases produced are mainly hydrogen, carbon dioxide, carbon monoxide, methane, and ethane. At ambient temperature and pressure, i.e., after pressure relief, these can be considered ideal gases. The carbon gasification efficiency is defined as

Figure 1. High-pressure laboratory equipment for supercritical water gasification of real biomass.

pV˙

different systems were used for the particle size reduction. The crushing units applied in this work were Macerator (Seepex 25-I-1-F12-2), Colloid Mill (PUC N100/E/RD/15kW), Meat Chopper, Ultra-Turrax, and Grindomix (Retsch GM 200) Maize Sort. Three sorts of maize with different ripeness degrees, Doge (700), Mikado (500), and Gavott (240), were used to investigate the influence of the composition of the maize plant on the gasification of biomass. In the literature has been found that the lignin content of the biomass affect the gasification efficiency.4 The lignification’s grade is related to the ripeness of the plant. The influence of the growing conditions were investigated with two charges (1 and 2) of corn silage harvested in different years. Apparatus. A continuous down-flow reactor (1000 mm long and 8 mm inner diameter) with preheater (250 mm long and 8 mm inner diameter) was used (Figure 1). The reactor material was the nickel alloy Inconel 625. The reactor was heated to a maximum temperature of 700 °C by heating coils. The reactor effluent was cooled to ambient temperature with a chiller. A pressure tank was needed to store the prepared biomass; inside, a piston was installed, which acted as a pump driven by water. With an HPLC pump, water was compressed to system pressure at the desired flow. Pressure was controlled by a back-pressure regulator (Tescom). After cooling and expansion, the reactor’s products were separated in a glass recipient into two phases: liquid and gas. Analysis. Gas composition was measured with a GC HP 6890 with two columns (80/100 Hayesep Q, 2 m long and 60/80 Molesieve 5 Å, 4 m long), one for H2 and the second one for CO2, hydrocarbons (CH4 to C4H10), and

Y)

∑i Ri Ci RT M

(1)

wm ˘

Gas production results as

P)

V˙ wm ˘

(2)

For the calculation of the residence time, variation of density of the main component (water) with temperature along the reactor is considered. Residence time is defined as

τ)

V V˙ i

(3)

Results and Discussion Influence of Potassium on the Gasification of Biomass in Supercritical Water. Figure 2 demonstrates the influence of potassium in the process of supercritical water gasification. With increasing potassium concentration from 0 to 500 ppm, the gasification yield rises from 0.82 to 0.92. Further increase in potassium concentrations up to 3000 ppm does not improve the gasification yield significantly. It is also shown that the carbon concentration (TC) in the reactor effluent increases with the potassium concentration. The positive effect of potassium addition on the gas yield has been described in the literature as a catalytic effect of potassium in the water-gas shift reaction. When studying the gasification of 5 wt % pyrocatechol at 500

Table 1. Elemental Composition (in wt %) of Biomass Educts educt

C

O

H

N

K

S

Si

Ca

P

Cl

Fe

corn starch charge 1 charge 2 Doge Mikado Gavott

44.44 43.40 45.1 44.3 44.2 44.9

49.34 46.70 45.0 42.8 43.6 44.6

6.22 6.17 6.61 6.30 6.30 6.40

1.02 0.8 1.05 0.80 0.95

0.98 1.14 3.20 2.80 1.64

0.93 0.19 0.30 0.26 0.28

0.35 0.48 0.55 0.51 0.38

0.20 0.18 0.38 0.44 0.25

0.14 0.16 0.30 0.24 0.24

0.13 0.20 0.74 0.77 0.33

0.01 0.02 0.05 0.05 0.02

Mg

0.119

Table 2. Weender Analysis, Dry Matter Content, and Calorific Value for the Different Sorts of Maize maize

% crude ash

% crude fiber

% crude protein

% DM

calorific value (kJ/kg)

Doge Mikado Gavott charge 1 charge 2

5.0 4.9 3.4 3.9 4.5

22.5 23.1 16.4 18.8 17.5

7.8 6.5 7.5 7.1 7.8

24.0 26.2 37.4 30.9 38.0

18 448 18 614 19 149 18 301

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Figure 2. Effect of potassium on the gasification efficiency (Y) and total carbon content in the liquid effluent (TC) at T ) 700 °C, p ) 25 MPa, m ˘ ) 3 g/min, and 5 wt % DOM corn starch.

