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Energy So urces, Part A, 30:11661178, 2008

Copyright Taylor & Francis Group, LLC

ISSN: 1556-7036 print/1556-7230 online

DOI: 10.1080/15567030701258246

Hydrogen Production from Biomass Wastes byHydrothermal Gasification

L. KONG,1 G. LI,1 B. ZHANG,1 W. HE,1 and H. WANG1

1State Key Lab of Pollution Control and Resource Reuse, College of

Environmental Science and Engineering, Tongji University, Shanghai,

P. R. China

Abstract Biomass is a useful feed material for energy and chemical resources.

Hydrothermal gasification of biomass wastes has been identified as a possible system

for producing hydrogen. Supercritical and subcritical water has attracted much atten-tion as an environmentally benign reaction medium and reactant. The main objectiveof this study is to assess and introduce the hydrothermal gasification of biomasswastes containing various quantities of the model compounds and real biomass. The

decomposition of biomass, as a basis of hydrothermal treatment of organic wastes,is introduced. To eliminate chars and tars formation and obtain higher yields ofhydrogen, catalyzed hydrothermal gasification of biomass wastes is summarized.

Keywords biomass waste, hydrogen, hydrothermal gasification, subcritical and su-percritical water

1. Introduction

Biomass is a substance made of organic compounds originally produced by absorbing

carbon dioxide in the atmosphere during the process of plant photosynthesis. As long as

the original biomass species are reproduced, cyclic follow of carbon dioxide and other

forms of carbon that we use as energy or materials in the atmosphere can be realized.

Since the concentration of carbon dioxide in the atmosphere theoretically remains constant

in this cycle, biomass is expected to become one of the key sources of renewable energy

in the sustainable society of the future. In the past decades, the interest to use biomass as

energy and resource production has increased. Energy and resource from biomass may

contribute in a considerable amount to the growing future energy and resource demand

(Matsumura et al., 2006). Energy and resource from biomass can additionally avoid the

increase of carbon dioxide in the atmosphere and would help to meet the obligations of

the Kyoto Protocol to reduce carbon dioxide release.

In the past, for dry forms of biomass, such as wood and straw, conventional thermo-

chemical gasification processes are applicable. At the same time, combustion of agricul-

ture wastes was the most important method for warming in the Chinese countryside. For

wet forms such as sewage sludge, cattle manure, and food industry waste, biomethanation

has been the only method applied. Biomethanation is a slow reaction taking almost

Address correspondence to Li Gunangming, State Key Lab of Pollution Control and ResourceReuse, College of Environmental Science and Engineering, Tongji University, 1239 Siping Road,Shanghai, 200092, P. R. China. E-mail: ligm@mail.tongji.edu.cn

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24 weeks, and the treatment of fermentation sludge and wastewater from the reactors

is now a large problem in China.

Nowadays, large amounts of hydrogen gas are used in the petrochemical and chemical

industry. Future developments of fuel cells will also stimulate the need of this gas.

However, hydrogen is a gas that cannot be directly available in nature, and thus mustbe produced from other substances. Most industrial processes for hydrogen production

use reforming techniques, which require hydrocarbons, and stem from the oil industry.

Thus, hydrogen produced in that way cannot any longer be considered a clean gas,

especially because of its bonds with oil production, which is limited by carbon dioxide

formation and by geopolitical aspects. As the reduction of greenhouse effect and economic

dependence on fossil fuels is highly dependent on reduction of fuels from oil and gas,

new ways of hydrogen gas production have been studied all over the world in the last

several years. One of the processes is biomass gasification, which has the advantage of

recovering wastes. This process can be used to synthesize not only hydrogen but also fuels

and a large number of different chemical compounds. Thus, gasification offers flexibility

toward both feedstock and final products. However, this well-studied process, which

usually uses high temperature steam (>973 K) under atmospheric pressure, produces

not only hydrogen but also carbon monoxide. In efforts to surmount these problems,

bioenergy researchers are focusing on a technology called hydrothermal gasification.

