Analytical Chemistry Study on Hydrogen Formation from Biomass by Hydrothermal Process

Noriaki Sugimoto *, Yasuyuki Ishida †, Yuta Shimizu ‡, Kuniyuki Kitagawa §, Tatsuya Hasegawa **

EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan

Ashwani K. Gupta ††

University of Maryland, Colleage park, MD 20742, USA

Hydrogen formed during hydrothermal reaction of biomass was determined by isotope labeling combined with cryogenic gas chromatography (cryo-GC) with differentiating two possible sources of hydrogen, biomass and water. At first, cellulose and wasted wood samples were gasified in supercritical D2O to label the hydrogen gas produced from water. Then the product gas was subjected to cryo-GC, where a separation column was maintained at -196°C by immersing it in liquid N2 to resolve hydrogen isotopes. On the resulting chromatogram, the peak of D2, generated from D2O, was mainly observed while that of H2 derived from biomass was almost missing. This result suggests that the main source of hydrogen in the product gas is not biomass itself but water as a medium under the gasification conditions used in this work. Furthermore, a residual water sample after hydrothermal reaction was also analyzed by ordinary GC to examine the organic degradation products from biomass.

I. Introduction IOMASS is a renewable resource, and it has been focused on as an alternative energy for fossil fuels. Supercritical water gasification is a promising technology for gasifying biomass with high moisture content.

Use of water as a reaction medium obviates the need to dry the feedstock and allows a fast reaction rate. Many researchers have investigated supercritical water gasification. The hydrothermal process using super- or subcritical water as a reaction medium has become a powerful method to gasify biomass with a high efficiency. There have been several reports for the formation of various fuels, such as methane (1,2) and hydrogen (3-5), from biomass through the hydrothermal reaction. Recently, the authors have also reported that the hydrothermal process in the presence of an alkali and Ni catalyst was significantly effective for selective hydrogen formation from wasted wood and sewage sludge even at relatively low temperatures around 400°C (6).

B

During the hydrothermal reaction, hydrogen can be formed not only from biomass itself but also from water used as a reaction medium. Therefore, it is important to determine the amount of hydrogen with differentiating its sources in order to optimize gasification conditions. In this work, the authors tried to determine hydrogen gas with discriminating its origins by means of isotope labeling followed by cryogenic gas chromatography (cryo-GC). Furthermore, the organic degradation products from biomass through hydrothermal reaction were also analyzed by subjecting a residual water sample to an ordinary GC system.

* Division of Energy Science. † Assistant Professor, Division of Energy Science, ishida@esi.nagoya-u.ac.jp. ‡ Division of Energy Science. § Professor, Division of Energy Science, kuni@esi.nagoya-u.ac.jp. ** Professor, Division of Integrated Research Projects, t-hasegawa@esi.nagoya-u.ac.jp. †† Professor, Department of Mechanical Engineering.

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II. Experimental Materials; A microcrystalline cellulose (Wako) was used as a model sample of biomass. Wasted wood supplied

from Furuhashi Co. was also used as a real biomass sample. Deuterium oxide (99%d) purchased from Aldrich was used as a reaction medium to label hydrogen generated

from water. Sodium carbonate (Na2CO3; Waco) and nickel catalyst (Ni/SiO2; 50 wt% Ni) were used as additives for the hydrothermal reaction. The Ni catalyst was prepared by impregnating NiNO3·6H2O (Kishida Chemicals) onto SiO2 supports followed by calcining at 400 ºC for 24 hrs in air. This catalyst was reduced with H2/N2 at 400 ºC for 5 hrs before the usage.

Hydrothermal process; hydrothermal reaction was performed in a stainless steel reactor (10 ml). About 0.1 g of a cellulose or real biomass sample was added into the reactor with 3 ml of water and additives such as Na2CO3 and Ni catalyst. After the remaining air was purged by the flow of N2 stream, the reactor was introduced into the hydrothermal furnace (Akico). The furnace temperature was programmed up to 400°C under a pressure of 25 MPa, and then maintained at the temperature for 30 min. After cooling down to a room temperature, the evolved gasses were sampled by a micro syringe through a gas sampler, and then subjected to GC measurements.

Cryogenic GC measurements of gas product; Cryo-GC system equipped with a thermal conductively detector (TCD) was used for the analysis of hydrogen isotopes. In order to resolve hydrogen isotopes, a separation column packed with an alumina (coated with 19 wt% of MnCl2) was maintained at -196°C by immersing it into liquid N2. As a carrier gas, neon was selected in order to prevent its condensation at such a low temperature. Furthermore, ordinary GC-TCD was also utilized to determine other gas species such as CO, CO2, and CH4.

Ordinary GC measurements of degradation products; the organic compounds in residual water were separated into two fractions, water-soluble and acetone-soluble ones. Then each fraction was analyzed by GC equipped with a flame ionization detector (FID). A metal capillary column [30 m × 0.25 mm id, coated with 0.25 μm of 5% phenyldimethylsiloxane immobilized through chemical cross-linking (Frontier Lab Ultra ALLOY PY-1)] was used.

III. Results and Discussion

A. Analysis of gas products from biomass samples; At first, cellulose was subjected to the hydrothermal gasification using H2O as a reaction medium. Figure 1 summarizes the amounts of each gas species obtained through the hydrothermal process. Here the values for the cellulose sample obtained without addition of any catalyst were also shown as reference data. As shown in this figure, the addition of the catalysts drastically increased the amount of hydrogen generated from the cellulose sample. Furthermore, it should be noted that the formation of CO2 in a gas layer was significantly suppressed presumably by the addition of Na2CO3 due to the dissolution of CO2 into an alkaline liquid layer.