°C, Kruse et al.2 found that the CO concentration in the gas product dropped from more than 40 vol % to less than 1 vol %, by increasing the amount of KOH from 0 to 5 wt %. The hydrogen content in the gas product rises by up to 3 times with an increasing potassium amount in the reaction mixture. Sinag et al.9 explained the catalytic effect of potassium by the formation of formate salt. CO reacts with KOH to a formate salt which reacts further with water to produce hydrogen. Influence of potassium addition on the gasification of real biomass has also been investigated. It is known that real biomass, like clover grass, contains approximately about 1 wt % potassium (based on dry matter biomass), which is equivalent to 500 ppm [K] for 5 wt % (dry matter) biomass concentration (see Table 1). Further potassium addition does not influence the gas production (3.22 L of gas/g of C without K and 3.12 L of gas/g of C with 500 ppm K at 700 °C). Kruse et al.10 also found that the addition of potassium did not influence the gasification yield in case of supercritical water gasification of wood and straw. To better understanding of the potassium effect, the salts contained in the biomass, which could have a catalytic effect, should be washed, an then potassium could be added to the educt mixture. Influence of the Particle Diameter on the Gasification of Biomass. The reaction rate depends directly on the diameter when the mass transport is the limiting step. It is expected that the gasification efficiency increases at lower particle diameter. To investigate the influence of the particle diameter, four different crushing methods where applied, as expressed in the Experimental Section. In Figure 3 is shown the particle distribution of three different educts, crushed with different equipment. In Figure 3 is observed that the best size reduction was achieved with the system Meat Chopper/UltraTurrax. Although in the specification of the colloid mill the particle size could be reduced until 0.04 mm, in this case only a mill gap of 0.7 mm could be set, because the fiber of the corn silage plugged in the mill at lower gaps at the maximal engine power. For this reason the laboratory meat chopper was used. For the investigation of the influence of the particle diameter on the gasification efficiency, the temperature selected was 600 °C. At this temperature the reaction could be limited by the mass transport. In Figure 4 the results of the influence of the diameter are presented. The gasification of the finer educt prepared with the system Meat Chopper/Ultra-Turrax results in higher

Figure 3. Distribution density (q3) for the output of the crushing units: (a) Ultra-Turrax (UT); (b) Grindomix (GM); (c) colloid mill (cm).

Figure 4. Influence of the particle diameter on the gasification efficiency (Y) for (a) Ultra-Turrax (UT), (b) Grindomix (GM), and (c) colloid mill (cm) at T ) 600 °C, p ) 25 MPa, and DOM ) 5 wt %.

gasification efficiencies as shown in Figure 4. Lower gasification efficiency were reached with the system Meat Chopper/Grindomix, although the specification of this cutting mill defined an end size reduction of 0.3 mm as possible. Finer particle sizes were not possible with the different crushing units proved in this work. An improvement of the crushing method is needed to reach better reduction levels. As observed in Figure 4, reducing the particle diameter in it half from 1.2 mm to 0.6 mm improves the gasification efficiency in 20%, which means that the reaction rate depends on the particle diameter and it is limited by the mass transport at the investigated temperature of 600 °C. Influence of the Corn Sort on the Gasification of Biomass. In Figure 5is represented the gasification efficiency of two different corn silage charges as a function of the residence time. It can be observed that the biomass type influences the gasification efficiency. Charge 2 was cropped in 2003. In this year the summer was too hot and dry. The dry matter of the charge 2 was higher than in the charge 1, and the elemental composition of the plant changed. More carbon were found in the charge 2 (45.08 wt %) than in charge 1 (43.4 wt %). Also more protein and ash are presented in the charge 1. Some investigators11 defined that the proteins affect the gasification efficiency. The ash content influences also the gasification efficiency, because it cannot be gasified at the reaction conditions. This increase on the percentage of such components led to the observed decrease in the gasification efficiency of charge 1. The different ambient conditions influence also the composition of the gas product. In Figure 5b can be observed that charge 1 produces more hydrogen than

