Hydrogen Production from Biomass Wastes by ... - CiteSeerX

Energy Sources, Part A, 30:1166–1178, 2008 Copyright © Taylor & Francis Group, LLC ISSN: 1556-7036 print/1556-7230 online DOI: 10.1080/15567030701258246

Hydrogen Production from Biomass Wastes by Hydrothermal Gasification L. KONG,1 G. LI,1 B. ZHANG,1 W. HE,1 and H. WANG1 1

State 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 attention as an environmentally benign reaction medium and reactant. The main objective of this study is to assess and introduce the hydrothermal gasification of biomass wastes 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 of hydrogen, catalyzed hydrothermal gasification of biomass wastes is summarized. Keywords biomass waste, hydrogen, hydrothermal gasification, subcritical and supercritical 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 thermochemical gasification processes are applicable. At the same time, combustion of agriculture 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 Resource Reuse, College of Environmental Science and Engineering, Tongji University, 1239 Siping Road, Shanghai, 200092, P. R. China. E-mail: [email protected]

1166

H2 Production from Biomass Waste HTG

1167

2–4 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 must be 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 > 374ıC 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 the dielectric 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 gasification or 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.

1168

L. Kong et al.

Recently, researchers focus on the utilization of biomass wastes in subcritical and supercritical water. Through hydrothermal treatment, they obtain useful chemical feedstock, 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-step hydrothermal 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 85–90%. 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 Brønsted 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 welldefined single reaction steps cannot describe the degradation of biomass in supercritical and subcritical water. One reason is that biomass is a combination of cellulose, hemicellulose, 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


1169

Figure 1. The procedure of biomass decomposition.

the temperature is higher than 400ı C, the breakage of interior and intermolecular H–H 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 dihydroxyacetone. 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 1–3 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

1170

L. Kong et al.

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.


1171

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 für 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 330ı C–410ıC and 30–50 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.

1172

L. Kong et al.

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 from subcritical 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 480ıC–750ıC and 28 MPa. The hydrogen yield increases sharply with increasing temperature over 660ı C. It is believed that the water-gas shift reaction occurred significantly at temperatures over 660ıC. Methane is identified as a very stable compound in supercritical water at temperatures as high as 700ıC. Carbon gasification efficiency remained 100% at 700ıC for a wide range of reactor residence times of 10–50 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 C2 H4 and C2 H6 . 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 yields of 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 subcritical 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.


1173

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 the metal surface, steam gasification of small molecules to produce CO and H2 , 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, and Pd/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 50–60% methane, 30–40% 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-gas shift 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

Table 1 The state of catalytic hydrothermal gasification of biomass Feedstock

Catalyst

Reaction condition

Main product gas

1174

Glucose

Ni, K2 CO3

500ı C

Hydrogen

Organic wastewater

Ni/carbon

360ı C, 20 MPa

Cellulose, softwood, hardwood, and grass lignin Sawdust, rice straw, alkylphenols

Ni

400ı C, 25 MPa

Ru/carbon, Rh/carbon, Pd/carbon, Ru/c-alumina Pt/c-alumina, Pd/c-alumina Ni

D-Glucose lignin Corn- and potato-starch gels, sawdust, cornstarch gel, potato wastes Glycerol, glucose, cellobiose, bagasse, sewage sludge, DoD wastes N-hexadecane Organosolv-lignin Glucose, catechol Vanillin, glycine Straw, sewage, sawdust, sludge, lignin pyrocatechol

Reactor

Reference

Methane, hydrogen —

Tumbling batch autoclave —

Tang and Kuniyuki, 2005 Osada et al., 2006

Microreactor

Takuya et al., 2004

400ı C

Methane, hydrogen

Tube bomb reactors

Takafumi et al., 2003

400ı C, 25.7 MPa

Hydrogen

Carbon

650ı C, 22 MPa

Hydrogen

Continuous flow reactor Tubular flow reactors

Takuya and Yoshito, 2004 Michael et al., 2000

Charcoal activated carbon

600ı C, 34.5 MPa

—

Supercritical flow reactor

Xu et al., 1996

NaOH ZrO2 KOH

400ı C 30, 40 MPa 600ı C, 25 MPa

Hydrogen

Batch reactor

Masaru et al., 2003

Hydrogen

Batch autoclave, tubular flow reactors

Sharma et al., 2006

K2 CO3 KOH

600ı C–700ıC

Hydrogen

Batch and tubular reactor

Andrea et al., 2000


1175

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 650ıC 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 include glycerol, 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 h 1 at 600ı C, 34.5 MPa. In the presence of the carbon catalyst, temperatures above about 600ı C 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, the addition of base catalyst (ZrO2 and 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 H2 from 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 O2 . 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 K2 CO3 at 250 bar and temperatures higher than 550ıC–600ıC, 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

1176

L. Kong et al.

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 the experiments 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 600ıC with a 2-min reaction time or at 700ı C 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 thermochemical 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 of the hydrothermal gasification of biomass wastes will be realized.