The main objective of this study is to assess the decomposition progress of biomass

wastes and model compounds under hydrothermal conditions. At the same time, the

recent advances of biomass gasification under hydrothermal conditions are investigated.

The developments of hydrothermal utilization of biomass wastes are also introduced.

2. The Characters of Hydrothermal Treatment

The interest of hydrothermal treatment, i.e., water with temperature and pressure near

and above its critical point (T > 374C and P > 22 MPa), serves as a reactive medium

due to its specific transportation and solubilization properties. Indeed, in such conditions,

water undergoes significant variations of its physical properties, like a decreasing of thedielectric constant, thermal conductivity, ion product, and viscosity, while the density

only decreases slowly. Thus, water acts as a homogeneous non-polar solvent of high

diffusivity and high transport properties, able to dissolve any organic compounds and

gases (Masaru et al., 2004; Peter and Eckhard, 2001; Phillip, 1999; Marc et al., 2004).

In such a process, hydrogen can be produced at thermodynamic equilibrium because of

the operating conditions. Chemical reactions with high efficiencies can be obtained in

the case of a water organic mixture without interfacial transport limitations. Therefore,

the conversion yields become significant (>99%) with a rather high (up to at least 50%)

percentage of hydrogen in the formed gas when model wastes are treated. Furthermore,

the hydrogen is produced at high pressure directly, which means a smaller reactor volume

and lower energy to pressurize the gas in a storage tank. A large portion of biomass

wastes, e.g., from agriculture and food industries, is wet biomass containing up to 95%

water. This wet biomass causes high drying costs if classical gas-phase gasificationor liquefaction processes are used. This can be advantageously avoided by using a

gasification or liquefaction in near-critical and supercritical water. The use of water in

hydrothermal conditions instead of atmospheric pressure steam could be advantageous

for converting biomass into pure hydrogen. Indeed, in such high pressure and temperature

conditions, it is possible to get high conversion levels of biomass thanks to the specific

properties of supercritical and subcritical water.

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Recently, researchers focus on the utilization of biomass wastes in subcritical and

supercritical water. Through hydrothermal treatment, they obtain useful chemical feed-

stock, such as acetic acid and lactic acid. Jin et al. (2001, 2003, 2005) carried out a

series of experiments to research the hydrothermal conversion of biomass water and

control pathways of hydrothermal reaction to improve the acetic acid yield. A two-stephydrothermal progress to increase the yield of acetic acid was discussed. The first step

was to accelerate the formation of HMF, 2-FA, and lactic acid (LA), and the second step

was to further convert the furans (HMF, 2-FA) and LA produced in the first step to acetic

acid by oxidation with newly supplied oxygen. The acetic acid obtained by the two-step

process had not only a high yield but also better purity. The contribution of two pathways

via furans and LA in the two-step process to convert carbohydrates into acetic acid was

roughly estimated as 8590%. At the same time, lactic acid, glucose, and acetic acid

were also produced by others research groups (Armando et al., 2002; Motonobu et al.,

1998, 2004; Lourdes and David, 2002; Shanableh, 2000).

3. Decomposition of Biomass Waste

The proton-catalyzed mechanism, direct nucleophilic attack mechanism, hydroxide ion

catalyzed mechanism, and radical mechanism play important roles in the hydrolysis

of biomass wastes. The most likely source of hydroxide and hydroxide ions is the

high temperature water itself because subcritical and supercritical water have a stronger

tendency to ionize than ambient water, which makes water a Brnsted base acid and acts

as an effective catalyst (Jin et al., 2005). The decomposition of biomass wastes under

hydrothermal conditions including hydrolysis, dissolution, pyrolysis, and all of them

favor decomposition and gasification. At the same time, the process under hydrothermal

conditions shows similarities to other methods as well as significant differences due to the

presence of water as the reactant, reaction medium, and catalyst (Peter, 2004; Noam and

Ronald, 2003; Oka et al., 2002). Usually, detailed chemical reaction pathways with well-

defined single reaction steps cannot describe the degradation of biomass in supercritical

and subcritical water. One reason is that biomass is a combination of cellulose, hemicel-lulose, and lignin. These components interact with each other, leading to a very complex

chemical mechanism. The chemical mechanisms inducing hydrogen formation from raw

biomass and decomposition are very complex and cannot be easily summarized (Minowa

et al., 1998, 1999). It is possible to say that pyrolysis, hydrolysis, steam reforming,

water gas shift, methanation, and other reactions play a role in the gasification chemistry.