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Figure 6. Influence of the dry matter content (DOM) on the gasification efficiency and reactor effluent (TC) at T ) 700 °C, 35 MPa, and m ˘ ) 5 g/min.

Figure 5. Influence of the biomass type on the (a) gasification efficiency and (b) gas composition for charge 1 (2002) and charge 2 (2003) at T ) 700 °C, p ) 25 MPa, and DOM ) 5 wt %.

charge 2. The CO2 concentration obtained in the charge 2 is higher than by the charge 1. The growing ambient conditions for both plants were different; a drier ambient was characteristic for the charge 2. This charge has more carbon which can be easier converted to gas components (CO2, CH4, etc.) by the supercritical water. Increase in the carbon content of the biomass produces a gas with more CO2. The gasification yield of charge 2 is higher than the one of charge 1; however, the gas product of charge 1 contains more hydrogen, which is the goal product of the supercritical water gasification process. For a better evaluation of which charge could be the optimal one for this purpose, an economical analysis has to be done, which includes both options of biomass and consideration of the units needed for cleaning of the gas product. Another variable that could affect the gasification of biomass in supercritical water is the lignin concentration. Idea et al.4 found that the presence of lignin in the educt mixture affect the gasification efficiency. A plant with early ripeness will have a higher lignification grade. Visual differences could be observed during the cultivation phase and in the end product (corn silage); for example, Doge was the greenest plant and highest plant. Each corn silage type was prepared with the same method (same crushing unit, etc.). The concentration, pressure, temperature, and residence time were the same for each plant sort. It was found that the type Doge could reach better gasification efficiencies; this type had the ripeness grade of 700, which means that it was the less mature plant. The other plants presented a reduced gasification efficiency. The differences between all plants are less than 5%, although each plant was strongly differentiated from the visual observations. The plant were genetically different, but the compositions were similar as listed in Table 1.

The reaction temperature was set at 600 °C, because it is possible that at higher temperature the gasification efficiency is independent of the type of plant as discussed above. The yield curves are independent of the type at longer residence times. At the shortest residence time at both temperature of 600 and 700 °C, the gasification efficiency of the type Doge was improved compared to the other biomass. The effect of the type of raw plant on the gasification efficiency is negligible when the plants were grown at the same ambient conditions. The effect of the type of corn on the gas composition was investigated. It was observed that the gas composition is almost independent of the biomass type. The corn plant type Doge produced more hydrogen and less carbon dioxide than the other plants; however the variation of the gas composition was less than 5% and the dependence on the type of biomass can be neglected. Influence of the Dry Matter Content on the Gasification of Biomass. Another variable of the educt preparation is the concentration of biomass. The dry matter content was varied from 5 to 20 wt %. Higher concentrations were not possible because plugging of the equipment were observed. The biomass concentration influences the gasification yield, which sinks exponentially with increasing dry organic matter content (DOM) of the educt stream, as observed in Figure 6. The carbon content of the reactor effluent (TC) rises with increasing biomass concentration. At concentrations of 10 wt % the educt was too viscous and higher pressure variations were found in the equipment. The yield curve in Figure 6 seems to approach a constant value at higher concentrations; therefore, the concentration of biomass does not influence the gasification efficiency at higher concentrations. In the experiments with DOM 20 wt % some plugging was observed at longer residence time. This plugging problems at higher concentrations and longer residence times can be explained with the results obtained by Kruse et al.12 They found that the phenol content in the reactor effluent increases at higher dry matter and longer residence time. Phenols are though to be the intermediate for the formation of higher molecular weight products.

