Energy & Fuels 2006, 20, 1259-1265
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Subcritical and Supercritical Water Gasification of Cellulose, Starch, Glucose, and Biomass Waste Paul T. Williams* and Jude Onwudili Energy and Resources Research Institute, The UniVersity of Leeds, Leeds LS2 9JT, UK ReceiVed September 23, 2005. ReVised Manuscript ReceiVed March 14, 2006
The subcritical and supercritical water gasification of cellulose, starch, and glucose as representative biomass model compounds and biomass in the form of Cassava waste has been investigated in a heated batch reactor. Cellulose and starch are two polysaccharides which have identical chemical compositions based on the monomer glucose but which have different chemical structures and physical properties. The influence of temperature in the subcritical and supercritical regimes of water were examined in relation to the yield of the product gases, oil, char, and water. For the model compounds and the Cassava waste, the main gases produced were carbon dioxide, carbon monoxide, hydrogen, methane, and other hydrocarbons, and there was significant production of oil and char. There were, however, distinct differences between the yields of the different products and the trends in relation to temperature. Cellulose produced a higher yield of char, carbon monoxide, and C1-C4 hydrocarbons compared to starch and glucose, but glucose produced the highest hydrogen yield. The Cassava biomass waste produced a char yield similar to that produced by starch, but a lower hydrogen yield.
Introduction Hydrothermal gasification processes are currently being investigated both as a waste treatment process and as a means of energy and materials recovery from biomass and biodegradable organic wastes. The supercritical water gasification process has advantages compared to conventional gasification, in that higher gasification efficiencies at much lower temperatures of approximately 400 °C are achieved. Since the process takes place at high pressure, smaller reactor volumes can be used and the resultant pressurized gas product can be stored directly in pressurized storage tanks resulting in a significant energy saving.1 Water near or above its critical point of temperature 374 °C and pressure 22.1 MPa has unique features with respect to its density, dielectric constant, ion product, viscosity, diffusivity, electric conductance, and solvent ability compared to ambient water.2-6 The vapor and liquid phases become indistinguishable, and the water behaves as a dense gas with a consequent removal of any interphase mass transport processes. Organic compounds have high solubilities and complete miscibility with supercritical water.4-6 There is considerable interest in the use of biomass as a renewable fuel which can be utilized in a range of energy conversion technologies and which also has the added advantage of being CO2-neutral. There is interest in the use of novel technologies which can convert biomass into useful fuels for * Corresponding author. Tel: 1133432504. Fax: 1132440572. E-mail:
[email protected]. (1) Matsumura, Y.; Minowa, T.; Potic, B.; Kersten, S. R. A.; Prins, W.; van Swaaij, W. P. M.; van de Beld, B.; Elliott, D. C.; Neuenschwander, G. G.; Kruse, A. Biomass Bioenergy 2005, 29, 269-291. (2) Mizuno, T.; Goto, M.; Kodama, A.; Hirose, T. Ind. Eng. Chem. Res. 2000, 39, 2807-2810. (3) Savage, P. E. Chem. ReV. 1999, 99, 603-621. (4) Clifford, T. Fundamentals of Supercritical Fluids; Oxford University Press: New York, 1998; pp 22-23. (5) Marshall, W. L.; Franck, E. U. J. Phys. Chem. Ref. Data 1981, 295304. (6) Hao, X. H.; Guo, L. J.; Mao, X.; Zhang, X. M.; Chen, X. J. Int. J. Hydrogen Energy 2003, 28, 55-64.