Another reason is that it is mainly a heterogeneous process, proceeding inside and,

in particular, on the surface of biomass particles. The heterogeneous reaction cannot be

directly compared with homogeneous reactions of other organic compounds. Here, studies

of a pure component like crystalline cellulose lead to more detailed information. The

biomass wastes can be decomposed through hydrothermal treatment into aqueous phase,

oil, gas, and residue. The procedure of biomass decomposition is shown in Figure 1

(Minowa et al., 1999).

At low temperature regions, the oligomer is the main liquefaction product of biomass,is the most part lower organic compound. At the same time, the conversion rate of

oligomer is much faster than the hydrolysis rate of biomass wastes. Thus, even if the

hydrolysis products such as oligomer or glucose are formed, their further decomposition

rapidly takes place, and thus a high yield of hydrolysis products cannot be obtained.

However, around the critical point, the hydrolysis rate jumps to more than an order of

magnitude higher level and becomes faster than the oligomer decomposition rate. When

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Figure 1. The procedure of biomass decomposition.

the temperature is higher than 400C, the breakage of interior and intermolecular HH

bonds happen with ease and produce a large number of H2, CO, CH4, and tar (Mitsuru

et al., 2000, 2003; Kim et al., 2004).

To discuss the chemistry passing off, here key compounds such as crystalline,

cellulose, glucose, and organic acids are identified and quantified in studies of biomass

conversion in water. These compounds are formed by different and typical reaction

pathways and are therefore a tool to compare complex chemical processes. The key

compounds make it possible to compare the results of model compound reaction with

those of the reaction of real biomass. A comparison of the changes in the key compound

concentration for different types of biomass should give hints about the influence of

biomass composition on chemistry. For example, through the basic study cellulose model

compounds (crystalline, cellulose, glucose) and their decomposition products and the

comparison with literature data, the main reaction pathway has been elucidated. Figure 2

shows the result of cellulose decomposition (Mitsuru et al., 1998; Jin et al., 2004; Bicker

et al., 2005).

Cellulose hydrolysis produces oligomers and glucose. Glucose epimerizes to fructose

by the Lobry de Bruyn-Alberda van Ekenstein (LBAE) transformation or decomposes

to erythrose plus glycolaldehyde or glyceraldehydes plus dihydroxyacetone. Produced

fructose also decomposes to erythrose plus glyceraldehydes or glyceraldehyde plus dihy-

droxyacetone. Glyceraldehyde converts to dihydroxyacetone and both glyceraldehyde

and dihydroxyacetone dehydrated into pyruvaldehyde. Pyruvaldehyde, erythrose, and

glycolaldehyde further decompose to smaller species, which are mainly acid, aldenydes,and alcohols of 13 carbons.

4. Biomass Gasification

Biomass can be very effectively utilized when converted into gas fuel, particularly

hydrogen gas (Knoef, 2005). To produce hydrogen from water using biomass, the biomass

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Figure 2. The main reaction pathway of cellulose decomposition.

is first properly gasified, and then the product gas is reformed into hydrogen via reactions

with water. Biomass gasification technologies are summarized in Figure 3.

Most of the research spurred by this interest has been of economic technology in

nature, based on gasifier performance data acquired during system proof of conceptual

test. Less emphasis has been given to experimental investigation of hydrogen production

via biomass gasification. Until now, all process equipment needed to produce hydrogen

has been well established in commercial use, except for the gasifier. Comparison with

other biomass thermochemical gasification such as air gasification or steam gasification,the hydrothermal gasification can directly deal with the wet biomass without drying and

have high gasification efficiency at lower temperature.

Figure 3. The main methods of biomass gasification.