Ind. Eng. Chem. Res., Vol. 44, No. 24, 2005 9075 Table 3. Linear Regression of the Gasification Efficiency (Y) as a Function of the Residence Time (τ) and Dry Organic Matter Content of the Biomass (DOM)

Figure 7. Influence of the dry matter content (DOM) on the composition of the gas product at T ) 700 °C, 35 MPa, and m ˘ ) 5 g/min.

In the literature it was reported that the solid formation decreased with pressure when using a model substance.2 Kruse et al.12 found that the phenol concentration in the reactor effluent decreases with rising pressure. For this reason the pressure in the reactor was increased to 35 MPa at DOM 10 wt % and 700 °C. In this experiment, the reactor did not plugged and the gasification yield was almost constant. It is possible that the higher hydrogen partial pressure in the reactor prevents solids formation and consequently plugging of the reactor was inhibited. The gas composition changes with the biomass concentration as shown in Figure 7. CO2 and methane concentration increase with the dry organic matter. The hydrogen concentration decreases at higher dry organic matter. At higher biomass concentrations (DOM >15 wt %), the gas composition remained constant. The effect of the concentration was studied also by Yu et al.7 in the gasification of glucose. They found that the hydrogen yield drops and the methane yield rises as glucose concentration increases. The observed methane concentration increase with the dry matter content of the biomass can be explained with the Le Chatelier principle.12 Hydrogen is produced primarily through the water-gas shift reaction. Increasing water content favors the hydrogen production. Methane formation does not need water and can be produced at lower water concentration. Kinetics. If the gasification efficiency does not change with the dry matter content of the educt, then the approximation to a zero-order kinetics as presented in ref 8 is applicable. In this case the gasification efficiency remained nearly invariable at higher biomass concentrations (>10 wt %). For lower concentrations the effect of the dry organic matter content on the gasification yield has to be taken into account. In this case, a pseudofirst-order kinetics could be assumed, as used by Lee et al.13 to model the gasification of glucose in supercritical water. It was found that each yield curve has a region with a more or less linear growth with the residence time. This part could be approximated with a zero-order kinetic as explained in ref 14, although the yield curve changes with the dry matter content. A linear regression can be applied to each curve; the resulting equation has the form

Y ) kRτ + c

(4)

where kR is the reaction rate constant in min-1 and c is the value of the axis intersection at zero residence time (τ ) o), which should be also zero if the reaction order were constant as a function of the residence time. In

DOM (wt %)

equation

R2

2.5 5 7.5 10

Y ) 0.2946τ + 0.6256 Y ) 0.2138τ + 0.6358 Y ) 0.1636τ + 0.6262 Y ) 0.048τ + 0.7934

0.9833 0.9826 0.9534 0.9831

Table 3 are presented the yield equations obtained for each concentration. The reaction constant of each equation can be expressed as a function of the concentration. In Figure 8 this constant is plotted as function of the DOM of the feed. The equation obtained for 10 wt % was not used here, because at this biomass concentration the reactor plugged and the obtained reaction constant diverged from the other values at lower concentrations. The constant c of eq 4 can be approximated as a constant equal to the average 0.6293. The linear regression of the reaction constant kR as function of the DOM can be observed also in Figure 8. The R2 was almost 1.00, and the approximation is exact.

Figure 8. Approximation of the reaction constant kR as function of the dry matter content of the feed (DOM) at T ) 700 °C and p ) 25 MPa.

