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Hydrothermal gasification of biomass and organic wastes
Journal of Supercritical Fluids 17 (2000) 145 – 153 www.elsevier.com/locate/supflu
Hydrothermal gasification of biomass and organic wastes H. Schmieder *, J. Abeln, N. Boukis, E. Dinjus, A. Kruse, M. Kluth, G. Petrich, E. Sadri, M. Schacht Forschungszentrum Karlsruhe, Institut fu¨r Technische Chemie, Postfach 3640, 76021 Karlsruhe, Germany Received 15 July 1999; received in revised form 20 October 1999; accepted 4 November 1999
Abstract Wet biomass and organic wastes can be efficiently gasified under hydrothermal conditions to produce a hydrogen rich fuel gas. New experiments in two tubular flow reactors and in two batch autoclaves with carbohydrates, with aromatic compounds, with glycine as a model compound for proteins and with real biomass are reported for different residence times, temperatures and pressures. It was found that at 600°C and 250 bar all compounds are completely gasified by addition of KOH or K2CO3, forming a H2 rich gas containing CO2 as the main carbon compound. Concentrations of CO, CH4 and C2 –C4 hydrocarbons are low in the product gas (B 1, 3 and B 1 vol%, respectively). Carbon balances for the smaller flow reactor are closed to better than 96%. Ranges of product concentrations are given as estimated from experimental reproducibility. Some reflections for the engineering of a pilot plant are presented. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Gasification; Biomass; Waste; Supercritical water; Flow reactor; Batch experiments
1. Introduction Energy from biomass may significantly contribute to the growing future demand for energy. Energy from biomass avoids the net increase of carbon dioxide in the atmosphere and would help to fulfil the obligations of the European Union to reduce carbon dioxide release. Today the main part of the energetic use of biomass comes from the incineration of wood with a rather low energy yield. An increase in the * Corresponding author. Fax: +49-7247-822244. E-mail address: [email protected] Schmieder)
(H.
efficiency can be expected by gasification and use of the fuel gases in turbines or even in fuel cells. The supply potential is considerable as shown in Table 1 for the example of The Netherlands [1]. Table 1 does not mention manure production and only considers in part wet organic wastes arising from different branches of industry. Such wastes may have high negative costs (disposal costs) which are a benefit for the gasification process. The estimated energy potential represents 6.1 billion m3 natural gas or 15% of the natural gas consumption of The Netherlands. Although the larger part of the amount is subsequently used today, some of the 4000 kt/a should be available for energy use. The estimated supply poten-
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H. Schmieder et al. / J. of Supercritical Fluids 17 (2000) 145–153
tial of rest biomass (DOM) for Germany amounts to 50 000 kt/a [2] and for the European Union to 200 000 kt/a (J.M.L. Penninger, private communication) or more [3]. A large part of the biomass is wet biomass containing up to 95% water. For water contents of more than 40% the thermal efficiency of a traditional steam (reforming) gasification plant decreases drastically to 10% at 80% water (J.M.L. Penninger, private communication). A very promising alternative for wet biomass is the less investigated hydrothermal gasification (expected thermal efficiency 70% at 90% water; J.M.L. Penninger, private communication). First experiments to convert carbohydrates to liquid products in supercritical water were performed by Modell and Amin in the mid 1970s at MIT [4]. Systematic experimental investigations for the conversion and gasification of biomass, for waste model compounds and for real wastewater under hydrothermal conditions were carried out at the Pacific Northwest Laboratories (PNL), at the university of Hawaii and at the National Institutes for the Resources and Environment, Japan (NIRE). At NIRE the conversion of cellulose was studied in autoclaves with residence times of up to 1 h at 200– 400°C and 80 – 180 bar [5,6]. It was found that sodium carbonate as a catalyst suppresses the formation of char and oil and mainly water soluble products were formed. At 400°C with Ni catalysts CH4 and CO2 were found as major products in the gas phase. Laboratory tests at PNL [7,8] were directed to the formation of a CH4-rich gas from biomass,
waste model compounds and real wastewaters. Batch reactor experiments were performed at 350°C, 200 bar and reaction times of 60–120 min. It is shown that aromatic and aliphatic hydrocarbons as well as oxygenates are converted to a methane-rich fuel gas in the presence of hydrogenation catalysts. The results were confirmed in continuous-flow reactor experiments with residence times of 10 min and more for conversions of 90% and higher. Antal investigated the coke-catalyzed gasification of model substances, biomass (aquatic plants and others) and wastes of the Department of Energy at 600–650°C, 280 or 345 bar and reaction times of 30 s in a tubular flow reactor at the University of Hawaii [9–11]. At 600°C and 345 bar, glucose and other feeds at concentrations of up to 0.2 M were gasified completely to a hydrogen-rich gas. Coke or tar formation was not observed. Department of Defense wastes were also completely gasified under these conditions. Liquefaction experiments of biomass under supercritical conditions were also done at the University of L’Aquila [12]. At the Forschungszentrum Karlsruhe experiments for hydrothermal conversions of organic material started in the early 1990s with a kinetic investigation of the thermolysis of tert-butylbenzene [13] and were continued with the treatment of model compounds and real biomass and wastes with batch and tubular flow reactor experiments preferably aiming at the production of a hydrogen-rich fuel gas [14,15]. Since hy-
Table 1 Supply potential of rest biomass – The Netherlands [1] Material
Moisture, % of wet material
Agriculture (straw, bulbs…) Organic wastes (food and beverages, waste paper…) Wood (thinnings, prunings…) Sludges (water treatment…) Total
15–80 10–80 15–50 60–95
Ash, % of dry material
Supply of dry organic material, kt DOM/a
Energy potential (heating value), PJ/a
1–10 1–20
870 6080
17.4 124.3
1–5 40–65
1840 760 9550
35.8 11.5 189
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Fig. 1. Miniplant.
drothermal gasification of aromatic compounds and amino acids at temperatures above 500°C was not known from the literature we studied glucose as a model compound for cellulose; aromatic compounds as model compounds for lignin; and glycine as a model compound for protein. We observed a pronounced effect of the addition of potassium on the reforming and shift reactions and this was therefore studied in some detail. In our latest experimental set-up we succeeded in closing the balance to better than 96%. The paper presents our results and points out future work to be done for a technical plant.
experiments by the addition of a certain amount of the material to the autoclaves.
2.1. Autocla6e I Designed for temperatures of up to 700°C and pressures of 1000 bar, it is built from Nimonic 110 with an inner volume of 100 ml and is equipped with a magnetic stirrer for mixing. The educts (model compounds, real biomass or wastes) were injected after heating of the water filled autoclave to the desired temperature.