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The steam reforming of biomass in steam is proposed as a viable source of hydrogen

by Antal et al. (1994) in the Hawaii Natural Energy Institute (HNEI). Subsequent research

revealed that steam pyrolysis of biomass results in the formation of many gaseous

products, as well as a refractory tar. Hydrogen yields are not high. Consequently, interest

in biomass pyrolysis as a source of hydrogen declined. After a decade of disinterest,work on the steam reforming of biomass in water as a source of hydrogen commenced

again, focusing on hydrothermal gasification. Systematic experimental investigations for

the conversion and gasification of biomass by hydrothermal treatment were carried out

by Elliott and Sealock (1996) in the Pacific Northwest Laboratory (PNL) of the United

States, Minowa in the National Institute for Resources and Environment (NIRE) of Japan

(Usui et al., 2000), and Schmieder et al. (2000) in Forschungszentrum Karlsruhe Institut

fr Technische Chemie (FKITC) of Germany. Simultaneously, others groups also carry

out much research on the gasification of biomass wastes and their model compound.

4.1. Gasification Progress

Kruse and Gawlik (2003), Kruse and Henningsen (2003), Kruse et al. (2005), and Sinag

et al., (2004) studied the degradation of biomass in the ranges of 330C410C and

3050 MPa and at 15 min of reaction time. Comparing the results from earlier studies

of model compounds, e.g., glucose or cellulose, with biomass degradation is to identify

chemical reaction pathways. The simplified reaction mechanism of cellulose degradation

during hydrothermal gasification is shown in Figure 4. The results show that the key

compounds are phenols (phenol and cresols), furfurals, acids (acetic acid, formic acid,

lactic acids, and levulinic acid), and aldehydes (acetic aldehyde and formic aldehyde).

Through gasifying biomass in a continuously stirred tank reactor (CSTR), they identify

that the results concerning the dependence of the dry matter content on the gas formation,

total organic carbon content, and phenols concentration are very different. In the CSTR

the increase of the dry matter content leads to an increased gas yield, in particular of

CH4, no char/coke and no increased tar formation with increasing dry matter content,

and the phenols yield increases. The reason may be the very fast heating and the back

Figure 4. The schematic representation of the gasification of cellulose.

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mixing, which leads to the presence of reactive hydrogen during every step of biomass

degradation. Here the phenol formation is the last hurdle for complete conversion. This

is not found in the batch reactor. Biomass is much more complex because biomass

contains a lot of different substances. Especially, the influence of salts is significant and,

in addition, rather complex. At the same time, the influence of water properties fromsubcritical to supercritical conditions on the biomass degradation is also obvious.

4.2. Model Compound and Real Biomass Gasification

Some researchers chose glucose as a representative biomass model compound to gasifying

under hydrothermal conditions. The main gases that Paul and Jude (2005) found in their

experiments were carbon dioxide, carbon monoxide, methane, and hydrogen, and there

is significant production of oil and char. As the temperature and the concentration of the

oxidant, hydrogen peroxide, are increased, there is an increase in the yield of gas. The

increase in the concentration of the oxidant, hydrogen peroxide, will decrease the amounts

of char, oil, and water-soluble products. The product yield and composition do not

significantly change with the temperature (and pressure) and residence time. The increase

of glucose in the reactor system causes a decrease in the gasification of glucose and results

in significant formation of char and oil. Lee et al. (2002) reports the gasification of glucose

using tubular-flow reactors at 480C750C and 28 MPa. The hydrogen yield increases

sharply with increasing temperature over 660C. It is believed that the water-gas shift

reaction occurred significantly at temperatures over 660C. Methane is identified as a

very stable compound in supercritical water at temperatures as high as 700C. Carbon

gasification efficiency remained 100% at 700C for a wide range of reactor residence

times of 1050 s. Hao et al. (2003) utilized glucose as a model compound of biomass

to form a product gas composed of H2, CO, CH4, CO2, and a small amount of C2H4and C2H6. Glucose at low concentrations (ca. 0.1 M) can be completely gasified in