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

References Andrea, K., Danny, M., Pia, R., et al. 2000. Gasification of pyrocatechol in supercritical water in the presence of potassium hydroxide. Ind. Eng. Chem. Res. 39:4842–4848. Antal, Jr., M. J., Manarungson, S., and Mok, W. S.-L. 1994. Hydrogen production by steam reforming glucose in supercritical water. In: Advances in Thermochemical Biomass Conversion, Bridyewater A.V. (Ed.). London: Blackie Academic and Professional, pp. 1367–1377. Armando, T. Q., Muhammad, F., Kilyoon, K., et al. 2002. Low molecular weight carboxylic acids produced from hydrothermal treatment of organic wastes. J. Hazard. Mater. B93:209–220. Ayhan, D. 2004. Hydrogen-rich gas from fruit shells via supercritical water extraction. Int. J. Hydrogen Energy 29:1237–1243.


1177

Bicker, M., Endres, S., Ott, L., et al. 2005. Catalytical conversion of carbohydrates in sub-critical water: A new chemical process for lactic acid production. J. Mol. Catal. A: Chem. 239:151– 157. Elliott, D. C., and Sealock, Jr., L. J. 1996. Chemical processing in high-pressure aqueous environments: Low-temperature catalytic gasification. Trans. IchemE. 74:563–566. Hao, X. H., Guo, L. J., Mao, X., et al. 2003. Hydrogen production from glucose used as a model compound of biomass gasified in supercritical water. Int. J. Hydrogen Energy 28:55–64. Jin, F. M., Ausushi, K., and Heiji, E. 2001. Oxidation of garbage in supercritical water. High Pressure Res. 20:525–531. Jin, F. M., Takehiko, M., and Heiji, E. 2003. Oxidation reaction of high molecular weight carboxylic acids in supercritical water. Environ. Sci. Technol. 37:3220–3231. Jin, F. M., Zhou, Z. Y., and Enomoto, H. 2004. Conversion mechanism of cellulosic biomass to lactic acid in subcritical water and acid-base catalytic effect of subcritical water. Chem. Lett. 33:126–127. Jin, F. M., Zhou, Z. Y., and Takehiko, M. 2005. Controlling hydrothermal reaction pathways to improve acetic acid production from carbohydrate biomass. Environ. Sci. Technol. 39:1893– 1902. Kim, I. C., Park, S. D., and Kim, S. 2004. Effects of sulfates on the decomposition of cellobiose in supercritical water. Chem. Eng. Process. 43:997–1005. Knoef, H. 2005. Handbook Biomass Gasification. Netherlands: Biomass Technology Group Press, 22–23. Kruse, A., and Gawlik, A. 2003. Biomass conversion in water at 330ıC–410ıC and 30–50 MPa. Identification of key compounds for indicating different chemical reaction pathways. Ind. Eng. Chem. Res. 42:267–279. Kruse, A., and Henningsen, T. 2003. Biomass gasification in supercritical water: influence of the dry matter content and the formation of phenols. Ind. Eng. Chem. Res. 42:3711–3717. Kruse, A., Krupka, A., and Schwarzkopf, V. 2005. Influence of proteins on the hydrothermal gasification and liquefaction of biomass. 1. Comparison of different feedstocks. Ind. Eng. Chem. Res. 44:3013–3020. Lee, I. G., Kim, M. S., and Ihm, S. K. 2002. Gasification of glucose in supercritical water. Ind. Eng. Chem. Res. 41:1182–1188. Lourdes, C., and David, V. 2002. Formation of organic acids during the hydrolysis and oxidation of several wastes in sub- and supercritical water. Ind. Eng. Chem. Res. 41:6503–6509. Marc, H., Philip, A. M., Glenn, T. H., et al. 2004. Salt precipitation and scale control in supercritical water oxidation—Part A: Fundamentals and research. J. Supercrit. Fluids 29:265–288. Masaru, W., Hiroshi, I., Mitsumasa, O., et al. 2003. Catalytic effects of NaOH and ZrO2 for partial oxidative gasification of n-hexadecane and lignin in supercritical water. Fuel 82:545–552. Masaru, W., Takafumi, S., Hiroshi, I., et al. 2004. Chemical reactions of C1 compounds in nearcritical and supercritical water. Chem. Rev. 104:5803–5820. Matsumura, Y., Sasaki, M., Okuda, K., et al. 2006. Supercritical water treatment of biomass for energy and material recovery. Combust. Sci. Technol. 178:509–536. Michael, Jr., J. A., Stephen, G. A., et al. 2000. Biomass gasification in supercritical water. Ind. Eng. Chem. Res. 39:4040–4053. Minowa, T., Zhen, F., and Ogi, T. 1998. Cellulose decomposition in hot-compressed water with alkali or nickel catalyst. J. Supercrit. Fluids 13:253–259. Minowa, T., Zhen, F., and Ogi, T. 1999. Liquefaction of cellulose in hot-compressed water using sodium carbonate: production distribution at different reaction temperature. J. Chem. Eng. Jpn. 30:186–190. Mitsuru, S., Bernard, K., Roberto, M., et al. 1998. Cellulose hydrolysis in sub-critical and supercritical water. J. Supercrit. Fluids 13:261–268. Mitsuru, S., Tadafumi, A., and Kunio, A. 2003. Fractionation of sugarcane bagasse by hydrothermal treatment. Bioresour. Technol. 86:301–304.