2.2. Autocla6e II 2. Experimental The model compounds glucose for cellulose, catechol and vanillin for lignin and glycine for proteins were used as aqueous solutions. Real biomass (wood as saw dust, straw) and wastes (sewage sludge and lignin) were treated in batch
Designed for temperatures of up to 500°C and pressures of 500 bar, it is built from stainless steel containing an inlet made from Inconel 625 with an inner volume of 1000 ml. Mixing is achieved by a tumbling device. The educts were heated together with water to the desired temperature ( 2–3 h).
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2.3. Miniature plant
2.5. Analytical
The experimental set-up of the tubular flow reactor is illustrated in Fig. 1. The flow reactor was fabricated from Inconel 625 with a tube length of 500 mm, an outer diameter of 14.4 mm and an inner diameter of 8 mm. The reactant solution is fed into the reactor by a Bischoff 2250 HPLC pump. The feed is quickly heated to 600°C with an electrical heater which is coiled around the entrance part of the reactor and maintained at this temperature by two additional heaters downstream of this entrance heater. The ‘reaction zone’ of the reactor has a length of 350 mm. A stainless steel rod with a length of 80 mm and a diameter of 7.8 mm is placed at the entrance and the exit of the reactor. In this way, backmixing is suppressed and a fast heating of the feed flow at the reactor entrance and a fast quenching of the product flow at the reactor exit are guaranteed. The temperature profile along the reactor axis is measured by six type K thermocouples mounted on the reactor’s outer wall. A movable type K thermocouple inside a capillary tube fixed inside the reactor provides the temperature of the feed stream in the reaction zone. The pressure is measured by a Burster type 8201 pressure sensor. After the gasification reaction, the effluent is cooled down to room temperature by a cooling water jacket at the exit of the reactor, depressurized by a TESCOM model 26-1761/ER3000 electro-pneumatically controlled back pressure regulator and transferred into a phase separator. The liquid phase was collected at the bottom of the phase separator, while the gas phase was passed to a water-filled gas flow meter to determine the volume of the formed gas mixture. Samples of the gas phase were taken by a gas sampling tube between the phase separator and the gas flow meter.
After identification by HP GC-MS of the individual compounds, liquid samples were analyzed with an HP 5890 gas chromatograph combined with a Varian autosampler for S6 olid-P6 hase M6 icroe6 xtraction, SPME fiber: 85 mm polyacrylate, 20 min extraction; column: DP5, 30 m× 0.25 mm, 0.25 mm; carrier: 1 ml/min He; detector: flame-ionization detector, 250°C; oven: 40–260°C (12 min) at 10°C/min. In addition, the total organic carbon contents of the liquid phase were determined (TOC Analyzer DC 190 by Schmidlin). Besides the gas flow measurement, CO and CO2 were analyzed on line by IR spectroscopy and O2 by a paramagnetic sensor (Uras 10 E by Hartmann and Braun). H2 was analyzed by thermal conductivity (Caldos 5B by Hartmann and Braun) for the experiments in the bench scale plant. For the miniature plant, samples were taken from the gas phase and analyzed by gas chromatography. Two HP 5880 gas chromatographs were used, one with N2 carrier gas for hydrogen analysis, the other with He carrier gas for all other gases. For the column switching procedure used, a Porapak Q column (2 m long) and a mole sieve column (13× mesh 60–80, 2 m long) were connected in series while the second column was bridged for the analysis of CO2 and hydrocarbons during the analysis run (carrier: 22.8 ml/min He, 22.8 ml/min N2 for hydrogen analysis; thermal conductivity and flame-ionization detector connected in series, 200°C; oven: 80°C (19 min) to 160°C (60 min) at 20°C/min).
2.4. Bench scale plant The plant was originally built for experiments to oxidize pollutants in supercritical water (SCWO). A tubular flow reactor (Inconel 625, inner diameter 8 mm, length 15 m) heated by a fluidized sand bath is used. A description is given in [16].
3. Results and discussion The predominant part of the experiments was performed in the tubular flow reactors to investigate the influence of the main parameters (temperature, pressure, residence time and alkali metal addition) on the gasification efficiency to produce a product gas with an optimum hydrogen yield. In these experiments, only model compounds could be used because no reliable slurry (suspension) feeding system has been available up to now for the bench scale plant.
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only with traces of solid and oily by-products. Samples of the aqueous effluent (TOC(res)) contained mainly phenol and derivates despite low aliphatic ketones identified by GC-MS. This is in contrast to the results at MIT [19] where besides the low carbonic acids and aldehydes, furan derivates were also found by HPLC chromatography. These investigations were done at very low feed concentrations (10 − 3 M) without addition of alkali metals. A similar temperature trend was observed with gasification of the aromatic compound catechol [15]. The addition of potassium also significantly influenced the CO content in the product gas as illustrated in Table 3. The experiments with glucose show that by addition of KOH the CO concentration in the product gas decreases more than 20-fold. A similar influence is observed in the experiments with vanillin where potassium carbonate was added instead of KOH. It can be concluded that the addition of alkali metals, probably as acid-base catalyst, increases the rate of the shift reaction (CO+ H2O= CO2 + H2) regarding the shift as a follow reaction of the reforming. The effect is significantly less marked with addition of alkaline
Table 2 shows the temperature influence on the total organic carbon (TOC) destruction efficiency of glucose at feed concentrations of up to 0.3 M in both plants (TOC(res)= TOC of the aqueous effluent). At low temperatures soot and tar are formed which are not considered in the TOC destruction efficiency. The values in brackets (column two) indicate the total gasification efficiency considering soot and tar formation. For the experiments in the miniature plant generally a closure of the carbon balance better than 96% could be achieved. Several authors studied the reforming reaction of different organic materials in sub- and supercritical water with addition of catalysts (for reviews see Refs. [17,18]). Obviously, the addition of alkali metals increases the reaction rate and suppresses the formation of soot and tar. The best effect as comparing other alkali metals was in our screening experiments achieved by the addition of potassium. The influence is shown in Table 2 by the experiments at 600°C with and without added KOH. The results show that at temperatures higher than 550°C a complete gasification of glucose can be achieved without the formation or
Table 2 Average TOC destruction efficiency of glucose as function of temperature T, °C
1−TOC(res)/TOC(feed)
P, bar
Feed, M
[K], M
400 500a 550b 600b 600a
0.35 (0.12) 0.6 (0.46) 0.83–0.9 0.88 0.99
300 300, 450 270, 310 210, 310 100(!)–300
0.2 0.2 0.1–0.3 0.1 0.1
1.8×10−3 (KOH) 1.8×10−3 (KOH) 1–3×10−3 (K2CO3) 0 1.8–5×10−3 (KOH)
a b
Miniature plant; residence time: 60–140 s. Bench scale; residence time: 60–140 s.