The gasification yield as function of the concentration and residence time can be described with the equation

Y ) (-0.0023(DOM) + 0.3409)τ + 0.6293

(5)

This equation can be substituted in the one presented in ref 8. The gasification yield can be estimated with the following equation for the temperature 300-700 °C, DOM 2.5-7.5%, and residence time validity regions depending on each temperature:

(

)

-0.0023(DOM) + 0.3409 102 × 0.2316 -47.9 [kJ] τ + 10-2.8 exp(6.1 × 10-3T [K]) (6) exp RT [K]

Y)

(

)

The results obtained with eq 6 are presented in the Table 4. It can be observed that for the first values of residence time the calculated value of the gasification efficiency (Y*) is similar to the measures values (Y). For longer residence time (longer than 1.5 min), the calculated efficiencies are higher than expected. It is because the yields have a linear increase at shorter residence time and reach a maximum. Prolonging residence time

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Table 4. Results of the Model as Function of the Biomass Concentration and Residence Time τ (min)

Y2.5

Y2.5*

τ (min)

Y5

Y5*

τ (min)

Y7.5

Y7.5*

τ (min)

Y10

Y10*

0.68 1.02 1.28 1.49 1.72 2.24 2.82 3.20

0.82 0.95 1.00 1.06 1.02 1.03 0.90 1.03

0.84 0.95 1.03 1.09 1.17 1.33 1.51 1.63

0.67 1.03 1.29 1.49 1.71 2.26 3.30

0.82 0.91 0.97 0.98 1.02 1.03 1.04

0.79 0.89 0.95 1.00 1.05 1.19 1.45

0.68 0.68 1.04 1.29 1.47 1.67 2.27 2.75 2.78 3.27

0.72 0.75 0.81 0.82 0.84 0.90 0.87 0.92 0.83 0.89

0.75 0.75 0.82 0.87 0.90 0.94 1.05 1.14 1.14 1.23

0.69 1.05 1.72 2.15 2.74 2.74 2.78 3.15 3.21 4.61

0.75 0.84 0.88 0.89 0.93 0.92 0.84 0.76 0.79 0.79

0.71 0.76 0.84 0.89 0.96 0.96 0.96 1.01 1.02 1.19

from this value does not improve the gasification efficiency at each reaction temperature.8 Conclusions The influence of the educt preparation on the gasification of corn silage in supercritical water was investigated. Potassium, up to a concentration of 500 ppm, influences the gasification of model biomass. Further addition of potassium did not improve the gasification yield. The addition of potassium has no effect in the gasification of potassium-containing natural products. For the preparation of biomass, a reduction of the particle size was needed. Fine particles improve the gasification efficiency, and the composition of hydrogen in the gas product increases. The influence of finer particles such as powder has to be investigated. Finer particle could not be obtained with the crushing units used in this work because of the presence of raw fibers in the corn silage. Other crushing units such as the ball mill have to be tested. It is expected that the fiber will not be plugged in this system for size reduction. The use of different corn plants to produce corn silage was investigated. The plants were different in their maturity grades. It was observed that the corn type does not influence the gasification of corn silage when the growing ambient conditions are the same. However, two different charges of corn silage, which were cultivated in two different years, produce different results. The ambient conditions (soil, weather, etc.) are the most important variable to take into account. The gasification yield decreases exponentially with rising dry matter content of the prepared feed up to 10 wt %. At higher biomass concentration, the gasification yield remained constant. Hydrogen content decreases at higher DOM. Methane, ethane, and CO2 concentrations increase with rising biomass concentration. A model for the gasification efficiency based in a zero-order kinetic with concentration influence was obtained. This model applies only in reduced residence time regions. To model each component production as a function of residence time, a more complicated kinetic approach (i.e. fractional order) has to be applied. More experiments at shorter residence times are needed to prove the change of the kinetic order with the residence time. Acknowledgment We appreciate the contributions of LAP Forcheim and University Kassel in the corn silage supply and LUFA Karlsruhe for the analysis. Thanks go to Mr. Drexler for the preparation procedure of the educts, to Mr. K. Weiss for the mechanical work during the experiments, and to Dr. Diem and Dr. Kruse for discussions of the results.