the future. Supercritical water gasification of biomass has been shown to produce high conversion rates to a gas composed mainly of hydrogen and carbon dioxide with, in addition, carbon monoxide and C1-C4 hydrocarbons.7,8 The production of high concentrations of hydrogen gas has led to much research into this conversion technology as a route to produce the fuel from a renewable source for the hydrogen economy. Biomass consists of mainly the polymers cellulose, hemicellulose, and lignin. In the study of supercritical water gasification of biomass, model compounds and real biomass samples have been examined,7-11 and smaller monomers which make up the three main biomass polymers have also been investigated. For example, there have been several studies of glucose as the representative monomer of cellulose.6,8 In addition, it is of interest to investigate the influence of compound chemical structure on the yield and composition of products from the supercritical water gasification of biomass components. Cellulose and starch are natural polysaccharide polymers of the glucose monomer, C6H12O6. However, for cellulose, the glucose molecules are linked via β-1-4 linkages, whereas for starch, the glucose monomers are linked via R-1-4 linkages. The cellulose polymer is regarded as being stronger than the starch polymer. Their comparative behaviors under the conditions of supercritical water gasification have not been investigated in detail. In this paper we report on the sub- and supercritical water gasification of cellulose, starch, and glucose as representative biomass model compounds. The influence of the chemical structure of the model compounds in relation to the yield and (7) Demirbas, A. Int. J. Hydrogen Energy 2004, 29, 1237-1243. (8) Schmieder, H.; Abeln, J.; Boukis, N.; Dinjus, E.; Kruse, A.; Kluth, M.; Petrich, G.; Sadri, E.; Schacht, M. J. Supercrit. Fluids 2000, 17, 145153. (9) Yoshida, T.; Matsumura Y. Ind. Eng. Chem. Res. 2001, 40, 54695474. (10) Yoshida, T.; Oshima, Y.; Matsumura, Y. Biomass Bioenergy 2004, 26, 71-78. (11) Minowa, T.; Ogi, T. Catal. Today 1998, 45, 411-416.
10.1021/ef0503055 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/12/2006
1260 Energy & Fuels, Vol. 20, No. 3, 2006
Figure 1. Diagram of the reactor assembly.
composition of products was the main aim of the study. The influence of temperature in the subcritical and supercritical regimes of water as well as the residence time were examined in relation to the yield and composition of the product gases. In addition, biomass in the form of Cassava waste has been gasified under sub- and supercritical water conditions, and the results have been compared to those for the model biomass compounds. Materials and Methods Experimental Reactor and Reagents. The subcritical and supercritical water gasification of the samples was investigated using a stainless steel autoclave reactor obtained from the Parr Instrument Co. A schematic diagram of the experimental system is shown in Figure 1. The reactor had a volume capacity of 500 mL designed to a maximum temperature and pressure of 500 °C and 35 MPa, respectively, with a T316 Bourdon tube mounted on the bomb head using a standard attachment fitting. This gauge measured the internal pressure of the reactor. The reactor was externally heated by a 3-kW ceramic heater. A type J thermocouple was fitted into a stainless steel sheath which monitored the internal temperature of the reactor. The reactor was fitted with a cooling loop fixed into the reactor, to be used in cooling the reactor fairly rapidly with water, to ambient temperature at the end of an experiment. The gas-sampling system was comprised of a cylindrical tube attached at one end to the gas outlet valve via a two-way high-pressure valve. The other end was connected to a stainless steel tube, which had two low-pressure valves attached. The pressure of the gas released into the sampling system could be monitored using a pressure gauge. Cellulose, starch, and glucose were obtained from Sigma Aldrich, UK. The oxidant used, hydrogen peroxide (30% w/v), was obtained from Supelco, UK. Hydrogen peroxide (30 wt %) was used for the experiments and was obtained from Sigma Aldrich, UK. Hydrogen peroxide has been used in the hydrothermal oxidation of organic compounds and has been reported to be a much more effective oxidant than pure oxygen or air.12 The amount of hydrogen peroxide represented about 36% of the stoichiometric oxygen requirement for the complete theoretical oxidation of 5 g of the sample. The biomass material used in this study was Cassava waste. Cassava is a tropical root tuber cultivated mainly in Africa and South America and other parts of the world. It is used mainly in the preparation of starchy meals and flakes as well as industrial starch. The edible part of the tuber is covered by two skins; the outer skin is thin and brownish. and the inner skin is thicker and whitish. The skins comprise about 12% of the total mass of Cassava. During processing, the skins are removed for disposal, and they represent the waste biomass material used in this investigation. The proximate analysis of the Cassava waste was 76 wt % moisture content, 22 wt % volatiles, and 2 wt % ash. The ultimate analysis (12) Croiset, E.; Rice, S. F.; Hanush, R. G. AIChE J. 1997, 43, 234323252.