Then the gas product, formed from the hydrothermal reaction using D2O, was analyzed by cryo-GC to obtain information on the sources of hydrogen. Figure 2 shows typical chromatograms of (a) a standard hydrogen mixture consisting of H2, HD and D2, and (b) gas component obtained from hydrothermal reaction of the wasted wood sample. On the chromatogram of the standard sample (a), the peaks of each isotope were clearly observed as well-resolved ones. Furthermore, the good reproducibility of their peak intensities (R.S.D. < 1%)

0

0.4

0.8

1.2

1.6

H2

H2H2

CH4

CH4

CH4

CO

CO2

CO2 CO2CO CO

cellulosewith no additive

Wasted woodwith additives

cellulosewith additives

yiel

ds o

f gas

spe

cies

(mm

ol)

0

0.4

0.8

1.2

1.6

H2

H2H2

CH4

CH4

CH4

CO

CO2

CO2 CO2CO CO

cellulosewith no additive

Wasted woodwith additives

cellulosewith additives

yiel

ds o

f gas

spe

cies

(mm

ol)

Figure 1. Amount of gas species formed through hydrothermal reaction of cellulose and wasted wood.

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suggests that the composition of hydrogen isotopes can be determined precisely by means of this GC system. Next, in the chromatogram of the gas product from the wasted wood sample (b), the peaks of D2 and HD, which were presumed to be formed from D2O, were clearly observed while that of H2 was almost missing.

Table 1 summarizes the relative yields of hydrogen isotopes for cellulose and wasted wood samples, determined from the peak intensities observed on the chromatograms. As shown in this table, the hydrogen isotopes were mainly consisting of D2, derived from D2O used as a reaction medium, for both the samples. This result suggests that the main source of hydrogen in the product gas is not biomass itself but water under the gasification conditions used in this work. Here, D2 was thought to be formed though the water-gas shift reaction [1], which had been reported to be the main route of hydrogen generation in the supercritical gasification of wood biomass.

H2

HD

D2

0 10 20

（a） standard hydrogen isotopes

（b） gas product from wasted wood

Retention time, min

H2

HD

Figure 2. Chromatograms obtained by cryo-GC;(a) standard hydrogen isotopes, (b) gas product from wasted wood sample.

CO + D2O → CO2 + D2 [1]

B. Analysis of degradation products in liquid layer; The residual water sample obtained after the hydrothermal reaction of the cellulose sample was subjected to GC-FID in order to examine the species and amount of degradation products. Figure 3 shows the typical chromatogram of water-soluble fraction. In this chromatogram, a series of organic compounds such as furan derivatives and phenolic compounds were observed as typical degradation products from cellulose. Similarly, as for the acetone-soluble fraction, various phenolic compounds were observed as main degradation products on the resulting chromatogram. However, the sum of yields for the water- and acetone-soluble fractions

Table 1. Relative molar yields of hydrogen isotopes formed from hydrothermal reaction of biomass.

cellulose

wasted wood

H2 HD D2 total

2.4 10.4 87.2 100

0.7 10.0 89.3 100

Relative molar yieldsa (mol%)

a determined from the peak intensities corrected by sensitiveness to TCD.

OH

0 5 10 13Retention time, min

OH

OHOO

OHO

OHO

O OH

Figure 3. Chromatogram of water-soluble fraction obtained after hydrothermal reaction of cellulose.

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was only ca 20wt % with respect to the amount of the cellulose sample introduced into the reaction cell. Moreover, it was revealed that carbons in the original cellulose sample were present mainly in a form of CO2 resolved in a liquid layer (ca. 40wt %) from pH measurement of water before and after hydrothermal reaction. This fact indicates that reforming of CO2 resolved in a liquid layer into H2 should be effective to improve the gasification efficiency of hydrothermal reaction.

IV. Conclusion Isotope labeling followed by cryogenic GC was successfully applied to the determination of hydrogen generated

during the hydrothermal reaction of biomass. By using this technique, it was shown that hydrogen was mainly derived from water as a reaction medium under the hydrothermal conditions used in this work. Furthermore, analysis of residual water revealed that carbons in the original biomass samples were present mainly in a form of CO2 resolved in a liquid layer, suggesting that reforming CO2 in a liquid layer into H2 should be effective to improve the hydrogen yield.

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Energy, vol. 5, 1994, pp 813-815. 2Xu, X., Matsumura, Y., Stenberg, J., Antal Jr., M. J., “Carbon-Catalyzed Gasification of Organic Feedstocks in Supercritical

Water” Ind. Eng. Chem. Res, vol. 35, 1996, pp 2522-2530. 3Minowa, T., and Ogi, T., “Hydrogen Production from Cellulose Using a Reduced Nickel Catalyst” Catalysis Today, Vol. 45,

1998, pp. 411-416. 4Minowa, T., and Fang, Z., “Hydrogen Production from Cellulose in Hot-compressed Water Using a Reduced Nickel

Catalyst: Product Distribution at Different Reaction Temperatures” J. Chem. Eng. Jpn., Vol. 31, No. 3, 1998, pp. 488-491. 5Yoshida, T., Oshima, Y., and Matsumura, Y., “Gasification of Biomass Model Compounds and Real Biomass in

Supercritical Water” Biomass and Bioenegy, Vol. 26, 2004, pp. 71-78. 6Ishida, Y., Hata, K., Tanifuji, K., Hasegawa, T., Kitagawa, K., “Effective and Selective Hydrogen Formation from Biomass

through Hydrothermal Reaction” Proceeding of the 3rd International Energy Conversion Engineering Conference, AIAA-2005-5542, 2005.

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