Table 3 Average CO and CH4 product gas content with and without addition of KOH or K2CO3
Glucose without KOH Glucose 0.18–2×10−2 M Vanillina without K2CO3 Vanillinb 0.7–1.5×10−3 M a b
CO, vol%
CH4, vol%
T, °C
Feed, M
20 0.6 36 1.5
3.71 10.0 90.11, 3.3 90.52 9.4 90.25 12.6 9 1.4
550, 600 600 600 600
0.1 0.1–0.6 0.067 0.067
Bench scale; residence time: 60–120 s; P: 200–310 bar; [KOH]: 2×10−2 M. Miniature plant; residence time: 60–120 s; P: 200–310 bar; [KOH]: 1.8×10−3 M.
H. Schmieder et al. / J. of Supercritical Fluids 17 (2000) 145–153
150 Table 4 Average gas compositions
CO2
CH4
C2H4
Average gas composition of glucose gasification at 600°Ca Vol% 59.79 0.55 0.439 0.08 Mol gas/mol glucose 9.1 B0.1
31.8 91.1 5.1
3.2 9 0.3 0.6
0.65 9 0.2 B0.2 C2–C4
Average gas composition of catechol gasification at 600°Cb Vol% 61.59 2.4 0.529 0.04 Mol gas/mol catechol 10.6 B0.1
29.3 92.0 5.0
2.2 9 0.7 0.5
0.52 9 0.26 B0.2 C2–C4
H2
a b
CO
TOC(res)=(19 918) ppm; residence time: 30–120 s; P: 250 bar; [KOH]: 1.8×10−3 M. TOC(res)=(68 919) ppm; residence time: 30–120 s; P: 200–300 bar; [KOH]: 1.8×10−3 M.
earth metals [18]. The difference in the formation of CH4 between the experiments done in the bench scale or miniature plant may be caused by the different concentrations of potassium used and/or by the different geometry of the reactor tubes. The influence of residence time on the gasification efficiency was tested for both glucose and catechol (feed: 0.2 M) at 30, 60 and 120 s with the miniature plant (600°C, 250 bar, KOH). The results show that for such feed concentrations a minimum residence time of 60 s is required. At 30 s, the gasification efficiency (TOC(res)) drops to 98% and a decrease of the CH4 yield is observed. At feed concentrations higher than 0.6 M and the low potassium concentrations used a drastic decrease of the gasification efficiency is observed with formation of soot and tar at residence times of 120 s as well. A marked pressure effect is observed for the CH4 yield. The CH4 content in the product gas increases from 3 vol% at 250 bar to 8 vol% at 450 bar whereas H2 decreases from 60 vol% at 250 bar to 50 vol% at 450 bar. Table 4 shows the average product gas compositions of glucose and catechol gasification experiments in the miniature plant at 600°C and feed concentrations of 0.2 M. For these experiments the closure of the carbon balance is better than 96% and no or only traces of solids or oily material were found. Besides methane C2 –C4 hydrocarbons with C2H4 as the major component were detected in the product gas with a yield of below 1 vol%. The results show that under these conditions water takes part
(splits) as reactant with a large portion. For glucose 76% of the theoretical hydrogen formation is found according to the equation: C6H12O6 + 6H2O= 6CO2 + 12H2, perfect gas: DH = 158 kJ/mol
(1)
The experimental result corresponds approximately to the following equation: 2C6H12O6 + 10H2O= 11CO2 + CH4 + 20H2, DH = 152 kJ/mol
(2)
For the gasification of catechol 82% of the theoretical hydrogen formation is achieved if only carbon dioxide and hydrogen are considered as products. Glycine was used as a model compound for proteins and polyamides in gasification experiments using the miniature plant (600°C, 250 bar, KOH) [15]. A TOC(res) of 1.5% was found in the aqueous effluent at residence times of 60 and 120 s. The nitrogen of the amino acid is completely converted to ammonia and a hydrogen rich gas is produced because a large part of the carbon is bound in the aqueous effluent ( 45%). With real biomass and wastes, only batch experiments have been done in our laboratory up to now. Table 5 shows results of experiments performed with the stirred autoclave I with vanillin as a model compound for lignin and with lignin itself at 600 and 570°C. The achieved TOC destruction efficiency is high although the real gasification efficiency could not be determined because the traces of solid and oily product are not quantitatively recovered. In the aqueous effluent cate-
H. Schmieder et al. / J. of Supercritical Fluids 17 (2000) 145–153 Table 5 Screening experiments with autoclave Ia
Vanillinb Ligninc
Educt (res)/Educt (feed)
T, °C
P, bar
\0.99 \0.99
600 570
200–600 400
a
Reaction time: 1800–3600 s; [educt]: 2500 mg/kg. Without addition of K2CO3. c [K2CO3]: 1.8×10−3 M. b
chol, methoxyphenol and phenol were identified as the major compounds by GC-MS. For vanillin more than 99% destruction efficiency is measured even without added potassium. Compared to the tubular flow reactor experiments, both the low feed concentration and the higher reaction time, might explain this finding. Table 6 shows results of the screening experiments with straw, wood and sewage sludge at temperatures of 450 and 500°C using the tumbling autoclave II. For straw a significant increase of the TOC destruction efficiency is found as was to be expected with increasing temperature but no difference is found between the experiments with and without addition of potassium. This finding can be explained by the high potassium content of straw (ash: 4.6 wt% with 15 wt% K). The high yield of methane is striking compared to the tubular flow reactor experiments. The lower temperature and the longer reaction time of 2 h and more with the tumbling reactor is probably an explanation. Altogether the results confirm the findings above as well as the results of Antal
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[9–11] that for a complete gasification temperatures higher than 550°C are required. The current knowledge about reaction fundamentals in hot high pressure water does not allow an attempt of the interpretation of the experimental observations on the basis of reaction mechanisms and kinetics. Thermodynamic calculations were employed instead to improve the understanding of the reaction paths occurring during gasification. The equilibrium constants were calculated with the aid of the BENSON code/data base (NIST) and for the variation of pressure and educt concentration our own code VISCO (G. Petrich, private communication) was used. Besides the primary dehydration/cracking step three reactions are assumed for the gasification of ethanol: reforming,
e.g.
C2H5OH+ H2O= 2CO+ 4H2,
DH = 256 kJ/mol the shift
(3)
CO+ H2O= CO2 + H2,
DH = − 41 kJ/mol
(4)
and the methanation
CO+ 3H2 = CH4 + H2O,
DH = − 210 kJ/mol.
(5)
The results of the preliminary calculations show that for the model compounds the temperature and pressure trends found experimentally are confirmed. The methane yield is predicted too high compared to the experimental results found with the tubular flow reactors operated at short residence times. The calculations as opposed to the experimental findings will be discussed in a further publication.