Williams and Onwudili of the waste was 43 wt % carbon, 6.4 wt % hydrogen, 2.4 wt % nitrogen, and 48 wt % oxygen. The Cassava skins were dried in the oven at 105 °C for 12 h and then cooled in a desiccator. The dried samples were pulverized with a mortar and pestal and sieved through a 300-µm mesh sieve. Prior to experiments at the subcritical and supercritical water conditions, the samples were dried again at 105 °C for 1 h and the required portions were weighed out. Experimental Procedure. The cellulose, starch, glucose, and Cassava (5 g) were weighed into the reactor, a known volume of water was added, and this was followed by the addition of a known volume of hydrogen peroxide to bring the total volume of liquid to 100 mL. The amount of hydrogen peroxide was less than the stoichiometric requirements for complete oxidation. Nitrogen gas was used to purge the reactor for 10 min. After purging, the reactor was heated at 20 °C min-1 to the final temperature. A series of experiments were undertaken to determine the influence of temperature between 330 and 380 °C; since the reactor was sealed, there was a consequent increase in pressure of between 9.3 MPa (95 bar) and 22.5 MPa (225 bar), respectively. Reaction time was varied between 0.0 and 120.0 min. These times represent the experiments, where as soon as a set temperature was reached, the reactor was held for a specified time. Where the effect of temperature was monitored, the heating was stopped at the designated temperatures of 330, 350, 374, and 380 °C. At the end of each experiment, the heating was stopped and the reactor was removed to cool. After cooling to room temperature, the final temperature reading from the thermocouple and final pressure reading from the pressure gauge were taken. Liquid, solid, and gaseous products were obtained. These included oil, water-soluble products (WSP), char, permanent gases, and C1-C4 hydrocarbon gases. The gaseous effluent was sampled via the gas/liquid sampling system (Figure 1). The characteristics of the batch reactor and experimental procedure resulted in a significant preheating period during which some reaction was inevitable. However, experiments carried out at lower temperatures of below 330 °C (280 and 300 °C) showed that at the lower temperatures, the main reaction was hydrolysis of the feedstock to produce water-soluble products (see later). Gas Analysis. The sampled gases were immediately analyzed using two separate Varian gas chromatographs. A Varian CP-3380 gas chromatograph with a thermal conductivity detector (GC/TCD) and fitted with two columns was used. One column was used to separate hydrogen, oxygen, nitrogen, methane, and carbon monoxide, while the other was used for the analysis of carbon dioxide. The columns used were 2-m long with a 2-mm diameter, packed with a 60-80-mesh molecular sieve. Argon was used as the carrier gas. The column oven was held isothermally at 30 °C for the analysis; the injector oven was at 120 °C. The detector temperature was 120 °C with a filament temperature of 160 °C. Hydrocarbons from C1 to C4 were analyzed using a second Varian CP-3380 gas chromatograph with a flame ionization detector (GC/FID). The column used was 2-m long by 2-mm diameter and packed with 80-100 mesh Hysesp. The injector was held at 150 °C, while the detector temperature was 200 °C. Nitrogen was used as the carrier gas. The oven temperature program was 60 °C for 3 min, with a heating rate of 10 °C min-1 to 100 °C, held for 3 min, finally ramped to 120 °C at 20 °C min-1, and held for 9 min at 120 °C. The injector was held at 150 °C, while the detector temperature was 200 °C. Liquid and Solid Analysis. The reaction products produced during the supercritical water gasification of cellulose, starch, glucose, and Cassava were gas, oil, char, and water. The water also contained water-soluble products (WSP) which were also analyzed. The char was separated by filtration. The oil was extracted from the water using a separating funnel, with the pH adjusted to pH 2 with 0.6 M HCl, and extracted three times with 3 x aliquots of dichloromethane by shaking vigorously for 5 min each time. The combined organic and aqueous phases were allowed to stand in a separating funnel for about 30 min to 1 h, followed by separation of the immiscible phases. The dichloromethane extract containing the oil components from the liquid-liquid extraction