923.15 K, 25 MPa, and 3.6 min resident time and no char or tar is observed. The raw

biomass feedstock of sawdust with some CMC is also gasified in this system and the

gasification efficiency reaches in excess of 95%. Ayhan (2004) investigated the yieldsof total extraction products from supercritical water extraction, which increased with

increasing temperature for all runs. The yields of hydrogen (YHs) increase with increasing

temperature and pressure for all runs, and the increase of YHs with pressure are higher

than those with temperature. Takuya and Yukihiko (2001) examined the gasification of

cellulose, xylan, and lignin mixtures in supercritical water at 623 K and 25 MPa. Their

results indicate that a decrease of gas production is observed for the mixtures containing

lignin. Thus, they surmise that cellulose or xylan is likely to function as a hydrogen donor

to lignin. The reaction of intermediates from cellulose and xylan with lignin results in

a decrease in H2 production. A set of equations develops to estimate the amount and

composition of the product gas to accurately predict the actual results using only the

lignin fraction as a parameter. This confirms the importance of the lignin fraction effect

on hydrothermal gasification characteristics.

4.3. Catalyst Gasification

However, in the real case, all of the biomass does not react with supercritical and subcrit-

ical water, although its reactivity is higher in this specific medium than in atmospheric

pressure steam. Every organic molecule is not transformed into hydrogen or carbon

dioxide gases. Significant amounts of tars and chars can be formed during the reaction.

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This shift from thermodynamic expectations has, however, been reduced by the use of

a catalyst. The catalysts that used in the experiments are summarized in Table 1. The

table shows us that base catalyst is the most important catalyst, and through base catalyst

we can enhance the hydrogen yield and rate in the gas. A conversion mechanism is

suggested which consists of decomposition of big molecules to small molecules on themetal surface, steam gasification of small molecules to produce CO and H 2, followed by

CO methanation and CO shift reaction to produce CH4 and CO2. The catalyst is found

to be highly active and stable with no sintering (Tang and Kuniyuki, 2005; Osada et al.,

2006).

4.3.1. Metal Catalyst. Takuya et al. (2004) gasifies lignin, cellulose, and their mixture

with a nickel catalyst under hydrothermal conditions at 673 K and 25 Mpa. When

softwood lignin is included in the feedstock, gasification efficiency is low but increases

with the amount of the catalyst. Sufficient amount of catalyst achieves high gasification

efficiency even for the mixtures of cellulose and softwood lignin. One possible mechanism

is the catalyst being deactivated by tarry products from the reaction between cellulose

and softwood lignin. But the gasification of hardwood and grass lignin is much easier.

Takafumi et al. (2003) conducts gasification of alkylphenols in the presence of various

supported metal catalysts at 673 K. The results show that activity of the catalyst is

in the order of Ru/c-alumina > Ru/carbon, Rh/carbon > Pt/c-alumina, Pd/carbon, andPd/c-alumina. The main gas products are methane, carbon dioxide, and hydrogen. The

analysis of liquid products shows that dehydroxylation occurs easier than dealkylation for

supported ruthenium and rhodium catalysts. The sum of the yield of gases and the ratio

of methane from 4-propylphenol with a Ru/c alumina catalyst increases with increasing

water density, while the yield of liquid products shows a maximum at 0.1 g/cm3. The

gasification of various alkyl-phenols is investigated over a Ru/c-alumina catalyst at 673 K

and 0.3 g/cm3 of water density for 15 min. The yield of gas is above 10% and in the

order of 4-isopropylphenol > 2-isopropylphenol, 2-propylphenol > 4-propylphenol >

3-isopropylphenol. The composition of gas is 5060% methane, 3040% carbon dioxide,

and 10% hydrogen.Takuya and Yoshito (2004) developed a flow reactor system that smoothly gasifies

glucose and glucose-lignin mixture solution at 673 K, 25.7 MPa. The reactor system

consists of three continuous reactors, which are a pyrolysis reactor, an oxidation reactor,

and a catalytic reactor with nickel catalyst. The reactions occur in each reactor as

follows. In the pyrolysis reactor, there are mainly two kinds of reactions: decomposition

and polymerization. The decomposition proceeds in the early stage of the reaction.