Table 6 Screening experiments with autoclave II
Strawa Strawa Woodb Sewageb Sewagec a
Educt (res)/Educt (feed)
T, °C
H2, vol%
CO, vol%
CO2, vol%
CH4, vol%
TOC(feed), ppm
0.8 \0.9 0.9 0.552 0.853
450 500 450 450 450
33–43 34–42 30 49 47
1.2 0.3 1.7 2.8 1
40–47 40–45 49 31.2 37
17.5 16.2 19 17 15
4600 4600 4900 2300 2300
With and without addition of K2CO3; 1.7x10−3 M; reaction time: 7200 s; P: 315–350 bar. Without K2CO3; 1.7x10−3 M; reaction time: 7200 s; P: 315–350 bar. c [K2CO3]: 1.7x10−3 M; reaction time: 7200 s; P: 315–350 bar. b
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Fig. 2. Autothermal gasification pilot plant.
4. Conclusion and outlook Our experiments in two tubular flow reactors and in two batch autoclaves show that in the presence of KOH or K2CO3 at 250 bar and temperatures higher than 550 – 600°C 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; concentrations of CO, CH4 and C2 – C4 hydrocarbons are low in the product gas ( B 1, 3 and B 1 vol%, respectively); at lower feed concentrations (50.2 M) residence times of 30 s are required. At higher feed concentrations (]0.6 M) and constant potassium concentrations soot and tar formation appears; the addition of potassium decreases the CO concentration and increases CO2 and H2 in the product gas; carbon balances for the miniature plant are closed to better than 96%;
the screening experiments with the real feedstock confirm the experiments done at the University of Hawaii. Compared to the traditional gasification process for the hydrothermal gasification 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 tar formation can be suppressed, and the heteroatomes (S, N, and halogenes) leave the process with the aqueous effluent avoiding expensive gas cleaning. Further experiments have to be done to optimize the process parameters (pressure, additives) especially in view of higher feed concentrations (\ 10 wt% organic) necessary to achieve a thermal efficiency high enough to establish an economic process. For the process development the next indispensable step is the construction of a pilot plant in a representative scale to optimize the technical components and to demonstrate the
H. Schmieder et al. / J. of Supercritical Fluids 17 (2000) 145–153
interconnected operation. The following technical hurdles have to be overcome by R&D: make available a reliable high pressure feeding system for slurries, fouling problems of the heat exchanger, preheater and reactor caused by salty precipitates, find a construction material resistant also with regard to hydrogen embrittlement, and make available a cheap treatment of the aqueous effluent to meet the release standards. To heat up the feed to the reaction temperature including the introduction of the rather low gasification heat besides the sensible heat after the effluent/feed heat exchange two alternatives are possible: (i) autothermal, where the required heat is introduced by partial oxidation of the feedstock (oxidant addition) and (ii) allothermal, where the required heat is introduced by a preheater where a portion of the product gas is burned. Fig. 2 shows a first process flow diagram for an autothermal concept with hydrogen enrichment in the high pressure separator by addition of diluting water. We think that for the pilot plant a minimum feed rate of several tens of liter per hour is required for the following reasons: reasonable scale-up factor to a later technicalsized plant; reduced influence of wall effects; high pressure slurry pumps are not available for smaller throughputs.
References [1] A.P.C. Faaij, Energy from biomass and waste, Ph.D. thesis, Utrecht, 1997, p. 31. [2] D. Wintzer, et al., Wege umweltvertra¨glicher Verwertung organischer Abfa¨lle, Schriftenreihe Abfallwirtschaft in Forschung und Praxis, vol. 97, Erich Schmidt, Berlin, 1996. [3] M. Kaltschmitt, L. Dinkelbach, Biomass for energy in Europe. Status and prospects, in: M. Kaltschmitt, A.V. Bridgwater (Eds.), Biomass Gasification and Pyrolysis – State of the Art and Future Prospects, CPL Press, Newbury, UK, 1997, p. 7. [4] M. Modell, Gasification and liquefaction of forest products in supercritical water, in: R.P. Overend, T.A. Milne, L.K. Mudge Jr (Eds.), Fundamentals of Thermochemical Biomass Conversion, Elsevier, Amsterdam, 1985, p. 95.
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[5] T. Minowa, F. Zhen, T. Ogi, Cellulose decomposition in hot-compressed water with alkali or nickel catalysts, J. Supercritical Fluids 13 (1998) 253 – 260. [6] T. Minowa, T. Ogi, Y. Dote, S. Yokoyama, Methane production form cellulose by catalytic gasification, Renewable Energy 5 (1994) 813 – 815. [7] D.C. Elliott, L.J. Sealock Jr, E.G.C. Baker, Chemical processing in high-pressure aqueous environments 3. Batch reactor process development experiments for organics destruction, Ind. Eng. Chem. Res. 33 (1994) 558 – 565. [8] D.C. Elliott, M.R. Phelps, L.J. Sealock Jr, E.G.C. Baker, Chemical processing in high-pressure aqueous environments 4. Continuous-flow reactor process development experiments for organics destruction, Ind. Eng. Chem. Res. 33 (1994) 566 – 574. [9] D. Yu, M. Aihara, M.J. Antal Jr, Hydrogen production by steam reforming glucose in supercritical water, Energy Fuels 7 (1993) 574 – 577. [10] X. Xu, Y. Matsumura, J. Steinberg, M.J. Antal Jr, Carbon-catalyzed gasification of organic feedstocks in supercritical water, Ind. Eng. Chem. Res. 35 (1996) 2522 – 2530. [11] X. Xu, M.J. Antal Jr, Gasification of sewage sludge and other biomass for hydrogen production in supercritical water, Environ. Prog. 17 (1998) 215 – 220. [12] G. Del Re, G. Giacomo, V. Brandini, Oxidation of aryl-alkyl hydrocarbons in CO2 based supercritical fluids, in: Proceedings of the 4th Italian Conference on Supercritical Fluids and their Applications, Sept. 7 – 10, Capri, Italy, 1997, p. 35. [13] A. Kruse, K.H. Ebert, Chemical reactions in supercritical water 1. Pyrolysis of tert-butylbenzene, Ber. Bunsenges. Phys. Chem. 100 (1996) 80 – 83. [14] A. Kruse, J. Abeln, E. Dinjus, et al., Gasification of biomass and model compounds in hot compressed water, in: N. Dahmen, E. Dinjus (Eds.), Proceedings of the International Meeting of the GVC-Fachauschuß ‘Hochdruckverfahrenstechnik’, Forschungszentrum Karlsruhe, 1999, 1999, pp. 111 – 114 Wissenschaftliche Berichte FZKA 6271. [15] D. Meier, P. Rimbracht, A. Kruse, M. Schacht, Gasification of pyrocathechol in supercritical water in the presence of potassium hydroxide, submitted. [16] H. Schmieder, E. Dinjus, H. Goldacker, A. Kruse, Experiences with supercritical oxidation for hazardous waste treatment, in: Proceedings of the 4th Italian Conference on Supercritical Fluids and their Applications, Sept 7 – 10, Capri, Italy, 1997, p. 343. [17] M. Siskin, A.R. Katritzky, Reactivity of organic compounds in hot water: geochemical and technological implications, Science 254 (1991) 231. [18] L. Sealock, D.C. Elliott, E.G. Baker, R.S. Butner, Chemical processing in high-pressure aqueous environments 1. Historical perspective and continuing developments, Ind. Eng. Chem. Res. 32 (1993) 1535. [19] H.R. Holgate, J.C. Meyer, J.W. Tester, Glucose hydrolysis and oxidation in supercritical water, AIChE J. 41 (1995) 837.