Sub- and Supercritical Water Gasification of Biomass
Energy & Fuels, Vol. 20, No. 3, 2006 1261
Table 1. Product Mass Balances from Cellulose Gasification in Relation to Temperature and Reaction Time T (°C)
time (min)
CO2 (wt %)
CO (wt %)
H2 (wt %)
C1-C4 (wt %)
oil (wt %)
char (wt %)
water (wt %)
WSP (wt %)
total (wt %)
330 350 374 380
120 120 120 120
35.8 38.9 40.0 47.4
6.21 6.21 5.16 4.32
0.42 0.42 0.42 0.42
0.74 0.74 0.84 1.05
2.74 2.42 2.32 2.11
12.42 10.42 9.16 7.05
37.3 38.4 38.6 41.2
0.11 0.11 0.11 0.00
96 98 97 103
330 350 374 380
0 0 0 0
36.8 38.9 38.9 38.9
6.74 6.95 6.63 5.79
0.32 0.32 0.32 0.42
0.63 0.63 0.74 0.84
4.11 3.26 3.26 3.26
10.3 9.58 9.47 8.42
37.5 38.4 38.4 38.5
1.05 0.84 0.84 0.84
97 99 99 98
Table 2. Product Mass Balances from the Gasification of Starch in Relation to Reaction Temperature and Time T (°C)
time (min)
CO2 (wt %)
CO (wt %)
H2 (wt %)
C1-C4 (wt %)
oil (wt %)
char (wt %)
water (wt %)
WSP (wt %)
total (wt %)
330 350 374 380
120 120 120 120
43.1 44.3 44.4 45.5
3.37 4.00 3.89 2.74
0.95 0.95 0.95 0.95
0.32 0.32 0.32 0.32
3.89 3.26 2.95 2.95
6.00 6.21 6.63 6.53
39.8 40.2 40.2 40.6
0.32 0.32 0.32 0.32
98 100 100 100
330 350 374 380
0 0 0 0
40.2 41.5 42.1 43.4
5.68 5.47 4.84 4.53
0.95 0.95 1.05 1.05
0.32 0.32 0.32 0.32
4.21 4.00 4.11 4.11
4.95 4.95 6.21 5.68
38.7 39.3 39.5 39.9
0.95 0.84 0.84 0.74
96 97 99 100
was blown to dryness with a gentle stream of nitrogen gas, placed in a desiccator to dry off the moisture overnight, and then weighed to determine the total mass of oil produced during supercritical water gasification. The water still, however, contained some watersoluble products (WSP) since it was colored light to dark brown. The mass of the water-soluble products was determined as total dissolved solids. A sample of the liquid was taken and placed in a crucible on a water bath to evaporate off the liquid. After evaporating the liquid, the crucible was dried in an oven at 105 °C for 1 h to determine the mass of water-soluble products. Since the supercritical water gasification of the model biomass samples and the Cassava was carried out in a large excess of water, the mass of water produced during the reaction could not be measured and was therefore calculated. Calculations were based on an approximation to the major overall reaction C6H12O6 + 6H2O2 f 6CO2 + 6H2 + 6H2O The amount of water produced from the decomposition of hydrogen peroxide to water and molecular oxygen was also calculated. The mass balance was based on the total mass of the reactants (feed), including the biomass compound, water, and hydrogen peroxide deployed, rather than water “by difference”. The product oil was analyzed using Fourier transform infrared spectrometry (FT-IR). The FT-IR instrument was used in the absorbance mode and only for qualitative identification of functional groups. The analytical system was a Nicolet 560-m FT-IR spectrometer equipped with an Omnic PC-based software system for the recording of results and calculation of absorbance spectra. The software was programmed to subtract the background spectra automatically from subsequent analyses and produce a percentage absorbance spectrum for each sample. The sample oil was prepared and presented to the instrument as a KBr disk containing the oil. The spectrometer scanned the sample from 450 to 4000 cm-1 wavenumber, and spectral peak heights were normalized to the major C-H peak. The product char and oil were analyzed for carbon, hydrogen, nitrogen, and oxygen content using a Carlo Erba Flash EA 1112 compact analyzer. Automated determination of CHNS in one sample and oxygen in another sample was carried out. Between 2.5 and 4.5 mg of the char was used for analysis. The elemental analysis of the oil and char was used in the calculation of the carbon and hydrogen mass balances.