1178

L. Kong et al.

Mitsuru, S., Zhen, F., Yoshiko, F., et al. 2000. Dissolution and hydrolysis of cellulose in sub-critical and supercritical water. Ind. Eng. Chem. Res. 39:2883–2890. Motonobu, G., Ryusaku, O., Tsutomu, H., et al. 2004. Hydrothermal conversion of municipal organic waste into resources. Bioresour. Technol. 93:279–284. Motonobu, G., Takatsygu, N., Akane, O., et al. 1998. Supercritical water oxidation for the destruction of municipal excess sludge and alcohol distillery wastewater of molasses. J. Supercrit. Fluids 13:277–282. Noam, E., and Ronald, M. L. 2003. Review of materials issues in supercritical water oxidation systems and the need for corrosion control. Trans. Indian Inst. Met. 56:1–10. Oka, H., Yamago, S., Yoshida, J., et al. 2002. Evidence for a hydroxide ion catalyzed pathway in ester hydrolysis in supercritical water. Angew. Chem. Int. Ed. 41:623–625. Osada, M., Sato, T., Watanabe, M., et al. 2006. Catalytic gasification of wood biomass in subcritical and supercritical water. Combust. Sci. Technol. 178:537–552. Paul, T. W., and Jude, O. 2005. Composition of products from the supercritical water gasification of glucose: A model biomass compound. Ind. Eng. Chem. Res. 44:8739–8749. Peter, K. 2004. Corrosion in high-temperature and supercritical water and aqueous solutions: A review. J. Supercrit. Fluids 29:1–29. Peter, K., and Eckhard, D. 2001. An assessment of supercritical water oxidation (SCWO): Existing problems, possible solutions and new reactor concepts. Chem. Eng. J. 83:207–214. Phillip, E. S. 1999. Organic chemical reactions in supercritical water. Chem. Rev. 99:603–621. Schmieder, H., Abeln, J., Boukis, N., et al. 2000. Hydrothermal gasification of biomass and organic wastes. J. Supercrit. Fluids 17:145–153. Shanableh, A. 2000. Production of useful organic matter from sludge using hydrothermal treatment. Water Res. 34:945–951. Sharma, A., Nakagawa, H., and Miura, K. 2006. A novel nickel/carbon catalyst for CH4 and H2 production from organic compounds dissolved in wastewater by catalytic hydrothermal gasification. Fuel 85:179–184. Sinag, A., Kruse, A., Rathert, J. 2004. Influence of the heating rate and the type of catalyst on the formation of key intermediates and on the generation of gases during hydropyrolysis of glucose in supercritical water in a batch reactor. Ind. Eng. Chem. Res. 43:502–508. Takafumi, S., Mitsumasa, O., Masaru, W., et al. 2003. Gasification of alkylphenols with supported noble metal catalysts in supercritical water. Ind. Eng. Chem. Res. 42:4277–4282. Takuya, Y., and Yoshito, O. 2004. Partial oxidative and catalytic biomass gasification in supercritical water: A promising flow reactor system. Ind. Eng. Chem. Res. 43:4097–4104. Takuya, Y., and Yukihiko, M. 2001. Gasification of cellulose, xylan, and lignin mixtures in supercritical water. Ind. Eng. Chem. Res. 40:5469–5474. Takuya, Y., Yoshito, O., and Yukihiko M. 2004. Gasification of biomass model compounds and real biomass in supercritical water. Biomass Bioenergy 26:71–78. Tang, H. Q., and Kuniyuki, K. 2005. Supercritical water gasification of biomass: thermodynamic analysis with direct Gibbs free energy minimization. Chem. Eng. J. 106:261–267. Usui, Y., Minowa, T., Inoue, S., and Ogi, T. 2000. Selective hydrogen production from cellulose at low temperature catalyzed by supported group 10 metal. Chem. Lett. 346:1166–1167. Xu, X. D., Yukihiko, M., Jonny, S., et al. 1996. Carbon-catalyzed gasification of organic feedstocks in supercritical water. Ind. Eng. Chem. Res. 35:2522–2530.