Hydrothermal gasification of biomass and organic wastes H. Schmieder *, J. Abeln, N. Boukis, E. Dinjus, A. Kruse, M. Kluth, G. Petrich, E. Sadri, M. Schacht Forschungszentrum Karlsruhe, Institut fu¨r Technische Chemie, Postfach 3640, 76021 Karlsruhe, Germany Received 15 July 1999; received in revised form 20 October 1999; accepted 4 November 1999
Abstract Wet biomass and organic wastes can be efficiently gasified under hydrothermal conditions to produce a hydrogen rich fuel gas. New experiments in two tubular flow reactors and in two batch autoclaves with carbohydrates, with aromatic compounds, with glycine as a model compound for proteins and with real biomass are reported for different residence times, temperatures and pressures. It was found that at 600°C and 250 bar all compounds are completely gasified by addition of KOH or K2CO3, forming a H2 rich gas containing CO2 as the main carbon compound. Concentrations of CO, CH4 and C2 –C4 hydrocarbons are low in the product gas (B 1, 3 and B 1 vol%, respectively). Carbon balances for the smaller flow reactor are closed to better than 96%. Ranges of product concentrations are given as estimated from experimental reproducibility. Some reflections for the engineering of a pilot plant are presented. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Gasification; Biomass; Waste; Supercritical water; Flow reactor; Batch experiments
1. Introduction Energy from biomass may significantly contribute to the growing future demand for energy. Energy from biomass avoids the net increase of carbon dioxide in the atmosphere and would help to fulfil the obligations of the European Union to reduce carbon dioxide release. Today the main part of the energetic use of biomass comes from the incineration of wood with a rather low energy yield. An increase in the * Corresponding author. Fax: +49-7247-822244. E-mail address: [email protected] Schmieder)
(H.
efficiency can be expected by gasification and use of the fuel gases in turbines or even in fuel cells. The supply potential is considerable as shown in Table 1 for the example of The Netherlands [1]. Table 1 does not mention manure production and only considers in part wet organic wastes arising from different branches of industry. Such wastes may have high negative costs (disposal costs) which are a benefit for the gasification process. The estimated energy potential represents 6.1 billion m3 natural gas or 15% of the natural gas consumption of The Netherlands. Although the larger part of the amount is subsequently used today, some of the 4000 kt/a should be available for energy use. The estimated supply poten-
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tial of rest biomass (DOM) for Germany amounts to 50 000 kt/a [2] and for the European Union to 200 000 kt/a (J.M.L. Penninger, private communication) or more [3]. A large part of the biomass is wet biomass containing up to 95% water. For water contents of more than 40% the thermal efficiency of a traditional steam (reforming) gasification plant decreases drastically to 10% at 80% water (J.M.L. Penninger, private communication). A very promising alternative for wet biomass is the less investigated hydrothermal gasification (expected thermal efficiency 70% at 90% water; J.M.L. Penninger, private communication). First experiments to convert carbohydrates to liquid products in supercritical water were performed by Modell and Amin in the mid 1970s at MIT [4]. Systematic experimental investigations for the conversion and gasification of biomass, for waste model compounds and for real wastewater under hydrothermal conditions were carried out at the Pacific Northwest Laboratories (PNL), at the university of Hawaii and at the National Institutes for the Resources and Environment, Japan (NIRE). At NIRE the conversion of cellulose was studied in autoclaves with residence times of up to 1 h at 200– 400°C and 80 – 180 bar [5,6]. It was found that sodium carbonate as a catalyst suppresses the formation of char and oil and mainly water soluble products were formed. At 400°C with Ni catalysts CH4 and CO2 were found as major products in the gas phase. Laboratory tests at PNL [7,8] were directed to the formation of a CH4-rich gas from biomass,
waste model compounds and real wastewaters. Batch reactor experiments were performed at 350°C, 200 bar and reaction times of 60–120 min. It is shown that aromatic and aliphatic hydrocarbons as well as oxygenates are converted to a methane-rich fuel gas in the presence of hydrogenation catalysts. The results were confirmed in continuous-flow reactor experiments with residence times of 10 min and more for conversions of 90% and higher. Antal investigated the coke-catalyzed gasification of model substances, biomass (aquatic plants and others) and wastes of the Department of Energy at 600–650°C, 280 or 345 bar and reaction times of 30 s in a tubular flow reactor at the University of Hawaii [9–11]. At 600°C and 345 bar, glucose and other feeds at concentrations of up to 0.2 M were gasified completely to a hydrogen-rich gas. Coke or tar formation was not observed. Department of Defense wastes were also completely gasified under these conditions. Liquefaction experiments of biomass under supercritical conditions were also done at the University of L’Aquila [12]. At the Forschungszentrum Karlsruhe experiments for hydrothermal conversions of organic material started in the early 1990s with a kinetic investigation of the thermolysis of tert-butylbenzene [13] and were continued with the treatment of model compounds and real biomass and wastes with batch and tubular flow reactor experiments preferably aiming at the production of a hydrogen-rich fuel gas [14,15]. Since hy-
Table 1 Supply potential of rest biomass – The Netherlands [1] Material
Moisture, % of wet material
Agriculture (straw, bulbs…) Organic wastes (food and beverages, waste paper…) Wood (thinnings, prunings…) Sludges (water treatment…) Total
15–80 10–80 15–50 60–95
Ash, % of dry material
Supply of dry organic material, kt DOM/a
Energy potential (heating value), PJ/a
1–10 1–20
870 6080
17.4 124.3
1–5 40–65
1840 760 9550
35.8 11.5 189
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147
Fig. 1. Miniplant.
drothermal gasification of aromatic compounds and amino acids at temperatures above 500°C was not known from the literature we studied glucose as a model compound for cellulose; aromatic compounds as model compounds for lignin; and glycine as a model compound for protein. We observed a pronounced effect of the addition of potassium on the reforming and shift reactions and this was therefore studied in some detail. In our latest experimental set-up we succeeded in closing the balance to better than 96%. The paper presents our results and points out future work to be done for a technical plant.
experiments by the addition of a certain amount of the material to the autoclaves.