However, long residence time in this reactor causes undesirable polymerization of biomass

fragment. Consequently, moderate residence time in the pyrolysis reactor is favorable in

their reactor system. In the oxidation reactor, tar and/or char products are effectively

decomposed via radical reaction led by oxidant to low molecular weight products that

can be decomposed in the catalytic reactor. With residence time in the oxidation reactor

that is too short, high molecular weight compounds such as tarry products decompose

insufficiently. In the catalytic reactor, CO is converted to H2 and CO2 via water-gasshift reaction, and low molecular weight liquid compounds are also decomposed to gas.

However, heavy molecular compounds such as tarry and/or char products are not easily

decomposed via catalytic reaction. They reveal that supercritical condition is suitable

for gasification of biomass because the tar and/or char products are decomposed easily.

By employing an oxidation reactor even at low temperature (around 673 K), they settle

the char plug problem and enhance gasification ratio and content of hydrogen gas in its

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Table 1

The state of catalytic hydrothermal gasification of biomass

Feedstock CatalystReactioncondition

Mainproduct gas Reactor

Glucose Ni, K2CO3 500C Hydrogen Tumbling batch

autoclaveOrganic wastewater Ni/carbon 360C, 20 MPa Methane,

hydrogen

Cellulose, softwood,hardwood, and grass lignin

Ni 400C, 25 MPa Microreactor

Sawdust, rice straw,alkylphenols

Ru/carbon, Rh/carbon,Pd/carbon,Ru/c-aluminaPt/c-alumina,

Pd/c-alumina

400C Methane,hydrogen

Tube bombreactors

D-Glucose lignin Ni 400C, 25.7 MPa Hydrogen Continuous flowreactor

Corn- and potato-starch gels,sawdust, cornstarch gel,potato wastes

Carbon 650C, 22 MPa Hydrogen Tubular flowreactors

Glycerol, glucose, cellobiose,bagasse, sewage sludge,DoD wastes

Charcoal activatedcarbon

600C, 34.5 MPa Supercritical floreactor

N-hexadecane NaOH 400C Hydrogen Batch reactor Organosolv-lignin ZrO2 30, 40 MPaGlucose, catechol KOH 600C, 25 MPa Hydrogen Batch autoclave

tubular flowreactors

Vanillin, glycine K2CO3Straw, sewage, sawdust,

sludge, lignin pyrocatecholKOH 600C700C Hydrogen Batch and tubu

reactor

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production gas. They have succeeded in achieving high gasification efficiency based on

carbon up to 96% at 673 K, 25.7 MPa, with total residence time of about 1 min. The

main gaseous products are H2 and CO2.

4.3.2. C Catalyst. Michael et al. (2000) studied the gasification of biomass feedstocks,

including corn- and potato-starch gels, wood sawdust suspended in a cornstarch gel, and

potato wastes, which are delivered to three different tubular flow reactors by means of

a cement pump. The organic content of these feedstocks is vaporized at temperatures

above 650C and pressures above 22 MPa. A packed bed of carbon within the reactor

catalyzed the gasification of these organic vapors in the water; consequently, the water

effluent of the reactor is clean. The gas is composed of hydrogen, carbon dioxide,

methane, carbon monoxide, and traces of ethane. The gas composition and gas yield

are strongly affected by the reaction temperature. High entrance temperatures favor

the methane steam-reforming reaction and result in the production of a hydrogen-rich

gas (57 mol%) with yields exceeding 2 L/g. Xu et al. (1996) utilized spruce wood

charcoal, macadamia shell charcoal, coal activated carbon, and coconut shell activated

carbon as catalyst to gasify organic compounds. Feedstocks studied in this article includeglycerol, glucose, cellobiose, whole biomass feedstocks (depithed bagasse liquid extract

and sewage sludge), and representative Department of Defense (DoD) wastes (methanol,

methyl ethyl ketone, ethylene glycol, acetic acid, and phenol). Complete conversion

of glucose (22% by weight in water) and the whole biomass feeds to a hydrogen-rich

synthesis gas is realized at a weight hourly space velocity (WHSV) of 22.2 h1 at 600C,

34.5 MPa. In the presence of the carbon catalyst, temperatures above about 600C are

needed to achieve high gasification efficiencies for concentrated organic feeds in water.