Nomenclature Ci ) concentration of component “i” in the gas product (vol %) C0 ) initial concentration of organic and inorganic carbon in the feed (mol/L) CA ) concentration of “A” (mol/L) m ˘ ) feed flow (g/min) M ) molar mass of carbon (g/mol) pa ) ambient pressure (Pa) P ) gas production (l gas/g C in educt) R ) universal constant of gases Ta ) ambient temperature (K) at ambient conditions V˙ ) gas flow under ambient conditions (l/min) V˙ i ) internal flow under reaction conditions (l/min) V ) reactor volume (l) w ) carbon concentration in the feed (wt %) Y ) carbon gas efficiency Greek Symbols Ri ) number of carbon atoms of component “i” in the gas product τ ) residence time (min)

Literature Cited (1) Yoshida, Y.; Dowaki, K.; Matsumura, Y.; Matsuhashi, R.; Li, D., Ishitani, H.; Komiyama, H. Comprehensive comparison of efficiency and CO2 emissions between biomass energy conversion technologies-position of supercritical water gasification in biomass technologies. Biomass Bioenergy 2003, 25, 257-272. (2) Kruse, A.; Meier, D.; Rimbrecht, P.; Schacht, M. Gasification of Pyrocatechol in Supercritical Water in the Presence of Potassium Hydroxide. Ind. Eng. Chem. Res. 2000, 39, 4842-4848. (3) Sergio Rapagna` and Ajmal Latif. Steam gasification of almond shells in a fluidized bed reactor: the influence of temperature and particle size on product yield and distribution. Biomass Bioenergy 1997, 12 (4), 281-288. (4) Idea, T.; Matsumura, Y. Gasification of cellulose, xylan and lignin mixtures in supercritical water. Ind. Eng. Chem. Res. 2001, 40, 5469-5474. (5) Yoshida, T.; Oshima, Y.; Matsumura, Y. Gasification of biomass model compounds and real biomass in supercritical water. Biomass Bioenergy 2004, 26, 71-78. (6) Boukis, N.; Diem, V.; Habicht, W.; Dinjus, E. Methanol Reforming in Supercritical Water. Ind. Eng. Chem. Res. 2003, 42, 728-733. (7) Yu, D.; Aihara, M.; Antal, M. J. Hydrogen production by steam reforming glucose in supercritical water. Energy Fuels 1993, 7, 574-577. (8) D’Jesu´s, P.; Boukis, N.; Kraushaar-Czarnetzki, B.; Dinjus, E. Influence of the Process Variable on the Gasification of Biomass in Supercritical Water. Ind. Eng. Chem. Res. 2005, submitted for publication. (9) Sinag, A.; Kruse, A.; Schwarzkopf, V. Key Compounds of the Hydropyrolysis of Glucose in Supercritical Water in the Presence of K2CO3. Ind. Chem. Eng. Res. 2003, 42, 3516-3521. (10) Kruse, A.; Abeln, A.; Dinjus, E.; Kluth, M.; Petrich, G.; Schacht, M.; Sadri, E.; Schmieder, H. Gasification of biomass and model compounds in hot compressed water. High Pressure Chem. Eng., Proc. Int. Meet. GVC-Fachausschuss Hochdruckverfahrenstech. 1999.

Ind. Eng. Chem. Res., Vol. 44, No. 24, 2005 9077 (11) Kruse, A. Hydrothermale Wasserstofferzeugung aus Biomass-Einfluss von Inhaltstoffe und Optimierungsansa¨tze. Chem. Ing. Tech. 2004, 9, 1266-1267. (12) Kruse A.; Henningsen. T.; Sinag, A.; Pfeiffer, J. Biomass Gasification in Supercritical Water: Influence of the Dry Matter Content and the Formation of Phenols. Ind. Eng. Chem. Res. 2003, 42, 3711-3717.

(13) Lee, I.-G.; Kim, M.-S.; Ihm, S.-K.: Gasification of glucose in supercritical water. Ind. Eng. Chem. Res. 2002, 41, 1182-1185.

Received for review July 25, 2005 Revised manuscript received September 14, 2005 Accepted September 29, 2005 IE0508637