2.1. Autocla6e I Designed for temperatures of up to 700°C and pressures of 1000 bar, it is built from Nimonic 110 with an inner volume of 100 ml and is equipped with a magnetic stirrer for mixing. The educts (model compounds, real biomass or wastes) were injected after heating of the water filled autoclave to the desired temperature.
2.2. Autocla6e II 2. Experimental The model compounds glucose for cellulose, catechol and vanillin for lignin and glycine for proteins were used as aqueous solutions. Real biomass (wood as saw dust, straw) and wastes (sewage sludge and lignin) were treated in batch
Designed for temperatures of up to 500°C and pressures of 500 bar, it is built from stainless steel containing an inlet made from Inconel 625 with an inner volume of 1000 ml. Mixing is achieved by a tumbling device. The educts were heated together with water to the desired temperature ( 2–3 h).
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2.3. Miniature plant
2.5. Analytical
The experimental set-up of the tubular flow reactor is illustrated in Fig. 1. The flow reactor was fabricated from Inconel 625 with a tube length of 500 mm, an outer diameter of 14.4 mm and an inner diameter of 8 mm. The reactant solution is fed into the reactor by a Bischoff 2250 HPLC pump. The feed is quickly heated to 600°C with an electrical heater which is coiled around the entrance part of the reactor and maintained at this temperature by two additional heaters downstream of this entrance heater. The ‘reaction zone’ of the reactor has a length of 350 mm. A stainless steel rod with a length of 80 mm and a diameter of 7.8 mm is placed at the entrance and the exit of the reactor. In this way, backmixing is suppressed and a fast heating of the feed flow at the reactor entrance and a fast quenching of the product flow at the reactor exit are guaranteed. The temperature profile along the reactor axis is measured by six type K thermocouples mounted on the reactor’s outer wall. A movable type K thermocouple inside a capillary tube fixed inside the reactor provides the temperature of the feed stream in the reaction zone. The pressure is measured by a Burster type 8201 pressure sensor. After the gasification reaction, the effluent is cooled down to room temperature by a cooling water jacket at the exit of the reactor, depressurized by a TESCOM model 26-1761/ER3000 electro-pneumatically controlled back pressure regulator and transferred into a phase separator. The liquid phase was collected at the bottom of the phase separator, while the gas phase was passed to a water-filled gas flow meter to determine the volume of the formed gas mixture. Samples of the gas phase were taken by a gas sampling tube between the phase separator and the gas flow meter.
After identification by HP GC-MS of the individual compounds, liquid samples were analyzed with an HP 5890 gas chromatograph combined with a Varian autosampler for S6 olid-P6 hase M6 icroe6 xtraction, SPME fiber: 85 mm polyacrylate, 20 min extraction; column: DP5, 30 m× 0.25 mm, 0.25 mm; carrier: 1 ml/min He; detector: flame-ionization detector, 250°C; oven: 40–260°C (12 min) at 10°C/min. In addition, the total organic carbon contents of the liquid phase were determined (TOC Analyzer DC 190 by Schmidlin). Besides the gas flow measurement, CO and CO2 were analyzed on line by IR spectroscopy and O2 by a paramagnetic sensor (Uras 10 E by Hartmann and Braun). H2 was analyzed by thermal conductivity (Caldos 5B by Hartmann and Braun) for the experiments in the bench scale plant. For the miniature plant, samples were taken from the gas phase and analyzed by gas chromatography. Two HP 5880 gas chromatographs were used, one with N2 carrier gas for hydrogen analysis, the other with He carrier gas for all other gases. For the column switching procedure used, a Porapak Q column (2 m long) and a mole sieve column (13× mesh 60–80, 2 m long) were connected in series while the second column was bridged for the analysis of CO2 and hydrocarbons during the analysis run (carrier: 22.8 ml/min He, 22.8 ml/min N2 for hydrogen analysis; thermal conductivity and flame-ionization detector connected in series, 200°C; oven: 80°C (19 min) to 160°C (60 min) at 20°C/min).
2.4. Bench scale plant The plant was originally built for experiments to oxidize pollutants in supercritical water (SCWO). A tubular flow reactor (Inconel 625, inner diameter 8 mm, length 15 m) heated by a fluidized sand bath is used. A description is given in [16].
3. Results and discussion The predominant part of the experiments was performed in the tubular flow reactors to investigate the influence of the main parameters (temperature, pressure, residence time and alkali metal addition) on the gasification efficiency to produce a product gas with an optimum hydrogen yield. In these experiments, only model compounds could be used because no reliable slurry (suspension) feeding system has been available up to now for the bench scale plant.
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149
only with traces of solid and oily by-products. Samples of the aqueous effluent (TOC(res)) contained mainly phenol and derivates despite low aliphatic ketones identified by GC-MS. This is in contrast to the results at MIT [19] where besides the low carbonic acids and aldehydes, furan derivates were also found by HPLC chromatography. These investigations were done at very low feed concentrations (10 − 3 M) without addition of alkali metals. A similar temperature trend was observed with gasification of the aromatic compound catechol [15]. The addition of potassium also significantly influenced the CO content in the product gas as illustrated in Table 3. The experiments with glucose show that by addition of KOH the CO concentration in the product gas decreases more than 20-fold. A similar influence is observed in the experiments with vanillin where potassium carbonate was added instead of KOH. It can be concluded that the addition of alkali metals, probably as acid-base catalyst, increases the rate of the shift reaction (CO+ H2O= CO2 + H2) regarding the shift as a follow reaction of the reforming. The effect is significantly less marked with addition of alkaline
Table 2 shows the temperature influence on the total organic carbon (TOC) destruction efficiency of glucose at feed concentrations of up to 0.3 M in both plants (TOC(res)= TOC of the aqueous effluent). At low temperatures soot and tar are formed which are not considered in the TOC destruction efficiency. The values in brackets (column two) indicate the total gasification efficiency considering soot and tar formation. For the experiments in the miniature plant generally a closure of the carbon balance better than 96% could be achieved. Several authors studied the reforming reaction of different organic materials in sub- and supercritical water with addition of catalysts (for reviews see Refs. [17,18]). Obviously, the addition of alkali metals increases the reaction rate and suppresses the formation of soot and tar. The best effect as comparing other alkali metals was in our screening experiments achieved by the addition of potassium. The influence is shown in Table 2 by the experiments at 600°C with and without added KOH. The results show that at temperatures higher than 550°C a complete gasification of glucose can be achieved without the formation or
Table 2 Average TOC destruction efficiency of glucose as function of temperature T, °C
1−TOC(res)/TOC(feed)
P, bar
Feed, M
[K], M
400 500a 550b 600b 600a
0.35 (0.12) 0.6 (0.46) 0.83–0.9 0.88 0.99
300 300, 450 270, 310 210, 310 100(!)–300
0.2 0.2 0.1–0.3 0.1 0.1
1.8×10−3 (KOH) 1.8×10−3 (KOH) 1–3×10−3 (K2CO3) 0 1.8–5×10−3 (KOH)
a b
Miniature plant; residence time: 60–140 s. Bench scale; residence time: 60–140 s.