The carbon gasification efficiency remained near 100% for more than 6 h when a swirl

generator is employed in the entrance of the reactor.

4.3.3. Base Catalyst. Masaru et al. (2003) studied partial oxidation of n-hexadecane and

organosol-lignin by using a batch type reactor in supercritical water. As the result, theaddition of base catalyst (ZrO2and NaOH) does not increase the conversion of n-C16 and

promotes the formation of 1-alkenes and H2. Since the H2=CO2 ratio is almost or more

than unity, partial oxidation into CO and base catalyst enhances water-gas shift reaction.

The experiments with and without O2 are also conducted for lignin. The yield of H2from lignin with zirconia and sodium hydroxide (NaOH) is 2 and 4 times, respectively,

the same as that without catalyst at the same condition for both with and without O 2.

Thus, a base catalyst has a positive effect on decomposition and partial oxidation of

lignin to gaseous products such as H2. In the case of lignin studies, the enhancement

of decomposition of the carbonyl compounds (aldehyde and ketone) by catalytic effect

of NaOH and ZrO2 inhibit char formation and promote CO and thus H2 formation.

Schmieder et al. (2000) shows that in the presence of KOH or K 2CO3 at 250 bar

and temperatures higher than 550C600C, carbohydrates, aromatic compounds, glycine

as a model compound for proteins, and real biomass are completely gasified to a H2-rich product containing CO2 as the main carbon compound. The addition of potassium

decreases the COX concentration and increases CO2 and H2 in the product gas; carbon

balances for the miniature plant are close to better than 96%. Compared to the traditional

gasification process for the hydrothermal gasification has the following advantages for a

wet biomass: organic waste feedstock can be expected, much higher thermal efficiency,

a hydrogen-rich gas with low CO yield can be produced in one process step, soot and

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tar formation can be suppressed, and the heteroatomes (S, N, and halogenes) leave the

process with the aqueous effluent avoiding expensive gas cleaning.

Andrea et al. (2000) uses pyrocatechol as a model compound for lignin in biomass

and for aromatic compounds in wastewaters to research the chemical reactions occurring

during gasification and their dependence on reaction conditions. They carry out theexperiments in two different reactor types, a batch and a tubular reactor, to achieve long

reaction times at low temperatures as well as short reaction times at high temperatures.

More than 99% of the pyrocatechol is gasified at 600C with a 2-min reaction time

or at 700C with a 1-min reaction time. The addition of KOH and other salts increase

the relative yields of hydrogen and carbon dioxide and decrease the relative CO yield

by acceleration of the water-gas shift reaction. Thermodynamic calculations and the

experimental results show that an increase in temperature and time and a decrease of

pressure leads to an increase of hydrogen formation as well as a decrease in the methane

yield. Doubling the concentration of pyrocatechol leads to a decrease in hydrogen yield

and gasification efficiency.

5. Conclusions

Hydrothermal gasification of biomass wastes provides a new idea for the treatment and

utilization of organic waste. Hydrogen can be obtained as the main production under

hydrothermal conditions when water serves as a potential environmentally benign medium

and reactant for industrial chemical reactions. Compared with other biomass thermochem-

ical processes such as pyrolysis, gasification, air gasification, or steam gasification, the

supercritical water gasification can directly deal with wet biomass without drying and

have high gasification efficiency at lower temperature. Catalysis should be the solution to

obtain higher yields of hydrogen and to decrease the amount of chars and tars. Carbon and

base catalyst play an important role in the increase of yields and ratio in produced gas.

Hydrothermal process is also one of the most promising processes for the conversion

of biomass waste into useful materials among several biomass conversion processes.

Through carrying out the basic research and developing catalyst, the industrialization ofthe hydrothermal gasification of biomass wastes will be realized.

Acknowledgment

The work was supported by Fund of Science and Technology Commission of Shanghai

Municipality.

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