Results and Discussion Supercritical Water Gasification. Table 1 shows the reaction products from the sub- and supercritical water gasification
of cellulose. The mass balance was based on the total mass of the reactants. The main product was a gas composed of hydrogen, carbon dioxide, carbon monoxide, and C1-C4 hydrocarbons (mainly methane). Increasing the temperature (and therefore the pressure) in the system from subcritical to supercritical conditions resulted in a decrease in the oil and char yield for the reaction time of 0.0 and 120 min. For example, the oil yield decreased from 2.74 to 2.11 wt % and the char yield decreased from 12.42 to 7.05 wt % as the temperature was increased from 330 to 380 °C at a reaction time of 120 min. There was a corresponding increase in the gas yield, mainly through an increase in the yield of carbon dioxide. The carbon content of the char varied from 75 wt % at 330 °C to 88 wt % at 380 °C. Table 2 shows the product yield for the sub- and supercritical water gasification of starch. The yield of char was significantly lower compared to cellulose supercritical water gasification, and the yield of carbon dioxide was generally increased. There was a significantly higher production of hydrogen from the starch than from cellulose. The trends in relation to an increase in process temperature were a small increase in char yield, which compared with a decrease for cellulose hydrothermal gasification. Increasing the temperature from 330 to 380 °C showed a decrease in the yield of oil and a corresponding increase in the yield of char and also an increase in carbon dioxide yield, as was found for cellulose. The resistance to hydrothermal degradation of cellulose compared to starch, reflected in the higher char yield and lower gas and oil yield, may reflect the differences in chemical structure of the two polymers. Table 3 shows the product mass balance for the sub- and supercritical water gasification of glucose. Table 3 shows that the main products from the sub- and supercritical water gasification of glucose consisted of a gas composed of hydrogen, carbon dioxide, carbon monoxide, and C1-C4 hydrocarbons (mainly methane). In addition, an oil product, char, water-soluble products, and water were formed. As the temperature of reaction increased from 330 to 380 °C, the carbon dioxide and carbon monoxide increased for both the zero- and 120-min experiments. However, there was a decrease in carbon monoxide at 380 °C, supercritical water gasification conditions. The char yield decreased with increasing temperature for the 120-min reaction time, but it showed an increase at the zero-minute time. The
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Table 3. Product Mass Balances from the Gasification of Glucose in Relation to Reaction Temperature and Time T (°C)
time (min)
CO2 (wt %)
CO (wt %)
H2 (wt %)
C1-C4 (wt %)
oil (wt %)
char (wt %)
water (wt %)
WSP (wt %)
total (wt %)
330 350 374 380
120 120 120 120
42.9 44.2 46.8
5.68 6.53 6.86
1.26 1.26 1.27
0.32 0.43 0.42
2.47 2.34 2.32
4.24 3.47 2.84
41.7 42.1 42.2
0.21 0.21 0.21
99 101 103
330 350 374 380
0 0 0 0
41.9 41.2 42.1 45.3
6.00 8.21 8.89 6.32
1.16 1.16 1.26 1.26
0.32 0.32 0.74 0.11
4.11 3.58 3.47 2.32
2.68 3.16 4.00 4.00
42.5 42.3 41.9 43.7
0.42 0.42 0.32 0.32
99 100 103 103
Table 4. Product Mass Balances from the Gasification of Cassava Biomass in Relation to Reaction Temperature and Time T (°C)
time (min)
CO2 (wt %)
CO (wt %)
H2 (wt %)
C1-C4 (wt %)
oil (wt %)
char (wt %)
water (wt %)
WSP (wt %)
total (wt %)
350 350
0 120
44.7 46.2
6.21 4.84
0.32 0.42
0.42 0.53
2.32 3.26
4.42 5.16
40.3 40.8