Table 3 Average CO and CH4 product gas content with and without addition of KOH or K2CO3
Glucose without KOH Glucose 0.18–2×10−2 M Vanillina without K2CO3 Vanillinb 0.7–1.5×10−3 M a b
CO, vol%
CH4, vol%
T, °C
Feed, M
20 0.6 36 1.5
3.71 10.0 90.11, 3.3 90.52 9.4 90.25 12.6 9 1.4
550, 600 600 600 600
0.1 0.1–0.6 0.067 0.067
Bench scale; residence time: 60–120 s; P: 200–310 bar; [KOH]: 2×10−2 M. Miniature plant; residence time: 60–120 s; P: 200–310 bar; [KOH]: 1.8×10−3 M.
H. Schmieder et al. / J. of Supercritical Fluids 17 (2000) 145–153
150 Table 4 Average gas compositions
CO2
CH4
C2H4
Average gas composition of glucose gasification at 600°Ca Vol% 59.79 0.55 0.439 0.08 Mol gas/mol glucose 9.1 B0.1
31.8 91.1 5.1
3.2 9 0.3 0.6
0.65 9 0.2 B0.2 C2–C4
Average gas composition of catechol gasification at 600°Cb Vol% 61.59 2.4 0.529 0.04 Mol gas/mol catechol 10.6 B0.1
29.3 92.0 5.0
2.2 9 0.7 0.5
0.52 9 0.26 B0.2 C2–C4
H2
a b
CO
TOC(res)=(19 918) ppm; residence time: 30–120 s; P: 250 bar; [KOH]: 1.8×10−3 M. TOC(res)=(68 919) ppm; residence time: 30–120 s; P: 200–300 bar; [KOH]: 1.8×10−3 M.
earth metals [18]. The difference in the formation of CH4 between the experiments done in the bench scale or miniature plant may be caused by the different concentrations of potassium used and/or by the different geometry of the reactor tubes. The influence of residence time on the gasification efficiency was tested for both glucose and catechol (feed: 0.2 M) at 30, 60 and 120 s with the miniature plant (600°C, 250 bar, KOH). The results show that for such feed concentrations a minimum residence time of 60 s is required. At 30 s, the gasification efficiency (TOC(res)) drops to 98% and a decrease of the CH4 yield is observed. At feed concentrations higher than 0.6 M and the low potassium concentrations used a drastic decrease of the gasification efficiency is observed with formation of soot and tar at residence times of 120 s as well. A marked pressure effect is observed for the CH4 yield. The CH4 content in the product gas increases from 3 vol% at 250 bar to 8 vol% at 450 bar whereas H2 decreases from 60 vol% at 250 bar to 50 vol% at 450 bar. Table 4 shows the average product gas compositions of glucose and catechol gasification experiments in the miniature plant at 600°C and feed concentrations of 0.2 M. For these experiments the closure of the carbon balance is better than 96% and no or only traces of solids or oily material were found. Besides methane C2 –C4 hydrocarbons with C2H4 as the major component were detected in the product gas with a yield of below 1 vol%. The results show that under these conditions water takes part
(splits) as reactant with a large portion. For glucose 76% of the theoretical hydrogen formation is found according to the equation: C6H12O6 + 6H2O= 6CO2 + 12H2, perfect gas: DH = 158 kJ/mol
(1)
The experimental result corresponds approximately to the following equation: 2C6H12O6 + 10H2O= 11CO2 + CH4 + 20H2, DH = 152 kJ/mol
(2)
For the gasification of catechol 82% of the theoretical hydrogen formation is achieved if only carbon dioxide and hydrogen are considered as products. Glycine was used as a model compound for proteins and polyamides in gasification experiments using the miniature plant (600°C, 250 bar, KOH) [15]. A TOC(res) of 1.5% was found in the aqueous effluent at residence times of 60 and 120 s. The nitrogen of the amino acid is completely converted to ammonia and a hydrogen rich gas is produced because a large part of the carbon is bound in the aqueous effluent ( 45%). With real biomass and wastes, only batch experiments have been done in our laboratory up to now. Table 5 shows results of experiments performed with the stirred autoclave I with vanillin as a model compound for lignin and with lignin itself at 600 and 570°C. The achieved TOC destruction efficiency is high although the real gasification efficiency could not be determined because the traces of solid and oily product are not quantitatively recovered. In the aqueous effluent cate-
H. Schmieder et al. / J. of Supercritical Fluids 17 (2000) 145–153 Table 5 Screening experiments with autoclave Ia
Vanillinb Ligninc
Educt (res)/Educt (feed)
T, °C
P, bar
\0.99 \0.99
600 570
200–600 400
a
Reaction time: 1800–3600 s; [educt]: 2500 mg/kg. Without addition of K2CO3. c [K2CO3]: 1.8×10−3 M. b
chol, methoxyphenol and phenol were identified as the major compounds by GC-MS. For vanillin more than 99% destruction efficiency is measured even without added potassium. Compared to the tubular flow reactor experiments, both the low feed concentration and the higher reaction time, might explain this finding. Table 6 shows results of the screening experiments with straw, wood and sewage sludge at temperatures of 450 and 500°C using the tumbling autoclave II. For straw a significant increase of the TOC destruction efficiency is found as was to be expected with increasing temperature but no difference is found between the experiments with and without addition of potassium. This finding can be explained by the high potassium content of straw (ash: 4.6 wt% with 15 wt% K). The high yield of methane is striking compared to the tubular flow reactor experiments. The lower temperature and the longer reaction time of 2 h and more with the tumbling reactor is probably an explanation. Altogether the results confirm the findings above as well as the results of Antal
151
[9–11] that for a complete gasification temperatures higher than 550°C are required. The current knowledge about reaction fundamentals in hot high pressure water does not allow an attempt of the interpretation of the experimental observations on the basis of reaction mechanisms and kinetics. Thermodynamic calculations were employed instead to improve the understanding of the reaction paths occurring during gasification. The equilibrium constants were calculated with the aid of the BENSON code/data base (NIST) and for the variation of pressure and educt concentration our own code VISCO (G. Petrich, private communication) was used. Besides the primary dehydration/cracking step three reactions are assumed for the gasification of ethanol: reforming,
e.g.
C2H5OH+ H2O= 2CO+ 4H2,
DH = 256 kJ/mol the shift
(3)
CO+ H2O= CO2 + H2,
DH = − 41 kJ/mol
(4)
and the methanation
CO+ 3H2 = CH4 + H2O,
DH = − 210 kJ/mol.
(5)
The results of the preliminary calculations show that for the model compounds the temperature and pressure trends found experimentally are confirmed. The methane yield is predicted too high compared to the experimental results found with the tubular flow reactors operated at short residence times. The calculations as opposed to the experimental findings will be discussed in a further publication.
Table 6 Screening experiments with autoclave II
Strawa Strawa Woodb Sewageb Sewagec a
Educt (res)/Educt (feed)
T, °C
H2, vol%
CO, vol%
CO2, vol%
CH4, vol%
TOC(feed), ppm
0.8 \0.9 0.9 0.552 0.853
450 500 450 450 450
33–43 34–42 30 49 47
1.2 0.3 1.7 2.8 1
40–47 40–45 49 31.2 37
17.5 16.2 19 17 15
4600 4600 4900 2300 2300
With and without addition of K2CO3; 1.7x10−3 M; reaction time: 7200 s; P: 315–350 bar. Without K2CO3; 1.7x10−3 M; reaction time: 7200 s; P: 315–350 bar. c [K2CO3]: 1.7x10−3 M; reaction time: 7200 s; P: 315–350 bar. b
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Fig. 2. Autothermal gasification pilot plant.
4. Conclusion and outlook Our experiments in two tubular flow reactors and in two batch autoclaves show that in the presence of KOH or K2CO3 at 250 bar and temperatures higher than 550 – 600°C 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; concentrations of CO, CH4 and C2 – C4 hydrocarbons are low in the product gas ( B 1, 3 and B 1 vol%, respectively); at lower feed concentrations (50.2 M) residence times of 30 s are required. At higher feed concentrations (]0.6 M) and constant potassium concentrations soot and tar formation appears; the addition of potassium decreases the CO concentration and increases CO2 and H2 in the product gas; carbon balances for the miniature plant are closed to better than 96%;
the screening experiments with the real feedstock confirm the experiments done at the University of Hawaii. Compared to the traditional gasification process for the hydrothermal gasification 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 tar formation can be suppressed, and the heteroatomes (S, N, and halogenes) leave the process with the aqueous effluent avoiding expensive gas cleaning. Further experiments have to be done to optimize the process parameters (pressure, additives) especially in view of higher feed concentrations (\ 10 wt% organic) necessary to achieve a thermal efficiency high enough to establish an economic process. For the process development the next indispensable step is the construction of a pilot plant in a representative scale to optimize the technical components and to demonstrate the
H. Schmieder et al. / J. of Supercritical Fluids 17 (2000) 145–153
interconnected operation. The following technical hurdles have to be overcome by R&D: make available a reliable high pressure feeding system for slurries, fouling problems of the heat exchanger, preheater and reactor caused by salty precipitates, find a construction material resistant also with regard to hydrogen embrittlement, and make available a cheap treatment of the aqueous effluent to meet the release standards. To heat up the feed to the reaction temperature including the introduction of the rather low gasification heat besides the sensible heat after the effluent/feed heat exchange two alternatives are possible: (i) autothermal, where the required heat is introduced by partial oxidation of the feedstock (oxidant addition) and (ii) allothermal, where the required heat is introduced by a preheater where a portion of the product gas is burned. Fig. 2 shows a first process flow diagram for an autothermal concept with hydrogen enrichment in the high pressure separator by addition of diluting water. We think that for the pilot plant a minimum feed rate of several tens of liter per hour is required for the following reasons: reasonable scale-up factor to a later technicalsized plant; reduced influence of wall effects; high pressure slurry pumps are not available for smaller throughputs.
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[5] T. Minowa, F. Zhen, T. Ogi, Cellulose decomposition in hot-compressed water with alkali or nickel catalysts, J. Supercritical Fluids 13 (1998) 253 – 260. [6] T. Minowa, T. Ogi, Y. Dote, S. Yokoyama, Methane production form cellulose by catalytic gasification, Renewable Energy 5 (1994) 813 – 815. [7] D.C. Elliott, L.J. Sealock Jr, E.G.C. Baker, Chemical processing in high-pressure aqueous environments 3. Batch reactor process development experiments for organics destruction, Ind. Eng. Chem. Res. 33 (1994) 558 – 565. [8] D.C. Elliott, M.R. Phelps, L.J. Sealock Jr, E.G.C. Baker, Chemical processing in high-pressure aqueous environments 4. Continuous-flow reactor process development experiments for organics destruction, Ind. Eng. Chem. Res. 33 (1994) 566 – 574. [9] D. Yu, M. Aihara, M.J. Antal Jr, Hydrogen production by steam reforming glucose in supercritical water, Energy Fuels 7 (1993) 574 – 577. [10] X. Xu, Y. Matsumura, J. Steinberg, M.J. Antal Jr, Carbon-catalyzed gasification of organic feedstocks in supercritical water, Ind. Eng. Chem. Res. 35 (1996) 2522 – 2530. [11] X. Xu, M.J. Antal Jr, Gasification of sewage sludge and other biomass for hydrogen production in supercritical water, Environ. Prog. 17 (1998) 215 – 220. [12] G. Del Re, G. Giacomo, V. Brandini, Oxidation of aryl-alkyl hydrocarbons in CO2 based supercritical fluids, in: Proceedings of the 4th Italian Conference on Supercritical Fluids and their Applications, Sept. 7 – 10, Capri, Italy, 1997, p. 35. [13] A. Kruse, K.H. Ebert, Chemical reactions in supercritical water 1. Pyrolysis of tert-butylbenzene, Ber. Bunsenges. Phys. Chem. 100 (1996) 80 – 83. [14] A. Kruse, J. Abeln, E. Dinjus, et al., Gasification of biomass and model compounds in hot compressed water, in: N. Dahmen, E. Dinjus (Eds.), Proceedings of the International Meeting of the GVC-Fachauschuß ‘Hochdruckverfahrenstechnik’, Forschungszentrum Karlsruhe, 1999, 1999, pp. 111 – 114 Wissenschaftliche Berichte FZKA 6271. [15] D. Meier, P. Rimbracht, A. Kruse, M. Schacht, Gasification of pyrocathechol in supercritical water in the presence of potassium hydroxide, submitted. [16] H. Schmieder, E. Dinjus, H. Goldacker, A. Kruse, Experiences with supercritical oxidation for hazardous waste treatment, in: Proceedings of the 4th Italian Conference on Supercritical Fluids and their Applications, Sept 7 – 10, Capri, Italy, 1997, p. 343. [17] M. Siskin, A.R. Katritzky, Reactivity of organic compounds in hot water: geochemical and technological implications, Science 254 (1991) 231. [18] L. Sealock, D.C. Elliott, E.G. Baker, R.S. Butner, Chemical processing in high-pressure aqueous environments 1. Historical perspective and continuing developments, Ind. Eng. Chem. Res. 32 (1993) 1535. [19] H.R. Holgate, J.C. Meyer, J.W. Tester, Glucose hydrolysis and oxidation in supercritical water, AIChE J. 41 (1995) 837.